Help With Geometry Settings For Sprint Car

[fliprio 9]

I believe that due to not having a good diff ( tran x/ Kazz etc ), your missing out on that final piece of the puzzle. Which would be the extra grip and exit speed your missing. With the setup you have now. Having the plate diff will pull the car tight mid/exit of a corner and transfer more weight to the rear, where the roll stiffness ( not sure of correct term here. Cameron or someone please confirm this as dont want to lead anyone stray here, with miss information ) of the car is. Then its a matter of learning how to add the power at the correct times etc ( not assuming your already doing this ) to 'balance' the car.

 

This was from experience in my car. At the time was running a Quaife diff, 21.3mm rear torsion bars, 24mm arb in a 309 beam, Koni Yellows with eibach springs. Once i changed to the Plate ( Tran x ) diff it was a world of difference. Your able to get on the power earlier, brake later and exit like a slingshot.

 

Cam

 

Yep, a proper diff would be a very good addition, but with trying to buy a house its not affordable at the moment. Even on mild power applications though, the turn in is rubbish and worse than a standard GTI set-up.

 

The softer springs should help, as a few people in this post had pointed out, the springs like Eibachs and much softer and for the standard rear beam, the fronts are just too hard.

 

Disconnecting the front ARB made a big difference, but made it quite twitchy under breaking, so im going to soften up the front springs and reconnect the ARB.

 

The spring platforms are quite close to the tyres at the moment (Just about get my finger in between them), so I think I will just have to take the shocks off and free up the adjustment platforms and go with 8" springs again.

[camgti 56]

Yea,ess spring on the front will certainly help. 200lbs and under would be a good start.

 

Cam

[Henry 1.9GTi 36]

try a little toe out on the front. And a fair bit of negative camber if you can, should help turn in until it rolls it off. Should help turn in. Did you try disconnecting the front arb? Being a sprint car with very tight turns Id imagine removing it would benifit you loads.

Edit: woops should of read your post fully. Twicthy under braking eh? interesting. Never run with it disconnected myself as just do circuit stuff.

Edited September 3, 2013 by Henry 1.9GTi

[fliprio 9]

Yea,ess spring on the front will certainly help. 200lbs and under would be a good start.

 

Cam

 

I have ordered some 9" 180lb and see how I get on, this in combination with the 24mm rear ARB should help

[fliprio 9]

try a little toe out on the front. And a fair bit of negative camber if you can, should help turn in until it rolls it off. Should help turn in. Did you try disconnecting the front arb? Being a sprint car with very tight turns Id imagine removing it would benifit you loads.

Edit: woops should of read your post fully. Twicthy under braking eh? interesting. Never run with it disconnected myself as just do circuit stuff.

 

it was more under high speed breaking that it would require a bit more steering wheel input when the ARB was disconected.

 

It looks like the top mounts will run about -2° of camber, but atthe moment it doesnt have any caster. As soon as the top mounts and springs are replaced I will get a proper alighnment done on it to see where it is. A local place is happy to do it for me and get it to a certain set of perameters - I just need to decide what they are!

[Anthony 1,003]

180lb might be a little on the soft side with an iron block XU engine (GTi-6) fitted, but give it a go and see how you get on.

 

Certainly more sensible spring rate wise than 250lb, which are simply too stiff to match standard torsion bars on the back IMO

[fliprio 9]

180lb might be a little on the soft side with an iron block XU engine (GTi-6) fitted, but give it a go and see how you get on.

 

Certainly more sensible spring rate wise than 250lb, which are simply too stiff to match standard torsion bars on the back IMO

 

I will give them a go and see how they fair, I have gone for a longer spring to bring the ride height up a bit as its quite low at the moment. Worst case they dont work and I can stick them on ebay.

[camgti 56]

Ive not realised your still running standard rear bars. Its going to be unbalanced until you can beef up the rear bars if you have stiffer springs in the front. Otherwise, revert back to something closer to standard until you can. 180lbs is still going to be too stiff.

[fliprio 9]

180lb might be a little on the soft side with an iron block XU engine (GTi-6) fitted, but give it a go and see how you get on.

 

Certainly more sensible spring rate wise than 250lb, which are simply too stiff to match standard torsion bars on the back IMO

 

The 180lb springs have made it much better at the sharp slow twisty bits, its given it that extra bit of compliance in the suspension.

 

Booked in for a geometry set-up next week, aiming for;

 

-2° camber, 5° castor, 1mm toe out

[camgti 56]

Great news! I guess the next step is to fit bigger rear bars! Glad we have got it getting around there better, bet its fun!

 

Cam

[fliprio 9]

Great news! I guess the next step is to fit bigger rear bars! Glad we have got it getting around there better, bet its fun!

