XR4Ti Archived Tech Articles

    Rear Suspension Shimming - The technique for aligning the rear suspension in camber and toe; and my choice in using washers instead of shims for this alignment.


Post Date:    Sun, 18 Jun 2000
Subject:      Re: looking for rear wheel shims guy at Carlisle

Hello all,

Larry Davis mentioned that BAT sells shims.  They are pretty pricey, though. 
It's BAT pn M423R, $30.00.  It states that it will take about 1.5' degrees of
camber out (or put it in), and adjust the toe back to factory spec.

I did a rear suspension alignment to my race car about a month before 
Carlisle.  From about -2.75' to about -0.85' gave me a big change in rear 
traction in corner exit (approximately 2 seconds improvement on a 40 second, 
moderate/light push-inducing autocross course).

I didn't use shims for the alignment, but instead, M10 washers.  I used a 
digital caliper to measure individual washer thickness.  I actually had a 
thickness range of about 0.0585 to 0.0920, in 0.0010 increments, sorted into 
lots of little baggies.  It took about 500 washers to make sure I had enough.
Keep in mind, this only did the camber, I still had to do the toe.

For toe, I didn't have a washer thin enough, as I only needed one about 0.025
thick.  So for this, I used aluminum roof flashing, sold at Sears for about 
$10 for a 1' x 10' roll.  The flashing was 0.010 thick.  I trimmed and 
drilled out a few "washers" and used them to adjust toe (drill then trim, and
use a GOOD drill bit).  I am not pleased that I had to use aluminum for 
racing conditions, but it's held up well.

So far the car has fared pretty well with plenty of firsts to it's title (DSP 
SOLO II).
 
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Post Date:    Thur, 13 Jul 2000
Subject:      Complete Post on Rear Suspension Shimming

Hello again,

I posted a condensed version of the technique I used for suspension shimming,
and now it's time for the complete version.  Bear with me, it's a little
long, but filled with content.

I autocrossed my car twice with the rear negative camber at about -2.75' for
each side and oddball toe measurements (one slightly in, the other slightly
out).  The excessive negative camber was due to BAT's 40mm lowering springs
that I installed on the car.  Straight line traction was laughable and
traction coming out of a corner was terrible (I have no LSD, yet).

So I thought about it for a while and finally I figured that I would be much
better off with about -0.75' camber.  However, I had to know what size shims
I'd need to make what change.  Off to Allan Slocum's Merkur Encyclopedia I
went.  It's currently at the following URL (as of June 2000):

http://www.company23.com/aslocum/

On his rear suspension alignment page is a listing of theoretical shim sizes
and the resulting changes to be expected.  I found the numbers to be close,
but not on the money.  They work perfectly to get you easily in the ballpark
of where you want to be.  More on why I suspect Allan's theoretical numbers
were off the mark later.

As I mentioned in the title of this article, I did not use shims, as in the 
buy-them-at-NAPA shims, but instead I used washers.  What's the difference?
Well, a shim is essentially washer with one open side.  Using a washer
requires you to remove the bolt completely from the female threads to get the
washer around the bolt.  With a shim, you just loosen the bolt enough to get
the shim in.

As I found out, washers were hardly difficult to install.  In fact, I doubt
they aren't much harder to install at all.  More on the technique of
installation later.

Now it's time to discuss parts and tools.  Here is a list of the tools I used
for the project.  Some tools aren't essential, denoted with an asterisk, but
they make the job easier (and potentially more expensive if you don't already
own them):

- 13mm socket (proper size, as I recall)
- jack and jack stands
- wheel removal tools
- shims/washers
- digital caliper, vernier micrometer, or other thickness measuring device
- camber and toe measuring setup (I used a digital camber gauge* and a
  homemade toe setup* my racing teammate had built).

In theory, you don't need to have anything to measure the alignment, it'll
just take a little longer to do.  I had the rear alignment measured when I
had the front toe done after some front suspension work.  They gave me a
printout of all the specs at NTB.  If you took that as a baseline, threw in
some shims and then went back to NTB for an alignment check (probably free),
then you have essentially the same thing, just a little less convenient.

I am not going to go in depth with the specific way to set up your own
alignment measuring system, as it is on the web (no URL right off hand).  I
will however, skip the basic setup and include what I have done, with
whatever is needed to be clear.

I had the baseline (I keep refering to "I", but there were indeed two of us)
of the NTB printout, but I wanted to make sure my measuring system was close
to accurate.  So I measured the camber and toe, just to be sure I had a solid
baseline.

With those figures it was time to pick appropriate washers (shims) to do the
job, so I guess I'll talk about those now.  I picked up two different kinds
of M10 washers at my local fastener supplier.  Be careful when choosing
washers, as the outside diameter of the washer will affect the alignment
results.  A larger washer will change it more, and a smaller washer will
change it less.  I suspect that Allan calculated the changes in camber and 
toe based on changes along centerline of the bolt, when in fact, the change
is really made at the outside edge of the washer around the bolt.

The two different washers I chose were identical in outside diameter, but
were of different thicknesses (herein refered to as washer #1 and washer #2).
My next step was to find out exactly how thick each washer was and catagorize
them by thickness.  I borrowed a digital caliper to do this.  All in all, it
took about 500 washers to get a complete set of ten washers in each group.  I
grouped washers in increments of 0.0010.  With washer #1, I got a range from
0.0580 to 0.0710.  With washer #2, I got a range from 0.0730 to 0.0920.  I
bagged and labeled each increment with x.xxx0 - x.xxx5, since the difference
between 5/10000 and 10/10000 is inconsequential.  The only size I was missing
was 0.0720.  I would have liked to get a washer that was thinner, preferable
down into the 0.0350 range, but I was unable to find anything in that range
with the appropriate outside diameter.

Based on Allan's charts as to what shim size equals what change in camber, I
picked the washers I would need to shim each side the proper amount to result
in the -0.75' camber I was looking for.  Washers in hand, it was time to start
wrenching.

The job of installing the washers was sooo much easier than I first thought it
was going to be.  Jack up the rear of the car and set it on jack stands.  Then
take of the wheels and the brake drums.  You are now looking at your main focus.
You'll see the heads of the four blue bolts that hold the bearing carrier to the
trailing arm.  To install the shims (for camber), loosen the bottom two bolts 
about 4-5 turns and back the top bolts completely out of the trailing arm.  I
let the bolts stay in the bearing carrier.  You can access the bolts through the
provided two holes in the drum mounting plate.

The shims will be placed between the trailing arm and the bearing carrier, so
you'll be looking over the top of brake drum backing plate.  You can't stick the
shims inside the brake setup.  If you did, you'd get the brake drum and the
shoes out of alignment.  This is why you should be able to shim with a rear disc
conversion as well, you just shim between the adapter and the trailing arm.
This keeps things in alignment.

With one hand holding on to the bolt that was about the receive the washer, my
head peering over the top of the brake backing plate, my other hand slowly
positioned the washer in place.  The area is too tight to hold the washer with
pliers, so I just held the washer against the backing plate with a finger and
lowered it into place.  When it was in the right spot, I pushed the bolt
through and threaded it back into the trailing arm.  Repeat with the other bolt,
and then on the other side of the car and that's it.  It takes me about 20
minutes to jack up the car, put in the shims and put the car back on the ground.

After I'd shim, then I'd measure the change in camber using the digital camber
gauge.  If the camber wasn't to my liking, I'd jack the car up again and reshim.
This work was cake.

The harder part of the job was doing the toe.  I wanted my toe set at 0.00" toe
in.  I shouldn't say this was the hardest part.  Setting up the alignment rack 
(twopieces of angle iron on on brackets to get them to the height of the
centerline of the wheel with fishing line strung front to rear) was the most 
time consuming part of the process.  It was also difficult to find the proper
shim for the job, since the change had to be very small, versus the big change
to the camber.  I ended up not using a washer at all, but a piece of aluminum.

The toe shims were fabricated (I use this word loosely) from a sheet of roof
flashing purchased at Sears Hardware for about $10.  The sheet was 10' x 1' and
was 0.0100 thick.  I idea was to drill holes in the sheeting cut it into small
sqaures.  I wasn't happy that I had to use aluminum in a stressed suspension
component, but I checked the shims out after the first race with them in and
they appeared in fine shape.  I'll be checking them again in the near future.

I repeated the same technique for the camber shim installation, but this time, I
was installing the shims in the front bolts, not the top (to reduce toe in).  Toe
change is much more fine tuned, as I didn't need the big 2 degree change.  I was
only looking for a change of less than 1/16th of an inch as measured at the lip
of the wheel.

Once I was satisfied that my toe was were it needed to be, I put everything back
together and went racing.  As noted in my previous post, I estimated my car to be
about 2 seconds quicker per 40 second run.  This entire gain came from my ability
to get the power down to the inside tire in a corner.  Now, this change was done
in late April, and I still haven't been up against my normal competition to
confirm the actual gains, but I'm hoping the estimation is about right.  I'll 
keep everyone posted on the actual results.

