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Discussion Starter #21 (Edited)
Body Weight

In the sports car world, the Genesis Coupe is a heavy beast. While weight does help the car stay grounded, the momentum that the car builds up when it is moving tends to make the car act like a sledgehammer trying to change direction mid swing. While it is possible to do, it takes far more effort to change direction than a lighter car. The additional weight means that more torque is being used to propel a larger mass down the road that affects acceleration and gas mileage. Heavier weight means that the brakes must work harder to bring the car to a stop.

Shedding weight is a tradeoff. In many cases you can shed weight simply by removing things that you aren’t using… seats, trim, carpeting, spare tire, and etc. The obvious problem with this methodology is that using a stripped down car for daily driving could present a significant comfort issue.

At this point you are probably thinking that there is really nothing you are willing to do to shed weight, so what would be the point of this section of the thread? The point is that if you cannot lose weight you should at very least understand how to manage the weight or more importantly how the weight is distributed from front to back and from left to right. Ideally, you want a perfectly balanced car that shares and equal amount of weight among all of the wheels. To accomplish this, you will need a set of adjustable coilovers.

Sprung Weight vs. Unsprung Weight vs. Rotating Mass weight

Sprung weight is everything from the springs up, and unsprung weight is everything from the springs down (the stuff that moves with the road). Wheels, shock absorbers, springs, joints and tires contribute to the unsprung weight. The car, engine, fluids, you, your passenger, the kids, the bags of candy and the portable Playstation all contribute to the sprung weight. Reducing unsprung weight is the key to increasing performance of the car. If you can make the wheels, tires and control arms lighter, then the suspension will spend more time compensating for bumps in the road, and less time compensating for the mass of the wheels etc. The greater the unsprung weight, the greater the inertia of the suspension, which will be unable to respond as quickly to rapid changes in the road surface. In other words, heavy suspensions tend to feel less responsive and "heavy".

While it is unlikely that the average tuner will be able to change out whole suspension components, putting lighter wheels can lighten the unsprung rotating weight of the car which can increase your engine's apparent power. Why? Well the engine has to turn the gearbox and driveshafts, and at the end of that, the wheels and tires. Heavier wheels and tires require more torque to get turning, which saps engine power. Lighter wheels and tires allow more of the engine's torque to go into getting you going. That's why sports cars have ultra light alloy wheels. On the other side of the coin, a heavy rotating mass takes much more energy to slow down, so braking is less responsive and effective. Less intuitively, high wheel/tire weights can contribute greatly to a phenomenon known as "wheel hop". Overall, heavy tires and wheels are going to have a dramatic effect on the overall performance of the car, not just in handling performance.

The sprung weight is what is thrown around when the vehicle is subjected to heavy acceleration, deceleration, and turns. Reducing the sprung weight will reduce how much weight is being transferred when the body rolls; however, it must be noted at this point that your springs and your dampeners were designed to control the sprung weight and how that weight transfers when the body rolls.

The lesson to be learned here folks is that stripping the interior of the car for "weight reduction" may not always lead to better overall performance if unsprung weight is not addressed. In fact, unsprung weight will have a greater influence as to how well a car accelerates, stops, or handles corners more so than the sprung weight of the car. Consider that before you decide to pull the back seat out of your car to "reduce weight".

Corner balancing

The corner balance process changes the weight on a tire by adjusting the spring height at each corner of the car; this in turn can also affect the ride height of the car. Lengthening the spring decreases the force or weight on the tire (lowering that corner of the car) and shortening or compressing the spring increases the force or weight on the tire (raising that corner of the car). Adjusting the weight at any corner of the car will also affect the weight settings at the other three suspension corners. If the corner balance is not within an acceptable range, the car is rolled off the weight scales and one or more of the spring perches (heights) are then adjusted, the car is then rolled back onto the scales and the corner weights are re-measured. This process is repeated until the imbalance is brought to within the lowest value possible. Ideally, the car is balanced while you are sitting in the driver’s seat to compensate for your weight. If you typically drive with a co-driver, the co-driver should also be present when the car is adjusted. It is possible to get nearly a perfect static distribution of weight among all four wheels using the corner balance method on a Genesis Coupe.



Corner balancing the car ensures a more predictable shift in dynamic weight transfer when the car is moving. Corner balancing should always be done in conjunction with alignment since camber adjustments will affect ride height and ride height will affect alignment. The two activities cannot be conducted separately effectively. The result of an effective corner balance is an optimized suspension system that will deliver the best performance given the limitations of the suspension design.

