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Old 03-30-08, 11:06 AM   #1
larry_llama
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Big Dummy & Avid BB& Discs: clearance problem!?

Hey BD nuts... I have a question... Have any of you installed BB7 brakes on your frame yet? I tried to transfer my old (first generation perhaps) BB7s from my mountain bike (temporary donation until I buy new ones) and ran into trouble on the rear.

The front went on no problem, but on the back, the disc is too close to the frame so the mounting bracket will not fit correctly between the frame and the disc. Does the mounting bracket come in different thicknesses if I buy a new brake? I figured that these clearances would be standard... Any ideas?
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Old 03-30-08, 12:56 PM   #2
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I ran into that problem on both my Karate Monkey and on my Dummy. The problem is crappy alignment when they assembled the frame. It's not just with Avids, you will have the same problem with any disc brakes.

With the Monkey I was able to rig up a brace and bend the dropout out enough to make everything work. With the Dummy, my LBS used a facing tool to cut it back and give the brake tab clearance. With so many tubes attached to the dropout on the Dummy, I don't think trying to bend the dropout area is the best option. I still had to clean up the tab with a Dremel tool to get a clean mount but the facing tool gave me a flat, properly aligned surface where the adapter bolts to the frame.

I also made a caliper guard from a piece of aluminum I had lying around the garage. The blue is tool-dip to try and protect the bag a bit. You can kind of see where the tab has been cut back in the photo. It's actually not cut back as far as it looks like. Part of what you see on the edge of the tab is black Sharpie ink. There's plenty of steel left to support the braking force.
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Old 03-30-08, 01:02 PM   #3
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Oh man... what a mess. Is it just me or does that seem to be a pretty big oversight?

Maybe I will just run V brakes in the rear. I was going to do that originally, but figured at $50 for BB7s at pricepoint I might as well run them both front and rear.
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Old 03-30-08, 01:40 PM   #4
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On a normal frame it's not really a big deal to tweak the frame back to proper alignment, nor is it really that uncommon to have to do so, especially on a low-cost steel frame such as most Surlys. On something like the Big Dummy, however, it's not nearly as easy to do.

My LBS had already cut/faced the brake tabs before I picked up the frame. The first I knew about it was when they presented me with the frame and showed me that they "fixed" it. I knew as soon as I looked at it that the adapters wouldn't fit and I would have to do some grinding. I also gave serious thought to refusing the frame. However, since I knew that I could make it work with a bit of effort, and since it was extremely unlikely that there was another frame immediately available without the problem, I accepted the frame.

Because the Big Dummy has such a long frame and because the cargo is carried over the back wheel, it breaks the generalization that ~80% of the braking is with the front brake. Even so, a good V-brake should be just fine on the rear in conjunction with a good disc brake up front. Since you are only borrowing a disc brake this probably isn't a super big deal. If you had already bought new, expensive brakes it would be worse.

Since I was able to deal with it myself and hadn't heard of anyone else having the problem, I never bothered to tell Surly about it. Now that there's at least two of us tho, I think I'll email a link to this thread to those nutters in Minnesota. They do deserve to know if there's a QC problem they need to deal with.
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Old 03-30-08, 03:09 PM   #5
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Yes, and at least 2 people who need t-shirts for their trouble :-)
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Old 03-30-08, 03:30 PM   #6
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Thanks for the heads up. I haven't mounted my brake rotor on the rear as I was waiting for the Rohloff hub to come in before I bothered. I might do it on my current rear wheel just to see if I have the same problem and how bad it is. I think I have an Avid rear caliper adapter that I have already removed some material from since I had this issue with another frame. Hopefully if I need to move the caliper over the modified adapter will work without having the bother messing with any further mods.
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Old 03-30-08, 03:51 PM   #7
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Oh man... what a mess. Is it just me or does that seem to be a pretty big oversight?
Surly already acknowledged this problem on the Surly blog. Surprised more people have not mentioned it.
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Old 03-30-08, 04:06 PM   #8
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Surly already acknowledged this problem on the Surly blog. Surprised more people have not mentioned it.
Really? I have been reading their blog religiously for BD news and don't remember that at all. At any rate, I'll probably just stick with V for now and see how it goes. I'm still waiting for the LBS advice. Unfortunately they are less than local -- about 5 hours away -- so I won't get a chance to bring it to them for quite some time...

