Inboard/Outboard Rear Brakes

[Adam D. Bell]

I've read plenty of posts about what has been written on this forum concerning this topic. But I want to know about the following:
Why have teams run with dual rear inboard and/or outboard brakes?
What engineering struggles did you encounter?
What were the pro's and cons?
I saw some teams in design final with outboard rear so I know that has to be at least a decent explanation as to why or why not in doing this.

The goal of this post is to understand how inboard vs outboard stacks up.

I know that inboard helps keep weight upspring and near the center of the vehicle and the suspension doesn't have to deal with the braking forces. What have you all learned?

Outboard will allow potential for easier packaging, Greater braking control, and can be more effective system.

Anything you know that can be added to this list of pro's and cons.

[Adam D. Bell]

I've read plenty of posts about what has been written on this forum concerning this topic. But I want to know about the following:
Why have teams run with dual rear inboard and/or outboard brakes?
What engineering struggles did you encounter?
What were the pro's and cons?
I saw some teams in design final with outboard rear so I know that has to be at least a decent explanation as to why or why not in doing this.

The goal of this post is to understand how inboard vs outboard stacks up.

I know that inboard helps keep weight upspring and near the center of the vehicle and the suspension doesn't have to deal with the braking forces. What have you all learned?

Outboard will allow potential for easier packaging, Greater braking control, and can be more effective system.

Anything you know that can be added to this list of pro's and cons.

[RacingManiac]

Dual Rear Outboard:
Pros:
-Safer in case of a drive shaft failure if you have a diff with no preload
-Not putting driveshaft in reversal of loads
-Potentially better braking performance

Cons:
-more complicated rear upright packaging
-more weight(vs a single rear setup)
-more unsprung mass

At least those were some of our consideration. We did run a dual outboard setup in 2006, a change from 2005's single rear. I say potentially better performance because for us thats kinda untrue but that's not due to the layout but the OEM part we chose this year vs the custom parts we've had in 2005.

[drivetrainUW-Platt]

packaging, you need to mount the calipers and brake disks to something, and thats a lot harder to do then you might think since things like cv joint housings and driveshafts are hardened and I wouldnt feel safe welding on them, depending on your uprights you might have more room outboard then inboard, but its all dependent on your car, and all the above things said

[drivetrainUW-Platt]

also, theres the question do you want duel rear brakes if you go inboard????

[Adam D. Bell]

Thanks, your comments have helped me make a decision. I think I might piss a person off but I'm going to do it anyway.

http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[Conor]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by drivetrainUW-Platt:
also, theres the question do you want duel rear brakes if you go inboard???? </div></BLOCKQUOTE>

That's a packaging nightmare. I don't see how you could possibly gain any design points for having dual inboard. The only thing you're doing is decreasing your unsprung weight, but you're increasing your headaches ten fold. I say stick with the tried and true method of one brake on each wheel. That's one thing the design judges won't question, but if they do, you can tell them that the evidence to support your decision is right behind all four wheels of their own personal vehicle.

[Greg 08]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> I don't see how you could possibly gain any design points for having dual inboard. The only thing you're doing is decreasing your unsprung weight, but you're increasing your headaches ten fold. </div></BLOCKQUOTE>

We have found quite the opposite with our car. I have not looked into how the atv diffs work and how a sprotor would work on that, but on a torsen diff it makes absolute sense. Since the torsen diff essentially locks up when torque is applied. Upon corner entry you want to yaw the car in order to get around the corner. Having a locked rear end makes this difficult.

As for packaging we had a fairly decent system. In the Rockford CVs that we run, there are large lightening holes drilled almost all the way through already. We simply turned some adapter plates and screws to fit and bolted them through that hole. CNC some slightly different diff mounts and a little bit of measuring and they mount right up fine. We found packaging to be no worse, and servicability better. In addition it would allow you to do some very creative things with the rear wheels and uprights. We ended forgetting about that whole time constraint thing and ended up running old wheels, however make no mistake about it, inboard dual rear brakes have definite tangible advantages.

If anyone from Michigan State is reading, they actually made their own diff so that when torque was applied one way, it locked and the other way stayed open. I had considered this myself, but we decided against it for simplicity and ease (we already had a good standard diff housing).

