I have a lovely 1998 Indy Fab SS Deluxe that I’ve been planning on building up and riding again, but it being rim brake only on the back has been putting me off.
I considered having a disc mount brazed on (or maybe doing it myself), but I decided to go down the non-permanent route.

I’ve seen similar designs to this on t’interweb, but none that I could actually find for sale. So I decided to have a go at my own using a Boxford milling machine we have at work.

My only slight worry is that the forces on the dropout might be a bit much. I’ve fitted a 140mm disc to start off with to minimise the braking force a bit.

CAD designs:

[url=https://flic.kr/p/223M1zP]SS Dropout 1[/url] by tom harrison, on Flickr

[url=https://flic.kr/p/DL69q4]SS Dropout 2[/url] by tom harrison, on Flickr

[url=https://flic.kr/p/223M1vk]SS Dropout 3[/url] by tom harrison, on Flickr

Prototypes – first was hand cut and milled in aluminium to get the shape/dimensions, and then CNC milled out of wood to make sure my CAD drawings worked:

[url=https://flic.kr/p/Fhd3LN]CNC SS Disc Dropout[/url] by tom harrison, on Flickr

[url=https://flic.kr/p/FhcS8o]CNC SS Disc Dropout[/url] by tom harrison, on Flickr

Final dropout – finish isn’t perfect, but it’ll do to test with and I can always make a ‘nicer’ one at some point!

[url=https://flic.kr/p/23nj3bU]CNC SS Disc Dropout[/url] by tom harrison, on Flickr

[url=https://flic.kr/p/JtcRbp]CNC SS Disc Dropout[/url] by tom harrison, on Flickr

[url=https://flic.kr/p/23nj8MJ]CNC SS Disc Dropout[/url] by tom harrison, on Flickr

[url=https://flic.kr/p/23nja7C]CNC SS Disc Dropout[/url] by tom harrison, on Flickr

And fitted ready for a ride/test tomorrow:

[url=https://flic.kr/p/DL5SiV]CNC SS Disc Dropout[/url] by tom harrison, on Flickr

[url=https://flic.kr/p/23njdg3]CNC SS Disc Dropout[/url] by tom harrison, on Flickr

I’m liking the wood ones 🙂

Means you can whack up the speed on the milling machine to test the pattern quicker.

I might turn them in to Christmas tree decorations for next year 😉

Nice work!

As long as the bracket you made is a snug fit in the dropout slot I should think it will be ok. A brace running from the bracket’s upper caliper mount up to the the frames V brake mount would solve any such issues.

EDIT; Hmm no it probably wouldn’t looking at it again. Something to let it lean on the seat stay right under that upper caliper mount would though.

A lot of torque is generated from the disc caliper.I’d be looking at some way of bracing it to the seatstay to prevent the dropout taking all the force.I’ve seen what a Rohloff can do to a dropout without the torque arm!

As long as the bracket you made is a snug fit in the dropout slot I should think it will be ok.

It is a very snug fit, no movement at all with the brake on and rocking the bike back and forth.

You’re right though, I am a bit worried by the amount of torque that will be going through the dropout.

From experience of trying something similar, I suspect it will work but you may get an annoying squeal as you brake… hope I’m wrong!

BB7s – they squeal whatever you do anyway!

I kind of agree with you OP – I think the dropout will be fine but I think your Aluminium mount will get chewed up by the braking torque.

Just for clarity – a smaller disc will not reduce the braking torque 8)

Looks good. The A2Z adapter I have also rests on the seatstay where the upper caliper bolt mount is, so perhaps that braces it a bit there too.
Generally retrofit braze on disc tabs have a brace between seatstay and chainstay. Not something you can do with a bolt-on adapter, I’ve had no issues using the A2Z adapter on a few different frames with 160mm rotor though.

.I’d be looking at some way of bracing it to the seatstay to prevent the dropout taking all the force

That was my initial thought too. In fact, both the steel disk bikes I have have a brace between the seat and chainstays to spread the load between them too. Would be a shame to damage that nice frame.

edit – I was a bit slow. Oh yeah, and nice work too OP.

Just for clarity – a smaller disc will not reduce the braking torque

Will it not?

I thought that torque = radius × force.
So if you increase the radius of the disc, braking torque will increase.

Or am I being dumb (probably!)

Looking at the drop out area of the frame you could position the caliper in the triangle between the chain and seat stays like the original mounts on the On One carbon 456 ie rotate your mount around 90 degrees anti-clockwise. You could then make some form of extended pad areas to brace it against them with some form of adjuster or adjustable clamp – ie turn the mount into the seat/chainstay brace itself.

Little bit more work but would look a lot neater and not stick out on the frame so much.

Did you use the official IS brake mount measurements or work them out from another frame?

The A2Z adapter has a little foot which rests on the seat stay but i think you could make something better. The problem you do have the the sliding drop out means it needs to be adjustable as you tension the chain and the gap changes but can be worked around.

Look at B and D options here: https://www.on-one.co.uk/i/q/FSOOCNCSO/on-one-cnc-anodized-swap-out

The singlespeed design has slots to adjust the brake manually after tensioning the chain.

