Shimming lower strut bolts

I had planned on getting camber plates from Motor Force, but the parts I needed wouldn't be in stock until after the upcoming track day, so I needed to do something in the interim. After reading much about the pros and cons of shimming the lower strut bolts, I decided it would be useful for me. The only real draw back is that it pushes the top of the tire/wheel inward towards the strut. With camber plates, the wheels and strut assembly move as one, and stay in the same position relative to one another. Since I need 15mm spacers for my street wheels anyway, I figured I'd be fine even with the reduced clearance of shimming.

Thinking about it a bit, it's fairly obvious why shimming the lower strut bolts causes a camber increase. But how much? Well, obviously it all depends upon what your ride height is set at, and the geometry of your suspension. I found some general numbers to go off of for E36 based cars, and they seemed fairly accurate for me.

I made a trip to Park Rose Hardware since they apparently have an amazing selection of bolts. The stock bolts are M12x1.75x20 (diameter x thread pitch x length) and grade 10.9. When installing these bolts while doing the H&R Coilover suspension, I was kind of surprised how short they were since there's plenty more room for them to advance beyond where they do. I guess it's just not necessary? In any case, Park Rose Hardware had everything one could desire. Basing things on the above numbers, I figured I wanted to add about 2 degrees of negative camber, so roughly 5mm of shims, so I needed at least 25mm bolts, if not 30. I bought four M12x1.75x25 bolts with the same 18mm head (and hardness, 10.9) as stock (in case I only needed one shim) at $1.34 each. I also picked up four M12x1.75x30mm bolts with a 10mm hex head in hardness 12.9 at $1.24 each (go figure).

They were out of stock of the 10.9 hardness washers, but 8.8 will do just fine since they aren't carrying any load. These washers are designed to be .1 inch thick. I measured them to average around .95 inches, or 2.48mm. I started out by measuring the camber of my car as is. Dennis instructed me on a cheap and easy method to do so. Basically:

1. Park car on level surface (otherwise your readings are totally bogus).
2. Create a plumb bob out of string and a bolt. I just found any old string laying around in the garage, tied a heavy socket to one end, and wrapped the other end around my tripod. Wait until the string stops moving.
3. Measure the distance between the bottom of the wheel and the string (I used dial calipers to do so). This is X₁.
4. Measure the distance between the top of the wheel and the string. This is X₂.
5. Measure the vertical distance on the wheel between your two measurements. This is Y.
6. Camber = ArcSin( ^(X₁-X₂)/_Y )

I first did this to measure the camber with my stock wheels on, and only bothered with the driver's side.

Came out to be -1.168°. Okay, going on the numbers above, and based upon the thickness of the washers, I did some calculations and discovered that with three shims, I'd be roughly at -4°, which is a bit much. With two, I'd be a few ticks below 3°, and I was hoping for more! Oh well, two it is. I always get a kick out of how rigid the M Coupe is when I jack it up. Here I am, on just one jack stand, but the other front wheel is also off the ground! You can always get one whole side up, but one whole end is cooler.

Now that the measurements and math are mostly done, here comes the easy part! The three collar nuts at the top of the strut (in the engine bay) need to be loosened. Just make them flush with the bolt. This allows you to move the strut assembly up and down a bit. Next, remove the wheels (duh), and disconnect the two lower strut bolts. Just above those, the hub assembly pivots on the strut via a fit bolt, with 18mm head as well (and a self locking hex nut on the other side). This simply needs to be loosened. A wrench needs to be on both sides of it, one to hold the nut, and the other to undo the bolt. The strut is now free to pivot away from the hub. This gives you enough room to slip your washers between the strut and the hub. Keep one washer on the other side of the strut (same as stock). Be sure to measure the shims, together, that you're installing! These are not exactly precision made, and they all have varying thicknesses. It's probably not critical, but I used some dial calipers to make sure all four sets were roughly the same overall thickness. I decided to go with the 30mm bolts, simply because there was plenty of threads for them, they were harder/stronger (not that it really matters), and because they looked cool (hey, playing with bolts is actually fun!). Now things can go back together. The two lower strut bolts need to be torqued to 107NM, as does the fit bolt at the pivot point.

Now hit repeat on the other side. Once the three bolts are torqued to 107NM, the car can be lowered. Do not tighten the three collar nuts at the top of the assembly until there is weight on the struts (I also rolled the car back and forth). Once everything is on the earth again, you can torque these 6 (total) nuts down to 22NM.

Once everything was done, I measured the camber on both sides of the car again, using my plumb bob technique. From one reading to the next I obviously got slightly different measurements, since this string thing isn't exactly precise, but of course they were roughly the same. The left side measured out at -2.85°, while the right side was -2.97°. At first I was a bit concerned about the difference, but in reality this is totally fine. I guess the tolerances from BMW are much greater than this anyway. As I was packing up all my string stuff, I thought, "What the hell am I doing all this for? I work in machine vision!" The next day I whipped out a sensor, set it up, took the acquisition, set up the job, and done!

Pretty slick! One thing I was concerned about was the increased toe out which this causes. I figured I'd have to adjust it. I used my Longacre Racing Toe Plates to measure before and after the shimming.

Before, the wheels had ¹/₆₄" of toe out, each. after shimming, they had ⁵/₆₄". Not too bad at all! In fact, just about right. Good deal. Since the toe doesn't need to be changed after this process, I may wind up just continuing to shim the struts before each track day instead of buying those camber plates! We'll see if that process gets old or not. Really, now that I know exactly the steps to follow, it shouldn't take longer than 20-30 minutes. Especially if I start changing to track pads before events, this may be a very viable solution to the camber issue!