Something to consider, I never got our design sheet back but the judges seemed quite receptive of the idea.

Greg Ehlert
Michigan Tech

[Conor]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Greg 08:
In the Rockford CVs that we run, there are large lightening holes drilled almost all the way through already. We simply turned some adapter plates and screws to fit and bolted them through that hole.

Greg Ehlert
Michigan Tech </div></BLOCKQUOTE>

You mounted your caliper or your rotor to your CV joints????

[Adam D. Bell]

Greg,

What are the tangible benifits of running dual inboard brake with a limited slip diff? From what I understand the diff is going to try to drive both wheels at about the same torque so If I brake on the diff won't this help distribute braking torques also?

[Greg 08]

Yep sure did. We actually thought about it a little bit too. The way the RCV cups are designed, we thought it plenty safe. We also haven't broken them yet either (knock on wood). http://www.rcvperformance.com/saeaxlekit.html If you look axially, you can see the dual lightening holes. These were replaced in a newer version with one large hole, about .404". This goes almost the entire way down, about .375" from the bottom. We continued the holes through at 5/16" diameter and bolted them through the entire cup with a washer on the end. Machined a plate out of billet that matched the curve and adapted the 3 hole pattern to the 4 hole pattern in the rotor. The caliper was mounted to the diff mount, rigidly mounted to the chassis. What is your objection to mounting them there?

Greg Ehlert
Michigan Tech

[Greg 08]

Adam,

The way a torsen diff works can be found on the Zexel Gleason website, http://www.torsen.com/files/Traction_Control_Article.pdf The article will explain much better than I can, especially when hampered without bar napkins and sharpie.

At any rate, it would help to distribute the torques to keep the wheels moving in a straight line. The problem is that is not what you want. If you go through a corner, the car accelerates laterally as well as angularly (yaw). Having the wheels fight turning at different rates causes an understeer moment, which makes it harder for the driver in our opinion. We thought this was a big deal for our driver, so we ran dual inboards.

One reason the torsen is so popular with fsae teams is it works really well for our application. Speaking in generalities, in corner entry with no torque on it, it acts like an open diff. This allows you to get the car yawing like you want. As you roll through the corner, it behaves the same until you get on it as you come out of the corner. When you apply the torque of the motor it locks up and causes the wheels to try to turn the same rate, thus causing you to yaw back into a straight line. This logic is definitely just our opinion, so you can take it or leave it, but there are real tangible benefits to running dual inboard rear brakes. Just about your design tradeoffs right?

Greg Ehlert
Michigan Tech

[Adam D. Bell]

The biggest thing is that the system works together. Doing inboard brakes looks just as challenging as outboard brakes. I think we have more to win with inboard Less unsprung weight. Weight close to the center and I don't have to worry about the suspension links taking braking loads which opens up more opportunity for the rear suspension to be lightened. With the added system performance. I think it's worth investigation with CAD and a preliminary design with review of feasibility. What kind of testing to you recommend to evaluate this yaw characteristic? I would like to know how to prove it's effects.

[vandit]

check out on this forum only...one of the university has given a decent reason for putting inboard brakes....they explained something about the equal braking torque due to limited slip differential.....i read it quite long time back and was not able to understood but they got appreciation from judges for their reasoning

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by shanky:
one of the university has given a decent reason for putting inboard brakes....they explained something about the equal braking torque due to limited slip differential </div></BLOCKQUOTE>

This sounds backwards. You only get equal torque with an open diff. I am assuming you are refering to a single rear brake on the diff?

I agree with Adam, inboard brakes (assuming a brake each side) done properly will be no easier to design than outboard, but potentially have some advantages in packaging, suspension design etc. I would be concerned about the driveshafts and joints reacting the brake torque though so needs careful analysis.

Regards, Ian

[Conor]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Adam D. Bell:
Weight close to the center and I don't have to worry about the suspension links taking braking loads which opens up more opportunity for the rear suspension to be lightened. </div></BLOCKQUOTE>

Your suspension will still be seeing braking loads. No matter if you brake inboard or outboard, you'll still be experiencing deccelartions that will be absorbed by the suspension. I wouldn't rush into lightening the suspension just yet. Are you planning on mounting the dual inboard near the diff or further on the shafts?

