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  1. #1
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    Clearance between disc rotor and front fork. Soma Double Cross

    I'm in the middle of building a Soma Double Cross Disc and am using components from a Trek Portland, (forks on the Trek went bad, and I missed the feel of a steel frame) I have Bontrager Select Disc wheels, and when I put the wheel in the front fork, there is a millimeter or less clearance between the rotor and the fork. (I have not installed the brakes (Avid BB7)). Is that normal?

    On the Trek the distance between the drop outs was smaller, but the distance between the forks at the place where the rotor was, was greater.

    I'd feel more comfortable with couple more mm of clearance. Is there a way to increase the effective length of the front hub so the forks are farther apart when the wheel is locked in place? Or should I not worry about it?

  2. #2
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    What size rotor are you using ?

  3. #3
    Noob mikezs's Avatar
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    The gap can be as small as you want, the rotor isn't going to warm and the forks aren't going to get any closer, so the clearance is a moot point as long as they aren't touching now.

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    Senior Member Matt Gaunt's Avatar
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    Quote Originally Posted by mikezs View Post
    The gap can be as small as you want, the rotor isn't going to warm and the forks aren't going to get any closer, so the clearance is a moot point as long as they aren't touching now.
    Less than 1mm clearance shouts of using components that were not designed to be used together. Either the rotor is too big (disc-compatible forks are warrantied only to a stipulated maximum rotor size) or some other abnormality is being played out.

    Any chance of pictures?

    I would not ride on a fork with less than 1mm clearance, FWIW.
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  5. #5
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    Quote Originally Posted by cobba View Post
    What size rotor are you using ?
    160 mm.

  6. #6
    Senior Member BCRider's Avatar
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    That's the small size then. So assuming your wheel is the correct flange to flange spacing of 100mm's and that the axle is definetly seated in the dropouts fully then it is what it is. Although it's tighter than any bike that I can recall seeing.

    Something to check is that the dropouts are evenly spaced and that one side doesn't have any extra thick paint preventing the axle from seating to the same depth on both sides. The paint in the U of the dropouts will get chewed up anyway so why not take a moment to carefully scrape it out without doing damage to the metal. With the paint in the curved portion of the U in the dropouts gone I would not be surprised to see the rotor to leg clearance get a little larger.
    Model airplanes are cool too!.....

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    coprolite fietsbob's Avatar
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    Steel fork blades you can always make a judiciously located dent, there,
    at closest point of potential contact.

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    I have a suspension fork with a 160mm rotor and I probably have ~2mm of clearance, and it has never ever been a problem. This is the beauty of disks - the tolerances are much much tighter and so you need less space. If the rotor were to be so warped that it hit the fork leg it would jam in the caliper the first time through.

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    [QUOTE=BCRider;13504287]That's the small size then. So assuming your wheel is the correct flange to flange spacing of 100mm's and that the axle is definetly seated in the dropouts fully then it is what it is. Although it's tighter than any bike that I can recall seeing.
    QUOTE]
    The outside distance between the cones nuts is ~ 98 mm. The inside distance between dropouts is ~100mm. This sounds like they are compatible.
    The distance between the rotor and the fork is only 0.55 mm. I did email Soma and the rep said this really is too close, and that he's seen it before. I'm torn between finding a fix (longer cones perhaps?) or working with the distributor in getting replacement forks. Replacing the forks is a lot more work since it requires specialized tools which I can use (I belong to a bike co-op) but it is sort of a long process.

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    Quote Originally Posted by DCB0 View Post
    I have a suspension fork with a 160mm rotor and I probably have ~2mm of clearance, and it has never ever been a problem. This is the beauty of disks - the tolerances are much much tighter and so you need less space. If the rotor were to be so warped that it hit the fork leg it would jam in the caliper the first time through.
    I see your point. Do you think I'll have trouble getting the calipers to fit given the distance is only 0.55 mm between the rotor and fork? The calipers are not the easiest to get adjusted, but I've done it.