I went for my first ride today.. just around town a bit with only a front brake and only a rear derailer. The handling sure takes some getting used to (quick swerves are a bit less quick than I'm used to). Hopping up onto a curb is also a new challenge!
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Old 03-30-08, 04:07 PM   #9
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Surly already acknowledged this problem on the Surly blog. Surprised more people have not mentioned it.
Can you point me to the blog post on the Surly blog you are talking about? In the BD Spew they talk about the caliper rubbing the left side freeloader, but I don't recall reading anything about the caliper mount being misaligned with the rotor... and I read the Surly Blog regularly.
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Old 03-30-08, 05:25 PM   #10
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Being a fan of empirical data I mounted up a rear disc on my BD and the avid BB7 caliper. I can get the rotor centered in the caliper with no troubles and no modification to stock components.
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Old 03-30-08, 06:50 PM   #11
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Lucky!
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Old 03-30-08, 07:16 PM   #12
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Speaking of empirical, I just got the calipers (for measuring) out ... clearance from inside of mount to the disc: 8.9mm, thickness of CPS mount: 11mm - So I'm looking at removing 3mm of material from that 6mm thick disc mount if I want to clearly clear the disc. Seems sketchy....
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Old 03-30-08, 07:27 PM   #13
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Speaking of empirical, I just got the calipers (for measuring) out ... clearance from inside of mount to the disc: 8.9mm, thickness of CPS mount: 11mm - So I'm looking at removing 3mm of material from that 6mm thick disc mount if I want to clearly clear the disc. Seems sketchy....
Hey Larry I checked my BD and I have ~2mm of clearance from the rotor to the unmodified avid caliper mount. If you have to remove 3mm of material than the difference between our BD setups is ~5mm. That seems way too much of a difference in tolerance. BTW - my caliper mount is ~10.5mm thick. This is all with current generation Avid BB7 stock parts.

It might be worth grabbing a new BB7 rear brake and trying it out rather than making such a major mod to your disc caliper mount. I agree removing 50% of the material seems excessive.

I actually have the old original Avid mech disc brakes [same as BB7, but before there was a BB5] on a mtn bike - probably exactly what you have, but I don't have the time/energy to swap it onto my BD to see if I can replicate the problem.
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Old 03-31-08, 09:40 AM   #14
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Update... Since your clearances are much wider, I got worried that the frame member might be bent inward. So I took a closer look and all of the frame parts seemed way too close to the rotor. In some places barely a mm of clearance at the very edge of the rotor. So I double checked that there weren't any axle spacers or anything like that getting in the way... nope, there's no way the cassette could go any closer to the right-hand dropout.

Then guess what I found:


YES - A 3mm DISC SPACER!

So my mtb frame must have had the disc mounts slightly wide, and the BD slightly narrow, and trying to use the same wheel clearly doesn't work!

So removing this, there is just enough width to get the older CPS mount in. The newer ones (I have a spare for the front) are definitely a bit narrower. So it looks like all I hve to do is get a minor refacing to the mount holes and a new brake will jusssst fit.

Thanks for all of your help and measurements in this thread guys... disaster averted!
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Old 03-31-08, 11:12 AM   #15
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Good to hear Larry! I'm glad it was something fairly simple.
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Old 03-31-08, 11:40 AM   #16
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Yep! There was just enough dirt on the aluminum spacer that it blended well into the hub flange and looked like part of the hub. Now to find someone to face those mounts for me ;-p
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Old 03-31-08, 12:40 PM   #17
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Now you two have got me wondering...

When I picked up my Dummy frame the LBS had already faced the mount tabs, easily removing a couple mm of material. Exactly how much they removed I don't know off hand. I'll have to measure this evening to satisfy my curiosity. It's possible that they really didn't need to face the tabs at all. The Avid mount system has a fair amount of adjustment so that the tabs could be a bit out of whack and the brakes will still align ok.