[Chuck Maddocks]

the oil in the diff also acts as a heat sink for the heat generated by the brakes. if you have two rotors (rather than one), you may not be generating a whole lot of heat, but something to keep in mind. my preference is outboard. in design this year a judge mentioned the safety issue when an axle fails and you can't brake. less load on the axle = smaller, lighter axle = faster

[Mike Cook]

You don't need rear brakes to stop. Also, braking loads are less than forward acceleration loads.

[Adam D. Bell]

Heat generation is an energy issue not a function of how many rotors you have. If you have one brake rotor doing all of the rear braking vs two rear brakes doing the braking the energy into the system is the same. So the energy into the diff will be the same due to braking. If i'm wrong please tell me where I'm screwed up in my thinking.

This is the best I can think with my daughter hounding me http://fsae.com/groupee_common/emoticons/icon_razz.gif

[vandit]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
the oil in the diff also acts as a heat sink for the heat generated by the brakes.
</div></BLOCKQUOTE>

i didnt understood that how is the heat gettting transfered from disc to diff casing and that too inside of diff casing....the area that inboard disc shares with diff casing like circumfrence * thickness....that's too less to conduct heat to diff casing........and i think more or less the basic function of oil inside diff is lubrication....

plz check me....even i am learning

[Buckingham]

Heat generation is the same, but if you have different surface areas and surface speeds, you will have a different overall heat transfer coefficient which means the operating temp of the brakes will be different which means the energy flow into the diff will be different. (the trick is to get the heat generated to leave the brakes before it has a chance to get to the diff)

Easiest way to find out if your diff can handle your brake heat: go drive a few laps record lap times, come back to pit and take a torch to your diff housing until it is as you are comfortable with and go back out for a few more laps. Who knows, maybe you will be faster with a cherry-red diff... http://fsae.com/groupee_common/emoticons/icon_smile.gif

[jdstuff]

Heat in your braking system is not a bad thing. In point of fact, heat (in the correct proportions) will increase your braking performance....just look at pad friction material coefficent curves. Irregardless whether you have 1 or 2 rotors, the system should be designed to operate within the same temperature range to maximize the coefficient of friction of the pad material.

As far as heat transfer into the diff is concerned (with a single rotor), I think that it is much less than most people assume it is....or at least it can be made small depending on how you choose to mount the rotor. I have heard many say that judges frown upon diff mounted rotor because of the heat transferred into the diff. This is not true! Judges simply want to see that you have considered this in your design criteria, and have a justified reason for your choices. This means testing....which can be pretty simple. Temperature sensitive paint can tell you a lot about what is happening on the track....no need for fancy IR temperature sensors which are out of the budget of most teams. The paint is cheap and you can buy it from McMaster-Carr. The point is that just about any system can work well, which is obvious just by looking at the 100+ cars at competition. What you need to show is that you understand why it works, and prove that it was the best choice for your car.

[Adam D. Bell]

Good Point. Thanks for the comment.

[Conor]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jdstuff:
Heat in your braking system is not a bad thing. In point of fact, heat (in the correct proportions) will increase your braking performance....just look at pad friction material coefficent curves. Irregardless whether you have 1 or 2 rotors, the system should be designed to operate within the same temperature range to maximize the coefficient of friction of the pad material.

As far as heat transfer into the diff is concerned (with a single rotor), I think that it is much less than most people assume it is....or at least it can be made small depending on how you choose to mount the rotor. I have heard many say that judges frown upon diff mounted rotor because of the heat transferred into the diff. This is not true! Judges simply want to see that you have considered this in your design criteria, and have a justified reason for your choices. This means testing....which can be pretty simple. Temperature sensitive paint can tell you a lot about what is happening on the track....no need for fancy IR temperature sensors which are out of the budget of most teams. The paint is cheap and you can buy it from McMaster-Carr. The point is that just about any system can work well, which is obvious just by looking at the 100+ cars at competition. What you need to show is that you understand why it works, and prove that it was the best choice for your car. </div></BLOCKQUOTE>

I definetely agree with you on this. I don't think the judges are looking for a "right or wrong answer" so much as they are a solution that you believe best fits your application and the evidence to support your conclusion.

