Brake Rotor Size

[Brian Smith]

How do most teams chose their front brake rotor size? What we normally do is just put the biggest rotor we can fit in the front (10.25" or so). The rear is sized to give proper balance due to weight transfer. Is this the best solution? A smaller diameter disc would weigh less and have smaller moment of inertia. It would also run hotter.

Do any of you look at your brake temps? How hot are you running and what size disc are you running? Are you using drilled or slotted disc?

Also how do you determine disc thickness? We just felt like 3/16" was a practical thickness to prevent warping.

Do most teams run a floating disc (allowing for diameter to increase and not put stress into mounting bolts) or solid mount?

[Mi_Ko]

Just take the biggest you can fit.

You'll gain much more if you have good brakes as if you woul'd use discs with smaller moment of inertia.

Anyone ever tried carbon fiber discs? (I know they aren't worth spending money on it, but aniway?) Delft??

Brian I think I have seen much more solid mount discs as floating ones. I've heard floating discs are good for the response on the pedal and a slightly better brake power and not to eliminate stress on mounting bolts. Is that correct?

[Brent Howard]

One of the California teams had carbon rotors last year. I think it was Pomona, but i'm not 100% sure.

Brent

[Dave M]

no carbon brakes for Cal Poly Pomona, just steel

Dave

[RaID]

our team went with 200mm disc at front lastyear
with small callipers and the brakes overheated
esepcially in the enduro event

this year were going as big as possible at the front which is 10", the like you the rear will be sized accordingly, and both disc will be floating

[Denny Trimble]

We've run as small as 7.5" x 1/4" discs up front, with Wilwood Dynalite Single calipers, with no overheating issues, and that was in our '99 car which had a 2nd place endurance time, with two large drivers at the wheel.

This year we're at 9" up front, 10" in the rear, 3/16" thick, floating, and cross-drilled. So far, so good!

[jack]

we have 9.6in rotors, in 10in wheels, a little less than a .25 thick, dont over heat, but never really worked that well in past designs. maybe this year...

[RaID]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Denny Trimble:
We've run as small as 7.5" x 1/4" discs up front, with Wilwood Dynalite Single calipers, with no overheating issues, <HR></BLOCKQUOTE>

one of the reason ours over heated is that the rear brake was pretty much doing nothing as it was usually covered in diff oil.

[Brian Smith]

RaID,

You said your fronts overheated. Do you know what temps they were reaching or what pads you were using. Most wilwood pollymatrix pads look good up to about 1300 F.

[RaID]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Brian Smith:
RaID,

You said your fronts overheated. Do you know what temps they were reaching or what pads you were using. Most wilwood pollymatrix pads look good up to about 1300 F. <HR></BLOCKQUOTE>

sorry cant tell you the temps, 03 team didnt take any temp readings of them, the calipers were 2 piston floaters of a Kawasaki 125 offroad bike and the during the event they had install air ducts since they were loosing braking, the air ducts improved the braking but it still wasnt very good, the pads were sinterd metallic

sorry about the vague info but thats all that was passed onto us, when we have used the car this year we havent driven it long enough for the brakes to over heat, however there definately isnt enough braking power

[TomF]

Hi, Using Carbon fiber brake discs is not an idea. In formula one Carbon-carbon brakes are used wihich is mat from a different material. It is pure Carbon reïnforced with carbon fire. Unfortunately working temp are extremely high. With FSAE cars You'd never reach it. You could use carbon fibre, but I don't see the benefit here. The stuff can't stand heat. So even Delft will use steel this year.
Greetings from Holland

[madman]

I did the brakes for the UWA car last year and I thought that I would add my two cents worth here.

I sized the front and rear rotors based on the thermal loadings as I found this to be the main constraint. To do this you need a model with a reasonable accuracy - I found that the solid rotor model described in Limpert (1999 Brake Design and Safety, 2nd Ed.) was pretty good. You just have to modify the model to account for your particular cooling airflow regime and the stopping frequency. FSAE can be pretty brutal because of the high frequency of brake application. The limpert model uses similar criteria to the ADR brake test and assumes the cooling during the stopping time is insignificant compared to the total cooling time - which doesn't appear to be the case for FSAE.

