Rear Brake Myths

D

dave1

New member
Jul 23, 1998
460
0
0
A number of times I have heard it said that the mid wheels bear more of the
breaking load than the fronts. Here is some hard data that tends to refute
that notion. This table shows the remaining pad (distance from the surface
of the pad to the top of the rivet) on each of my rear brake shoes after
30,000 miles:

Left (Drivers) Side Right Side

Mid Front Shoe .115" Mid Front Shoe .100"
Mid Rear Shoe .126" Mid Rear Shoe .129"

Rear Front Shoe .122" Rear Front Shoe .106"
Rear Rear Shoe .128" Rear Rear Shoe .140"

Given this sample of one, it looks like the brake wear on the mid wheels is
very close to that of the rear wheels. This suggests that the loading on
the mid and rear brakes are very close to the same.

Dave
73 Sequoia
 
All it may show is that you did not have the brakes adjusted correctly.It
proves nothing. Its a fact that the mid axel contributes more braking power
than the rears. that is no myth
Why, because there is more of the weight there during braking

>A number of times I have heard it said that the mid wheels bear more of the
>breaking load than the fronts. Here is some hard data that tends to refute
>that notion. This table shows the remaining pad (distance from the surface
>of the pad to the top of the rivet) on each of my rear brake shoes after
>30,000 miles:
>
>Left (Drivers) Side Right Side
>
>Mid Front Shoe .115" Mid Front Shoe .100"
>Mid Rear Shoe .126" Mid Rear Shoe .129"
>
>Rear Front Shoe .122" Rear Front Shoe .106"
>Rear Rear Shoe .128" Rear Rear Shoe .140"
>
>Given this sample of one, it looks like the brake wear on the mid wheels is
>very close to that of the rear wheels. This suggests that the loading on
>the mid and rear brakes are very close to the same.
>
>Dave
>73 Sequoia
>
>
>
 
Dave,

Thanks for sharing this data and your measurements are of interest to me for
I think this kind of research adds to our knowledge pool and supplies one
more piece of evidence toward finding the correct answer.

I agree with you, under Normal Braking the rear axle brakes do closely share
the load with the mid axle brakes.

I believe the confusion you mention is because in a panic stop situation,
things are different. Under Heavy Braking the rear axles will lock up
before the mid axles due to weight transfer. This is behind those efforts
to more evenly balance the lock up by changing to different sized components
on the two axles.

In Normal Braking, it makes no difference if one wheel is carrying more load
than the other.

In Normal Braking, as long as enough weight remains on both axles for both
mid and rear wheels to turn the same speed, braking loads will be closely
shared. This assumes equal swept areas, pressures on shoes, etc.

Anything short of locking up the rears, rear and mid axles will closely
share their braking loads with stock brakes.

My opinion, subject to revision, if relevant facts should appear or
others shout very loudly.

Don Miller
Shenandoah Valley of Virginia

>
> Date: Thu, 24 Jun 1999 16:10:04 -0400 (EDT)
> From: Tom Warner
> Subject: Re: GMC: Rear Brake Myths
>
> All it may show is that you did not have the brakes adjusted correctly.It
> proves nothing. Its a fact that the mid axel contributes more braking
power
> than the rears. that is no myth
> Why, because there is more of the weight there during braking

> >Date: Thu, 24 Jun 1999 11:51:08 -0400
> >From: Dave
> >Subject: GMC: Rear Brake Myths

> >A number of times I have heard it said that the mid wheels bear more of
the
> >breaking load than the fronts. Here is some hard data that tends to
refute
> >that notion. This table shows the remaining pad (distance from the
surface
> >of the pad to the top of the rivet) on each of my rear brake shoes after
> >30,000 miles:
> >
> >Left (Drivers) Side Right Side
> >
> >Mid Front Shoe .115" Mid Front Shoe .100"
> >Mid Rear Shoe .126" Mid Rear Shoe .129"
> >
> >Rear Front Shoe .122" Rear Front Shoe .106"
> >Rear Rear Shoe .128" Rear Rear Shoe .140"
> >
> >Given this sample of one, it looks like the brake wear on the mid wheels
is
> >very close to that of the rear wheels. This suggests that the loading on
> >the mid and rear brakes are very close to the same.
> >
> >Dave
> >73 Sequoia
 
Easy way to tell if your logic is true arch. Everyone take their coaches
out on a level highway in the country get going 40MPH or so and stomp on the
brakes as hard as you can and see which wheels leave rubber marks. anyone
want to speculate on the results. I can't do it because none of mine will.
YET!~

