I recently ordered a road uni from Mad4One without a disc brake, because it was expensive, but I feel I need a brake. Could I just order a $25 hydraulic disc brake from amazon instead of a $100 one? Would it damage the hub? Would it work? Don’t want to damage my brand new uni.
The brake is a piece of safety. I wouldn’t go for a no-name brake if I wanted to stay unharmed. However, some Shimano brakes can be really cheap and work great. You shouldn’t need to spend $100 to get one. E.g. you could go for the Altus model, which is listed at less than €30 where I live.
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I also use inexpensive brakes. My preferred brake is the Shimano MT200, and it meets my needs.
It also has the following advantages:
- 3-finger brake lever, easily accessible from the front
- Easy to install thanks to the conveniently located clamp screw
- Easy to align because it offers plenty of space between the brake pistons
This brake doesn’t cost much more than a big sandwich
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Sandwiches cost a lot in Switzerland 
By the way, the Shimano MT-200 range name is “Altus”. So, we’re recommending the same brake 
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What size rotor should I get? 160mm or 180mm?
This looks great, yep. Regarding the rotor size, I would say the bigger the better. A 180mm rotor provides a bit more power than a 160mm rotor, and dissipates the heat better. Be careful that you’ll probably need a front IS>PM adapter as well to fix the caliper. This should match your rotor size.
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Have the MT200 on one of my unis, and so far am happy with it. Also, the rotor it comes with in your linked set should be just fine. Have this on one of the unes and have the more expensive ….66 on an other one. The 66 is stamped in a way that the edges aren‘t sharp. Shimano probably thought this to be a cheap way to make a rotor with smooth edges (for safety reasons), which is mostly true. But first, there are still sharpish edges on the outer rim. And second, all the cutouts have smooth corners. So the disc sort of vibrates because of this, which I feel is a bit disturbing on a unicycle - on a b!ke probably less so. And instead of helping to move the dust away, those rounded edges cause the dust to collect… If I was to chose between the …56 and the …66 discs again, I would clearly opt for the cheaper …56.
As far as disc size, I‘d go with 180mm. Should usually be enough (I remember you aren‘t tall, so I reckon not heavy either). I changed for a 203mm on my muni, but because a day of downhill caused rather cramped fingers. Normally, 180mm is perfectly fine.
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Are you sure the disc vibrates because of what you have observed? This may be due to the pads not being correctly bedded in, or to a Nimbus DBrake. I mean, Shimano is a really good brand and I don’t imagine them selling a rotor that vibrates because of its shape, even if it’s cheap.
I’ve already installed the RT56 and 66. The RT66 is also designed for metal pads and is probably more hardened or something. The RT56 seemed much more suitable for the MT200 brakes; the braking performance was better. I get even better braking performance with my cheap discs. They seem a bit rougher. My MT200 brakes best with them (I know, as a dealer, I shouldn’t write such things).
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Absolutely sure. It‘s not that typical high pitch vibration that you sometimes get with disc brakes. Also, there’s no d’brake adapter on this Flansberrium uni. Yesterday I switched wheels between two frames, and the disk in question is now indeed on a uni with a d’brake (without Hugo strut). IWell, it behaves just the same.
Here are two pictures that should make my point pretty obvious, I think:
The white “halos” around the cutouts were never touched by a brake pad 
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That was one reason for me to buy it. In case I want to try out metal pads later.
I don’t have complaints about the braking performance other than that disturbing vibration thing.
I have one of your disks and it does indeed an impeccable job.
Is that from stamping the hole during fabrication?
It feels like the braking trace is a bit too low (isn’t it supposed to run over the cut outs??).
Maybe the IS adapter for your brake is too small and you need the next step up 
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That‘s possible. But then, it does appear to run over the cutouts, even though the trace doesn‘t appear black towards the outer rim.
I have one brake where I already put shims between the adapter and the caliper, but I don‘t know on which uni it is. I’m sure that I didn’t mount a 180mm disc and a 160mm adapter, but maybe I mounted the adapter the wrong way around?
I don’t think it is possible to mount an adapter the wrong way around. However, you may have mounted a rear adapter whereas a front adapter is required
Typically, Hope adapters work for rear as well as for front brake, but they do not provide the same distance for both. E.g., there is a 200 front/180 rear adapter (labelled “HBMN”).
In this particular case, this would do the opposite of what you observe, but there may be variants from other manufacturers
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Either from stamping, or from polishing. When new, the disc was very shiny - not the surface with grinding marks.
It’s actually possible to mount the adapters incorrectly. I make this mistake occasionally, too. Disc rotors aren’t all exactly the same height. I also have combinations of brakes with discs from other brands, where the brake rotor touches the surface. It also seems to me that the heights of the brake mounts on unicycle forks are sometimes different.
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As has been said already, you will be fine with a budget brake but I would say still go for a name brand, ie Shimano, SRAM, tektro.
The really cheap ones on Amazon from brands like ztto, meroca or suchlike are ok when they work but it can be difficult to get spares. Even the ones that are an obvious copy of branded models have minor differences like piston seals being slightly different sizes or bleed port threads being different. So even routine maintenance can be difficult.
If money is tight it could also be worth speaking to a bike shop or looking at eBay or Facebook marketplace, many bike riders will upgrade a perfectly good brake, sometimes without even using it so there are good deals to be had if you look.