one or two rear discs

[Rob (ANU)]

Hi guys,

Just wandering what teams were running a single disc on the rear with the torsen diff... And how do you justify it... Also those teams who understand why you shouldn't please enlighten me??

Cheers,
Rob Wootton
Australian National University

"In general, the greater the understanding, the greater the delusion: the more intelligent, the less sane." George Orwell

[Rob (ANU)]

Hi guys,

Just wandering what teams were running a single disc on the rear with the torsen diff... And how do you justify it... Also those teams who understand why you shouldn't please enlighten me??

Cheers,
Rob Wootton
Australian National University

"In general, the greater the understanding, the greater the delusion: the more intelligent, the less sane." George Orwell

[Sam Zimmerman]

We run a single rear rotor. We justify it by saying it works and it makes the packaging and manufacturing easy.

Sam Zimmerman
Vandals Racing
2002-2003 Team Leader

[Denny Trimble]

Most US teams run a single rear brake on a torsen, but Wollongong was praised this year for the dual-brake setup. Jay O'Connell mentioned that a single rear brake on a torsen creates an understeer torque on corner-entry. I think this is the biggest argument against the single-brake setup.

Here's my hand-wavy interpretation:
In a dual-brake setup, for a given brake line pressure, the left and right wheels receive the same braking torque regardless of their normal load. So, as you trail-brake, the inside tire becomes light. Before it locks up, the inside and outside tires are generating similar braking forces, so any oversteer/understeer moment is zero. After the inside tire locks-up, it is still producing braking force, just less than the outside. This is a small understeer torque, but we don't like to see locked wheels so it's not important.

With a single-brake on a torsen, as you trail brake, the torsen initially sends 50/50 torque to each wheel. Then, as the inside wheel becomes lighter and starts to lose traction, the torsen transfers more braking torque to the outside wheel. It continues to do this "proportionately" to the difference in normal load on the inside and outside tires, also factoring in the torque bias ratio. So it sets up a large difference in inside and outside braking forces, which is a large understeer moment.

Any other explanations out there?

University of Washington Formula SAE ('98, '99, '03)

[Rob (ANU)]

Sam,
This is the exact reason why we were thinking of changing from two to one disc... last year packaging was an issue but nothing a file and a bit of elbow grease couldn't fix http://fsae.com/groupee_common/emoticons/icon_smile.gif

Cheers,
Rob Wootton
Australian National University

"In general, the greater the understanding, the greater the delusion: the more intelligent, the less sane." George Orwell

[Rob (ANU)]

Danny,
Thanks... but how many teams with one disc have really noticed this when driving the car...

Cheers,
Rob Wootton
Australian National University

"In general, the greater the understanding, the greater the delusion: the more intelligent, the less sane." George Orwell

[Denny Trimble]

Good question. I've only driven single-disk cars, so I can't tell you. But understeer is our enemy, and this may contribute to it.

If you have to tweak the car to cancel this understeer affect, it might oversteer too much somewhere else, and then you might tune it back to where it was and have a car that understeers on corner entry.

Understeer/Oversteer torques from an LSD are an important way to tune the car. The judges like to see torque bias ratio testing/adjustment. Also, look back a few years in Racecar Engineering (I believe), there was talk of the special active diffs (tracked vehicle steering transmission concept) that F1 teams use to help rotate the car on corner exit. Not braking related, but it shows the significance of diff US/OS torques on car performance.

University of Washington Formula SAE ('98, '99, '03)

[Engelbert]

Hi Denny, that was a good explanation of the difference between the single and dual discs....highly logical. Makes sense to me !

My only experience with fsae:
Due to many factors (dont start!), our first -and my only http://fsae.com/groupee_common/emoticons/icon_frown.gif )car understeered out of tight corners like you wouldn't believe, but could be oversteered during corner entry more easily, so I would assume in this case, our torsen setup (with a single disc) would have been helping with the initial stability...

Changing the topic to general vehicle behaviour:

I spose generally speaking, cornering is broken into entry, mid-corner, and exit.(Sorry if I offend any pro's out there...) Though I found it more of a 4 or 5 stage process.

I found the initial braking and turn-in caused oversteer, but once the car was a little more settled (just before mid corner, still during transfer of weight though), it changed quickly to understeer, which stayed till after the exit of the corner.

In this way, the car basically had to be thrown sideways from the start of the corner (whilst the rear is still 'light') just to get it pointing in the right direction during exit. (very sharp corners only. larger radii corners did not encounter this problem).

