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  1. #1
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    does the front suspension compress with rim brakes more than discs?

    HI,
    Is it just me, or does the front suspension compress under hard front braking more so with a rim brake than disc?

  2. #2
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    has anyone experienced this?

  3. #3
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    I don't know from personal use, but I would think it would matter with the amount of power used to slow one down, and weight transfer to the front end?

  4. #4
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    Good question. I was just looking at a discussion on the differences of forces on spokes on a disc vs rim brake, makes sense there's a difference from what I saw there. OTOH I've never used the same fork with both rim and disc brakes, so no meaningful personal experience. Hopefully a kindly engineer will educate us....
    "...the people get the government they deserve..."
    suum quique

  5. #5
    Meh.
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    Please do not bump your own thread.

    There are many different things to consider.

  6. #6
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    There is no reason to the fork to compress differently.
    The only thing that matters is that disc brakes may brake more, and the weigh transfer to the front makes the fork compress.
    But if you brake the same with both brake types, it should compress the same.
    The brakes don't apply any force in the direction of the compression of the fork.

  7. #7
    TNC
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    I have no business wading into this engineering swamp, but I've seen the difference described in this way. With disc brakes, the rotor is the round, relatively small metal disc that is clamped by the pads in the caliper. With rim brakes, the rotor is the wheel rim, obviously clamped by the pads on the brake arms.

    Rim brakes would be unbelieveably powerful if they had the clamping force of a good hydro disc brake...surely too much...probably be like an ejection seat. While there are some forces at the hub, spokes, and fork that are somewhat different between the two brake systems, I've always understood resulting brake dive or fork compression to be the sole and equal result of weight transfer based on the braking power...regardless of the system. Of course, all this is assuming that all other factors are equal...same fork, same spring rate, same bike, same rider...well...you get the drift. If there are other factors at work here, I'd be glad to hear about them. Interesting discussion.

  8. #8
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    Ah - but I can think of why disc brakes would compress fork less under same braking load.
    - Rim brake distributes braking load (a torque) on both fork legs
    - Disc brake put that torque on only one leg --> more bending of that fork leg --> more resistance to travel --> less fork compression

    Someone's gonna argue that rim brake would then bend both legs and increase resistance on both. I would counter that the single leg stress bend it more so result in more resistance than when both legs are torqued. Of course, this difference would be very small, even smaller if you fork has a stiffer arch - probably negligible for most of us. But hey, if you can detect it, I've got the explanation for it.

  9. #9
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    My experience with rim and disc brakes is that disc brakes seem to have more stopping power than rim brakes, so I'm more careful to feather or modulate disc brakes when I am stopping. Think about your first few times riding with disc brakes, did you go or nearly go OTB?

    From an engineering perspective, rim brakes are going to put a lesser torque on the fork than disc brakes will. Modern v-type rim brakes are mounted on the fork a short distance from the crown, so will have a relatively short lever arm. Disc brake calipers are mounted near the axle which is a much longer lever arm. Torque is calculated as the product of force and distance.

    Even though a disc brake caliper is mounted on only one fork arm, the bending torque will still be distributed or spread to the other fork arm through the axle, so both arms will experience approximately the same amount of torque.

    Sorry, I thought I was going to be able to resist posting, but I couldn't help myself!

  10. #10
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    Quote Originally Posted by 4slomo
    Even though a disc brake caliper is mounted on only one fork arm, the bending torque will still be distributed or spread to the other fork arm through the axle, so both arms will experience approximately the same amount of torque.
    I kind of agree. However, that's not 100% true, only 99.9%...
    The axle will also flex.
    Wether or not the flexing of the axle is enough to allow one side to flex more than the other and affect the fork performance... it could be debated. I personally don't think it would affect.

    My personal experience... I had a bike with v-brakes. It had a 28 mm stanchions 100 mm travel fork (a long travel fork for such stanchions, it was very, very flexy). I installed disc brakes and didn't notice any change.

  11. #11
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    Quote Originally Posted by 4slomo
    From an engineering perspective, rim brakes are going to put a lesser torque on the fork than disc brakes will. Modern v-type rim brakes are mounted on the fork a short distance from the crown, so will have a relatively short lever arm. Disc brake calipers are mounted near the axle which is a much longer lever arm. Torque is calculated as the product of force and distance.
    Yes, torque is product of forceXdistance; only, you're using the wrong distance. Every torque is expressed with respect to a pivot point, in the case of the crown as pivot point, the force that acts to make that torque comes from your axle, not brake. The pivot that your brakes act on is the axle - the torque about the axle will be identical if the deceleration is the same.

  12. #12
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    Forget all the (complicated) physics here....

    most likely explanation is that braking force is better modulated through discs (applying frictional force very accurately)... if you grab a fist-full of v-brakes, not usually as much modulation... leads to a bigger change in inertia, etc

    the other possibility is that the fork with v-brakes is poorly set-up (wrong preload or poor compression damping) or just old and clapped-out
    You better just go ahead and drop that seatpost down to the reflector... the trail gets pretty rough down there.

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