P30 installing rear brakes?

[TeeJay30]

Hi everybody,
New on here today not very good at forums so sorry for any mistakes but I have a Winnebago Warrior on a chevy p30 chassis please can anybody tell me which way the brake shoes go on as one shoe is thicker than the other and there must be a reason for it.
Many thanks in anticipation. TeeJay.

[DaveVA78Chieftain]

You have a primary and a secondary shoe. Primary is long pad and secondary is short pad. Long is to the back and is considered primary because of the auto-adjuster. short is front shoe.  A difference in thickness is due to wear differences as they both should be same thickness when new.

[TeeJay30]

Hi Dave,
Many thanks for your answer I can see what you are saying but these shoe's are new and are different thickneses I am told that the thinner shoe goes to the front and when the pedal is applied that moves out and then puts the pressure on the thicker one because there is no plvot on the bottom of the shoe's and in theory they are floating so I stil dont know.Many thanks anyway. TeeJay.

[DaveVA78Chieftain]

Who is the mfg of these "new style" shoes and I would also like to know how the brake hardware was modified so they operate differently.  The hardware design dictates how they operate.  The pivot point is the star wheel assembly at the bottom.



This animation shows how it all works.

https://www.youtube.com/watch?v=bMg_j5_AGMg

Just substitute the vacuum booster for the hydroboost unit you have installed on your rig.

Dave

[jeno]

I'm curious also because I had just replaced mine and they were the same thickness.

[TeeJay30]

Hi again all,
Like I said earlier not very good at these forum's dont know who I am talking to but yet again Dave many thanks dont know mfg but being as the star wheel is not fixed I  assume thats what they mean by floating and your discription sounds good to me so thats the way I am going.Thanks again TeeJay.

[TeeJay30]

Hi Jeno,
Sorry but cant help you on that one I get my parts from an Engish company over here and they have always been a perfect fit which these are its just a case of which way round they go but they are definetly different thicknesses and a friend of mine who lived in Canada knew that they were different but cant remember which way they go! so think its a case of watch this space! Regards TeeJay.

[Rickf1985]

I am thinking you better keep a close eye on those brakes. I am thinking the thin one is going to wear out rather quickly.The fact that one is thicker than the other and they are supposed to be the same thickness means that you will have to adjust you adjuster up pretty far unless the thick shoe is thicker than a standard shoe. I have to say that I have been a master mechanic for 45 years and I have never seen this. I would love to see the part number for those shoes.

[TeeJay30]

Hi Rickf,
Thanks for your concern I tried to attach photos of shoes but the files vere too big and me being dumb dont know how to make them smaller anyway the shoes are marked on the edge of lining with the thicker one marked NSS-4240-EE and the thin one AE 4200-11
also the thicker one has UBB1 stamped in the metal part of the shoe. Once agin many thanks for your concern. TeeJay.

[DaveVA78Chieftain]

After a little research I came up with the following for the JB7 Rear Brake shoes (FMSI 2042BT-S655 or FMSI 2042AT-451)

Primary shoe: Shoe that goes on the forward side of the mounting plate and has the shorter length friction pad with a 0.25" thickness

Secondary Shoe: Shoe that goes on the rear side of the mounting plate and has the longer length friction pad with a 0.375" thickness

Formula Blue Set P/N: E117-0451-0

Interchange Part Numbers

[TeeJay30]

Hi Dave,
I just cant thank you enough for that information I only been a member for a couple of days and got instant responses from you all  you guys are amazing many thanks to you all I only hope I can help you all sometime.Thanks again from a very happy new member.
TeeJay.

[Rickf1985]

Well I will say it again, I have never seen it but quite obviously it does exist. It is interesting because I have worked on fleet vehicles including  P-30 delivery trucks.

[DaveVA78Chieftain]

We tend to look for the 2 different lengths of the the friction material and may not notice the small 1/8" thickness difference between the primary and secondary shoes.

[TeeJay30]

Well done to all of you once again many thanks from over the pond.TeeJay.

[Froggy1936]

Upon application going forward The frt shoe contacting the drum transfers the force thru the adjuster to the larger shoe wich is stopped from turning with the drum due to the top anchor pin ,thereby being forced into more forceful contact with the drum & actually doing most of the work ! Frank

[Rickf1985]

I don't totally disagree but keep in mind that the wheel cylinder is applying equal force out on the tops of both shoes so that means the one with less surface area actually has more force per square inch on it thereby equalizing the length difference.

[rustyzman]

Froggy nailed it.  On this style brake you not only have the pressure from the wheel cylinder on each shoe, but also the transfer of force from the front shoe's engagement through the adjuster to the rear shoe.  The additional force from the self energizing effect of the front shoe on the rear shoe with Bendix or Duo-Servo style drum brakes will substantially increase the pressure on the rear shoe vs. the front.  Likely the temperature of that rear lining will be higher as well.  Normally the length difference between the primary and secondary shoes makes up the wear differential alone, but I know I have seen this scenario before.  I just don't remember on exactly which vehicle.  Most vehicles with this style brake don't have a thickness variation, especially the lighter ones. 

Very possible that this size of brake may need the additional lining thickness simply because the length difference alone is not enough to overcome the wear difference between the two shoes.
Anyway, just some thoughts on the matter.

[lngfish]

 

Way to help him Dave.