Flat Spots on Rearmost (new) Tires

[SteveW]

Greetings from Southern California -

Here's an odd one. Not my coach - a 26' Royals belonging to a dear friend.

Two new tires on the rear most wheels. Alcoa wheels. Drum brakes. Driving this weekend and we feel a new and unusual noise from the rear. Thump.
Thump. Thump...

Upon inspection - we find flat spots on both rear tires !!

As I understand things - locking up under heavy braking could flat spot the mid wheels - the rears would lift - no ??

And the parking break - if previously modified to operate only the rear drums isn't strong enough to allow wheel dragging - is it ??

I'll have to check the load rating of the tires - but if the load rating is less than E - might the tires get a flat spot from just storage parking
??

Your opinions and comments are appreciated. Thank you all for the expertise shared here.

Be safe,
Steve W
Southern California
1973. 23. Yellow

--
Steve W
1973 : 23'
Southern California

 

[Larry Davick]

The mid wheels do most of the braking back there. The rear ones can easily skid as the pogo action of the front bogie lifts the rear.

Larry Davick

Sent from my iPad

>
> Greetings from Southern California -
>
> Here's an odd one. Not my coach - a 26' Royals belonging to a dear friend.
>
> Two new tires on the rear most wheels. Alcoa wheels. Drum brakes. Driving this weekend and we feel a new and unusual noise from the rear. Thump.
> Thump. Thump...
>
> Upon inspection - we find flat spots on both rear tires !!
>
> As I understand things - locking up under heavy braking could flat spot the mid wheels - the rears would lift - no ??
>
> And the parking break - if previously modified to operate only the rear drums isn't strong enough to allow wheel dragging - is it ??
>
> I'll have to check the load rating of the tires - but if the load rating is less than E - might the tires get a flat spot from just storage parking
> ??
>
> Your opinions and comments are appreciated. Thank you all for the expertise shared here.
>
> Be safe,
> Steve W
> Southern California
> 1973. 23. Yellow
>
>
>
>
>
>
>
> --
> Steve W
> 1973 : 23'
> Southern California
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

 

[hal stclair]

The rears will drag first and sometimes with very little warning.
Hal
--
1977 Royale 101348,

1977 Royale 101586, Diesel powered,

1975 Eleganza II, 101230,

1974 Eagle Bus 45',w/slideout,

Rio Rancho, NM

 

[Jim Hupy]

You kinda are looking at this backwards. When heavy breaking occurs, the
rear of the coach unloads the rear/rear tires due to forward weight
transfer. The rear/rear tires lose traction and stop rolling. They then
slide along the pavement locked up, not rotating. The front /rears and the
front disc brakes are doing all the stopping.
Solution? Reaction arm brakes in the rear. Stop gap fix? Heavier
calipers in the front discs and more aggressive linings. Fixing the
forward/rears so that they are less effective by changing the wheel
cylinders to a size that effectively reduces their braking effort, while
leaving the rear/ rears alone.
Lastly but not least, Make Very Sure your front ride height is
correct. Too low and you negate the anti-dive feature built into the
alignment specs. Your coach should appear to be higher off the ground in
the front of the coach than it does in the back.
Stop following closely and avoid panic stops (very hard to do anymore)
if you leave adequate intervals to allow for proper stopping distances,
some unthinking 4 wheeler will change lanes in front of you.
This will give you some things to consider.
Jim Hupy
Salem, Oregon

On Wed, Mar 18, 2020, 3:32 PM Steve Weinstock via Gmclist <

> Greetings from Southern California -
>
> Here's an odd one. Not my coach - a 26' Royals belonging to a dear friend.
>
> Two new tires on the rear most wheels. Alcoa wheels. Drum brakes.
> Driving this weekend and we feel a new and unusual noise from the rear.
> Thump.
> Thump. Thump...
>
> Upon inspection - we find flat spots on both rear tires !!
>
> As I understand things - locking up under heavy braking could flat spot
> the mid wheels - the rears would lift - no ??
>
> And the parking break - if previously modified to operate only the rear
> drums isn't strong enough to allow wheel dragging - is it ??
>
> I'll have to check the load rating of the tires - but if the load rating
> is less than E - might the tires get a flat spot from just storage parking
> ??
>
> Your opinions and comments are appreciated. Thank you all for the
> expertise shared here.
>
> Be safe,
> Steve W
> Southern California
> 1973. 23. Yellow
>
>
>
>
>
>
>
> --
> Steve W
> 1973 : 23'
> Southern California
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[emerystora]

No way from storage. It had tobe mechanical. Very likely from the parking brake being left on. Yes, it would be strong enough to lock the wheel. How would it lift if only the rearmost locked?