Higher spring rates

Higher spring rates increases the amount of force required to compress the spring, reducing the amount of vertical movement and weight shifting during acceleration, braking, and turning. Higher spring rates are a trade off with comfort… so the advice is to go with the highest spring rates that you can tolerate on your daily drive. If you have a dedicated track vehicle, go with the highest spring rates you can find for your car.


Stiffer damping rates

This is where adjustable dampeners pay for themselves. That ability to change damping rates means that you can make on the fly adjustments to account for road conditions and driving environments. Stiffening the damping rate reduces the speed at which weight is transferred. While a very stiff dampener may be good for a dry flat road, it may be less than ideal for roads that are not quite smooth and you require a faster rebound rate in order to keep your tires on the ground. You want your tire to be able to quickly conform to the road surface while at the same time minimize the bounce and weight transfer. There is no “ideal” setting for damping as the damping adjustment will vary according to the car’s configuration, weight, weight distribution, and road conditions. It is probably fortunate that making damping adjustments are so easy to do as the process of finding the ideal damping setting is truly a trial and error effort.

Tire load rating

While not directly related to the car’s weight, tires are rated for a maximum load. Ensure that the tires you purchase for your car have a maximum load-rating equivalent to the load that you will be placing on it or better. This is a dynamic load computation as opposed to a static load rating. As a result it would be difficult for you to compute the number yourself to determine whether the appropriate tire is installed. With that said, you do have a guide. If you purchase a tire with an equal or greater load rating than the OEM tire, you are good to go. Overloading the tires will result in a buildup of excessive heat in the tire, which could lead to tire failure and reduced grip performance.

Tire selection and configuration

Earlier in this guide I wrote extensively about different types of tires and how to read the various markings on the tire. The intent of this section is not to repeat that information; rather it is to discuss tires as it relates to maximum performance. Typically, your best performing tires will always be a summer tire. Although manufacturers have also come up with other descriptors such as ultra-high performance, high performance, or performance ratings, these are only typically applicable to that manufacturer’s line of tires and may have little or no bearing on tires made by other manufacturers. In other words, just because a tire manufacturer rates their tire at the “Ultra High Performance” level, that rating only applies to how that tire compares to other tires that manufacturer produces. There is no industry wide standard for rating tires on the basis of performance other than speed or load rating. Given this disparity among manufacturers, a tire manufactured by one manufacturer that has been rated as a “performance” tire may very well be a better performer than a tire made by another manufacturer that has been rated as an “ultra high performance” tire. To find a tire that fits your needs it’s time to do some research and start reading reviews. Keep in mind that reviews done by consumers, such as those that can be found on Amazon, are very subjective and may not represent the actual strengths and weaknesses of a given tire. Try to stick to objective reviews that quantify things like dry and wet performance, lateral grip, stopping distances, and etc.

Tire configuration

Tires come in various configurations and sizes and for most folks the choice comes down to what looks good or what fits as opposed to the tire that will deliver the best performance. There are two primary things you want to look for when you are looking for the tires that are going to give you the performance you are looking for… that is, tread width and profile height. Tread width is pretty much a given. You want to look for a tire that is going to provide the maximum width that will reasonably fit on the wheel. Maximum width means maximum amount of rubber on the road. More rubber = more grip. Tire width is measured in millimeters where wheel width is typically measured in inches which means that you will probably have to do some math to figure out how wide of a tire to go with. For instance, 8.5 inches = 215.9mm. Always round up to the nearest tire width. In this particular case the next standard tire width above 215.9mm is 225mm. If you have a 10.5 inch wide wheel, go with a minimum of a 275mm tire.

The tire profile height is a percentage of the tire width… so a 35 profile tire is 35% of the total width of the tire. Because the profile measurement is a proportional number as opposed to a set number, you have to be careful not to confuse a 225/35 tire height with a 275/35 tire height. They are not the same as 35% of 225mm is 78.75mm or about 3.1 inches tall and 35% of a 275mm tire is 96.25mm or 3.75 inches tall. Now, knowing all that, how does tire profile fit into the performance metric?

The tire profile is a direct reflection of how stiff the sidewall is. A tall tire will have much more deflection than a narrower one. What that means from a performance aspect is that the narrower the tire’s profile, the less deflection it will have, and the more the tire will grip until it gives. Now that sounds like a no brainer right? We’ll just go with the lowest profile that we can to get the maximum amount of grip and our problems are solved? Well… not so fast junior, there’s more to it than that.

