Building My Own Oset 20 Alternative

[betarambo]

I know this thread might be a bit mis-categoriized, but I hope it doesn't get moved as this is the place to get to the audience I need.

I started to build my daughter's 16" Oset up to a 20" about 6 months ago. Then I heard that Oset was coming out with the 20" so I decided to wait patiently for it. However, I just can't wait any more. She is getting too big and Oset doesn't have a firm date or price for the 20. I hate to do it as I am a huge fan of Oset, but my daughter is really getting into trials and this summer is the perfect opportunity for her to do a bunch of riding, but the 16 is now way too small and underpowered.

So I need your help. I see a lot fo very smart people on these threads and a lot of people from the e-bike community helping out with their experiences. Any thoughts, suggestions, or ideas are greatly appreciated.

The one project like this that I have seen worked out very well. It was basically upgrading the 16 with new forks, a 20" front wheel, lengthening the swingarm and stringing the rear hub to a 14" motorcycle rim. I was going to head this way, but have changed my mind. Since I want to add more power and she is now too tall even with the big neck I put on it I think I will build a new bike. This also lets me allow her to keep riding while I sort this out. The final argument for this aproach is that when all is done I should be able to recoup some of the money by selling her gently modified 16" Oset.

I got my hands on a Gas Gas 50 Boy. In comparing them it has everything I want in general. It is a bigger frame, and has motorcycle rims of 14" rear and 16" front. For those who don't know, motorcycles measure wheels differently than bicycles. It works out that a 20" bicycle has tires that are about 20.5" diamter whereas both tires on the 50 are about 21" diameter. The GG has pretty nice suspension and geometry and enormous discs front and rear.

I started out by weighing both bikes. I found that the 16 Oset with Lipos is 52.6 pounds but the GG 50 is 101.6 pounds dry! No wonder Oset took all the market share overnight. Putting my 60 pound kid on a hundred pound bike is close to putting me on a 400 pound bike. I really think the light weight of the Oset has been the key to her riding really coming on. Her newest trick is to take advantage of a dab by moving the front end. On the Oset she moves the front whereever she wants it. I had her try it on the 50 and she cannot even get the front wheel in the air with just muscle.

So to make this work I need to shave some seriosu weight. I took out all the bits I won't need like engine, radiator, clutch, exhaust, air box etc and got it down to 67.2 pounds. I figure to run it at 60V so I will add back in maybe 7 pounds of batteries. This puts me at 75 pounds without a motor or controller yet, so still some work to do.

For the motor I have been poking around the web a bit. She has the 24V motor but we run it at 36V. My meter says I am pulling 42 amps peak so that gives me about 1500 watts. The motor is rated for 600 watts. I am figuring that this means we are looking at 600 watts sustained and 1500 peak when trying to compare to other motors.

Finding a motor is a bit of a challenge. You don't want your average industrial motor as wieght is no big deal to them. So I looked mostly in electric vehicle (EV) circles. I figured I wanted something with a nominal rating of maybe 1,000 watts to get a bit of a bump over the nominal 600 watts in her existing motor.

Kelly makes one that would mount up pretty well and sells for about $200. However, I got very interested in the idea of a hub motor. Golden Motor makes a kit for converting a bicycle to electric called the Magic Pie 3. The motor and controller both live in the hub of the rear wheel. This keeps things pretty clean and keeps wiring to a minimum. It is rated for 1,000 watts at 48V and you can run it up to 60. You can run it all the way down to 24V which might be good for low traction situations (I currently run her at 36V normally but 24V when things are slippery). The controller has an optional USB cable to let me program some options. It even has regenerative braking although for trials I will probably disable that as the complication and risk are not worth it for trials. I think I can get the hub motor, controller, a 20" cast bicycle wheel, disk brake and throttle for about $600 US. Getting rid of the chain is pretty neat from the stand point of maintenance, safety, and efficiency.

I am trying some of their forums to decide if the power will be enough. I am a bit concerned that the normal behavior has it kind of soft off the line. I think it might be focused on top speed rather than acceleration which is pretty much the opposite of what we want for trials.

The other big concern is the weight. The whole thing is about 20 pounds. Having that much weight, that far back and being unsprung wieght could do some really odd things to the handling of the bike. Then again, it might make wheelies much easier, and having the weight all unsprung could be fabulous for traction.

