Ill handling decelleration
01-07-2009, 06:22 AM
#1
Junior Member
JOURNEYMAN
Thread Starter
Join Date: Oct 2004
Posts: 29
Ill handling decelleration
My partner and I took his freshly built Vega to the track Saturday for shake down runs and discovered a problem.
The car rolls from side to side on decelleration. It goes straight but it can be a little unnerving ay 140+ mph. This happens as soon as you let off the throttle, before braking. Brake application doesn't seem to affect the rolling. It keeps up the motion until you're almost stopped.
The car accelerates hard and straight all the way down the track, but as soon as you let off of it, the rolling begins.
We thought it might be spring/shock adjustment, but everything is adjusted properly (tripple checked). Front and rear spring rates measure out evenly and so to the shocks. There does seem to be more suspension travel on the right front corner which we can't account for and that may be at the heart of the problem.
Anyone have any thoughts?
The car rolls from side to side on decelleration. It goes straight but it can be a little unnerving ay 140+ mph. This happens as soon as you let off the throttle, before braking. Brake application doesn't seem to affect the rolling. It keeps up the motion until you're almost stopped.
The car accelerates hard and straight all the way down the track, but as soon as you let off of it, the rolling begins.
We thought it might be spring/shock adjustment, but everything is adjusted properly (tripple checked). Front and rear spring rates measure out evenly and so to the shocks. There does seem to be more suspension travel on the right front corner which we can't account for and that may be at the heart of the problem.
Anyone have any thoughts?
01-07-2009, 06:34 AM
#2
Senior Member
DYNO OPERATOR
Join Date: Apr 2008
Posts: 504
Topping the shocks out is very wild on the big end. look to see at ride height how much shock is showing between the bumper and shock body. If you have 3" travel shocks or near that I never run more than 3/4" of ram showing. I use the travel for tire application not bump which the bumper does plenty well by it self without much shock travel in the bump or compression. If that checks out the next it a broken or weak spring in the front. A scale report will detect this easilly. Excessive preload put in the car will permit the car to behave seeming well under acceleration than wild under decell. Is the driver 500#? Could your car be built lob-sided, are the shock mounts square to each other and square to the rear suspension brackets? If you are within .030" that would be ok.... check these things including tire pressure front and rear too and you should have the problem by the neck.javascript:emoticon(':shock:') I hope this gives you something to start with.
01-07-2009, 09:41 AM
#3
Senior Member
RACING JUNKIE
Join Date: Feb 2007
Location: Michigan
Posts: 1,900
Another thing you could check is the shock valving on each shock. Take each shock off and if it's a coilover remove the spring and fully compress then fully extend, change your shock settings and see if it changes if not you may have a bad shock. I had this happen once brand new shocks and one had no resistance on rebound compared to the other. Just a thought.
01-07-2009, 09:46 AM
#4
Senior Member
DYNO OPERATOR
Join Date: Apr 2008
Posts: 504
Beast477 is correct, I forgot about that issue potential. having them adjusted differently side to side will raise Hell to. Sounds like the answer is in the question. The right side travel being greater needs not to be glossed over.
01-07-2009, 10:55 AM
#5
Senior Member
RACING JUNKIE
Join Date: Nov 2004
Location: ILLINOIS
Posts: 2,190
IT COULD ALSO BE FRONT END ALIGNMENT WHEN YOU LET OFF QUICKLY IT THROWS THE CASTER/ CAMBER OFF ENOUGH TO START A VIOLENT MOVEMENT-YOU MIGHT ALSO CHECK FOR A CRACKED BRACKET WELD ON THE SUSPENSION[ SAME THING IT WON'T SHOW UNTIL THE CAR DROPS BACK DOWN]
UNEVEN AIR PRESSURE ON REAR SLICKS OR FRONT TIRES
I WOULD CHECK EVERY POSSIBILTY BEFORE YOU PUT IT IN THE WALL OR ON IT'S ROOF!!!!!!!!!!!!!!!!!!!!!!!!!!!
SOMETIMES A SIMPLE LITTLE THING CAN KILL YOU VERY QUICKLY
LIVELY
UNEVEN AIR PRESSURE ON REAR SLICKS OR FRONT TIRES
I WOULD CHECK EVERY POSSIBILTY BEFORE YOU PUT IT IN THE WALL OR ON IT'S ROOF!!!!!!!!!!!!!!!!!!!!!!!!!!!
SOMETIMES A SIMPLE LITTLE THING CAN KILL YOU VERY QUICKLY
LIVELY
01-07-2009, 01:16 PM
#7
Senior Member
EXPERT BUILDER
Join Date: Jan 2004
Location: Shepherdsville, KY
Posts: 313
Sounds like bump steer to me. If this "freshly built vega" included any mods to the front steering/suspension bump steer is the likely culprit. If the front end geometry isn't correct the toe will change as the ride height of the car changes. When this happens it will steer the car in different directions as the ride height changes.
01-08-2009, 02:17 PM
#10
Member
JUNIOR BUILDER
Join Date: Mar 2006
Location: Illinois
Posts: 96
Did you check that right front shock off the car? Even if they are adjustable, if there is a bad seal they will do this.
Other than that, Tell us about your suspension. Stock, Ladder, 4link? Are spring/shocks set narrow in the rear? What kind of lateral link do you have, and how high or low is it? Anti roll bar? Pre loaded weight on cross corner springs? Spring rates?
With out knowing much about the car, my list of maybes would be bad shock, preloaded right rear and left front spring, no anti roll bar front or rear, too low a instant roll center in the rear for the center of gravity.
Other than that, Tell us about your suspension. Stock, Ladder, 4link? Are spring/shocks set narrow in the rear? What kind of lateral link do you have, and how high or low is it? Anti roll bar? Pre loaded weight on cross corner springs? Spring rates?
With out knowing much about the car, my list of maybes would be bad shock, preloaded right rear and left front spring, no anti roll bar front or rear, too low a instant roll center in the rear for the center of gravity.
