After about a year of pressure to join you goofs I finally did it, so here is my first post.
I can confirm that running a 24V Oset on 36v works well. When I converted my kid's 24V 16" to Lipo we started with a 6s2p setup which gives 10 aH of around 24 volts. We found however that as the voltage dropped the bike lost a lot of power once we got below 24v. She had me changing batteries when we had only used 40% of nominal capacity.
6s of Lipo gives you 6X4.2V=25.2V hot off the charger which sounds great. However, you get 6X3.8V=22.8V when they are about dead. The 24V controller the OSet uses appears to get unhappy when you go that low.
My solution was to buy some 3s Lipos and add those in parallel. This gives a voltage range of 37.8-34.2. With this setup we found more power but also we could run the batteries as deep as I felt comfortable with and saw no power loss. I think that for a 36v bike you generally use 10s2p to get the same effect (2 five cell packs parallel, 2 in series for 42v-38v 10 ah).
As she got taller I got by for a while by swapping the handlebar stem out for something with more reach. This bought us about a year, but eventually she was just too tall for a 16" bike. The 20" Oset was about a year away at the time so what is an father to do? A gas bike wasn't really an option as those things are way too heavy for a kid and those little engines are a major pain to keep running well.
So I did the sensible thing. I built her a custom electric bike using a Gas Gas 50 as a donor. I will have to do a write up on that for this forum at some point. I ended up with a bike she loved. She named it "Sparky". It used a hub motor (no chain required) and came in about 15 pounds lighter than a gas bike. You can read the build write up I did for an e-bike forum here:
Along the way I setup as an Oset dealer and finally got my hands on a 20" Oset. This thing is awesome! I just finished converting it to Lipo today. I invited my friend over to try it out. After many warnings about the extreme power of this bike, my friend made me turn it all the way up. He made it 20 feet before he looped it. I won't mention any names, but it ryhmes with Jim Nugan.
Some specs I collected along the way for your amusement:
Gas Gas 50 Boy: 103 lbs.
20" Oset with SLA batteries (stock): 91 lbs.
Sparky (with Lipos): 85 lbs.
20" Oset converted to Lipo (with stock capacity): 68 lbs.
16" 24v Oset run on 36v of Lipo pulls about 36v at 40a for 1.44 kW (even thoguh the motor is nominally rated for 650W)
Sparky runs 60v at 40a for about 2.5kW
20" Oset runs 48v up to 100a for 5kW but the gearing or controller makes it feel like way more than twice Sparky's power
I have heard from some friends and customers that the Lipo conversion is to much of a pain. It can be indeed. You need to be careful with them. You need to pull them out to charge them. It does take away the convenience of the normal, plug it in when you can and ride when you want thing. I got a tip the other day for a nice compromise.
Earthxmotorsports.com sells Lifepo4 batteries to replace SLA (selaed lead acid) batteries. Lifepo4 is a bit safer than Lipo (although my reading and experience has me convinced that Lipos are only dangerous if you don't pay attention). It is also just a tiny bit heavier and a tiny bit less efficient. However, they are still WAY lighter and WAY mroe efficient than SLA.They are also more environmentally firendly somehow.
These guys sell a direct kit for 12" and 16" Osets. The kit gives you new batteries and a new charger. The cool thing is that they have the BMS (battery management system) built into the battery. That is how they allow you to leave them in the bike and let you not worry about a fire from overcharging a cell or a failure from runing the battery too deep. I would think that they will have a kit for the 20" soon, but I am sure you could call them and they would put a kit together for you since the 20" is just one more battery. I have not tried it yet and don't know anyone personally who has, but it sounds like a great solution if you want to shave a ton of weight but don't want the hassle of learning Lipo. Let me know if anyone tries it out. It would make particular sense if you needed to replace your batteries anyway as the SLAs are not particularly cheap.
While we had the 20" out today we had my friend's kid try it (14 year old). He normally rides a Beta 125. On the 20" he was balancing it and hopping the front end no problem. He also threw down some nice 90 degree nose wheelies. He can't do any of this on his Beta. He can do it on his trials bicycle. He rode the 20" a lot like his bicycle. I guess that makes sense since it has the same wheels and tires and the weight is closer to his bicycle than to his motorcycle.
My daughter has ridden the 20" a few times now. Wheelies are the main thing we have been working on lately and this bike is kind of unleashing that skill. Especially after the Lipo conversion, the front is really light for her. With her 16" she had learned to move the front end over when she dabbed. With Sparky she lost that skill because the front had those huge forks and way overdesigned front brake (disc and caliper off of adult sized bikes) so she just didn't have the strength for it. With the 20" that skill is back just like what she used to do. She is 9 years old and I think this bike will work well for her for quite a while.
For the Lipo conversion I am using 4 six-cell packs of 5 ah each, 2 in series 2 in parallel (12s2p). I am using 6 cell packs because I have a bunch of them from the other projects. Also, I have not been able to nail down a max voltage on the new controller. This setup gives me 50.4v-45.6v. This makes me think that I might be back to not getting all the charge out of batteries before the bike slows, but people in Britain who did this report that it works fine. I figure that this means that the new controller is happier when voltage goes belwo nominal. We have not yet run through a whole charge so we will see. The next step up would be to add in some 3 cell packs but that would get my voltage up to 63v hot off the charger. I am 95% sure the controller would handle this, but I don't want to let the smoke out if I am wrong. I think that there are some common FETs (the big switches in the motor controller that switch power to the motor as it rotates) that have a limit of 63v so this might be too close for comfort.
This thing is so much fun to ride I am seriosuly considering selling my TY80 and getting a 20" for me. I think with a taller stem I could ride it in upper class. It is odd to get used to no clutch and so much power, but you just can't stop grinning when you get on one. You can't beleive how cool a 68 pound bike with massive power is until you try it.
I will be allowing test rides on at our next trials but be warned! You will pay for that broken fender when you do what Fearless did in my driveway today!
Anyway, I am glad I finally joined the asylum. I have a pretty good understanding of Osets, Lipo and electric bikes in general after all the reading I did to build Sparky so throw me some questions if you like.