Out of curiosity, are there many teams out there who have an understeer issue on tight corner entries (ie. whilst still braking)? Has anyone else had similar handling characteristics to my description above, and if so, did you find a solution ? Our car could definitely have been improved (damn lack of time!), but I still dont know if the problem could have been completely removed.

It seems to be an inherent trait in this form of vehicle/track combo, but I'd like to hear other drivers perspectives...

Comments ? Abuse ?...

Sam McFerran.
UQFSAE-2001.

PS. SamG, you still owe me money from Bathurst last year!

[Travis R]

One disk saves weight (unsprung and overall) and cost. It also makes easy packaging, and simplifies design/manufacturing of the rear upright.
It seems to me that corner entry understeer is better than corner entry oversteer... at least from a safety stand point. I'd rather have to brake a little earlier, than worry about spinning and fight the car through every corner.
Just my opinion.

Travis Rouse - Test Pilot - The University of Texas (Austin)
http://www.ls1power.net/Travis/RX7/autox4_slide.jpg

[Denny Trimble]

Engelbert,
Not sure what's causing the handling problems you describe. Could be just too much rear brake bias and trail-braking too deep, or perhaps a "funny" alignment (takes a while to get it right for some people... took me a while the first time!). Rear shocks set too stiff could be a problem too.

Getting the car to take very tight corners well means very high yaw rates. Since not many teams are using 4ws http://fsae.com/groupee_common/emoticons/icon_smile.gif that means the front tires have to initiate the yaw process, and they need lots of extra grip at low speeds to initiate this yaw. Solving this problem while keeping high-speed corner stability is a major challenge of FSAE, once you have a running car http://fsae.com/groupee_common/emoticons/icon_smile.gif

Hope this helps.

University of Washington Formula SAE ('98, '99, '03)

[Kevin Hall]

Engelbert

Your problem sounds similar to the problem that I had with my Daytona......it had soft suspension, and would give alot of weight transfer going into and out of corners. Coming in, the dive effect lightens the rear, and causes oversteer, but then when power is applied again, the back end tightens up, only to have the front end let loose under power due to squat. Too low of a spring rate, or not enough front anti-roll are my suggestions.

Kevin Hall
University of Saskatchewan
'03-'04 Team Director

[caroline]

{I know it's been forever since anyone's posted here... }

As far as Englebert's oversteer/understeer issues, the weight transfer and suspension explanation makes a lot of sense (and seems the much more likely culprit, as Denny said, it's tough to get suspension right). However, I wonder if it doesn't tie back into our question of one disk v. two with the torsen.

Denny's explanation of the dual-brake setup seems correct to me- standard braking response without accounting for differential effects.

However, having unequal braking torques on each side messes with this picture when there is a torsen dif. As we know, when the torque available from tires changes relative to each other, a torque-sensing differential actually transfers up to the TBR multiple of the low-torque side to the high-torque side.

The following are my (very limited) understanding, and would appreciate critiques/comments:
Seems to me that if the single disk brake transfers all braking torque to the low-torque inside wheel, the diff will sense the braking torque and not realize that the actual wheel is actually not providing that much torque. Then, the torque differential between the two sides will not be as high and therefore torque transfer will not be as high, and there will be significant torque lost compared to if there were no disk on that wheel. This makes sense especially as it is common knowledge that if you are in a torque-sensing vehicle with one side having 0 torque (one wheel in the air, or mud or what have you), then if you apply the brakes you can regain the torque you lost. I would think this would create a slight oversteer yaw moment because the rear, esp outside rear, would not have as much braking as it ought...

If, on the other hand, the brake is on the high torque side, there could be potentially too high or premature torque transfer to the outside wheel while the inside wheel still has some weight on it, but the diff thinks that the differential is higher than would be otherwise. This too would cause torque loss (albeit less than for having the disk on the inside wheel) because the inside wheel is not getting the torque it should have.

If Englebert's system has a single disk, maybe it is creating an oversteer moment during times of weight transfer to the front and outside, which is removed as the vehicle reaches the apex of the curve because that is when the weight is transferred to the rear and the braking is removed, therefore the diff is able to transfer torque appropriately again.

BTW, saying that something "works" is perfect for dynamic events but should not be acceptable for DESIGN event. Packaging is fine, but think about the rotational inertia argument- rotational inertia increases to the square of the radius- two smaller radiused disks might have lower rotational inertia than one big disk. do the math. (now I'm sure my team is thinking- caroline- go do it yourself...)

