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Old 02-10-17, 11:04 AM   #1
dabac
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Mounting a disc brake mount - what went wrong?

Hi guys,
So I finally got around to begin the disc brake conversion of my commuter.
Got a Hayes interface drawing off the 'net.
Made what I still think is a reasonably chunky fixture out of M10 threaded rod, a piece of 1/2" aluminium and went at it.
However, when I mounted a wheel to the reworked fork, I had to push the caliper to the very end of its adjustability to get the rotor to center in the caliper and clear the pads.

The assembly is parallel enough, but its about 3 mm off sideways.
The drawing states that the inside face of the rotor mount should be 4 mm outboard of the inside face of the dropout.
Which it is, as close as me, my Vernier caliper and two stacks of washers could make it.
Might be/probably is off by some tenths, but nothing like the error I now have.
Any idea where the offset might be coming from?

Can't see that it should matter, but I'm using a Shimano mechanical caliper.
The wheel has a centerlock hub w/an Alligator 6-bolt adapter.
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Old 02-10-17, 02:42 PM   #2
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Have you tried a different wheel? I wonder if the offset of the rotor could be different for centre lock vs 9 mm quick release hubs. Could the 6 bolt hub adaptor be to blame? How is the axle offset? Thickness of the cone nuts?

You would think everything would be set to a standard, but you never know. I would try a different wheel and see what happens if you are sure you located the tabs right.
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Old 02-10-17, 09:17 PM   #3
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I have no gleams but to say that this is why one should consider mocking up the actual components. Andy
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Old 02-11-17, 07:32 AM   #4
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A picture of the setup you used, with your fixture bolted to the disc tab, might provide a clue. Just looking at the picture of your fixture, I see a nut that looks like it was between the dropout face and the fixture. Did you account for that thickness, plus 4mm, when you bolted the tab to the fixture?
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Old 02-11-17, 11:48 AM   #5
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can we see a picture of your fixture in use? Also, a picture of the wheel/disk/fork?

The theory that the wheel is off somehow seems to have some credibility, can you try it on an existing fork?
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Old 02-14-17, 01:11 PM   #6
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I have tried with another, regular 6-bolt wheel. It's a little less off but still far from mid-adjustment.
Haven't tried the centerlock wheel in another fork yet.
Can anyone confirm that the 4mm offset is correct?

Can't get the next pic to upload...
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Old 02-14-17, 01:14 PM   #7
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OK, let's try this workaround...
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Old 02-14-17, 01:20 PM   #8
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Quote:
Originally Posted by dsaul View Post
A picture of the setup you used, with your fixture bolted to the disc tab, might provide a clue. Just looking at the picture of your fixture, I see a nut that looks like it was between the dropout face and the fixture. Did you account for that thickness, plus 4mm, when you bolted the tab to the fixture?
Yep. Used matching nuts, and two pieces of 4 mm plywood when I bolted the bracket to the fixture.
To the best of the combined ability of me and my vernier caliper, I thought I was heading for factory spec.
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Old 02-14-17, 01:26 PM   #9
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I have no gleams but to say that this is why one should consider mocking up the actual components. Andy
I was aiming for factory spec, and future trouble-free replacements of parts.
Didn't think it'd be a problem with that reliable-looking drawing and the fixture I'd made.
Building off the components - w/o knowing how they measure up - might have resulted in a unique, not-compatible-with-anything-else alignment.
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Old 02-14-17, 07:40 PM   #10
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Quote:
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I was aiming for factory spec, and future trouble-free replacements of parts.
Didn't think it'd be a problem with that reliable-looking drawing and the fixture I'd made.
Building off the components - w/o knowing how they measure up - might have resulted in a unique, not-compatible-with-anything-else alignment.

Having components in hand allows one to measure them up. It also allows one to find the min/max of range and aim in the set up for where along this range you feel is correct for your needs.


