Short brake linings to the rear!

[DavidGB]

Recently I have been working on the brakes on my '38 Dodge D8 (sedan.) These brakes are very similar to Power Wagon Brakes. I have done brakes many times over the years on PW's etc, but each time is like the first and I always tend to review available literature at the time of the current job. Once I had the drums off and could look at what I had, I made a couple of observations. 1) The wheel cylinders are of the type with unequal size pistons and were mounted with the larger piston facing aft. This jives with all the various manuals etc. that I had read. 2) The brake shoes had unequal length linings with the shorter ones on the rear side. (Other 38 D8's I have seen did not have this variation.) This issue was not covered in my service manual for this car and is not mentioned in TM-9 etc.
Page 2 of the "Brakes" technical help on the Texas Power Wagons site shows the short lining to the rear. If you shop brake linings on eBay, applicable NOS/NORS linings that have different lengths show the short ones labeled "Reverse" and the longer ones labeled "Forward." Now, the reason I bring all this up is that in the "Dodge Service" publication that I have (Brakes-Front Drive Axle-Transfer case-Rear Axle-Differential) printed May 2001, 3rd Printing by PWA, it states in the 5th paragraph of page 6 "They may not be marked, and in that case you need to know that the short lining goes to the front." This caused me much angst. At this time I have concluded that that statement is incorrect. However, I would love some affirmation (or correction.)
Thank you,
David

[Clint Dixon]

Recently I have been working on the brakes on my '38 Dodge D8 (sedan.) These brakes are very similar to Power Wagon Brakes. I have done brakes many times over the years on PW's etc, but each time is like the first and I always tend to review available literature at the time of the current job. Once I had the drums off and could look at what I had, I made a couple of observations. 1) The wheel cylinders are of the type with unequal size pistons and were mounted with the larger piston facing aft. This jives with all the various manuals etc. that I had read. 2) The brake shoes had unequal length linings with the shorter ones on the rear side. (Other 38 D8's I have seen did not have this variation.) This issue was not covered in my service manual for this car and is not mentioned in TM-9 etc.
Page 2 of the "Brakes" technical help on the Texas Power Wagons site shows the short lining to the rear. If you shop brake linings on eBay, applicable NOS/NORS linings that have different lengths show the short ones labeled "Reverse" and the longer ones labeled "Forward." Now, the reason I bring all this up is that in the "Dodge Service" publication that I have (Brakes-Front Drive Axle-Transfer case-Rear Axle-Differential) printed May 2001, 3rd Printing by PWA, it states in the 5th paragraph of page 6 "They may not be marked, and in that case you need to know that the short lining goes to the front." This caused me much angst. At this time I have concluded that that statement is incorrect. However, I would love some affirmation (or correction.)
Thank you,
David

Due to the nature of the orientation of the brake parts on our trucks, the leading shoes are naturally more productive than the trailing shoes (assuming the vehicle is traveling forward). That is, the leading shoes apply more pressure to the drums, create more friction, and result in more stopping force than the trailing shoes. The reason for this discrepancy is because of the difference in relationship between the heel and toe ends of the shoe - all relative to orientation. I forget the correct terminology that describes our brakes, but you will probably find the official descriptions in the Dodge Service publication you mention anyway. If not, I am sure someone else here will chime in.

I made the following observation some years ago much to Gordon Maney's delight:

In trying to simplify and make sense of the whole mechanics of the brakes in my own mind, I compared it to coasting down a hill in a little red wagon. I think what I originally said went something like this, "If you get going forward really fast down that hill in that little red wagon, and suddenly throw the wagon tongue forward, letting it flop to the ground ahead of you, the tongue will dig into the sod and will stop the wagon in a big hurry. One the other hand, if you choose to ride down the hill with the wagon going backwards, and throw the tongue rearward behind you, the tongue will not dig into the sod but will bounce and drag along behind and not stop the wagon nearly as fast."

In the above analogy, the heal of the wagon tongue is the end closest to the wagon - the part very close to the pivot point. The toe of the wagon tongue is the handle at the extreme other end - the part furthest away from the pivot point.

The brake shoes are the same. The heal is close to the pivot, the toe is at the other end close to the wheel cylinder. On the leading shoe the toe wants to dig into the brake drum while the heal kind of just drags along. The toe is doing the majority of work because of leverage, and with the addition of force from the wheel cylinder, is doing a very good job. While the trailing shoe is exactly the opposite. The heal is trying to do all the work while the toe is just kind of dragging along, even with the added force applied to that end from the wheel cylinder. So the leading shoe does a lot more work than the trailing shoe and will wear much faster because of it.

To make both shoes work more equally and share the load, the design of some of our brake systems use wheel cylinders of unequal bores. The larger bore is directed to the trailing shoe where the extra force is needed. If one brake shoe's lining is longer than the other, the longer one gets to be the trailing shoe where the extra length increases its efficiency even further.

I think I went on and made mention of the Calvin and Hobbes cartoons.

I stand by the information in the PWA published material.