I am mainly hoping for a critique of my thinking, but hope this helps someone!

best!
caroline
university of oklahoma- sooner racing

[Nihal]

I'm not sure if you guys are just discussing 1 inboard brake disc vs 2, or just inboard vs. outboard.
The reasons I've heard/thought about and seen for inboard vs. outboard mainly have to do with movement of the chassis in pitch. With an inboard brake the car pivots about the front axle/tire contact patch (like a motorcycle). With dual outboard brakes I would think that with anti-squat/lift your side view pitch center would be farther aft of the front axle. This would make sure the rear end of the car rises less.

[John_Burford]

Denny

This is an old thread! But I remimder the Design finals when Jay memtioned Wollongong's Daul rotor setup. I wanted to ask Jay what his reasoning was because I was at a loss.

The understeer/oversteer explaination makes perfect sense. BUT !!!!!! he is wrong to beleive that daul rotors is the right choice for FSAE. Once again FSAE creates a unique situation that leaves the Road Racing and Passagner Car engineer drawing the wrong conclusion.

Road racers don't have tight turns (low steering angles)

Passenger Cars have torsionally soft frames (limited cross weight transfer)

FSAE has high steering angles; torsionally stiff frames; add some caster and scrub radius, and you have all the turn-in you want. The weight, cost and complexity of dual brakes are not nesessary. Plus your maximum braking potential is going to be higher with a single disk on the LSD.

Oh well, so Jay missed one his first year as Senior Design guru.

John Burford

[Buckingham]

I wouldn't think the effect on pitch would be great enough to be the deciding factor on whether to go inboard/outboard. There are other ways of generating anti-dive/lift and there are many other factors to base the inboard/outboard decision on.

(Just because your radiator fan DOES produce a thrust force accelerating your car, doesn't mean that this fact should drive your radiator placement)

Besides, especially on a non-aero car, any form of deflection (pitch being one) isn't necessarily a bad thing as long is it lies within your intended design range.

The majority of this discussion, i believe, is on how that mysterious little round thing called a torsen actually works, and which method of packaging (1 or 2 disc) will yield the better dynamic performance.

[Denny Trimble]

Donavan - I agree.

Hey John! Good to see you on the forum.

One benefit of dual outboard rotors, at least for our '05 car, was the ability to move the engine 2" further rearward (the single-rotor to engine clearance was limiting before). Also, removing the caliper mount from the diff mount made eccentric chain tension adjustment much simpler.

I'd argue that your maximum braking potential will be very close with four brakes as compared to three; I see where you're going with that, though.

Perhaps more importantly, what is your car's braking capacity at partial lateral acceleration (trail braking)? Depending on your chassis setup (warp stiffness, steering-induced weight jacking, CG location), unweighting the inside rear tire with a single brake on a torsen can artificially reduce your braking capacity. Once the inside wheel start spinning backwards, you have zero total brake force at the rear end, which then pushes the effective brake bias (ratio of actual contact patch braking forces) to the front.

Having driven good cars with single (UW through 2004) and dual (UW 2005) rear brakes, there's not a huge difference, but I prefer the dual brakes. Knowing that the outside rear will always provide braking force, no matter what the inside rear is doing, is confidence inspiring.

Caroline,
I got the impression you're imagining a brake attached to either the left or right halfshaft. This wouldn't work well; all teams with single brakes run them to the diff, just like the sprocket, so the diff can then distribute braking torque however it wishes, instead of trying to sense torques in the halfshaft, which it can't. The torsen can only sense the total torque, and generates friction (which prevents differential halfshaft speeds) accordingly.

[Marshall Grice]

Another thing we noticed with the dual brakes is you have the ability to control inside wheel spin on corner exit with the brakes, just like you would with an open diff in anyother low traction environment. particularly useful on very tight turns where there is a lot of geometric weight transfer (caster, large steering angles).

i feel that the single brake on a torsen is overall less effective then a single brake on a spool. all of the US tendencies go away when the inside wheel lifts off the ground which is pretty easy to do if the roll centers are designed accordingly. This opens up a completly different set of problems for the torsen that don't exist with a spool. So my question is if you're main reason for running a single rear brake is for weight and packaging why are you running a torsen?

[Denny Trimble]

We only got inside rear wheelspin on corner exit when the front swaybar broke, which was sometime during the Detroit '05 event, and the entire SVSU GP http://fsae.com/groupee_common/emoticons/icon_smile.gif

I am apparently not a ninja behind the wheel, because I never used the brake on corner exit.