While a low profile tire will give you the most lateral grip, they also have a tendency to lose grip suddenly, which can cause you to lose control of your car. What you want is a tire that gives you the maximum amount of grip but will also give you plenty of warning that you are at the edge of where it is going to give. The way that tires do that is that they will typically start to squeal before they lose all grip. So… the answer to the question is that you want enough profile to provide enough deflection to allow for a tiny bit of slip before all grip is lost while at the same time holding the ground for the longest possible time.

The correct answer to the question of how large a profile should I get is that you should get a tire with a profile that gets as close to the OEM tire height as possible. The overall rolling diameter of the tire should never be significantly different from the OEM tire/wheel combination. If you have lowered your car so much that you cannot fit an OEM sized tire in the wheel well, consider raising your car so that it will fit. From a performance standpoint, you never want to compromise performance for looks.

Tire pressure

There are few things that you can configure on your tire than making sure that the outside of the tire is actually mounted correctly, and that directional tires are actually mounted to spin in the right direction. Tire pressure is something that can be easily configured to adapt to road conditions to adjust the gripping force that a tire has on the road.

What is the “ideal” tire pressure? Answer: For most daily driving situations the ideal tire pressure is 35 psi without regard to what the diameter or size of the tire is. If this sounds familiar it is because 35 psi is the tire pressure that is recommended by the manufacturer and is printed on the placard on the driver’s side door jam. With that said, the “ideal” tire pressure may not be the ideal pressure in all circumstances and conditions.

Tire pressure can be adjusted to change the handling characteristics of the car. Typically, a lower pressure will increase grip by allowing the tire to conform better to the road surface. The trade off to lower tire pressure is increased friction, rolling resistance, heat, and wear. Increasing the tire pressure can improve wear and reduce tire rolling temperatures at the expense of decreased grip. Tire pressures on each axle should always be the same; however, you can increase or decrease the tire pressure front and back to compensate for oversteer and understeer conditions. By increasing the tire pressure in the front and dropping the tire pressure in the rear, you increase the tendency of the car to understeer. Likewise and opposite condition will tend to increase oversteer. Few things can be adjusted on the car that can be done relatively easily that will have such a dramatic effect on the car’s overall handling characteristics than simply adjusting the tire pressure in your tires.
 

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Discussion Starter #22 (Edited)
[B]Suspension accessories[/B]

All Genesis Coupes have a MacPherson strut front and 5-link rear suspension with front and rear stabilizer bars, to improve ride and handling. R-Spec and Track (Ultimate) models have track-tuned suspensions with improved roll control and the ability to adjust the front struts to a maximum of 1.5 degrees of negative camber for sharper, more responsive turn-in and reduced understeer. In spite of a few threads out there that highlight the downside of the suspension system in the Genesis Coupe, in reality Hyundai has done a good job of giving us fairly high end performance on a budget with regard to our suspension system. With that said, we have to remember that the car off the showroom floor was designed to please the masses and not necessarily to perform well on the track. With this one limitation, there is room for improvement where performance is concerned. In this section, we will look at various aftermarket options that can be purchased which can improve handling and performance of the Genesis Coupe on the track.

Roll (sway) bar

Stabilizer bars are part of a car's suspension system. They are sometimes also called anti-sway bars or anti-roll bars. Their purpose in life is to try to keep the car's body from "rolling" in a sharp turn. Normally, without a sway bar when the car corners the weight of the chassis shifts toward the outside of the turn compressing the springs on that side. The springs on the inside generally extend a little, or do nothing. In relation to the chassis, it appears that the outside suspension compresses and the inside doesn't.

A sway bar couples the suspensions on each side to each other, relative to the chassis. If you could put the car up on a lift and actually compress the suspension on one side by hand, the sway bar makes the compression of one side also try to compress the suspension on the other. Ok... it's still not really obvious why that's useful so let me say the same thing a different way.

A sway bar effectively increases the spring rate on whichever side is compressed the most. If the sway bar were solid with no twist so there's a 100% coupling between each side then an attempt to compress one spring actually becomes an attempt to compress both springs. It doubles the spring rate. If the bar has some twist, then it may only increase the spring rate by say 50% on whichever side is compressed the most. The animation below illustrates how a sway bar uses the opposite spring to reinforce the spring rate of the spring that is being compressed in a turn.


All trim levels for the Genesis Coupe have both front and rear sway bars; however, as mentioned earlier, R-Spec and Track (Ultimate) models come with beefier sway bars. The 2013+ models had actually reduced the size (stiffness) of the sway bar to improve comfort.

