07-25-2011, 10:44 PM
Join Date: Feb 2010
Total Cats: 3
Bump Steer Measurements...
I found this on the Spec Miata site…has anyone measured their own cars? I think the figures below are for an NA Miata 1.6 with the normal tierod ends (not the taller ones, but I can check what the SM suspension uses: maybe it does use the taller ends) and as noted a 12 3/4 ride height. You can see that the toe change per unit travel increases with lower heights.
<<<<I couldn't find these numbers posted anywhere on the forum. So FYI: These are numbers from a 1990 Miata with the Mazdaspeed#: k-spec-m5-susp. The car was aligned as close to the recommended settings as it could be without any "tweaking". The base ride height was 12 3/4" as measured hub centerline to fender lip. 1 3/4 degrees neg camber. 3 degrees pos caster.
----------camber-----bumpsteer---rod travel to BS
1.25 bump**-3.0******.045" out*****Firm on stop
1.00 bump**-3.0******.039" out*****.170"*******
1.00 droop**-1.0******.048 in
2.00 droop**-.25******.105 in
This might help you figure out your set up. As you can see, the front end toes-out on bump moderately. (This is not as bad as toeing-in on bump.) For other circle track racers in the crowd, it might even be thought of as a good thing...kind of like Ackerman steering. But, most racers prefer to adjust the bumpsteer to zero and then add toe-out to the static alignment if they want better turn in.
However, just knowing that the front end toes-out when loaded makes me reconsider adding any more static toe-out to the car's alignment.
The rear was fine. It varied much less. From .004 toe out @ 1" bump to .002"in at 1" droop.
Rear camber went from -2.0 degrees at ride height to -2.75 degrees @ 1" bump.
It went from -2.0 @ ride height to -1.4 degrees @ 1" droop and then to -.8 degrees @ 2" droop.<<<<