Handling for Audi Quattros 1

[Hueyoz]

Can anyone tell me how to improve the handling of an all-wheel drive Audi, particularly in regard to the fairly chronic understeer they suffer from. I would like to understand the physics of car handling a little better too, if someone can spare a little time to let me know how a stiffer rear sway bar affects the cars handling, and how lower springs, or softer springs etc affect it too. Thanks.

 

[Chumley]

Hueyoz:

Which year and platform Audi Quattro are you referring to? Understeer is a common handling characteristic and a safety feature of Audis which began their lives as front wheel drive vehicles. Their AWD systems have improved significantly since they first appeared in production in about 1980.

Many guardrails have modified the graceful lines of Audis in the wrong hands with weight and balance characteristics that favor oversteer. Driver technique and wheel alignment are critical and it's tough to beat the factory specs. Not every shop out there is qualified nor has the equipment to properly align Audi quattros. If you are sure that all your suspension components are functional and your alignment is exactly to spec, and I do mean exactly, go to this site do a little reading before you register. There's a few guys there who regularly race them and can get them to stick to the pavement a little better. Ride quality will suffer though. After you read a few thousand posts you'll find a few guys who are pretty good with them. As Evelrod often points out though, the real hot racing tricks are pretty tightly guarded secrets, so, don't expect to learn everything the first day.

http://www.audifans.com/mailman/listinfo/s-car-list

Chumley

 

[Hueyoz]

Thanks Chumley, it is a 1998 A4 Quattro, 2.4litre V6. What I'd also like to learn is how the different suspension components interract and affect handling. I've read a few comments on this website, but I'd like to learn more, both for general knowledge, and to tune my Audi's suspension a little better. It handles OK, but I think it could handle considerably better, without a lot of work. You are correct, it does react a lot to driver input - you have to drive in "correctly" or it will understeer quite badly. Having said that, I think with a little work, it would handle better, and with the right driver input, better again.
Hueyoz

 

[GregLocock]

Yeah those ride and handling guys at Audi are a lazy lot, they probably didn't even bother to do any development work on the Quattro. You'll probably be able to improve the car out of all recognition in a couple of weekends, maybe just one if you are really good.

OK, sarcasm mode off. If you want to learn about interactions then the best book is Race Car Vehicle Dynamics by Milliken and Milliken, available from the SAE. That has a longish section on both primary and secondary interactions.

I haven't found much on the web that rises above the very basics for interactions.

The simplest thing you could do to reduce understeer would be to remove the front sta bar, or drop the front spring rates. But you would be unwise to do so without understanding the other likely effects - I suspect you'll find that the 'lazy guys' I mentioned before were very concerned by throttle off in turn characteristics, and the behaviour under braking, and mixed mu surfaces. They might also have decided that newer drivers in a turbo would spend a lot of time backing off, so a tendency to plough under most conditions might be built in.

You do realise that all practical 4 wheeled vehicles with rubber tyres understeer in steady state cornering at anything less than 8/10 don't you?

Cheers

Greg Locock

 

[Chumley]

Hueyoz:

I'm with Greg on this. The best engineering minds in the automotive industry have been continuously working on improving the handling of the Audi Quattros now for about 25 years. Those same minds somehow managed to finish 1,2,3 this year at Le Mans and basically thrash the competition in most of the professional level races that they are still allowed to compete in.

It's interesting that you think the Audi Quattro handling is "OK, but could be considerably better . . . without a lot of work." I'll be looking forward to reviewing your suggestions for improvement.

Chumley

 

[Hueyoz]

Greg, that book sounds good, what/where is the SAE? Don't misunderstand me guys, like I said it handles nicely, but I've also driven a BMW (for example) and it just "likes" to go through a corner, as opposed to the Quattros which you have to really drive it "accurately" to get it through a corner at speed and in good shape. One thing I did learn in a hurry, is that you really have to be on the power all the way through the turn. They don't like entering a corner with no power at the wheels, I can tell you that - particularly in the wet.
I've already added a larger rear sway bar which has improved the balance of the vehicle quite a lot. It still understeers quite a lot though, which is why I'd like to learn a bit more about suspension b4 I do much else. I suspect, from what I've read, that reduced front springs would make quite a difference, although I'd probably have to reduce the rear as well, given the meatier rear bar. Otherwise I might be going through the corner backwards!
Greg, if I reduce the spring rates all around, do u have any idea what that will do to the overall handling? I assume there is a good reason the springs are rated at whatever it is they are rated at now.

