Caster vs Bumpsteer

[another69]

I have a 1st gen camaro with a rear steer rack, and i have toe out bumpsteer on suspension compression (jounce). I have not adjusted caster yet, just have it mocked up without springs. Anyway, caster affects bumpsteer, right?

[onechev]

All adjustments can effect other. Set the caster,camber and toe then fix the bump

[killer69]

yes because it moves the outer tie rod end down as you add caster

[another69]

I was planning on setting caster and camber with no weight on the suspension, and without springs installed. Is this a bad idea? I'll recheck once the springs are in and the weight of the car is on all 4 wheels, but it seems simpler to set now, rather than a weighted car. plus, I can set caster without turnplates by measuring the angle of the spindle via the steering arm bolts. I'll set toe last, with weight on the car. Feel free to shoot down my idea if its flawed. Like I said, I'll recheck camber once the springs are in and the car is sitting normal, but bumpsteer and caster should not change, right?

[SLO_Z28]

It wont change much, but it will change. Wait till the car is together to set this.

[David Pozzi]

Caster changes with ride height. Do it after the car is together. Some rack kits come with special steering arms to fix bumpsteer.

[another69]

The rack I have is a Unisteer, and it did come with arms "to minimize bumpsteer". Their specs call for 2 deg of caster, and I'm closer to 6 deg now, so I'll change it and see if bumpsteer improves. BTW- I am at ride height, just up on a 4 post lift. I guess I'm really just trying to avoid buying turnplates.

[killer69]

you can do it without turn plates, just have to make sure you turn the wheel tha same amount both ways 20 deg is about 1 turn ??? give or take, so just turn the wheel 1 full turn left then 1 full turn right and that will do it with out turn plates.

[David Pozzi]

Less positive caster will make toe out bump steer worse, plus you dont want to reduce caster below 4.5 degrees unless you dont care how it drives. You need the outer arms to be lower to get less toe out in bump. Or raise the rack higher, which probably isn't practical.

[another69]

Thats a good idea about counting turns of the wheel for caster. Is 20 deg critical, or does the wheel angle for setting caster just need to be equal and opposite? My longacre gauge specs 20 deg. I know its not rocket science, but I can't seem to wrap my head around it. Thats why I was planning on using a straightedge across the steering arm bolts and checking that angle. If its possible with the wheels, brakes, etc on at ride height is yet to be seen.

My tierod angles up away from the rack toward the steering arm, so it makes sense that raising the rack would reduce it. To bad its not really practical. As far as bumpsteer is concerned, is there a difference between lowering the tierod via "bumpsteer kits" or just adding more caster? I have a lot of adjustment, so it should not be a problem to add 7 deg or more of + caster. I originally wanted to reduce caster to try and make my car feel more nimble, but I don't even know if I was on the right track with that. Even if it does make the steering feel heavier, I'd rather get the geometry right, or as close as possible. BTW- I am using Howe tall upper balljoints with SPC upper arms.

[Norm Peterson]

Unless you want to do some extra math to correct what the gauge reads out to you to what it should be, you really do want to use the 20°, as that's what your gauge is set up for (there is a factor that you multiply the camber difference by to get caster, which is dependent on the angle(s) steered). I'd guess that your caster calculation error would be roughly proportional to the accuracy with which you actually hit the 20° considering both directions, as long as you don't miss it by too much. IOW, if you are off by 10% in the angle steered, your caster will be off by about the same 10%. At an intended setting of +2° it probably doesn't matter, but if you're looking for 5° you'd miss by half a degree or so. I think when you steer less than 20° the gauge reading ends up lower than what's actually there. IOW if you consistently steered 18° and read +5° caster off the gauge, you'd really have about +5.5°.

There used to be a paper on the Hunter Engineering site that went into this in great detail. I'm pretty sure that there are a few copies floating 'round the 'net and elsewhere.


One of the tricks for reducing bumpsteer on the '78 - '87 G-body cars actually is to crank in some "extra" positive caster, so yes, you can use your caster setting to reduce bumpsteer. Don't forget that when cornering, +caster also helps the camber situation on the outside front tire.

Just a semi-random data point, but the OE caster setting for the current Mustangs is +7.1°, with a tolerance of 0.75° either way from that.


Norm

[another69]

Thanks, everybody-
I went back in the garage to play around with caster settings- I went from one extreme to the other. I only did 1 side, and only 4" total wheel travel. 2" up and 2" down from ride height. With 0 deg caster, I had a toe change of over 3/8" !! With my adjustment pretty close to maxed out at 8.5 deg, it had improved to the point that I'll have to check it with feeler gauges, because my 1/32" rule was too big to fit in the gap. I'll check a few more times, but the positive caster REALLY helped out. My only thought about it now is how much caster is too much caster? If I need to compromise, which is the lesser of 2 evils- too much caster or some bumpsteer?

[Norm Peterson]

It doesn't have to be a hard "either-or" situation, if you use some combination of caster setting and a bumpsteer kit.

Somewhat more positive caster than OE spec is preferable to big stacks of shims in the bumpsteer adjustment (at least looking at the steering from structural stiffness and strength points of view). Caster that's way too high might develop a shimmy.


Norm

[David Pozzi]

Check toe change at 5 or 6 deg positive caster. It doesn't sound like it will have a significant amount at that setting.

