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  1. #1
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    Forks and rotor size

    I have a question about using rotors larger than what a fork is rated for.

    I have a Trek Soho (2008) and put on a 180mm rotor not realising it could be an issue. I later found out from Trek that the fork is only rated for 160mm rotors. I was a bit concerned and began reading through various thoughts on the matter.

    Surely the forks are built to withstand the braking force of a variety of differently weighted riders (the dropouts on this Soho fork are not puny). Wouldn't the extra 15% or so of torque caused by the 180mm over the 160mm be be negated because of this? Additionally, better braking systems will produce more torque because of better stopping power. I bet the Shimano M415 mechanicals that came stock on the Soho with a 203mm rotor couldn't stop as well as my current SLX using a 160mm rotor.

    Make sense? Any thoughts?
    Last edited by jsilva; 07-26-2011 at 10:34 AM.

  2. #2
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    Basically lets assume an asphalt flat road....

    You hit the brakes...the rear wheel should come off the ground....even if you move a little back from the seat.....

    That kinda sets the max torque the forks will be subjected to.

    And that goes for the mounts as well.

    So basically front wheel traction limits the braking torque.

    Now there maybe some high speed transients if you stab at the brake, anyway I have been running 203 rotors on 2005 Fox 32 forks and everthing is just fine.

  3. #3
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    I'm sure you are ok with using 180mm on your fork and probably would never had any issues with it but their lawyers only feel comfortable with 160mm in order to warranty the product.

  4. #4
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    There was an excellent discussion on this topic last year, and Andrwswitch posted a really nice picture of how the forces at the adapter mounts changes as the rotor gets larger
    Here

    (Read the whole thread if you really want to get into the theory of it all)

    If you look at that picture you can see that the top bolt of the mount is under compression with the small rotor, but there is more shear on it (and more torque) with the large rotor.

    I think there are some legitimate reasons why there are recommended rotor size limits for forks, but really, the limit should be about the same for all fork brands, unless one brand uses a vastly softer and weaker material that would allow the caliper to rip out of the mounting points. A lot of forks recommend up to 203mm. I didn't know some recommended only 160mm, and that doesn't really make sense to me. What kind of fork is it?

  5. #5
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    Fantastic thread, thanks!

    This is the stock fork on my Trek Soho which I use for commuting and fitness. I know people might think SLX hydraulics with a 180mm rotor is overkill, but I ride in London at a brisk pace and want all the braking power I can get. I want to feel like I'm in control of the bike.

    I've attached a photo that might be interesting in light of Andrwswitch's picture and his ideas on post vs. IS mount. As you can see, my IS mounts have an extended bit which might offset the extra distance of the calliper. What do you think?

    But there was also concern over fork flex. Perhaps it is just as simple as that Trek (or whoever) didn't test the fork for anything over 160mm and therefore can't recommend it. The Trek representative actually said, "I recommend using a 160mm rotor on this particular setup..."

    In any case, I still wonder how over-engineered the forks are. I imagine they must be in order to account for different braking systems and different weight riders...
    Attached Thumbnails Attached Thumbnails Forks and rotor size-photo.jpg  

    Last edited by jsilva; 07-27-2011 at 08:26 AM. Reason: smaller image

  6. #6
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    Quote Originally Posted by smilinsteve View Post
    There was an excellent discussion on this topic last year, and Andrwswitch posted a really nice picture of how the forces at the adapter mounts changes as the rotor gets larger
    Here

    (Read the whole thread if you really want to get into the theory of it all)

    If you look at that picture you can see that the top bolt of the mount is under compression with the small rotor, but there is more shear on it (and more torque) with the large rotor.No the torque is limited by front wheel traction

    I think there are some legitimate reasons why there are recommended rotor size limits for forks, but really, the limit should be about the same for all fork brands, unless one brand uses a vastly softer and weaker material that would allow the caliper to rip out of the mounting points. A lot of forks recommend up to 203mm. I didn't know some recommended only 160mm, and that doesn't really make sense to me. What kind of fork is it?
    The torque applied is constant....

    Indeed since the larger rotor has a larger lever arm the force applied is smaller with a larger rotor, to generate that constant torque. Agian except for short term transients.

    The basis for larger torque and larger forces comes from assuming that the clamping force of the caliper and the friction factor of the caliper.pads is constant this results in a larger torque and force....

    BUT even with a 160mm rotor one can cause the front wheel to skid....so again the max torque is constant regardless of rotor size.

