Rick,
Very thorough explaination, you should do a "white paper" on the subject
and let people not on the net make their modification decisions based on
this input. Thank you.
Jim Bounds
www.gmccoop.com
>
> To all,
> Sorry, but I've just gotta jump into this discussion with some brake system
> basics.
>
> 1. Disc brakes vs drum brakes: When drum brakes overheat, the drum tends to
> expand and "bell-mouth". (The open side of the drum expands more than the
> side with the mounting web.) This causes the shoes to lose contact with that
> open side, reducing the shoe contact area, accelerating lining fade, etc.
> Things go downhill fast after that, and so do you. 8-0
> By contrast, the disc brake disc (or "rotor" as the ever-pretentious American
> manufacturers like to call them) gets much hotter than a typical brake drum
> (Ever see pictures of a race car at night, its brake discs glowing cherry
> red?), but they don't distort in such a way to lose contact with the pads.
> Also, the disc brake pads operate at much higher presure per square inch of
> lining. This makes them less susceptible to fade due to reduced coefficient
> of friction from heat. Further, they can cut through a film of water to stop
> better when wet, and their harder denser material doesn't absorb water as much
> as typical drum brake linings.
>
> 2 Disc brake disadvantages: Aside from higher initial cost, the main
> problem is the difficulty and expense of adding a parking brake to a disc
> brake caliper. Since the pads require very high pressure to apply them, you
> must contrive a very strong lever system with great mechanical advantage.
> (The service brakes just use a really big piston with large surface area.)
> This mechanism must somehow apply pressure more or less concentric with the
> hydraulic piston. Some cars (Subaru comes to mind) use a pushrod right into
> the cylinder, but this can cause leaks. I once owned a Citroen with a
> completely separate set of pads operated by the hand brake on the front discs.
> Some cars (Corvette, Porsche, et al) put a separate drum brake in the middle
> of the rear brake discs just to get a parking brake.
> One other problem which has come to light recently is brake fluid boiling
> due to the higher temperatures involved. This of course causes total loss of
> braking, and excessive excercise of the driver's sphincter muscles! Metallic
> and semi-metallic brake pads, introduced to replaced the now-banned asbestos
> friction material, do a fine job of providing friction at high temperatures,
> but they transmit much more of that heat to the caliper piston and the fluid.
> (Asbestos of course is a good insulator.) Numerous accidents have resulted
> from the EPA's drive to clean the air of the last particle of asbestos. This
> is why we all go to our friendly Ford dealer to buy their super high-boiling
> point brake fluid. (You do know this, don't you?)
>
> 3 Duo-servo plusses and minusses: The rear drum brakes on our GMCs (as on
> most American cars for the last 40 years) are Bendix duo-servo brakes. These
> brakes derive much of their application force from the forward rotation of the
> drum. The wheel cylinder (or E-brake lever) pushes the leading shoe out
> against the drum, whereupon it is dragged in the direction of rotation
> slightly. This motion is transmitted through the adjuster strut to the rear
> shoe, pressing it firmly against the drum. This allows a simpler hydraulic
> system with smaller wheel cylinders, and at least the possibility of stopping
> should the power booster fail. OTOH, when things start to fade, that fade is
> multiplied, just as the apply force was, so........
>
> Bottome line: If you drive in the mountains and/or with a heavy load or
> toad, the rear disc brakes may reduce your chance of brake fade. Otherwise,
> you shouldn't need them IF your stock system is in good condition, properly
> adjusted, etc. The GMC was noted for its great brakes when it was new, we
> just need to get it back to that condition, IMHO.
>
> Returning the soapbox to the next speaker,
>
> Rick Staples,
> Burned-out certified Master Technician.
atri63
