I received a few questions about checking bumpsteer, and rather than reply to
each, I'll throw it on the list and you can hit delete any time you've seen
enough...
<OPINION mode ON>
M cars do not seem as bad as the earlier cars, but almost all cars will really
improve on the "kickback" and nervous darting around on an uneven road if you
correct the bumpsteer. I have seen cars that need the steering arms lowered at
the front (or racks raised), but the majority of cars need the steering arms
raised at the front (or rack lowered). The correct way to measure the bump
steer (toe in or toe out through the range of wheel movement as if it were
going over a bump...hence the name "bump steer") is to note the wheel position
at static ride height, jack up the car, remove the bottom bolt on the shock so
it is not attached to the a arm, reset the wheel to ride height, mount 2 dial
indicators against the tire set at 1/2 of their travel, raise and lower the
wheel as if going over a bump, then let it drop down as if going into a hole.
When you do this, note the difference in the values on the dial indicators to
tell you if you are toeing in or out. If the value of the front indicator is
increasing and the rear one is decreasing, you have toe out eg. if the front
one started at 2" and went to 3" (and if the opposite occurs, toe out). Value
differences should be in the "Thousands of an inch" range. Toe out under bump
means you have to raise the front of the steering arms or lower the rack. Toe
in under bump means you have to lower the front of the steering arms or raise
the rack.
Good Luck...
Now for a "real world" backyard sort of approach...
If the bump steer is really bad (and many are), open the hood and jump up and
down on the frame. Watch the wheel to see if it toes in or out when you push
down on the frame (it is more important which way it goes under bump rather
than droop as that wheel will do more steering when heavily loaded). Follow
the direction of the fix as above. If your car looks pretty stable with very
little movement of the tire in the above scenario, you're ready for a little
finer measurements. You'll need 2 pieces of 1/4" plywood about 18" square and
a piano hinge. Mount the 2 pieces to the hinge so that the hinge is inside of
the 2 plywood pieces when they form a 90 degree angle. Now, as above, note the
static ride height, disconnect the shock and with a jack under the frame, put
the car back at normal ride height. Put one side of the plywood against the
tire and put something heavy on the other piece on the ground. Square the
standing piece up against the tire. Now lower and raise the car to simulate
bump and droop. You can measure the toe in or out against the standing plywood
piece with a machinists ruler (bigger gap at the front is toe in, bigger gap
at the rear is toe out). You are never going to get it perfect, but closer to
zero change under bump, the better. Remember that a little bit of toe in
under bump is better than any toe out.
After you have heated and bent the arms a few times. remember to get the tie
rod end mount parallel to the ground again.
I have seen cars with no apparent damage to any suspension pieces with as much
as 2" of toe out under normal bump ! Those cars felt like a whole new car
afterwards. My favorite comment was "Hey, I can even drive one handed now"
Good luck, Have fun, YMMV...
Nick in Nor Cal
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