Society of Automotive Engineers paper on brakes and fluid temps

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Tom F&L GoR

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Delphi has written a few papers on brake caliper components; surprisingly the design is still a lot of trial and error, so papers like this, where they create a computer model and compare results to measurements from a vehicle, are still fairly leading-edge.

The paper uses multiple braking events during a mountain descent with periodic cooling periods as the simulated case. Design targets are to keep the brake fluid below 140C, which is when wet brake fluid may boil. They point out it may not boil while driving, but up to 10 minutes after stopping, due to heat soak back into the caliper body.

In this paper, front wheel braking power is 7 times that of rear wheel braking. (Don't take the number of 7 as gospel, but it points out that the fronts do most of the work.)

Figure 8 shows measured temperatures; disc at 475C, pad surface at 475C, caliper bridge peaks at 150C, brake fluid at 120C. The fluid temp hits it's peak after the vehicle is stopped. These are with a cast iron caliper and semi-metallic pad. The computer model predicts well, as Figure 9 shows.

First simulation (figure 10) is to change to an aluminum caliper. Now the caliper bridge peaks at 200C and the brake fluid at 160C. The higher heat transfer rate of aluminum over cast iron pulls heat into the fluid, and this would predict a fluid boiling situation.

Second simulation (figure 11) is to keep the aluminum caliper and change to non-asbestos pads. Non-asbestos pads have much lower heat transfer rates, so the disc surface temperature rises to 525C, the pads surface to 500C, but the caliper bridge to only 110C, and keeps the fluid below 100C.

Third simulation (figure 12) is with 2/3 worn down non-asbestos pads. Now the pads conduct more heat, so the disc peaks at 500C, the pads surface at 425C, the caliper at 175C, and the fluid at 150C.

Summary: Use of aluminum calipers will cause higher brake fluid temperatures than with cast iron calipers. Use of non-asbestos pads will produce much lower brake fluid temperatures compared to semi-metallic pads. Worn down pads will allow higher brake fluid temperatures than new pads. Soak-back may boil fluid even after stopping.

What did we get out of this? (1) We're not going to change aluminum calipers back to cast iron, I know. (2) Cool down laps are important. (3) When you shop for pads, maybe the right question is how good of a heat barrier (insulator) are they rather than how much heat can they physically tolerate. Sometimes pads are "rated" as to what temperature they operate best at, which is important, but how much of that heat they transfer is also important. If you had two pads that "work" at 500C, you'd want the one with a lower heat transfer coefficient. (4) Another thing is that worn pads will transfer more heat into the caliper and brake fluid. Some of you knew this from experience, some of us found out by accident, maybe some actually blamed the pads as being no good. But pads that are 2/3 worn should probably be retired from track use.
 

ViperRay

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Good information Tom.

I wasn't aware of the heat soak problem with the brake fluid.

With the brake fluid recirculator system I have, it means I'll want to pump the brakes a little after I stop for a few minutes :laugh:
 
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Tom F&L GoR

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Chuck, this system has two sets of brake lines and a special valve. When you step on the pedal, fluid from the MC travels to the caliper like normal, but when you lift your foot, the hot fluid from the caliper comes back through a second set of brake lines. There's no cooling, really, just replacement of brake fluid so it doesn't get overheated.
 
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Tom F&L GoR

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Check into the link in my post above. It's a dual check valve system that was "discovered" for NASCAR short track races, where brake heat is tremendous. Think if it as bleeding the brakes while you drive, just replacing a little of the fluid every time you pump the pedal. No difference in pedal feel, fairly straightforward to install. Not sure about use with ABS, though.
 

Steve-Indy

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Thanks for the great links, Tom...one wonders about the plumbing necessary to adapt this to ABS cars...noting that the system shown was NOT for use with ABS, as I understood the text...does that mean it takes another type of valve?
 

Nadine UK GTS

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Just got a chance to get to the VCA board and browse through posts, (shhhhh...I'm at work, but I've been rushing around and working long shifts all over Christmas!) Anyway, interesting reading, thanks for the links Tom.

Hey Tom, are you interested in making up any such brake fluid recirculating kits? It's something I've been wanting to do ever since you told me about it, after your rear 40mm caliper upgrade, but I've just not had the time to give to it! If need be, PM or email me, as I'd like to do this. Thanks and Seasons greeting to you, Nadine and '97.
 

GTS Dean

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You can obtain a Viper brake fluid recirculator from Woodhouse. The best way to ensure survival of the stock brakes is to use AP600, Castrol SRF or other top-quality REAL RACING FLUIDS - and keep your foot off the pedal so they can cool between applications.
 
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Tom F&L GoR

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Nadine, I will find my old notes and diagrams and send you those details. The only "custom" part is the brake line "T" that connects what were the two brake bleeder screw locations and routes the fluid back up to the special valve. The picture in the link below makes it very obvious what it has to do; one needs to look at the caliper and decide what direction to aim the fitting so that the brake line doesn't rub or get pulled during suspension travel. The fitting then needs to be soldered or welded on with care. I silver soldered mine and it has done fine. You also have to bend a few replacement hard lines at the master cylinder, but that can be done with purchased short tubing.

I copied the information in the link here:

The idea itself is patented, so you can read about it here and also here . In layman's terms, their operation is described about half way down this page. The valve itself costs $139 from www.cvproducts.com

You'll also need three flex brake lines for the fluid return. I measured out (and successfully used) two 22" and one 42" length on my Gen 1, which cost about $11-$13 per line. You'll need a "t" fitting and some other small hardware, which for me came up to about $90.

The difficult part is that you have to make one of these yourself, since you can't buy them for Viper Brembo calipers. It was easy (using an 8" metric line, bending to fit) then silver solder a -3 fitting in the right place. Obviously you want to make sure it's leakproof, headed in a direction so the brake line doesn't rub, and stiff enough that it doesn't break off. (Hint: solder on the fitting first, then drill the hole through the fitting and into the tube.) I made mine more like the one on page 36 (bottom right corner) of this catalog.

Here's a diagram of the setup; I have the fluid inlet in the stock location and the exit at the two bleeder valves, as in diagram 1.

Do-it-yourself types can do this for $250 or so, if you want to go to someone that's done it before, then Woodhouse can sell it to you.

The other "cool" thing is that now you can bleed all four screw locations at once by removing the return line to the valve.
 
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Tom F&L GoR

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Chuck, I don't know the exact ID measurement, but AN-3 lines are common for brake lines. Use stainless? Not sure which line you are referring to.

For the new "return" lines I used steel braid covered teflon lines (what people call steel braided lines.) They are therefore all "flexible" which helps with installing them.

At the master cylinder to install the 2-way valve, I bought several sizes of of pre-made lines so I didn't have to futz with the flaring the ends.

The "T" on the caliper can be made from an 8" length (the shortest one they sell at the parts store) of metric brake line. Bend it to fit from bleeder hole to bleeder hole. Then decide where you want the third leg (back to the valve at the master cylinder) to attach. I filed a slot into the bottom of a fitting so it would straddle the brake line. Wled or solder this fitting on your bleeder to bleeder pipe. LASTLY drill through the fitting and into the pipe to make your "T" connection. If you drill first and then weld or solder, you might plug the hole.

By the way, Chuck, this only works with 40mm rear calipers... ;)
 

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