Posted by Hnry on February 20, 04 at 11:39:51:
In Reply to: Re: wheel cylinders - skid all FOUR? posted by Ben on February 19, 04 at 10:37:27:
Remember the Astro van? It came out before abs systems were cheap, so didn't have one. It was very nose heavy , so to prevent rear brake lockup Chevy engineers weakened the rear brakes a lot. But there was an untintended consequence. I found the following description in a complaint in NHTSA's files. There were many others very similar. "I started up my Astro and drove down the block to pick up neighbor's kids for carpool to school. I pulled into their driveway and applied brakes to stop in front of their garage. The Astro wouldn't stop. It kept moving, slowly, for another 10 feet and finally came to a stop just inside the garage -fortunately the garage door was open and the space was empty, so there was no damage or injury."
What had happened was that the front tires of the Astro were on a patch of ice that had formed overnight under the eave of the garage roof, so the front brakes were ineffective. The rear tires were on clean concrete. The engine, having just been started, was on high idle and was able to turn the rear wheels with enough torque that the designed-to-be-weak rear brakes couldn't stop them from turning. They verified this later by putting the Astro up on stands at the dealer, starting the engine cold, and applying the brakes. Couldn't stop the rear wheels from turning!
Midwest utility companies had bought thousands of Astros, had complained to NHTSA, and there was heavy pressure on Chevy to put bigger dia wheel cyls, but they never did - probably because insurance companies would pay a few bucks each for the low speed rear-enders, while GM would have to pay big court settlements for rollovers if the vehicle had strong enough rear brakes to make it oversteer.
I think that the reason that we have some vehicles with weak rear brakes could be a hangover from that pre-abs era. Or, maybe they continue to design the rear brakes weak so that the rear abs won't be noisily activating all the time and scaring some drivers. I know it wakes me up when mine activates. It has also saved my a** a couple times.
Hnry
: Agree that the OEM bias is not very good.
: Issue is that by moving closer to 50/50, there is greater potential for it to over steer. Worse when towing, as jack knife in an instant.
: I've raced many types of vchicles, but never a truck. Plus never a truck this sized.
: Have driven Ford Expeditions. Three of them. One live axle rear, other two IRS. All high end with all orderable bells and whistles. All were brother-in-law's, he's manager of a Ford dealership. All three oversteered at the limits. Transition is not very forgiving either (meaning abrupt with little warning).
: They like the "sports car" handling and main selling point. WRONGO for the types who mainly drive SUVs.
: Consider myself knowledgable and able to handle myself on the track and on public roads. Those Expeditions, especially the first one, caught me by surprise and think someone without track time would have spun it and totalled or hurt or killed. Recovery was NOT easy either, as with the very high CG, it was pitching wildly, even with rear anti-lean bars and tires aired above sidewall list by about 10psi (knew was going to be messing around with them).
: So, think GM or any SUV manufacturer will do anything to keep away from that very small percentage chance of oversteering...aka liability!!!!
: When read/hear of folks wanting/going to rear anti-lean bars and how it "handles" better (aka, more like a car), just shake my head and want to know where they drive so I'll be far from them...
: Anyway, back on topic. Yes have been noodling this ever since first Expedition skid and also finding that my fronts were wearing pads fast, one set of Performance Friction Z's lasted less than 8K miles. Confirmed positioning of thoughts after getting the rears hot enough to burn off the paint one trip coming back from Lake Tahoe. Had set the E-brakes a bit to help as the fools in front of me were overloaded and weaving/swaying all over the place (Ford F150 towing humongo TT AND GMC 1500 ditto too big TT...they seemed to aline themselves along highway 80...all the time).
: In playing with various rear adjustments found slight drag best, for me & my setup. Actaully needed new rear shoes at around 55K-60K miles.
: Finally, note that when towing the rears actually do more than most think. The same weight transfer on the TV to the front axle while braking happens to the TT....but....it's tongue then places that weight transfer to the TV's rear axle...keeping the TV from transfering ANY weight to it's front axle...