[scottie]

we (michigan state) took some heat this year for the dif mounted rotor/caliper, i dont know if thats because we didnt have enough testing to back up some of our design or not but as far as heat transfer to the diff correct me if im wrong but that seems like a minor point. oil and grease act as a lubricant as well as a coolant for your diff.

[jack]

i agree, the heat issue in my opinion is not a big deal. the big deal is how it makes your car behave on corner entry.

(i dont know if that was covered earlier in this thread, i was to lazy to go back and read it http://fsae.com/groupee_common/emoticons/icon_wink.gif )

[jack]

this is what i was taking about:


<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Greg 08:
Adam,

The way a torsen diff works can be found on the Zexel Gleason website, http://www.torsen.com/files/Traction_Control_Article.pdf The article will explain much better than I can, especially when hampered without bar napkins and sharpie.

At any rate, it would help to distribute the torques to keep the wheels moving in a straight line. The problem is that is not what you want. If you go through a corner, the car accelerates laterally as well as angularly (yaw). Having the wheels fight turning at different rates causes an understeer moment, which makes it harder for the driver in our opinion. We thought this was a big deal for our driver, so we ran dual inboards.

One reason the torsen is so popular with fsae teams is it works really well for our application. Speaking in generalities, in corner entry with no torque on it, it acts like an open diff. This allows you to get the car yawing like you want. As you roll through the corner, it behaves the same until you get on it as you come out of the corner. When you apply the torque of the motor it locks up and causes the wheels to try to turn the same rate, thus causing you to yaw back into a straight line. This logic is definitely just our opinion, so you can take it or leave it, but there are real tangible benefits to running dual inboard rear brakes. Just about your design tradeoffs right?

Greg Ehlert
Michigan Tech </div></BLOCKQUOTE>

[jack]

this guy is retarded:

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
You don't need rear brakes to stop. Also, braking loads are less than forward acceleration loads. </div></BLOCKQUOTE>

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Conor:
I definetely agree with you on this. I don't think the judges are looking for a "right or wrong answer" so much as they are a solution that you believe best fits your application and the evidence to support your conclusion. </div></BLOCKQUOTE>

Exactly. If you've got a single rear brake I'm going to ask how much lighter it was than two brakes. What's the effect of less unsprung mass and what does it do to the corner entry balance of the car?

If you have outboard brakes I'm going to ask why you've got a heavier package than some other teams with a single inboard unit and why you've chosen to have more unsprung weight.

In the end, how you answer those questions is more important than the system that's on the car...

Ben

[CappyUMD]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jack:
this guy is retarded:

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
You don't need rear brakes to stop. Also, braking loads are less than forward acceleration loads. </div></BLOCKQUOTE> </div></BLOCKQUOTE>

In Cook's defense, braking with two front wheels and one rear wheel is less stable than two front wheels. Rear wheel acceleration loads are greater than braking loads because of weight transfer.

Still, he is retarded.

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by ben:
In the end, how you answer those questions is more important than the system that's on the car...
Ben </div></BLOCKQUOTE>

Same here. Although personally I dislike the single-rear-brake-on-diff concept, if a team can explain clearly why they have chosen that solution, justified with approprate data, then you cannot fault them.

Why don't I like it? Because to do the analysis properly you require a lot of data that I think is hard to get, and even harder to have confidence in. Like true TBR characteristics for the diff under all loading conditions. Like accurate friction and compliances for CV / tripode joints. Like accurate tyre data to calculate the slip ratios and yaw moment resulting from the diff TBR. etc etc...

Regards, Ian Murphy
Formula Student 2006 Design Judging Team Leader

[CappyUMD]

I just realized...if the dual brakes are outboard you wouldn't lose braking on either wheel if a halfshaft failed. Guess I'm the retard.

[jdstuff]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by CappyUMD:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jack:
this guy is retarded:

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
You don't need rear brakes to stop. Also, braking loads are less than forward acceleration loads. </div></BLOCKQUOTE> </div></BLOCKQUOTE>

In Cook's defense, braking with two front wheels and one rear wheel is less stable than two front wheels. Rear wheel acceleration loads are greater than braking loads because of weight transfer.

