Hi All,
I'm building a sim, and I have what should be a very basic mechanics
question, but it's still confusing me! :)
It's all about pitching torques. So, this part I do understand:
Under braking, a rearward longitudinal force is developed in the tyre CP,
and this force is passed to the sprung mass through the suspension linkage
(let's have zero anti-dive/lift here for simplicity). Because the centre of
mass is above the CP, this results in a pitching torque on the sprung mass
that causes a longitudinal load transfer, and makes the sprung mass 'dive'
forward.
That's all fine. My question is what happens to the reaction torque from the
brake itself? As far as I can see, when braking, the caliper applies a
torque to the disc to slow the wheel, and according to Mr Newton there
should be an equal but opposite reaction torque acting on the caliper, and
so from there through the suspension linkage (assuming outboard brakes) onto
the sprung mass (and the being a torque, this can simply be translated to an
identical torque about the centre of mass). This torque should also be a
pitching torque, giving longitudinal load transfer and making the sprung
mass dive down at the front, in addition to the CP-force-induced load
transfer.
Now, my problem is that if I apply this reaction torque in my sim, most of
my cars will easily lift their rear wheels right off the ground under
braking (ie there is an unrealistic and excessive longitudinal load
transfer). Also, in all the reference books I have (RCVD and Gillespie for
example) wheel loads are calculated under braking assuming that the only
longitudinal load transfer comes from the longitudinal tyre force, with no
mention of brake reaction torque at all. So my question really is, where has
my analysis of brake reaction torque gone wrong? Why shouldn't I be applying
a reaction torque from the brake?
Any help appreciated,
Cheers,
Steve.
I seem to remember something about the brake caliper being placed in front of
the axle center line or behind it making a difference with the effect you're
speaking of. The rotational force would having a lifting effect one way and a
diving effect the other. I seem to remember it being why they changed the
location of the calipers on motorcycle's front forks.
Bill Berry