Other models worth a look at are: Mackin T.J. et al. (2002, Thermal cracking in disc brakes) and Jones R.A. (1995, Modelling the cooling of an automotive brake rotor).

With regards to making the system work. Do your hydraulic calcs!! And don't make my mistake of using less than quality components - it doesn't take much to make a pedal spongy. Recommended reading is Carrol Smith (as always) in 'Engineer to Win' about pedal box design and the whitepapers at www.stoptech.com (http://www.stoptech.com).
Other factors to consider with brake fade are material compatibility, rotor design and pad selection. Slots and holes will reduce fade and increase bite. Incompatibility between materials can make the problem insolvable. If your rotor material is too hard and your pad material too soft, when the pad gets hot enough for the adhesive friction regime to stop working (pretty hot), the rotor is relatively too hard for the high temperature abrasive friction regime to work - bye-bye pedal. Oh, and cheap pads are that for a reason. I've had our brakes glowing red-hot (very pretty) and still achieving friction coefficients of 0.4.

Ok, that's a bit more than 2 cents of company time.

Brian

[roadrunner]

Were running 230mm fronts with 250mm single rear on our new car. Drilled, floating and 5mm thick. As for over heating issues, difficult to tell as the car isnt built yet!!!!!

Last years car runs a similar size all round (230mm i think) and they`re all fixed. Will be interesting to see a comparison between the two. Especially as theyre running the same engines, wheels and tyres.

Is it possible to get enough heat into carbon brakes in events other than the enduro considering how short they are?

[Dr Claw]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Is it possible to get enough heat into carbon brakes in events other than the enduro considering how short they are? <HR></BLOCKQUOTE>

we used titanium rotors with ceramic coatings for '04 (got hammered in design semis, but thats another story and another solution)...and we didnt get up to temperature to UNWARP them. you probably wouldnt even touch the temp you'd need for a carbon setup. I'd like to know how anyone driving an Enzo stops where he wants to, when he rolls it outa his driveway in the morning.

anyways, all i have to say, is watch out for LTU's brakes for next year. it's going to be something no one has ever seen before http://fsae.com/groupee_common/emoticons/icon_smile.gif.

[The guru of guru's]

question to guys at adelaide...how heavy was your car?
thanks
dave.

[gug]

adelaide was either 238 or 248 kilos wet, cant quite remember.

will be a fair bit lighter this year. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[CornellGixxer]

We ran a 10" rotor up front and out back, both made from cast iron and drilled. Both were a floating rotor design with the front keys in double shear and the rear in single.

Although we got an IR temp sensor to mount on the car during testing to measure brake temps we never actually got a chance to use it before competition. From some testing done in '02 our front discs run between 800 and 900F on a warm day in the dry.

We've been running Wilwood calipers and pads for a few years now and they have performed excellently. This year we swapped between the A and E compounds and found that the only real difference was driver preference.

[madman]

I suggest carefully considering the accuracy of IR temperature sensors for brake rotors. I found the cheap IR sensors good for comparitive purposes, but not sufficiently accurate for good modelling.

Expensive (AUD$2000 per sensor) self calibrating, multiple-spectrum sensors work well (0.2% accuracy), but the cheap sensors assume a material emmisivity of 0.95 (which is roughly correct for organic compounds), however cast iron has a typical emmisivity of 0.55 and polished aluminium of down to 0.05.

The good part is that most brake pads leave an organic surface film on the brake rotor - thereby at least giving something like an organic emissivity. I conducted some hand-wavy tests and got accuracies of approx. 10% compared with a contact thermocouple (with Ferrodo DS2000 brake pads).

The cheap, accurate way to measure temperaure on the cheap is with an imbedded thermocouple (SAE standard J79 I think). You just need a slip ring to take off the temperature data (noise here is surprisingly low, accuracies within 1% are achieveable).