[Marshall Grice]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> I am apparently not a ninja behind the wheel, because I never used the brake on corner exit. </div></BLOCKQUOTE>

yeah, don't worry you'll figure it out when you learn to drive fast. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Z]

Possible reason for Englebert's corner-entry-oversteer and mid-corner-and-exit-understeer (one of many).

It is usually recommended to have zero-bump-steer, or else a small amount of bump-steer is "acceptable" if it gives understeer (sometimes this is built-in on passenger cars).

For the rear wheels, bump-toe-in, droop-toe-out, gives understeer during steady cornering ('cos of body roll). In this case, on braking/corner entry, with soft suspension and no rear anti-lift, the rear of the car rises, both rear wheels go toe-out, and as soon as the front of the car turns-in, the rear of the car steps out (inner-rear loses load, outer-rear gains load and pulls rear of car outwards). Rear toe-out is very unstable!

Once the rear drops to normal or low ride height (mid-corner and exit) the car reverts to understeer - the more heavily loaded outer-rear-wheel has toe-in.

Significantly "wrong" toe angles, measured in tenths of degrees, are nearly impossible to gauge by eye (whereas, say, a "bad" camber angle - measured in degrees - is easily seen). So make sure you accurately measure your toe-angles (static, bump, and due to compliance)!

Z

[Mike Cook]

Just to reiterate: I think that a single rear brake is pretty good in fsae. Weight, complexity, cost etc. goes down. Understeer on entry is NOT a bad thing. Also I think most cars have enough brake bias adjustment to control understeer/oversteer on entry. Now, if a car had massive understeer on entry due to the single rear brake setup, then i'm not sure that increasing the rear brake bias would be the way to go... That said with our taylor setup and single rear brake I have found that brake bias has wayyyyy more affect on the corner entry characteristics than a single rear brake and that understeer due to the single brake was not really a big factor.
Seriously, people go on and on about the single rear brake thing and it is geting lame. In my experience there are better things to focus on.

[McFly]

From my experiences and driving fsae cars with both single rotor and dual inboard rotors, the dual rotor was by far much better. Part of it may have been a lot of time went into the design of the rear rotors instead of just using something that worked. We found that with the single rotor acting on the diff, it would under hard braking unevenly distribute the braking force to one wheel and that wheel would lock up. This is under straight line braking. Also we found that going to the dual rotors significantly reduced the rear wheel hop under hard braking thus increasing over all braking capabilities. Like I said, most of this may because we did a lot of designing and testing on the dual rotors, you might be able to design a single rotor just as well.

Also, I wouldn't say that going to a single rotor will reduce weight because our dual rotor system was just a touch lighter when everything was considered. Also, rotating inertia was significantly reduced, went from a 10" rotor to dual 5" rotors.

[kwancho]

5" rotors!? what calipers did you use?

[Storbeck]

I've seen Madison's brake setup, and the calipers are tiny little guys, AP two piston rear's I think. My question is how high is the line pressure.

[McFly]

Yes we used AP racing calipers. They do not suggest using rotors that small. Since the pads are smaller, and with tight tolerances on everything you can still contact the whole pad.

As far as line pressures, I don't know the numbers off the top of my head right now but with the bias by set at neutral position the line pressures are the same front and rear. It's nothing that some goodridge steel braided brake lines can't handle.

[Frank]

one disk + spool, easy

[Nihal]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I wouldn't think the effect on pitch would be great enough to be the deciding factor on whether to go inboard/outboard. There are other ways of generating anti-dive/lift and there are many other factors to base the inboard/outboard decision on.

(Just because your radiator fan DOES produce a thrust force accelerating your car, doesn't mean that this fact should drive your radiator placement)

Besides, especially on a non-aero car, any form of deflection (pitch being one) isn't necessarily a bad thing as long is it lies within your intended design range.

The majority of this discussion, i believe, is on how that mysterious little round thing called a torsen actually works, and which method of packaging (1 or 2 disc) will yield the better dynamic performance. </div></BLOCKQUOTE>


I was thinking it would have an effect on the sprung weight transfer. The rear tires will be more effiecent than the fronts under braking, you are thus allowing more rerward braking force.

[Buckingham]

Nihal,

Sorry for the confusion, i was referring to vehicle deflection (pitch). You are correct that forces (weight) do have a significant role in dynamic performance.