I've been thinking about the Torque Arm Impact to breaking. I am still
planning to order and use a Torque arm but wanted to think out the issue and
the physics of what was happening. I drew out a simple force diagram to see
if I can understand what's happening under braking vs. under acceleration.
The torque Arm design is to STOP rotation of the rear axle under acceleration
that leads to wheel hope from spring wind up. In my force diagram I assumed
springs as ridged for transfer of force as spring wind up/release isn't a
common braking problem.
Envision a static diagram of a wheel, leaf spring, leaf spring mounting
points, and the torque arm mounting points it might provide logic to what is
taking place.
- Assuming fwd motion as a wheel rotating counter clockwise.
- Brake forces result in an opposite clockwise force
o This leads to a Clockwise moment of force applied around the axel
o This leads to forces on the axel mount points to the body
* Rear Spring point force is down
* Front spring point force is UP
* Torque arm mount point force is up
Maybe because the force is UP during braking and the Torque arm is Longer than
the springs it has a great mechanical advantage in acting to LIFT the car and
transferring the center of gravity further forward than it would be without
the torque arm? The act of braking is adding energy to the system so it's
not a balanced equation and its dynamic with additional loadings beyond what I
have thought out by making it a simple static system.
- A simple assumption might be that the arm takes up whatever forces are
normally applied due to spring wind up. But likely it also takes some of the
leaf spring loads as well and then applies it more forward in the car body.
- Reports that Torque arms decrease braking performance indicate some
kind of lift of the rear or that the center of gravity is shifted forward
under braking because of the upward force causing the rear of the car to
unweight?
Possible improvements for braking while maintaining acceleration performance
would be to allow for only Downward force application of the torque arm at the
spring point.
- One way Hinged arm that goes open not allowing an upward force to be
applied?
o Drawback is that a one way hinge would likely be a weak point with
frequent failure
- Oval shaped dampener (rubber bushing that was shown at the front
transmission mount) that allows for upward travel of the torque arm under
braking but preloaded to always make contact to the bottom for acceleration?
o Drawback on availability of an oval dampener, also likely to be noisy
rattling from top to bottom under transitions from stopping to going.
- Mounting point? Further rearward mount of the torque arm link?
Closer to spring perch?
o Drawback negative impact to suspension travel/ride quality
- Stiffer springs? Less wind up and less load under braking?
o Drawback negative ride impact.
What's interesting is the opposite force diagram for acceleration shows how
the torque arm shifts weight to the rear of the car allowing for more force on
the rear wheels and better traction at launch. What are others thoughts?
It's been a long time since I've had static and dynamic classes and I've never
had to use them for work.
dan
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