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MG MGB Technical - Please explain 'leading edge' on brake shoes

I've read to put the "leading edge" forward, correct? Just what is the leading edge? I'm trying to adjust my new shoes and I can not get rid of the scraping noise. I have the emg brake off (it's new) and I have them adjusted to the smallest setting on the adjuster, won't go closed anymore. Thought maybe the leading edge had something to do with it. Any ideas how to adjust with less scraping? Or is some scraping exceptable? It scrapes for about 6" of the turn of the wheel. Any help? Thanks, Jeff
theBman

By definition, the leading edge is always forward. In drum brakes there are two (sort of) types of brakes. Twin and single leading shoe (see link). The leading edge is the point of first contact and is the point on the shoe furthest from the shoe pivot. If you were looking at the left rear drum, to have the leading edge forward, the shoe pivot would be at 3 o'clock and the brake cylinder would be at 9 (toward the front bumper). That would be true only for the top shoe. The bottom shoe would be trailing edge forward. Typically, rear brakes are single leading shoe and front drums are twin.

http://www.dansmc.com/drum.jpg
Kemper

I forgot to mention that you'll have some drag after adjustment until the shoes seat. The drag/scraping is more pronounced when adjustment is combined with new shoes. If you adjust the brakes (especially new ones) with little to no drag, they won't be working efficiently and the e-brake work work well either.

Let them wear in and then adjust again and they'll work better and make less noise after adjustment. Because the brake load it applied first at the wheel cylinder, the shoes wear faster there until more of the shoes make contact. Hope that helps.
Kemper

Thank you, Not sure I completely follow that but I'll give it some thought, thanks alot
theBman

Ok, I'll drive and adjust, thanks again
theBman

I should have mentioned that my example about the left rear wasn't specifically for any car (I can't remember how the 'B brakes are). I think the 'B pivot is between 7 and 8 o'clock and the wheel cylinder is between 1 and 2.

Since the 'B (and midget) shoes have a mechanical lever e-brake, the shoes go on one way. That's possibly what was meant by "leading edge forward". Anyway, I've never heard a working drum brake that didn't make at least a little noise.
Kemper

Hi Jeff.

Were you thinking of leading edge chamfering, which reduces the tendency to grab ?. I always chamfer both ends of both shoes.

As advised above, don't worry about it unless the binding is bad enough for the hubs to get hot after some open road (ie no braking) driving.

Did you remember to apply a little proper brake grease on the ends of the shoes ?.

It isn't obvious, but a brake shoe needs to be able to move slightly in a fore-and-aft direction as it is applied and released, and a little grease can make quite an improvement in braking smoothness in my experience. It also helps the shoes to release more easily, and might allow your shoes to pull off a little further.

I wonder if it is time for some new 'pull off' springs ?, they are exposed to frequent heating and do tend to deteriorate with age.

Don
Don

Jeff - I believe that you are confusing terms. Leading "edge" is the end of the shoe that contacts the drum first. Each shoe has a leading edge and a trailing edge, the leading edge of one shoe is pointing forward and the on the other shoe is pointing back. The leading edge of the shoe pointing back engages the drum first when the ca is moving forwand and vica versa for the shoe with the leading edge pointing forward. This is a true situation on drum brakes as found on the MGB rear wheels, with a double ended brake cylinder that pushes against both shoes. The shoe with the leading edge engaging the drum first when the car is moving forward is pulled toward the drum by the rotating drum itself, creating a small amount of servo action and make that shoe somewhat more active in the braking process. That shoe is refered to as the leading "shoe". Years ago when drum brakes were the only thing available, the two shoes for a wheel were made of two different materials, the one with the longest wearing material was designated the leading shoe. That is no longer done because if drum brakes are used on a car, they are always the rear brakes, which only do about 30% of the braking anyway. During the change over period from drum to disk brakes, some cars, like the TDs, TF, and MGAs used twin leading shoes on the front wheels and had two seperate wheel cylinders on each wheel. I even had a Ford Courier (Mazda) pickup with twin leading shoes on all four wheels. Cheers - Dave
David DuBois

My 69 VW Bus had twin leading shoes on front (with two wheel cylinders per wheel), as does my 55 TF. Many motorcycles also used twin leading shoes on the front brakes, until front disks became common. The TF has a single piston on the rear brakes, but the entire wheel cylinder can slide back and forth a bit on the brake backing plate.
Mark B.

Ok, if each shoe has a leading edge and a trailing edge how does one tell which is which? And does it really matter?
theBman

To use the clock analogy again, If the shoes are fixed on a pivot at 6 and a lever at 12 and if the wheel is rotated clockwise and the brake appied...the leading edge on the left shoe is at the pivot (6 o'clock) and the leading edge of the right shoe is at the lever (12 o'clock). This reverses if the wheel rotates anti-clockwise. One shoe is leading and one is trailing. This is a single leading-shoe brake and makes the brake work equally well regardless of wheel rotation.

