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  1. #1
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    Avid BB7s - MTB vs Road feel

    Just built up my first road bike with discs (Soma Fog Cutter). I went with BB7s because I have used the mtb version extensively over the past 16 years on many mountain bikes (also have XT hydros on one bike), and have always been happy with them. And they are cheap (already had brifters with cable brake levers).

    First, I'll say that I am totally sold on discs for road use. These are not even the best discs out there and I like them more than any road rim brakes I've owned or demoed. This was not a big surprise to me, though.

    What I DID find interesting was the difference in feel from all of the BB7s I've used on my mtbs. Not quite as solid feeling, and I think it is due to road brakes (including the road BB7 calipers) using a shorter brake cable pull, and therefore more tension in the cables and housing.

    I've always felt that brakes on road bikes never feel quite as solid (more flex in the system) as any of the V-brakes I've set up on my bikes, even pretty cheap ones (in some cases I have used brake boosters on particularly flexy frames or forks). At first I thought it was because I used compression-less brake housing on my MTBs, but even after setting up both mine and my wife's road bikes with compression-less housing (which did help), I still felt that way.

    I have long suspected a lot of the extra flex in the system on the road bikes was in the cable housing, due to the higher cable tension.

    The fact that compression-less housing always makes such a big difference for me suggested to me that the cable housing was responsible for a lot of the flex in any cable braking system. I have especially noticed the difference between regular and compression-less housing with mechanical discs (on mtbs) where you have much longer cable/housing runs.

    But I was never sure, because the caliper designs are different between mountain and road. But with this setup, it is the closest I've gotten to an apples-to-apples test. Same caliper design (but different cable-pull ratio), same brake cables/housing model, roughly same length cable/housing runs, different levers.

    Anyway, I am still very happy with the brakes. Just thought the difference in feel between these and the mountain version was interesting.
    Last edited by kapusta; 04-07-2017 at 12:18 PM.

  2. #2
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    I agree with your observation that less cable pull means more cable tension, which exagerates cable stretch and housing compression. I'm curious though what brake levers you're using. Shimano has made 3 generations with successively more cable pull (all 11spd levers pull the most, then 10spd with internal shift cables, then everything previous). Prehaps they've noticed the same thing and have been gradually remedying the problem.

  3. #3
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    Quote Originally Posted by Lelandjt View Post
    I agree with your observation that less cable pull means more cable tension, which exagerates cable stretch and housing compression. I'm curious though what brake levers you're using. Shimano has made 3 generations with successively more cable pull (all 11spd levers pull the most, then 10spd with internal shift cables, then everything previous). Prehaps they've noticed the same thing and have been gradually remedying the problem.
    These are 10 speed 105 levers. Pretty sure the 5600 model. They do not have the hidden shift cables.

    So I guess these would fall into the "everything previous" category.

  4. #4
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    Well newer levers will sharpen up the feel and compensate for the cable stretch.
    A stretching spring (stretching cable) works like a lever, same force at both ends but one end moves more.
    Same with a compressing spring (compressing housing).

  5. #5
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    Quote Originally Posted by Lelandjt View Post
    Well newer levers will sharpen up the feel and compensate for the cable stretch.
    .
    In some sense that is true. However, if all I do is switch to levers with more cable pull, but I keep the brake calipers the same, it may "feel" more solid, but I am going to have to pull harder on the lever for a given amount of braking force. In the end, the tension on the cable is going to need to be the same for a given amount of braking force, as will the flexing of the housing. the leverage (cable pull) at the levers just mitigates that (in terms of lever feel and travel) in different ways.

    Using a V-brake lever with a road caliper would be an extreme example.

    The only way to reduce the tension needed from the cable for a given amount of braking (clamping) force is to change the leverage (pull ratio) at the calipers.

    Question: On the later 10 and 11 speed group-sets, (where you mentioned they increased the cable pull of the levers) did they also change the pull ratio of the brake calipers?

    A stretching spring (stretching cable) works like a lever, same force at both ends but one end moves more.
    Same with a compressing spring (compressing housing)
    Not sure about that. By definition, a lever changes the force needed or exerted from one end to the other. If it were a lever, and you pulled on the end that travels farther, then the other end (shorter travel) would have more force on it (I guess that would be higher tension in this case?).

