Geometry of the Tie-rods and Lower Control Arms (Bump Steer)
10-03-2003, 11:44 AM
#1
Burning Brakes
Thread Starter
Geometry of the Tie-rods and Lower Control Arms (Bump Steer)
My car is slowly being put back together, but I haven't gotten to the front suspension and steering yet. I can't remember what the geometry looks like, and I can't find any pictures to look at.
Basically, I'm wanting to understand in detail what it is about our outer tie-rod/lower control arm geometry that causes bump steer, so that I can find the best solution for me and my car. Can someone post a picture of the relationship between the tie-rod and lower control arm for me to look at, or can someone describe the geometry in detail? Is the tie-rod shorter than the control arm, or is it longer? Is the tie-rod mounted at a different angle than the lower control arm, and which end of the tie-rod is off geometrically? Lastly, what are your thoughts on the best solution for fixing bump steer? Thanks for the input.
Mark
Basically, I'm wanting to understand in detail what it is about our outer tie-rod/lower control arm geometry that causes bump steer, so that I can find the best solution for me and my car. Can someone post a picture of the relationship between the tie-rod and lower control arm for me to look at, or can someone describe the geometry in detail? Is the tie-rod shorter than the control arm, or is it longer? Is the tie-rod mounted at a different angle than the lower control arm, and which end of the tie-rod is off geometrically? Lastly, what are your thoughts on the best solution for fixing bump steer? Thanks for the input.
Mark
10-03-2003, 12:01 PM
#3
Melting Slicks
Re: Geometry of the Tie-rods and Lower Control Arms (81vette)
Hi, Mark. Sorry, no pics. I did a complete suspension rebuild on mine last year. It's like driving a new car. I always wondered what it was like to drive a new Corvette (but I could never afford one! :) )
Basically, it is that as the control arm goes through its arc of motion, the tie rod would need to change its length slightly in order to maintain the same steering angle on the wheel. But of course the tie rod length is fixed, so as the control arm goes up and down, the steering angle has to change a little.
I can tell you that one way to minimize the issue is to minimize that range of motion that the control arm moves through. You do this with stiffer springs and sway bars, and new (preferably stiffer) bushings in the control arms themselves. I went from worn out, sagging stock coil springs to new VB&P "GT" coils, rated at 460 lbs/inch. I cut 2/3 of a coil off the top, and should have cut a little more. However, my engine has aluminum heads and some other minor weight-saving things which affect ride height, so don't take the coil-cutting as an automatic thing.
I used VB&P's "super" front end rebuild kit with new everything, plus polyurethane bushings to replace the worn out rubber. I kept the stock 1.125" front sway bar, but remounted it in poly bushings. Bilstein sport shocks, too.
The rear end got a similar treatment. New 330 lb/inch monospring in place of the 9-leaf steel spring, plus all new poly bushings, and added a 0.75" sway bar and the Bilsteins.
With the Pirelli V-rated tires, which have a very stiff sidewall, the car corners like it's on rails, to use an old phrase. It is amazingly fun to drive.
Good luck with your '81! :seeya
Basically, it is that as the control arm goes through its arc of motion, the tie rod would need to change its length slightly in order to maintain the same steering angle on the wheel. But of course the tie rod length is fixed, so as the control arm goes up and down, the steering angle has to change a little.
I can tell you that one way to minimize the issue is to minimize that range of motion that the control arm moves through. You do this with stiffer springs and sway bars, and new (preferably stiffer) bushings in the control arms themselves. I went from worn out, sagging stock coil springs to new VB&P "GT" coils, rated at 460 lbs/inch. I cut 2/3 of a coil off the top, and should have cut a little more. However, my engine has aluminum heads and some other minor weight-saving things which affect ride height, so don't take the coil-cutting as an automatic thing.
I used VB&P's "super" front end rebuild kit with new everything, plus polyurethane bushings to replace the worn out rubber. I kept the stock 1.125" front sway bar, but remounted it in poly bushings. Bilstein sport shocks, too.
The rear end got a similar treatment. New 330 lb/inch monospring in place of the 9-leaf steel spring, plus all new poly bushings, and added a 0.75" sway bar and the Bilsteins.
With the Pirelli V-rated tires, which have a very stiff sidewall, the car corners like it's on rails, to use an old phrase. It is amazingly fun to drive.
Good luck with your '81! :seeya
10-03-2003, 12:57 PM
#4
Burning Brakes
Thread Starter
Re: Geometry of the Tie-rods and Lower Control Arms (Fevre)
Maybe I wasn't too clear. I know what bump steer is, and I know what can cause it. However, since I don't have my car back together I cannot see what it is that causes it on our C3's. Is the tie-rod too short? Is it mounted to high/low at one end in relation to the control arm? Once I have this information I can start planning for a solution before I begin putting the front suspension back together. Just gives me a head start on the thinking process.
BTW, I had the VBP 550 lb front springs on mine originally, but I am now going to the VBP fiberglass mono spring. One of the reasons I've become interested in bump steer is because I want to reduce the spring rates to give me a more comfortable ride, but to also increase handling by allowing the tire to follow the road instead of bouncing all over it with the 550 springs. The problem with lower spring rates is that I'll have more movement of the front suspension which results in additional camber/toe changes to contend with. I'm planning to address the camber issue by extending the spindles. Now I need a plan for bump steer, but I need to understand why we have bump steer on our cars. Thanks for the input.
:cheers:
Mark
[Modified by 81vette, 12:48 PM 10/3/2003]
BTW, I had the VBP 550 lb front springs on mine originally, but I am now going to the VBP fiberglass mono spring. One of the reasons I've become interested in bump steer is because I want to reduce the spring rates to give me a more comfortable ride, but to also increase handling by allowing the tire to follow the road instead of bouncing all over it with the 550 springs. The problem with lower spring rates is that I'll have more movement of the front suspension which results in additional camber/toe changes to contend with. I'm planning to address the camber issue by extending the spindles. Now I need a plan for bump steer, but I need to understand why we have bump steer on our cars. Thanks for the input.
:cheers:
Mark
[Modified by 81vette, 12:48 PM 10/3/2003]
10-03-2003, 09:51 PM
#5
Burning Brakes
Thread Starter
Re: Geometry of the Tie-rods and Lower Control Arms (81vette)
Has anyone got a picture of the geometry, none of my books show what I'm looking for?
Mark
Mark