So the plan would be to strip some more weight off the 50, maybe get rid of those huge disks and such, maybe even replace the front suspension and wheel with some lighter bicycle stuff. Put the hub motor on with a 20" Maxxis Creepy Crawler bike trials tire, and give it a try. If things work, I think I can later swap the rear over to the 14" motorcycle tire so I can run the new Vee Rubber tire (which is a fair bit softer than the old Cheng Shin although still not near a real trials tire). The rear wheel thing will be tricky as bicycles use 36 spokes but motorcycle 14" uses 28, It will also be tricky since the hub motor is big so the spokes will be really short, but I think I can get it to lace up.

I am starting with 67 pounds and adding 27 pounds of electronics, but I will be ditching the old rear wheel. Maybe in net I would land at 85 pounds or so. I think I need to find another 10 pounds or so to make it really work but I guess I can figure that out once the trickier bits are sorted.

What do you think? Am I nuts to try a hub motor? Would i be smarter to just do the 20" conversion to her existing Oset and upgrade the motor a bit and maybe drop the pegs some to get her riding position more correct? Please help me out before I start placing orders and firing up the welder!

[gwhy]

I think the GG will make a very good conversion as you say, as a bike, its all good and well tested, hub motors are a very easy conversion but as you have pointed out they are very heavy and yes they are more for the top speed and not torque so this would be one of the main problems with using a hub, the lack of torque can be got around by getting a hub that will do a speed of around 35/40mph in a 26" bicycle wheel @48v if this was then laced into a 20" wheel this will bring the speed down and increase the torque @48v , 600W will make it a bit under powered with a larger wheel if you are comparing it to the oset. It would be very difficult to find the balance between torque and speed with a hub motor unless you can get a hub motor that is custom wound for a top speed of around 25mph that produces boat loads torque in the 20" wheel @ 48v or 60V. To have a bike of this size you will be looking for around 1kw sustained with maybe 2kw peak as a minimum for a small rider. On another negative side to hub motors is how reliable it would be in a trials environment. Geared hub motors generate more torque than a DD hub but again in a trials environment a geared hub will not tolerate to much abuse before the gears strip in the hub, the gears can be replaced with metal gears which will resolve this issue but this will also add even more weight. These are some points that would need considered carefully before selecting a hub motor to use on a trials bike.

[telecat]

I can't remember the name of the guy but In North Yorkshire a guy built a 20" from scratch and has his son competing regularly I'll see what I can dig up. I also have seen the 20" OSET as one of the prototypes competes locally. It is very good and gets close to the REV50's with a good rider.

[telecat]

I've been told the 20" is in production and is to be launched at the British WTC round

[ninefives]

Betarambo - I like where you are going with this - my thoughts have been around taking apart a mountainbike frame and rewelding to the right size and geometry then cannabalising the OSET for the clever bits. Thinking of the smallest real trials wheels and tires like on the smaller Sherco 80. - But that was as far as I got - waiting for someone more industrious and clever I think that upping the motor power by about 50% would do. I run 42v through the 36v motor and for the bigger steeper hills (we are talking 30ft+ high and steep) he is throttle wide open to maintain speed.

My preference is for simplicity in the motor department.

Good luck.

[juanroberts]

How is the project coming along?

Ever thought of putting the hub motor and controller combination on a 20" mountain bike?

[betarambo]

The project is almost done. We rode it for the first time yesterday and then I blew the motor controller while trying to hook up a new throttle. I am going with a bigger, more fun controller which should be here in time to pack it up and head to the youth nationals in Colorado Wednesday. She will probably ride the 16" Oset for the nationals but maybe I can finish the new bike for the last day. I still am a giant fan of Oset.

Check the link below for the place where I got a lot of advice and the "build log" I am working on.

http://endless-sphere.com/forums/viewtopic.php?f=3&t=41746&p=610472#p610472

I kind of got distracted from making it work and ended up making it pretty. Lots of polished aluminum, custom graphis, and S3 hardware.

[betarambo]

Apparently posting a jpg is not allowed here. Click the link above to see pics of how she looks.

During the test ride she pulled my fat ass a bit harder up my driveway hill than the 24V Oset running at 36V did. With the new controller I can run up to 48V at 80A which is WAY too much for her so I will have to limit it in software.

[betarambo]

It is done and runs good. She loves it. It took a ton of work and learning but I love the end product. Let me see if I can embedd some youtube videos.

The hub motor did work out good for balance as it puts the center of gravity quite a bit lower than a gas bike or a motor with chain.

