IRS alignment - please explain it to me in kindergarten terms

[JohnK]

The engine is about to go in this weekend. I know there's not much point in doing an alignment before the engine is in, but I at least needed to get it to the point that it could roll straight(ish) and not damage the tires as we're moving it on and off the lift to install the engine. One of the rear tires had so much negative camber when I set the car down on its wheels that the top of the tire was touching the shock tower. That wouldn't have been good.

So anyway, I've got front toe-in at ~1/16". I don't really care about camber/caster at this point. I'm calling that good enough to be able to roll the car around.

On the rear, I started by setting toe-in to 1/16". I then set camber to -0.5*, which totally threw off the toe-in. I've read that the two are inter-related so I was expecting this. I went back and started playing with toe-in again, but I'm seeing two issues. They're probably both symptoms of one larger issue but I'm not smart enough to know what that larger issue is.

- First, the gap from the top of the tire to the coil-over is very different on the two sides. On the DS I can stick my fingers easily between them. Probably 5/8" space between the tire and the coil over. On the PS I probably have less than 1/4" space. I can't stick my fingers between the tire and the shock.

- Second, As I'm trying to get the toe in spec, I've run out of adjustment on the DS toe adjustment arm. The threaded part has literally come out of the end of the arm.

So clearly I've buggered this all up but I have no idea what I'm doing wrong. If I re-thread the toe adjuster on the DS it will just throw the toe-in numbers even more out of whack.

Thoughts?

[edwardb]

Three thoughts from my side: (1) I assume you have it roughly at ride height? Understand you're not trying to get it perfect at this point. But will be way off if not at least in the ball park for the ride height. (2) I always do the camber first. Not the toe-in. Yes the two affect each other. But I've found the camber is the big mover. At least until both get close. (3) What's missing from your question is how you're getting the rear suspension aligned with the chassis. Typically called thrust angle. That could be the reason you're running out of adjustment. If one side is way off compared to the other. For DIY alignments, often strings are used. I use a laser pointer on toe plates and confirm both sides point the same amount from the chassis tubes.

In my experience doing two of the 2015+ IRS, the adjusters all end up about in the center of their range. You might want to return them to that point and start over. If you're off as far as you described, you'll spend forever chasing the adjustments. Good luck.

[CraigS]

A diagram of thrust angle
https://live.staticflickr.com/65535/50460127426_8b4a7397ce_w.jpg (https://flic.kr/p/2jSZkT1)top view (https://flic.kr/p/2jSZkT1) by craig stuard (https://www.flickr.com/photos/152454123@N04/), on Flickr
Your toein (the relationship between the two wheels) could be perfect but if the wheels are like this (the relationship between the wheels and the car) you might have your current result.
https://live.staticflickr.com/65535/51359873407_da963ce296_w.jpg (https://flic.kr/p/2mfuM1K)top view 2 (https://flic.kr/p/2mfuM1K) by craig stuard (https://www.flickr.com/photos/152454123@N04/), on Flickr
For a quick check get a piece of wood trim, or anything straight and long enough, hold it up against the outside of the rear tire w/ the long end toward the front. Measure from it to the frame just behind the front tires. Hopefully that measurement is close to equal on both sides. A=B
https://live.staticflickr.com/65535/51359884582_9580a1e416_w.jpg (https://flic.kr/p/2mfuQkq)top view 3 (https://flic.kr/p/2mfuQkq) by craig stuard (https://www.flickr.com/photos/152454123@N04/), on Flickr

[JohnK]

Thank you both! I had not contemplated thrust angle at all, so that's probably the larger issue that I was missing. I hadn't bothered to set ride height, so I'll do that first. Right now the adjusters are threaded up the shock body only enough to be snug up against the spring so it's probably off. Paul, I like your suggestion of setting everything back in the middle of its adjustment range and starting over from there. Much appreciated!

-John

[J R Jones]

John, I have exchanged alignment ideas with a CO MKIV owner, his alignment shop report included wheelbase comparison, and it was off. I do not know if he pursued the issue.
Comparing wheel center to wheel center distance right and left is complicated by symmetric toe settings front and rear. and (front) symmetric castor left and right.
My suggestion is as stated above, attempt to get symmetry, and measure wheelbase. If wheelbase differs, the rear track bars may adjust it out.
This is an additional detail, and likely has no effect on your tire/spring clearance.
I have used the string perimeter method of roughing-in toe settings, and also a laser, but you must lay-out the chassis centerline which is another complication. I use suspension mounts as data points; again the laser helps.
jim

[CraigS]

When you set ride height count the # of threads visible above/below the adjuster collar.
https://live.staticflickr.com/65535/51361789577_8812a2669d_z.jpg (https://flic.kr/p/2mfEACa)coilover thread count (https://flic.kr/p/2mfEACa) by craig stuard (https://www.flickr.com/photos/152454123@N04/), on Flickr
You want the same number right and left but the front and rear will most likely be different. Once you have them equal all further adjustments should be done w/ the same number of turns on both sides.

