Home » Public Forums » GMCnet » Re: [GMCnet] Gmclist Digest, Vol 29, Issue 94
| Re: [GMCnet] Gmclist Digest, Vol 29, Issue 94 [message #108956] |
Sun, 19 December 2010 09:44  |
Gary Casey
Messages: 448
Registered: September 2009
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Lots of good comments on the braking system - most of which I agree - but there
are a lot of things not explained. Sorry if I don't correctly acknowledge each
one. For example, Some might mistake brake sensitivity for brake
effectiveness. Sensitivity means that decceleration is available for less pedal
force. But a vacuum booster is capable of only so much force - at what pedal
force does it reach its limit and is it capable of locking the wheels at that
point? Don't know and no one has said. Cars of that era often required the
driver to "push through" the booster to get a maximum performance stop. I don't
know what the situation is on our coaches with either system. One can go to a
smaller diameter master cylinder and get lower pedal effort, giving the illusion
of increased brake effectiveness. But if one side of the system leaks is there
still enough pedal travel to stop on the others? I don't know and no one has
said. And some have implied that with a heavy vehicle like ours longer stopping
distances are to be expected - I disagree. Stopping distance is a function of
tire-to-road friction coefficient and our hard tires have only a little lower
coefficient than car tires. Certainly less than 10% different.
There have been lots of explanations given regarding the behavior of the
standard system with a leading/trailing arm design and some are good and some
are needlessly complex. But some lead to incorrect conclusions (example
below). First, all the braking effort (torque) created by the brake goes into
slowing the vehicle - none is magically "lost" by being converted to vertical
force. And the middles never generate enough force to lift the rears off the
ground. The geometry is such that almost 2 G's worth of braking would be
required to do that. I would guess that the traction coefficient of our tires
is less than 0.9, about half that required. Even the friction generated by a
locked rear is still slowing the coach - it's not "totally wasted."
Yes, during normal braking (before the rears lock) the reaction of the middle is
balanced by the reaction from the rear - no net lift is created. If the brake
balance is even everything is fine. Until the rears lock, that is. From then
on the imbalance is only the difference in braking between the middle and the
rears.
Why was such a seemingly inferior design allowed to be produced? I was working
at GM developing an advance braking system during that period. The prevailing
attitude was: First priority of a brake system was low pedal effort during
normal braking. Also first was noise reduction - no noise allowed. Also first
was low rotor/drum wear. For disks another first priority was no dusting
allowed. Next criteria was that only one stop from the speed limit on level
ground was required. Going down mountains? Just go slow. Faster than the
speed limit? Not required. The fact that the rears will always lock
prematurely certainly wasn't considered to be a big problem as it didn't effect
any of the top priority items.
There was one poster that admitted he was descending a 6% grade at high speed
(at low speed the resulting panic stop wouldn't have been required). He
reported that the brakes were faded at initial application, so they must have
been almost faded already. It is important to know the limitations of the
braking system and all will fade, even disks. But he was right in that with 4
or 6-wheel disks he probably would have had a larger fade margin. (short
anecdote: I was driving my car down Pikes Peak in second gear, not in a
particular hurry. At the halfway point I stopped at the "brake checkpoint" and
a guy shot an infrared temperature sensor at my front rotors. He waved me on,
but I asked him what he read. He said "150", so I asked about the usual
temperatures he sees. He said, "500 or more." So most people, probably
unknowingly, had used up way more than half their brake heat capacity at that
point.) My point is that a lot o)f people are in this mode - "using up" too
much of the heat capacity of the brakes without knowing it.
The one big advantage of disks is that they have more heat absorption/rejection
capability than drums.
The question still remains - can all 6 be locked up with the standard system?
With a 4-wheel disk system? With a 6-wheel disk system? With a system
failure(leak) is there still enough pedal travel? With a reaction arm system on
2 wheels? On 4? Apparently no one knows for sure, or at least are unwilling to
post the results. When someone gets around to measuring stopping distance from
60 mph, I suggest measuring stopping time - it's easier to measure. And measure
pedal force, too. If a constant 0.9 G's is attained the stopping time from 60
is 3.04 seconds - I don't think you'll get better than that with non-racing
tires. Distance is hard to measure, as the starting point is difficult to
establish. Then if anyone wants to get really thorough, measure the G's
attained at all different pedal forces and plot the results. I'd really like to
see that.
Thanks for reading.
Gary Casey
Previous post:
The rear wheel brakes act just opposite to the mid-axle brakes in
that the force vector from the tire/road contact point is pulling
down in a straight line on the rear suspension arm pen. Same as the
mid wheel, only approximately 1/2 of the forces generated by the
brakes is pushing rearward horizontally and is helping slow the
vehicle. The downward vertical force on the pen is reacting the
nearly opposite vertical force generated by the mid wheel and cancel
each other out during normal braking conditions. Thus, during normal
driving only approximately 1/2 of your braking ability is being
utilized in slowing your vehicle and the other 1/2 is totally wasted.
Things get much worse when you do a panic stop. Within a fraction of
a second after applying full brake pressure, the mid wheel brakes
lifts the rear of the coach until the shock bottoms out, shifting
most all the rear coach weight to the mid wheels and lifting most of
the weight off the rear wheels. That is the screeching that everyone
hears making them think they have good brakes. It is almost
impossible to slide the mid wheels with drum brakes unless you have 2
3/4" wide CM shoes and much larger brake cylinders.
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