What is the point behind having 2 different length shoes per drum on drum brake equipped cars? I have heard that you can use 2 long shoes to help increase braking power, but was wondering what effect this would have over the short/long combo from the factory, besides the obvious more shoe contact.

 

The only reason there's a longer one, is for parking brake contact.
Giving the confinement of the shoes in the drums, the lowest part (parking brake part) doesn't exert as much force as further up on the shoe.
There could be more to it though, like the direction the drum is spinning, and the relationship to the shoes initial bite???


Ron

 

does seem silly to have 2 differant lining sizes when the metal it's attached to is the same, or could be made the same. i was under the impression, and i could be wrong, that the size had to do with equallizing the gripping due to the rotation of the drum. it's been that way forever but now days weight reduction and cost factor into every single part that goes on a car.

 

You know, I worry more about getting rid of the heat from these drums.
I seen some old photo's of racecars with drums that had little vanes all the way around with ducking...hmmm now thats a topic Drums out performing disk - oppps rear drums out performing rear disk. Shouldn't be to hard with stock 3rd gen disks.... right?


Ron

 

no **** right......... :sillylol:

 

I’m not sure why 2 diff size shoes either but here’s what I did to improve rear drum braking. First, rebuilt the wheel cylinders. Then went to the boneyard and found like new rear shoes for $2 a shoe. Got 1 set of organic, 1 set semi-met, all full length, no ¾. Cleaned everything up and lubed the wear spots on the backing plate. I put the organic’s on the rear side because I’ve heard that they don’t need to heat up to work like semi-met, and thats where the long shoe is stock. I put the semi-met shoes on the front side. So far so good. Now only when doing a panic stop do I hear the rears starting to squeak/skid as on the brink of locking up. So at this point I think they’re working as best they can and I am not considering a rear disk swap. E-brake pulls to about 3-5 clicks. So far no prob on a wet road either.

 

The two different sizes has to do with the "duo servo" action of drum brakes under a braking condition. The shoes expand out, then the primary shoe (the front-small one) is wedged down into the drum because of drum roation. This is "servo" or self serving action. The secondary (the rear-larger one) shoe is wedged up into the drum because of drum rotation. Since two pads have self serving action, they are called "duo servo", deal self serving.

Since the primary pad wedges down, the top of the shoe is of no importance to braking and therefore there is less pad at the top of the primary shoe. The secondary shoe wedges up, so the bottom of the pad is of little importance, except for when the parking brake is applied. This is why the secondary shoe has more material, it uses the whole pad under different conditions, where the primary one does not.

 

So then would replacing the smaller shoe with the same larger shoe increase braking power at all?

 

NO. Because the servo action he's talking about increases the force that pushes the shoes into the drum. Thus they dont need as much power boost. This is why cars got away without vacuum boosters for so many years. Also, drum brakes actually apply more braking force then disc brakes. Part of this is because of the servo action, and part is because drums have more friction material. All of their friction material is at the farthest part of the drum too which gives it more leverage. The lever distance of disc brakes varies from the inside edge of the pad to the outside.

This all makes it sound like drums are dandy and better then rotors. Except drums are crappy at getting rid of heat. So at best you get one good stop, then you keep gettin worse till you have nothing but the smell of burnt brakes. I've seen some old school brake upgrades where people have drilled holes through the drum friction surface. Kindof similar to drilled rotors. I guess this really improves drums, almost makes them like discs. Problem is, if it rains or you go through a puddle... bye bye brakes! Wet drum brakes dont do anything. I'd also be afraid of drilling a drum myself, could change the balance or even weaken the drum.

 

In theory...no. Besides, they're only drum brakes. Don't get too obsessed with upgrading them, just drive the car and enjoy it.

 

Im going to say that yes it will, but the secondary shoe is going to wear out quicker. The reason is, the primary (front) shoe with more lining material is going to grab the drum better (due to the increased friction surface) and therefore also exert more force on to the secondary shoe. The front (primary) lining has less material in order to balance the forces of both the primary and secondary shoes, and like fisherbody86 and chevymad said, it's due to the self energizing nature (or servo action) of this type of drum brake. Another way of looking at it is: the primary shoe only has the wheel cylinder pushing it against the drum. The secondary shoe has the wheel cylinder and the bottom portion of the primary shoe pushing it against the drum (drum turning in a forward direction). The secondary shoe can't push on the primary shoe because the pin at the top of the backing plate won't allow it to.