Improving suspension geometry?

[bse53]

I lowered the car in the front to 5.5" chassis height. That of course puts both the lower and upper control arms at the wrong angles, though the LCA is pretty close.
I created a spacer for the UCA to raise it an inch and move the mounting point rearward .75", to make it easier to get the 6-6.5* caster I've read is optimal with a small amount of static negative camber. While the dynamic camber gain looks good, it has raised a couple of new problems.

The hub is tilted back with the added caster, which puts the tie rod at about a 6* angle, which has created a lot of bump steer.

Anybody have an idea how much bumpsteer is acceptable? By lowering the steering arm .4 inches (according to the suspension program) I can get the bumpsteer down to around .5* (the program lists it as degrees).

One way to do that would be to reduce the anti-dive by raising the rear pivot point, which has the effect of lowering the steering arm.

What would be the effect of raising the rear pivot point .5"? That would still leave the tie rod at a 4.5* angle, which seems high, but it doesn't seem to affect the bump steer.

Another way to lower the steering arm is to heat it and bend it down. I think that can be done without weakening the arm too much.

[The Dark Side of Will]

http://www.speedwaymotors.com/Adjustabl ... ,3333.html



Also:
http://www.summitracing.com/parts/aaf-a ... Ogod2GEA0g
http://www.spohn.net/shop/Replacement-P ... OgodJXoA2g
http://www.summitracing.com/parts/afc-1 ... OgodOwMADg

[Series8217]

I lowered the car in the front to 5.5" chassis height. That of course puts both the lower and upper control arms at the wrong angles, though the LCA is pretty close.
I created a spacer for the UCA to raise it an inch and move the mounting point rearward .75", to make it easier to get the 6-6.5* caster I've read is optimal with a small amount of static negative camber. While the dynamic camber gain looks good, it has raised a couple of new problems.

"chassis height" is dependent on your tire. That is not a good way to measure ride height. Please measure from the fender arch (highest point) to the center of the axle/hub. It's a measurement that's independent of tire size so we can easily compare other setups to it.

If you don't have the stock fender, pick another chassis or body element from which to measure the axle/hub center.

[bse53]

Thanks for the link to the bump steer bolts. These look like I'd have to change the ball joint to a rod end to use.

I can't measure the hub to fender distance, since I don't have any fenders right now. I'll do that soon.
Surprisingly to me, I running the tie rod at about 4* will minimize toe changes the best. As I've read, you want the tie rod to intersect the instant center.

I'm finishing up the spacers and will start working on figuring out how to mount the Koni shocks that are coming. I went with the 30-5436, which should be close to what I should need. One of the things the spacer will allow is a way to him the UCA. It's been frustrating trying to set up a camber and caster combination.

I was going to try and paint it before the season started, but that's not going to happen. I did remove the doors and am going to attach the plastic skins to hinges and pin them. According to my reading of the rules, it's legal and will save about 50 pounds. I'm not too worried about not having much side protection at autocross. But I've already decided I wouldn't track the car without a cage.
I do have a C4 vette that I do do HPDE's with, though I haven't done much lately.
The final drive ratio of the transmission is 3.73 (3 speed auto), so you'd run out of gear pretty quick. I've thought about switching back to a manual, but that would mean going to 18" rims to find a gear ratio that would work for autocross which means raising the CG 2".

[The Dark Side of Will]

Are you running stock height knuckles?

Looks like it in your photo. Are you concerned that raising the UCA inner pivot will hurt your camber gain?

[bse53]

Yes, they're stock knuckles. But with the added caster, I'll get some camber gain.
Here's some data I recorded. I'm using a bubble camber gauge, so some of the readings are a little sketchy, but looking at the camber gain at 10* steering angle, at 1" compression it looks like running the caster at around 6.5% will give me plenty of camber. One of the drawbacks to lowering the car with the LCA below level, is some funky camber readings in droop.



All those numbers are degrees.

The bump steer is a bigger issue, but I think using those bolts you linked to with heim joint rod ends, I can deal with that.

