Rotor and Drum Temperatures

[2002#3]

Gang,

 

'74 tii, if that's important...

 

Finished complete new brake system: rotors, pads, hard/soft lines, new handbrake cables, and upgrade to new 250 mm drums, cylinders, and shoes.  All suspended wheels seem to spin freely with minimal subjective drag.  Rears seem to be adjusted well.  Stops well.  HB works well.  Test drive was 10 miles of 30-60 mph with frequent stops (nothing drastic; no threshold).  Then decided to feel rims and perform temp checks with IF temp gun.

 

Observation 1:  L and R drums are ~110 F and ~125 F, respectively

Observation 2:  L and R rotors both are 120-170 F (depending on spot measured) and very hot to touch

 

I can provide better temp info if you like.  I do not anticipate any track days.

 

Questions:  Is measuring brake component temps a good additional way to evaluate correct brake adjustment and excessive drag?  What brake component temps might one expect on a street car?

 

I fully expect aswers to these question to depend on numerous variables and, probably, to have no firm answers.  I am simply trying to learn the utility of measuring the temps of these components.

 

Thanks.

 

Larry

[02Pilot]

If nothing's dragging, temps only matter as you approach the high end of the spectrum for failure, in this case boiling your brake fluid. You are very far away from that. Don't worry about it until you start getting up into the 350degF+ range.

[Simeon]

The presence of tools like the IR thermometer let us collect too much data now. With an objective measurement of the actual temperature of a rotor or drum, I have no idea what that actually equates to. 

 

On the basis of the subjective measurement of ‘put your hand on the wheel rim near the hub and it will be warm but not so hot you pull your hand away’ - how does it measure?

 

Brakes are designed as a means to convert kinetic energy into heat. They will always be hot  

[ray_]

I'd worry only if one were significantly hotter, or less hot, than the others.

 

[TobyB]

Quote

The presence of tools like the IR thermometer let us collect too much data now

 

Yeah, the good old days when we didn't know shit were much better!

 

But in a way, a number without a reference is pretty darned useless, it's true.

I can tell you that at much over 600f max at the rotor edge, you'll start having pad and cracking problems,

that 400f at the drum isn't a problem with Carbotec Green shoes, and that if one side of the car is in the sun,

your tire temperatures can vary up to 50f, and several psi.

 

I shot IR sensors at the tires when I was on track

a few times, and the data that came back from THAT was a handful to digest....

...but ultimately, after I made a few small changes and evened things out,

it merely corroborated what other sensors, including my butt, were saying.

 

At those low temperatures, it merely tells you that you have brakes. 

Like Ray, I wouldn't worry until you had a much larger differential between sides.

(no, you do NOT need a 188mm diff! heh)

 

t

 

[Schnellvintage]

A better gauge for your brake analysis, would be, if the car pulls, at any temperature and/or driving conditions.  Brakes get hot when they are being used, basic physics.  The brakes start to fade, when the brakes/pads and/or fluids can't handle the excessive heat..

At least that's how it was done before all these hi tech tools became so inexpensive.

 

[John76]

The best way to use a IR gun is to compare drums and rotor temps from left to right. The actual temp measured is subject to too many variables, such as the rear drums being painted have a much higher emissivity than the shiny surface of the front rotors. The field of view of the particular IR gun is also a huge variable (you are not measuring the surface temperature of the area of the laser dot!).

Good way to check the parking brake adjustment is to pull on the handle slightly while coasting at about 25mph. Then measure both drums with your IR gun and compare. Should be equal (or close to it). I found a dragging e-brake lever on the right rear....temp was 100 degrees higher than the left side.

[2002#3]

Thanks, kind folks, for your aforementioned comments, wise counsel, and sage advice.  My temps be low, so on I go, driving with toasty, dependable brakes.

 

Funny how these things work.  While driving an '02 for 38 years, I never thought about brake temps until...I thought about it.  Today.  Then, I really thought about it.  Then, I measured the temps (because I could) and tossed a thought-coin into the Fountain of 2002 Knowledge to see what you guys would say.  

