I’m designing a new bike for my daughter and want to use a disc rear brake. As it’s going to be a single speed with sliding dropout I’m struggling a bit with the mount design.
The problem is that the caliper needs to be able to slide with the wheel to maintain the position relative to the disc.
I’d like to mount the caliper on the horizontal chainstay so thought this might work:
If I base the fixed mount on the dimension for a 140mm here:

I think I could then use a 140-160mm adapter that I could slot to enable me to slide the caliper.

Does that sound like it would, in principal, work?

Few people use disc mount attached to a sliding drop out so it moves with the wheel.

Use a slotted ISO mount on the frame.

Attach PM caliper to ISO-PM adapter.

Could you do it with a long slot and a couple of barrel nuts to slide along and bolt into?

I’d go for the sliding dropout with built in disc mount so it all moves as one.

Actually I’d go for a fixed dropout and a built in chain tensioner near the cranks, far easier and neater solution.

Use a slotted ISO mount on the frame

That could work. Could I do that on a horizontal chainstay rather that an angled seat stay? ie: the slots running in line with the stay?

I’d go for a fixed dropout and a built in chain tensioner near the cranks

That sounds even better, should help make the design of the dropout easier.
Any links to good tensioners?

You’re making the whole bike from scratch and you want to stick an off the shelf tensioner on there?

I used Blackspire Stinger style one on my singlespeed and found it far more reliable and far less faff than any of the sprung ones. I’d imagine you could adopt a similar idea but incorporate it into the chainstay somehow.

Yeah, I know….I should make something 😉

Been looking and came across this:

Would be pretty easy to incorporate a mount into the chainstay to use a single roller.

steve – you’re bolting your rear triangle to the front yes?
Why not have some kind of adjustment (pivot at top? or packers between front and rear joining plates?) there that moves the rear triangle away from the BB shell?

Isn’t the Ibis Tranny designed along those lines?

If you integrate the dropout and caliper mount into a sliding component and copy the geometry in the drawing above, then everything will slide together.

How about a Paragon Machine Works style slider?

Is this for your daughter to progress to after the balance bike you made for her? Does it really *need* a disc brake? One more sharp spinny thing for her to chop her fingers off with? Also can you get disc brake levers small enough? A lot of kids bikes after balance bikes come with coaster brakes for good reasons.

Good point, my little lad has a brake on his balance bike but there is no way his little fingers are going to reach it never mind be able to pull it controlled enough not to skid instantly if he could.

I did wonder about that kickstarter brake thing which was basically a cable running along the bars but not sure you’d get enough cable pull being so close to the bars.

Slotted mounts work fine.

You can also slot them on two arcs which keeps caliper in iso position but gives you approx 2:1 wheel movement to disc slot length. It is too complicated to explain here but drop me a mail and I’ll send some photos or cad.

Saying that – putting the adjustment at the front is a good idea and keeps the wheel spindle / stub axle mount simple and solid.

I use a home-made Stinger thinger on my bike-

It works really well, and the chain wraps further around the front and back rings than it would with a sprung tensioner.

Only problem I found with a front tensioner was on one frame I couldn’t get the chain any shorter but the tensioner hit the chainstays before taking up enough slack but that was only a problem on one frame of about five I’ve run like that.

steve – you’re bolting your rear triangle to the front yes?

Yes, that’s the plan. Could make the location for the swingarm slotted to allow some movement.

Does it really *need* a disc brake? One more sharp spinny thing for her to chop her fingers off with? Also can you get disc brake levers small enough?

Good point but I think with a 140mm rotor it should be OK. Most levers have reach adjustment now so hoping something would work. Could always make a shorter lever.

Saying that – putting the adjustment at the front is a good idea and keeps the wheel spindle / stub axle mount simple and solid.

Yeah, I think that’s the way to go. Would like to keep the rear as simple and strong as possible.
BB mounted tensioner would be the simplest method but will look at a couple of other options. It would be really easy to make a slotted mount for an adjustable roller, fitted to the chainstay..

If you know the ring / sprocket sizes then there are online calculators to tell you exact singlespeed chainstay length. You only need a small amount of adjustment beyond that for chain wear.