 

Cam

 

The rear beam will need a rebuild soon, so im sure the opportunity will arise

 

I have a sprint next week, so we will see how it goes

[camgti 56]

Perfect mate. Yea a rebuild will make a good difference, tighten enerything up.

 

Cam

[fliprio 9]

Got the 4 wheel alignment done today, best we could get with the top mounts was -1.5 degrees camber, 3.8 degrees castor and 1mm toe out, moving it around a few holes didn't seem to make a whole load of difference, it just started to loose camber for a very small increase in castor.

 

Interestingly the rear end is a bit out, near side rear has 5.3mm of toe in and the drivers side 0.7mm toe out

[welshpug 1,658]

a bit? that's MILES out!

 

 

should be 3 per side from memory, you might be able to get that by loosening the bolts and giveing it a good wiggle/lever about.

 

are the bushes solids or o.e rubber still?

Edited October 2, 2013 by welshpug

[fliprio 9]

The data he had on the system said +1.9 with a 0.8mm tolerance, so not far off 3.

 

Its in on the total toe angle, which would indicate its on there a bit wonky.

 

Its still on original rubber bushes. There is the start of a very small amount of play in the bearings, so it needs to be rebuilt at some point which is when I was going to do the mounts.

[fliprio 9]

The guy doing the alignment said most of the time that amount of toe in on the rear is caused by a bent stub axel caused by a knock to the wheel, it is strange though that the other side has some toe out

 

Can you get much movement in the beam via the bolts , or does it always pull itself into the same position once the bolts are tightend up?

[welshpug 1,658]

a knock to the rear will typically given lots of positive camber and toe out, not in.

[jackherer 543]

Every time I've seen that much variation at the rear it's been caused by mismatched trailing arms fitted when rear beams have been rebuilt.

[fliprio 9]

a knock to the rear will typically given lots of positive camber and toe out, not in.

 

The camber seems spot on, at some point in time it has had a nock to the side with the large toe in as that side is a slightly different white and you can see where some of the creases to the rear quarter pannel have been pushed out

Every time I've seen that much variation at the rear it's been caused by mismatched trailing arms fitted when rear beams have been rebuilt.

 

hmm, worth looking into. I bet there is no way of telling what arms it has though

[jackherer 543]

hmm, worth looking into. I bet there is no way of telling what arms it has though

 

They're not marked AFAIK. Graham posted a list of the different 205/309 arms when I first had this issue - http://forum.205gtidrivers.com/index.php?showtopic=80362 - and 306/Xsara/ZX ones fit too so there are a lot of different possibilities. Fitting a matched pair is the best bet IMO.

[allanallen 528 2 Cars]

There are slightly different casting marks on a lot of trailing arms so you can often distinguish a pair.

[fliprio 9]

 

They're not marked AFAIK. Graham posted a list of the different 205/309 arms when I first had this issue - http://forum.205gtidrivers.com/index.php?showtopic=80362 - and 306/Xsara/ZX ones fit too so there are a lot of different possibilities. Fitting a matched pair is the best bet IMO.

 

The toe values in that thread seem to be the total toe values, so none of them are similar to what I have . But as you say, there could be others that give something different. The bearings are on there way out, so I expect a beam rebuild is on the cars for the winter and replace the arms at this point.

[fliprio 9]

There are slightly different casting marks on a lot of trailing arms so you can often distinguish a pair.

 

I will have a look tomorrow and see what they look like

[camgti 56]

Get the beam built with new shafts and bearings. Then you can start to question other things? Surely that makes more sense? Play in bearings will have a knock on effect to the actual values your getting read out. A small movement at the bearing will result in bigger movement at the wheel. Its all speculation until this variable is taken out. Fits good solid mounta while its apart.

 

Cam

[fliprio 9]

Get the beam built with new shafts and bearings. Then you can start to question other things? Surely that makes more sense? Play in bearings will have a knock on effect to the actual values your getting read out. A small movement at the bearing will result in bigger movement at the wheel. Its all speculation until this variable is taken out. Fits good solid mounta while its apart.

 

Cam

 

Thanks the plan - winter project me thinks

 

on the plus side, the car handled 300% better at todays sprint, turned in nicely and has made the back end a bit more twitchy, which is nice and drivable which meant I could just work on my lines between each run rather than wrestle it around with understeer - I just have to stop the back end coming around when braking hard ( I new I should have left the old Knackered brakes on)

 

I have driven it after each mod, rear ARB, front springs, post geo check and the thing that I found made most the difference was going to 1mm toe out rather than 1mm toe in which it had.

 

Thanks all for the help getting it handling nicely - im sure its just the start though.