  11. #11
    Get on your bikes & ride! xB_Nutt's Avatar
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    What fork are you using? The disc version of the Double Cross fork has a flattened area on the inside leg on the rotor side.
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    Quote Originally Posted by xB_Nutt View Post
    What fork are you using? The disc version of the Double Cross fork has a flattened area on the inside leg on the rotor side.
    It's the IRD Tange chromoly steel forks that Soma sells to go with its Double Cross DC frame.

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    Wondering how you resolved the issue. I am currently building up a Double Cross and have the same issue using the Soma Straight Blade CX fork, an Avid BB7 with a 160mm rotor and 29er DT swiss wheels with 350 Hubs (100mm spacing).

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    Quote Originally Posted by ernst96 View Post
    Wondering how you resolved the issue. I am currently building up a Double Cross and have the same issue using the Soma Straight Blade CX fork, an Avid BB7 with a 160mm rotor and 29er DT swiss wheels with 350 Hubs (100mm spacing).
    I am also using hte Avid BB7s, and as I recall my spacing is about the same as yours. Are your forks black? The Soma Rep said that he has seen this issue with the black forks. The black forks were new for the fall of 2011. Actually he had seen a case where the rotors actually touched the fork blade. Some have suggested that as long as the rotor doesn't touch, it should be OK: the rotor won't get any closer. But since Soma said this would be covered under the warranty, I went ahead and sent it back and asked for an exchange. The only ones they had in stock where midnight silver. I'm guessing they had black ones in stock but maybe they realized there was a problem with a lot of the black forks. It was a pain to send them back, especially since the distributor (bikemania.biz) could not send me the replacement forks until they received the forks I sent back. Plus I had already applied frame saver and installed a star nut into the forks. I had to do this again for the new forks at my expense.

    For the defective forks, the inside distance between the fork blades was about 86.8 mm. This distance was measured at the blade's vent hole, which is near the lower moutning hole for the caliper.

  15. #15
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    you can always add a washer or two between the locknut and cone. that should force the forkends apart...

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    Quote Originally Posted by hueyhoolihan View Post
    you can always add a washer or two between the locknut and cone. that should force the forkends apart...
    I considered this, but this decreases the length of axle that sits underneath the drop outs. I'm not sure what the minimum length this should be, but I didn't like how it looked.

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    Quote Originally Posted by truthseeker14 View Post
    I am also using hte Avid BB7s, and as I recall my spacing is about the same as yours. Are your forks black? The Soma Rep said that he has seen this issue with the black forks. The black forks were new for the fall of 2011. Actually he had seen a case where the rotors actually touched the fork blade. Some have suggested that as long as the rotor doesn't touch, it should be OK: the rotor won't get any closer. But since Soma said this would be covered under the warranty, I went ahead and sent it back and asked for an exchange. The only ones they had in stock where midnight silver. I'm guessing they had black ones in stock but maybe they realized there was a problem with a lot of the black forks. It was a pain to send them back, especially since the distributor (bikemania.biz) could not send me the replacement forks until they received the forks I sent back. Plus I had already applied frame saver and installed a star nut into the forks. I had to do this again for the new forks at my expense.

    For the defective forks, the inside distance between the fork blades was about 86.8 mm. This distance was measured at the blade's vent hole, which is near the lower moutning hole for the caliper.
    Wondering what clearance the new fork has. Mine is black also and the rotor actually touches the fork very so slightly. I have contacted SOMA and they have been very responsive so far. Looks like I will be able to replace it under warranty.

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    Quote Originally Posted by ernst96 View Post
    Wondering what clearance the new fork has. Mine is black also and the rotor actually touches the fork very so slightly. I have contacted SOMA and they have been very responsive so far. Looks like I will be able to replace it under warranty.
    The clearance is about 2.4 mm. This may sound small, but when I look at it installed, it seems huge in comparison to the defective forks. Glad to hear Soma is responsive. Where did you buy the frame and forks?

