brakes: how hot is too hot?

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DougJ

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Joined
Mar 7, 2005
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549
Hi folks,

I've been chasing a problem of dragging brakes for a year.  I've tried most every fix available--but that's not the point in this post.

In this post I'm asking: how hot is too hot?

I've just bought an IR thermometer (haven't attempted to calibrate it even roughly) and today after about a 100km run at highway speeds ending with a 10 to 20kph run down a 5%??? hill into a ferry terminal the front disk rotors were pretty even at about 260F.  After getting off the ferry with a 30km run at about 70kph and a little more braking activity along the way, the rotors were 460-ish after pulling into my driveway.

Are temperatures over 400F too hot?

Any commentary is appreciated.

TIA and Ciao,

Doug
 
Doug,

The length of your trip and speed don't matter much; it's how much you used the brakes during the last few miles/minutes of travel. Linings/pads and rotors can handle 400F without any trouble, but if they're reaching that temperature with just normal stop and go braking (no long grades or heavy braking) you may want to check for drag. Brakes can 'fade' with heavy use, but it's normally because the brake fluid is boiling and not a problem with the brakes themselves. Always use high-quality, new (never used) brake fluid as specified by the manufacturer.

OOPS! Almost forgot: Some brake pads can out-gas under extreme use and will form a thin layer of gas between the pad and the rotor, keeping the pad from actually contacting the rotor. It will feel like a hard pedal but little braking is being done. Fluid boiling will produce a soft pedal with little braking.
 
Doug,

A question - did you start having problems with drag after you replaced the rotors or pads? If so, here are three more possible problems you may want to look into: 1. The new pads were too thick,  2. the new rotor(s) were too thick,  or 3. you didn't 'bed the brakes'. For the first two, check the shop manual for the proper pad and rotor thickness. Check with a micrometer. The rotors can be turned if too large, but pads need to be replaced. For the last one, bedding the brakes is a technique not normally done, but can be very effective. New or used rotors are not as flat as they may seem. They contain hills and valleys and when you replace either pads or rotors or both, it is wise to match them up; pad to rotor. It's a simple process. Accelerate to a speed of about 30 mph, then step on the brakes quite firmly while keeping your foot on the gas. Release when you're almost stopped, accelerate back to 30, wait several seconds to cool the brakes, then repeat about 8-10 times. You can use this technique to try to reduce drag on old pads/rotors, but it won't be as effective because a glaze will have been built up on the rotors. This can be removed by having them ground, but may not be necessary. 
 
Hi Karl,


The length of your trip and speed don't matter much; it's how much you used the brakes during the last few miles/minutes of travel.

Well, I could well have used the brakes more in the shorter trip after the ferry than the longer trip before the ferry.  After the ferry, it's not highspeed highway travel, but a relatively narrow, somewhat windy road, with the ferry load of vehicles speeding up and slowing down almost all of the way.  But here I may be reaching for an explanation of the higher rotor temperatures at the end of the shorter trip.  I try to use the brakes as infrequently as possible, and do this by "reading" the traffic four or five vehicles ahead of mine.

you may want to check for drag.

Well, my friend, I've been doing that for a little over a year now, and no one seems able to truly fix the problem--or perhaps there is no problem, it's just me worrying about hubs that are hot to the touch when that is quite normal.  Of course, they are not always hot to the touch; sometimes they are just barely luke warm.

Always use high-quality, new (never used) brake fluid as specified by the manufacturer.

I don't do this work myself and have to depend on the service places I use--not the jiffy joints, BTW--to use good quality materials; certainly I'd expect a Ford dealership to use the Ford recommended products.

because the brake fluid is boiling

Would you know offhand what the boiling point of DOT 3 is (which is what I think is recommended for the brake fluid)?

Thanks for your interest in this question of mine.

Ciao,

Doug
 
Doug,

Dot 3 requires that the dry boiling point be 401F or higher and a wet boiling point of 284F. Because brake fluid is somewhat hygroscopic (absorbs water), the boiling point is reduced from the dry boiling point temperature proportional to the amount of water it has absorbed; up to a maximum of about 3%. Incidently, it is wise to continue drive a vehicle for several minutes after heavy brake usage. If not, the hot rotor can continue to transfer heat thru the pads to the calipers and brake fluid and cause fluid boiling and a reduction in caliper piston seal life due to the excessive heat.
 
Karl, thanks for this info:

Dot 3 requires that the dry boiling point be 401F or higher and a wet boiling point of 284F. Because brake fluid is somewhat hygroscopic (absorbs water), the boiling point is reduced from the dry boiling point temperature proportional to the amount of water it has absorbed; up to a maximum of about 3%. Incidentally, it is wise to continue drive a vehicle for several minutes after heavy brake usage. If not, the hot rotor can continue to transfer heat thru the pads to the calipers and brake fluid and cause fluid boiling and a reduction in caliper piston seal life due to the excessive heat.

Do you have an IR thermometer? and if so do you check your tires and rotors, etc., at stops--even on a from time to time basis?  And if yes, what sort of temperatures do you typically observe after normal driving and normal stopping?

Ciao,

Doug
 
Doug,

Do indeed have an IR thermo. Radio Shack had them a couple of years ago, but I can't find them listed anymore. Around $30 IIRC. Temp of rotors varies considerably, but seeing as I don't stop immediately after a long downhill, they've had a chance to cool down some (see, I take my own advice) but 250-300 is not uncommon. Normal driving;  150-200.
 
Hi Karl,

but 250-300 is not uncommon. Normal driving;  150-200.

Thanks for this.

Seems my second temperature of 460-ish is way too high; even the first temperature of 250-ish could be high.

Looking back at an earlier post of yours, I'm wondering if I'm just too gentle with the brakes.  The brakes on my minivan last for ages I think because I don't tailgate (have lots of vehicles cut in front of me) and get my foot off the gas well before I HAVE TO brake.  In the case of the rig, may be they need some use to be sure the pads are out-gassed and properly seated.

Ciao,

Doug
 
Doug,

The out-gassing isn't something you can eliminate once and then forget about it. Depending on the pad material, even old pads can out-gas, but this is rarely a problem and usually only happens under extreme braking and very high rotor temperatures - not something to worry about.

460 is on the high side, but shouldn't be a problem if it's only the rotor. When you measure the rotor, also measure the caliper 'cause that's where the fluid boiling can occur. When coming down a long grade, make sure you aren't riding the brakes. That can cause them to heat up considerably. It's better to apply them moderately from time to time to slow down rather than using them constantly and lightly to maintain a given speed. When you do that, you don't give the brakes a chance to cool down. 
 
460 is on the high side, but shouldn't be a problem if it's only the rotor. When you measure the rotor, also measure the caliper 'cause that's where the fluid boiling can occur.

I was definitely measuring the rotor, Karl; I'll make sure to measure the caliper next time.  Also, my plan is to measure both wheels, hoping and expecting the readings to be more or less even.

When coming down a long grade, make sure you aren't riding the brakes.

My practice is neither to ride them nor pump them.  I will gear down at the top of the grade (on the principle of don't come down any faster than you could be going up) and then use the brakes decisively as often as may be required.

Ciao,

Doug
 
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