Converting 3x7 kids MTB bike to 1x9

[fasteasyfree]

24" wheels, 150mm cranks. My son is 4'8".

Bit of an experiment, but I'm hoping to convert my lad's bike to a one-by system. I already have a microshift Advent derailleur, cassette and shifter, plus a 104bcd narrow-wide chainring.

Unfortunately, I made the mistake of not inspecting the crankset closely enough: What appear to be hex bolts are in fact fancy rivets. I've also managed to completely strip the threads of the square taper arm on the drive side.

So I'm looking for some advice / sounding board.

It's past the 14 day return window to send the items back, and I still want to make this work. Plus he's pretty stoked on the upgrade.

I'm resigned to grinding the drive-side crank off; there's no real way around that, and I can't use them anyway (that I'm aware of...). The question now is how to get something on the front to replace it. I have a powerspline BB and SX Eagle crank arms, but they're 170mm - maybe a little too long. I also have a Deore hollowtech BB/crankset, but again 170mm.

So unless someone knows of a relatively cheap set of square-taper cranks, 104 bcd and less than 170mm, it's looking like the powerspline will be the way we go and simply have to put up with the longer arms and hope he grows fast!

Am I missing anything in this?

 

[chris667]

Lots of inexpensive kids bike chainsets at under 170mm. Look for folding bike chainsets.

 

[Ajax Bay]

"I've also managed to completely strip the threads of the square taper arm on the drive side." (Assume arm = spindle)
Not sure why you think you'll need to "grind the drive-side crank off"? Assumes the current crankset is a 'normal' square taper.
Screw in a crank extractor and 'do it'. The crank extractor doesn't care if the spindle thread's stripped. Would need to replace anyway with a BB with spindle the correct length for new cranks.
I agree with you: 170mm cranks really are too way long for a 142cm tall rider. They grow.

 

[fasteasyfree]

"I've also managed to completely strip the threads of the square taper arm on the drive side." (Assume arm = spindle)
Not sure why you think you'll need to "grind the drive-side crank off"? Assumes the current crankset is a 'normal' square taper.
Screw in a crank extractor and 'do it'. The crank extractor doesn't care if the spindle thread's stripped. Would need to replace anyway with a BB with spindle the correct length for new cranks.
I agree with you: 170mm cranks really are too way long for a 142cm tall rider. They grow.

The spindle and bolt are fine. The crank pulling tool wasn't all the way in when I tried extracting the crank itself. I went finger tight like I've always done in the past, instead of using a spanner for more leverage that I realised was required with hindsight. So the non drive side came off fine, but it's too late for the other - there are no threads.

 

[Reynard]

Mmmm, I'm 4ft 11 and ride with I think 160mm cranks on my 24" wheel vintage MTB. Bought mine from Spa Cycles when I upgraded the original 3x5 drivetrain to 3x8. I think it were the shortest ones that came with their own brand budget touring chainset. They do kids cranksets as well, so worth looking there for something suitable.

You might also want to check whether the rear triangle will actually take the bigger cassette without either it or the chain fouling the chainstays. If the frame is steel, spreading the rear triangle to get the clearance you need is a relatively easy thing to do - albeit a tad nerve-wracking. If the frame is ally, then a re-think might well be required.

 

[Cycleops]

So the non drive side came off fine, but it's too late for the other - there are no threads.

Don’t worry, get a sturdy length of steel, like car tyre lever, behind the crank at the base and give it a good few whacks with a hammer, it’ll come off eventually. It’s what they do down here, my LBS had never seen a crank extractor before I gave him one and still insists on using the ‘traditional’ method.

 

[Big John]

^^^ You're 'traditional' method is sometimes used in our workshop and the other week I got a crank arm fair and square on the shin as it flew off. I was working some distance away on another workstand. This is why health and safety in the workplace was brought in

 

[FishFright]

Spa Cycles and SJS Cycles are my got to for short cranks.

 

[Ajax Bay]

I'm a 'finger tight' man myself, @fasteasyfree - sounds a bit unlucky or muck causing friction earlier than 'normal'.
Agree with all above re Spa Cycles and a variety of crank lengths, but I doubt a crankset with a (4-bolt) 104mm BCD has ever entered their hallowed doors. Why, they might ask, is 4-bolt better than 5-bolt? Except for boys that are "pretty stoked on the upgrade".
https://sheldonbrown.com/cribsheet-bcd.html
https://www.spacycles.co.uk/m8b17s109p0/Drivetrain/SPA-CYCLES/Chainsets