just wondering how many teams have/are running outboard rear brakes? why is in board better than out (other than weight) or vice versa. how about front brakes? just want to know what you people think.

JACK
www.etec.wwu.edu

Either inboard or outboard brakes work okay, but the arguments in my book for a single inboard brake is as follows.
1. Weight and packaging. It is a fairly simple matter to put a single disk on the opposite end of the differential carrier but you must use a locking or locked differential.
2. Inboard reacts the brake loads directly into the chassis, outboard requires the brake torque to be passed into the chassis structure via radius arms or the A arms.
3. Reduction in spng weight. When the rotor and caliper is moving with the wheel, that weight must be controlled and damped.

I hope this helps
Slan Leat

Sleeque

We run a 4 brake system because it provides even braking and the judges like it. We had a 3 brake inboard system before and they didn't like ti too much so we switched. I'd go with the 4 brake system over teh 2 front/ 1 rear set-up simply because the judges like it.

Brent

www.ucalgary.ca/fsae

We simply found that 4 brakes was a complete waste for our car. The rear brakes do so little that we found that the diff generated as much heat as the rotor. Putting something on the car because "the judges like it" is not the way to win. The way to win is to prove to the judges that you made the right decision.

Marshall McLean
University of Western Ontario

UWO FSAE Racing Team
Western Ontario Sports Car Association

quote:

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Originally posted by Brent Howard:
I'd go with the 4 brake system over teh 2 front/ 1 rear set-up simply because the judges like it.

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Wow. First, I seriously doubt that the judges would prefer outboard to inboard, unless you run an open diff? Were there any cars in the design finals with outboard rear brakes? I'd be suprised. Secondly, the judges sure don't like it if you do something without knowing why. We've resisted judges opinions before, and from my experience they are usually right. But you've got to understand why they are right or else you are just lost.

-Charlie Ping
Auburn University FSAE 1999-present

Charlie,

Brent never mentioned anything about outboard brakes. Its possible to have dual inborad brakes at the rear.

Regards,

Scott Wordley & Roan Lyddy Meaney
Monash FSAE Wingmen
http://www-personal.monash.edu.au/~fsae

Good point. You may very well be correct, however I would still infer from his post that he meant outboard.

quote:

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We had a 3 brake inboard system before and they didn't like ti too much so we switched.

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-Charlie Ping
Auburn University FSAE 1999-present

Knowbody said anything about inboard rear brakes stressing the driveshafts? Torque from outboard brakes would be reacted at the tire-road interface (ie is not reacted through driveshafts)

Aren't your driveshafts designed to be stressed?? If you have enough torque at your engine, you can theoretically produce more torsional stress in you driveshafts in acceleration than breaking (wheel loads). I see no real argument for outboard rear brakes in FSAE...

Aaron Johnston
University of Waterloo FSAE

www.eng.uwaterloo.ca/~fsae

I think the usual choice of a Zexel-type diff for FSAE is sound, but just as a point of discussion, a single brake works fine with an open diff. In fact, it works most like well-matched individual brakes when the diff is open. Or at least that's true as long as neither wheel locks. If one wheel locks, we can add more braking force on the other wheel with individual brakes, whereas with a single brake through an open diff, the rolling wheel is limited to the torque of the locked one.

This is largely academic, because if one wheel locks, we probably want to release the brakes anyway, to avoid flat-spotting the tire.

Frictionally connecting the two wheels is good in terms of avoiding flat-spotting. One penalty is that the diff locking force tries to resist attempts to turn the car. This tends to add understeer on turn-in if we are braking, which is probably undesirable for autocross, though it might actually improve trail-braking behavior on an oval or a road course.

In practice, single rear brakes acting through Zexels seem to work just fine.

Mark-

If we were running open diffs (obviously not), couldn't there be an issue when an inside wheel becomes very unloaded? Wouldn't the inside wheel be turning backwards at the same speed as the wheel that is moving forwards, while the rear brake is simply stopping the carrier?