Since I most certainly did have clearance problems with my Karate Monkey, I was not at all surprised that the Dummy may also have the problem.

We shall see...
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Old 04-01-08, 07:34 AM   #18
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Another quick note - my old (first gen) rear BB7 had initially used a washer on each bolt between the CPS and the frame. I cannot use that on the BD frame. I think the old CPS design is overall just too wide for this setup. The new ones look better though.
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Old 04-01-08, 05:46 PM   #19
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What size frames are you guys having issues or lack of them on? I'm still waiting on a 20" because of a QC issue keeping some from being sent out. Maybe this is it?
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Old 04-01-08, 08:15 PM   #20
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What size frames are you guys having issues or lack of them on? I'm still waiting on a 20" because of a QC issue keeping some from being sent out. Maybe this is it?
I've got an 18" BD and no disc brake issues.
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Old 04-02-08, 10:04 AM   #21
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Mine is 20, and it's less of an issue than I previously thought. I contacted Surly and got a speedy reply which stated among other things that they were not aware of and disc clearance issues but wanted as much info as possible in order to resolve it for me asap. nice guys! Also, there is some question about whether there is any QC issue at all (check the threads here). At any rate, I wouldn't think given the info we have that the QC issue (if there is one) is related to this at all.

By the way, I installed my old avid with a newer mount last night and went for a ride with no problems. but the clearance is literally paper thin, so I'm definitely going to need to get it faced soon (which is a good idea anyway I guess).
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Old 04-02-08, 04:45 PM   #22
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Good to hear that Surly is now aware. When I get mine (August?) I want it to be perfect.

Also: I was away for over 24 hrs and this is the only BD thread with any new posts. What the heck is wrong with you guys?
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Old 04-02-08, 09:49 PM   #23
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OK, so I finally had time this evening to get in there with calipers and figure out how much metal was removed when the LBS faced the brake tabs and if it was really necessary to make the brakes fit and work.

As it is right now, the clearance between the brake adapter and the rotor is even along the length of the adapter block so the plane of the brake mount is properly perpendicular to the axle. The clearance between the rotor and the brake mount adapter is ~2.25mm. (A 2mm allen wrench fits in the space with just a little play and a 2.5mm won't fit in without flexing the rotor just a bit.)

The upper mount tab was cut back ~1.5mm and the lower tab was cut back about .5mm. From all this it seems that the brake assembly would have fit without all the messing around. Towards the outer/upper edge of the rotor where it would have been tightest, there would have been ~.75mm clearance and it wouldn't have been perfectly straight but the adjustment in the Avid mount should have dealt with that just fine.

So, even though it is true that the brake tabs weren't welded on perfectly straight, in the end it seems that so far none of us has really had cause for real concern.

Oh, and my frame is an 18".
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Old 04-07-08, 04:37 AM   #24
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Looks like I'm another BD owner with rear caliper mount alignment issues. I have an 18", was mounting a Shimano Deore M535 caliper. The caliper appears to be unparallel when mounted. I've already dropped it off at the shop to have things faced, I'm going to let them align the rear mount. Front caliper mount appeared to be just fine.

Here is where Surly mentions facing rear disc brake mounts in their blog section. Maybe this is what spambait11 was referring to, however they do not mention it being a "problem".

http://www.surlybikes.com/spew9.html
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Old 04-07-08, 07:52 AM   #25
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Looks like I'm another BD owner with rear caliper mount alignment issues. I have an 18", was mounting a Shimano Deore M535 caliper. The caliper appears to be unparallel when mounted. I've already dropped it off at the shop to have things faced, I'm going to let them align the rear mount. Front caliper mount appeared to be just fine.

Here is where Surly mentions facing rear disc brake mounts in their blog section. Maybe this is what spambait11 was referring to, however they do not mention it being a "problem".

http://www.surlybikes.com/spew9.html
I'm not familiar with the Shimano disc calipers - do they have some method for adjusting the position of the caliper relative to the mounting bolts? I know the Avid calipers have quite a lot of movement to allow you to get the caliper parallel to the rotor.
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