mstateglfr 

This winter, I am going to tear down and build up a mountain bike with my 10yo. We have a 26” wheel 13.5” frame rigid Nishiki and a 24” wheel rigid Trek 220 Mountaintrack to choose from.

I wanted a rigid fork as any suspension fork we got was going to be a few pounds(at least) heavier than rigid and would have been pretty cheap in quality. I don’t want a heavy entry level RST or SR fork that doesn’t rebound or doesn’t compress due to lack of rider weight.

We will change out the seatpost, stem, handlebars, saddle, derailleurs, grips, and shifters(from twist to trigger). Everything else will be cleaned, greased, etc.


Questions-
Anyone know of a good way to set up 2x7 for a kids mountain bike? Both bikes are 3x7 currently and have heavy riveted cranks. The large ring simply wont be used. A 32-24 or something similar double up front would be ideal.
- Anyone have suggestions for a 152mm triple to use as a double? The only kids length(152mm or so) triple crank I have found is from Trailcraft and its has 32T and 22T rings with a bashguard. Its $80…not cheap. Trailcraft Cycles
- For shifting, is there a 2x7 trigger system than I am not finding or is this just a matter of having 3x7 shifters and locking out the front derailleur so it cant swing past what is effectively the larger of the 2 rings?
- For chainline, Ill base it off of the distance to the middle of the 2 actual rings, correct?






This wont be a cost effective project, itll mean more money is in the bike than the bike will be worth, etc etc etc. Totally cool with that. It will still be cheaper than any new kids MTB, lighter than most all LBS options(not lighter than Islabikes or Woom) and be higher quality than most options.

Bill Kapaun

Why not just leave 3 rings and avoid the cost/hassle?
He doesn't HAVE to use the largest ring, or you could just lock it out with the H screw turned in.

gsa103

Simply set the front derailleur high limit to not shift into the largest gear.

BBG bash guards are about $10 and good replacement for the outer chainring.
https://www.bbgbashguard.com/

I did a similar conversion with a Shimano XTR (3x9) to (2+bash x 9). Locking out the high limit was all that was required to get good shifting.

mstateglfr 

On the 26" bike, its a 170mm crank arm which is too long and a new crank is needed.
On the 24" bike, its a full steel crank and riveted rings. Things a tank.

SHE doesnt have to use the largest ring, yes, but part of this project is an intro into how the bike works and how bikes can be adjusted to the user.

mstateglfr 

Thanks, this is what I figured would be the route to go, I was just hoping there was a 2speed front shifter to go with a lower end system(7 cog) to make it real easy and uniform.
Oh well, this will be an easy enough setup to just lock out the FD.


...now to find an entry crank thats short and not riveted.

Bill Kapaun

If she's 10, when do you expect a possible growth spurt?
Maybe stay a bit more moderate on the budget and then "splurge" a bit when she's a bit more "height stabilized". Stay with the short (if she is) cheap cranks. IF you don't already have a 152mm, look for a 24" X mart type junker for salvage. That's where I found one for my "experiment" with shorter cranks for my bad knee. They'll typically be a cheap cup/cone that looks kind of like a cartridge BB. IF you are quite lucky, the spindle/cups will still be serviceable.

I'd probably go with the 26" frame just because of available tires. That assumes you want something with more of a "street" tread and maybe skinnier (26X1.50?) to save some weight.
My 26X1.25" street slicks" should, in theory, only be 1/2" taller than a 24X2.00 tire. (1/4" bike height)

Darth Lefty 

Some MTB left shifters have a "mode converter" to lock out the top shift for 2x, but many do not. You just treat the low/mid shift like normal and the FD travel adjustment will have the effect of locking out the top. Example from a Shimano manual:Another thought on weight loss. A really nice pair of 26er tires is about 2 lb lighter than a cheap nasty pair.

You can get some crank shorteners.

gsa103

Some of the older Shimano shifters lock-out the bottom shift for 2x using only mid & high.
I tried setting up my bike using 2x mode and found that I got better shifting with 3x mode and locking out the top shift, since I was effectively using a 3x crank without a top chainring. I suspect that the cable pulls might be slightly different for 1->2 than 2->3.

See this mtbr thread for details:
Shimano 10sp: flipping the switch to convert from 3x to 2x- Mtbr.com

Darth Lefty 

That seems weird. It sort of diminishes your options for tuning. Inner to middle is the most tolerant shift because the indexing only needs to match the upper position, the lower one has the cable slack and the derailleur is set by the limit screw. Same deal at the small cog in the rear.

rlacy

Hi have a look at the SR Suntour XTC JR-T202 Crankset. I'm pretty sure the rings are bolted not riveted and the whole set is 800gms with the three rings. I'm just buying one for a 24" Full Sus upgrade project for my 8yr old who's been on the trails since he was 5. It has 152mm arms which should be fine for a 10 yr old. cost around €19. It's a 42/32/22 set.

mstateglfr 

thanks! Looks like its $37 delivered from xxcycle over in France. shipping is almost as much as the crank, but thats a lot cheaper than the other options so far.

rlacy

No problem, hope it works out.

Retro Grouch 

You might want to think about the top tube lengths of the two bikes that you are considering. I'd be concerned about the potential for toe overlap too, especially for a mountain bike because I'd expect a lot of low speed maneuvering.

mstateglfr 

Agreed on the toe overlap concern. The 24" bike doesnt have an issue as its geometry is designed for a smaller frame.
The 26" wheel bike, though a small frame, doesnt look to be a big issue from the limited test riding. With that said, the 26" will be garaged for a year or so since its just a little too big. I would rather she ride a smaller bike on the trails than larger. Easier to handle and whatnot.