Hope it goes well – well done for all your efforts . .

https://www.tensile.net/tensile-disc-brake-converter.html#PhotoSwipe1515792930247

Might be worth having a look at this adapter. I’ve got one on an old hardtail – it bolts through the qr adapter like yours, but then has a bolt that braces it against the seatstay, and a torque arm which goes up to the v brake mount. Works perfectly.

Needs more spoons

No tom!

Bad Tom!

Bad! Tom!

I’d be bracing it for sure, especially in a high end frame.

The torque on it will Shirley vary with rotor size. Simples physics innit. Unless your point is that it’s about traction between tyre and found being the limiting factor.

Just use an xtr v brake and get rid of that nicely made monstrosity . I rode miles and miles with a v out the back and disc on front.

No tom!

Bad Tom!

Bad! Tom!

Bring a camera along on Sunday Joe and film me going down in a massive fireball 😉

cynic-al – Member

I’d be bracing it for sure, especially in a high end frame.

*ctrl-F* “spoon”

Rob Hilton – Member

Needs more spoons

Ah, good, carry on. 😉

He’s right though OP, I’d not ride that, not very far anyway. I think it will bend or snap and bad things will happen. I bet you could add some sort of triangulation to it to support it without too much modification of the design though, and it looks quite nicely made, well done.

Brace it. Like this:
http://singletrackworld.com/forum/topic/diy-disc-mountstay-brace-on-old-steel-frame-pic-heavy

I modelled something very similar intended for my slot dropout commuter, I was going to do it in mild steel though…

You do have a fair bit of support from the slot faces to react against brake torque so it’s not a given that bracing back to the stay is necessary, let us know how you get on…

Nice work.

I did design, make and sell something similar for very early on-one frames.

I seem to recall I had bracing under the dropout too.

I didn’t use a CNCd lump to locate it. I used nuts from m6 bolts which are 10mm a/f located by 6mm holes. Much easier to fabricate.

I will see if I can find pics.

didn’t use a CNCd lump to locate it. I used nuts from m6 bolts which are 10mm a/f located by 6mm holes. Much easier to fabricate.

I did think of that – seen some bmx disc converters that use M6 bolts like that.

i’m glad you’re handy on the mill cos i don’t think much of your filing 😉
i’m also happy you’ve built the indy fab back up. we’ll need something to ride to the bog next time we go camping.

Bring a camera along on Sunday Joe and film me going down in a massive fireball

Will do.
I suspect the dropout will bend given what happened with the torque nuts on the alfine and a similar dropout.

i’m also happy you’ve built the indy fab back up. we’ll need something to ride to the bog next time we go camping.

It’s a beautiful bike serving a vital purpose…

The torque comes from the wheel at 13″ and ultimately will ultimately get reacted at the dropout as there’s no other load path it can take. What happens between, ie the rotor size is irrelevant. Big rotors are “better” because they can both apply more torque at any given instance, have more surface surface area to dissipate heat and the surface moves slower relative to the pad at a given rpm. All that is only applicable up to the point where you can’t generate any more grip and at the rear that point is quite low due to weight transfer.

I’d try to avoid skidding over braking bumps and no back hops but there’s lots of similar products being used in BMX right now so you may be ok.

Good luck, it’s a neat solution to have it outboard of the dropout. Not one I’d have thought of.

Brace added…..what do we think? Still a flaming ball of death?

[url=https://flic.kr/p/22mYAh7]Brace added![/url] by tom harrison, on Flickr

[url=https://flic.kr/p/23oUzEG]Brace added![/url] by tom harrison, on Flickr

I approve of the P-clip usage!

It’s how coaster brakes are braced, so I thought why not!

Brace added…..what do we think? Still a flaming ball of death?

That’ll be great, for going backwards.

I can’t help thinking that any force strong enough to damage the dropout/bracket would open up that clip like a tin of sardines.

Just a pad to push against the back edge of the seat stay would probably resist movement better.

bringing the camera…

P clip should help, I guess the new extra tab of aluminium also braces against the stay.

Reverse braking is actually quite a relevant comment – a friend peeled off a disc mount that he’d silver soldered. It happened on a failed attempt at a steep banking, braking whilst rolling backwards. With weight transfer, that is probably the worst loadcase for your mount (but doesn’t happen very often)

Bar adding a spoon….that’ll do pig.

Big rotors are “better” because … the surface moves slower relative to the pad at a given rpm.

Eh?

The outside of a bigger rotor moves past the pad faster at a given rpm.

thenorthwind – Member
Big rotors are “better” because … the surface moves slower relative to the pad at a given rpm.
Eh?
The outside of a bigger rotor moves past the pad faster at a given rpm

Doh! Of course! I worded that wrong didn’t I! 😳 Anyway there’s more contact area per revolution which means better dissipation of heat and better cooling.

So DDay then Tom?

If you die in a horrible ball of flame today, can I get the Love/Hate frame I sold you back?

You know, the sensible single speed with real actual disc mounts on?

Good luck !

😉 if I die it’s yours!

Not sure if I’ll get a ride in today……teething baby + tired wife + snotty 4 year old + pile of books to mark = busy tom 🙁