Emery Stora

>
> Greetings from Southern California -
>
> Here's an odd one. Not my coach - a 26' Royals belonging to a dear friend.
>
> Two new tires on the rear most wheels. Alcoa wheels. Drum brakes. Driving this weekend and we feel a new and unusual noise from the rear. Thump.
> Thump. Thump...
>
> Upon inspection - we find flat spots on both rear tires !!
>
> As I understand things - locking up under heavy braking could flat spot the mid wheels - the rears would lift - no ??
>
> And the parking break - if previously modified to operate only the rear drums isn't strong enough to allow wheel dragging - is it ??
>
> I'll have to check the load rating of the tires - but if the load rating is less than E - might the tires get a flat spot from just storage parking
> ??
>
> Your opinions and comments are appreciated. Thank you all for the expertise shared here.
>
> Be safe,
> Steve W
> Southern California
> 1973. 23. Yellow
>
>
>
>
>
>
>
> --
> Steve W
> 1973 : 23'
> Southern California
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

 

[Jim Hupy]

All 4 rear brakes are the same size and effectiveness in theory. When the
weight transfers forward, the coach suspension unloads more on the rear
most bogies. The braking effort is the same, so the rears stop turning. Jim
K has some really clear video evidence of this. It is not a mystery. It
happens with 4 wheel vehicles, too. Watch an unloaded flatbed trailer
sometime during heavy braking. There will be so much tire smoke coming off
the tires, you will think something is on fire.
Jim Hupy
Salem, Oregon

On Wed, Mar 18, 2020, 4:16 PM Emery Stora via Gmclist <

> No way from storage. It had tobe mechanical. Very likely from the parking
> brake being left on. Yes, it would be strong enough to lock the wheel. How
> would it lift if only the rearmost locked?
>
> Emery Stora
>
>
> > On Mar 18, 2020, at 4:32 PM, Steve Weinstock via Gmclist <

> >
> > Greetings from Southern California -
> >
> > Here's an odd one. Not my coach - a 26' Royals belonging to a dear
> friend.
> >
> > Two new tires on the rear most wheels. Alcoa wheels. Drum brakes.
> Driving this weekend and we feel a new and unusual noise from the rear.
> Thump.
> > Thump. Thump...
> >
> > Upon inspection - we find flat spots on both rear tires !!
> >
> > As I understand things - locking up under heavy braking could flat spot
> the mid wheels - the rears would lift - no ??
> >
> > And the parking break - if previously modified to operate only the rear
> drums isn't strong enough to allow wheel dragging - is it ??
> >
> > I'll have to check the load rating of the tires - but if the load
> rating is less than E - might the tires get a flat spot from just storage
> parking
> > ??
> >
> > Your opinions and comments are appreciated. Thank you all for the
> expertise shared here.
> >
> > Be safe,
> > Steve W
> > Southern California
> > 1973. 23. Yellow
> >
> >
> >
> >
> >
> >
> >
> > --
> > Steve W
> > 1973 : 23'
> > Southern California
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[john phillips]

The rear axel seems to lock up befor the others. There is weight
transfer from the rear to the mid axel under heavey breaking. There have
been many comments about different breaking system and some going so far as
to not break the the back axel at all.
My coach has smaller slave cylinders for the 3rd axel. There is a point
where the right ratio between 2 and 3rd axel slave cylinders would case all
the wheels to break loose at the same time but that would only work for a
certen road serface. Unless you go with some kind of computer
controeld breakeing that will not lock the wheels up you will have problems
trying to get maximum breaking without breaking traction.

