DougMan

This post was already made once, but buried as a reply to some old ones. Several people gave me good feedback (and my ego a boost!) so I will re-post it as a stand-alone.<p>I know this issue has been covered in detail in the past, but I thought I'd add my perceptions, having spent several years as a vehicle dynamics engineer (ride and handling) at Ford. I am no longer in Detroit and hoping for my TTQ sometime this fall.<p>There are two key contributors, in my assessment, to the "instability". The first is aerodynamics. Most know that this car was designed first, analyzed later. The beautiful shape is actually not very wind-friendly. Take a look at a pure side view. Have you even seen a car which looks more like an airfoil section? The bernoulli effect (the air traveling over the top of the car follows a longer path than the air underneath) creates lower pressure on the the top of the car, resulting in lift. The smoothly shaped rear further contributes to the problem because the air stays "attached" (boundary layer) further down the rear of the car. In fact, the name "spoiler" comes from the use of these devices on airplane wings--they "spoil" lift by tripping the boundary layer and causing airflow seperation earlier (watch the wings on your next airline flight during the descent). A spoiler on the TT might in fact also reduce drag because it would trip the boundary layer and prevent the low-pressure area from creating a rearward force (a low pressure area behind the car is a major contributor to drag). I imagine that the underbody treatments, designed to improved the drag coefficient, actually contribute to more lift by smoothly routing air underneath the car and furthering the "airfoil" effect. It should be noted that both the FWD and Q should have the same basic effect with respect to aerodynamics. <p>The second effect has more to do with suspension dynamics, and here is where there are significant differences between FWD and Q. The "toe curves" of the rear suspension (the way that toe-in changes as a function of both vertical travel and lateral forces) are a huge contributor to handling behavior. The twist-beam rear suspension on the FWD is probably quite predictable in toe, but also contributes to the car's understeer on the track. A multi-link suspension gives the designer more ability to "push the envelope" of oversteer because it can be tuned to give unique toe characteristics by changing bushing rates and link geometry. There is also likely a difference in rear suspension roll stiffness (more means less understeer). Now, if the designer has made all these adjustments based on an assumption of weight distribution, and the weight distribution suddenly changes, the car behavior changes dramatically. This can actually be good--by "lifting", the driver shifts weight towards the front, usually resulting in a car understeering less. This gives the experienced driver the ability to fine-tune their line with the throttle, as steering input becomes less and less effective as you approach the limit. In the case of the TT, the weight distribution also changes as a function of speed, because of the aerodynamics. The result is that the understeer/oversteer balance of the car changes as rear lift develops. The TTQ is closer to the edge to begin with, and thus perhaps more subject to issues at high speed.<p>My assessment is that the biggest problem is not that the car all of a sudden becomes unstable, but that its handling feel changes, and many drivers are not ready for it. Audi, or any other manufacturer, for that matter, would not release a car that was fundamentally unstable at speeds it can attain, even if above the legal limits.<p>I want this car, and one of the reasons is that Audi did indeed push the envelope everywhere. I am tired of cars that are "dumbed down" for the general public, with a resulting lack of depth for people that really understand performance automobiles. The easy way to avoid lawsuits is to dial in tons of understeer. Only a small percentage of the buyers will ever notice. Audi took a risk on the styling, and most of the people on this board are happy they did. It is consistent that they've pushed the envelope in other areas. I do not mind a car that demands a lot of me, and I am not at all scared of the "high speed instability" issues being thrown around (although I admit to having track training at high speed). If you drive your TT (or any car) at the kind of speeds we're talking about here, you should be an informed, trained driver, you should be doing it under controlled conditions, and you should be expecting that your car may behave very differently than it did at 60 mph.<p>If I've bored the hell out of all of you let me know so I don't ever make another post like this.

B Collins

nt

RW

Why couldn't they design a stable car?

SAF

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smiTTy

I agree completely.<p>Thanks again, DougMan!<p>

Perry

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Marius B

The strange underbody spoilers have been explained by Audi top brass in the european press as specifically aimed at creating downforce, to counter the extra lift the top of the body would produce. Some have also warned against lowering the TT, as this would reduce the downforce produced by those spoilers.<p>Looking at them, it's hard to imagine much effect at all. Those little "mud-guards" in front of the wheels may perform as vortex generators. The almost flat horizontal panels just look like ordinary streamlining to me. On the other hand, they knew there was a lift problem, and they had access to the best science in the world, so there's probably something in it.<br>

angel

Brad,
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Take care --Angel