Base model 2010: F 24mm R 19mm
R-Spec/Track model 2010: F 25mm R 22mm

Base model 2013+: F 23mm R 19mm
R-Spec/Track model 2013: F 24mm R 20mm

Front sway bar – why bigger is better

Our cars are set up with a McPherson setup up front. Like all suspension systems on cars they have their pluses and minuses. Some of the pluses for the McPherson setup are weight, and simplicity. The big con, that concerns us and what front sways address, is poor dynamic camber curve.

What happens is this – when you compress the outside front corner (the one that’s loaded when in a turn) the suspension compresses as it should. When it compresses the dynamic camber curve causes the wheel to roll more toward positive camber. Push more and the more it rolls toward positive. This is bad because you start to loose grip as the inside edge of the tire gets “light.” Ideally you want the full contact patch evenly on the ground (or close to evenly) to maximize grip. This is the reason that folks try to run a decent amount of static negative camber. In other words, if you start with negative camber when it rolls over toward positive camber you have enough of a camber “budget” to allow for more contact patch on the ground.

So, where do bigger front bars come into play? Well, the front bar acts as a spring (torsion spring in this case) to help hold the static camber curve as you compress the front suspension. As you compress the loaded corner, the bigger bar will resist that camber roll over to positive on the loaded corner. This is why bigger is better.

Ok, so what size should I use? Well, it really depends upon what you want to do. My thoughts are this – HUGE make for every quick turn in, but darty handling. The moderately huge bars make for very good turn in, but not so darty so things are smoother, more predictable and more controllable.

Hey, that’s great, but still what bar should I get?

For autocross or somewhere where you want stupidly quick turning that is a bit darty then I suggest the HUGE bars. For something like canyon driving or street only use (or track work with nothing but nice sweeping turns) I suggest something like the 24-26mm adjustable bar.

Rear Sway bar – why getting the biggest bar you can may not be the right solution

Yes, it is true that a larger rear sway will increase oversteer, BUT there is a point of diminishing returns. If you go too big you will overpower the capabilities of the springs, and you will get inside rear wheel lift on hard turns at the worst possible time – at the apex. This means you won’t be able to get back on the gas until the tire in the air gets back on the ground. Sure, our cars have Torsen LSDs, but they do not provide enough of a locking factor to account for a wheel in the air. Having a wheel in the air WILL hurt your handling.

What if I get coilovers or super stiff springs? I can run the biggest bar in the world, and be cool right?

Well, you could, but you reach a point of diminishing returns with sway bars. Remember springs affect both ride and roll. If you go with stiff springs, you will not need as much rear bar. The reason is that the stiff springs will now provide that roll stiffness that you want. In fact, with stiff springs you could easily find yourself needing to soften the rear bar. Keep that in mind.

Coilovers

As with the recent discussion about tires, the purpose of this section is not to rehash what has already been covered; rather, it is to discuss the performance benefits of coilovers. Technically speaking, the Genesis Coupe already has coilovers installed on the front suspension. The main difference between the factory coilovers and the coilovers you can purchase in the aftermarket is that aftermarket coilovers are height adjustable and most of the mid-high end coilovers are also provisioned with dampening controls.

While there are many folks out there who purchase coilovers so that they can lower their car for aesthetic purposes without completely trashing their ride quality, coilovers can actually be a valuable performance tool to actually improve handling. If you remember earlier in the thread where we were discussing corner balancing, coilovers makes corner balancing your car possible. Additionally, most aftermarket options also allow you to specify spring rates. Couple these features with an ability to increase and decrease dampening rates and you have a fully customizable suspension that can be tuned and optimized for the way you drive.

Last, but certainly not the least consideration is that the strut can be adjusted to the spring ride height. In other words, the strut travel does not change as you lower your car. On a non-adjustable strut, if the spring is shortened, it reduces the travel length of the strut piston. Strut manufactures install what is known as a “bump stop” that protects the shock by preventing the piston from bottoming out in the tube. If you drop your car 2 inches, you reduce the strut travel by 2 inches… meaning that you are 2 inches closer to bottoming out your strut than you were at stock height. If you compress the spring to the point where the strut bottoms out and then compress it a bit further you will be riding on the bump stop. In other words, your spring cannot compress further than the strut will allow. At that point 100% of any additional travel is absorbed directly into the body (the strut is attached to the body at the strut towers). This doesn’t just make for an uncomfortable ride, but also can cause “bump steer” where your steering wheel reacts to bumps in the road. At high speed bump steer can be quite disconcerting… especially if it happens in a turn.