John Phillips
Citrus Heights CA.

On Wed, Mar 18, 2020 at 3:32 PM Steve Weinstock via Gmclist <

> Greetings from Southern California -
>
> Here's an odd one. Not my coach - a 26' Royals belonging to a dear friend.
>
> Two new tires on the rear most wheels. Alcoa wheels. Drum brakes.
> Driving this weekend and we feel a new and unusual noise from the rear.
> Thump.
> Thump. Thump...
>
> Upon inspection - we find flat spots on both rear tires !!
>
> As I understand things - locking up under heavy braking could flat spot
> the mid wheels - the rears would lift - no ??
>
> And the parking break - if previously modified to operate only the rear
> drums isn't strong enough to allow wheel dragging - is it ??
>
> I'll have to check the load rating of the tires - but if the load rating
> is less than E - might the tires get a flat spot from just storage parking
> ??
>
> Your opinions and comments are appreciated. Thank you all for the
> expertise shared here.
>
> Be safe,
> Steve W
> Southern California
> 1973. 23. Yellow
>
>
>
>
>
>
>
> --
> Steve W
> 1973 : 23'
> Southern California
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--

*John Phillips*

 

[Carl Stouffer]

Old nylon ply tires used to take a set (flat spot) during storage or even being parked overnight in cold weather, but they would round off again after
several miles of driving. I seriously doubt that that is the problem if the tires are new. Probably skidding due to the rear wheels unloading under
hard braking. It's kind of hard not to notice when that happens. That's why I put the Reaction Arm system on my coach.
--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member

 

[Matt Colie]

I revised this slightly last year (it would seem), but the truths are all from a mathematical study that I did after I first flat spotted new rear
tires. What still bothers me personally is that I did not see the application of the floating backplate that I had been so successful with while
racing dirt bikes only two plus decades before.

While weight transfer when retarding (less than a full brake stopping effort) Is often cited as an issue, with the ratio of the CG elevation to Wb
(wheel base), this just cannot be a major factor in the situation.

I wrote this, so it necessarily is largely written in "Engineerese". If there is anything that someone should like clarification of, please come back
and tell me.

Here Goes........

GMC Motorhome Brake issues
A discussion of the situation when the as-designed hardware is modified.

From this point, coach refers to the GMC built TZE chassis Motorhomes and Transmode models built between 1973 and 1978.

Executive summary:
There is little risk with the increase of brake power on the intermediate wheels/tires. There is however a significant cost for not much advantage
and in all cases, the floating backplate mod is still very advantageous in all situations reviewed. A modification that should be reviewed in detail
would be increase of the rear wheel cylinders to a one and one-eight bore. Either this or a means to increase the available service brake pressure.
The system is currently limited by brake pedal pressure alone.

I did the first full dynamic analysis years ago and my attention was centered on stopping the all the way rear wheels from locking and flat spotting
tires. This is an issue with which we are all to familiar. At that time, I built and used a mathematical model that required a software license
that I no longer maintain. While this is unfortunate, the software just makes the task faster and easier.

At one time, I had a goodly number of reliable contacts in both the tire and brake worlds, but alas, those that have survived this long have retired
and only one had enough of his references available to be any help at all. The greatest help actually came from Tom Pryor as he had secured some of
the brake data from an aftermarket supplier that was very useful. I still had to use just the rough numbers collected years ago in lots of places.
These estimates may be in error, not very a large error and none represent and order of magnitude that would be required to make the final evaluations
seriously invalid.

There are no diagrams because including them without the analysis package to assist would require the addition of a great deal of effort that I do not
have time to contribute.

The actual discussion of the analysis.

While considering the many applications of more powerful braking systems on the intermediate axle, I began to wonder if there could be a hidden
problem there. And there are two that show up in extensive analysis, but it is my opinion that neither is likely to cause a critical issue.

