Thoughts on Brake Linings

[Steve Moskowitz]

So today I removed all the brake shows on my 1941 Olds race car.  I was surprised to see that the linings were the same size on the primary and secondary shoes.  Of course factory shoes had a shorter length on the primary shoes.

 

I am guessing someone decided to engineer this with more lining  on the front shoe figuring better braking.  Sounds sensible (although I am leery) but I am not an engineer so I may be missing something in the evaluation.  I am having competition linings installed and the company makes whatever I want and will arc them to my drums.

 

 

[pmhowe]

Hi Steve,

The conservative approach is to go with what the original specs called for, especially since you are redoing them.

You can speculate that greater front area will give better braking. Might be. Without knowing the engineering analysis for that choice, you have no reason to expect an improvement from the change. At the very least, it is worth looking at more modern shoe designs prior to accepting the change. I'll be interested to learn what you decide.

Phil

[Spinneyhill]

I was always told and read that the primary shoe was the one that is pulled on by drum rotation, which is the front shoe when going forward with one cylinder.  https://en.wikipedia.org/wiki/Drum_brake

 

MoToR's manual, 1947, on p. 195 shows a drawing of Ford and Mercury 1946 brakes. There is one two-piston cylinder and two anchors. It is a LHS diagram: drum rotation is counter clockwise. The long shoe is the leading or primary shoe on the front and the short shoe is the trailing or secondary shoe on the rear. The short end of the trailing shoe is the bottom.

 

Here is a diagram of the braking force with different systems, from Wikipedia (ref. above).

[Rusty_OToole]

I always thought the primary shoe put more pressure on the secondary shoe causing it to wear faster. This is why they put more lining on it, to equalize wear.

 

When they did away with asbestos in the 80s it really loused up brake linings. Don't know if they have come up with anything yet that is as good as your car had when new. Most of the time new linings are too hard, don't grip and you have to stand on the pedal with both feet if you want to stop fast.

[Bloo]

A 41 Olds is probably Bendix-style "servo action" brakes. The pivot point at the bottom is movable. The leading shoe helps apply force to the trailing shoe. Unlike Fords of the period, the short shoe is always the leading shoe. A picture of the shoes on the backing plate would show if the pivot is movable. Many Pontiacs and Buicks of the period have this, but not Chevrolets.

 

 

[Bloo]

4 hours ago, Rusty_OToole said:

When they did away with asbestos in the 80s it really loused up brake linings. Don't know if they have come up with anything yet that is as good as your car had when new. Most of the time new linings are too hard, don't grip and you have to stand on the pedal with both feet if you want to stop fast.

 

I had the opposite experience. The "asbestos free" linings of the 90s worked better than anything I ever had, and especially on drum brakes, They did have one annoying quirk, expanding as they broke in.

 

Are asbestos brakes banned in Canada? They were banned down here (US), but I don't think they still are.

Edited April 16, 2018 by Bloo (see edit history)

[Steve Moskowitz]

"In the real world the rear longer was used to modulate the self adjusting during back up" this was from an engineer friend of mine.  If this is the case then I probably will stick with both shows being long as this car is a beast to stop.

[edinmass]

Use a brakedoktor to set the brakes up, you will think you have power brakes.......

[JACK M]

Not uncommon for reman shoe to kits come with all longer linings.

Easier to not get them mixed up I guess, Also fewer pieces to keep track of.

[Bloo]

7 hours ago, Steve Moskowitz said:

"In the real world the rear longer was used to modulate the self adjusting during back up" this was from an engineer friend of mine.  If this is the case then I probably will stick with both shows being long as this car is a beast to stop.

 

That doesn't sound right. I'm no engineer, but I have never seen a Bendix style (moving pivot) system that did not have a short leading shoe. I have worked on plenty of cars that were too old to have self adjusters. I am not necessarily trying to talk you out of the long shoe.

 

(I am assuming you have a moving pivot system because I haven't seen them.)

