On Sat, 15 Mar 2008 13:55:00 -0700, Bruce L. Bergman
wrote:

On Sat, 15 Mar 2008 08:57:03 -0600, "Pete C."
wrote:
Roger Shoaf wrote:
"Pete C." wrote...

Yes, but for a pickup in particular, the braking gets better with more
weight due to the much improved rear wheel traction.

The problem is not between the rubber and the road, it is in the brakes
themselves.

A brake converts the energy of the rolling vehicle into heat, and the heaver
the load, and the faster the vehicle is rolling, the more heat gets
produced. When the brake exceeds the ability to dissipate the heat that is
built up, the brakes cease to function.

This is why they have those runaway truck ramps on steep hills.

That is not the issue with a pickup.

Not normally, no - but a properly motivated idiot driver can get the
brakes hot enough to heat-fade to nothingness on almost any vehicle.

Passenger cars have enough reserves that getting the brakes that hot
takes a concerted effort. But pickups and small rental trucks are
often overloaded far above safe levels and taken down mountain passes
at "normal highway speeds" that are WAY too fast for conditions
considering the overload.

Actually, over the last few years this is not true. Most passenger
cars today have borderline braking capacity at best. They are made as
light as possible, and with ABS you never need/want enough braking to
lock the wheels on dry pavement anyway. I could fade the brakes out
on any of my last 3 cars in virtually NO time, without more than half
trying.
Overloaded on a mountain pass you can take out ANY amount of brake if
you don't know how to drive.In normal operation, trucks tend to be
closer to overbraked than most cars. (some sports cars and light duty
pickups excepted, of course).

And they don't normally pull over small pickup trucks and box vans
on a whim and send them to the scales if they suspect an overload, or
run them all through the regular highway scale stations and issue them
tickets for overloading. It usually only becomes an issue at the
coroner's inquest after the "K-injury" (fatal) accident...

The rear brakes on a pickup have
far more capacity both in braking action and in heat dissipation, than
the tires they work on have traction to the road surface without cargo
in the bed. Add significant weight in the bed and that braking capacity
becomes available.

And that's why good trucks (Toyota for one) put a simple and cheap
all-mechanical Load Sensing Proportioning Valve on the rear axle brake
line, with a ride-height sensing arm going up to the chassis.

Empty truck, springs full up, LSPV arm sets it to minimum pressure,
and the brakes cut off before rear wheel skidding really gets started.
With weight transfer to the front axle, you can practically lift the
rear wheels off the ground in a panic stop, and this would lead to
serious control and stability problems.

Full truck, springs full down, LSPV set to wide open, and the rear
brakes will do all they can to help. And if the CG is very high or
far forward of the axle centerline, and the weight shifts forward in a
full panic stop, the LSPV will still kick in to keep the rear from
skidding.

-- Bruce --


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