[Vector006]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by madman:
I suggest carefully considering the accuracy of IR temperature sensors for brake rotors. I found the cheap IR sensors good for comparitive purposes, but not sufficiently accurate for good modelling.
<HR></BLOCKQUOTE>

where'd u find the cheap IR sensors? we are looking for some for recording tire temperatures.

Would there be any advantage to adhearing an large washer anodized black aluminum ring (or equivalent) on the inner part of the rotor to record temps off, to get the right emissitivity?

Also...as far as sizing brakes...what front to rear biasing ratio have teams chosen and why? I sized them last year based on a 1G decel and sized them based on the load transfer only. Is there anything else i should be considering? And what rotor material has people used? We've had problems with glazing ours in the past.

And to Dr.Claw...sorry to bust your bubble, but cutting a hole in the floor pan and having the drivers wear espestos shoes has been done before...watch the Flinstones.

-jer

[madman]

I found cheap (AUD$200) sensors by trawling the net and scientific cataloues somewhere - sorry, I forget where.

Wollongong and UQ (I think) use IR tyre sensors. Where'd you guys find 'em?

Black paint on a non-scrubbed area of the rotor is a common trick. Just check that A) The paint can handle the temps, B) the measurement spot is not larger than your paint spot - cheap sensors tend to have wider angle measurement cones.

As far as deceleration numbers - do some testing. The judges will love you for it. Match your maximum braking effect (maximum utilisation of the available traction) to the maximum deceleration of your car (with some tweaking for driveability).

And sorry - you glazed your rotors? Not your pads?? It sounds a little bad pad match to the rotor/application/temperatures/something.

Brian

[Steve-Imperial]

Ive been advised by an aftermarket brake rotor manufacturing company to use 420 grade stainless steel for the rotors, mainly due to the fact that it is magnetic and so can be held in a magnetic chuck. Can anyone verify this advice?!
Our brakes will have 220mm non-floating rotors.

[Colin]

Doesn't SS have pretty ordinary heat transfer characteristics?, it is also bad for fatigue I'd be worried about cracks. I've never heard of 420 grade SS I know 2205 is magnetic where as 316 and 304 are not, hope that's some help

[Steve-Imperial]

Well we had cast iron discs on our old car which cracked so this makes me think that there must be a better alternative.
The company I talk about sells aftermarket discs for superbikes so I get the impression that this material cant be that bad!
Another company which specifies that they use it: http://www.galferusa.com/products/brake_discs.htm

Im thinking myself that cast iron is best suited to thicker discs whereas stainless steel has the durability for thin discs. That might be completely wrong though does anyone really know?

[jack]

mountain bike disc brakes are all stainless and very thin, i allways figured it was so they dont rust, but maybe there are other reasons? they get pretty hot too.

[MikeWaggoner at UW]

Cheap IR Sensor

http://www.omega.com/ppt/pptsc.asp?ref=OS540&Nav=temj04

[Vector006]

Thanks for the reply Mike, but i am looking for a cheap IR sensor that i can hook up to a Data Aquisition box (ie not hand held)...anyone know what price should i be looking to spend per sensor? Omega has some for $175 each...

[Igor]

Search for "thermopile" sensors, but I think it will be hard to find cheap ones. It will probably be cheaper to hack one of the previously mentioned handheld units.

For the quick and dirty method you could also just slap on one of those stickers that record the maximum temperature.

Igor

[Steve-Imperial]

The front brake rotors on our first car, which we're running at this years competition, have some small cracks in (about 8mm long). Will the scrutineers look for this and if they notice will they stop us running?
Just in case they do stop us we were thinking of making some reserve discs from mild steel (as advised by our head of dept.!). He said that they wouldnt be the ultimate but they would work, and that the only advantage of stainless over mild steel is that it doesnt rust. What do people think of that, has anyone tried mild steel rotors in the past?

[Garbo]

We're running mild rotors now... 10.5" dia, 0.125" thick. There have been no warping or cracking problems noticed yet... I will check again since the topic has been raised.

