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)

[technicalninja]

Designing some inboard brakes would be the best solution. I think the pushrod for floating calipers would be hard to locate with wide wheels and all the other stuff that's under there. You also get an unsprung weight reduction that would likely have a real world benefit too.

Yep!
The swingarms are at angles too.

Building reaction bars would be an exercise in futility IMO.

Only thing I'd do is inboard mounted stuff.

Now I'm putting "weird ass" torque on the tranny housing, mounts. and motor mounts.

That will create other issues...

[Shivers]

Designing some inboard brakes would be the best solution. I think the pushrod for floating calipers would be hard to locate with wide wheels and all the other stuff that's under there. You also get an unsprung weight reduction that would likely have a real world benefit too.

Yep!
The swingarms are at angles too.

Building reaction bars would be an exercise in futility IMO.

Only thing I'd do is inboard mounted stuff.

Now I'm putting "weird ass" torque on the tranny housing, mounts. and motor mounts.

That will create other issues...

Those Big cars you speak of, most seem to have at least some bracing in the rear trunk. could you drop down from there through the trunk floor with your caliper mount and put the stresses into the cage/chassie?