Adjustable end links

Most vehicles do not need adjustable sway bar end links but there is a small number of people who definitely can benefit from them. Adjustable end links are for those who purchased coilovers and have made the investment to have the car corner balanced. You cannot corner balance your car correctly without adjustable sway bar end links.

The first step in corner balancing your vehicle is to disconnect the front and rear sway bar. In order to corner balance your car, the preload from the sway bar has to be released to get a correct reading. After you are done balancing the car it is time to reconnect the sway bars. If you check your car on the scales after you have attached the sway bar using fixed length end links, you will see that all of your weights have changed because the sway bar loads the coil springs. With the adjustable sway bar end links you can adjust the sway bar to eliminate tension to prevent the sway bar from compressing the higher spring (remember that corner balancing will often make one spring on an axle longer than the opposite spring to shift the weight of the car). Once you make this adjustment and the weight is the same with and without the sway bars connected you have managed to set your sway bar to zero preload, which is exactly where you want them to be.

Note: The end link length must be set with the weight of the car on its wheels. You should be able to easily slip the sway bar end of the end link into the sway bar mounting hole. If both sides slip in without force, you have effectively removed all the preload off of your sway bar.

If you aren't planning on corner balancing your car,
there's little point in spending your hard earned cash on adjustable end links.
 

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Discussion Starter #23 (Edited)
I think wheel fitment may be the most asked question on this forum, so I suppose that it deserves a bit more explanation. There are two ways you can go about finding the perfect fitment:

1) You can copy someone else. Find someone who has the wheels you want and ask them what their wheel specs are. This is the easiest way to ensure that you are getting the fitment you want for the wheels you want.

2) Do the math yourself. Wheel fitment is a science, and as with any other science, the results can be replicated by understanding the problem and the method by which the problem is solved.

For #1 , there's not much more to do than research, find what you want, and find out what the fitment was. A good place to start is: http://www.gencoupe.com/wheels-tires-brakes-2008-2012/18859-official-genesis-coupe-wheel-fitment-thread.html. There are numerous wheel and photo threads throughout this forum and in many cases the owners of the cars depicted will have their specs in their signatures or if you check their garage, the wheel specs are almost always there. If all else fails, you can always try to ask.

For #2 , doing the math is simply just understanding where your constraints are. This starts by measuring some clearances.

3) The hub face on the car is where all measurements need to be taken since this is the mounting surface for the wheel. There are several things that you'll need to measure in order to get the fitment you are after:

a) Brake clearance. This is especially important if you have a big brake kit such as Brembos because the wheel has to be large enough in diameter for your brake caliper to fit inside of the wheel (unless you have enough negative offset so that no part of the wheel overlaps the brake). This measurement is taken by measuring from the center of the hub out to the most extended part of the brake caliper like this:

In the drawing above, the inside radius of wheel must be at least 6.59 inches (inside wheel diameter of 13.18 inches)

b) Hub to fender clearance. Draw two straight lines... one down from the fender where you want the outside of the wheel to sit and one vertically at the hub face and then measure the distance between the two. The measurement is illustrated in the figure below (8).

Note: Always use millimeters for your offset measurements because offset is always measured in millimeters.

c) Hub to strut clearance. The hub to strut measurement is a little less straight forward because the strut is rarely vertical. Because of this, it is important to understand that the part of the wheel that is most likely to rub on the strut is either going to be the tire or the rim of the wheel. Generally speaking, if the tire is flush (not wider than the wheel) the tire won't be the issue, it will be the wheel. With that said, you want to measure the clearance by drawing two lines. The first line is vertically from the hub face. To find where you need to measure from on the strut, you have to take the radius of the wheel you intend to install and measure vertically from the center of the hub face up by the radius and over to the strut from that point as illustrated in in the figure below (9).



4) Once you have determined the minimum inside diameter to clear your brakes, the offset from the hub to the fender, and the offset from the hub to the strut, you are ready to do some math.

a) You will alway start by assuming a wheel offset of zero. This will make the math simpler. A wheel offset of zero means that that the mounting hub of the wheel is at the exact centerline of the wheel... meaning that when it is mounted you can assume that 1/2 of the wheel width will be on the fender side and 1/2 of the wheel will be on the strut side.

b) Take the wheel width you want to install you your car and divide it by 2. That number should be less than the clearance measurements from hub to fender and hub to strut. If one is less and the other is greater, fret not, this is where offset will be your friend.

c) You can rob Peter to pay Paul by playing with spacers if the wheel you want has too much positive offset. Adding spacers will push the wheel out from the hub increasing the backspacing (the distance from the hub to the strut) while reducing the fender spacing. To determine the maximum size of the spacer, you will need to determine the difference between where the outside of the wheel sits (assuming a zero offset) and where you want it to sit. The difference is your spacer size or your offset spec.

d) Ensure that when you push the wheel out that it creates the clearance necessary to clear the strut. If, after pushing the wheel flush with the fender, you find that you still don't have enough clearance for the strut, you need to choose a narrower wheel.