    The fact that you pull farther on one end of the spring than the other end moves means that you are spending energy stretching the spring itself. It is inefficient.

  6. #6
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    Yes, the 3 different generations of calipers are made to have a different amount of cable pull. I paired gen 1 brakes with gen 2 levers and the action was very sharp but once the pads hit the rims it was wooden feeling. You had to squeeze very hard.

    Remember that in stretching a spring you aren't losing any energy except a very small amount to internal friction. Since you are pulling on one end of the "spring" with a lever, not directly (the brake lever is a lever), spreading the work across a greater distance means less force is required for the same force output at the other end of the spring compared to replacing the spring with a rod. It's a weird thought experiment but it's why the company I work for (Santana Tandems) has a special low leverage caliper for the rear cable disc brake. It's engineered to work with the extra cable stretch of a tandem length brake cable. Put that caliper on a shorter cable and it feels like my mix-matched Shimano caliper brakeset did.

    I'm not very good at explaining this sort of thing so let me know if I've lost you. It took about a half hour at NAHBS for me to explain to my friend at TRP that a cable disc system (or even rim brake) transfers every bit of power that a hydro system does (minus a tiny bit lost to internal friction happening during stretching/compression) but eventually I got it across.

  7. #7
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    Quote Originally Posted by Lelandjt View Post
    Remember that in stretching a spring you aren't losing any energy except a very small amount to internal friction. Since you are pulling on one end of the "spring" with a lever, not directly (the brake lever is a lever), spreading the work across a greater distance means less force is required for the same force output at the other end of the spring compared to replacing the spring with a rod.
    This is fairly basic physics. What you are saying here, while an understandable misconception, in not really true.

    A think a few things need clarification, here:

    1- While it is technically true that you don't "lose" energy when stretching a spring, the implications are not what you seem to think they are.

    Lets put internal friction aside for now (we seem to be in the same page on that, anyway).

    When you deform a spring (stretch, compress, bend...) you don't "lose" the energy.... it is stored IN THE SPRING. It stays in that spring until the spring is released. Now, if you can harness that energy, great. A trampoline or bow and arrow are two great examples. However, in the case of brakes and the spring being the cable and housing, this means you get the energy back when you RELEASE the brakes. All that energy is doing is is helping the brake lever on the bar snap back. That is ALL it is doing for you, and it is AFTER you are done braking. So unless you have plans to use your brake lever as some sort of catapult or slingshot, that energy is not doing you any good.

    2- While it is true that in a lever (with mechanical advantage) the input end travels farther than the output end, this does NOT mean that something is a lever JUST BECAUSE the input end travels farther. I think this is where you are being tripped up. There are other reasons, besides mechanical advantage, that the input end may move farther than the output end. And the point I made above may explain what you may be missing, here.

    Here is a thought experiment: Pull on two ends up a spring with a tension meter on both ends. I think it should be intuitive that the tension (force) on each end would be the same. Or press a spring together between your hands. You push equally hard on both ends. It does not matter which end "moved" more, the force on either end will always be the same. And this is true from the beginning of the compression, where needed force is low, the the end, when you have reached the maximum force you are wishing to exert.

    You seem to be saying that because you exert that force over a distance, that means it takes less force. No it does not mean that. Whether you are pushing with a spring or a rigid rod, the force is the same on both ends of that spring or rod. However, in the case of pushing on the spring, you are exerting that force OVER A DISTANCE. And this is where the extra energy (or work) comes in....

    Force X Distance = Work (or put another way, putting energy into something)

    In this case, that "something" is the spring, which, as explain above, stores it until you release the bake lever (which is not useful for purposes of braking).

    To repeat what you said, I don't know if I am explaining this well enough. However, the bottom line is that a spring is in no way lever-like in terms of mechanical advantage.

  8. #8
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    Quote Originally Posted by Lelandjt View Post

    I'm not very good at explaining this sort of thing so let me know if I've lost you. It took about a half hour at NAHBS for me to explain to my friend at TRP that a cable disc system (or even rim brake) transfers every bit of power that a hydro system does (minus a tiny bit lost to internal friction happening during stretching/compression) but eventually I got it across.
    If you change "power" to "force", then I would sort of agree with that with some clarification.