[JohnK]

Thanks everyone for the input. Armed with this additional insight, I gave it another go yesterday. I thought I'd post an update and share what I learned in case anyone stumbles across this thread down the road.

- I started by setting ride height. I had previously just run the adjusters up until they were snug against the coil springs and thought that was "good enough for now." It turns out that with no engine in the chassis, I was at 6.5" ride height front and rear. No wonder I was struggling with alignment. I ended up having to run the adjusters nearly all the way to the bottom of the shock but was able to get the ride height to 4" in the front and 4.25" in the rear. This will all get thrown off and have to be re-done once the engine is in. BTW - the issue of clearance between the tire and the top of the shock is a non-issue when the chassis is at the proper ride height, as the top of the wheel is above the top of the shock.

- I then re-set the toe adjusters to the middle of their range. I backed them all the way out and measured the exposed lengths (~28mm of threaded length on the inner and outer adjusters) then threaded them in half way and used that as my starting point.

- I then set camber to -0.5*. The wheels were very toed-in at this point.

- Having seen how toe-in and camber adjustments are interrelated, I decided to try a different approach this time with toe adjustment. I started off on the drivers side. I would adjust toe a few turns, then bring camber back in spec, and repeat. Both adjustments had the net effect of toeing the wheels out, so this let me "sneak up" on the proper setting. Once the wheel looked like it was roughly straight I set up my laser. I set it behind the wheel and aligned it with the Fastrax alignment gauge, then used the laser to take measurements against the 4" tube at two spots - directly in front of the rear wheel and just behind the front wheel. I kept adjusting toe and camber simultaneously until I had the thrust angle of the wheel relative to the frame at 1/16" toed in.

https://thefactoryfiveforum.com/attachment.php?attachmentid=151760&d=1628347267

https://thefactoryfiveforum.com/attachment.php?attachmentid=151759&d=1628347267

- Once I had the DS more or less where I wanted it, I repeated the same thing on the PS with one shortcut. I measured the final exposed length of toe-adjuster on the DS with my caliper and set the PS toe adjuster to the same position as my starting point. I then set camber to spec, and then followed the same procedure on the PS from there.

- Once I had both sides individually with 1/16" toe in relative to the 4" tubes I measured toe-in with the Fastrax gauge and it confirmed that I had a total of 1/8" toe in. I'm calling that good enough for now. Definitely OK for just wheeling it around the garage.

I will have to repeat all of this once the engine is in, and again once the body is on but now I have a working knowledge of how this is all supposed to go, so thank you all again!

Jim - that's interesting insight on wheelbase differences side to side. I didn't bother to check this time around, but when I re-do alignment "for real" I will check. Not sure how one would go about addressing a wheelbase discrepancy with an IRS rear end though.

Craig - I'm assuming that counting threads and having the same number of threads exposed side-to-side is a proxy for having even L/R weight balance. My plan is to eventually have the car aligned and corner weighted by a local shop that does professional race chassis tuning. For now, I made sure to back the adjusters down the same number of turns on each side.

[Indy Shu]

I will have to repeat all of this once the engine is in, and again once the body is on but now I have a working knowledge of how this is all supposed to go, so thank you all again!

The alignment should not change. Just reset the right height and you should be back to your previous settings. Hth

[CraigS]

Wheel base is tough to measure. Looking at measuring front to rear wheel at the outside edge of the rim is almost impossible. When a front wheel is steered it moves forward or back a bunch. I have measured to the front lower coil over mount bolt back to the rear axle on a solid axle car but haven't tried it on the IRS.
Counting threads is exactly what you suspect. My experience is that FFR does a pretty darn good job welding up the frame in their jig so I make the assumption that the frame is square. So then making the coil overs equal length works fine. If you or a shop have the ability to measure actual corner weights then you can adjust from there. These things are kind of a pain to get right. The car itself is slightly rear heavy and then the driver sits dang near on top of the LR wheel so the car ends up LR heavy. So you try to adjust to get cross weight right, but then the L and R front wheels are different so under braking the RF tends to lock up. So it ends up a compromise.