Note: The widest wheel that you can reasonably fit on a Genesis Coupe is 11 inches in the rear and 9.5 inches up front; however, there is almost no tolerance for error with a wheel that wide. The maximum ideal wheel width is 10.5 inches in the rear and 9 inches up front which gives you some play room for adjustments if you need to make them. You can go with a wider wheel than the specs I just provided; however, it will require modifications to the fender and possibly additional spacers to make them usable.

Up to now, we've been talking about the wheels and not the tires. Where the wheels determine how far out the overall wheel/tire will stick out from the wheel hub, tire diameter will play into how low you can drop your car. There are three numbers that you need to concern yourself with. They are the bead diameter, the width and the profile. All three work together to determine the overall rolling diameter of the tire.



The bead diameter is kind of a no brainer. If you have 20" wheels, you need 20" tires. There's no room for fudge factor on this one. The tire bead diameter and the wheel diameter have to match in order for the tire to fit the wheel.

There is some play room with regard to tire width and profile. But first, let's talk about the basics:

• Tires width is measured in millimeters... so a 275 profile tire has a tread width of 10.8 inches.... 255 has a tread with of 10 inches... and so forth. You want to try to get a tire that has a tread width that is the same or slightly larger than the wheel width for a proper fit. You can, however, install a smaller tread tire on a wider width wheel... this is known as "stretching" the tire.

Tire stretching gained popularity in Europe where it is illegal to run with a tire that extends outside of the fender well. To run wider wheels, Germans and other European owners would install a smaller width tire on a wide wheel that would allow the tire to stay inside of the fender while the wheel itself poked out.

Tire stretching taken to its extreme can result in popping the bead and deflating the tire when the tire is subjected to a shock (bump) while you are driving. A small stretch is generally considered safe; however, keep in mind that the purpose of installing wider wheels is to increase the rubber to ground contact area. If you run a narrower tire, you are actually decreasing the contact area.

• The profile of the tire is expressed as a percentage of the tread width. In other words, if you are running a 275mm tread and a 35 profile tire, the sidewall will be 35% of 275mm or 96.25mm (3.8 inches) tall.

The profile of the tire will have a dramatic effect on performance and handling in turns. The narrower the profile of the tire, the less deflection the sidewall will experience when subjected to lateral forces... or in layman's terms, turns. Although reducing sidewall deflection will result in the tire holding firm in turns giving you maximum grip, it also has a nasty side effect of breaking loose with less or even no warning. On a standard sized tire, there is enough sidewall deflection that the tire slips gradually, giving you that telltale tire squeal sound before you actually lose grip. With narrow profile tires, you go from grip to no grip very quickly.

The sidewall deflection will also impact how smooth the car is when it is driven over bumps. The narrower the profile of the tire, the more energy that will be transmitted through the tire and wheel and into the suspension.

With very narrow profile tires, you run the risk of cracking or bending the wheel on potholes and other bumps as the tire will deflect when subjected to a sudden change in the road and that shock will be absorbed by the wheel instead of the tire.

• You can play with the tread width and profile of the tire to adjust the overall circumference of the tire. A 275/30R20 tire will have a sidewall height of 3.25 inches and an overall diameter of 26.5 inches. This translates to 761 rotations per mile. A 285/30R20 tire will have a sidewall height of 3.37 inches and an overall diameter of 26.73 inches. Although the difference seems small, the number of rotations per mile on the 285s will be 754 rotations per mile. For comparison, a stock 245/40R19 will rotate 755 times per mile. This difference will become significant in the following paragraph.

• You want to stay as close to the overall stock wheel/tire diameter as you can for two reasons:

The diameter of the wheel will determine how fast the wheel completes a rotation. Changing the speed at which the wheel rotates by either reducing or enlarging the spec will result in inaccurate speed measurement and speed indications.

