Help With Geometry Settings For Sprint Car

[fliprio 9]

It was first sprint event yesterday in the 205, whilst excellent fun I was a bit plagued by under steer which was shortly followed by the front end pushing on when putting the power down. The surface wasn’t that great but the initial bite/turn in just didn’t seem to be there, but a few other 205’s seemed to be doing the job so it’s my car or my driving

 

The car is very stable at high speed – track work etc, but the small twisty stuff doesn’t seem to be its thing, which has gotten worse since putting some 15” A048’s on. I tried reducing the pressures down through the day to 25psi which helped with the traction a bit.

 

It’s running AVO coilovers, adjustable top mounts and standard GTI for everything else I think. Shenpar originally set it up for the previous owner, with what they said is -2° camber and set it up as per their race cars. It doesn’t self centre on the steering, so I would assume it’s not running standard caster/toe. Damping settings set to about half way, although I can never feel much difference. Its running a GTI-6 lump and standard diff. Standard rear beam on Avo dampers

 

Where would be a good place to start with the geometry to help once I establish what it actual is? I haven’t had a look though a good few search results and found lots of different answers based on personal preference.

[welshpug 1,626]

IMO and experience, it'll be the standard rear beam causing your issues, pressures should be higher than 25 hot, but 25 cold is a good starting point we found with the rally cars.

 

the self centering should still be apparent so there might be an issue with the rack or steering column.

[fliprio 9]

Thanks for the reply

 

I started on about 28, but couldnt get any heat into them, so dropped the pressures down a bit, which a few people seemed to think would help get some heat into the tyres, the lap is only 60 seconds, so not much time to get lots of temperature into them.

 

The only time I have felt it self centre is under power when one of the wheels is spining up, other than that you cant feel it at all, would that be stiffness in the rack/column? I had thought it would be the caster angle causing it - I suppose a good 4 wheel alignment would help just to see what it is. I have seen these settings mentioned quite often around -2degs camber, 10-15mins toe out, 4-5degs of caster

 

When you say the rear standard beam, I take it you mean the standard stiffness? or do you mean the height, its been lowered, but doenst look lower than the front.

[welshpug 1,626]

the stiffness specifically.

[allanallen 528 2 Cars]

What weight front springs are you using?

[fliprio 9]

What weight front springs are you using?

 

No idea, I would expect the ones that came with the AVO kit unless you have to specify what you want. I used to have 225lb on my Ibiza coilovers and these feel stiffer than those.

 

the stiffness specifically.

 

thanks, I will have a look through some posts to see what kind of combination might be best

[allanallen 528 2 Cars]

I've no idea what springs came with avos but I'd imagine they're too firm for the standard rear setup. Unfortunately everything you'll be recommended to improve the car is going to cost you it all depends on how much you want to spend. IMO the things you wanna be looking at are--

tracking set to neutral or slight toe out

top mounts set to 1:30/10:30 ie as close to max caster/camber as possible

check your front spring poundage, bare in mind standard springs are around 100lb so wacking 225lb+ springs on a car with standard rear bars/arb isn't ever going to work!

Bigger rear arb or tb's or even better both.

 

A048s need a bit of heat In them as you well know so they're probably not ideal for sprinting on. There's a few sprinters on here (whitepugs is instantly springing to mind) that will hopefully spot this topic and point you in the right direction tyre and setup wise.

[fliprio 9]

I've no idea what springs came with avos but I'd imagine they're too firm for the standard rear setup. Unfortunately everything you'll be recommended to improve the car is going to cost you it all depends on how much you want to spend. IMO the things you wanna be looking at are--

tracking set to neutral or slight toe out

top mounts set to 1:30/10:30 ie as close to max caster/camber as possible

check your front spring poundage, bare in mind standard springs are around 100lb so wacking 225lb+ springs on a car with standard rear bars/arb isn't ever going to work!

Bigger rear arb or tb's or even better both.

 

A048s need a bit of heat In them as you well know so they're probably not ideal for sprinting on. There's a few sprinters on here (whitepugs is instantly springing to mind) that will hopefully spot this topic and point you in the right direction tyre and setup wise.

