Ill handling decelleration
#11
Senior Member
RACING JUNKIE
Join Date: Jun 2003
Location: Hobbs, NM
Posts: 842
Originally Posted by gearhead1011
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.
#13
Junior Member
SHOW GUEST
Join Date: Jan 2007
Posts: 1
poss. tire balance
had problem at one time after installing larger engine and car went faster found rear tires out of balance., got balanced and corrected problem , we had pulled complete front end apart,sent parts all over the country to have checked. and it came down to tire balance. some times we look for the hard problem, when its the little one we forget. good luck
#14
Senior Member
DYNO OPERATOR
Join Date: Aug 2007
Location: Lost Creek, WV
Posts: 533
Ill handle in decel
I see some comments about bump steer. If it were a bump steer problem wouldn't it show up when the front end settles after the launch? I am very interested in what you find your problem is because I have a car doing the same thing. I did find on mine the left lower control arm was very tight. I haven't had it back out yet to see if this fixed my problem. I'll let everyone know if it does.
Thanks TS1955
Thanks TS1955
#15
Senior Member
RACING JUNKIE
Join Date: Jun 2003
Location: Hobbs, NM
Posts: 842
Re: Ill handle in decel
Originally Posted by TS1955
I see some comments about bump steer. If it were a bump steer problem wouldn't it show up when the front end settles after the launch? I am very interested in what you find your problem is because I have a car doing the same thing. I did find on mine the left lower control arm was very tight. I haven't had it back out yet to see if this fixed my problem. I'll let everyone know if it does.
Thanks TS1955
Thanks TS1955
Most car builders design their cars so that the effects of bump steer are minimal. However, you must still take care to bolt on your suspension carefully so as not to create unwanted bump steer. Make sure that you are always using the correct components for a particular car. Bump steer must be designed into the car and cannot be adjusted out if improper parts are used or if pivot points are moved without considering bump steer design principles.
In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm.
The instant center is an imaginary point that is created by drawing a line from the upper a-arm ball joint through the a-arm pivot where it is intersected by an imaginary line that extends from the lower ball joint through the inner control arm pivot. Where the two imaginary lines intersect is the instant center.
Sounds complicated? Really it is very simple. To achieve zero bump the front end must be designed correctly. The tie rod must travel on the same arc as the suspension when the car goes through travel. Simply matching lengths and arcs to prevent any unwanted steering of the front tires.
To exaggerate, if the tie rod were only 10" long and the suspension were 20" long then when the suspension traveled the tie rod angle would shorten much quicker than the suspension arc. In this scenario the tie rod would shorten much quicker through travel than the suspension and the car would toe in drastically over bumps. The shorter arc of the tie rod would pull on the spindle and toe it in through travel.
Bump Simplified
When designing a car, if the centerline of the outer tie rod lines up with the centerline of the lower ball joint, and the inter tie rod lines up with the lower pivot point then the length and angle of the tie rod and suspension will be the same resulting in zero bump. Most car builders design their cars in this way.
#17
Senior Member
DYNO OPERATOR
Join Date: Aug 2007
Location: Lost Creek, WV
Posts: 533
dparker,
Thanks for the explanation on bump steer. I looked at my car and it seems to be OK. The outer tie rod is in the center line of the ball joints & the inner is in line with the control arm pivot point and is parallel with the lower control arm.
Thanks TS1955
Thanks for the explanation on bump steer. I looked at my car and it seems to be OK. The outer tie rod is in the center line of the ball joints & the inner is in line with the control arm pivot point and is parallel with the lower control arm.
Thanks TS1955
#18
Junior Member
SHOW GUEST
Join Date: Jun 2006
Posts: 5
I agree with the bump steer answer. heres the thing. check the pivot point of the rack at the inside of the rod. if it is bending(piviting) at the same point of the inside lower contol arm then that part is correct, offen a rack is to short or too long for the frame width.(not in line with the lower control arm ) if this bending point is the same, then check the angle of the tie rod from pivot to steering arm. if this rod rises or falls at a different angle to the lower control arm it will toe in or toe out the tire on deceleraration. there are tie rod spacer kits to correct the angle and lenth of the rod. second thing to check is if the control arms and shock have been installed correctly. meaning they need to be tilted back 7 to 10 degrees, simular to the factory. if they were welded on level(which I have seen) then it will pitch the front end forward ahead of virtical center.tossing the weight on the front end causing the back end to wash out when you lift at the top end.