Rear brake torque arm? Options

[technicalninja]

I have not seen mention of one of the 914s rear brake engineering issues yet.

This regards locating a brake caliper on a swing arm.

During braking the forces acting on the swing arm try to move it UP away from the ground.

This is more pronounced with a dual rear swing arm design like the GMC class A motorhome from the 70s (I have one).

In the GMCs case the rear control arm is similar to a 914 but the front arm faces forward.

In a full-on panic stop the front arms torque DOWNWARD and the rear arms torque UPWARDS. The forward wheels try to lift the coach up and the rear arms lift the back wheels off the ground. This equals stupid bad rear brake action.

This vid shows what happens in the case of the GMC but it doesn't mention the front arms forcing the coach up which is the main issue with these.

https://www.gmcrvparts.com/product-p/apk5-.htm

This is also an issue for motorcycles. Here's a too long vid on fitting a rear torque arm to a Hayabusa.

https://www.youtube.com/watch?v=hTC-Zk5k6NY

The gist of the mod is to mount your caliper on a rotating plate and then secure the plate to the chassis with a brake torque bar. The stopping force is applied to the chassis, not the swing arm.

Don't believe me?

Put you 914 in the air and spin the rear wheels up to 30mph, have a helper nail the brakes and watch what happens to the rear suspension. It will JUMP UP bigtime!

I've seen these on mountain bikes (human powered).

I'm toying with an inboard caliper mounted on the transmission to alleviate this issue.

Anyone else have ideas, comments.


(IMG:style_emoticons/default/stirthepot.gif) (IMG:style_emoticons/default/stirthepot.gif) (IMG:style_emoticons/default/stirthepot.gif)

[fixer34]

From a purely scientific/physics standpoint, it appears your comments have merit.
But in the real world...

Unless you are racing, how often does the average driver do a 'full-on panic stop', or even lean on the brakes hard enough to bring this into play?

Your example also didn't mention if the trailing arm has been 'loaded' with the weight of the car like it would be normally when the tires are on the ground.

Lastly, yes we see race cars with calipers/rotors mounted next to the transmission. Is this to address the issues you mention or reduce unsprung weight? With the rotor mounted directly to the hub/wheel, all of the braking force is transmitted to the tire(s). Having the rotors/calipers mounted inboard means the braking force has to be transmitted thru both CV joints (in the opposite direction of driving force) and the axle before getting to the tire.

Go for it if you wish, but it sounds a bit like a solution in search of a problem.