TurninConcepts

Genesis Track Suspension Revealed

Generally speaking, the suspension the track edition Genesis is pretty good from a driving standpoint. There is little roll to the body, decent grip and a sporty feel. In daily commuting, one can feel the stiffness and willingness of the suspension, but it doesn't go too far to become a kidney buster.

This all changes however, when the car is pushed. A few people have reported loads of understeer when autoxing or pushing it hard. When driven over undulations in the road at speed (65-80) the car can be felt bobbing and bouncing with the front bobbing at a different rate than the rear. There is also a lack of precision that can be felt mid corner when additional steering input is needed and when bumps are encountered. Our goals is to put cause to these issues and to explain our plans for fixing these issues.

Basic Overview

The Genesis Coupe uses a MacPherson Strut front suspension and multilink rear. These are very common forms of suspension that have been around for some time. The MacPherson strut front is favored by many car manufacturers for its simplicity in design and compact fitment. The multilink rear is a proven setup that is used on Lancer Evolutions and the latest Subaru Impreza STI among many others. While these are favored by manufacturers, neither are without faults or a lack of means of improvement.

If there is one major problem with the MacPherson strut suspension, it can be summed up by the term camber loss. Camber is the angle at which the top of the tire tips in towards the car. Camber is also directly related to the amount of grip a suspension has. Up to a point, the more camber that is added the more grip the tires will have at that location. (I can cover this in another paper if folks need - Clint) When a car with MacStrut (short for Macpherson) based front suspension is thrown into a turn the amount of body roll the car has directly affects the amount of camber loss and subsequent grip at that front of the car. This camber loss will result in lower negative camber numbers to the point where the front loaded tire can actually go to positive camber relatively quickly. Positive camber is not good for overall grip when in a turn and positive camber in the front of a car during a turn will lead to understeer.

As an example, a car with -.5* of static camber in the front (much like the Gencoupe) that encounters 3* of body roll in a turn will result in a dynamic camber number of +2.5*. 0* of camber is what we shoot for when the car is fully loaded in a turn and where max grip on the tire will occur. Any positive camber beyond that is leading to understeer and is not good. There are a few ways to combat this camber loss with the most basic being adding additional static negative camber to the front. Using the same example with a static camber setting of -3* instead of -.5* with the same 3* of body roll would result in 0* of dynamic camber and maximum grip on the loaded tire. There are however problems with adding that much static camber to the car including twitchiness under hard braking and increased inside tire wear. (the single larger factor in tire wear however is toe, not camber)

The rear suspension is a multilink setup with five separate arms to control the motion of the rear tire. It is quite complex in design and function and works much differently than the front. While a Macstrut design loses camber under compression, the multilink design tends to gain camber on compression. Too much rear camber to front camber can cause a car to understeer as well. There are also issues with the control of toe in the rear of a multilink setup as it causes the rear toe to change in and out as it compresses and rebounds. Because the springs and shocks are quite a ways inboard from the wheel, precise calculations must be done to figure out damping profiles of the shocks and spring rates.

Problem Conditions/Areas

There are several problematic conditions/areas that we have identified in the suspension of the Genesis Coupe. They are as follows:
-Severe understeer
-Lack of suspension travel
-Imprecise steering at times
-Soft bushings
-Bobbing/uncontrolled undulations
-Improper spring rates

These are the major areas that need tweaking right off. More will surface as we dive more deeply into the suspension design of the car, but these need to be addressed now. Without resolving these issues, a proper handling sports car the Gencoupe does not make.

Understeer and Lack of Travel

In part of our new car buying ritual, we completely remove the suspension and measure everything. In the first week we had our Gencoupe the suspension was removed 5 times! We know we're nuts, but this is what we love. Our first major discovery when measuring the suspension was the lack of bump travel in the front. With the car at standard right height, there is only 2.25" of total bump travel (including the bump stops) and only .375" of free strut travel before engaging the bump stops. The results of this? Major undsteer when the car is pushed. As the car rolls and the suspension is loaded in the front the strut is compressed and the bump stop is engaged. When the bump stop is engaged the spring rate of the suspension is increased. Softly at first, but very quickly increasing as the bumper is compressed further. Once enough of the bumper has been compressed, the damper (shock) can no longer control the spring movement resulting in the car's tires skipping across the ground...understeer.

After driving the car for a few hundred miles and measuring the suspension, there is no doubt in my mind that the majority of the understeer issues with the car come from not having enough bump travel. Unfortunately, there is not a great way to fix this. We have changed the stock bump stop out in the front for a shorter unit. Overall it is 1" shorter than the stock bump stop giving us a total of 1.375" of bump travel before engaging the bump stop. This change is immediately noticed when driving down the road as the car does not "bounce" as much over small dips. It is much more smooth leading me to believe that the stock bump stops are engaged most of the time during regular driving. The overall understeer characteristics of the car were not affected by this shorter bump stop as it is still engaged quite heavily under hard cornering.

There are two other ways in which bump travel can be gained on the car, but at this point the products are still in early development phases. More to come as we get closers to working prototypes.

The stock alignment specs on the Gencoupe call for +.5* to -.5* of camber in the front. This is no where near enough for performance driving on a Macstrut equipped car. We changed out the stock lower strut bolts, both non-adjustable units, for 14mm camber bolts from SPC. With both bolts set for max negative camber we were able to get close to -1.5* on both sides. This definitely helped the overall grip when entering corners and made the care turn in faster, but did not help the overall grip at the limit of adhesion due to the lack of bump travel.