The Genesis Coupe is designed to work with a staggered wheel setup... meaning that the front and the back wheels are different widths (remember that width affects profile height). The traction control is looking for a difference in speed between the front and back wheels to determine if the back wheels are slipping. If the diameter ratio front to back are different, it will make the traction control think that the rear wheels are slipping and the traction control system will cut power for 1-2 seconds by slamming the throttle plate on the intake shut. This power cut is both alarming and abrupt, and can be dangerous in certain circumstances.

There is no magic number that specifies how much of an aspect ratio change needs to happen before traction control starts messing with your ego as every car setup is different; however, I can say that the further you get away from the stock spec, the less tolerant traction control is to limited slippage. The best advice I can give you is to stick as close to the overall rolling diameter of the stock wheel setup to keep traction control at bay and prevent a potentially embarrassing scene where you and the passengers of your car are violently thrown forward when the ECU cuts power while you are trying to accelerate.

• Tire manufactures seem to use different rulers when measuring tread width and profile height. Just like clothing and shoe manufactures have different versions of the same sizes of clothes, tire manufacturers don't always quite get that width and profile thing exactly right.

What this means to you is that every brand of tire will fit slightly different on the same wheel. For instance... a BF Goodrich Comp Sport II measuring 275/30R20 may fit quite nicely in your fender well on a 20 x 10.5 inch wheel; however, if you switch to a Michelin Pilot Super Sport, you may find that the exact same sized tire is actually bigger than the BF Goodrich tires you replaced them with.

The fix for fitting is to adjust the camber of the tire to accommodate a wider width or slightly taller tire. Increasing negative camber will tilt the top of the tire in towards the fender; thus, increasing the amount of clearance from tire to fender. This all presumes that you have enough backspacing between the tire and the strut to allow for more negative camber. Rubbing on the strut is as bad for the tire as rubbing on the fender.



• Lastly, let's talk about camber for just a bit. The Genesis Coupe is designed to run normally with some negative camber. If you run your car hard in corners on a regular basis, you can probably benefit from adding more negative camber on the front wheels; however, you need to stay within the manufacture spec range. If you exceed the manufacturer spec range you can expect that things are going to start wearing out faster because you are adding stresses to the suspension and the powertrain (drive axles and wheel bearings in particular) which are going to wear things down much faster than they normally would. Keep in mind also that most High and Ultra High performance tires have an asymmetric tread pattern.... meaning that they are either directional (designed to rotate in one direction only - typically to evacuate water quickly) or they have an "outside" on them (designed with a different tread pattern towards the outside of the wheel than the inside). What this means to you is that the tire may not be able to be rotated left to right (you cannot rotate tires on a Genesis Coupe front to back) or if you do, you don't really change the wear pattern because the camber left to right should be the same and you are still wearing on the same part of the tire. Because camber cants the tire in or out, the tire does not sit flat on the ground and camber will cause either the outside of the tire (positive camber) or the inside of the tire (negative camber) to wear faster than the rest of the tread.

If what you got out of all this was that wheel/tire fitting on a car is more of a science than just simply finding the correct bolt pattern and hub bore, then you probably read it correctly. If you find that my explanation is far more to absorb than you are willing to invest in the effort, then Option 1: Copy someone else is probably the method you should stick with (welcome to the 90th percentile). When you start getting into the realm of custom wheel setups, you need to educate yourself so that you can make informed decisions. Keep in mind that wheel setups are as much a performance decision as they are an aesthetic choice. What looks good may not perform well and what performs well may not always look good. There is often a trade off between aesthetics and performance. While it is possible to have both, it doesn't happen by accident.
 

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Discussion Starter #24 (Edited)
Chassis braces and you...

Even though it is made of steel (and whatever else), the chassis and body of every car flexes. As a car progresses through a turn, the forces involved will cause the car to twist. Chassis flex is experienced by the driver as vague or inconsistent responsiveness to steering and brake inputs and an unsettling feel when driving hard. As the suspension reacts to irregularities in the road surface such as potholes, bridge joints, railroad tracks, driveways, etc. the shock or impact can be imparted to the chassis via flex. Ideally these obstacles are soaked up by the tires, shocks, springs and bushings of the car, but some of the energy can be transmitted to the chassis and is felt as body shake and shudder during chassis flex. During enthusiastic or spirited driving, chassis flex can be experienced as poor response to steering input. Another common characteristic is the inability of the car to take a smooth “set” in long sweeping corners requiring constant correction to hold the line. In extreme cases, the flex can be severe enough to alter wheel alignment while under load, significantly reducing the cornering ability.


