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.

[T.Rick6]

I also have a classic 77 class A GMC , I’ve studied this for quite a while, ended up with a hydro boost brake system, works awesome. If I had an extra 2k I might try this. (IMG:style_emoticons/default/beerchug.gif) Todd

[73-914]

Inboard brakes work well on Jags + HMMWVs (IMG:style_emoticons/default/agree.gif)

[technicalninja]

I'm in agreement regarding the hydroboost on the GMC (or a BAD ASSED vacuum pump).

They are unstoppable if the engine dies, and you lose your vacuum source.

I'm also "in" on the changing the front hub assemblies out to 1 ton GMC 4wd truck stuff.

I wouldn't add the rear brake bars without also adding rear disc to it.

Engine will be a LSX block with twin turbos.

Tranny is the weak point with the GMC IMO. I haven't found something strong enough for longitudinal mount front wheel drive.
Maybe an Allison out of a Diesel pusher would work.
I'm betting it's too long.

My GMC is a little bit cooler than others.

It has "Team McLaren", the correct Kiwi Birds, in the correct colors painted nicely on the sides.
I don't have its history however...
It may be a fake!



Putting inboard brakes on a sports car has multiple improvements and multiple drawbacks.

98% of the 914s out there will NOT need this...

I'm going to build a 2% car.

Clay's big car...

Maltese falcons 1100 hp car...

Tygaboy's stuff...

These are the cars that might benefit with something like this.

[gnomefabtech]

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.

[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?

[Superhawk996]

Inboard brakes are suicide devices.

Investigate why they aren’t used on other than a few vintage / specialty vehicles. The answer is not cost. See F1.

[technicalninja]

Inboard brakes are suicide devices.

Investigate why they aren’t used on other than a few vintage / specialty vehicles. The answer is not cost. See F1.

@Superhawk996 please expand on "suicide devices".

I REALLY would like your insight!

I've thought of a "deal breaker" already.

The output flanges on the tranny are NOT designed for side loading.

The output stubs on a Jag rear end (with inboard disc) are more like wheel bearings than normal differential output flanges. They have a pair of bearings on each side.

Run-out is going to be a bitch and I believe the current bolts are going to loosen up regardless of anything I do.

The there is the HEAT that is going to be applied to the output flanges.
That might prove problematic...

What I would like to know is what OTHER "gotchas" you can come up with.

I'm NOT trying to argue here!

I want to "Vulcan Mind Meld" you...

In regard to the Jag trash. If you've EVER had to do rear brakes OR an alignment on a Jag rear set up you will learn to HATE the Jag shit.

I have...

(IMG:style_emoticons/default/ninja.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.

[arbitrary]

While researching this topic I came across https://www.eng-tips.com/viewthread.cfm?qid=291696