  7. #7
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    Quote Originally Posted by jeffscott View Post
    No the torque is limited by front wheel traction
    The torque applied is constant....
    Nope. Don't confuse the torque on the rotor with the torque on the mounting hardware.

    The caliper grabs the rotor and a torque is generated through the rotor to the hub. Since the caliper doesn't move, there is an equal and opposite force on the caliper at the mounting hardware. The direction of the force and the distribution of the force through the adapter and fok leg depends on the spacial arrangement of the caliper to the rotor and mounts. The force on the hardware could be compressive, tension, shear, etc, depending on where the caliper is at. With a larger adapter, the torque on the upper bolt is greater, even though the total torque on the rotor is not greater.

    Quote Originally Posted by jeffscott View Post
    Indeed since the larger rotor has a larger lever arm the force applied is smaller with a larger rotor, to generate that constant torque. Agian except for short term transients.

    The basis for larger torque and larger forces comes from assuming that the clamping force of the caliper and the friction factor of the caliper.pads is constant this results in a larger torque and force....

    BUT even with a 160mm rotor one can cause the front wheel to skid....so again the max torque is constant regardless of rotor size.
    I agree with all that.
    Last edited by smilinsteve; 07-27-2011 at 11:30 AM.

  8. #8
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    I wonder if the max rotor size is because they are worried about the QR and dropouts more than the strength of the leg.

  9. #9
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    Quote Originally Posted by smilinsteve View Post
    Nope. Don't confuse the torque on the rotor with the torque on the mounting hardware. Torque is Torque it does not change with location

    The caliper grabs the rotor and a torque is generated through the rotor to the hub. Since the caliper doesn't move, there is an equal and opposite force on the caliper at the mounting hardware. The direction of the force and the distribution of the force through the adapter and fok leg depends on the spacial arrangement of the caliper to the rotor and mounts. The force on the hardware could be compressive, tension, shear, etc, depending on where the caliper is at. With a larger adapter, the torque on the upper bolt is greater, even though the total toque on the rotor is not greater.

    Again the torque is constant throughout the support system.
    The forces change yes, since the torque applied is constant and the shape and location of the bracket is also constant the forces are also the same.

    The asumption that is made to increase the forces is that the torque that the brake appliew to the whell increases and that increases the forces...

    So again with the torque limited by wheel traction (or OTB), the forces don't increase appciable..

    We know this because the adapter for a 160mm rotor or a 203 mm rotor fit the same post mounting brackets....

    It is possible that some forks are made of weaker materials and therefor the threads in those forks cannot take the load.....

    But again the loads are equal becasue the applied torque is the same. So the same limit would occur with a 160mm rotor or a 203 mm rotor.

    Again the false assumption that a bigger rotor applies a greater torque is where the error is made.

  10. #10
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    Quote Originally Posted by smilinsteve
    With a larger adapter, the torque on the upper bolt is greater
    So would you say that the extra bit on my caliper mounts provide extra support for the upper bolt? I thought part of the issue was the change of angle for the lower bolt, more force pulling up as compared to a smaller rotor. On mine it looks like there would still be compressive force on the upper portion, but not as much on the lower.

    Quote Originally Posted by jeffscott
    Again the false assumption that a bigger rotor applies a greater torque is where the error is made.
    Aren't you assuming a 160mm rotor can always provide enough torque to accomplish traction loss or OTB? This hasn't been my experience, and I know I'm not alone. In a situation where a 160mm rotor cannot accomplish this, a larger rotor could provide more leverage and more stopping power, no? Like a long wrench vs. a short one...

  11. #11
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    Quote Originally Posted by jeffscott View Post
    The forces change yes, since the torque applied is constant and the shape and location of the bracket is also constant the forces are also the same.
    So the forces change, or the forces are the same?


    So again with the torque limited by wheel traction (or OTB), the forces don't increase appciable..
    The torque generates force on the caliper which creates a reaction at each of the caliper mounting bolts. Since those are the only 2 points of contact between the frame (adapter) and caliper, the sum of the forces at the bolts equals the frictional force from the rotor on the caliper. The total force is less with a larger rotor.

    Even if you assume the torque on the rotor is limited by traction, the individual forces on each of the bolts change, as does the direction of the force, when the caliper is farther from the mounting points on the frame. With a small rotor a larger component of the force is directed along the axis of the bolt (compression) and generates no moment about the mounting point.

    See? The direction of the force is what's important. A bolt can take more force in compression than in tension or shear. And a force along the bolt axis generates no torque, but that same force directed perpendicular to the bolt axis creates the torque on the mount.