TurninConcepts

Imprecise Midcorner Steering and Soft Bushings

I have noticed at times that while the steering can be very quick at turn in, it lacks some response mid corner. This is most often noticed just after entering a corner as a feel of understeer (not to be confused with the scenario discussed earlier). In my typical response to and undsteering car I was slowing down in an attempt to regain front grip. I did once however decide to add more steering input with the wheel and was rewarded in more front grip and a change in direction. What I am to realize is the suspension bushings are so soft they deflect enough to change the direction of the car. Upon inspection, the bushings were found to be incredibly soft. We are going to go through and measure all bushings in the suspension for a complete overhaul. The change that stiffer suspension bushings make is often amazing. Stiffer suspension bushings act as a base for all other suspension modifications. If the bushings are soft and easily deflected, the precision from all other suspension mods will be masked somewhat and maximum performance will not be achieved.

Bobbing/uncontrolled oscillations and Improper Spring Rates

The bobbing problems or uncontrolled oscillations can easily be felt over large dips and humps at highway speed. The car will hit these imperfections in the road surface with the result being a certain number of oscillations of the suspension. The number of oscillations is referred to as the frequency with the front and rear frequencies being considered independently.

The unwanted motion we are concerned with is expressed in a difference of frequency between the front and the rear that results in a pitching motion where the front and rear oscillate at different rates and for too long a period of time. This pitching motion will result in the front finishing it's oscillations or movement before the rear does. Ideally the rear would finish at the same time as the front for best overall control at speed (very important when on track). In order to determine the cause, we first looked to the spring rate being used on the track suspension as well as the motion ratio of the front and rear suspensions.

The ratio can be described as the difference in amount the wheel travels compared to the amount the spring/shock travels. For example, if the wheel travels 1" upward and the spring perch travels 3/4" upwards the motion ratio would be .75:1 or 75%. This means that the spring is moving only 75% of what the wheel is moving. This calculation is one of the factors in determining the correct spring rate that should be run on the car. Other factors include wheel base, sprung weight, unsprung weight, and desired frequency among others. (If needed I can go into wheel rate and frequency in further detail - Clint)

Given the design of the front and rear suspension and their inherent motion ratios it is clear that for optimal performance the rear spring rate should be higher than the front as the rear motion ratio is lower than the front. In testing, we found that the rears are stiffer than the front, but by not a high enough margin. They should be close to 30% more stiff given the stock rate of the front. More will come of this as we get damper plots of the stock shocks.

Conclusions

So what does all of this info tell us?

1. The stock suspension isn't too bad, but definitely has it's short comings.
2. Any lowering springs that reduce bump travel (all lowering springs by themselves) will result in more understeer and worse handling
2a. Ideal aftermarket spring designed to enhance performance would be stock height (or a little higher!) with higher spring rates
3. There needs to be more travel in the front
4. There needs to be more negative camber in the front
5. The stock bushings are way too soft for a performance oriented car

Next Steps

1. Dyno the stock dampers
2. Confirm spring rates
3. Work out products for increasing front bump travel
4. Develop stiffer suspension bushings

More info will be added to this and more threads created as we get more info.

TurninConcepts

Random pictures we took along the way.

Pic of a front strut and knuckle with two camber bolts installed. This allowed us to achieve roughly -1.5* of camber in the front on each side.


Stock bolts.


Standard camber bolt. 14mm


Pic of the lack of travel in the front. This is with the car at ride height.

chris@vtengineering

woah nice write up man.

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cant_stop_spinning

awesome info!

So if i would just buy stiffer bushings from you, would i notice less understeer?? I dont plan on tracking the car but i would love to make it handle even better.

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TurninConcepts

Stiffer bushings will help with feel, but not necessarily reduce understeer. The soft bushings can give a feel similar to understeer that by some definitions is not actually understeer.

Tony

Brillo

Just curious, could the larger 19'' wheels be making the problem worse due to their weight?

Awesome writeup btw.

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TurninConcepts

They do add to unsprung weight, but not enough that it would have drastic effects on the suspension tuning.

GennyBK

I just want to fix the bobbing up and down and possibly stiffen some parts of the suspension. Not really meant for this to be track since this is my daily.

Andyroo

Thanks Tony and Clint. This is great stuff....looking forward to the next steps.

EDIT: Firmer bushings could help reduce understeer, depending where they are and what type of dynamic alignment changes they would be minimizing, but it definitely seems the lack of travel is problem #1.

- Andrew

TurninConcepts

Believe me - we're going to be addressing that one.

TurninConcepts

Agreed and that may be one of the problems with the front control arm bushings. They are SOFT!

Paulicat

Can I ask why they would use such a soft bushing?
Is there any scenario where you would want soft bushings?

Great writeup, love the detail level!
Keep it coming!

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Andyroo

Soft bushings = comfort!

- andrew

TurninConcepts

8020 rule. 80% of the market want's a soft car. Car companies want to make money. Go with the wants of the larger part of the market.

Most of the folks who are active on boards such as this fall into the 20% rule, and of those about 5% will seriously track the car at some point.

Paulicat

Interesting, I would've figured the 80% would just not get a Track (or GT as its called up here in Canada). Seems to negate the point of offering up a "stiffer gt calibrated" suspension when its deflated with soft bushings.

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FRCDAIR

what kind of wait can we expect on some of the parts you discussed....maybe a list of each.

Anyway we can reasonably expect to fix the mentioned problems yet still drop the ride height by a mile 1.2-1.5" or so?

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cant_stop_spinning

so you think to reduce the understeer new suspension will be needed? for more travel? or is there something you can do to the stock setup?

BTW thanks for answering all these questions!!!

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ArchAngel916

I thought the 8020 rule was that 80% of profits come from 20% of the market... lol

Great right up, though. I'm anxious to follow your progress with the Gen Coupe.

TurninConcepts

yeah - I'd think so too, but believe me - a factory built race car would not sell well to the general public.

That's a different 8020 rule. Same name though.