light duty truck brake 4

[LAL2004]

For a light duty pickup truck, what is the brake duty cycle?
Going down a long 6% grade, with the brake pedal lightly held with a constant light pedal force only to maintain freeway speed, no load in truck except driver, my 1/2 ton 2005 Chev Silverado truck brakes begin to chatter slightly with increasing excitation, then after about 1/4 mile distance they shudder increasingly more and are audibly louder as heard in the cab. Note that ABS is not engaged (pedal has no ABS pulsation) and the roadway is smooth. No other automobile we have owned nor does our other vehicle (2006 SUV) exhibit this symptom. Brakes are front disc and rear drum (Bosch, I believe) and have been replaced, remachined, etc. under warranty but the problem remains.
Dealer now advises to downshift to 3rd gear to use engine braking, but I believe trucks, meant to be loaded, should have sufficient brakes to enable light pedal force-downhill braking, especially unloaded!
Brake design/test engineer suggestions are most welcome!! Thanks.

 

[GregLocock]

How long is this long grade?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

Your rotors either have hot spots or are warped. Hot spots are not permanantly removed by re-machining. What is the history of those front brakes?

 

[MikeHalloran]

Same results with the ABS system fuse(s) removed?



Mike Halloran
Pembroke Pines, FL, USA

 

[LAL2004]

Hi to all who responded to my post!! Thanks! Still wondering if SAE or someone can describe industry standard brake duty cycle for this type of vehicle (light duty half-ton full-size pickup). In original post, I should have stated that the steering wheel does not vibrate/shake as warped rotors would/should cause.
Answers to several questions I received are: 1. Length of downhill 6% grade is >1 mile, but I only keep the brakes on for approximately 1/4 mile at a time, in order to let them cool a bit before re-applying. 2. Re: Rotor hot spots: Problem existed when truck was new. Dealer has installed new front rotors and machined the rear drums, then re-machined the front rotors under warranty, and replaced rear drum brake adjusters. 3. Have not tried ABS system fuse removal, which is a good idea.... am applying only light pedal force; feel no ABS pedal pulsation as ABS should produce if ABS is in effect. Any follow-up ideas are appreciated. Thanks again, all.

 

[Fabrico]

Dealer has installed new front rotors and machined the rear drums, then re-machined the front rotors under warranty...

Was the re-machining done for the problem described? That again points straight to messed up rotors. This condition is not always felt in the steering wheel and you can't machine the problem away. You need new rotors.

It happens to many kinds of cars and trucks. You just have to deal with it. If the problem comes back someday after installing new rotors, you may have to try a different brand of rotors and/or pads.

 

[patprimmer]

I suspect that if you get the brakes really hot then park the truck, you heat treat the spot under the pads differently to the rest of the rotor.

I have a situation where I drive down a mountain to catch a ferry. I always stop short and move forward a foot or two every few seconds so as to even out the cool down rate and the deposit of sublimated resins from the hot pads.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

You need to look up the FMVSS for braking. It isn't my field, so I'm not going to spend any time on it. The EEC regs are to hand, it specifies 6% at 30 kph, for 6 km.

Incidentally, I doubt it is the rotors themselves. I'd look at the pads.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SMOKEY44211]

I've had a number of customer complaints with the same circumstances you described on that style vehicle. The best method I have come up with is premium pads (ceramic or semi metallic) and new rotors. Drive the vehicle for a week or two and machine the rotors. The rotors come out of the box dead on but after going through a number of heat cycles they are always distorted. After machining they seem to run true for the rest of the brake pad life. My theory is the material of the rotor doesn't achieve a closest pack crystaline structure until after it has gone through the heat cycle. My explanation may be off target but people who are suppossedly "in the know" that I have talked to via my suppliers haven't really offered anything of substance to explain why it works. Not much comfort in your case as the dealer honoring the warrantee may not have the luxury of being able to try it.---------Phil

 

[LAL2004]

A big thanks again to everyone for the follow-ups and the original responses...all your comments and info are great.
I will digest the tips/tech info you have given me, and work on this using your input. I will re-post separately later if I resolve this brake problem, as it may shed some light on this topic for you responders + anyone who has a similar brake issue. I really appreciate your time and effort and hope I can do the same for eng-tips members!! Again, my sincere thanks! LAL2004

 

[Knap]

May be of interest

http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

http://www.powerbrake.co.za/downloads/tech_01_judder.pdf

 

[Rob45]

I've also had luck with simply taking new rotors and having them stress-rellieved and then re-machined prior to use.
What you're doing, riding the brakes down a long grade, is putting enough heat into them to stress-relieve them: initial stresses, when relieved, distort the rotor surface. Hence the need for re-machining them.

And no, distorted rotors do not always produce a steering wheel vibration; rotor thickness variations too small to be measured with an ordinary micrometer - e.g., less than .001" - will produce bad pedal pulsation, and no steering wheel motion.

 

[OP800]

The dealer has to use the factory pads and lining to be in compliance with their obligations.
I would try a different set of frictions since there has been no improvment with OEMs. The factory is not always best on everything.
The heat vs friction ramp rate is importent as well as the material.
I have replaced my frictions with aftermarket and would not go back to OEM now with such good brakeing. Some of the reason is towing nearly 12,000 lbs gross combined weight pulling a trailer with it's own electric brakes and a controller in the cab.
I use just enough braking on long hills to keep the truck at a reasonable speed as well as consideration to traffic ahead and behind.
OBTW it's a Ford.

 

[Rob45]

What model Ford truck?

You might call Dearborn and ask for Sandy Moody; she's the manager of brake development, and she might direct you to someone who could answer your questions and solve your problem.

Or not.

regards,
- R