Adding disc mounts to a 120mm frame

Hi,

Has anyone ever tried to weld/braze sliding disc brake mounts on to a steel frame with a 120mm rear end, e.g. a Pompino? I'm considering buying the frame and parts I'd need to give it a go, but was wondering if anyone has done it and can give advice.

Thinking of running an ISO / 6-bolt flip-flop 120mm hub, with a disc rotor screwed to the ISO side, and a freewheel screwed on to the threaded side.

My reasoning is that I want a bike I can run single-speed when I want to, but I hate caliper and v-brakes, and I'm not a fan of any disc single speeds on the market, plus they are hard to find for cheap 2nd hand and their 135mm rear ends make running them fixed a pain.

The only issue I can foresee is a chainstay-mounted caliper clearing the seatstay (see attached), as track-ends rule out seatstay mounted calipers since you'd have to remove the brake to take the wheel out.

Cheers!

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Does 120mm disc rear hubs even exist?

You can get 116mm hubs for trials bikes with disc mounts for either threaded or splined sprockets, but they don't tend to have lockring threads as trials riders who use fixed sprockets combine them with crank-mounted freewheels. (better weight distribution, fewer teeth for the same ratio, ability to backpedal while the bashring is in contact with an obstacle...)

I did have a quick look into this, but considered using a 135mm wheel (wasn't planning to use it for fixed). Winston Vaz said he'd have to take a look when I mentioned cold setting from 120 -> 135mm.

You can use this type of disc tab on the seatstay no probs - you just loosen the bolt(s) and slide up to take the wheel out and no need to readjust the caliper positioning bolts when you put it back in.

120mm fixed/disc iso bolt hubs:

Fixieinc shuri hub
only 32h

aliexpress novatec d166sb, also only 32h and not always with lockring threading (so only freewheel)

....or any double sided track hub with a 6 bolt adapter on:

I am not sure of the discline/chainline of the last combo as I havn't tried it yet.

Yeah I would use a hub like that, and run a freewheel on the threaded side with a disc on the ISO fixed cog side (I am talking about using it for singlespeed, not fixed, but with the option to flip the hub over and run it with an ISO cog).

Surely chainline would be fine as its a thread on freewheel on a regular threaded hub?

That's a really good design actually, cheers. I can't see how it would allow you to move the caliper as you tension the chain though?

Surely chainline would be fine as its a thread on freewheel on a regular threaded hub?

The issue is that disc line ≠ chain line no?

I know the 135mm trials hub (freewheel thread one side, 6 bolt the other) I had gave a much wider chainline on the 6 bolt side than on the threaded side.

So what at first seemed like a simple job of flipping the wheel and swapping the rotor for a cog actually ended up as a total pain in the arse job involving shimming the chainring out.

I think on a 120mm hub disc line does equal chainline, because the hubs pictured above are designed for fixed / fixed use, with a 6-bolt cog (pictured).

They aren't designed to be used with a disc rotor but I don't see why it wouldn't work, to allow a 120mm frame to run disc brakes.

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It’ll still put something in the wrong place, at least if it’s the rotor and it’s too far inboard then it should be easy enough to work around it

Im only talking about the last suggestion with a normal double sided track hub plus loose disc adapter.

The disc adapter is probably 7mm thick so the disc line is probably going to be 44 mm instead of 41.5mm.

Could be a problem with rearstays (or maybe not :) ).
and as @M_V says if you want to change things regularly.

also I found spoke clearance can be a problem (caliber hits spokes on some combination) so maybe you want a wide disc line (44-45mm)

Maybe buy a bunch of all the various spacers for every combination!

look here I have done related stuff: Dolan plus disc brake

Ah yeah, that is a good point. Those disc rotor spacers are a good shout.

That's interesting about caliper-spoke clearance, I didn't consider that - only seatstay clearance. Your project is pretty interesting!

I would use MTB calipers instead of flat-mount because I like to ride flat bars, which would change things a bit as they are wider but shorter.

Traditional IS mounts are flush with the inner dropout and put the disc-line ~14.3mm in from the dropout so that is 45.7mm measured from the center on a 120mm hub.

If the pompino has room for a 160mm disc there (vs chainstay/seatstay clearance) that would be a good spot to aim for.

then calipers and caliper adapters could be standard versions.

some drawings of disc brake standards: http://www.peterverdone.com/wiki/index.php?title=Shimano_Framebuilder_Info

Ok cool, thanks for those standards.

I suppose a 160mm disc would put the caliper further away from the spokes and seatstay than a 140, as long as there is space in the rear triangle for one. Cool!

I suppose I need to design a tab to weld on to the chainstay now, that will fit the standards. I think Peter Verdone has some on his website actually.

Wouldn’t it be easier to buy a Pompetamine? It’s pretty much a Pompino with disc brakes.

Yeah it would be but they're a lot rarer, more expensive and can't be easily ran as a fixed gear. Plus that would be no fun haha.