Jump to content

Drums...


Anderzander
 Share

Recommended Posts

Something’s confusing me, in how some single leading shoe drums are set up, which I can’t get my head around...
 
On British motorcycles, with single cam/cylinder drum brakes - a common mod was to shorten the friction material on the trailing shoe at the cylinder end.
 
We did it to make sure we had the servo affect: where if the leading edge of the leading shoe touches the drum first, then the rotation drags the shoe away from the cylinder - reducing the effort to brake.  i.e. the servo affect. Whereas if the trailing shoe touches first, then the rotation of the drum drives the shoe against the expanding cylinder - and is the exact opposite effect.
 
So we used to cut a section of material off that back shoe and it would improve the brakes wherever they had not been contacting with the leading edge of the lead shoe.
 
However the Standard set up on say a 109 rear is the opposite....
 
FD843DF8-E355-45C2-9F06-7E979F03974C.thumb.jpeg.404486996237a98e7089271592eba4df.jpeg
 
Where the description is:
 
The leading shoe, which goes towards the front of the car, has the lining fitted so that the distance from the wheel cylinder to the shoe lining is greater than for the trailing shoe.”
 
So, I’m really confused?
 
Anyone shed any light on this please ?
Link to comment
Share on other sites

Your explanation of how it works is incorrect. The leading shoe has a self engaging effect.  The location of the lining material on the pad has only a slight effect on this.

Touching first is meaningless.  Nothing happens until both touch and then pressure is applied evenly to both shoes by the cylinder which floats to automatically adjust to the location of the shoes.

Link to comment
Share on other sites

37 minutes ago, Red90 said:

Your explanation of how it works is incorrect. The leading shoe has a self engaging effect.  The location of the lining material on the pad has only a slight effect on this.

Touching first is meaningless.  Nothing happens until both touch and then pressure is applied evenly to both shoes by the cylinder which floats to automatically adjust to the location of the shoes.

Have a look here:

http://theengineersjunction.blogspot.com/2011/02/drum-brakes.html?m=1

When the brake is applied with the vehicle stationary, hydraulic pressure pushes each shoe outwards and an equal force is applied by each shoe to the drum. But this applied force does not remain equal when the vehicle is moving (Fig. 28.14A). The drag of the moving drum on the friction linings causes one shoe to be applied hard and the other to be pushed towards the 'off position. The shoe that does more work is called the leading shoe, and the other shoe is called the trailing shoe.

Its not equal - that’s why they say 75% of the braking force is from the front shoe.

Here’s some more ok the servo affect and the pad location - there is lots of stuff on the net about this.  Though here it seems to be saying two contradictory things? 

Position of shoe tip. The movement of leading edge of the friction lining towards the hydraulic expander increases the self-serve action.

which was my understanding ?

and 

the friction lining of the trailing shoe is normally placed nearer to the expander than in the case of the leading shoe.

which is how Land Rover have it
 

Factors affecting self-servo torque. 
These include : 
(a) Position of shoe tip. The movement of leading edge of the friction lining towards the hydraulic expander increases the self-serve action. 
(b) Coefficient of friction (u) of the lining. An increase in the value of u also increases self-servo action. 
(c) Position of the shoe anchor. Movement of the anchor towards the centre of the drum increases the self-servo action. 
Therefore the L&T shoe brake that has a large self-servo characteristic develops high braking power that means it has a large drum drag for a given application of force. This is made possible due to the use of energy from drum rotation to minimize the effort applied by the shoe expander. 
Conversely, the negative servo action on the trailing shoe must be minimized for obtaining powerful braking action. In order to achieve this, the friction lining of the trailing shoe is normally placed nearer to the expander than in the case of the leading shoe. When this feature is incorporated, proper attention must be given during reassembling the brake, otherwise if the shoes are reversed, a fierce braking action with possible grab (lockup) results during application. 

My experience from British bikes is that the brakes can go from woeful to acceptable when you cut the material in the way I was taught. 
 

So I expect I am missing something - but the applied force isn’t even and the pad position is a factor, and the front shoe plays a primary role that can be offset by poor timing of pad contact.

Link to comment
Share on other sites

24 minutes ago, Soren Frimodt said:

Maybe you dont want the servo effect on a hydraulic setup because it is not needed and it might even make the brakes drag a bit as they wont want to let go right away when you release the pressure

Isn’t that what the springs are for though Soren?  And why on the 10” drums on the front shoe is sprung the way it is? 

Link to comment
Share on other sites

46 minutes ago, Soren Frimodt said:

Maybe you dont want the servo effect on a hydraulic setup because it is not needed and it might even make the brakes drag a bit as they wont want to let go right away when you release the pressure

I think your right:

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

If the leading edge of the leading shoe were to touch the drum first the brake would grab.

Perhaps on those British bikes the braking was so woeful that setting the brakes up to grab was the only way to make them work ? 

Link to comment
Share on other sites

55 minutes ago, Anderzander said:


Position of shoe tip. The movement of leading edge of the friction lining towards the hydraulic expander increases the self-serve action.

which was my understanding ?

Yes, but it is not a huge difference.  If you buy new shoes today, you will see that they use evenly spaced linings.  In use, you won't notice any difference when changing from the old ones with the offset linings.

The vector drawing on that link is incorrect. It is the tangent from the center of the friction surface that determine the amount of servo action, not the location of the leading edge. 

Link to comment
Share on other sites

Few musings: 

A good reason for removing material from the trailing shoe would be to increase pressure by making the contact area smaller to compensate for the anti servo effect. The logical place to do this is the trailing end of that shoe, but we're talking about Land Rover here :hysterical:. Don't expect it to have any credible thought put into the design or construction  After market parts even less so. Every time I look at something and think "well that's just jeffing stupid", I have to remind myself what I'm looking at, move on and call it "charm".

  • Haha 1
Link to comment
Share on other sites

I would assume the the brakes are set on the rear that way so that when moving backwards there is at least 2 shoes out of all 8 that will add some bight, assuming the 109 has a twin leading shoe setup

how fast do old bikes move backwards?

 

Edited by ian_s
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy