4 replies to this topic

[red300zx99]

Ha, I must love to argue or somethin. Now the subject is on motion ratio's, always been taught, disregarding angles at the moment

a = dist from lower arm axis to ball joint
b = dist from lower arm axis to spring mount

MR = b/a

While reading over Milliken on the subject it becomes pivot point to wheel, not ball joint, which makes sense as the torque is reacted at the wheel and we are tryin to find wheel rate, not ball joint rate, that thing is just along for the ride. But then I figure that the wheel doesn't actuallt rotate around the lower arm axis, but the IC.

Then I come across Herb Adams in Chassis Engineering with his equation of

a = dist from lower arm axis to spring mount
b = dist from lower arm axis to ball joint
c = dist from IC to ball joint
d = dist from IC to wheel center

MR = (a/b)^2 * (c/d)^2

Sounds about right, but something still doesn't seem to be clickin on all cylinders, anyone have any insight? I've got some MR numbers from Bill Mitchell's program for our car, but Mr. Mitchell is tight lipped at the moment over the subject. But I have seen many a program use the first equation to figure out motion ratio's, and that seems scary.

[RDV]

Try doing a force moment graphic diagram and all will be clear.

There is a huge amount of confusion over motion ratio and velocity ratio, have seen many a MR being mentioned as VR and the concept has become so fuzzy in my mind that I always have to think twice about it... sort of a mental stutter.

In the end on ranges used very similar to sin and tan , values will be approximately correct, so you can get away with murder...

[RDV]

... and when balljoint is on a narrow rimmed production car its as near as dammit the same as contact patch.... even on simulation the use of a stick, disc or cylinder model produces huge differences in MR values depending on camber change curves, etc. will dissert on this later but work calls.

[Greg Locock]

We don't really use a single figure, as I get a plot of shock displacement/wheel displacement directly, but as a way of talking about things I'd use the a/b formula to get the mind working on the right lines. The IC argument is a furphy, I think. What you are really interested in is force (displacement, velocity) at the CP as a function of force (displacement, velocity) in the shock. Um, all your calculations seem to be based on a vertical shock, we rarely have that luxury.

[red300zx99]

Not really worried about the shocks at the moment, more worried about steady state at the apex and figuring out instantaneous motion ratios to calculate roll stiffness and the like. Personally I'm waiting for RDV's dissertation, as we use relatively short uppers that cause the IC length change relatively fast, and from this is figure the real motion ratio is changing relativly fast too. But ok im out now, first race of the season, and this is my first race as a crew chief, so wish me luck