I looked at drop spindles, but you still haven't lowered the CG, and looking at the stock geometry, you still have to deal with the bump steer, and the lack of static caster. I would like to know how Series8217 got so much caster. I couldn't get more than 5*.

The first autocross is the middle of April, so that will be a pretty good test. According to the suspension program i have, I can reduce bump steer to about 1*, with is about .2". I haven't measured it, which I will when I start putting the suspension back together.

Can anyone confirm that the end of the tie rod is 14 mm-1.5 pitch?

[Series8217]

What swaybars are you using? What's your max body roll?

[bse53]

I'm running 600 lb springs front, 400 lb springs rear, with a 1" bar on the front with a 5" link. No bar on the rear.

I don't know what kind of roll I'm getting, but the calculated roll is 1.15*/g. I have an app on my phone that says I'm pulling 1.4g momentarily and can hold about 1.2 g steady state (that was at 34 mph if you want to calculate the radius).

I was going to try 700 lb up front, and bought some 8" Landrums, but couldn't get them mounted. I should have bought the 7"!

This is all new to me, in the sense I autocrossed the stock vette for several years, and you are very limited to what you can do in the stock class. But I'm having a blast trying to optimize the suspension.

[bse53]

Does anyone know of a source for 14mm-1.5 pitch heim joints? If I'm going to use the bump steer adjusters that Will linked to, I need to find some. Everything I've found googling are 14mm-2 pitch. Hopefully I can find female, but male with a coupling nut would work.

[Series8217]

I'm running 600 lb springs front, 400 lb springs rear, with a 1" bar on the front with a 5" link. No bar on the rear.

I don't know what kind of roll I'm getting, but the calculated roll is 1.15*/g.

1.15*/g sounds reasonably accurate. From pics of my car while it's cornering I see what looks like around 1 inch of compression which equates to about 1 degree of roll for a 60 inch track width.

Does anyone know of a source for 14mm-1.5 pitch heim joints? If I'm going to use the bump steer adjusters that Will linked to, I need to find some. Everything I've found googling are 14mm-2 pitch. Hopefully I can find female, but male with a coupling nut would work.

I don't think rod ends come in more than one thread pitch. Everything I have seen uses the "fine" thread pitch for a given diameter, and that's all you get. Unfortunately you may be stuck with making a coupling nut. Maybe something exists in the GM performance aftermarket though.

[Series8217]

I found some examples of bolt-on steering arms added to factory knuckles:

C4:


C5:


I don't see any reason why this couldn't work with the '88 Fiero knuckle.

[The Dark Side of Will]

AMC Eagles (and maybe their RWD cousins) used the same knuckle and steering arm left and right... the arm was just designed to bolt on two different ways.

I've been thinking about a way to build a knuckle so that the steering arm itself was adjustable for caster in a deterministic way... but don't have any good ideas thus far.
A similar theme might be to make a knuckle on which the steering arm was a wide channel which could be bridged by threaded rod, allowing built-in bump steer adjustment without the special studs and torsional loads on the steering arm

Idea... instead of making the steering arm adjustable, make the upper ball joint mount adjustable for both caster and camber/SAI. That could be done via a two-axis shimming system. Of course that doesn't help you much right now.

That C4 arm is BEEFY and looks like stainless.

[bse53]

I found a source for rod end heim joints in the 14mm-1.5 pitch from a store in the UK. I'll deal with that after the shocks get here and get new shock mounts made. I went with the Koni 30 series.

Have the spacer/bracket finished and it certainly allows way more adjustment than I need. I'm thinking of around -.75* static camber with 6.5 to 7* caster, which will allow for a fair amount of dynamic camber gain.





I added those bolts on the spacer to a plate welded on the inner fender well that I tapped, so I can finely set the arm and not have it move while tightening the bolts. It works well.

It turned out to be a little more of a chore. I needed to notch the spacer so I can add a upper shock tower mount when the shocks get here.

One of the things raising the UCA was to bring its angle closer to the stock angle. I think Bloozeberry measured it to be 4* stock and right now, it's slightly more than 4*.