[another69]

I'll check it again- I'm just pumped that I'm able to improve it so easily. I'll try 5 or 6 deg, but if it has less bumpsteer at 8 or 9 would that be too much caster? I know that you can feel too much caster, but at what point can bumpsteer affect you? Would .050 toe change over 4" - 5" of wheel travel really be noticable? I guess I'm looking to find a good compromise without a lot of trial & error.

[JRouche]

I'll check it again- I'm just pumped that I'm able to improve it so easily. I'll try 5 or 6 deg, but if it has less bumpsteer at 8 or 9 would that be too much caster?

No, 8 or 9 is not too much. But be ready to scrub the tires in a slow parking lot and if you dont have power steering it will be pretty tight. And at speed it will feel really heavy. The tires will want to stay forward. With power steering its not an issue (besides tire wear).

When I initially tried to set up my front end without turn plates it turned into a measuring nightmare. I was chasing numbers all over the place. I really recommend making up a simple table with all the measurements each time you change something so you can back track and recreate the setup that looked best.

I setup my front unloaded also. I have air springs so it was just a matter of deflating the springs and supporting the car. I got some great numbers after ALOT of changes and finding the correct camber, caster and toes setting. But after I got the turn plates and loaded the suspension up and rechecked the numbers they changed.

So I re-did all my adjustments with the car on the plates. Made a new table (about twelve different fields) to show all the variables and picked one that had the best amount of camber that I wanted, the maximum amount of caster and a constant toe.

My final numbers (at ride height) were .5 degrees of camber, 3.5 degrees of caster and 1/16" toe in. I wanted more caster but after driving it (manual rack) it was pretty heavy at speed, too heavy. Felt like I had 20" wide tires up front at speed.

I did have to use some baer tracker bumpsteer correctors to lower the steering rods. I had to use the full shim pack just to get the steering rod low enough so it would track the lower control arm closer. Bumpsteer is not there until full compression or droop.

Anyway, nice to see a guy doing his own alignment. Here are some links to what I did. Kinda alot of pics and text but you might get some info. JR

My home grown caster-camber gauge..
http://www.stevesnovasite.com/forums...73#post1372973

Some mechanical work to make it all happen....
http://www.stevesnovasite.com/forums...d.php?t=154977

[another69]

Well, I set it up unloaded with 5 deg caster and 1/2 deg camber, but I will double check everything on the ground. There are too many variables, and I was getting thrown by using the spindle thread for my camber guage. I should stop wasting time and just do it "right". I might just have to go ahead and buy turnplates, I'll just have to find a cheap pair. I hate buying cheap tools, but $600+ is too much to spend when I use it maybe once a year. Any turnplate suggestions??

[JRouche]

Well, I set it up unloaded with 5 deg caster and 1/2 deg camber, but I will double check everything on the ground. There are too many variables, and I was getting thrown by using the spindle thread for my camber guage. I should stop wasting time and just do it "right". I might just have to go ahead and buy turnplates, I'll just have to find a cheap pair. I hate buying cheap tools, but $600+ is too much to spend when I use it maybe once a year. Any turnplate suggestions??

I havent tried it but it sounds like it would work. Linoleum type 12x12" flooring sheets you can get for cheap. Im not sure how they do it but what I would do is use two per wheel. Get the panels that have the PSA on one side, They are only a buck or so each. Get the smoothest ones you can find. Not the ones that have texture, and whats cool is the smooth ones are actually cheaper.

Set the car up so the tire is just above the ground and place the sheets just below the tire. Im not sure but I think the tire will move more forward and out (loaded tire) when you are checking so you might have to place the sheets off center by a lil amount. And you can do this with the suspension springs loaded to get real numbers.

I would place the sheets like this. PSA side down on the concrete floor. Clean the floor pretty good. Mainly for loose debris and dust. Dust and loose dirt will ruin the glue fast. If there isnt anything loose on the concrete floor the tiles stick really well, yeah, thats their intended purpose. Then for the other tile it is used upside down on the floor tile (face to face). But with some slippery lube in between. I would use a fairly high pressure lube, not oil and not some greases. Im thinking STP oil treatment would be a good candidate, pretty thick and sticky so it will stay in place and not squish out. Im trying to think of a grease that would work but they all seem to want to squish out (not enough stickiness).

So with a liberal coating of lube in between the two smooth surfaces and the loaded tire on the other sticky side of the upside down tile you will prolly have a decent sliding surface. And it will slide in and out and fore and aft.

Id go slow with the turning of the wheel so you dont slide the tire on the top tile, just a nice and easy turning from straight to the degrees of turn you want. And for some insurance to see if the tire slid on the top tile I would run a piece of tape on the top tile that was inline with the tire so you could see if the tire slipped on the top tile.

And really, it sounds like a hack type job. But believe me, folks have done even simpler alignment jobs that will sound even worse, and they worked!!

My thinking is do what you can NO matter how goofy the setup sounds. Cause really, before there were digital alignment rigs there were folks doing their own alignments how ever they could. And it WORKED!!! Folks were going 100+mph before our grandfathers were driving. JR

[JRouche]

Oh, one other thing. If you are close to me (93012 zip) Id be happy to have you out for an alignment day. Im no expert, might be some learning for the both of us but I think we could figure it out. Id love to help a guy out. JR

[robertjra]

$53.95 front end alignment