[Steve-Imperial]

Excellent thats what I like to hear! Since our budget has gone overdrawn we cant afford fancy materials like stainless!

[Vector006]

I was advised by Wilwood tech support not to use stainless. Im not a materials guy, but i believe it has a much lower thermal conductivity than mild steel. Having such a large temperature difference on the surface compared to the core may be the source of the cracks forming. Youd have to check this out with someone more knowlegable though. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Steve-Imperial]

I forgot to say that the rotors which are cracking are cast iron (with good thermal conductivity). The reason why theyre cracking is probably due to differing expansions of the centre and edge. I reckon with the thin brake discs used for Formula Student the conductivity of the material might not be that great an issue.

[MikeWaggoner at UW]

Most of the rotors I've seen with cracking problems are on aluminum hubs, bolted against hubs without room for expansion. The hubs get warm and create a high radial stress.

[Denny Trimble]

Yeah, we did some thermal expansion FEA and showed up to .010" radial expansion of the hub relative to the rotor, at operating (measured) temperatures. That's a pretty good argument to go for floating rotors.

Mild steel has worked fine for us for the past few years. We just haven't been able to get our hands on cast iron in the right size.

[Marc Jaxa-Rozen]

How does cast iron stack up to alloy steel? We used machined solid 4140 rotors and didn't have any problems, I'm guessing the difference is mostly based on thermal conductivity?

Marc Jaxa-Rozen
École Nationale d'Aérotechnique

[Draksia]

This year we will be using a mild steel 7.25 inch vented rotor on all four corners.

[Ben Beacock]

We used 3/16" mild steel, mostly because the laser cutting place would cut them free of charge. Otherwise, they have to change a bunch of settings to make a nice cut. They were hard bolted to aluminum hats and we had problems with runout. We'll be going with a floating design next year

[drivetrainUW-Platt]

Ben,
from the picts i took it looks like u guys were running the blue Wilwood cart calipers, is this correct? we had no success trying 3 different masters, they wouldnt even have come close to stopping our car, had bleeding issues too, but brakes killed us at comp this year.

[Ben Beacock]

We aren't very satisfied with the kart calipers. We have'nt disassembled them yet, but I think part of the problem is an internal spring to pull the pads back off the rotor. Might work in a kart, but we had a spongy pedal problem. Other brake problems nearly took us out of the endurance as we were using engine braking for the last few laps. The fronts were covered in CV grease and one of the rear CVs shifted, causing the rotor to start machining the caliper. Added to all that was a bending problem with the pedal assembly during the brake test. You'll see where we're spending alot of design time this year http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Schwanger]

What about aluminum brake rotors. Is it worth the effort and can you find the right pads?

[rjwoods77]

Wilwood PS-1 calipers. 7.1" MCP rotors. PS-1 calipers from disc centerline to edge closest to mounting hub is 1.18" Hard to find any dual opposed piston caliper that slim. Decent cast units. Dual 1" piston. Good retraction. I know the rotor size and caliper will work nice. Using 10 inch rims. Just dont know if the friction compound(only one available) is right for the competition. If it isnt then we can always bond different material to the pad plates. Saves 1.5lbs in caliper weight if you use these instead of a dynalite single. But you can get dynalites anywhere and they have a zillion pad compounds.

[running man]

For the people who are designing floating rotors: Who is your supplier for the floating buttons and where have you purchased them from? I have not had any luck finding a supplier of the button alone, without the entire assembly.

[madman]

Made 'em. Just machine simple buttons from a metal of your choice then fasten the the assembly with a circlip and washers. If you want to be really schmick you can use spring loaded wave-washers to stop the rotors rattling on their float.

Just note that your average washer is a cheap, stamped item and therefore has tolerance variations that are probably greater than your axial float.

Happy machining
Brian

[Chase]

I believe speigler was who we used last year, but ill check and make sure.

-edit- apparently they weren't who we used a guy on our team told me that for sure.

[Andy Horner]

We purchased ours from Spiegler the buttons, washers, and e-clips