I've always had rim-brakes on my bikes but now I'm assembling a bike with a frame that has disc mounts only. What features or qualities am I sacrificing if I opt for the $40 mechanical disc brake caliper over the $160 one? Reliability? Longevity? Lightness? Shorter stopping distances? Easier installation and adjustment?
It depends a lot which specific brakes you are comparing.
If you are looking at $100+ brakes they are probably hydraulic. The high-end market for cable disc brakes is limited because hydraulic seems to be taking over. You can still find boutique brakes like Paul brakes, and there is the Hy/Rd brakes from TRP which cost slightly more. The question of whether to use hydraulic brakes is a discussion unto itself. Hydraulic is favored for low lever effort and not having to be adjusted under heavy use. However hydraulic is also more expensive and has disadvantages of more complicated setup and requiring more special tools and parts.
Within cable disc brakes, the biggest disadvantage of very cheap (generic) cable disc calipers is that sometimes they don't work well, and you may have a hard time finding pads that fit once your current pads wear out. I'm talking about truly generic calipers here, meaning they came with a bike, or they are $10-20. The low end offerings from Shimano and Tektro are in the $40 range, and they work fine. Then again some types of cheap calipers use common (i.e. Shimano) pads, so it really depends on your specific brakes.
Anecdotally, I bought a cargo bike with unbranded mechanical disc brakes. The brakes required too much lever effort for my taste, so trying to improve them, I first replaced the cables with compressionless cables. Then I upgraded to oversized rotors. The brakes were better, but then I installed Shimano cable disc calipers, BR-M375. The brakes became immediately much better and I am very happy with them several hundred miles later.
I bought a different bike with Tektro mechanical discs and the brakes are great. I would say equal to the Shimano. Neither the Tektro nor Shimano calipers are very expensive..$40-50 each. For $60 each you can get TRP Spyre or Spyke brakes which have some nice features but won't really work better than the $40 Shimano.
In addition to the usual weight/materials/finish factors, mechanicals can vary in:
- Whether the arm uses a bearing
- Whether one or both pistons move
- Parallelism quality
- Threaded versus bendy pad retention pin
- Stock pad quality/fanciness
- Whether the outer pad has a pad adjuster, as opposed to just the inner
- What else you get in the box in terms of adapters and cables and housing, and especially whether or not it comes with compressionless housing. (Using compressionless housing is by far the biggest single factor in mechanical disc caliper performance, assuming proper adjustment, and anyone who says otherwise is seriously shooting from the hip.)
Time was there were quite a few mechanicals with parallelism issues, including some pricey ones (BB7s) but the market has thankfully grown less tolerant of that and you usually have to go to the very worst brakes to see much of it, well below the $40US price point. Between the numbers you mention, it won't be a major piece of things usually. Also, as far as mechanicals go, $160US would usually imply a set.
The biggest difference between cheap and the best mechanical (=cable actuated, not hydraulic) disc brakes will be, whether both brake pads are movable. Cheap mechanical calipers only move one piston that pushes on one pad. The other is fixed. That means that the rotor flexes a lot and it is all rather unsatisfactory and harder to set up exactly without moving the caliper in the mount during the lifetime of each pad pair.
The better ones can freely adjust both calipers with a hex key and move both prake pads with two pistons when the brake is pushed.
Unlike with hydraulical brakes, one has to adjust the brakes often as the pads wear.
With hydraulic brakes you get automatic adjustment, bigger power, better and more consistent modulation and also messy fluid and need to bleed them.
Another point is that cable-actuated calipers tend to be single-sided, whereas hydraulics are dual sided.
That is, the piston moves on one side or both sides respectively. Your rim brakes of yore are "double sided" too because both pads move in together.
On a single sided caliper, the one moving pad has to subtly bend the rotor to squeeze it between the moving and stationary pad.
In a dual-sided caliper, both pads are pushed in equally to squeeze the rotor.
Here I compare 2 similar cable disc brakes. Even the more expensive of the two isn't at the top of your range.
My bike came with Promax Render-R DSK-717. They're essentially a cheap copy of the Avid BB5, which I now have. Both have one moving pad and one fixed.
The cheap ones are much harder to adjust, specifically the fixed pad. On both you're supposed to be able to reach through the spokes and turn a knob by hand, or reach a tool through to turn the screw head in the centre of the knob. The Promax ones are so much stiffer that it can be hard to know whether you've run out of adjustment or it's just being difficult. That makes a quick precautionary tighten much harder - and this design relies on the fixed pad being sufficiently adjusted. Combined with unexpectedly severe pad and rotor wear, failing to make that adjustment was implicated in a rather unpleasant crash.
The stopping power with the same type of pads was indistinguishable