Gencoupe Suspension Revealed
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.
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.
There are several problematic conditions/areas that we have identified in the suspension of the Genesis Coupe. They are as follows:
-Lack of suspension travel
-Imprecise steering at times
-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.
The TiC Staff
Designers of Performance Suspension Components