I want to put my HS3540 into a 20" wheel on a kids mountain bike most likely. I'm wondering what the standard rear dropout width is on kids mountain bikes? Do they even come in 135mm widths? I've got a 6 speed kids mountain bike here with 20" wheels and without being exact the dropout width measures 127mm (didn't remove the wheel to measure proper). Maybe if I got a 7 speed kids mountain bike the dropout size would increase to 135mm? Could I get away with using a 5 speed freewheel to accommodate the narrower dropout width?
Dropout width on 20" kids mountain bike?
- Thread starter electr0n
- Start date
Chalo
100 TW
electr0n said:
It could be spaced 126mm (the old 6-speed standard), 130mm (the original freewheel MTB standard), or 135mm.
A shop can spread the rear stays to a wider size and align the dropouts afterwards. You can do this yourself adequately; there are instructions for it at sheldonbrown.com. (http://sheldonbrown.com/frame-spacing.html) But whether it's a good idea to do so depends on the bike's chainline. You want the middle chainring to line up with the middle of the cassette or freewheel. Spreading the stays without moving the chainrings outwards can change the chainline-- sometimes for the better, sometimes for the worse.
Switching freewheels without altering the axle spacing does not have any effect on the hub's width.
Why do you want a high powered motor on a kid sized bike? Even if you want 20" wheels, you should have a bike that fits reasonably well. Is it for a child?
Chalo
It's for me and I'm pretty small at only 5'9". I tried a 20" kids mountain bike today and it felt all right, at least for the few minutes I was riding it. Steering was a lot different than my 26" mountain bike. The other option would be to run a rear 20" wheel in a 24" mountain bike but then I'll have to deal with the pedals being too low and the bike will look strange. I'd like to still be able to pedal.
Chalo
100 TW
electr0n said:
Have you seen the Origin 8 Bully? It's a 20" bike built for average sized adult humans:
Chalo
Chalo
100 TW
electr0n said:
Don't sweat the dropout width-- that's easy to amend. Focus on getting a bike that fits you. Something designed for a four and a half foot child isn't it.
Look at it this way: You're going to spend many hundreds of dollars on batteries and motor, even though you can get AA penlight batteries for $0.50 each and electric hobby motors at Radio Shack for $5. Think of a department store bike, or a kidbike that's way too small, as the equivalent of the Radio Shack motor or the AA batteries. Yes, it's technically a bike and it doesn't cost much, but it's also wholly inadequate for what you're trying to accomplish.
If you can find a full-sized 24" BMX cruiser bike (not a kidbike with 24" wheels) and it has a bottom bracket height of at least 12", then you can put a 20" rear wheel on it and still have plenty of ground clearance with normal cranks. You can have low but probably adequate ground clearance even if you swap both wheels for 20". It's easy to tell if the BB is high enough, because the axle of a 24" wheel is about 12" high. If the chainstay slopes uphill to the crank, then the BB is higher than 12".
In some cases, linear-pull brake bosses located for 24" wheels are usable as U-brake bosses on 20" wheels (a tall braking surface on the 20" rim makes this more likely). If that works out, you'll have to sleeve them up from 8mm to the 9mm diameter employed by U-brakes. If that doesn't work, you can chop them off and weld on new bosses, or attach a disc brake tab.
Chalo
I'm gonna go with your advice and avoid the kids mountain bike. I do still want to be able to pedal. I had no idea there were bikes with so much bottom bracket clearance so that's another thing to consider. I may just end up going with a 24" rear wheel to keep things simple. I'm debating between the 20" and 24" rear wheel.
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