Chassis Braces are the most overlooked part of building a performance car. People will buy an intake then exhaust then coilovers when they begin the process of modifying their car. Chassis Braces are viewed as the last piece of the puzzle or a "bonus part." This logic has some flaws in it however. Chassis braces provide the best "bang for your buck" when attempting to extract everything out of your ride! These parts shore up deficits in the stock platform and don't impact the drive-ability of the car. Now you may say, "yeah but I want a little more power and braces don't add any horsepower or torque!" By having a unified chassis you are able to put more power down to the ground! These products help extract every ounce of power by making sure none is lost in chassis slop, wheel hop, or flex. Installing a full set of braces will not only improve handling, and drag times, it also make it easier to learn how to drive your car to the best of its ability. If the chassis becomes more predictable you will have an easier time taking your car to its limits because you will have a more tangible limit.

What do chassis braces DO?

Well, chassis braces, contrary to popular belief, are not going to measurably increase your G-force handling in the corners. They won't even measurably increase your cornering speed. They may, in some cases, improve your transitional speed (ie, your slalom speed, or perhaps in better layman terms - the speed at which the car changes directions). However, their primary contribution to your car is to HANDLING (or feel). Now, before I continue, handling and grip are two different things. Handling really refers to how the car feels to the driver, how well it communicates what is going on to them, and also how stable it is - especially at the limit of grip. Grip is how well the car holds the road.

There are many cars that I've driven that were extremely fast through curves, but were not exactly confidence inspiring. Because of their vague handling, they were very difficult to drive to the limits of the car and they were also somewhat frightening to drive rather than "fun to drive". The biggest reviewer complaint about the Genesis Coupe is that the steering "feels vague" even though the numbers all say that the car grips well in a turn. Chassis braces will improve the handling of the car, but understand that handling is highly subjective. As a general rule, the stiffer the chassis, the more predictable the handling, the longer the life of the chassis, and the easier the car will be to drive at the limit. Poor handling cars are not necessarily lacking in grip, they are lacking in feel or transitional response.

Finally, chassis braces do two other things that I won't talk too much about in this article because they are more theoretical. They are however important to understand. The first is that the chassis is basically just as much a spring as the springs in your suspension. Though, the chassis is more like the sway bars in your suspension in that it acts as a torsion spring. This is why you'll often hear people say that sway bars increase torsional (twisting) strength of the chassis. Anyhow, all this means is that as you go down the road or turn into a corner, the steel body of the car will bend with the forces applied to it by the suspension and things like centrifugal force. Obviously it doesn't flex much, even in factory form, or you'd not only notice but you'd have a rather dangerous vehicle.

Modern vehicles especially are quite stiff from the factory, if for no other reason than it generally makes them hold up better in accidents. The other reason is letting the suspension do its job. You see, when the body is twisting, it's changing the location and angles of the suspension as well. This messes with the alignment of the wheels in a corner or going over a bump. The body's flex is also difficult to monitor so it can lead to troublesome suspension tuning. In other words, if the body is flexing, it's much harder to tune out undesirable suspension traits because it's much harder to pinpoint what's causing that handling problem. A stiff chassis makes it very easy to isolate suspension problems and tune them out. It also keeps the tires in the location the suspension engineer intended, with the desired behavior as well.

So are they useless?

It depends. Chassis braces can be extremely worthwhile, or they can be an excessive waste of money depending on what you want to do. Chassis braces are primarily going to be an improvement to the feel of your car. If you're trying to improve measurable performance numbers, you probably won't see much there. If you're trying to improve times, you may or may not see huge improvements, depending on you as a driver - not so much the brace in use. The variables are many here, but as a general rule - chassis braces allows more mortal drivers the ability to drive their cars closer to the limit by giving the car a more predictable and stable feel. A true professional driver would probably prefer a really stiff chassis, but could most likely turn similar if not identical times with and without the added stiffness. That said, if you want to build a fun to drive "driver's car" or if you want to generally improve the handling and predictability of your car, chassis braces can really be a great addition to your car.
 

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Every time I refer to this thread I should post a thank you. But then again it would be a bunch of posts bumping the thread for no reason lol But thank you lol
@GenCoupeGeek
 

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This is seriously the most useful thread on this site. Amazing. Nice Job =)
 

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Thank you!

I've just started getting into cars, and a BTR Stage 3 + extras has been my launching point. You, and this forum, have been an absolute wealth of knowledge. Thank you so much for writing all this up!
 
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Been reading through this to prep myself, luckily my dads a retired mechanic if I screw something up lol.
 
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