  12. #12
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    Quote Originally Posted by smilinsteve View Post
    So the forces change, or the forces are the same?




    The torque generates force on the caliper which creates a reaction at each of the caliper mounting bolts. Since those are the only 2 points of contact between the frame (adapter) and caliper, the sum of the forces at the bolts equals the frictional force from the rotor on the caliper. The total force is less with a larger rotor.

    Even if you assume the torque on the rotor is limited by traction, the individual forces on each of the bolts change, as does the direction of the force, when the caliper is farther from the mounting points on the frame. With a small rotor a larger component of the force is directed along the axis of the bolt (compression) and generates no moment about the mounting point.

    See? The direction of the force is what's important. A bolt can take more force in compression than in tension or shear. And a force along the bolt axis generates no torque, but that same force directed perpendicular to the bolt axis creates the torque on the mount.
    The force on the caliper with a larger rotor is reduced. Proportional to the larger lever arm.

    The force on the fork braket is the same since it has the same lever arm regradless of rotor size.

    The upper bolt carries shear load only the compresive load is taken by the face of the bracket.

    The lower bolt carries shear load and tension load.

    The higher the braking torque the higher the shear load and the tension load.

    Geiven the same wheel braking torque the shear and tension loads are equal

  13. #13
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    Quote Originally Posted by jsilva View Post
    So would you say that the extra bit on my caliper mounts provide extra support for the upper bolt? I thought part of the issue was the change of angle for the lower bolt, more force pulling up as compared to a smaller rotor. On mine it looks like there would still be compressive force on the upper portion, but not as much on the lower.
    Yes it looks like your mount is reinforced to resist the forces coming from the caliper.



    Aren't you assuming a 160mm rotor can always provide enough torque to accomplish traction loss or OTB? This hasn't been my experience, and I know I'm not alone. In a situation where a 160mm rotor cannot accomplish this, a larger rotor could provide more leverage and more stopping power, no? Like a long wrench vs. a short one...
    Yes. You make an excellent point that I also brought up in that other thread. If your current brake system with 160mm rotor can decelerate your wheel from 30 mph to zero instantly (at which point you skid or fly over the bars), then a bigger rotor will do the same thing and generate the same torque on the hub when doing it. But more realistically, it takes some time to decelerate to zero and the larger rotor would allow you to to it more quickly, generating a larger max theoretical torque.

  14. #14
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    After working on some bikes with v-brakes I was testing the power of them and accidentally did a stoppie, and one of bikes was my wife's comfort hybrid! The other was my MTB. I was shocked because it never happened on my Soho with disc brakes. So I intentionally tried to do one on the Soho and it happened.

    My point is that I realised it is an issue of weight distribution. I can easily stop myself from doing a stoppie on my wife's bike and my MTB by distributing my weight a little differently. On my Soho, perhaps it's not necessary because the rider position and larger wheels (700c) makes it less likely to happen even when I pull the brakes hard.

    So, jeffscott's thoughts on OTB/stoppie being a limiting factor, regardless of rotor size, seems to rely on the distribution of the weight of the rider. On one hand this makes sense, but if the OTB/stoppie is intentionally avoided wouldn't this create more force on the fork? It sure feels like there is more stopping force when I don't do a stoppie vs. when I do.

  15. #15
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    Quote Originally Posted by jsilva View Post
    After working on some bikes with v-brakes I was testing the power of them and accidentally did a stoppie, and one of bikes was my wife's comfort hybrid! The other was my MTB. I was shocked because it never happened on my Soho with disc brakes. So I intentionally tried to do one on the Soho and it happened.

    My point is that I realised it is an issue of weight distribution. I can easily stop myself from doing a stoppie on my wife's bike and my MTB by distributing my weight a little differently. On my Soho, perhaps it's not necessary because the rider position and larger wheels (700c) makes it less likely to happen even when I pull the brakes hard.

    So, jeffscott's thoughts on OTB/stoppie being a limiting factor, regardless of rotor size, seems to rely on the distribution of the weight of the rider. On one hand this makes sense, but if the OTB/stoppie is intentionally avoided wouldn't this create more force on the fork? It sure feels like there is more stopping force when I don't do a stoppie vs. when I do.
    Of course weight distrubution has alot to do with howmuch braking force is required to do a stoppie...(I do a light stoppie every day when I get home to my garage)...

    However there is a limit..if your weight is too far back then the front wheel will skid rather than bite and do the stoppie....So there is a limit again.

    And as you say if your weight is forward no skid just a gently stoppie....

    Somewhere between the two is the maximum possible braking force.

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