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I own an old MTB bike, it's beaten up and I'm planning to restore it. Everything is already done, groupset, crankset, the fork, etc.

The MTB itself is a 29 inch frame and since I want to give this bike to my adult friend with height only 158 cm, I want to change the rim to 27.5 inch

Here lies the problem, the V brake is not low enough to grip the tire when I try to brake

Any solution?

Thanks

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    How is changing the wheel diameter going to accommodate a smaller rider? It wont change the stack and reach of the frame.
    – Michael
    Commented Dec 7, 2020 at 11:40
  • Reassemble it on 29" wheels. Even leaving it assembled on small wheels could fool someone into riding it with inoperable brakes.
    – Criggie ♦
    Commented Dec 7, 2020 at 12:17
  • @Michael It does help somewhat by lowering the bike a little for stand over reasons. It can also eliminate toe overlap if that’s a problem. Otherwise, you’re right, the stacks and reach won’t be affected.
    – MaplePanda
    Commented Dec 7, 2020 at 16:19
  • @MaplePanda: If stand-over height is a problem then the frame is probably way too large to begin with. Toe overlap should be less of a problem for small people with smaller feet and shorter crank arms.
    – Michael
    Commented Dec 7, 2020 at 17:28
  • @Michael reach is okay, the only problem is the height of the bike it self
    – Shanse
    Commented Dec 8, 2020 at 4:52

3 Answers 3

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The best solution is to use a frame that is designed for 27.5" wheels. For other problems related to wheel size change, see Do 27.5" wheels fit a 29" mtb?. When riding bike you touch the handlebars, saddle and pedals. If the frame is too large, smaller wheels don't help

If you really have to use this exact frame and either have cash to throw around or live somewhere where labour is cheap, it might be possible to add a second set of brake mounts (but find someone who understands welding and heat treatment to do this, especially if the frame is aluminum).

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  • If the wheels haven't been purchased yet, new wheels with drum brake hubs could be an option
    – Paul H
    Commented Dec 7, 2020 at 21:01
  • Yeah, seems the only solution is to add another brake set mount to the frame
    – Shanse
    Commented Dec 8, 2020 at 4:53
  • @Paul H yeah, I'll for another wheels with drum brake, but it seems like there's not many options here for drum brake
    – Shanse
    Commented Dec 8, 2020 at 4:57
  • I'd like to add that it's an incredibly bad idea. If you absolutely have to do it, ride the bike yourself but don't give it to anyone who's going to be hurt when it fails. Note when, not if.
    – ojs
    Commented Dec 8, 2020 at 12:51
  • For the front changing to a disc brake fork and disc brakes could be an option. On the rear welding should be less risky or (if this were a city/trekking bike) one could think about an internal gear hub with coaster brake.
    – Michael
    Commented Dec 8, 2020 at 17:58
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To figure out whether it's possible, you do the rim radius math and see whether it's possible given the amount of slot you have to play with in on your current brakes, or on any available brake.

So, (622-584)/2. You'll be running a rim with 19mm smaller radius. In the usual case scenario of a common v-brake in the middle-ish of the slot, you won't have the adjustment range you need. V-brakes don't differ much in their slot length or their slot position relative to the stud mount, and those numbers aren't given by manufacturers either. There are a few weirdo exceptions on the market that are capable of allowing this kind of conversion when other brakes can't, such as the Paul v-brakes. Going that route you'll be basically inventing your own mechanical advantage ratio; in this direction it will cause the brakes to need closer pad clearances and they'll feel squishy and bottom against the bar easier.

This sort of conversion will put the pedals the same distance closer to the ground, given the same width tire. Pedal strike will become a problem in some cases, but there are also bikes in the world with over-generous BB height to begin with, or more than needed for their application, so you'll need to determine for yourself whether it will be an issue. Don't ignore it though, because it can be dangerous if it's too extreme.

Very few 29er bike models have ever been made without disc brake mounts, and you're buying new wheels anyway if you do this, so just making it a disc bike is likely something to look at.

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Long reach caliper brakes on the flip side of the back might work. That is if you have a top hole for caliper brakes. I bought a 29 inch R4 and back wheel area is too small to fit a 29 inch wheel (go figure) so that’s what I had to do. Pork Cop BMX sells long reach caliper brakes. The best solution is 27.5 wheels with swapped out internal hub cable brakes.

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