I have always found that measuring Toe-in was a difficult and inaccurate
business but I
believe that I have now found some improved techniques which may be of interest
to others.
Several problems need to be overcome to make an accurate measurement:
1. There should be no play in the wheel bearings, king-pins etc and the wheel
spinners must be tight.
2. The wheels must be pointing straight ahead otherwise the Ackerman angles
will generate a false impression of toe-out.
3. Any run-out of the wheels must be removed from the measurement.
4. A suitable measuring gauge needs to available.
Checking the first problem is easy and rectification is outwith the scope of
this letter.
The second requires considerable care. I believe the best answer is to drive the
car along
a straight road having little camber and accurately note the position of the
steering wheel.
Other descriptions of toe-in measurement eliminate problem 3. by moving
the
car backwards or forwards so that measurements in front and behind the wheels
can use
the same point on the circumference of the tyres, but I found that the iterative
process
of adjusting and measuring required a lot of movements of the car with the risk
of displacing
the steering from the straight-ahead position.
My technique for eliminating run-out was to hold a biro pen or other marker
against the
tyre tread and to spin each front wheel until a mark extends around the full
circumference.
(Whilst it is necessary to jack up the front for marking, the car should be back
on the ground
for the measurement)
The final problem can be dealt with by using a measuring tape and an assistant,
but if working
alone then an adjustable roof bar makes an excellent gauge.
With steering wheel straight-ahead place the roof bar to the rear of the front
wheels and
adjust its outer support brackets to correspond with the lines drawn on the tyre
treads.
Now carefully withdraw the roof bar and position it in front of the wheels
comparing the
same points on the brackets with the lines at the front of the tyres and check
that the
roof bar brackets are that magic eighth of an inch wider than the distance
between the
lines on the tyres.
On SS and "Mark IV" cars the track rod ends are left and right threaded so
adjustment
is easily achieved by slackening the clamp bolts and rotating the track rod. I
haven't looked
at the thread pitch of the track-rod ends but a half a turn represents a fairly
gross change
in toe-in.
Whilst all this may seem rather trivial and obvious, the issues of
single-handed gauging with
a floppy tape measure, and eliminating run-out, have caused me much frustration
in the
past, so I hope this letter may be helpful to others.