Which wheels should skid first

[gary j zingle]

"Interesting logic, but counter to my understanding of the physics
involved.
Once a tire starts to slide on pavement, you've lost control of that
contact
patch. If the rears lock first, you still have steering control. If the=

fronts lock first, you've got an E-ticket ride coming."


While it is true that if the fronts skid you have lost steering control
locking
the backs can be even worse.

At least if the fronts lock up you can still see where you are going
because
you will still be going forward.

If the back tires lock up the back end can swap ends with the front real
fast.

So fast that you have trouble watching where you are going through the re=
ar
view mirror.

Probably less of a problem with the GMC. If even one wheel back there
keeps turning the back end should stay behind where it belongs.

The road conditions up here in the frozen North provide some interesting
opportunities to test these things out but I prefer to leave my testing i=
n
the past.

Therefore I tried to insure that the modifications to the GMC would not
result
in overbraking the rear.

Different road surfaces and conditions may reduce the likelyhood of
having the back slide around but where I live that is what I learned.

Regards

Gary Zingle
1973 GMC 26 foot
Edmonton, Alberta, Canada

 

[dave lowry]

Great post, Rick.
Clear, succinct and aaccurate.
Can't wait 'til tomorrow morning to read the dissenters views.
'Nite all, Dave

>
>> While it is true that if the fronts skid you have lost steering control
>> locking
>> the backs can be even worse.
>
>Gary,
> I have studiously avoided the ongoing brake debate, as it seems to me much
>of the disagreement is of the "blondes are better than brunettes" variety:
>ie: not amenable to scientific proof. Others are throwing facts at each
>other even though they basically agree on everything but terminology.
>Sheeesh!
>
> However, you have made a very good point which deserves reinforcement:
>Ideally, neither front nor rear wheels would skid, but if one or the other
>MUST skid, don't let it be the rears. This reminds me of the old adage about
>pickup trucks: "There are two kinds of pickup truck drivers; those who have
>slid off the road, and those who are about to." This was primarily due to
>the fact that a brake system adequate to stop a fully-loaded pickup would
>always have too much braking power in the rear when unladen. Nail the
>brakes, and around she goes! That's why pickups were among the first to
>apply anti-lock braking, and the only vehicles to use it just in the rear.
>
> I use pickups only to illustrate a point: lockup of both (all) rear wheels
>is to be avoided at all cost. Someone here pointed out that maximum braking
>is developed just short of lock-up, whereas once skidding a tire provides
>much less braking. This of course is axiomatic, and is the reason skidding
>rear wheels are so dangerous. When they start sliding, they'd just as soon
>slide sideways as ahead, and will try to pass those hard-braking front wheels
>right quick. Some have worried about loss of steering control if the fronts
>slide in a turn, but think what would happen if the REARS start to slide in a
>turn. In the former case you have understeer, where the vehicle continues in
>a straight line nose-first. In the latter you get oversteer, where the rear
>slides toward the outside of the turn, which can be exciting for most
>drivers. ;-) In both cases the driver must release the brakes to regain
>control, but with oversteer, (s)he must ALSO countersteer to catch the rear
>end before it gets ahead of the coach. BTW, we often try to use ONLY the
>rear wheels to brake on ice and snow, but that is different, as then the
>front wheels are doing NO braking and so stay ahead of the rears no matter
>what the rears are doing, and we never brake in a turn on ice.
>
> The combination valve in our GMCs and other vehicles does block
>appllication of the front brakes until the rears have received approximately
>130 psi of fluid pressure (GMCMH Manual X-7525A, pg 5-3). However, this is
>NOT to prevent front wheel lock-up on initial braking, but rather to overcome
>the force of rear drum brake return springs and get the rears working.
>Without it, the fronts would do ALL the braking in very gentle stops (most of
>the time for many drivers) and wear the front pads out prematurely. (This
>was a problem on the first disc-braked American cars.)
>
> Note that the combination valve is supposed to "blend" to full line
>pressure around 400 - 600 psi (panic stop pressure runs around 800 - 1000
>psi, IIRC), according to the same page of the manual. Above 600 psi, front
>and rear SHOULD get exactly the same pressure (although it sounds like Arch's
>don't). Note too that the combination valve is removed (or its hold-off
>function disabled) when installing rear disc brakes.
>
> I concede that locking up all 4 rear wheels (and hence losing directional
>control) is unlikely on a GMC. Still, I want the fronts to lock first if
>lockup occurs.
>
>HTH.
>
>Rick Staples
>'75 Eleganza
>Louisville, CO
>