Reading the specs about the Oset 20", it looks awesome. I am still going to buy one of those as well. I wouldn't if it was a scaled up 16", but it is so much more than that. In the end her new bike (named Sparky) could end up being my "fun in the pits the night before the trials" sled. Then again, if she likes Sparky more than the 20" then I will inherit the 20 for the same kind of injury seeking. Most likely I will use what I have learned on Sparky to make something even cooler when she is ready for the next size up.

We are running lipos at 12s2p to get 48V and 10 aH. I have it set to pull about 40A so she gets 2kW of power which is 60% more than what we ran the Oset at. I can go up to 3.2kW with the current setup so I have plenty of room to open it up for her as her skills increase.

I ended up with a Vee Rubber 14" rear tire which is way softer than the old Cheng Shins that were the only option in the past. That tire combined with the more gradual controller give this bike tons of traction.

I got to riding it today and it was a kick. I couldn't get myself off of it. Once my kid got on it she wouldn't quit until it got dark.

I am the happiest guy in the world right now. After 10 weeks or so, lots of money, lots of problems, tons of learning, and some very frustrating nights it works awesome!

Edited August 29, 2012 by betarambo

[gwhy]

looks good and looks like it rides ok . Hope it holds up well.

[dave horne]

The bike looks great. Well done not an easy feat to pull off. What kind on run time are you achieving?

[sparkalounger]

Nice work Mr. Butler!

I wish i would have know who uou were at the trials, i would have liked to talk to you some there.

( I am Carsen Sparks' father...)

[juanroberts]

It looks like the coolest thing, watching your daughter ride up the rock. The bike, and its torque look amazing. Congratulations!

Edited September 2, 2012 by juanroberts

[betarambo]

So far so good. We have maybe 12 hours of solid riding on it now. We are going through batteries a lot faster than I expected. I must have miscalculated something, somehwere. I ordered a couple more 5aH 6S lipos tonight. Right now I run it with 4 of those to get 12s2P for 50V 10 aH. I am going to try adding in some of my 3s1P batteries to run more like 63V 10 aH and see how that works. I will then have three "sets" of batteries.

Today she burned through 3 full sets of 50V batteries. It was pretty cool as she rode her first air log, but dang, that is a lot of batteries for one day! Our next trials is in a week so we will see how the run time works.

Just to be clear on the questions of range, it appears that we are getting maybe 30 minutes of riding (pure trials practice, no sitting) out of 12s2p 10 aH, but I stop the packs at maybe 60% of capacity because I am still a bit nervous about overdischarging and ruining a pack. I just got a Cycle Analyst (CA) which is a cool computer made just for e-bikes. I am still sorting through getting it calibrated right but that should give me some real data on run time and such.

Mr. Sparks, your kid is a crazy awesome rider for his age! I will see if I can figure out how to PM you on here so we can email. You gotta ditch those SLA batteries! Carson will be even that much better with 8 pounds less on the bike! I had the new bike (Sparky) at that event but was having issues with the first motor I bought so she didn't ride it in the trials. We did have it kind of running. I wish I had known you were interested and I could have shown it to you.

Let's see if I can link my newest video:

Edited September 3, 2012 by betarambo

[betarambo]

A couple of details:

I managed to ditch the 20" bicycle Creepy Crawler idea and go with a Vee Rubber 14"x3.0. This gives a bit more sidewall and more width so I think it finds more traction. Outside diamter of the tire is 20.5". This made it a heck of a deal to get on the hub motor but it worked. I had to use a 10" tube in order to get a TR87 valve stem (90 degree) because the rim is barely bigger than the hub so no air chuck is going in there. The tube required tire irons to install!

I took 1.5" out of the preload spacers on the forks by replacing them with some heavy duty 3/4" PVC pipe. I replaced the fork oil with 3.5 weight stuff (the lightest I could find). This has the forks working ok but still not perfect. These mini forks don't have brass bushings to let them run smooth so the rear suspension tends to give a lot better action than the front but for now we are calling it ok.

I built the battery box from 1/8" ABS plastic. It is absolutely amazing what you can do with plastic sheet and a heat gun (no, you cannot get away with your wife's hair dryer). I lined the inside of the box with blue foam from a pad made for laying on when you camp in a tent. I am thinking about a redesign that would use nomex (race care driver fire suit stuff) or some deconstructed Lipo safe bags to have some fire-proof-ness if things get sideways. I think the ABS is pretty darned impact resistance, but lipos are kind of scary! The current battery box is revision 3. Previous attempts included modifying a camper battery box, cutting open a new plastic gas can, and even one attempt based on a small trash can.