[DavidGB]

short to the rear

Clint,
Thank you for the response. I would first like to acknowledge that there is absolutely no doubt that your knowledge and experience of all things Power Wagon is far greater and superior to mine, and that I have the highest respect for you. However, when I am trying to get my head around something I can be a persistent pain in the butt.
I fully grasp the whole "self actuating" concept. Various ways of mounting the shoes though will affect which shoe gets the most pressure. When the shoes are anchored at the bottom (as in the case I am talking about,) the front shoe is the self actuating one with forward motion and the aft one is when in reverse motion.
Because of the "self actuation" less pressure is required for the front shoe and thus the smaller piston design. (By the way, VPW calls for the short to the rear and long to the front also.)
When the shoes "float" (not anchored at the bottom) the front shoe initially self actuates and causes both shoes to rotate slightly thus putting more pressure on the rear shoe. In all the literature describing this (latter) type of arrangement, the longer lined shoe goes to the rear and shorter to the front.
I have not found the logic of any explanation that I have so far come across for the unequal length of the linings to be convincingly definitive (i.e. for which there is no equal counter argument) and thus must still go on the weight of existing evidence in literature etc. to determine location - front or back. Admittedly equal length shoes are often used and I apologize for obssessing over an example that may not apply to many users of this forum, but the knowledge here is fantastic. Following are some supporting exhibits.
Thank you,
David

[W_A_Watson_II]

Man am I getting confused. But lets remember PSI.

All brake line pressure being the same:
A Large Bore piston on a smaller surface lining means more PSI at the lining to drum interface and assuming the same friction material more stopping power.
A small Bore piston on the larger surface lining means less PSI at the lining to drum interface under the same assumptions.

So if the front is self actuating (Meaning the wheels motion adds to the pistons work) I'd assumed the small piston and small lining front and large piston and lining to the rear. But I've not done the math of piston area vs lining area.

[DavidGB]

short to the rear

Good Morning Will,
Thank you as well for weighing in.
Since the front is self actuating (in forward motion) then less psi from the cylinder is needed. Ne c'est pas? This would follow the logic of your reasoning so far. The idea was to use the self actuating effect to require less pedal effort and the design needed to balance this out.
The PSI approach helps me a lot, but I too am not doing any math here.
David

[W_A_Watson_II]

Your welcome, the piston diameters and pad sizes and I can do the math.

[Matthew Welcher PWA]

http://constructionmanuals.tpub.com/...linder-249.htm

This is a very interesting read in reference to self energizing brakes and step bore wheel cylinders.

I keep it saved on my computer.

[DavidGB]

short to the rear

Good morning to you as well Matt,
That was indeed interesting reading. What gets confusing in all this is that many statements refer to "most cases" or the like, and thus one can not be sure of the specific applicability. The type of drum brake set up that does not anchor the shoes at the heels is more common than the type I have been referencing (in the automotive world.) These use "servo" action to self actuate the rear shoe. Thus when someone makes a comment about which way to mount different length linings and does not specify how the shoes are anchored it could well be that they are referring to the more common non-heel anchored variety.
No one has addressed why the sources I have mentioned that say short linings to the rear could be wrong, including the lining manufacturers. Are they?
Thank you,
David

[W_A_Watson_II]

Part of the issue is the picture you show of the small and large linings appear to be of different materials. That means one os more agressive (Higher friction coeficient) than the other, and as a result one will give greater brake power at the same area and pressure. perhaps they have already done the math and chosen the materials based on forces and cost effectivness?

[Matthew Welcher PWA]

I Had a very long and detailed conversation with Gordon in reference to exactly this. He had addressed this topic on Joe's Forum, a video I believe was included roughly 3 years ago. I was sitting beside him in his office and I need to be able to find that post he made.

It ended up being a late at night Coca Cola "from a glass bottle" and an entire jar of planters peanuts conversation. I am racking my brain remembering the details.

[Matthew Welcher PWA]

Take a look here and see if this helps.

http://www.t137.com/archives/wwwboar...s/4320410.html

[DavidGB]

short to the rear

Will,
That is only one picture from one manufacturer of brake linings. Looking on eBay and other sites that offer pictures of the items being sold, I have seen many others. I have not been able to see that the material always varies (short to long.) In some cases the short ones appear slightly thicker and in others not. They do often say "balanced" as well as forward and reverse. It is worth being clear that in the case where the rear shoe gets more pressure through servo action that the long lining is specified for the rear.
Thank you,
David

[DavidGB]

Short to the rear.

Matt,
Thank you. Also very good info. I do not find that it argues against my position that the short lining goes to the rear in the type of set up I have been talking about.
Ones things for sure, I am getting to know brakes more solidly than ever before.
David

[W_A_Watson_II]

Now I want a 1937 Plymouth! Room, Ride & Comfort.

[DavidGB]

short to the rear

That 37 is almost a dead ringer for my 38, and it is a pleasure to drive. Sadly the link to sanjuan.edu no longer works so I could not check out the referenced pages in that thread.
David