 

Larry  

 

"It's always something."...

Roseanne Rosannadanna, SNL, 1975-1980

 

"...with '02's."

Me, 2018

[Simeon]

1 hour ago, TobyB said:

 

Quote

The presence of tools like the IR thermometer let us collect too much data now

 

Yeah, the good old days when we didn't know shit were much better!

 

 

I can simulate the good old days by still not knowing shit. 

[TobyB]

Yup, me too.

 

now, where did I leave that pizza...

 

t

[Einspritz]

7 hours ago, 2002#2 said:

Observation 1:  L and R drums are ~110 F and ~125 F, respectively

Observation 2:  L and R rotors both are 120-170 F (depending on spot measured) and very hot to touch

 

The first question is what are you using for front pads and rear shoes?

 

Did you have the shoes arced to the drums? If not, then you may not be getting the whole shoe touching the drum surface when you apply the brakes, particularly when they are new. Ergo the low temperatures.

 

The fronts are hardly hot enough to get the pad material into the highest coefficient of friction (Mu) e.g. MetalMasters (IIRC 250 Deg.F). As a comparison, I use Hawk Hp+ on my M3 and it has a Mu of ~0.5 at 100 Deg F. which is great for quick bite and pesky things like the stray animals or traffic. The friction changes as the temperatures go up, but not by much, all the way to something like 600 Deg. F. Many other friction materials don't get to maximum friction until 250 deg.F or more, and certainly much less than 0.5 Mu....probably like 0.3-0.35Mu.

 

There are only two or three shoe manufacturers (that I know of) that publish their Mu curves, e.g. Ferodo and those temperatures are much higher than 110-125F for maximum Mu. I think that at 110-125F you aren't probably getting them hot enough, even for the "no name" brand.

 

One other thing to consider with your observations is the temperature of the brake fluid. You really don't want to exceed whatever temperature that is on the bottle e.g. 400Deg.F and of course it will be lower as it absorbs water over time.

 

Finally, did you add a brake proportioning valve between the MC and the rear brakes? You might be in a condition where your rear brakes will lock up WELL before the fronts. Under some conditions you can do without that and change the rear wheel cylinders to a different diameter to change the brake force back there.

 

It is important to consider the brake calipers, pistons, pads, M/C, rear wheel cylinders and shoes as a system and not just a collection of components.

 

HTH

[percy]

Bear in mind that brakes are designed to shed heat.  Consequently, even if you could obtain temp readings from the same location for any given part, readings would need to be taken simultaneously, for accurate comparisons.  Ball park readings might be good enough to determine resistance or dragging, but at that rate, the hand test is probably just as reliable.

 

I recall years ago observing a test car with drums fitted with thermocouples that supposedly gave real time temperatures.  No one shared any results, nor was I privy to exactly how the sensors were fitted, other than I noticed wires and tape.  The pictured thermocouple is embedded in brake pads.

 

 

http://thesensorconnection.com/brake-sensors/all-brake-sensors/embedded-brake-pad-thermocouple-sensor  

 

 

 

[Einspritz]

You can drill the metal of the pads or shoes and affix them there. also you can drill holes in the rear backing plates to allow for more air to circulate.

 

Alternatively, use temp dots or paint to check the highest temperature observed.

 

If you want to go all in you can have pressure transducers on the brake lines too to check that all is equal.

[Mike Self]

I worked the 12 hr Sebring race in 1965 from the timing shack--located at the end of the track's longest straight, on a 90 degree bend--and was amazed at the brightly glowing rotors on the cars as they braked for the turn--even in the pouring rain (4" fell on the track in less than an hour that year).  Don't remember any rotors disintegrating or falling off, but I suspect they were discarded after the race!

 

For our 4 hour shift, my timing partner and I were assigned to time Hap Sharp's Chaparral--which finished second overall--behind Jim Hall's winning Chaparral.  Quite an adventure for a couple of college kids who had never time a big-time race before!

 

mike

[irdave]

I found a sticking e-brake because my e-brake lever was hot- it's usually cool...  Just something else to be aware of.