When drag racing I get a load of squat and the front end lifts 4" + when using gas and running slicks. When on road rubber everythings fine but
when on drag slicks, the car lerchs to the left under hard acceleration in the first 2 gears. I previously blamed this on the diff but with the
slicks fitted there is a massive increase in traction which in turn give more squat and lift on the front. I am wondering if the lerch to the left is
due to track changes of the front wheels when effectively in droop. Im never heard of it befre but im going to call it "droop steer". I
havnt had time to get out in the garage to jack the car through the droop area to see if this occurs but just wondered if it was plausible.
Carnut
Torque reaction perhaps ?
If you have bags of traction and you're launching really hard - with a relatively soft rear end - it's almost sortova bound to happen.
GULVAL, why are all of your posts repeated?
it doesnt sound too likely to me, as the track change should remain symetrical. I suppose it could be a diff effect made more apparent by the incredible lift reducing front end grip - almost like a wheelie going off centre!
Is it possible that torque from the prop shaft is causing the rear axle to rise further at one side than the other side causing the back axle to steer due the track length changing from one side to the other (short trailing arms). Only applicable to live axle tho'
Its an indy with IRS
Start by checking the rear toe (on each wheel) and corner weights.
Also check the car isn't hitting one rear bump stop before the other ---
quote:
Originally posted by nitram38
GULVAL, why are all of your posts repeated?
quote:
Also check the car isn't hitting one rear bump stop before the other ---
What spring ratings are you running on the rear?
I would have thought for drag racing you want the rear locked up to stop it squatting, surely that is wasting power winding itself up before it launches batshit down the drag strip?
It's worth checking the bump-steer but you would normally experience really nasty instability all down the straight.
Bump-steer is the tendency of the wheels to turn in or out slightly as the wheels move up and down. You can check it by removing the front springs and
raising / lowering the car with dial gauges on the wheel rims.
Any movement at the rim is bad and can be removed by raising / lowering the steering rack or raising lowering the track rod end on the steering
arm.
I would check the car for squareness and that one side does not have more toe in than the other at the rear.
Ive checked the toe on the rear wheels and it is pretty much parallel.
I tryed to minimise bump steer at the front by shimming the rack but didnt look at the droop part of the travel.
I think Im using 175 lb springs at the rear but would have to check that. Im under the impression that soft is good for staight line racing but
obvisusly not if its hitting the bump stops.
Please not that this only occurs when using BOTH slicks and +75hp of nitrous.
I'll post a few pics soon so you can get some idea of how much it is squatting.
A few pictures of it squatting but couldt find any of when the slicks were on and when using gas so there is no where near the amount of squat
shown.
Rescued attachment squat.jpg
Another
Rescued attachment Silver kit3x.jpg
Note that the front bottom wishbones are parallel with the ground when stationary.
I think the reason drag racers set the rear up soft, is so all the weight transfers to the rear under hard launch, thus giving better traction.
Sounds like you may be getting too much traction, have you checked that the fronts are not actually leaving the ground ? sounds impossible, but IIRC
jon ison once had it happen. Also (and this is only a theory) if your wheels are not spinning, then the lsd will be working like an open diff and may
be transferring more power to one wheel, maybe this is what throws you off line?
Im under the impression that soft is good for staight line racing but obvisusly not if its hitting the bump stops.
I did initially bame the diff becaue when doing a burn out both wheels spin up together but then when they are both smoking, the drivers side wheel slows down to stationary. I thought this was the problem so Im going to make a spool to test and rule this out.
If you have a viscus diff, that's your problem. A viscus diff still has some slippage.
Yes it is a viscous diff. I know its slipping but would slip in the diff be sufficient to make you lerch sideways?
Even if it was slipping fully (ie acting like an open diff). Would an open diff cause this effect?
cheers
Carnut
possible, provided the front was lightening up enough to have only minimum grip left. The grip of a slick on a well rubbered drag strip must be immense, possibly enough to combine with the torque effect (one driveshaft is shorter) to add a twist. If this was so, ducking left would be caused if the longer shaft was on the left. Cant remember off the top of my head if this is so on a sierra.
Longer shafts on the right.
Already thought of that one.
so it probably isnt the diff IMHO 
all brakes free?! mine pulls hard right when i brake as the left caplier is siezed!
Everything is fine except when I crank up the gas to 75hp and have my drag slicks fitted. Pretty sure nothing is siezed or any bearings are
knackered.
Its realy annoying as its costing me loads of time.
I think I need to have all the below things checked and prepared ready for the start of season.
Hopefully one of them will solve the problem.
1. Build a spool diff
2. Get some stiffer rear springs to test (will check if bump stops are being hit by putting tape on damper shaft first)
3. Check front track changes in droop
4. Check all joints, bearing for slack and being siezed.
5. Corner Weights
you could try a run with no rear shocks, just a length of box section to hold it up. Obviously that would need a smooth stretch! Ive got a few spare shocks in the garage if you want to try them, its a set from GTS but im not sure of the spring rates.
I would check:
Rear toe is spot on equal.
Rear tyre pressure is equal each side.
Pull a measuring tape around the circumference of each rear tyre to make sure they are exactly the same diameter.
Cheers
Fred WB
[Edited on 13/2/06 by Fred W B]
check or uprate the rear bushes... if you are running slicks then you might have thrown them out very quickly.
They used to go every 1000miles in my Ultima due to the huge traction and heavy right foot.