Anderzander Posted April 23, 2020 Share Posted April 23, 2020 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.... 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 ? Quote Link to comment Share on other sites More sharing options...
Ex Member Posted April 23, 2020 Share Posted April 23, 2020 What you show is correct. The lining material is offset in the forward direction of rotation. Springs and shoes are in the correct location for a left install. Quote Link to comment Share on other sites More sharing options...
Anderzander Posted April 23, 2020 Author Share Posted April 23, 2020 Thanks for the reply - but my question is why is it like that ? When it makes it more likely for the trailing shoe to touch first and kill the self servo effect? Or is that just done with the cams.... Quote Link to comment Share on other sites More sharing options...
Ex Member Posted April 23, 2020 Share Posted April 23, 2020 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. Quote Link to comment Share on other sites More sharing options...
Soren Frimodt Posted April 23, 2020 Share Posted April 23, 2020 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 1 Quote Link to comment Share on other sites More sharing options...
Anderzander Posted April 23, 2020 Author Share Posted April 23, 2020 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. Quote Link to comment Share on other sites More sharing options...
Anderzander Posted April 23, 2020 Author Share Posted April 23, 2020 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? Quote Link to comment Share on other sites More sharing options...
Anderzander Posted April 23, 2020 Author Share Posted April 23, 2020 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 ? Quote Link to comment Share on other sites More sharing options...
Ex Member Posted April 23, 2020 Share Posted April 23, 2020 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. Quote Link to comment Share on other sites More sharing options...
lo-fi Posted April 24, 2020 Share Posted April 24, 2020 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 . 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". 1 Quote Link to comment Share on other sites More sharing options...
Daan Posted April 24, 2020 Share Posted April 24, 2020 Discs... 1 Quote Link to comment Share on other sites More sharing options...
Anderzander Posted April 24, 2020 Author Share Posted April 24, 2020 Don’t be naughty now ! Quote Link to comment Share on other sites More sharing options...
Mo Murphy Posted April 24, 2020 Share Posted April 24, 2020 1 hour ago, Daan said: Discs... This. Mo Quote Link to comment Share on other sites More sharing options...
Anderzander Posted April 24, 2020 Author Share Posted April 24, 2020 Philistine ! Quote Link to comment Share on other sites More sharing options...
smallfry Posted April 25, 2020 Share Posted April 25, 2020 Also, you need less brakes on the rear in any case. You have two cylinders on each wheel at the front, to create the twin leading shoe effect, which in theory gives more braking effort at the front. Quote Link to comment Share on other sites More sharing options...
Mo Murphy Posted April 25, 2020 Share Posted April 25, 2020 14 hours ago, Anderzander said: Philistine ! 😆 Quote Link to comment Share on other sites More sharing options...
ian_s Posted April 25, 2020 Share Posted April 25, 2020 (edited) 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 April 25, 2020 by ian_s Quote Link to comment Share on other sites More sharing options...
Anderzander Posted April 25, 2020 Author Share Posted April 25, 2020 Old bikes feel like they are moving backwards when in modern traffic - but you’re right ... 😊 Quote Link to comment Share on other sites More sharing options...
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