DIY Alignments
Date: Fri, 10 Jul 1998 19:13:18 -0700
From: Randy Chase (randyc2@home.com)
Subject: MR2 (GEN) DIY alignments
Regarding Davids recent post about doing your own alignments, this was
posted on Team.Net by ex-MR2 autocrosser Vince Bly (he went to the dark
side and bought a Miata) Some of it may be more Miata specific, but you
get the general idea.
>> Reading Brett Howell's account of his experiences with
N?? (ex NTW) reminded me of similar experiences I have had
with several auto shops, including NTW. I will spare the
bandwidth and not recount mine, other than to say that they
convinced me to do my own alignment. One spec. that can be
very critical to handling is toe (both front and rear). It
is also one that is prone to errors. After trying about
half a dozen techniques, I've settled on string! Although
this sounds crude, it's actually very precise and the
"equipment" is cheap. I've included a brief description of
the method below.
Initially, I'll describe the method assuming that the
front and rear track are the same. (This info should be in
your manual.) Park the car on level ground, put a loop of
nylon twine around all four tires at hub level. Nylon works
best because it will stretch--you want a little tension.
Adjust the loop so it is level and right at hub level across
each tire. Next adjust the steering wheel so the gap between
the twine and tire section is the same on both front tires.
Assume you have some toe-in at present. If this is so, the
twine will rest on the rear section of each front tire.
Measure the gap between the twine and the front tire section
on each side (if you have adjusted the steering wheel
correctly, it will be the same on each side). Add the two
measured gaps and divide the result by 0.80. This adjustment
is necessary since the measurement was made at the tire
section and the spec. refers to the toe at the tread. You now
have a reasonably accurate measurement of the front toe. More
importantly, you will find that, with care, the measurement is
very repeatable. Of course, you can also use it to measure
rear toe--just make sure you have the steering wheel exactly
straight ahead (measured front toe is exactly the same on both
sides). If you don't, and use this method while adjusting
rear toe, you can introduce setback (a side-to-side difference
in wheelbase).
There are three significant sources of error: errors due
to raised lettering on the tire section, the measurement of
the gap itself, and error in the adjustment factor to convert
from the section diameter to the tread diameter. On R type
tires, the raised letters are approximately 0.020". With care
and common sense, you should be able to compensate for this,
at least within 50%. This leaves a residual error of 0.010"
per side or less. If you measure the gap with a good steel
rule and take parallax into account (look it up, if you need
to), you should be able to measure the gap to within about
0.010" to 0.015". The worst case sum of these is 0.025" per
side which can be reduced by multiple measurements. (You need
to roll the car forward or backward between measurements to
change the effect of raised lettering). I have found that,
using the average of three measurements, I can come back
several days later and repeat the previous values to within
0.010" per side! I haven't worried too much about the
conversion to tread diameter (for my 50 series tires, I use
1/0.80), since I'm more interested in repeatable measurements
than absolute accuracy.
I realize that this is not a technique for those in a
hurry. What it is is a very precise measurement made by
someone you trust (you).
If your front and rear track are not the same, you'll
need small blocks, each with a thickness half of the
difference in track. For the Miata, these are 0.320" each
for the front. Just make sure that the blocks are actually
creating the gap they're supposed to (measure it).
If you don't trust someone else and are not comfortable
with you present method, try this--it doesn't cost much.
Vince Bly //Spiffy Wabba Racing //
Randy Chase
'91 MR2