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  1. #1
    Senior Member Bolo Grubb's Avatar
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    why the 5mm difference between MTB and road rear drop out spacing?

    If I under correctly modern mountain bikes have 135 mm space for the rear drop outs and road bikes are 130mm spacing.

    what is the reason for this? are the cassettes different size or is it the hub? Are MTB hubs wider to make them stronger?

    Yes I am a newb that knows nothing.

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    cyclist/gearhead/cycli... moxfyre's Avatar
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    Quote Originally Posted by Bolo Grubb
    If I under correctly modern mountain bikes have 135 mm space for the rear drop outs and road bikes are 130mm spacing.

    what is the reason for this? are the cassettes different size or is it the hub? Are MTB hubs wider to make them stronger?

    Yes I am a newb that knows nothing.
    Yes, it's because of strength. It's all about wheel dish. Take a look at your rear wheel: the right side spokes are at a steeper angle than the left side spokes, because the hub body must be off-center to accommodate all those cogs. The greater the difference between the angle, the greater the tension must be in the right side spokes. The asymmetry weakens the wheel. By making mountain bike dropouts 5 mm wider, mtb wheels have less dish than road bike wheels.

    Road/mountain cassettes are no different. Well, Campy cassettes are different from Shimano cassettes, but among the Shimano cassettes the only difference between mountain cassette and a road cassette is the gearing, and perhaps the choice of materials.
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  3. #3
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by Bolo Grubb
    If I under correctly modern mountain bikes have 135 mm space for the rear drop outs and road bikes are 130mm spacing.

    what is the reason for this? are the cassettes different size or is it the hub? Are MTB hubs wider to make them stronger?

    Yes I am a newb that knows nothing.
    I can think of a couple of likely possibilities. The first is just to reduce the dish and to maintain a better spoke braceing angle on mountain bike wheels. The second is to provide a little more space on the non-drive side of the hub to mount a disc brake.

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    Senior Member sydney's Avatar
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    Quote Originally Posted by Retro Grouch
    I can think of a couple of likely possibilities. The first is just to reduce the dish and to maintain a better spoke braceing angle on mountain bike wheels. The second is to provide a little more space on the non-drive side of the hub to mount a disc brake.
    But doesn't 135 predate disc brakes?

  5. #5
    Sometimes knows stuff. rmfnla's Avatar
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    Quote Originally Posted by sydney
    But doesn't 135 predate disc brakes?
    I was going to post exactly that; scarey, no?
    Today, I believe my jurisdiction ends here...

  6. #6
    meb
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    Senior Member meb's Avatar
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    Quote Originally Posted by moxfyre
    Yes, it's because of strength. It's all about wheel dish. Take a look at your rear wheel: the right side spokes are at a steeper angle than the left side spokes, because the hub body must be off-center to accommodate all those cogs. The greater the difference between the angle, the greater the tension must be in the right side spokes. The asymmetry weakens the wheel. By making mountain bike dropouts 5 mm wider, mtb wheels have less dish than road bike wheels.

    Road/mountain cassettes are no different. Well, Campy cassettes are different from Shimano cassettes, but among the Shimano cassettes the only difference between mountain cassette and a road cassette is the gearing, and perhaps the choice of materials.
    More specifically, strength in the vertical direction-a direction mountain bike terrains stress the wheel. The road wheel will be actually be stronger in the bike's lateral direction as the more scalene legs of the triangular truss formed by the spokes and the hub have more disparate angles so a change in one spoke's tension is not offset linearly by the complementary spoke, so the road wheel resist flexing in the lateral direction more than the mtb wheel. With higher cornering g's, dished road wheels are better than undished road wheels.

  7. #7
    Senior Member robo's Avatar
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    but does 5mm really make such a big difference? I would have thought they were just two standards that evolved independently and now they're entrenched.

    Rivendell makes a bike with 132.5mm spacing so you can use either, which i thought was cute.

  8. #8
    Spoked to Death phidauex's Avatar
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    The standards may have deviated early on due to a need to have wider tire clearance on mountain bikes, which, for straight or mostly straight chain stays and seat stays, would require a wider spacing at the hub side. That is pure conjecture however.

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    Road hubs didn't need to be any wider when they were 5 speeds...120mm. Then came along 126mm and 130mm to fit 6, 7, 8, 9, 10 cogs...therefore requiring wider spacing to make a stronger wheel. Wouldn't surprise me to see a standard 135mm for both road and mtn in the near future...or hopefully, at least, 132.5 Rivendell as well, huh? I knew Surly and Soma were doing 132.5 on some frames.

  10. #10
    cyclist/gearhead/cycli... moxfyre's Avatar
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    Quote Originally Posted by robo
    but does 5mm really make such a big difference? I would have thought they were just two standards that evolved independently and now they're entrenched.

    Rivendell makes a bike with 132.5mm spacing so you can use either, which i thought was cute.
    Yes, 5 mm is a significant difference. You can draw out a diagram of the rear wheel and work out the difference in tension between the right and left side spokes, taking into account the dish and the width of the cogs. There's a thread about it on rec.bicycles.tech somewhere...

    A lot of touring frames are made with 132.5mm spacing, for versatility. Nashbar even sells some touring WHEELS with the hub locknuts spaced to 132.5mm, so you can use them in either type of frame
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  11. #11
    I couldn't car less. jeff williams's Avatar
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    Some bikes are into 145+mm for dh wide hub bodies now.
    My mech was showing me, he's using 32 hole with thick spokes.
    Big...My rear is @ 140 already -so a new wheel, and we might coldset it out and build a bigwheel.
    Are not tandem bikes 140, 145? mm \less vertical dish on the drive side to increase the strength?
    Last edited by jeff williams; 07-03-05 at 11:28 AM.

  12. #12
    Chairman of the Bored catatonic's Avatar
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    It's all about space for the hubs.

    Older mountain bikes should have thinner rear triangles than modern ones. Not by much, but there will be a difference. Reason is as the number of cogs goes up, eventually you will hit a point where you have to make a wider hub.

    Now why they are wider than roadd is tires. Look at beach cruisers...they have rather wide rear triangles as well, especially for a singlespeed.

  13. #13
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    Quote Originally Posted by Bigwheel
    Rivendell as well, huh? I knew Surly and Soma were doing 132.5 on some frames.
    I think Riv was first, FWIW.

    My Kogswell model P is also spaced to 132.5. I've build a couple of rear wheels spaced to 135 for the P. Very nice.

  14. #14
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    Interesting about Rivendell...I didn't know that.

    Those DH solid rear axle hubs are sturdy looking ...DH rigs are getting closer to motocross every year

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