I’ve retrofitted disc brakes on both my Nimbus 36er and my Nimbus II 26 (same as yours).
You basically have three tasks:
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Mount the rotor. You have two options here:
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Mount it on the inside of the frame, on the left hand side of the wheel. You'll need to rebuild your wheel with a disc hub, which is the main disadvantage. The advantage is that you can use any cranks you like. This is how Nimbus does it.
Mount it outside the frame, on the right hand side of the wheel. The rotor is mounted to the right hand crank in this scenario. You can keep your current hub but you're locked into using KH Spirit cranks. This is the how KH does it.
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Mount the caliper. Again there are two options:
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Weld a mount onto your frame. This is the nicest option (KH and newer Nimbii use welded mounts) but is a bit of a hassle if you're not handy with that kind of thing.
Use a bolt on mount (the Nimbus d'Brake is the most commonly available). This replaces the bottom bearing cap on the side of the wheel with the rotor. It's a lot simpler than welding on a mount but is slightly less strong and is a bit of a pain to adjust when replacing the wheel in the frame.
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Mount the brake lever.
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If you use handlebars then just mount the brake lever there.
If you don't use handlebars then you need an underseat lever mount. Note that although that one says it's for KH saddles it will work for some Nimbus saddles too (I use it on my Nimbus gel saddle). There is a more universal two-bolt version if you're worried about compatibility.
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On my unis I’m using an external rotor with Spirit cranks and the d’Brake caliper mount. I’m pretty happy with them, but I’d prefer a welded caliper mount and may pursue this in the future. Here are some pics of my 26er and my 36er. I can dig out some more pics if you’re interested in anything specific.
One thing to note - there are actually two versions of the d’Brake in circulation. The original one is the one I have on my 26er and it fits with no worries. The newer version I have on my 36er, and it’s beefed up a bit from the original. I had some clearance issues with it hitting the spoke elbows and had to file it down a bit. This is the bare aluminium you can see in the photo. This is probably not a problem if you mount it on the left hand side of the frame, and indeed may be something specific to my particular 36er.