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)

[Superhawk996]

When the Lotus 72 and the Jag were using inboard brakes, there have been deaths due to failure of the driveshaft that results in no braking on one side and resulting instability.

https://en.m.wikipedia.org/wiki/Jochen_Rind...rash%20barriers.

See death and legacy section.

In the case of the Lotus 72 it was a front brake failure which would be far worse. I know we are taking about rear brakes here (and for Jag) but the same failure mode applies, just less severe. No thanks for me - especially on a street car. Though there are no rules in F1 banning inboard brakes they are not seen for other reasons I’ll touch on.

From an engineering perspective this (complete loss of brake function) is a very bad failure mode that would never fly in a modern court of law.

Besides that cooling inboard brakes is way more difficult due to underbody aerodynamics and hot engine air, exhaust heat wash that is much hotter than what the brakes get when in the wheels. Poor cooling means the need more bigger pads and thicker rotors. Weight begets more weight as the saying goes.

As you noted the need for robust mounting drives other requirements into the differential or transaxle they will be mounted to. Brakes are capable of absorbing 3x to 4x engine horsepower. That is a lot of torque and stress on whatever they are mounted to. Especially if that something is a magnesium 901 transaxle instead of a cast iron differential. More robust differential / transaxle mounting = more weight.

As you noted - maintenance is not fun at all.

It’s also with noting that when the car is on the ground, the brake reaction force into the trailing arm is downward at the pivot when the vehicle is traveling forward. This force under braking serves to help keep the rear of the vehicle from rising during braking. It’s a crude anti-lift.

Obviously the rear of the chassis will still lift with hard braking and lots of forward weight transfer under braking but it will be worse using a brake torque arm.

The way the wheel moves under braking when on jack stands or a hoist is irrelevant to how it behaves when the vehicle is on a ground reference plane with the body being a sprung mass instead of a rigid body.

To do the brake torque arm, the caliper carrier has to float relative to the hub. More complexity, more weight.

Food for thought.

@technicalninja

[edit] Forgot the F1 brakes thing. The biggest advantage to inboard brakes used to be the elimination of 12 or more lbs of unsprung weight from the wheel end. Carbon brakes now achieve that sort of weight reduction while leaving the rotors in a better location for cooling and service. On top of that carbon fiber wheels are now available (not F1) but they are out there. Neither of these technologies were available back in the day when reducing unsprung mass was the main case for going to onboard brakes.