Still, he is retarded. </div></BLOCKQUOTE>



Read the post above Mike's that he's replying to. He's just simply stating that you can still stop the car with only front brakes IN THE EVENT OF A REAR AXLE FAILURE if you have an inboard rear.....and that the reverse loading of the axles under braking is less than the forward acceleration loads.

[Storbeck]

I agree with Cook, you can still stop the car with just front brakes. I maybe wouldn't count on it for a track car, but in our situation you would have plenty of time to safely stop the car with only front brakes without hitting anything but cones. Our team has done it before when we had a circlip fall off on our rear master cylinder so the plunger fell out. I won't tell you how many runs I did at Saginaw before I figured it out. "...I keep adding rear brake bias and nothing seems to change..."

Also, the loads on the axle shafts are lower on braking than accel due to weight transfer.

[Mike Cook]

Jack, suck my ass. Improve your reading skills. I was replying to the post directly above mine. These cars stop fast- even with only two front brakes. Loosing rear brakes is not really a saftey concern of mine.

[Chuck Maddocks]

me:
the oil in the diff also acts as a heat sink for the heat generated by the brakes. if you have two rotors (rather than one), you may not be generating a whole lot of heat, but something to keep in mind. my preference is outboard. in design this year a judge mentioned the safety issue when an axle fails and you can't brake. less load on the axle = smaller, lighter axle = faster

mike:
You don't need rear brakes to stop. Also, braking loads are less than forward acceleration loads.

yeah, he was responding to my post, in which i was completely wrong on all accounts. mike's a really smart guy, but's he's not a drivetrain guy. still way smarter than me and most of the people on this post

i was saying that you should do a quick heat analysis (however pointless it may be..) just so you have it for the design comp.

i was told that a design judge didn't like the single inboard brake because it posed a "safety issue." this is debatable, and i'm sure a lot of other teams heard the same thing because they run a single inboard. just because a judge didn't like something on your car doesn't mean you have to change it. do the research, run the calcs, back it up in design.

and while braking loads are less than acceleration loads, some driveline components are designed to take both loads at the same time. i.e. diff with single brake rotor, axles with outboard brakes. that means you can make one lighter. i originally thought that if the brakes were mounted to the hubs they wouldn't produce reverse loading on the axles, but this is wrong. most people buy their half shafts so it doesn't make a difference anyway. I'M THE RETARD. but i'm also drunk, and i don't know why i still bother with this shit. i swear it's an addiction, but i'm getting over it

[drivetrainUW-Platt]

assuming an almost perfectly sealed differential....which we all hope for, but rarely accomplish, heat=pressure and pressure=leaking. We ran duel outboards and after endurance even with minimal oil and a small oil leak the diff was literally cold.....

[Geraint]

Hi guys. My names Geraint and I'm a first year student of Motorsport Engineering in the UK. I hope you dont mind me reviving an old topic but I aim to do my first year project on braking technology. I have been considering doing a research into the trade off's between outboard and inboard braking.
I may be picking your brains over the next couple of weeks as everything is new to me and may need some help understanding some points http://fsae.com/groupee_common/emoticons/icon_confused.gif
Anyways, just thought I'd introduce myself and prepare you guys for my future questions =) This thread has helped me greatly so far. Thanks.

[Jersey Tom]

I honestly see no reason to run two outboard instead of two inboard on a SAE car. The likelihood of a halfshaft failing at comp is about zero. And if you lose a halfshaft you're screwed anyway. If you lose one, you lose all your thrust. On an open or limited slip diff anyway.

Why make the weight unsprung if you dont need to?

[B Hise]

we're running outboard rears because of packaging issues. Our rear frame/diff assy doesnt really allow good mounting. You do run into the issue of dumping heat into the tripods and/or diff as well.

While we all know that minimizing unsprung weight is free grip, sometimes you have to make sacrifices.

Bryan

[Mike Flitcraft]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Conor:
You mounted your caliper or your rotor to your CV joints???? </div></BLOCKQUOTE>

Rotor. There's no feasable way to mount calipers to spinning CV joints, that have high pressure hydrualic lines going to them.

