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Softening rear disc

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by mattb, Dec 3, 2007.

  1. This might seem an unusual question: I want to make my rear disc weaker or softer! The 1978 SR500 came with a rear disc. The next year, and from then on, they came with a drum at the back - this particular rear disc is just too much on the bike. I frequently give myself a scare locking up the back, and I know others have the same problem with their '78, and I've never had this problem on any other bike (other's bikes with rear discs - I've always had drums) that I've ridden. So what can I do to weaken or soften the rear brake (aside from cross-drilled brake lines! :grin:) For instance, can one buy different grades of fluid, just as one can with fork oil to harden or soften up their action? Are there pads of different material on offer which vary enough to make a difference in this regard? I could of course source a '79 rear wheel with a drum, but I'd rather a less drastic and expensive fix of possible...

  2. OK so you don't want to make the disc softer, as that would make it break harder, you want to make it less responsive.

    two thoughts:
    Harder brake pads, or
    A larger master cylinder of a different bike.

    Alternatively you might try just adjusting the take up point. It may be it's just too high and therefore causing you troubles.
  3. There's a fairly good chance that your back tyre and your brake
    pads have both gone hard with age. Replace both of those, lube the
    brake pedal so you have a smooth action, and I'm sure it will be
    much more feel and harder to lock up.

    I have heard of people slicing a chunk off the brake pad material
    in order to weaken a rear brake, but that's under racetrack circumstances
    and I would never deliberately reduce braking power on a road bike.
    Also I have heard of people introducing a small amount of air bubble
    to the brake line to provide a more progressive/spongy feel, but that
    risks fluid boiling and having no brakes at all.

    +1 agree
  4. I think the most responsible thing to do would be to get out and practice regulating the rear brake pressure. The brake obviously works so its a matter of finding the bite point with a smooth action then regulating the pressure. :wink: (after making any adjustments)

    Your question is the opposite of someone saying that cant get away from the lights and keep the front wheel on the deck. Its just practice (unless you want to wheelie :LOL: )
  5. Thanks, that's a fair point as a general matter, but as with suspension, tyres, cable movement etc, the part itself can be problematic - ie in this case simply *too* snatchy. Part of the response needs to be, as you say, simply taking into account the problem and riding according to it, but equally it can be a problem that really does need to be mechanically sorted. I've heard other '78 SR500 riders complain about the same issue - I suspect Yamaha changed it to a drum after one year precisely because of this.

  6. Fair enough mate. I was just loathe to recommend hacking off half a pad.
    Have you spoken to any brake specialists? Maybe you can restrict the line pressure.
  7. Removing half the pad won't do anything and there is no need to put a restricter in. A larger master cylinder will give you less leverage.
  8. why would reducing the pad area not work to reduce braking effort?

    A shorter brake lever would also be easier to arrange than a new master cylinder as well - You could also play with the pivot point ratio of the existing master cylinder with the brake lever without having to change the erganomics of your foot.

    As a side note, I find the rear brake modulation of the dual sport much harder than the road bike, purely because of the different boots that I wear. A rigid heel boot with a stiff heavy sole means I lock the back brake a lot without meaning to.
  9. Because pad area doesn't go to braking effort only heat capacity. The ratio of piston areas is what governs lever effort.
  10. Yep, a harder brake pad that provides less grip. (Get expert advice on the selection of this.)

    Or, a larger master cylinder on the rear brake. No doubt there are units available with similar or the same mountings.

    But as said already, a simple cure may be to adjust the rear brake pedal so that you have to push down a long way with you toe to get any braking from it. I use this method on my bike to an extent. (Though I still boil the rear brake, and I've just worn out one rear pad, down to the metal, in 7200Km. :roll: :oops: )
  11. Only other thing that could be tried, would be to retrofit another caliper, with a smaller piston/braking area. Hell of a lot of effort though, I'd be going with the lever adjustment, maybe some stickier rubber for the rear, and maybe even consider dropping the rear a little. More weight on rear equals harder to lock up, at the expense of steering and front brake effectiveness.

  12. But hydraulics work on pressure - same pressure over a larger area equals more force retarding the disk. Granted, given the same slave cylinder area, changing the pad area a lot won't help you much because the effective pressure acting over the pad/rotor interface will change, but by reducing the pad area significantly, you can effectively overload portions of the pad and make them fade much faster than you should normally get - giving reduced brake effectiveness. Further, once you've glazed the pad, you'll get a fairly permanent change in friction coefficient until you fix it. I'll agree that for the same force applied to the pad, changing the area won't help you though.