As a final note on toe shimming, I have discovered that my local Lowe's home
improvement warehouse (equivalent to Builder's Square, Home Depot, Furrow, Hugh M.
Woods, etc.), carries aluminum plating in various thicknesses.  Taking a mic or
caliper in the store would be a convenient way to get the proper thickness you
need.  Just keep an eye out for resources.

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Post Date:    10 Sept 2000
Subject:      Re: XR Rear alignment
List:         merkurowners@merkurowners.org


Don and others,

First of all, to understand this email, you'll probably need a general working 
knowledge of how the rear suspension is setup.  You can gain this knowledge by 
visiting the only authorized version of Allan Slocum's Merkur Encyclopedia at:

http://www.company23.com/aslocum/

Click "Suspension" in the second from top frame, and then "Rear Alignment 
Proceedure" in the left frame.

On with content.  Don wrote:

> I added ... shims to the top bolts to 
> get the camber correct ... My problem 
> is that if I add more shims [to set 
> proper toe], I will have to add more 
> shims to the top front bolt which will 
> increase the camber that I just fixed.

Yes, this is a predicament.  If the bolts across from one another were level, 
then this wouldn't be a problem, since the front bolts would affect only toe, 
and the top bolts would affect only camber.  However, the trailing arm, to which 
the bearing carrier is attached, follows an arc as the suspension moves up and 
down.  This means that the position of the bolts, relative to one another, 
changes (call it a curve).  Sometimes one bolt is a lot higher than another, 
sometime it isn't as high.

Take a perfect world instance; the bearing carrier is level.  You shim the camber,
and then you shim the toe.  Because everything is level, one doesn't affect the 
other.  As soon as the bearing carrier moves up or down in it's curve, the camber 
and toe changes along their own separate curves.  Confused yet?

Let's simplify a bit.  Say your bearing carrier bolts are level with respect to 
one another.  Magically, you own an XR with zero camber and zero toe.  Now you want 
to increase your camber (tilt the top of the wheel inward).  I'll go a little wild,
for the sake of clear visual images.  For some reason you want (and can somehow 
achieve) 45 of negative camber.  Your wheel is now leaning inward at 45.  However, 
say you put your hand under the trailing arm and pushed upward.  In fact, you pushed 
it upward until it had amazingly rotated 90 degrees.  Now the bearing carrier is 
straight up and down, instead of the bottom being level (for the sake of 
simplicity, I'm ignoring the fact that the trailing arm is actually a semi-trailing 
arm and would pivot somewhat inward).  Guess what?  Your 45 of camber is now 45 
of toe in and no camber at all.

This is called your camber curve.  Your toe in has a curve too, as, for example, 
5 of toe in would turn to 5 of positive camber if you rotated the trailing 
from horizontal to vertical.

While this is a little exaggerated, you get the idea.  A camber setting has affect 
on toe as the trailing arm moves through it's arc, and likewise with the toe.

So there are three different curves involved (actually four, if you consider the
semi-trailing, rather than trailing arm complexity); curve one is "the path the 
trailing arm takes", curve two is "a function of camber and the curve the trailing 
arm takes", and curve three is "the function of toe and the curve the trailing arm
takes".

Yes, I am prattling on about something you didn't really ask.  However, what this 
boils down to is this...  Changing toe after setting camber will affect the 
camber.  However, jouncing the car up and down, or driving over bumps after 
setting the camber will affect camber too.  It's all about the curves.

You said you used a shim that was 0.044" for the camber change.  I used a shim 
that was closer to 0.0795" for my camber (it was at almost -3, and I wanted 
-0.75), and a shim that was 0.0200" for the toe (I really needed one at about 
0.0150").  I imagine your toe shim will be equally as small as mine.  Hmmm, 
maybe I should state that another way.

Here is what I did.  I set the camber, then set the toe.  I checked the change 
in camber and found it acceptably small.  If you are concern, and if you have 
the patience, and small enough shims (what are you using for toe shims anyway???),
you can run through the process of setting camber and setting toe and 
repeating the process until you feel as though you've gotten the effect of one on
the other significantly small enough.

Conclusion.  There is nothing you can do about the effect of camber on toe.  You
can minimize the effect by repeating the process more and more accurately.

Don't compensate on one corner.  Remember, the shim width on the two outside 
corners needs to add up to the shim width on the middle corner.

I think I covered everything and I hope I was clear enough to be somewhat 
understood.  If you need any questions answered, just contact me.  I've fooled 
around with it (mentally and physically) enough to have a good idea of what is 
going on.


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Published by Chris Anglin.