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I got a cheap second hand bike for my son as its not worth repairing his current one as he outgrew it. At the time, I knew it needed a new inner tube which I was going to replace.

Only now I noticed that the front brakes don't work. I can see they are disc brakes but the disc is missing. The only disc I can get locally has mounting holes. The bike has nowhere to mount it to. The calipers? open and close just fine.

I thought about changing the brakes to the type that works against the rim but the problem is the same and there is nowhere to mount those brakes (holes are missing and I don't want to make them)

Center of the rim with no where to install rotor Disc brake assembly with working calipers forks

Click images to embiggen

Assuming I can install a disc rotor do I need to get a specific kind or something else to mount it to? Do I actually need a different rim?

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    Someone replaced the front wheel with one designed for rim brakes. Jul 25 at 20:39
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    I assume this was a private sale. In many countries selling a bike without 2 functioning brakes is illegal as well as dangerous, and the seller could be prosecuted. There's not much chance of action being taken unless against a shop that makes a habit of it, but it might be worth checking the laws where you are
    – Chris H
    Jul 26 at 8:24
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    @Matt on a cheap 2nd hand runabout I'd do the same sort of thing - buy it, check it over, and deal with anything I find. It's possible that this bike was "fixed" for use in a flat area - where I work I see people riding with one or even both bikes disconnected because the wheels are so out of true, but not on hills. Further thought - it could have been stolen: 1 person locked their bike up by the front wheel only, and the rest of the bike was taken, to have another wheel bodged on to it.
    – Chris H
    Jul 26 at 10:37
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    Seeing the latest picture, I thought maybe the fork had been swapped, but no: that bike was sold with front disc/rear rim brakes and white forks on a black frame
    – Chris H
    Jul 26 at 10:45
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    @Criggie Not an expert by any means but the fork frame looks like it should support those brakes.. The v-brakes on all my other bikes require on the bottoms of the "horseshoe" that this one doesnt have. Perhaps this fork was used for multiple models and only certain ones were tapped.
    – Matt
    Jul 26 at 12:21
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You will need a wheel with a hub that is rotor compatible and fits your bike.

There are two main ways to mount a disk brake rotor to the hub.

A Shimano centerlock design has splines on the hub to allow a rotor to be attached.
enter image description here

Or a six bolt rotor mount
enter image description here

Your front hub does not have either of these mounting methods. If the fork is original to the bike then it is likely that the wheel is not.

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From what I can see, there is no serrated ring and the hub is not a disc-brake compatible one (for either center lock or 6-bolt rotors). Thus, for disc brakes you'll need a new wheel that mounts one or the other style of rotor. The rotor will cost probably an extra $20 or so if the wheel doesn't come with one.

It's hard to tell from the photos, but your wheel may have the flat surface next to the rim needed for V-brakes (pics, please). As you note, the issue is then mounting rim brakes.

We'd need photos of the whole fork, especially of the front just below the "crown" area where the Y of legs starts. If you have brake mounting bosses there, or threaded holes for them, it should be quite straightforward to mount V brakes. Decent quality Shimano V brakes run about $15-20 online in the US.

PS - I'm assuming this bike is a 26" wheel mountain bike - it would be good to know the wheel size for sure.

PPS - As wheels can be fairly expensive, in the past I have been known to buy a cheap used bike just for the wheels and then resell the frame.

Update: Thanks for the bike photo! It does seem to be a Walmart-level bike ( https://www.canadiantire.ca/en/pdp/supercycle-ascent-full-suspension-mountain-bike-26-in-0711038p.html ) and with no mounting holes for V brake bosses as you mentioned, I think you're stuck with buying a 26" wheel with disc brake hub and rotor. On the good side, a decent front wheel will make for safer riding and stand up to disc braking forces, and you can always keep it or sell it once your son outgrows this bike.

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    I've also been known to buy cheap 2nd hand bikes for wheels - but back wheels with rim brakes. Disc brakes tend to be on newer and/higher end (i.e. more expensive 2nd hand) bikes, and front wheels are cheaper new than back ones
    – Chris H
    Jul 26 at 8:22
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    Pick of forks attached.
    – Matt
    Jul 26 at 10:28
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Fortunately the brake is a cable-actuated non-hydraulic brake and you don't need to worry about fluid loss in this case. The hub looks as if it might have a lock-ring for the disk (that serrated ring on the left next to the spokes), but I'm not so sure when looking at the blown-up picture. In which case you'd need a new front wheel, unfortunately.

If there is a lock-ring, all you'd need is a fitting centre-lock brake disc, probably a 160mm version and a lock-ring tool. Both are quite inexpensive. It's not rocket science to install it and there is a number of how to videos on YouTube. Check the brake pads as they could be missing as well or be in a poor shape and adjust them.

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    That looks to be a Walmart-level el-cheapo wheel. I‘ll eat my dropper post if that hub actually has a centerlock interface.
    – MaplePanda
    Jul 26 at 5:58
  • @MaplePanda's right - there aren't any actual splines, and the part that looks like a bit splines gets too close to the spokes
    – Chris H
    Jul 26 at 16:37

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