critique my steering/suspension geometry please

[Guest bluex_v1]

That's an '89 mustang rack from my donor. I just finished the 3/4" up, 1/4" out JTR control arm pivot relocation. I was putting it off, but figured I'd regret it later because I'd have to move the power steering rack too to keep things in line. As you can see, the lower control arm is still almost parallel to the ground, and I would like to lower the car at least another half inch still. The steering arm is pretty close to parallel (I don't have the balljoint torqued yet, so figure it may pull up another 1/16th or so) to the control arm, and the articulating length of it is almost identical to the distance between the control arm pivot point and the balljoint. Should I leave the rack at this height? I kind of figured it would be safer to build in just a tiny bit of steering arm droop to be sure I don't get too close to a toe-in condition during bumps. It also will give me some room to shim with if I find I need to convert to a heim/spherical rod end instead.

Now, as to the parallelism of the control arm to the ground, is this going to be a good situation to use the MSA, etc. strut-to-knuckle spacer (aka, 'bumpsteer spacer')? I have 17s so if I get them, would the thicker ones be best so I can get close to stock camber gain rates?

 

The other question is how much differential steering angle (aka 'Ackerman') do I need to try for? I know this pic sucks, but I hope it can give someone an idea of what is going on. I'm eyeballing about 10 degrees forward. This is about the maximum I could get with my rack mounting scheme. I can easily bring the rack further forward with some washers to reduce that angle though. I also want to try to maintain enough range of aft adjustment so that if I decide to install the 1cm shorter quick steering knuckles, I will still have some ackerman remaining with the rack set fully back against the crossmember.

 

Also, with the front suspension not quite at full droop, I can not yaw the wheel fully. The bolt heads that hold the steering knuckle to the bottom of the strut will contact the bolt heads that go through the top of the control arm and thread into the balljoint. I'm concerned that on suspension rebound, I might turn and have my steering lock up because they have contacted. Is anyone else close to having this problem? Maybe my balljoints are not as tall as normal or something?

[Guest bluex_v1]

anyone?

I know all the regulars are probably burnt out on the bumpsteer topic, but I'm ready to finalize this setup unless someone has some feedback.

thanks

[j260z]

i think everything looks great and with the steering rod tightened up i don't think you could get any closer to optimum without going to a lot of trouble (ie. lowering the steering rack) and even doing that i couldn't see it affecting the geometry enough to warrant it.

[blueovalz]

This looks pretty good. What are you using for tie-rod ends at the steering knuckle?.

[Owen]

Also' date=' with the front suspension not quite at full droop, I can not yaw the wheel fully. The bolt heads that hold the steering knuckle to the bottom of the strut will contact the bolt heads that go through the top of the control arm and thread into the balljoint. I'm concerned that on suspension rebound, I might turn and have my steering lock up because they have contacted. Is anyone else close to having this problem? Maybe my balljoints are not as tall as normal or something?[/quote']

 

This doesn't sound right, I know the bolts are very close to each other, but it sounds like something is bent? I don't know what tho...

Owen

[JMortensen]

OK couple of things:

 

If you turn to full lock and there is interference, you need a limiter. Terry has some on his front control arms: http://www.fototime.com/ftweb/bin/ft.dll/detailfs?userid={7DC317B0-8EDB-4B2E-A837-F708D07C9769}&ndx=14&slideshow=0&AlbumId={17E71651-3EF8-4704-9954-22956DF10FCB}&GroupId={3B8751D4-D564-4405-8017-F14E1CDA9AF0}&screenheight=768

 

You can't really "see" bumpsteer problems when looking at the suspension, because the pivots are not easy to see, and small changes make a big difference. If you line up the arms and use an angle gauge or something similar, you'll be off, almost guaranteed. Also, lots of tie rods are bent, making it harder to tell by looking. The inner tie rod is inside the boot, so it is hard to tell on that one. You'll have to measure to be sure.

 

Ackerman on a front steer car would require the steer knuckles to be bent out towards the rim. Usually there is not a heck of a lot of room there. Much easier to change Ackerman on a rear steer car like a 510, because they then get bent inwards.

 

If you're not going road racing, I'd put some steering limiters on the control arms and drive it.

 

Jon

[Guest bluex_v1]

They are the original mustang tierod ends. On Alsil's site, he discusses using them and just tightening them and losening them a few times to get them to distort enough. I am going to try that, but it does already feel pretty snug. If nothing else, I'll convert to heims.

 

The bolts that go through the control arm and into the balljoint came with lock washers. Is that normal? If I removed them, I believe I'd have enough clearance. 30 ftlbs plus red locktite with no lock washers ok?

The arms themselves look fine...no obvious twisting or bending, no nicks or anything that might be the result of an impact.

To clarify: The wheels are not making it to full lock before the bolt heads impact. When the car is on the ground on its wheels, I can go to full lock without impact, the problem is only when the suspension begins to unload and the control arms start to droop.

 

I had to cut and reweld the inner tierods inside the boots to shorten them, but I did so with them clamped in angle iron and as close to the joint as possible to minimize any misalignment (plus tubing over the joint for reinforcement).

 

Jon,

You can actually get ackerman effect by placing the rack behind the ends of the knuckles too. The greater the angle of the tierods, the greater the effect (on a forward steer car).

I don't plan to roadrace the car, but there's a chance I might eventually, so I want to do as much as I can right the first time.

I get what you are saying about measuring errors, etc. 'Just drive it' and if it feels like it needs it, then tune bumpsteer by jacking it up in incriments and measuring toe right?

 

What about the MSA strut spacers though...seems like they would help me with the camber gain while not affecting my hopefully ok bumpsteer characteristics, or at least not any more than could be adjusted for later. I read in the past the people like JohnC saying they weren't the right shape...but isn't this what they were made for?

[JMortensen]

I guess your comment about Ackerman is a little confusing. I suppose you'd get a very mild cam action on the knuckles if they aren't in line with the tie rod ends. Still seems like the effect would be the same on both sides though. Maybe I'm just not picturing it correctly. I've always understood that by changing Ackerman one changes the imaginary line that goes through the tie rod and the ball joint, and that (at least for 510's) the goal is to get the lines on the left and the right to intersect at the differential at the axle line. But I've never heard of trying to adjust Ackerman by moving the rack fore or aft, so maybe I'm just totally ignorant of how that would be adjusted.

 

The MSA bumpsteer spacers are really camber curve adjusters. By moving the control arms down, you will gain more negative camber when the suspension compresses.

 

To measure bumpsteer you need to remove the front springs and disconnect front bar, measure with dial indicator on the front and back of the wheel (I just did it on the rotor) and either jack the body up and down or jack the control arm up and down. The suspension should be in the range where the car is driven, and the change in the dial indicators should be as close to the same as you can get as the suspension goes through its normal travel.

 

It is pretty easy to adjust if you use rod ends for tie rods, and a common bumpsteer spacer kit from a circle track parts supplier. Moving the control arm pivot is easy enough too if you slot the holes.

 

Jon

[johnc]

The only way you're actually going to know is to bumpsteer the car.

 

http://www.thedirtforum.com/bumpsteer.htm

[blueovalz]

Great link.