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Sunday, June 7, 2009

TT Suspension Geometry Analysis: How Bad is Lowering for Handling?

I've always known that lowering cars past a point starts to impact handling...yes, for those of you unfamiliar with this logic, it seems counter intuitive. Lower car = lower center of gravity = better handling, right?

Problem is most cars' suspension geometry is not designed to sit low, hence when you lower a car, it can introduce things like bump steer or it can throw off static and dynamic camber.

Recently, I decided to look into the relationship between lowering and handling specifically in the case of the TT. I discovered this thread:
http://forums.vwvortex.com/zerothread?id=1363022

I must commend pyce on all the work he did to make those simulations...absolutely brilliant.

While it mostly deals with Mk4 VWs, the TT's front suspension is identical to a Mk4's, except for one critical component: the spindles (and matching control arms and ball joints to accommodate them)...more on that later.

Note: In that thread, "A4" refers to the Volkswagen A4 chassis which the Mk4 Golf/Jetta and also the Audi TT are built off of. It does not refer to the Audi A4 sedan.

The whole thread is a good read if you understand basic suspension concepts...I'll summarize my findings from that thread here.

First off, what's on everyone's mind: how can lowering hurt a car's handling? The TT uses a MacPherson strut design in which as the suspension starts to compress, the camber goes more negative (the top of the wheel tilts towards the inside of the car more). This is what you want for cornering to compensate for the large load being placed on the outer wheels to maintain as great a contact patch as possible. However, past a point, the camber starts to go more positive (bottom of the wheel tilts inwards towards the car), which is the opposite of what you want.

Lowering a car can often put its suspension geometry at the point where the camber is already more negative and as the car turns and the suspension compresses, the camber on the outer wheels goes more positive, which is opposite of what's desirable.

So just how much does lowering affect camber? Here's a graph based on a simulation pyce ran in the thread above:



(the graph is VERY close to true values...my car is lowered right around 2" (50mm) and its static camber is 1.3 deg)

Now you can see why anyone driving a lowered Mk4 VW is liable to complain about the handling. The stock Mk4 spindles lowered anywhere over about a half inch are in the region where suspension travel causes camber to become more positive...and very rapidly! At just over 3" of travel the suspension is back to zero camber.

The TT, due to its different spindle design, has a far better camber curve. TT suspension can travel about 2" before the camber starts to become more positive and even then it doesn't go anywhere near as positive as Mk4 suspension. Notice how within an inch of travel (from 55mm to 80mm), it only goes about .1 degrees more positive.

The third spindle is the H2Sport spindle for Mk4s. It has an even more optimized design, as you can see in the camber curve. The key to the TT's highly improved camber curve over the Mk4 is in the placement of the ball joint on the spindle:



giving it far better camber characteristics.


The above graph however is with the wheels pointing straight, which is obviously not going to be the case going around a turn. When you turn the wheels, the camber changes, thanks to the ~8 degrees of caster built into the system. Here's a graph of what happens with the camber at various steering angles:



As you can see, when the wheels are turned the camber goes even more negative with the TT...meanwhile the Mk4 is still sucking at life and at one point actually sees positive camber.

So now, the bottom line...how much worse are the camber characteristics of a lowered TT versus stock ride height, taking steering and now body roll into account?

Here's the graph:



As you can see, surprisingly the 60mm lowered TT actually has more negative camber (!!!) than stock ride height up to 4 degrees of body roll. As mentioned before, this is generally desirable in a turn. Past 4 degrees of body roll, the OEM ride height starts to have an advantage that at most (at 6 degrees body roll) is a half degree of camber.

So it looks like lowering doesn't impact the camber characteristics too badly. However, one thing that does change is the roll center. I won't go into detail with roll centers, but shifting the roll center can actually increase your car's tendency towards body roll. The roll center is positioned ideally right around stock ride height. The rule of thumb is that the roll center is optimized when the control arms are at or close to parallel with the ground. This is something to consider.

So what do I think?
From what the study shows, it is possible to have a reasonably low TT and not have it handle like crap. A lowered TT still maintains good dynamic camber characteristics, as shown in the graphs. At lesser degrees of body roll, the lowered TT's camber is actually more desirable than a TT at factory ride height.

As far as the roll center is concerned, while it tends to make the car more prone to body roll, there's all kinds of things you can do to inhibit body roll anyway:

-By lowering your car, you lower the center of gravity, which decreases the car's tendency to roll.
-Most coilover kits have stiffer springs than stock to help with body roll. Many of the springs are also progressive rate, meaning their spring rate gets higher (stiffer) the more they compress.
-Swaybars, the rear bar being an ever popular TT mod, also work to inhibit body roll.
-Tires can reduce body roll. Tires act like springs - when you apply a given force to them they give way somewhat based on how stiff the sidewall is and how large it is...just like the spring rate and length of a spring. Low profile tires with stiff sidewalls can work wonders in inhibiting body roll. A slight tire stretch can stiffen a sidewall even further. I used to run 19" wheels (which I don't suggest you do, they're too heavy) the fronts being 8.5" wide with a 225 series tire. It cut an amazing amount of body roll off of the stock suspension.

So there you have some brilliant research by pyce and my TT specific conclusions based on it. Hopefully this will shed some light on the TT's suspension to help you all choose your setups.

-Filip

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