[Steve Moskowitz]

All the brakes I have worked on had a primary shoe that was shorter then the back shoe (secondary).  Everyone I have talked to, however, seems to feel it is worth the shot on a race car to get more braking surface.  I talked to a noted crew chief for NASCAR and he saw no issue. 

 

Ed, I vaguely remembered the brake doctor...I talked with a guy who has one and he does not enjoy using it so I cannot give it a try now.  If my new brake shoes do not work I just may have to find someone to try this with my car. 

[Spinneyhill]

I wonder if that short leading shoe is to prevent it jamming and grabbing on heavy application? May be able to remedy that on a longer shoe by chamfering the leading edge?

 

That's the thing. I have not played with such a system with moving pivot! Too modern.

Edited April 17, 2018 by Spinneyhill (see edit history)

[Rusty_OToole]

I remember the shoes being 2 different colors too, light and dark gray, as if they were made of different materials. That may have something to do with it. If they used to use a softer and harder material but now they are all the same.

[Curti]

The Bendix brakes that come on all 34-36 Auburn 6 & 8 have the shoe pad length the same.  There are three adjustments, up / down, for and aft, the star adjuster.  There is a little window on the outside of the brake drum that a feeler gauge is inserted to insure the proper clearance all around the 360.  Then use the star adjuster.  The material is  the same on both shoes and all four wheels. They will lock up all four wheels at or near 25 mph.  I know this, because of a little girl on a bicycle. 

[Steve Moskowitz]

Thanks, I am comfortable with the longer length on the front shoe.  On regular production it was about 2" shorter.  With arcing the shoes I should have better contact and hopefully better braking into the corners.  Either that or never lifting which could be a problem!  

[JACK M]

2 hours ago, Curti said:

The Bendix brakes that come on all 34-36 Auburn 6 & 8 have the shoe pad length the same.  There are three adjustments, up / down, for and aft, the star adjuster.  There is a little window on the outside of the brake drum that a feeler gauge is inserted to insure the proper clearance all around the 360.  Then use the star adjuster.  The material is  the same on both shoes and all four wheels. They will lock up all four wheels at or near 25 mph.  I know this, because of a little girl on a bicycle. 

 

That little girl on the bicycle stopped by and showed you how to adjust your brakes?   And we talk about the youth of today not being interested.

 

(snicker) 

[Frank DuVal]

On 4/16/2018 at 3:06 PM, Bloo said:

That doesn't sound right. I'm no engineer, but I have never seen a Bendix style (moving pivot) system that did not have a short leading shoe. I have worked on plenty of cars that were too old to have self adjusters. I am not necessarily trying to talk you out of the long shoe.

 

I'm with Bloo. The short/long shoe servo brakes started decades before self adjusters were common, so the short shoe has nothing to do with self adjusting parts on cars built before self adjusting brakes!

 

Your racing application is way far removed from factory stock parts driving the design. For factory stock, I would make a long/short shoe, and I do this  because all long shoes appear way too often in the aftermarket now. For racing, talk to racing people.

[Bloo]

After seeing that Auburn brake I'm going to have to look at the ones on my 36 Pontiac. They could be like the Auburn.

[Spinneyhill]

http://www.engineeringinspiration.co.uk/drumbrakes.html

It seems the shorter lining on the leading edge is to prevent spragging and grabbing. This can occur with irregular wear, such as can occur with repeated high deceleration stops, e.g. in racing.

 

So, Steve, take a bit of shoe off the leading edge of the leading shoe!

 

If you want a bit of mathematics, here is some http://www.iaeng.org/publication/WCE2010/WCE2010_pp1216-1223.pdf

 

Edited April 18, 2018 by Spinneyhill (see edit history)

[DavidAU]

I have always thought that by having a smaller shoe with less surface area on the leading shoe like the Bendix system has, the hydraulic system would automaticly apply more pressure to the smaller shoe causing more friction which starts to rotate it and that in turn drives the rear shoe harder into the drum.