 

[mloew]

Hueyoz,

SAE International is the "The Engineering Society For Advancing Mobility in Land Sea Air and Space" formerly know as the Society of Automotive Engineers. Thier internet locations is

http://www.sae.org.

Best regards,

Matthew Ian Loew

 

[GregLocock]

For a circuit car without spiffy aerodynamics you generally want the softest roadsprings you can get away with, ie just sufficiently hard to stop the bump stops engaging. This will maximise your grip from all 4 wheels at all times, which as you may remember is the fundamental driving condition in a race.

Since you've spent quite a lot of time and money making sure that all four wheels move independently, you can now spend some time and money stopping them from doing that by fitting an anti roll bar or two. Quite how you decide how much extra roll stiffness to add in, which may even allow you to go softer again on the road springs, is something that evelrod can describe with far more insight and experience than me.

On a road car in the first instance we use the relative roll stiffness of the two ends to change the way that each tyre weights up, which affects how much it saturates, which affects how much lateral force it generates, which affects the understeer of the car. We also need to keep an eye on the ride of the car which is strongly affected by the roll mode of the car, which can cause head toss. It is probably worth pointing out that in 'normal' driving (ie that which would not get you a ticking off, say 0.3 g) there is so little weight transfer that the handling is hardly affected by the a/r bar, in the sense I've discussed - the shocks probably have more impact.

I agree, the handling on a BMW is much nicer than the handling on the Quattro, I don't think a 4WD will achieve the crispness of a RWD, although it could and should have better objective performance.
Cheers

Greg Locock

 

[Hueyoz]

Greg, thanks for your input. A quick question or two though; who is evelrod? And can you tell me (in reasonably un-technical language) the answer to the following question: if we want the "suspension" to be as soft as possible in order to get as much traction as possible (to reduce understeer, presumably, in my case)then why do we then make the suspension stiffer by adding anti-roll bars? Obviously the anti-roll bars do something, but I don't understand what or why. Presumably they reduce body roll through a corner, but what does that do, and why is that important when it also reduces the amount of traction you are getting?

 

[GregLocock]

1) evelrod is a regular poster here who has raced and modified all sorts of cars, with some success.

2)The anti-roll bars change the way that the tyres across the car load up as you enter a corner. This affects the slip angles front to rear, and so affects the understeer. In an ideal world you would not use them, I think, since the whole point of an independent suspension is to maintain wheel contact as much as possible. Oddly enough on the active suspension cars they usually have antiroll, antipitch, and cross diagonal 'bars' in the software. I don't know how much of each they use. If you run no antiroll bars then the car will roll too much in corners, maybe even to the extent that the suspension tops or bottoms out. The enormous non linearity this would introduce would be very difficult to control. I'm not sure if a standard car would bottom out like this, but I guess it would (too lazy to work it out).

There is one proviso with antiroll bars - if the car does lean too much then the contact patch may cant relative to the road surface. In that case an antiroll bar may help traction by keeping the tyre flat to the road.

Cheers

Greg Locock

 

[Hueyoz]

Greg, you mentioned 'head toss' earlier as it was related to stiffer roll bars. I've noticed that there is a bit of head toss now that I've fitted the stiffer rear bar. Is this fixable by fitting softer springs, or is it only fixable by reducing the size of the rear bar? I actually have a 20mm bar fitted at present, but also have an 18mm bar (std is 15mm) I can fit too.
The stiffer bar has more effect on the understeer problem (I had the 18mm bar fitted first) but there was no head toss problem when I had it on (at least I didn't notice such a problem).
Thanks for your input,
Craig

 

[GregLocock]

Hard to say without sitting down and working out the total contribution of the anti roll bars and the road springs to the roll stiffness of the car. If anything I think that softer road springs would emphasise the mismatch between the bounce and roll characteristics of the suspension, but I would agree that softer road springs should reduce the total roll stiffness, which is, I think, the main cause of head-toss.

We typically get worried about head-toss when we go from our usual setup to our sports setup, but they do very strange things to the road springs as well as fitting eneormous anti roll bars and low profile tyres, which may exacerbate the problem.



Cheers

Greg Locock