Example- Back in the day canti brakes had no slotted/vertical pad adjustment, only the angle of the pad WRT the canti arm and rim to achieve pad rim contact. Of course this means that with differing rim widths (as in tacoing the original and the replacement is different) result is differing pad surface contact with the rim. fast forward to now and cantis have about 1cm of vertical pad placement (independent of pad angle). So now I can decide on the location of the pivot boss to provide a certain lever feel. AND if I'm off a tad the remaining range will still let me make the system work. I find this out by mocking up the system. Not by reading numbers on line. And when the range is especially small all the more need to be sure. Andy.
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Old 02-14-17, 09:36 PM   #11
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I wouldn't be using plywood as a spacer of any degree of accuracy. Get something solid.
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Old 02-14-17, 11:12 PM   #12
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I would check the fixture on an existing factory fork
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Old 02-15-17, 03:07 AM   #13
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I wouldn't be using plywood as a spacer ...

You don't seem to be aware of the proportions here.


How can a 4 mm spacer that isn't visibly compressed be responsible for an about 3 mm alignment error?


Quote:
Originally Posted by Canaboo View Post
.. any degree of accuracy. ...

I do have a vernier caliper, and I do know how to read it.
The plywood measures in at 4.0 mm to the limit of the caliper's resolution.
Should be more than accurate enough for this use.


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Originally Posted by Canaboo View Post
... Get something solid.

How would that improve things?


Unless the spacer compresses by the clamping force needed to hold the parts in position, what good would a stronger spacer do?


And this is just a place holder. It's not like its torqued down to 40Nm or something.


For the very moderate forces the fixture sees, you're gonna have a hard time convincing me that the plywood isn't good enough.
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Old 02-15-17, 05:22 AM   #14
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According to the Shimano Tech Docs the offset is 4mm, but the tolerance is listed as +1.3/-.3mm, so the working range would be 3.7mm to 5.3mm.

The tab is already mounted and its your bike, so I would just modify the aluminum adapter by removing some material from the face that mates with the tab. On my mill, this would be an easy task. It will be a bit more time consuming and tedious to get a good result with a file or grinder. Only remove material from the area where it contacts the tab.
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Old 02-15-17, 09:02 AM   #15
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Quote:
Originally Posted by dsaul View Post
According to the Shimano Tech Docs the offset is 4mm, but the tolerance is listed as +1.3/-.3mm, so the working range would be 3.7mm to 5.3mm..

Good to know that I was going for the correct offset anyhow.


I wonder why the tolerance isn't symmetrical?


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The tab is already mounted and its your bike, so I would just modify the aluminum adapter by removing some material from the face that mates with the tab.

It works as it is, but I'm all out of adjustment. Should a future replacement of parts drift further in the same direction, then I'd have to do something about it.


Filing the adapter is a better suggestion than extending the mounting slot in the caliper, thanks.


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On my mill, this would be an easy task.

If it comes to that, I'll do it in my drill press. I have a drill bit set for flat-bottom counter sunk holes.
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Old 02-15-17, 10:23 AM   #16
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You don't seem to be aware of the proportions here.


How can a 4 mm spacer that isn't visibly compressed be responsible for an about 3 mm alignment error?





I do have a vernier caliper, and I do know how to read it.
The plywood measures in at 4.0 mm to the limit of the caliper's resolution.
Should be more than accurate enough for this use.











How would that improve things?


Unless the spacer compresses by the clamping force needed to hold the parts in position, what good would a stronger spacer do?


And this is just a place holder. It's not like its torqued down to 40Nm or something.


For the very moderate forces the fixture sees, you're gonna have a hard time convincing me that the plywood isn't good enough.

Well if you're that sure it should be perfect then.
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Old 02-15-17, 10:28 AM   #17
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Well if you're that sure it should be perfect then.

The assembly isn't, but the plywood spacer seems like a very unlikely cause for the bulk of the misalignment.


I'd be ready to blame the plywood for minor imperfections, but not a whopping 3 mm off spec.
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Old 02-15-17, 11:14 AM   #18
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Not that it helps you but disk brake mounts are a can of worms. I was just trying to find a recent post where someone with a new fork had the center lock retainer rubbing on the caliper. Lots of posts where forks and calipers don't have sufficient clearance. Rock Shox had a lot of trouble on some of their forks. I've got a carbon fork where there were a lot of reports of insufficient clearance.
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Old 02-18-17, 04:47 PM   #19
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I sat this one out for a bit until something occured to me.