The lines would simply wrap around the axles and rip apart the lines or destroy the fittings.

~Mike Flitcraft
University of Cincinnati
Bearcat Motorsports

[Gareth]

Sometimes it surprises me how little knowledge is shared here...

First, the amount of energy input into your rear brakes, one or two rotors, is roughly fixed based on your bias bar settings. If you use a proportioning valve as well then it gets a little more complicated, but lets not worry about that now. So lets say your rear brakes do 40% of all the braking (arbitrary), given some velocity change you can figure out how much energy is being dumping into your rotors (1/2*m*v^2 = mc(T2-T1). To come up with a better guess of how much work is being done by the rear rotors you can do simple load transfer calc's and use the TTC tire data to figure out roughly how hard each wheel is braking. If you have them, you can use pressure transducers on your brake lines to develop a 'transient' brake application case, but it generally pretty close to a step input, or at least for this kind of crude analysis.

Generally you want a stiff brake pedal. If you've ever pushed a 'real race car's' brake pedal you'll know it's solid and this is coincidence. A stiff pedal gives the driver confidence and improves consistency. A simple extrapolation from this, is that you want the largest rotors you can fit, as your mechanical advantage goes up with the square of the radius. Yes, this means adding inertia so make an educated choice. I tend to feel that anything you can do to make the driver's job easier is a good thing. That and you're generally grip limited most of the time, at least those with reasonably tuned 600cc motors, so it's not exactly like you're throwing it all away (sorry for the generalization).

So, now you need to think about how big you want your rear rotors to be and how they could package in your chassis. The simple method I came up with was this:
- Fit the largest rotor I could in the front wheels, with the thinnest x-section I thought was reasonable.
- Thermally matched the mass of the rear rotors to the fronts, based on my assuptions of the f/r braking ratio. This was to try to get similar temperature rises between the front and rear rotors, giving me better brake bias consistency (brake pads cof's are temperature sensitive).
- Without doing any really complicated heat transfer, I basically assumed that the front and rear rotors would have roughly similar 'h' values, though I did place my rotors outboard (more on that soon), like the fronts. Then you know that the heat rejection is h*A(T-Tinf). With the same h and delta T, we need only to match the areas to our brake ratio. With a caliper selected (which gives pad height), a mass, and a desired area of the rotor, you have a first crack at the dimensions of your rear rotors. I used a solid disc approximation, but it's good enough for rough sizing.
- Now there's more steps for figuring out your MC sizes etc, but that's not relevant, though for the guys asking about adjustable pedal ratios, you could also just change your MC bore, if you have the $$. I made my own like the really nice AP Racing and Tiltons that have the 'trunnion' balance bar...so they were cheap. ;-)

So now you have to fit this, probably pretty big rotor into your car. I found it to be difficult to package inboard, so I went looking into the reasons for why/why not.

Pros:
- Less sprung mass

Cons:
- Harder to package (probably heavier with extra structure etc)
- Less air flow for cooling
- Puts tire on the end of a torsion spring (half shaft)

The last one I'm not totally convinced of, but I found the intertia of the wheel/tire and did the simple harmonic motion calc's, which told me that they have a NHF of about 15 Hz. I felt this was too close to my suspension resonant freq's and thought that it was a potential reason for the chatter we've felt in the past when the rear locks up (comments??).

So we put the rotors outboard. Incidentally, we felt that braking on the diff to be such a huge problem with respect to yaw damping, I designed a Salisbury LSD. It let me run very little lock under engine decel and lots under accel. It weighed several pounds lighter than our Torsen (with an aluminum housing) and was tunable with different ramp settings. One of the design judges from Champ car noted that they've been playing with their diff's for years, trying to get better performance. They use 'negative pre-load' springs to create some histerysis in the diff's behaviour. Essentially, under steady-state cornering you have some drag (tires, aero etc) that the engine has to overcome. It may not be large, but that torque starts to lock the diff, that is unless you modify the diff to prevent it from locking until a desired torque is reached. Then it could very aggressively lock as you transition to corner exit. Interesting stuff...

I hope this actually shines some light on the subject, instead of throwing more punches in the dark...