[emjay]

Mind your terminology.  You can't talk about leading/trailing shoes until you identify what brake design you have.  Assuming just the single point of force application, it matters whether the shoes are anchored at the other end or floating.  It is also important to identify mechanical actuation or single fixed hydraulic cylinder.  If the anchor is fixed, the front will self energize and the rear will de-energize, so without a doubt leading and trailing but what is the objective?  Even wear or equal braking effort from both shoes?  Generally equal wear would be the choice.  In general the classic friction equation of normal force and coefficient of friction applies but is independent of with surface area.  The trailing shoe has less normal force than the leading shoe so should it be long and soft or short with the same hardness, assuming soft linings have higher coefficients of friction.  The surface area must play a role just like wider tires somehow give more traction.  With that thought the trailing shoe should be longer to generate similar friction force to the leading shoe and then equal wear.  Changing the material is too confusing.  

 

However, the most common design is the floating anchor and single, double ended cylinder.  I see this design as one leading shoe that is in two pieces and works well in both directions of rotation, which is a limitation of other designs.  Don't lose sight of the point that the brake fluid acts on both pistons with the pressure and thus the same force.  The self energizing principle is an opposing force on the rear piston (cylinder on top) which in turns pushes both pistons forward within the cylinder.  That tells me the rear shoe is also self energizing and therefore the design has two leading shoes in both directions of rotation.  When you stop and think about it as the brake designers did over the years, it means both shoes should be alike and thus they are.  Now a fixed mechanical actuation may be something else again.  Those short and long, soft and hard sets may just have been part of the learning curve.

 

This is why I see drum brakes as the more eloquent design over discs, since discs need more massive components due to the high brute clamping force required, which can be readily supplied.

 

 

[Spinneyhill]

Of course!

 

Steve has a floating anchor (duo servo?) so has two leading shoes. So the short end should be the top of the front shoe and the bottom of the rear shoe, to avoid spragging and grabbing.

Edited April 18, 2018 by Spinneyhill (see edit history)

[kgreen]

On 4/16/2018 at 10:05 PM, Rusty_OToole said:

I remember the shoes being 2 different colors too, light and dark gray, as if they were made of different materials. That may have something to do with it. If they used to use a softer and harder material but now they are all the same.

I recall a recommendation for using two different brake lining materials when you had varied braking requirements.  Use a soft shoe for city driving which would grab well but also with a tendancy to overheat (not dissipate heat well) at highway speed braking.  Then swap the opposing shoe with a hard lined material shoe that would nt stop well or grab at slow speeds, but would not overheat on hard, high speed stops.  I read that years ago and don't know if that writer specified which material was to be used on the primary or seconday shoe.

[Larry Schramm]

The brake material is shorter on the one shoe to keep the brakes from grabbing and give a more uniform friction force from the leading / trailing shoes as applied to the brake drum. 

 

I have heard that this was pioneered in the 1924 French Grand Prix by the Dusenberg race team as they were the only team that had hydraulic brakes on their cars.   They also won that race which was the first American team to win that race.

[Ltc4748]

While in college during the 70's I remember the course on the braking system. We had theory and literature that spoke about the brakes in detail.  When the lining material went from the woven type to the asbestos base and each shoe was no longer anchored on one end, the manufacturers started making the linings different also. There is a Primary shoe (front) and a Secondary shoe(rear) at each wheel . The primary shoe was identified by the word primary marked onto the new lining or it was always the 2 shoes in a set with the shorter lining applied to them. The primary shoe lining material was a slightly different composition than the rear shoe. It was slightly softer or more "grabby" than the Secondary shoe. When the brake was applied both shoes move outward until they contact the rotating drum. The shoes then rotate toward the anchor pin. This action wedges the secondary shoe to the drum. For a more even brake wear and life they changed the asbestos mixture of material and the length on the primary versus the secondary shoes to get the most durability/life in the small space. I still have old lining sets that I keep for my antiques that are marked with ink "primary" and "secondary" in the boxes. Many times through the 70's, 80's and into the 90's I would see both primary shoes on one side of a vehicle or the primary shoe mounted on the rear of the wheel. They were always visibly worn harder than the secondary shoes were.