2 things. 1, would a shim under the disc be acceptable? I'm not sure which way it would need to go. A shim under the disc would move it outboard. A shim between the IS mount and the caliper adaptor would move the caliper inboard. Between the adaptor and the caliper would move the caliper further from center. (some floating rotors have rivet/caliper interference issues)

2: Is it possible to install a centerlock rotor backwards? I honestly don't know, I've only ever dealt with flat steel 6 bolt. But if it is *possible* then it might help to turn it around so the rotor spokes are under compression when the wheel turns in the forward direction.
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Old 02-18-17, 11:31 PM   #20
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The assembly isn't, but the plywood spacer seems like a very unlikely cause for the bulk of the misalignment.


I'd be ready to blame the plywood for minor imperfections, but not a whopping 3 mm off spec.
I agree, but I suspect your fixture is to blame for this problem. Hard to say why from here though. The other question I had was the centerlock adapter. I don't remember you saying that this setup worked with another fork, which is where I would start. Is there a place where you can use spacers to help?
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Old 03-01-17, 12:16 PM   #21
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I'm having trouble uploading full pics, but here's a clearer one of the fixture.
It measures in to the drawing to the resolution of my tools and instruments.
I'll shuffle some wheels around to see if it's the centerlock adapter that's causing this.
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Old 03-17-17, 05:19 AM   #22
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A pic of the caliper mounted on the fork, adjustment pretty much maxed out.
The rotor runs free, but with minimum margin.
Well enough to let me put off trying different wheels so far...
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Old 03-17-17, 02:19 PM   #23
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Another guess! It might be some variation if the following:
You indexed off the wrong sufrace. You are off nominal by the thickness of the drop out. The drawing shows the inboard face of the bracket 1.14 from the inboard face ot the drop out. My guess is you measured, built your jig from the inboard face then added the wood in the thickness needed as if it was for the outboard face. Thus moving the bracket inboard.

Is your pic the top looking down? It shows the mount in the front, on the top, on the right! Disc's go on the left! Though it shouldn't change the spacing, turning the jig around and flipping it over would add to the mental gymnastics.

You are within the adjustment range, so I'd roll with it. Double check with a tack weld and known parts before commiting next time.

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Old 03-20-17, 03:20 PM   #24
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Another guess! It might be some variation if the following:
You indexed off the wrong sufrace. You are off nominal by the thickness of the drop out. The drawing shows the inboard face of the bracket 1.14 from the inboard face ot the drop out. My guess is you measured, built your jig from the inboard face then added the wood in the thickness needed as if it was for the outboard face. Thus moving the bracket inboard.
No.
Inside face of the dropout is the reference. From there it's 4 mm to the inside face of the caliper (adapter) mount.
Thickness of dropout or thickness of caliper (adapter) mount isn't specified on the Hayes drawing I have.
My bet is on the hub, and/or the 6-bolt adapter.

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Is your pic the top looking down? It shows the mount in the front, on the top, on the right!
Yes, front mount.
It's what manufacturers should have done from the beginning.
Rear mounted calipers have the nasty habit of trying to eject the wheel during braking.
On an old fork with vertical dropouts and very shallow lawyer lips, it's even more the right thing to do.

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Disc's go on the left!
Left or right mount has no functional difference. Front or rear has.
But front-and-left would require mounting the caliper upside-down, which would make routing the cable/hose awkward.
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Though it shouldn't change the spacing, turning the jig around and flipping it over would add to the mental gymnastics.
Not really. As far as the brake goes, it's just like I'm running the fork backwards. If I were to turn the bars 180 degrees I'd get a bone-stock left/rear brake mount.
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You are within the adjustment range, so I'd roll with it.
Well, I'm certainly not going to cut it out again!

Might tweak the fit some and put a brake mount on the original threaded fork instead.

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Double check with a tack weld and known parts before commiting next time.
Really didn't think I'd need to do that when I'd found so clear drawings of how the brake mounts should be.
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Old 03-20-17, 03:43 PM   #25
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Dunno, looking at your adapter and the drawing, it's maybe the Hayes drawing is for the axis of the bolt holes, not the inside face of the adapter... I'd also be checking that the brake mount on the fork is at 90 deg to the axle axis.
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