Rear Disc... (mod, Horizontal Dropouts)

[Sponge]

Hello all,

I'm just curious as to how rear disc 'all works out' in the mod horizontal dropouts? On forks and stocks, they are easy enough since the hub is fixed straight and solid in the one and only possible position due to the vertical dropout, so the caliper alignment is only natural.

However, on mods, the hub is adjustable in the dropouts, but the caliper is fixed, so firstly we have to tension the chain with the right snail cam. BUT, this is where I start to get a tad lost... normally when I use rim brakes I just centre the wheel straight between the frame stays and then adjust the brake from there. However, with rear disc how should I go about adjusting the rear end? Should I always keep an eye on the rotor alignment with pads whilst turning the left snail cam?

Right now, the rotor is at a slight angle in the rear caliper, so I guess I don't get MAX power out of it.

Just incase this is of any help: I use, an '08 BB7 with those shoddy slippidy Echo snail cams.

Thanks in advance! I'm hoping this topic can be used by other rear mod disc newcomers for advice too.

Edited August 25, 2008 by Sponge

[cai]

Just don't wory about the disc allignment, and tension the chain as you normally would with a rim brake.

Once this is done, you can allign the BB7 clever.. To do this you find the to bolts on the end of the 'IS mount to post mount adapter' (they take a 5mm allen key, and have domed washers underneath - like on vee brake pads), un-do these.

Then follow this guide to allign it perfectly ;

Loosen brake wire pinch bolt if it is secured, then check that both pad adjusting dials are turned fully counter-clockwise to move pads fully away from rotor.

Turn the inner pad adjusting knob clockwise approximately 1 turn. Then turn the outer pad adjusting dial clockwise until the pad fully secures rotor. This aligns pads and caliper body to rotor.

Snug each caliper mounting bolt. Then alternatively tighten first one bolt, and then the other until both are fully secure.Draw slack from the brake wire and secure pinch bolt. Do not allow caliper arm to move upward when drawing slack from brake.

Set the pad clearance. The Inner pad (fixed pad) should have twice the gap to the rotor as the outer pad (moving pad). Loosen fixed (inner) pad by adjusting the dial 2-3 clicks counter-clockwise. Then Loosen the moving pad by adjusting the dial 4-6 clicks counter-clockwise.

Finally, squeeze the brake lever to test the caliper brake. Adjust the lever modulation setting by moving the pads inward or outward from the rotor by using both pad adjusting knobs.

To maintain the 2:1 ratio, turn the fixed pad by adjusting the dial twice as many clicks as the moving pad adjusting dial.

And your away

[PaRtZ]

Exactly right, but now what if you're NOT using a bb7 and have a hope or similar?

Theres ALWAYS a way, either pack it out with washers, tension the wheel differently, use a different chain etc etc etc.

[King C]

My friend has just bought a Ciguena and those have sliding mounts. That probabaly doesnt help if your talking about frames without sliding mounts like most Koxx's.

[Muel]

Exactly right, but now what if you're NOT using a bb7 and have a hope or similar?

Theres ALWAYS a way, either pack it out with washers, tension the wheel differently, use a different chain etc etc etc.

If you buy a hope, they're designed properly, so you don't need to mess about with silly spacers and cables.

I run a little baby tensioner because my chain stretched too much and I'm too tight to buy a new one.

[Captain Scarlet]

Some disc frames have very short drop outs (DOB, Neon) and so the wheel sits in one position, and everything is set up around it.

This appears to be quite good on the Neon frames due to the mount that is sloted for adjustment on the caliper.

Whilst on the subject of horizontal drop outs, why are they called drop outs?, isn't the name for vertical drop outs where the wheel actually "drops" out?

[JT!]

Do bb7's ajust pivitolally (is that a word?) when looking down on them. I know they ajust every other way but didn't think they could ajust tomake up for an out of line rotor.

[c.j]

If you buy a hope, they're designed properly, so you don't need to mess about with silly spacers and cables.

hope for the win

i wouldnt have thought it would matter toooo much.

Edited August 12, 2008 by c.j

[psycholist]

Since the Avids are post mount brakes and have side to side adjustment, the caliper can be mounted with one end slightly closer to the wheel than the other, allowing for the hub being off line. Having the hub off line probably isn't a good way to begin with the setup though... Vertical dropouts and a tensioner seems like a better idea where disks are involved, but I prefer stock bikes to begin with ...

[cai]

Do bb7's ajust pivitolally (is that a word?) when looking down on them. I know they ajust every other way but didn't think they could ajust tomake up for an out of line rotor.

Yeah boy! 'post mounts' / BB7's for the win

[Sponge]

Thanks for all the help! The BB7 guide is really useful. Very easy to setup brake!

But here's problem no.2: Wheel alignment.

Basically, I set it up so the hub is straight as possible in the frame. But this means that the rim is offset to the right hand side all the way through. Is this just bad 'dishing'? So yeah, it's a new wheel, factory-built, but when I line the hub up straight in the dropouts to get the best disc alignment, the problem is that the tyre scuffs the right stays. I tried tightening/loosening certain spokes in the rubbing areas, but it really doesn't seem to do much?

Maybe there's a method to totally shift a wheel? Right now it entirely leans to much to one side.

Any help'd be great

[cai]

I'm just going to explain this as if the wheel is rubing on the 'non-drive side'/disc side of the frame. If the wheel is rubing on the 'drive side'/chain side of the frame, do the oposite of what I say

Just tighten every other spoke that comes from the drive side of the hub about 2-3 complete rotations should do it!

But if your spokes are already quite tense, loosen the 'non-drive side' spokes say 1 turn - and tighten the 'drive side' spokes 1 maybe 2 turns.

Edit: It's really very easy! Once you've built a wheel successfully, truing etc.. is all very simple to do

Edited August 25, 2008 by cai

[Sponge]

Yeah that's the basic theory, but it seems to not work too well for some reason. Is there a proper 'dishing' method, to totally shift the alignment of the wheel?

An Adam Read or OBM answer would probably provide all that needs to be known! , but I'm also confident that this'll come in handy for others out there

[AdamR28]

http://www.sheldonbrown.com/wheelbuild.html

[JT!]

But surely if the avid BB7 aligns it's self no matter what angle the disc is at you should just align the wheel between the stays and then set up the avid?

[mike425]

to be honest it sounds like you need to take it into your local bike shop The disc can technically only sit in one place, and that is when you have put the wheel in straight regardless of the dropouts, it sounds like your wheel is built offset too far.

[Sponge]

Yeah sure does seem like it! What can be done for off-set wheels? I could take it to a shop or have some friends look at it. Is it just a case of a bit of lateral retruing? Or is it a bit more complex?

[AdamR28]

It's really not complex, 5 min job for someone experienced.

[mike425]

Yeah, like adam said, its not a big job, I've had countless DH frames where the properly dished wheel does not sit in the frame straight so a decent wheel builder has just had to pull it over a touch to one side. I wouldnt bother trying to do it yourself if you have no experience building wheels, a good bike shop wont charge you much anyway.