First of these was the "pole vault" issue of the leading link. The arm is eighteen inches long. In the standard configuration, it has a balanced
airspring that is supported by the trailing link. What is interesting is that this arrangement almost, but not quite, removes any change in ride
height during a hard braking situation. The analysis showed that the load that the leading link tried to apply to the main casting was largely what
it took away from the trailing link. So, the load on the leading tire increased and the load on the trailing tire decreased a very similar amount.
The larger portion of the irregularity is the result of the non-constant spring constant of the airspring.

Unfortunately, if the leading and trailing suspensions are isolated, as with some of the redesigned suspension systems, the change in ride height is
not so stable. The force that the leading link can create is not subtracted from the support provided by the trailing arm. The load supplied by the
trailing arm is only reduced by the increase in ride height and as soon as the tire slides, that is the limit. This limits the provides less possible
gain to the leading brake but it cannot make any significant gain for the effectiveness of the trailing brake as that can still achieve a sliding load
with relative little change in the situation.

Another concern that was raised is the possibility of overload damage to the leading link tire as might be caused by a severe stop situation. As the
maximum load time should be short and the real damage caused by overloading any tire is the heat involved and that takes some time to accumulate, the
maximum of about one thousand pound on a tire normally carrying twenty two hundred is just not that terrible. There are now many coaches running with
an isolated suspension system and a very large intermediate brake, but I have heard no reports of tire failures associated with this modification.

There is an issue with the plans that put much larger than design brakes on any of the rear wheels. This is a well understood control issue. One of
the big advantages of modern anti-lock brake systems is that wheels are prevented from locking. Some will claim that you might shorten a stop by
locking wheels. I can concur with this, but you also then introduce an serious control issue. Simply put, a sliding tire does not have any
particular direction in mind. The only reason that this can be survived in a wheeled vehicle situation is because the unstable vehicle can possibly
be controlled with active input by the operator. This does raise an interesting control issue. If one is running with this mod and in a panic stop
situation the rear brakes can be locked, the intermediate because of the larger brake and the rear because the load feedback unloaded it to
uselessness, then the rear of the vehicle could well have no control stability at all. As this easily become an unstable situation, it might require
a very skilled operator to maintain the vehicle control.

And, two interesting things that did fall out of the study.

The first is that the usefulness of the rear brake is so limited that it could well be ignored and used solely as a parking brake as long as the coach
is nose down a severe down hill as this could possibly induce sliding of the associated tire. If this brake is taken out of the service brake plan
and left to free roll in a panic brake situation, it will be very effective to maintain the control of the vehicle rear should the brake of the
intermediate axle lock and induce a sliding condition.

The second is that the 11*2 Bendix self excited brake is about a 400 horsepower device and in the motorhome application is not nearly fully utilized.
This is the reason that the floating backplate mod is so effective. As this can prevent the positive feedback on the leading brake from overloading
that suspension and also prevents the negative feedback that causes the trailing brake to become useless.

The above two bring in a new issue that I wish I had the time to investigate in the real world. The rear coach brakes are supplied with a 15/16 wheel
cylinder. This will make it about a 250 horse power brake. With tires available during the coaches design and even though most of manufacture, this
was a very well found choice. Things have changed. None of the four can live up to their full potential in a current application. When the coaches
wore bias ply tires, this was about all the tire patch could deal with effectively. While this was a good choice in my opinion, there is the option
of a 1-1/8 wheel cylinder that could increase the power capability of the OE brake by nearly half again (150%). There are, however, two immediate
issues with this modification. The first is that if the master cylinder displacement can handle the addition volume requirement. Even the master
cylinder is capable, there is a question if will it still be capable if these brakes are not maintained in proper adjustment. That really should be
an non-issue, but I have come to find that many do not understand what the automatic adjusters do require occasional hard stops in reverse.

I did not include any definitions as I did not know what might be helpful.

Matthew Colie 04 January 2019

--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

 

[SteveW]

Thank you, thank you, thank you all.

Extremely informative and helpful comments.

Steve W.
--
Steve W
1973 : 23'
Southern California

 

[Ken B]

I have learned my lesson. I cannot remember the number of tires I have flat spoted on landing my airplane by trying to stop before the first turn off
before the wheels have all of the airplane weight on them. Now I just let it roll and use the brakes sparingly.

Think about it on your GMC.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana