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  1. #1
    flatlander in paradise
    Reputation: joeycmsu's Avatar
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    disc brake sizing question

    Hey all,

    I'm switching to discs this winter, and my Marz Marathon SL fork says it can take a 6" rotor... so the stupid question is: Is a 6" rotor and a 160mm rotor generally interchangeable? 6" is more like 153mm, so that's why I'm asking.

    Also, is it possible to run a larger rotor even tho Marz says 6" is the max? I'm 6'4", 205lb and would like to run a bigger rotor in the front if i can.

  2. #2
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    Reputation: Bikinfoolferlife's Avatar
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    I wish the rotors would only be in mm, as there is variance in 6, 7 and 8 inch rotors when you look at the actual measurements in mm. 160 is generally a 6", there have been 165s as well. At 7 inch you can get 180, 183, 185. At 8 inch there's 200 and 203mm but think there's 205s as well, but not sure.
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  3. #3
    Fort Valley = Gnarl Fest
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    thats my feelings too. but I'm not 100% confident... I'll have to put a scale on my hayes "V6" at home

  4. #4
    Lionel Hutz, Esq.
    Reputation: Thirdrawn's Avatar
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    Quote Originally Posted by joeycmsu
    is it possible to run a larger rotor even tho Marz says 6" is the max? I'm 6'4", 205lb and would like to run a bigger rotor in the front if i can.
    I'm sure it's "possible" to run a larger rotor than recommended. But I would be curious why they put a limit on the rotor size in the first place. For example, a larger rotor may make the fork experience more torsion than it's designed to handle and will cause a failure. That would send your 6'4 205lb body flying through the air much faster than I expect you would like.

    There really isn't much of a reason to limit the amount of options on a component other than to prevent failure. Forks are pretty important. A fork failure is going to end your ride. Someone your size needs to be much more aware of the limits on components than guys who are smaller.

    It's probably "possible" to run a larger rotor, but I would be cautious to recommend it since you probably stress components enough as it is. Of course I'm not suggesting that you're fat, but your weight will place limits on your component choice that smaller guys don't have to deal with.
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  5. #5
    flatlander in paradise
    Reputation: joeycmsu's Avatar
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    No offense taken on the size/weight comments. In mountain biking terms, I'm a pretty big guy. The way the marz manual read, it sounded more like the fork was designed for 6" rotors because of where the IS mounts for the calipers were. I understand the difference in force applied to the lowers in the case of a larger diameter rotor, tho.

    So it sounds like I should be able to run a 160mm rotor in the place of a true 6" rotor. I might still confer with marz tech support.

  6. #6
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    Reputation: Bikinfoolferlife's Avatar
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    The size you'll find below 6"/160mm (despite they're not equivalent) is a 140mm which is a whole different ball of wax...
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  7. #7
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    joeycmsu....

    rest assured that a 160mm rotor is considered a 6" rotor, 180 to 185 are marketed as 7" and 200 to 203 plus some of the odd ball larger sizes, i.e. 205 and 210mm are considered 8" rotors. It's pretty much been this way since disc brakes for bikes were invented. The 160mm rotor is a 6" for all practical purposes. Nobody and I mean nobody markets a true 6, 7, or 8 inch rotor. There are those that are close, but no cigar. Don't sweat the mm to inches conversion quite so much, your over anylizing. The difference in torque moment between a 160mm (6.3") rotor and a true 6" (152.4mm) rotor is absolutely minimal. After all, we're only talking 3/10ths of an inch difference here.

    As for running a larger rotor on the SL. It is not recommended because of the construction of the fork. A Marathon SL is an XC/Race fork by design. And therefore quite light in both weight and contruction when compared to heavier duty Zocch forks. It's not designed to stand up to the higher stress loads placed on it by a larger diameter rotor. The twisting torque put on the fork by the larger rotor could cause the QR to loosen up, not a good thing. Also in rare instances some cracking of the dropouts has been reported. Nothing catastrophic, but it still ruins the fork. It's always better, esspecially for us biger guys, (I run 225 geared) to stick within manufacturers recommendations when it comes to our components.

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  8. #8
    flatlander in paradise
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    Thanks, I'm an engineer so I tend to be specific about the details. I've never run discs before, so I wasn't sure if the caliper would line up correctly. I'll give it a shot!

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