Rear ballast on a road car - why?

[djones]

I have just learnt the Audi TT (mk1) has 20kg of 'ballast' behind the rear bumber.

You may or may not know but originally the TT had no rear spoiler. Due to a number of high speed accidents a small spoiler was added. At the same time so was this ballast.

Why would this be?

Presumably it does not increase rear grip? I mean yes it pushes down but the tyres then have to contain that extra force when cornering???

Thanks for any ideas on this as it has me very confused.

[WhiteBlue]

Audi have been very secretive about the modifications. From the descriptions of the accidents one can conclude that the vehicle hit an eigenfrequency or mechanical resonance frequency. Changes in mass or downforce would obviously shift the frequency away from a critical point. All new vehicles of that type were build with the spoiler wing and electronic stability control ESP.

I found a rather untechnical description:
The car was deemed unstable when cornering at high speeds, due to "throttle lift-off oversteering". It was imperative that Audi rectify the situation as quickly as possible by altering the problem by changing the suspension settings to increase under-steer, and adding a rear wing spoiler to lower lift.

[Jersey Tom]

Can't speak for the ballast. With regard to spoiler and high speed accidents though.. the shape of the TT by itself seems like it would be conducive to generating lift at high speed. Spoiler may be there to mitigate some of that.

[WhiteBlue]

http://en.wikipedia.org/wiki/Lift-off_oversteer

Lift-off oversteer (also known as snap-oversteer, trailing-throttle oversteer, throttle off oversteer, or lift-throttle oversteer) is a form of oversteer in an automobile that occurs when the vertical load on the tires shifts from the rear to the front quickly due to throttle release while cornering. This decrease in vertical load causes a decrease in the lateral force generated by the rear axle, so the axle starts to accelerate towards the outside of the turn. This steers the car more tightly into the turn, hence causing oversteer. In essence, this means that easing off the gas causes the rear wheels to break away suddenly, with the potential for the car to leave the road tail first.
[edit]Causes and countermeasures

This type of oversteer is often more pronounced in rear-engined cars and cars with swing axle rear suspension. Rearward centers of gravity (such as older Porsche 911s) enhance this effect, though technically any vehicle can experience lift-off oversteer. Various suspension enhancements, such as a Weissach axle, Passive rear wheel steering, or a multi-link suspension, can limit a vehicle's tendency to oversteer in this situation. Even the handling of the Chevrolet Corvair improved in final years of production through the use of enhanced anti-roll bars, according to John DeLorean's book, On a Clear Day You Can See General Motors.

It is slightly different than I thought. I'm sure there isn't just this weight changed. But obviously the downforce and the weight will modify the way the dynamic weight shift will occur.

[Jersey Tom]

If you ask me, easiest way to cure off-throttle oversteer is to put on different tires. Everything else is a band-aid.

Is that really what the issue is though?

[WhiteBlue]

http://www.streetdirectory.com/travel_g ... _2006.html

This source says that the shape of the car was motivated by fashion rather than aerodynamical considerations. So the drag coefficient was bad and it probably also had a lack of downforce or even had lift. The design engineers J Mays and Freeman Thomas in California went for fashion rather than functionality.

[Pandamasque]

...it probably also had a lack of downforce or even had lift.

Most road cars have lift. That one most definitely had one. The original TT is shaped like an airfoil. So... unless you drive it upside down of course...


PS: wrong subforum, this should be in Automotive news and technology

[Giblet]

Well, at least the car started some design trends that we still see today, and have spawned some very nice designs (subjective, but anyways) like the 350/370Z, the pony car reissues, and the new Nissan GTR, and all the little mini vehicles like the Scion and the Kia Soul.

These trends are much higher door tops, or waistline as it were, and a steeply raked windshield.

Pretty much any car that uses these basic design practices does well these days. The eyes like them, and subtly set them apart from the standard wind-swept, low waisted, steeply raked 'cab-forward' windshield angle designs of the previous 10 years prior to he TT.

Other than that, it's a Mac of sports cars. Looks great, well designed, but a little fake and when the chips are down, can't pull it's weight (well if you relate racing to gaming).

[Pandamasque]

I have to agree with Giblet here, eve though he's riding an aerialgator. Here's the latest production TT-inspired car.