  19. #19
    Gear Combo Guru Chris_W's Avatar
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    Quote Originally Posted by truthseeker14 View Post
    I considered this, but this decreases the length of axle that sits underneath the drop outs. I'm not sure what the minimum length this should be, but I didn't like how it looked.
    I believe it is on Sheldon Brown's site where it says that in terms of structure and safety you actually don't need any of the axle to be in the dropouts, although it makes installation and alignment of the wheel a lot easier. He would have condoned using a wheel that had no axle protruding on either side.

  20. #20
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    if it's a standard hollow threaded axle, a longer axle can be cut down to any smaller size.

  21. #21
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    Quote Originally Posted by Chris_W View Post
    I believe it is on Sheldon Brown's site where it says that in terms of structure and safety you actually don't need any of the axle to be in the dropouts, although it makes installation and alignment of the wheel a lot easier. He would have condoned using a wheel that had no axle protruding on either side.
    I had wondered whether I could just rely on the lateral force exerted by the quick release on the lock nuts, but intuitively, it just seemed better to have some of the axle protruding. But you're right. Here is an excerpt from Sheldon Bronwn's web site:

    Axle Length http://sheldonbrown.com/frame-spacing.html

    Typical quick-release axles are 11 or 12 mm longer than the spacing of the hub locknuts. This gives 5.5-6 mm of axle protrusion on each side. You don't actually need nearly this much, so for respacing hubs to wider spacing, if you're not adding more than, say, 5-6 mm of spacers, you don't need a new axle. As long as you have 2 or 3 mm sticking out on each side, that's plenty.
    One of my own bikes is set up with the axle cut off flush with the locknuts, and even this is no problem in use, though it is slightly trickier to align the wheel when installing it.

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    I also just built up a Double Cross (V2.1) and had a similar problem. Although the disc didn't hit the fork arm, the right hand pad (Avid BB7's) drags on the disc even when adjusted all the way back. My solution was to use a small shim on the left hand axle, works fine and I still have 3-4 mm of axle protruding.

    I had a similar problem with the rear brake, in this case the disc drags slightly on the right hand side of the brake housing itself. Another shim between the mounting tabs and the brake fixed this as well. I noticed the following comment in this article:

    Foundry has also run into trouble with factory delivery, primarily because of finishing flaws on its disc and cantilever cyclocross frames at one of the brand’s two Asian contractors, according to Grantz.

    I suspect that Foundry and Soma Fabrications are using the same supplier.

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    Quote Originally Posted by ghsmith54 View Post
    Foundry has also run into trouble with factory delivery, primarily because of finishing flaws on its disc and cantilever cyclocross frames at one of the brandís two Asian contractors, according to Grantz.

    I suspect that Foundry and Soma Fabrications are using the same supplier.
    I think Foundry only makes carbon bikes, and Soma only makes steel. I'm wondering if the same factory would produce both. But I have not seen many carbon bikes with disc brakes; I'm interested in seeing how they hold up.

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    Quote Originally Posted by truthseeker14 View Post
    The clearance is about 2.4 mm. This may sound small, but when I look at it installed, it seems huge in comparison to the defective forks. Glad to hear Soma is responsive. Where did you buy the frame and forks?
    I purchased it directly from the Soma website. Had to send the defective fork back and am waiting now for the new one.

  25. #25
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    I'm just finishing up a Soma Doublecross for a customer, and, as usual, I used a Tange Prestige Lugged CX Disc only fork (They are chrome and I have them powder-coated to match the color of whatever Doublecross I'm building up). I use this fork because it's much nicer looking than any of the Soma CX forks and has more tire clearance. But most importantly, the left / brake side fork leg is slightly flattened to accommodate the BB-7's 160mm rotor. The only required modification is the need to use shorter bolts to attach the caliper to the bracket (The stock bolts strike the rotor's spider). The brake set-up and adjustment is straight forward and clearance & performance are excellent.

    PS: Granted the customer spent some bucks, but I just weighed the bike with pedals and it was 18.5 pounds - not bad for a steel 'cross bike with disc brakes.

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