Disc Brake Size?

This is most likely a stupid question but…
So to preface I know nothing about disc brakes. I have had some unis in the past with them but they came that way. I recently built a wheel for a hatchet and now need to put a brake on it. What size disc do I need? Is there even a correct size? I was thinking of trying out the Alligator Pizza because it is cheap and promises a smooth stop (or drag I am guessing) and I can get it in 180mm or 203mm.
Any advice on what size the Hatchet takes?

I have no firsthand experience with disc brakes, but the listing for the hatchet on unicycle.com seems to say it comes with a 180mm Shimano, and both the Shimano and Magura kits they sell loose are listed as having 180mm rotors, so it’s a reasonable guess that’s what the welded on mounts would match.

But there could be differences in mounting expectations for the brand your’re thinking of buying…

The Hatchet comes with an International Standard (IS) brake mount; the actual brakes are post mount and use an IS-to-post-mount adapter. You would use different adapters for 180mm and 203mm rotors.

The brakes sold on the UDC web site come with the proper adapter; if you buy a brake somewhere else you would need to get the adapter separately.

And 180mm should be fine; it’s what UDC uses, and what I’m using on my 26" and 27.5"+ unicycles. The 27.5"+ came with a 160mm rotor, and it felt a little underpowered, so I switched to the bigger one (using a different adapter).

I just talked about disk brake sizing with a friend who uses 203mm Rotors on his 27.5" uni. He said 180mm will be fine for almost anything, only on really long downhills (I think >600m of height difference without much stopping was what he was talking about) he ever managed to overheat a brake with 180mm rotors.

I use 160mm on my 29er and have had no issues. Remember they are designed for stopping bikes from speeds of 40mph+ with a freewheel, so a uni doing 10mph with the riders legs providing the bulk of the resistance anyway is nothing!

Also, its worth checking with the manufacturer if possible, certainly bike frames and forks are only warrantied for certain disc sizes. Larger rotors will place more leverage on the frame. As above, its unlikely to be an issue with a uni but worth checking to be safe.

Though with a unicycle you tend to drag the brake all the way down the hill, and so put more energy into it which heats up the disk (and personally I try and use the disk rather than my legs to keep the speed down, as that’s the whole point of having it). I also use a 160 on mine, but it’s worth bearing in mind as a valid reason for having a larger disk.

For a given stopping force at the tyre - which is something which tends to stay constant on a uni whatever size disk used, as it’s all about control - then the forces on the frame are the same for all disk sizes.

I’ve been trying to figure out if this is really true or not. If the caliper was directly attached to the frame at the rotor diameter (hopefully that makes sense) then for a given braking torque of x a larger rotor would put less pressure on the frame than a smaller one.

However, if we consider that an IS/post mount is always located at the same place on a frame then the caliper adapter becomes a lever arm and increases the torque being placed back into the mount. The question is whether the difference in rotor size is enough to matter. I suspect it isn’t, and a larger rotor results in a lower force at the mount, but I don’t have the numbers to prove it.

Also worth considering is the fact that on a unicycle we’re somewhat limited in how much braking power we can apply before it disrupts our balance. So it seems to me that the real change with a larger rotor would be lever feel and heat capacity. Plus a larger rotor is a little heavier and easier to damage.

I’ve been thinking about this because I currently have a 160mm rotor on my 36er and I’m debating whether to switch to a 180mm. The current rotor is fine when riding on the road, but there have been a few occasions where I’ve nearly run out of brake on steep offroad sections.

malte and i can show you how to overheat 180mm in 100hm.
you just need a steep downhill on wich you brake continously.

Maksym prefers 203mm.

As said before it highly depends on your weight, riding style and terrain. Beside the disc size the brake itself will make a big difference. A lightweight cc brake will heat up way faster than a downhill brake with a heavy caliper. Also pads can make a difference (e.g. sintered vs organic pads).