    An important point that I think we may agree on is that flex in the system does not change the amount of force at the lever needed to achieve a given amount of braking force.

    The difference is how far you need to move the lever (and deform the cable and housing) to reach that.

    I should also add that it is not necessarily a BAD thing to have some flex and extra lever travel, even if it does take some more energy to use (it is not much, really). Many people find that this makes the brakes easier to modulate.

  9. #9
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    Quote Originally Posted by kapusta View Post
    This is fairly basic physics. What you are saying here, while an understandable misconception, in not really true.

    A think a few things need clarification, here:

    1- While it is technically true that you don't "lose" energy when stretching a spring, the implications are not what you seem to think they are.

    Lets put internal friction aside for now (we seem to be in the same page on that, anyway).

    When you deform a spring (stretch, compress, bend...) you don't "lose" the energy.... it is stored IN THE SPRING. It stays in that spring until the spring is released. Now, if you can harness that energy, great. A trampoline or bow and arrow are two great examples. However, in the case of brakes and the spring being the cable and housing, this means you get the energy back when you RELEASE the brakes. All that energy is doing is is helping the brake lever on the bar snap back. That is ALL it is doing for you, and it is AFTER you are done braking. So unless you have plans to use your brake lever as some sort of catapult or slingshot, that energy is not doing you any good.

    2- While it is true that in a lever (with mechanical advantage) the input end travels farther than the output end, this does NOT mean that something is a lever JUST BECAUSE the input end travels farther. I think this is where you are being tripped up. There are other reasons, besides mechanical advantage, that the input end may move farther than the output end. And the point I made above may explain what you may be missing, here.

    Here is a thought experiment: Pull on two ends up a spring with a tension meter on both ends. I think it should be intuitive that the tension (force) on each end would be the same. Or press a spring together between your hands. You push equally hard on both ends. It does not matter which end "moved" more, the force on either end will always be the same. And this is true from the beginning of the compression, where needed force is low, the the end, when you have reached the maximum force you are wishing to exert.

    You seem to be saying that because you exert that force over a distance, that means it takes less force. No it does not mean that. Whether you are pushing with a spring or a rigid rod, the force is the same on both ends of that spring or rod. However, in the case of pushing on the spring, you are exerting that force OVER A DISTANCE. And this is where the extra energy (or work) comes in....

    Force X Distance = Work (or put another way, putting energy into something)

    In this case, that "something" is the spring, which, as explain above, stores it until you release the bake lever (which is not useful for purposes of braking).

    To repeat what you said, I don't know if I am explaining this well enough. However, the bottom line is that a spring is in no way lever-like in terms of mechanical advantage.
    It is when it's being pulled on by a lever. It essentially moves the fulcrum.

  10. #10
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    Quote Originally Posted by Lelandjt View Post
    It is when it's being pulled on by a lever. It essentially moves the fulcrum.
    No, it does not.

  11. #11
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    Avid BB7s - MTB vs Road feel

    Quote Originally Posted by Lelandjt View Post
    It is when it's being pulled on by a lever. It essentially moves the fulcrum.
    The actual lever here (the brake lever) is a red herring for the purposes of discussing the effect of internal flex (or spring) in the system, as it pertains to mechanical advantage.

    Consider this thought experiment:

    Picture a lever with an input arm of 2' and an output arm of 1'. That is a 2:1 advantage. If you hang a 10 lb weight on the output end, it is exerting 10lbs of force on that end, and you will need to exert 5 lbs of force on the input end to hold it in place.

    Now, hang that 10 lb weight from a spring. It is still exerting 10 lbs of force on that end. And it still takes 5 lbs of force on the input end to hold that up. THe lever has the same mechanical advantage. The spring offers no mechanical advantage. The result is the same overall mechanical advantage.

    If simply pulling on a spring gave mechanical advantage, then you would be able to lift heavy objects easier (less force) simply by hanging them on a spring. You would be able to tow things easier (less force) by pulling them by a spring. Neither of these are true.
    Last edited by kapusta; 04-12-2017 at 11:59 AM.

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