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  • Alfine 11 Input Gear Ratio -Talk to me about Torque
  • pistonbroke
    Free Member

    I like to think I’m fairly mechanically minded but the bit in the instructions for fitting an Alfine hub where they specify an input ratio of no lower than 1:1.9 i.e 18 tooth hub, no lower than 34 tooth chainring or 20 tooth hub 38 tooth chainring, has me stumped (sorry for the grammar). Apparently this is to prevent overtorquing the internal gears. Surely the stress on the gears is a function of your leg power, going for a lower input ratio will allow you to twiddle more thus reducing stress. Is this just a warranty getout by Shimano or really an issue? If I follow the rules and go for a 38/20 setup, the bottom gear is only about the same as 32 chainring/ 28 rear which is not low enough for a lot of the hills round here.
    I know I’ll get comments of running an 8 speed for years with 32/20 which is the setup I favour but am slightly worried by a few comments from the States of blowing up hubs with low gear options.

    geoffj
    Full Member

    Think about it this way. If you had a real biffer with a silly low ratio, stamping on the pedals on a very steep hill, then there is a chance that the mechanics in the hub may break due to the extreme forces. They have to draw the line somewhere, but I’d say your weight should be brought into the equation.

    fourbanger
    Free Member

    Tis only a guideline. Duly ignore it, but make sure you post up a good rant when your hub implodes.

    ir_bandito
    Free Member

    Rohloff specify a similar thing for the speedhub. Minimum ratio of 38:16 with a 26″ wheel (although they don’t specify what tyre size.

    Surely the stress on the gears is a function of your leg power

    No, its a function of force. Gears essentially give a leverage to increase that force, at the expense of speed . Which is why you can climb a steep hill in a low gear, but slowly. That increase in force can tear the iddy-biddy teeth of the internal gears, hence attempting to limit what the input force is. Obviously, its a bit generic, and probably pessimistic to cover uber-strong monster-thighs, so you could tweak it slightly if you’re gentle with it.

    fourbanger
    Free Member

    As for visualising the mechanics of the problem, it may help to think of an analogy with levers.
    Think of a seesaw with your legs pushing on one end and pivot somewhere in the middle. If the pivot is near you, it’s like having a higher input ratio. With the same force from your legs you can move a smaller weight on the other end of the see saw faster.
    Move the pivot further away from you towards the other end of the seesaw and for the force from your legs you can move a much greater weight slower. This is the same as reducing the input ratio.
    Now consider the overall force through the system in both scenarios and you can see which one is more likely to snap the lever/bugger the pivot/pop your hub

    packer
    Free Member

    The shimano ratio is a recommendation I believe – I don’t think they have said anywhere that you shouldn’t use a lower ratio or that the warranty is void if you do. Feel free to correct me if I am wrong.

    There are a coupe of stories on US forums of people breaking alfine hubs when using lower ratios, but there are also people breaking hubs with the recommended ratio, and a whole lot more people running lower ratios just fine for some time now.

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