keegreil

Hi everyone! So long story short, my 4 year old son has Spina Bifida and only partial use of his legs. He's got good quads, partial glutes, almost zero hamstring strength, and nothing below the knee. He started asking for a bike like his big brother a couple months ago, and I scratched my head for a while, and finally got him a KMX-K3 recumbent tadpole trike. The thing is absolutely awesome and has given him more independence than I ever dreamed possible. He's not real fast, and gets stuck a lot on small hills, but overall he loves the thing.

But...(here's where I need help), the bike is built with a freewheel hub. So he tends to pedal with only one leg. He'll push it forward through about 130 degrees of rotation, then pedal backwards the same amount, then push again with the same (usually left) leg. It seems he lacks the strength or particular muscles to push the pedals in a complete circle and alternate legs. he gets stuck where the pedals are both horizontal. A fixie hub would allow the momentum of the bike to push the pedals past this sticking point. His physical therapist says it would be really good if we could somehow make him pedal normally, alternating legs. It would double his power output so he could go faster and keep up with his brother, but also the forced alternating motion would benefit his walking therapy and cognitive development.

The rear wheel is 16", and I cannot for the life of me find a 16" fixie wheel. I'm reasonably handy and wouldn't mind tackling building up a custom wheel from a fixed hub and parts, but I'm not really sure where to start with selecting components. How many spokes do I use? What lacing pattern? Which hub to pick? Or is there an easier way to get a fixed gear 16" wheel?

My local bike shop is working on a possible solution (welding the gear fixed on an old coaster brake wheel), but I'm not sure if they're going to come through, so I wanted to explore some other options too. Thank you!

veganbikes

That thing is bad-ass...however it is a geared bike or at least has a chain tensioner from what I saw. You couldn't run that fixed with the tensioner and it didn't look like it had track ends or at least semi horizontal dropouts. It is possible that maybe a frame builder could do some welding but I wonder with that length of chain if it could work and if the routing could work.

If you find out what the dropout spacing is you could see if White Industries has a ENO hub in the right width? I believe they do 126mm as their smallest size.

You might try the Adaptive sub-forum here as well they might know. Or check with the manufacturer and see if they have options or ideas or ability to fab up something custom for relatively cheap?

Building a wheel isn't so bad you can probably do a 24-28 track hub on a BMX or maybe Velocity A23 or Dyad rim (though you may need to up the spoke count on the hubs) and if you have a good shop with a Phil Wood Spoke Cutter they can get you the short spokes you will need. If I can find lower spoke stuff I would do a 2x but if only 32h then 3x is usually recommended. However because of the issues above it may not actually work.

Good luck and hope your little dude is able to ride more and get stronger.

aggiegrads

You need a custom wheel, as this is a very specific situation. This should be no problem for a moderately sized shop with a wheel builder. You will most likely need to order parts, and it is very possible that the wheel builder will leave open holes in the hub - most 16 in rims have 24 holes or less, and most fixed gear hubs have 28 holes or more. One critical piece of information is the width of the rear dropouts. You need to know that to order a hub. To order a rim, you need to know the ISO diameter. You should see a marking on the existing tire with something like "40-349". One other problem that you will need a shop's help with is a way to tension the chain. Most single speed and fixed gear bikes have dropouts (holes in the frame where the wheel is mounted) that are adjustable so that the chain can be tensioned correctly. It isn't a problem on a geared bike because the rear shifter takes up the slack with a spring on the bottom (slack) side of the chain. A fixed gear has no slack side, so you need a way to tension the chain.

I would also post in the Mechanic's forum if you don't have a shop close by that will support you. It would be helpful to know where you are located.

My guess is that in your situation, you will find someone in the Mechanic's forum who will offer to help if you can't find anyone local.

keegreil

Thanks! Yeah, I forgot to mention the tensioner. It's used to make adjusting the boom length to the kid's leg length simpler, without having to break the chain each time. But I was planning to just remove it and deal with the chain, definitely can't run a fixed hub with that on as you said. I trialed it with a coaster brake wheel I had laying around, it did not work well, haha. For the fixed hub I was thinking of just 3D printing a nylon chain guide on the bottom of the frame where it turns the corner. Not ideal I know, but might work long enough to at least test the "fixie" idea, and maybe figure out how to add a proper idler gear later.

Dropout spacing is 135mm.

And sorry, what is a "2x" vs a "3x" in your last note?

aggiegrads

Looking at the photos, there may be some adjustment inherent in the chain idler in the center of the frame and the tubes that the chain runs through. You definitely want a competent mechanic to set the chain length or you will have problems with the chain falling off.

aggiegrads

2x vs 3x is the way that the wheels are built - how many times the spokes cross each other. I really don't think that the wheel build matters that much. It doesn't sound like your little one generates enough power to stress a wheel that small in diameter.

keegreil

Thanks aggiegrads! I'm located in Tampa, FL. There's tons of bikeshops, I will call around and see if any have a wheelbuilder that could do it, that'd definitely be the preferred option.
Rear dropouts are 135mm spaced. Existing tire says "16x1.90" and "F-252-1" (not sure what that second number means...).
Chain tension can be adjusted a bit with the boom of the trike, where the pedals are mounted. It can easily be pushed forward and back to account for chain length. That's not really how it's designed, you're supposed to adjust it to only the kids legs and the tensioner takes care of the chain. But if I get it close to my son's leg length and break the chain, I can push the boom out a little to tension it I think.
Thanks for the 2x/3x definition. He for sure does not generate much power (maybe someday!). Full throttle right now he can manage about 5 mph on level ground, a good brisk walk for me behind him.
I will check in with the mechanics section too. This forum is great!

veganbikes

If your dropout spacing is 135 then White industries is your best bet assuming the chain can work like that. I am not a 'bent expert so the idea of making it fixed is awesome but seems tough with routing. There is also non-eccentric hubs but again chain tension is the issue. Surly, Paul and possibly others do a 135mm fixed hub.

2 cross or 3 cross referring to the spoking pattern.

aggiegrads

The second number is probably a manufacturer's code. You may need a flashlight to find the ISO diameter on the tire, but all tires should have them. It is probably either 349 or 305.

If there are no markings, here is how to measure: https://www.sheldonbrown.com/rim-sizing.html

Russ Roth

I idea has merit, the biggest issue is really the non-direct routing of the chain; fixie hubs can't really have a lot of slack in them at all or they jump off the cog or skip teeth. The pics make it hard to tell but maybe get a freewheel attachment/adapter from a sun trike or similar and bolt it to the frame below the seat where the bottom of the curve is. Use a 2 cog freewheel like a dos enos which can then allow the use of 2 chains instead of one long one. Although the freewheel can spin the two cogs can't in relation to one another. If you put the adapter on a plate that can slide up and down or pivot somehow you can use it to tension both chains though you'll probably still need a half link to make it work perfectly. For something like this a basic 16" rim that most shops have laying around will work fine 28h 2x since both those and 28h track hubs are easy to find. Just have to space it to 135 with a new axle but not a big deal.

freewheel adapter: https://www.amazon.com/Sun-Bicycles-...425284&sr=8-21