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  1. #1
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    Bulding a disc front wheel -- any advice?

    I'm about to build a new front wheel for my Crosscheck w/ Avid BB7 front disc brake. I've never built a disc wheel before, so I'm wondering if there are any special gotchas or safety issues that I should be aware of (other than calculating the spoke lengths properly) before I start.

    I have a Deore LX M585 36h Centerlock hub (w/ DT Swiss 6-bolt adapter), Sapim Race 2.0/1.8 spokes (sized for 3x lacing), and plan to reuse the 36h Velocity Dyad rim from my existing (non-disc) wheel.

    First, the rim has not been particularly abused, has no cracks around the eyelets that I can see, and has about 2-3K miles on it. Is it safe to use this? I'd buy a new one but I'd rather not have the old rim sitting around taking up space.

    Second, I'm trying to figure out the best procedure for lacing the wheel (I usually just go by Sheldon Brown's tutorial where you start with the key spoke being a trailing spoke, heads out). The idea I have is that a disc wheel can sort of be thought of as the mirror image of a dished rear drive wheel. In other words, the "drive side" is now on the left, and the pulling spokes are now the leading spokes (when looked at from the right side), since the forces are reversed when braking. So if I flipped the wheel over so that the disc side was facing me, I _think_ that means I could lace it the same way that I'd normally lace a rear non-disc wheel (i.e. starting with the key spoke being a trailing spoke when viewed from the disc side, head out so that the spoke runs up the inside of the flange). Does that sound reasonable or do I have something backwards?

  2. #2
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    Quote Originally Posted by Metaluna View Post
    I'm about to build a new front wheel for my Crosscheck w/ Avid BB7 front disc brake. I've never built a disc wheel before, so I'm wondering if there are any special gotchas or safety issues that I should be aware of (other than calculating the spoke lengths properly) before I start.

    I have a Deore LX M585 36h Centerlock hub (w/ DT Swiss 6-bolt adapter), Sapim Race 2.0/1.8 spokes (sized for 3x lacing), and plan to reuse the 36h Velocity Dyad rim from my existing (non-disc) wheel.

    First, the rim has not been particularly abused, has no cracks around the eyelets that I can see, and has about 2-3K miles on it. Is it safe to use this? I'd buy a new one but I'd rather not have the old rim sitting around taking up space.

    Second, I'm trying to figure out the best procedure for lacing the wheel (I usually just go by Sheldon Brown's tutorial where you start with the key spoke being a trailing spoke, heads out). The idea I have is that a disc wheel can sort of be thought of as the mirror image of a dished rear drive wheel. In other words, the "drive side" is now on the left, and the pulling spokes are now the leading spokes (when looked at from the right side), since the forces are reversed when braking. So if I flipped the wheel over so that the disc side was facing me, I _think_ that means I could lace it the same way that I'd normally lace a rear non-disc wheel (i.e. starting with the key spoke being a trailing spoke when viewed from the disc side, head out so that the spoke runs up the inside of the flange). Does that sound reasonable or do I have something backwards?
    If you've got www.sheldonbrown.com open to the wheelbuilding page there's not much more you need to know... don't try to radially lace a disk wheel, get a copy of The Bicycle Wheel by Jobst Brandt (a good read in general even if you don't learn anything specifically useful) to get a complete picture of the underlying reasoning behind mostof the techniques suggested.

    Good luck! Report back on how it goes!

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    Lacing Rear Disc Wheels

    This post initially talks about lacing rears, but talks about disc wheels in general

  4. #4
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by Metaluna View Post
    The idea I have is that a disc wheel can sort of be thought of as the mirror image of a dished rear drive wheel. In other words, the "drive side" is now on the left, and the pulling spokes are now the leading spokes (when looked at from the right side), since the forces are reversed when braking.
    I doubt it makes much difference but, if it was my bike, I'd build it the same as the rear wheel. With disc brakes, the rim is trying to pull the hub around so the forces aren't really reversed. The same spokes are the pulling spokes.

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    Quote Originally Posted by Retro Grouch View Post
    I doubt it makes much difference but, if it was my bike, I'd build it the same as the rear wheel. With disc brakes, the rim is trying to pull the hub around so the forces aren't really reversed. The same spokes are the pulling spokes.
    The forces are reversed... when accelerating with a rear wheel the hub is trying to pull the wheel in a forward rotation... when braking with disks the hub is pulling the wheel to accelerate in the backwards direction.

    According to Sheldon Brown, though, it isn't that important which spokes are pulling and which are pushing - and there is considerable disagreement anyway.

    "This is not an important issue! There is a sizable minority of good wheelbuilders who prefer to go the other way around, and good wheels can be built either way."

  6. #6
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by LarDasse74 View Post
    The forces are reversed... when accelerating with a rear wheel the hub is trying to pull the wheel in a forward rotation... when braking with disks the hub is pulling the wheel to accelerate in the backwards direction.
    Yeah, you're right. I was thinking about rim brakes.

  7. #7
    Small Member maddmaxx's Avatar
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    If you can get your hands on a copy of Zin's "The art of Mountain Bike Maintainece" you will find a set of spoking instructions for a disc front wheel. He specifies leading spokes on the outside of the rim. He goes on to give instructions for a disc rear with trailing/outside on the drive and leading/outside on the non drive side to sort of spread the load.

    Most folks recomend against Revolution spokes for this application, opting instead for 2.0/1.8 double butted. The idea is that you can probably create more forces with the disc brake than you can with the pedals.

  8. #8
    Senior member Dan Burkhart's Avatar
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    Quote Originally Posted by maddmaxx View Post
    If you can get your hands on a copy of Zin's "The art of Mountain Bike Maintainece" you will find a set of spoking instructions for a disc front wheel. He specifies leading spokes on the outside of the rim. He goes on to give instructions for a disc rear with trailing/outside on the drive and leading/outside on the non drive side to sort of spread the load.

    Most folks recomend against Revolution spokes for this application, opting instead for 2.0/1.8 double butted. The idea is that you can probably create more forces with the disc brake than you can with the pedals.
    My belief is that all discussion about whether trailing or leading spokes should be heads in or heads out is so much mumbo jumbo. Even Sheldon conceded, if you bother to read his work carefully, that the only real issue was derailleur (sorry, derailer) clearance of outside (heads in) spokes deflecting under torque. For a front wheel, I don't believe it matters, disc brake or otherwise.
    Feel free to dissagree.
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  9. #9
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    Great discussion. Sounds like there's nothing special about the lacing as long as its not radial (I was going to do 3x anyway). I have Musson's wheelbuilding book and he agrees that there's not much difference between heads-in and heads-out for the pulling spokes. I kind of like the idea of having the pulling spokes running inside the flange, as the forces are closer to the centerline of the rim, but the difference is apparently small enough that it doesn't matter in practice.

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