Curious,

Travis Slagle
KU FSAE

As the suspension group leader for our new team here at Mercer, I've been doing a lot of research on the brake system. Looking at a book titled the "Brake Handbook", by Fred Puhn, and using a few numbers applied to some of his equations, it really seems that adding an extra brake to the rear would be useless. According to Puhn, about 2/3 of the braking is at the front, while only 1/3 of the braking is at the rear! That would seemingly make a three-rotor setup ideal to our needs!

I also ran some numbers through his equations for heat generation and found that the heat generation that we need to produce is far smaller than what 3 solid steel rotors of about 7 pounds each could handle. I haven't found a really good reason for why teams are cross-drilling their brakes... they're probably hurting performance by reducing their swept area. The additional cooling is not only not needed, but even if they did, it would be much better accomplished by using air ducts rather than weakening their rotors with holes. I haven't found any pictures from any formula car series that uses drilled discs.

If anybody else is in the know and has experience, which I'm not and I don't have, I'd appreciate their comments.


-Chris

Using inboard brakes can be just as effective as an outboard configuration if done properly. Even eliminating the need for a separate inboard rotor can be made, by utilising the rear sprocket itself as a braking surface.

quote:

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Originally posted by Matt Thio:
...utilising the rear sprocket itself as a braking surface.

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Matt Thio,
Have you done this before? I know that some teams have done it but I hear that it is a nightmare to keep the braking surface clear of chain lubricant. If you've done it could you share how you solved this problem?

James Waltman
waltmaj@cc.wwu.edu
http://dot.etec.wwu.edu/fsae/
Formula SAE
Vehicle Research Institute at
Western Washington University

wouldn't the heat from the rotor also cause chain stretching problems?

I saw someone mentioned the possibility of running the rear brake on the drive sprocket. We have actually done this for two years now. I think we may have been the first team to do it, please correct me if you know otherwise. I think Sydney Uni (Matts team) also did it this year. We've found it works fine. We actually built a test rig in the first year to check stopping power and temperatures etc. It all checked out fine. Saves heaps of weight as there is no rear rotor, less mounts etc. The things you have to think about are chain lubrication and keeping the heat away from your diff. See the photos below, the test rig rotor was actually glowing red hot in this shot.





Regards,

Scott Wordley & Roan Lyddy Meaney
Monash FSAE Wingmen
http://www-personal.monash.edu.au/~fsae

A few points

1) the loads on the axles when acelerating will always be higher when accelerating then braking. The weight on the rear tires once the weight transfers times the rolling radious times the cof will determine the max loads on the axle.

2) This all changes one more huge set of assumptions one you start turning in.

3) Torsen diffs act as an open dif once the inside unloadson turn-in but that may be a good thing.

4) a Torsen will behave as a poor mans ABS in the rear as long as there is load on all three peices of the equation the diff and two tires. If any of these three goes to zero load...all three go to zero torque.

5) if you question any of the above, just up the rear roll stiffness and remove front roll stiffness so that you can carry a inside rear tire at mid turn and try rolling on the power.

6) clutch pack diffs and spools change all of this to a new set of problems and advantages.

Hope this helps

AW

Tennessee Tech will be running a combination rotor/sprocket this year (I'm calling it the sprotor). Can't say much else about the design of the diff, except that it will be a huge departure from the 30 pound monster we had last year. Still trying to find a 12 tooth front sprocket before I can finalize the design of the rear....

I was a little dissapointed to hear that the idea had been used before, but what we have in mind will still hold a lot of originality.

JDF
Tennessee Tech

It's not a nightmare. An O-ring chain was used in order to reduce the amount of lubricant required. The Mechanical LSD was mounted with two CNC machined aluminium mounts either side of the differential. These aluminium mounts also housed roller bearings. diff mounted Turnbuckle-type adjustments were also used to keep chain tension, and the brake caliper is mounted directly to the aluminium diff mounts. Just be cautious not to heat the diff too much. I'll try and post a picture of USYD's soon, but the one already here looks nice as well.

we ran a strain gauge on a drive shaft..(radio transponding strain gauge $5000 US)

the torque on acceleration is limited by the clutch

the torque on de-acceleration is due to the INERTIA of the components from diff to tyre being brought to a halt rather suddenly

the result.. giant spikes on torque during aplication of the brakes.. (for us it was about 5 times the static breaking load) ((although we had rather heavy wheels.. 5kg))

just something to think about