In contrast, a TWIN leading shoe brake has a seperate pivot and lever for each shoe. In the example above, the left shoe now pivots at 12 and contacts a 6, while the right shoe still pivots at 6 and contacts at 12.

As mention by others the leading shoe gets better bite because friction actually puts it harder into the drum. Think of a pole-vaulter. He runs, jams the pole into the ground ahead of him and it sticks. If he jammed the pole BEHIND him, it would just slide along. The benefit of the twin leading shoe brake is that when rotating forward, both shoes grab rather than one grabbing while the other slide along (trailing). The downfall of twin leading shoe is that when rotation reverses, you now have twin trailing shoe brakes. Anyone that's ever stopped an old motorcycle with TLS front brakes (Bonneville or BMW) on an upward hill can comment on twin trailing shoes.
Kemper

What confusion !!!! Just look at what Dave DuBois has said. It's the the end of the shoe that first meets drum surface as the car travels foward and that's it. No clocks involved. This edge is pushed up by the wheel cylinder, however the rear is different with only one cylinder which obviously means that only one rotation facing shoe can be pushing and so leading which leaves the other to be trailing. This shoe however becomes the single leading shoes when reversing and also ensures that the handbrake has one leading and one trailing shoe to make sure it holds in either direction.
Iain MacKintosh

I think the key point is to recognise that the two brakeshoes in a pair are DIFFERENT and that they must be installed in the correct position. Refer to Haynes or other for a picture showing their relative positions.

Barry
ex 73 MGB
75 TR6
B.J. Quartermaine

"This edge is pushed up by the wheel cylinder, however the rear is different with only one cylinder which obviously means that only one rotation facing shoe can be pushing and so leading which leaves the other to be trailing."

"This shoe however becomes the single leading shoes when reversing and also ensures that the handbrake has one leading and one trailing shoe to make sure it holds in either direction."



If these two sentences are any less confusing than a clock analogy, something must be seriously wrong with my brain. The link in the first response explains it all.
Kemper

Errr, the shoes should be fitted so that a point on the brake drum, when the car is travelling forwards, passes over the empty bit of the shoe first before it reaches the friction material. Pics at http://www.mgb-stuff.org.uk/wn_brakesframe.htm and click on 'Rear Shoes'.
Paul Hunt 2

Thankyou Paul Hunt - you have addressed my point with a picture which, as always, is worth a thousand words or many e-pinions.

Regards,

Barry
B.J. Quartermaine

Thanks Paul and Barry, I just fitted the rear shoes to my car this weekend and completely didn't notice that they are supposed to go a certain way round. I will be rushing home later to check and of course I am sure to have them on the wrong way round! Oh well, am pretty good at getting them on and off now. First time I was so busy trying to keep the springs from flying away I forgot the handbrake lever. The second time I remembered the lever but got the sides muddled up! Luckily I don't have the drums on yet as I am waiting for new handbrake pull off springs to arrive. Paul, the pics help enormously. Thanks!

Simon
Simon Jansen

Actually I was thinking about this on the way home and as long as you have the springs and handbrake lever poking though the right holes the shoes will have the correct edge first. I checked mine and they are fine.
Simon Jansen

"Errr, the shoes should be fitted so that a point on the brake drum, when the car is travelling forwards, passes over the empty bit of the shoe first before it reaches the friction material. Pics at http://www.mgb-stuff.org.uk/wn_brakesframe.htm and click on 'Rear Shoes'."

THANK YOU for that photo reference. Jeff
theBman

This has been a great educational read. To take it a step further can anyone explain which is primary and which is secondary shoe. Seems that sometimes there are two types of linings in a set of shoes and they call them primary and secondary. Now that leading and trailing is clear It would be great to know ths also.
Thanks!
Bob

I'm assuming it's the same thing…just different terms. Leading edge=primary, trailing edge= secondary. The reason I asked the question in the first place was I thought there was a visual differents between the two shoes as you look at them off the car. Guess not.
theBman

Not to stir the pot again, but I think the word "edge" is still a little confused. Both shoes have a leading and trailing EDGE. The end of the shoe (see MGB-specific picture above) that contacts the drum first, determines which shoe is the lead and which is the trailing. Both shoes are the same and can go on in only one way.

I guess it was overlooked, but if you see Dave's post above:

"Years ago when drum brakes were the only thing available, the two shoes for a wheel were made of two different materials, the one with the longest wearing material was designated the leading shoe. That is no longer done because if drum brakes are used on a car, they are always the rear brakes, which only do about 30% of the braking anyway."

The primary is the leading shoe, which provides the most braking due to the design. The secondary is the trailing shoe, which provides less braking becuase of its mechanical disadvantage.
Kemper

Kemper,
Yes, I did miss that. It is the answer to what I was asking, thanks!
Bob

This thread was discussed between 03/12/2005 and 06/12/2005

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