    Just notching grooves into the pad would reduce the area acting on the rotor without affecting the heat sink mass of the pad significantly.
  13. In terms of pad fade, well that may actually work against you. The rear brake doesn't work much, so getting the pad surface up to temperature may actually increase grip.
  14. fairy nuff. I still think he's better off just not using it at all, else just a token light boot pressure without trying to reach a good peak braking force from the rear tyre! ;)

    btw, aren't road pads built to come up to temp at the first application anyway? Otherwise they aren't much use to you when you need to do an emergency stop at the end of the street.
  15. I think Roderick has summed up the thread.

    Not sure about street pads. I assume they'd get a bit better after a couple of applications.
  16. In much the same way that ladies' high heel shoe heels frequently break the edges off ceramic tiles, because an incredible amount of pressure is exerted over a much smaller area.

    However, if the pad is reduced in vertical size - say taking it down to one third of its height, then that, in spite of increasing the pad pressure, will rub against a much smaller part of the wheel.

    It's easy to try...

    Now that last bit is possibly the best suggestion so far.

    By moving the connection to the master cylinder closer to the pivot point the foot will move through a larger arc to achieve the same amount of pad pressure. The greater the movement, the more control you have.


    Trevor G
  17. I believe there are two problems involved - an over-sensitive brake system and one which also lacks feel.

    There are a number of possible cures:

    1) Change the pivot ratio as mentioned previously

    2) Change the master cylinder to one which has a smaller piston. This also changes the leverage ratio so that the pedal moves further for an equivalent pad movement. (This is the opposite of what others are suggesting - I believe their plans will make your brake even less sensitive)

    3) Reduce the height of the pad so that it is working on a much smaller diameter of disk. This must be done evenly around the mid-point of the pad - it must not be off-centre to the slave piston, otherwise the pad will skew when you apply the brakes.

    4) Convert to a fully floating brake caliper.

    This is the ultimate, sensitive braking system. The caliper is no longer rigidly fixed to the swingarm but has its centre machined out so that it can be mounted on a bearing. A torque rod is then connected from an outer edge of the caliper to a mounting point on the frame so that the rod is approximately parallel to the swingarm.

    This decouples the brake system from the suspension so that braking force is no longer affected by bumps or variations in terrain. It also makes braking a lot more sensitive - it slows and warns about lockup before it happens. Instead of trying to rotate the caliper around the axle and exert force on the suspension, braking forces act in parallel with the swingarm.

    A classic, custom bike like yours needs a classy floating caliper system.


    Trevor G
  18. Trev, the ladies shoe heel has a constant force (weight) acting on it. The smaller you make the area, the higher the pressure under the heel to support it. There is a problem with hydraulic brake systems as I noted earlier about the difference between the pressure exerted on the pad by the slave cylinder(s) and the pressure at the pad interface. For the same pedal feel, you will always get the same pressure at the slave (and thus force applied to the pad). Thus, you could use this effect to overwork the pad by reducing its area and causing its friction properties to change - it wouldn't be a direct effect like everything else that was mentioned. An added benefit of this would be to introduce more flex in the pad backing (especially for a single piston caliper) that would introduce more lever travel for the same effective pad interface pressure.

    Reducing the moment arm of the pad is a good idea if and only if you won't get the pad rolling in the caliper from the asymmetric load. You'd be safer to replace the whole disk with a smaller diameter version.
  19. Umm, Trev, you're not a physicist or engineer are you?

    A larger master piston does mean that the brake will begin to apply more quickly, so there will be less brake pedal travel. That is, the larger piston pumps more fluid for the same master piston travel distance, so it moves the slave piston a greater distance than with a smaller master piston.

    However, for the same force applied at the master piston (which translates to the same force at the brake pedal, or brake feel) a larger master piston applies that force over a larger area, which means a lower fluid pressure is generated.

    With no change to the slave cylinder, a lower fuild pressure produces a smaller force at the slave piston, which means a smaller force at the brake pad, which means less friction with the brake disc, which means less braking force generated. This, I believe, is what the OP wanted. You used the term "less sensitive", which could mean something different.

    Hence, a larger master cylinder means less braking force.

    A smaller master piston generates more fluid pressure for the same force applied at the master piston, but requires more pedal travel to engage the brake. More fluid pressure equals more force at the brake pad, which means more braking force.

    Think about a hydraulic car jack. You pump a tiny piston lots and lots of times, with little force, and you lift a whole car. It's the basic principle of levers, applied to hydraulic systems.
  20. About this pad area thing, I'm no physicyst, but why do some brake pads come with slots cut into the surface? The front pads on my Ducati are like this, surely they don't do that to make them less efficient?