 

Thanks, I have checked the ARBs and they are standard 19mm on the back and 17mm on the front. The rear beam has started to get a very small mount of play, so time for a rebuild anyway I expect and change the bars while im at it, I will have to keep an eye out for some second hand bars.

 

I have added a picture of the top mount, is it easy to say what kind of settings is been set to by the position or is every shell different?

 

 

The A048s were a bit of an impluse buy, at £170 on eBay for a set of 4 with 3.5mm of tread left, I got click happy :-) a fair few people running them at the sprint, or the R888s, I have also got some Parada Spec 2's which are rubbish and the whole car sqirms like mad when hard on the breaks becaus ethe sidewalls are so floppy.

[allanallen 528 2 Cars]

Top mounts just been set to max camber, 3 o clock if you like I'd move it to roughly the 1:30 position to gain some castor. 10:30 on the passenger side.

I've set a few cars on top mounts like these and they're rarely in the same positions to get the camber the same on both sides of the car.

[petert 554]

Standard diff and standard rear beam are your biggest issues. You need to fit 23mm torsion bars, solid rear mounts and an LSD before worrying about anything else. I start at 25 front/31 rear and finish at 33/33 after 6 or 7 hot laps. If your sprint is only one lap, I'd probably look at a Kumho V70A in soft. I'd set the front up at -2 camber, max caster and 0-1mm toe out.

Edited April 17, 2013 by petert

[Cameron 16]

I started on about 28, but couldnt get any heat into them, so dropped the pressures down a bit, which a few people seemed to think would help get some heat into the tyres, the lap is only 60 seconds, so not much time to get lots of temperature into them.

 

It's a common misconception that lowering tyre pressures helps get more heat into the rubber, when actually you need to do the opposite! Higher tyre pressure forces the rubber to work harder and heat up quicker (more deformation = more internal friction = more heat) so you should always run higher pressures in the cold / wet than you do in the hot & dry.

[Henry 1.9GTi 36]

how to you propose to make the tyre work harder by any other means than driving faster?

 

edit: obviously there are lots of geometry related things but assuming no change in geo we can only influence the tyres conrnering stiffness via pressure. Higher pressure = high cornering stiffness = less slip angle for a given lateral g. Also less contact area under load reducing frictional forces. Less deformation etc etc..

 

"Pressure, speed and operating forces will affect tire

temperature. High temperatures can be achieved with

excessive camber, running with low inflation pressures

(or a slow leak), or by using a tire made from a

compound that is too soft for the track (such as using

rain tires on a dry track"

Smith, N.D. (2004). Understanding Parameters Influencing Tire Modeling: SAE Paper.

Edited April 17, 2013 by Henry 1.9GTi

[fliprio 9]

It's a common misconception that lowering tyre pressures helps get more heat into the rubber, when actually you need to do the opposite! Higher tyre pressure forces the rubber to work harder and heat up quicker (more deformation = more internal friction = more heat) so you should always run higher pressures in the cold / wet than you do in the hot & dry.

 

The way it was described was that lower pressure = more movement of the tyre which helps to build heat. Obveously this is my first event, so just learning the ropes and understanding what changes to make.

 

Lowering the pressure did help with traction, but that may have just been a change in my driving as the day progressed.

[Henry 1.9GTi 36]

For the OP. Understeer coz your stiffened front and not the rear as said by many above. You wont get around that with subtle geo changes. You need to buy a big bit of metal! Man metal!

[fliprio 9]

For the OP. Understeer coz your stiffened front and not the rear as said by many above. You wont get around that with subtle geo changes. You need to buy a big bit of metal! Man metal!

 

Message understood, I purchaced the car with the suspension as is so im just working out whats been done to it and what changes do what, just covering some of the basics first before spending a load of cash.

[fliprio 9]

Standard diff and standard rear beam are your biggest issues. You need to fit 23mm torsion bars, solid rear mounts and an LSD before worrying about anything else. I start at 25 front/31 rear and finish at 33/33 after 6 or 7 hot laps. If your sprint is only one lap, I'd probably look at a Kumho V70A in soft. I'd set the front up at -2 camber, max caster and 0-1mm toe out.

 

Thanks for the info Petert

[petert 554]

I run hard compound on the front and medium on the rear but you might be better with med/soft or even soft/soft if you've only got 60 seconds of motorsport.

 

http://www.kumhotyre.co.uk/circuit_racing.php

[camgti 56]

Would also fit a thicker ARB. Will help with turn in initially and during power through a corner. As mentioned you need to match the rear springs rates ( torsion bars ) with the springs you have on the front. You can work out roughly how stiff they are by measuring the spring. Its diameter, amounts of active coils etc and then you can match them up ( or replace them ) to the rear. As Peter has pointed out 23mm bars would be a good starting point. I would definitely add ( uprate ) an ARB at the rear while you do this. You can use a GTi6 or Xsara VTS rear ARB and have it cut to match the width of the 205.

 

all these together will work nicely. BUT. Your still going to be behind unless you get a LSD in there! Its a bitch when you start modifying these cars. Its one thing after another!! $$$

 

Cam

[fliprio 9]

Would also fit a thicker ARB. Will help with turn in initially and during power through a corner. As mentioned you need to match the rear springs rates ( torsion bars ) with the springs you have on the front. You can work out roughly how stiff they are by measuring the spring. Its diameter, amounts of active coils etc and then you can match them up ( or replace them ) to the rear. As Peter has pointed out 23mm bars would be a good starting point. I would definitely add ( uprate ) an ARB at the rear while you do this. You can use a GTi6 or Xsara VTS rear ARB and have it cut to match the width of the 205.

 

all these together will work nicely. BUT. Your still going to be behind unless you get a LSD in there! Its a bitch when you start modifying these cars. Its one thing after another!! $$$

 

Cam

 

I take it you mean front ARB as well? I will have a go at investigating the front spring rate this weekend, in the past I have seen it engraved onto the top of the spring, but when I rebuilt the coilovers recently they werent marked.

 

I had an interesting chat with Andy at AB Motorsport today, he gave some good advice and recomended a 21mm rear torsion bar, but said to look out for a Pugeout Sport bars instead of the aftermarket ones as he had a few issues in the past.

 

Looking at places like pug1off the bars certainly arent cheap! although a 309 beam with 21mm partner van torsion bars and a shortend 306 GTi-6 ARB comes out well on cost, especially since my beam I have now would need a rebuild.

[petert 554]

No he definitely didn't mean a bigger front ARB. That would make the situation worse.

 

I get tb's and arb's through 309PUG rather than having them made locally. Excellent quality and excellent price.

 

If you still drive it on the street, then 21mm, but if you want the thing to handle then 23mm and a cage is required.

[fliprio 9]

thanks petert, I have sent 39PUG a PM to see how much he charges for his bars. Is there still a certain amount of compliance to the rear suspension with 23mm bars? I already have the full cage.

[petert 554]

You won't like it on long trips. You can't have your cake and eat it too. It will be harsh and unbearable on the street but superb on the track. Highly recommended if you have a full cage.

[Cameron 16]

Harshness will be down to what dampers you use; it's perfectly possible to have a firm but supple ride. I had 24mm bars on the back of mine and the ride quality was surprisingly good.

[fliprio 9]

You won't like it on long trips. You can't have your cake and eat it too. It will be harsh and unbearable on the street but superb on the track. Highly recommended if you have a full cage.

 

Some of the sprint surfaces are more road like than track like at times, not too worried about how it will feel on the road, I just want it to keep a bit of compliance to the suspension. First point of call is to find out what spring lb the fronts are

[fliprio 9]

AVO have confirmed that they are 250lb front springs.

 

From what I have found so far, getting new torsion bars and an ARB from anyone recomeneded is going to be £500+ which is beyond the current budget.

 

If the front to rear spring stiffness is the issue, how would getting softer springs for the front help, maybe in combination with just doing the bigger rear ARB?