| Re: [GMCnet] New RV Buddies article [message #112780] |
Sun, 23 January 2011 16:21  |
Gary Casey
Messages: 448
Registered: September 2009
Karma: 0
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Senior Member |
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Jim,
Good question, and it's true, but.... The objective of the braking system is to
be ABLE to lock up all 6. It's up to the driver to use that ability. And the
peak friction coefficient on dry paving is about 0.9 (maybe 1.0 for car tires
and even higher for race tires) and the friction of a locked wheel is only a
little less - maybe 0.8 to 0.85. So, yes, you would like to be able to apply
all the brakes just to the point of locking, but not quite. How do you do that
in the real world (without anti-skid) - and do it equally on all 6 wheels?
Answer is, you can't, so for braking on dry paving the best bet is simply to
lock them all up and see what happens. No, you won't be able to maneuver, but
the statistics I have seen say that it is better just to lock 'em up. As usual
though, every potential accident is unique. And the weight of the vehicl
doesn't change the objective. 2,000 pounds or 200,000 you still want to stop up
to the limit of the tiresw whether you have 4 or 18 wheels. Now for 2 wheel
vehicles it's a different story...
Gary
I only pose one question. Why would any reasonable, thinking, person want to
"lock up the brakes" on a 12.000 pound vehicle? The shortest stopping
distances are achieved at some point before wheel lock up occurs.
Jim Hupy
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| Re: [GMCnet] New RV Buddies article [message #112794 is a reply to message #112780] |
Sun, 23 January 2011 17:21  |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Gary, your ending comment reminded me of a Greeves Challenger 250 cc
Motorcycle that my Father in law had shipped in a crate from England from
Hyland-Crowe LTD., the exporter of them. It came with the most agressive
knobby tyres that I had ever seen, and the knobs extended clear around to
the sidewall of the tyres. The bike had earles type leading link front forks
that used hydroelastic biscuit in shear for impact absorbtion. The brakes
were of the internal expanding single leading shoe type and were cable
actuated. The cables traveled in a rather loose fashion past the front
knobby. When the front brakes were applied, under some circumstances, the
cable would come into contact with the knobby and the rotational motion of
the front wheel and tyre would fully apply the front brake to lockup, hands
on the brake lever or off made no difference. After the dust or mud settled
from the resultant endo and the wheel stopped turning, the cable would be
freed up, and no clues were left to tell what happened. I had the front
brakes off the bike several times to try to determine what caused the brakes
to lockup. Only when my brother in law went by me doing a front end wheeley
with the Greeves did we discover what was happening. Simple fix, once you
know the problem. Cable guides are a good thing.<Grin>
Jim Hupy
Salem, Or
78 GMC Royale 403 ( I still have a Greeves TFS and a bunch of spares)
On Sun, Jan 23, 2011 at 2:21 PM, Gary Casey <casey.gary@yahoo.com> wrote:
> Jim,
> Good question, and it's true, but.... The objective of the braking system
> is to
> be ABLE to lock up all 6. It's up to the driver to use that ability. And
> the
> peak friction coefficient on dry paving is about 0.9 (maybe 1.0 for car
> tires
> and even higher for race tires) and the friction of a locked wheel is only
> a
> little less - maybe 0.8 to 0.85. So, yes, you would like to be able to
> apply
> all the brakes just to the point of locking, but not quite. How do you do
> that
> in the real world (without anti-skid) - and do it equally on all 6 wheels?
> Answer is, you can't, so for braking on dry paving the best bet is simply
> to
> lock them all up and see what happens. No, you won't be able to maneuver,
> but
> the statistics I have seen say that it is better just to lock 'em up. As
> usual
> though, every potential accident is unique. And the weight of the vehicl
> doesn't change the objective. 2,000 pounds or 200,000 you still want to
> stop up
> to the limit of the tiresw whether you have 4 or 18 wheels. Now for 2
> wheel
> vehicles it's a different story...
> Gary
>
>
> I only pose one question. Why would any reasonable, thinking, person want
> to
> "lock up the brakes" on a 12.000 pound vehicle? The shortest stopping
> distances are achieved at some point before wheel lock up occurs.
> Jim Hupy
>
>
>
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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