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single rear brake disk question
kb58 - 9/11/05 at 04:05 PM

This is for those using a Quaife-style LSD, driving the LSD with a chain, to a sprocket where the ring gear would normally be.

I've seen setups where a single brake disk is used, on the input side of the differential. My question is, how well does it work? I ask because I'm unsure how well the LSD backfeeds torque, does the single disk in fact work perfectly? Is there an issue with brake torque transfering back and forth between the left and right tires under braking, unsettling the car and causing it to weave back and forth?

mark chandler - 9/11/05 at 06:38 PM

Sounds like a handbrake mechanism to me, ala landrover.

This is an easy way to save sprung weight by putting the brake on the prop, and it will pass MOT's. Lots of choice on calipers then.... food for thought if you have a BEC with reverse gearbox especially an electric one with a spocket, make something that meshes once parked.

chrisf - 9/11/05 at 06:47 PM

You've logged on there before, but I bet the FSAE boys on their forum would be great at answering this. I often wondered the same thing...

Let us know what you find.

--Chris

britishtrident - 9/11/05 at 08:50 PM

Called a transmission brake --- don't even go there. they were used on many cars until about 1930 but fell out of use for good reasons. Having owned and driven cars of the 1920s era I can testify transmission brakes can be nasty things, difficult to control.

kb58 - 9/11/05 at 08:52 PM

Understood, but they didn't have Quaife type differentials then, so I can see how an open diff would be big trouble. I'm just wondering how well Quaifes distribute braking torque.

Bob C - 9/11/05 at 09:25 PM

tis what I done for exactly the reasons mark describes - Some time next year I'll be able to report what sva thought of it......
Bob C

Chris Clarke - 10/11/05 at 08:52 AM

quote:

You've logged on there before, but I bet the FSAE boys on their forum would be great at answering this. I often wondered the same thing...


There are quite a few discussions on the FSAE board regarding the single inboard brake, run a search and you should find all you need.


There are a lot of teams that run with a single inboard brake, of which, probably 95% run a Torsen diff.

Having driven our FSAE car with our single, inboard brake on a Torsen diff, and watching many other cars run, I can say that I have never seen anything dramatic occur due to this.

For straight line braking, it doesnt act any differently than dual outboards. Even with braking into corner entry, it isnt really noticeable. (transients is where most of the arguements for/against come up)

Problems can arise with this set-up though. If you lift the inside rear wheel, you will lose all braking on the rear, if you watch, the inner wheel can turn backwards in the air also.

Also, all teams that have reported driveshaft failures (about 5) with this set-up have said that it occured in the braking direction. The best cars only see about 100 hours of running time, and most only about 10-15, so they shouldnt be used as examples for the reliability of components.

You should also note, that a lot of FSAE teams run it because "everyone else is", and at the competition last year, we had a braking design judge from Stoptech basically come out and tell us that they want to see dual outboard brakes on the cars.

Here is a summary of what he said.


quote:

Judge: Well.. your car isnt lightweight so thats not an argument, your unsprung mass is large so that you shouldnt care about that either with this setup. All you should care about is unbalanced breaking which you will see significant amounts of. Even if you had a lightweight car with low unsprung mass, you dont have any aerodynamics to insure that the rear wheels will always be on the ground. Even if you did have aerodynamics, this competition doesnt see enough speed that the aero will make a good argument. Ill tell you this right now... were looking for outboard brakes, thats what we want to see for this competition.


Another point is that generally we dont see very poor or changing surface conditions, which I would think would lead to more of the "unbalanced braking".

wilsongt - 10/11/05 at 01:11 PM

Hi there

Can someone please educate me as to why you would lose all rear braking with this setup, should a rear wheel lift? Loss of braking on one wheel I can understand...

I would have thought that the torsen diff was meant to handle such a scenario, or does it only handle torque distribution (whether from the brake or the engine) up until the moment one of the wheels leaves the ground?

Andy advice gratefully received. Having followed the dpcars.net saga, very interested in this way of reducing components

Regards,
Glenn

chrisf - 10/11/05 at 02:20 PM

When a Torsen lifts a wheel, it acts as an open diff. At least that's my understanding.

--Chris

The Shootist - 10/11/05 at 07:02 PM

That if you put the brake on the input side of the diff that the braking would be just as stable as the accelleration would be. On a conventional front engine/ rear diff car this would mean that the gear advantage would allow a very small disk to be used, but the high speed would require carefull thought as to pad material.

Chris Clarke - 10/11/05 at 07:27 PM

quote:

Can someone please educate me as to why you would lose all rear braking with this setup, should a rear wheel lift? Loss of braking on one wheel I can understand...


The Torsen is a torque-biasing differential, which means that the amount of torque that it is able to transfer to the wheel with traction is dependant on the amount of torque it can transfer to the wheel with less traction. Torsens come from the factory with a torque bias ratio (TBR) of about 2.6. What this means is that the wheel with more traction can apply about 2.6 times the amount of torque that the wheel with less traction.

ie. If one wheel can transmit 100ftlbs, the other can transmit 260ftlbs.

So if one wheel is lifted, it is able to transmit 0 ftlbs, then the other wheel can transmit 0 x 2.6 which equals 0 ft lbs. So it does act like an open diff. The same thing would occur if one wheel were on ice.

The newer torsens have a pre-load built in via some springs and clutches for this very situation, but I think they are harder to find, have a lower TBR, and lower strength.


quote:

On a conventional front engine/ rear diff car this would mean that the gear advantage would allow a very small disk to be used, but the high speed would require carefull thought as to pad material.


This would be used on a mid-engined car, with a chain driven diff, so the brakes would be after the final gear reduction, and at the same speed as if they were outboard.

tks - 10/11/05 at 09:50 PM

and don't think wiht using our sierra lsd, because its even VISCUS....

that means if you lock the input shaft you will loose all the LSD power....

i would mount bike examples on the outer side of the diff like Erwe on this forum.

Its lighter and it gives less sprung weight.

There where some early alfa's that also did this technique.... inluding placing the gearbox in the back for better weight distribution..

Tks

wilsongt - 11/11/05 at 09:26 AM

Many thanks Chris, that makes it very clear. Get all you braking over with in a straight line, as Stirling Moss would say

Have you seen the build at www.dpcars.net? The builder has recently indicated some reservations with his braking design. I think it will still be OK - I just don't see many situations on the track where you would have a whell dangling in the air at teh same time you are braking. Cornering/accelerating yes, braking no.

Disagree?

Regards,
Glenn