>Emery
>
>First: What is the story on the engine at this time?
>
>Second: Glad to see this subject talked about. I have no idea what
>the truth is. It seems to be a very widely held belief. When I was in
>that phase of my restoration I read all I could get my hands on about
>the subject. I even went to the 1 1/16 brake cylinder. I only did it
>because everything I read said to. I will also admit to spending a lot
>more than 1 hour setting looking at the rear suspension trying to
>figure out why this could happen. I agree with you that the front
>wheels on a 4 wheeled vehicle weight is transferred to the front
>wheels where a big percentage of the braking gets done. I also
>think this is where there may be a problem. We Got 6. I will say
>right up front I aint no engineer. Here is what my logic tells me is
>going to happen. When you hit the brakes weight will be transferred
>to the front wheels. That I buy into. Now back to the bogie. If weight
>is transferred to the middle wheel the cone is going to press into the
>air bag and raise the pressure. This pressure is going to be transferred
>to the back wheel. Now for a 4 bag system if the bags are isolated
>there would be a pressure differential. I can understand that. I was told
>by one person that I did not understand the ride height control. Once
>the front end went down the ride control valve would bleed off and try
>to level the coach. First it will not do anything for 4 seconds. Second
>if it did bleed off you still have the same pressure on both ends of the bag.
>I dont think the ride control will bleed off. It does not try to keep the
>coach level it just tries to keep the rear end the same height and I
>dont think it will climb high enough during braking to bleed off. I
>never figured out where the flaw was in my logic. So I took the
>Chicken S*** way out. I read that I should use 1 1/16 and 1 1/8
>wheel cylinders. I figured 1 1/16 was not enough to hurt if 1 /18
>could be ok. I honestly do not know what the answer is. I would like
>to hear some opinions so that I might have a better idea of what is
>going on back there. Nobody is going to hurt my feelings with any
>thoughts because I never figured out what was right or myth.
>
>Take Care
>Arch 76 GB IL
>
>In a message dated 6/24/99 6:02:54 PM Central Daylight Time,

>
>>
>> I think that we all agree that on a four wheel vehicle,the front breaks
>take
>>
>> a heavier stopping load that the rear partly because the weight of the
>> vehicle tends to transfer to the front wheels as the front of the vehicle
>> dips toward the ground when it stops. It would seem that the same effect
>> would occur as the rear bogie wheels brakes are applied.
>>
>> Anyone have any reference materials that apply to this so that we are not
>> all
>> speculating?
>>
>
>
 
>In a message dated 6/24/99 8:59:37 PM Central Daylight Time, FoxxG
>writes:
>
>>
>> Who really cares?
>>
>
>>Foxx

>
>Most of us do. You see some of us have loved this lady from a far for to
>long. Now we can be a steward to her for a while. That makes us happy.
>If you have to ask the question you have you will never understand the
>answer. There are a lot of us here who care. Like fine wine, good cheese
>or classic machines some of us have been chosen to be stewards of the best
>there ever was. If we can make them better then we will. Anyone who is
>a steward must ask if they are doing it right-----the future holds the
>answers not us. That is why places like this are so important. We dont
>have the answers we are seeking them.
>
>Take Care
>Arch
>

Well spoken Arch. You continue to be my GMC poet laureate.

Chuck
 
- ----- Original Message -----
From: Chuck Blanford
To:
Sent: Thursday, June 24, 1999 11:38 PM
Subject: Re: GMC: Re: Rear Brake Myths

>In a message dated 6/24/99 8:59:37 PM Central Daylight Time, FoxxG
>writes:
>
>>
>> Who really cares?
>>
>
>>Foxx

>
>Most of us do. You see some of us have loved this lady from a far for to
>long. Now we can be a steward to her for a while. That makes us happy.
>If you have to ask the question you have you will never understand the
>answer. There are a lot of us here who care. Like fine wine, good cheese
>or classic machines some of us have been chosen to be stewards of the best
>there ever was. If we can make them better then we will. Anyone who is
>a steward must ask if they are doing it right-----the future holds the
>answers not us. That is why places like this are so important. We dont
>have the answers we are seeking them.
>
>Take Care
>Arch
>

Well spoken Arch. You continue to be my GMC poet laureate.

Chuck

Even if you're unemployed with nothing to do, Arch!

Roger
 
>Who really cares?

There are a number of us looking at upgrading the rear braking system. It
helps to know what the loading is on the mid VS rear wheels. If the load is
the same then any upgrade done to the mids should also be done to the rears
. If the loading on the mids is higher, then it may make sense to upgrade
the mids to brake more than the rears.

Dave
73 Sequoia
 
Rick,

Thank you for a very good explanation of the dynamics of the rear
suspension during braking.

> Assuming you have stock brakes with identical cylinders and shoes, it
>could hardly be otherwise, unless you do a lot of locked-wheel braking.

Yes, I was thinking about that after I posted my measurements. As long as
all wheels are still turning at the same rate then the braking force, and
thus the wear, will be the same (assuming everything else is working
properly).

>...Which leads us to the point of all this speculation: How much braking
>power can each wheel develop before it locks up and skids?

Yes, I see that is the root of the idea behind changing the mids to brake
more than the rears.

> The leading rear wheels (mid wheels) are mounted on what amounts to a
>leading arm suspension. .... Just the opposite situation
>exists at the rearmost (trailing rear) wheels.

This makes sense. If one draws a little diagram showing the force vectors
at rest and during braking then its easy to see where the front forces add
up to more downward force on the mids and less downward force on the rears.
That would not be the case if the swing arm pivot pins where at the surface
of the road (where the braking force is applied). But since they are well
above the surface of the road one must take into account the angular force
vectors.

Its hard to reconcile the dynamic forces during braking with the way the
suspension works at rest. At rest all forces are kept in balance by
transferring any force differences through the airbag. During braking the
additional braking forces are applied in opposite directions with respect
to the swing arm pivots. That is where the difference in downward force
comes from.

Maybe a simpler way to think of it would be to imagine the mid swing arms
being able to rotate completely around their pins. If there where enough
friction (between the tire and the road), the mid wheels would tend to
swing under the pin.

This would happen because, when the wheels are locked up, you can think of
the whole system as a solid assembly. The the braking force is not applied
to the axal (in line with the pins) but to the surface of the tire (which
is well below the pins).

I hope I am thinking about this right....

Dave
73 Sequoia
 
You must log in or register to reply here.
Applied GMC
GMCMI Fall 2026 Rally

Share this page

Share this page
Top Bottom
Back