carnut
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 posted on 12/2/06 at 11:50 AM |
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Droop Steer
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
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zilspeed
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| posted on 12/2/06 at 11:54 AM |
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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.
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nitram38
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| posted on 12/2/06 at 12:13 PM |
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GULVAL, why are all of your posts repeated?
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JoelP
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| posted on 12/2/06 at 12:20 PM |
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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!
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daviep
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| posted on 12/2/06 at 12:39 PM |
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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'
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carnut
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| posted on 12/2/06 at 01:20 PM |
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Its an indy with IRS
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britishtrident
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| posted on 12/2/06 at 01:45 PM |
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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 ---
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zilspeed
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| posted on 12/2/06 at 01:55 PM |
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quote:
Originally posted by nitram38
GULVAL, why are all of your posts repeated?
Not here they aren't.
Sorry 
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carnut
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| posted on 12/2/06 at 02:09 PM |
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quote:
Also check the car isn't hitting one rear bump stop before the other ---
How would I do this then? It would be easy enough to see if 1 hits and the other doesnt but to find which hits first is puzzling me?
If it makes any difference I know the rear right has more weight over it because im sat at that side.
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Hellfire
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| posted on 12/2/06 at 03:23 PM |
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What spring ratings are you running on the rear?
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Triton
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| posted on 12/2/06 at 04:07 PM |
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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?
My Daughter has taken over production of the damn fine Triton race seats and her contact email is emmatrs@live.co.uk.
www.tritonraceseats.com
www.hairyhedgehog.com
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Nick Skidmore
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| posted on 12/2/06 at 06:46 PM |
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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.
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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carnut
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| posted on 12/2/06 at 07:13 PM |
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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.
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carnut
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| posted on 12/2/06 at 07:20 PM |
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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
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carnut
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| posted on 12/2/06 at 07:21 PM |
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Another
 
Rescued attachment Silver kit3x.jpg
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carnut
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| posted on 12/2/06 at 07:22 PM |
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Note that the front bottom wishbones are parallel with the ground when stationary.
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Wadders
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| posted on 12/2/06 at 07:28 PM |
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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.
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carnut
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| posted on 12/2/06 at 08:06 PM |
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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.
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The Shootist
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| posted on 13/2/06 at 03:42 AM |
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What kind of LSD?
If you have a viscus diff, that's your problem. A viscus diff still has some slippage.
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carnut
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| posted on 13/2/06 at 09:54 AM |
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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
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JoelP
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| posted on 13/2/06 at 10:05 AM |
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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.
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carnut
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| posted on 13/2/06 at 10:09 AM |
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Longer shafts on the right.
Already thought of that one.
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JoelP
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| posted on 13/2/06 at 10:48 AM |
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so it probably isnt the diff IMHO 
all brakes free?! mine pulls hard right when i brake as the left caplier is siezed!
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carnut
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| posted on 13/2/06 at 10:57 AM |
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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
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JoelP
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| posted on 13/2/06 at 11:02 AM |
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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.
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