[Garbo]

Dammit, Gareth, why do you alumni types feel like you have to write books on the forum. :-)

[Buckingham]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Without doing any really complicated heat transfer, I basically assumed that the front and rear rotors would have roughly similar 'h' values </div></BLOCKQUOTE>

'h' is not independant of rotor radius (fairly significantly). Think about parallel flow over a flat plate. Slots/grooves/holes/etc also change h very significantly and in a very non-intuitive manner. I have also found that h affects operating temp more than anything else.

Although with an adjustable bias bar, a thermally balanced system is really only important if you have VERY sensitive brake pads (find a new supplier) or if you run into boiling issues (and its not the fluid's fault).

[Kyle Walther]

dhaidinger:
How are you calculating your "h" value.

Every equation i run into in brake books for solid disks relates it to Re. The trouble i'm having is defining the Characteristic Length. I've been using the equation for a fin, the surface area/perimeter. What have you found is most accurate?

[Buckingham]

testing

[Kyle Walther]

boo. that's not the answer i wanted..

anyways i have another enquiry.

I have two equations that predict how much heat is transferred into the pad verses the rotor. The single stop equation relates the properties of the two materials, But the repeated stop equation is more a function of the "h" of the two parts, because of the higher temps, and doesn't even reference the rotor material. does anyone have a good equation for a repeated stop heat distribution that uses both "h" and material properties of both the pad and the rotor. Or does it not matter

[Buckingham]

Think in terms of a resistive network. Heat is generated at the surface between the pad and rotor. The pad and rotor are parallel circuits to ambient. Heat (like electricity) will follow the path of least resistance.

[Kyle Walther]

I know thats what the first equation is stating, but for repeated braking with cooling intervals the equation changes because of the higher heat. at least thats what it says in the book.

[Buckingham]

You can read all the books and have all the degrees you want, but it is still easier to measure HP vs RPM than to calculate it from raw theory. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Kyle Walther]

Yeah i'm coming to that conclusion too. http://fsae.com/groupee_common/emoticons/icon_frown.gif We have a brake dyno to test with, but i just wanted to take a large list of materials and narrow it down, So i didn't have to spend more money and time on testing than i had too.

[Garlic]

Donavan- Spoken like a true racer. Without testing theory is all but meaningless.

You might come up with the best thought-out theory and analysis of your concept ever, and then go testing, and you'll be back starting from square one.

People that get results, are the people that use REAL results as a guide. People that go by theory only are the ones who either end up proving themselves wrong, or (sadly) never even going that far and being proud of something unproven.

[MichaelH]

How did you guys attach the rotor to the hub or cv when running inboard brakes.

I was thinking of an insert into the hub but not able to come up with a good way to fasten it to the hub.

-Michael

[Mike Cook]

Can you not attach a brake hat to the inboard stub axle flange?

[MichaelH]

I was seriously thinking about modifying the cvs like someone said earlier to accomadate an insert for the floating rotor between the cv and the upright/hub. Using the three holes that are already present and just drilling them the rest of the way to attach the insert directly to the cv.

[Kyle Walther]

that kind of what we did last year. We mounted ours to the inboard cv. The only problem we had was with the extra forces applied to the stub shaft. you can kind of see it in the picture below
http://sae.ou.edu/gallery/images/photos/SRT-2006/cad/06_chassis_all_3.jpg

[MichaelH]

I was referring to the outboard cv, but the inside cv does pose an interesting idea. Did that work pretty well for the most part?

[Kyle Walther]

yeah. it kept braking forces out of the uprights and a-arms, which is nice. and if you plan it well, the packaging is very clean. lowers your unsprung, and it allows you to go with a smaller rear rotor, but so does mounting on the back side of the hub.

like i said just watch the added forces to the axle components

[Gatorman]

http://www.gatormotorsports.com/gms/formula/2006/images...20(Custom)%20(2).JPG (http://www.gatormotorsports.com/gms/formula/2006/images/Comp06pics/100_0008%20(Custom)%20(2).JPG)

[fade]

nice. and correctly mounted on the diff carriers. http://fsae.com/groupee_common/emoticons/icon_smile.gif