I won't know if it was a good change until I get a chance to drive it at an AX. Unfortunately I'm going to miss the first one on the 12th, since I also need to redo the transmission, so I won't get it out until May.

[Series8217]

I found a source for rod end heim joints in the 14mm-1.5 pitch from a store in the UK. I'll deal with that after the shocks get here and get new shock mounts made. I went with the Koni 30 series.

Have the spacer/bracket finished and it certainly allows way more adjustment than I need. I'm thinking of around -.75* static camber with 6.5 to 7* caster, which will allow for a fair amount of dynamic camber gain.

I was running about that much static camber but 9 degrees of caster and the front tires wear only the outer 30%. I was burning up my RS3s (bluing the outer shoulder). This was at both autocross and track days. You definitely need more camber and/or caster than that.

[bse53]

I may increase the caster once I get some tire temp. data. I want to keep the camber at 1* or less to maximize straight line braking.

[The Dark Side of Will]

I found a source for rod end heim joints in the 14mm-1.5 pitch from a store in the UK.

Would you mind posting the manufacturer, vendor, part number, etc. to the "Hard to Find Parts" thread?

[bse53]

I found a source for rod end heim joints in the 14mm-1.5 pitch from a store in the UK.

Would you mind posting the manufacturer, vendor, part number, etc. to the "Hard to Find Parts" thread?

Will do.

it will be difficult to make the Koni's work. The top eye has a 2" shoulder, which makes it nice for the extra large bump stop, but trying to thread the shock through the mount will be too difficult.

A couple of things that I'm dealing with and wonder if anyone else has similar constraints.

First, the ARB has a total travel of 4". I can't figure out what it's hitting that is limiting the travel. 4" is enough travel, given it's MR, but it will be important to get the bar centered in the total wheel travel.

The second issue is the total compression travel before the UCA ball joint runs out of travel. With the chassis set to 5.5" (which translates to 13.5" from the top of the hub to the inner fender (I don't have any fenders on the car right now)) I have maximum of 2.5" of compression before the steering knuckle limits the amount of steering angle (exactly one full turn of the steering wheel, which I casually measured to be 20*). At 2" of compression (distance from the hub to inner fender is 11.5") I can turn the steering wheel another quarter turn. As you raise the hub, the amount of total steering continues to go down.

Now, based on my estimated roll, it isn't a problem. First, how many times do you turn the wheel one full turn on an autocross course? Never. Although we occasionally have turnarounds on our somewhat limited venue that require 180* turns in about 40'. And compression in braking won't be an issue as the suspension can continue to compress when the wheel isn't turned.

This may be normal, and I've never noticed because I've never measured it before.

[The Dark Side of Will]

Will do.

Thanks

"chassis height" is dependent on your tire. That is not a good way to measure ride height. Please measure from the fender arch (highest point) to the center of the axle/hub. It's a measurement that's independent of tire size so we can easily compare other setups to it.

If you don't have the stock fender, pick another chassis or body element from which to measure the axle/hub center.

With the chassis set to 5.5" (which translates to 13.5" from the top of the hub to the inner fender (I don't have any fenders on the car right now))

Alternatively, you could measure from the floor to the height of the center of the hub, post that number, and continue quoting "chassis height" as you have been... although you'll have to be more specific about what you're measuring to when you quote chassis height. Knowing your hub height will give us a basis of comparison.

[bse53]

Alternatively, you could measure from the floor to the height of the center of the hub, post that number, and continue quoting "chassis height" as you have been... although you'll have to be more specific about what you're measuring to when you quote chassis height. Knowing your hub height will give us a basis of comparison.

The floor to center of the hub is 11.5".

[bse53]

I think the limiting factor are the ball joints and the amount of caster.

I suspect if I reduced the caster to 5*, I would have more compression travel at 20* steering angle. The upper ball joint is slotted, which limits the range of motion sideways. At least that's what I'm surmising.

Before I raised the UCA mounting points, I think the limiting factor was the extreme angle of the UCA. But then I was running less than 5* of caster with -1.8* static camber.