| [GMCnet] JUST W0NDERING & WONDERING & WONDERING [message #60324] |
Fri, 16 October 2009 12:50  |
Charles Aulgur
Messages: 78
Registered: March 2006
Karma:
|
Member |
|
|
Yesterday, Ken posted:
Charles,
A. It may be that my 10+ year old booster is not as effective as I
think it is, so maybe I'll try a new one before going to the
Hydroboost. But the numbers I've seen and developed myself indicate
that the line pressure with ANY vacuum booster will not be above
1100-1200 psi; that's not enough. I'm very anxious to SEE anyone slide
all the wheels on a GMC.
ANSWER. I agree with you on the maximum pressure a MC can produce
and it is the same pressure with or without the MC sensitized. This
statement will probably set off another fire storm but this is my story:
Many, many rears ago I had my brake booster sensitized by Bob Lamely
and it was a great help in not having to apply as much brake pressure
to slow the coach. At that time I was not aware what modifications
the brake booster required to "sensitize" them. My brother. who
lives in Missouri, also has a GMC so naturally I had to brag about me
having much better brakes then him.
He could not fine anyone in his local area that knew how to
synthesize a MC so he sent it to me to have it done in CA. After I
had my MC sensitized, a friend told me about a local mobil brake
specialist that has is shop in the back of his truck and he traveled
around to various car dealers and repair facilities when they needed
special brake service. I called the gentleman and ask if he did MC
sensitizing and he said "all the time". He said his truck was broken
down, but if I brought the MC out to his house He would do it and I
could watch. He also had a test bench in his truck to test the MCs.
What a surprise when he showed me that all that is necessary to
sensitized a MC was to change one Item. The MC booster has a round
rubber disc about the size of a quarter that he replaced with a disc
the same size but it wis made out of stiffer rubber. The major cost
in doing the sensitizing is replacing the large rubber diaphragm and
all the soft goods in the MC. He put a similar stock MC on his test
bench and showed me how much pressure it developed and he put my
sensitized MC on the test stand and showed it only developed a
similar pressure. He explained that less pedal force is require to
develop the same MC pressure as the MC pressure starts increasing but
it tapers off on the assist as the MC pressure increases to it's
maximum pressure. That's my story from a "Show Me State" kid.
B. Something don't compute here: You say the center wheels will slide
because they're only carrying 1/2 of the weight of the rear of the coach
-- but the reason they're only carrying that, instead of maybe 3/4,
is because you've put the 4-link on the rear, eliminating the weight
sharing I was ignoring earlier. So eliminating the rear 4-link should
put the center brakes back to working harder than the rears (where I'd
put smaller calipers). Thanks to the 4-link preventing pole-vaulting,
the rears will still carry about 1/2 of their normal load -- NOT lift
off the ground as in the past. What am I missing here?
I don't deny that 4-link on all 4 rear wheels would be optimal. But I
still think the marginal utility of the rears is not economically
justified. Maybe I can test myself into submission. :-)
ANSWER. The reason that OEM brakes on the mid axle can develop more
braking is because as the brake pressure increases more weight is
carried by the mid axle and the tire contact with the road surface
will resist more force before it slides.
The set of rear tires act just opposite to the mid axle. When the
brake pressure starts increasing on the rear tires, the torque
generated by the brakes is reacted by trying to rotate the suspension
in a clock-wise direction. The resulting action is the rear tire is
pulling back and down on the suspension arm rotation pen at about a
45 degree angle with the road surface. Thus, as the brake pressure
increases it pulls down on the suspension pen a proportional force
and this shifts weight from the rear tires to the mid axle tires. To
do this, the rear tires have to take weight off themselves to add
more downward force on the suspension arm bogie pen. As the weight
on the rear tires decreases to the point the tire friction at the
road surface decreases until it can no longer react the torque
generated by the brakes and the tire slides. If you will review the
video on the GMCWS web side in slow motion where I did the brake
testing with no reaction bar on the rear two wheels, You can hear the
rear tire start screeching long before the mid axle is raised to the
limit of the shock absorber travel.
Chuck Aulgur
La Mesa, CA
76 Royale which is one of two existing GMCs that the rear four tire
do an equal job of slowing my coach
.
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
GMCforum
Search
Help
Register
Login
Home
