Oset 26" Homebuild - Adult Sized

[betarambo]

I got the bike built and it is working pretty well. To make this work I:

1. Stretched the swingarm 3".

2. Bought the beefiest 36 spoke 26" bicycle wheel I could find.

3. Laced the wheel to the stock Oset rear hub in a 4-cross pattern using DT Swiss butted spokes (13 ga. at the end like stock, 14 ga in the middle).

4. Added 6" to the chain.

5. Bought a standard front wheel for downhill bikes (26" for 20mm through axle).

6. Put in 4 EarthX LifePo4 12V 24 aH batteries and made vacuum molded custom plastic shells for them.

7. Added a Trial Tech stem with 110mm length and 35 degree rise.

8. Flattened the downtubes a bit on the front with a torch and hammer to make clearance for the front under compression. Hoping that I am right that these downtubes are overkill considering the reduced loading here with no engine and only ten pounds of batteries.

9. Cut away most of the front most of two plates in front of the battery. Same as above.

10. Changed over to 1,000 lbs per inch rear spring that comes with the bike and added some preload.

11. Pumped 60 psi into the front forks.

12. Installed a HobbyKing watt meter.

It ended up 75.8 lbs. in weight so about half of my Beta. All the dimensions are pretty similar to my Beta. The 26" wheels are a little bigger, the skidplate and pegs are about half an inch higher. The only major difference in gemoetry is in the steering head area of the frame. The Beta puts the steering head a lot further forward, like 5".

I am still waiting until Tuesday for my good rear tire which is supposed to be 2.7" wide. Right now I am running something I had laying around which is a cheap standard mountian bike tire. This is my biggest issue so far.

As it sits now I am able to ride Intermediate to Sportsman sized obstacle (about class 4-5 of 9 locally). I think the new rear tire will be worth about one class.

[betarambo]

So, now for the help I need from you...

I foudn an issue today where I think the controller decided to protect itself and went into some safe mode.

What happened:

I was trying a big stack of logs. As I lifted the fornt the lame rear tire spun in the sand. I panic gassed it and caught traction. I easeentially did a splatter minues the un-weighting. This caused the rear wheel to fly right into the log face at max power. Luckily the wheel held up just fine despite the lound "bang" when it hit. I pulled the bike down and went to ride off put the throttle made the bike try to move but not actually go. The lights on the throttle showed good voltage so it was not the BMS in the batteries. I could feel it pushing forward about one pole of the motor when I gave it throttle. I figured the wheel or chain was jammed somehow but it rolled fine when pushed. I shut it off and turned it back on and the problem went away. A minute later my buddy was trying the bike and it did it to him under very low load. I shut it off and turned it on and everything was good again. It has not shown this problem again.

Theory:

I noticed on my wattmeter that I am pulling almost 140 amps rather than the 100 amp limit I used to see with 20" wheels. I have checked my watt meter somewhat and I beleive it even though it is a cheapo. I may need to put my beter meter on and confirm this. I am wondering if by gearing the bike up by about 30% with this wheel I have confused the firmware in the controller. Perhaps the firmware uses speed in its calculation somehow and now that this is off, the control is off.

I checked the controller after my ride. It is pretty hot but not at a level where most controllers would be freaking out. I forgot to check the motor temp. Without looking too closely I don't see any extra wires coming from the motor so I don't think the motor has a temeprature sensor going to the controller. I suppose it could have somethign internal but I doubt it.

So I think that either I am pulling too much current and the controller goes into safe mode or I am overheating the controller and causing the same thing. Then again maybe it is not a safe mode but a true temperature failure somewhere along the way. I live in the high desert. It was probably around 90 degrees F when I was doing this.

Questions:

1. Does anyone have any details on this controller?

2. Does it have any safe modes?

3. Does it use wheel speed for feedback?

4. Does this motor have anything "smart" in it?

5. Any other theories?

Moving forward:

It ocurrs to me that Oset gave me a handy way to test my wheel size theory. The speed limit switch is supposed to limit top speed to 70% which should be close to limiting it to max speed of a 20" bike. I might as well check the math here.

If top speed with the switch at 1 was 20 mph on a 20" wheel then it should now be 26"/20"X20 mph = 26 mph. Changing the switch to 0 give 70% X 26 MPH = 18.2 MPH.

So that means that flipping that switch will limit my speed down below stock. I guess I will try it and maybe rule that issue out. Of course the only way I made it happen was in a panic near crash so I am not sure how to repeat it!

[gwhy]

I don't know much about the 20" controller but pretty sure it has a thermal safe mode but I think this should just reduce output power and not really stop the bike from running. The motor do not have any feedback to the controller. There will not be any feed back to the controller about the speed of the bike. Most controllers (but not all) will peak higher than there rated current for a set time. What you need to do is check the watt meter and look for the min voltage reading, 48v controllers tend to have a 41-42v lvc . If the voltage is dropping below the lvc of the controller ( this could be down to the batterys, the bms in the battery or the main wiring from batteries to controller ) this may cause this sort of problem ( watt meter need to be as close to the controller as pos to test ) and the fact that you need to reset ( restart ) the controller could also point to a lvc problem, but Im not 100% sure it could be even something like a bad (intermittent ) connection on the throttle and this puts the controller into fault mode but gets cleared when reset.

[betarambo]

Cool, thanks for the info. I hadn't thought about a lvc on the controller. As the Earthx's runout I have seen the BMS cutout on me. Simple enough to check as I can run 9 or 10s lipos to see where the controller cuts out.

There must be some sort of logic about wheel speed as they have the max speed switch that lets you drop top speed to 70%. Even at 100% it is clear that the controller is limiting power based on RPM as once you reach top speed the power drops. You can even see it on the voltage readout on the throttle as the batteries bounce back up at top speed. But I get your point, it is motor RPM and not speed so it is not likely related.

Still, I don't think the lvc is the issue as the batteries were about halfway charged at the time. Then again, it was such a big burst that maybe the batteries really sagged. One way or another, it will be easy to figure out where the lvc is on the controller and on the batteries.

Too bad I can't ride it right now as p[aint is drying...

[copemech]

I know nothing about these bikes, but it would seem to me the controller is monitoring motor current(load) and you have exceeded the limit due to the effective gear ratio change of the larger wheels.

Good thing it is not set to reverse like a power door or window on obstacle detection!

[betarambo]

Gwhy, your thought about lvc on the controller helped me find the issue. Thanks once again for your brilliance!

Long story short, one of the 12V batteries I was running is a dud. Today the bike started cutting out on me after only using 2.5 aH from a 24 aH pack. I got it home, after a bit of a walk, and put it on the stand. I found that the controller will not spin the wheel if voltage is below 40v when first turned on or if it sags below 37v while the wheel is spinning. Wondering how the heck I could be that low I found that I had 3 batteries at 13.1v and one at 0v. I am playing around now seeing if the battery is savable. I suspect I will need to get one warranteed. That explains the odd cutout and also why I wasn't getting anywhere near the life out of my new batteris as I expected.

[betarambo]

EarthX replied after hours on a holiday and said they will overnight me a replacement battery. Very cool.

[mxoset]

Guys, I've had the same issue with my 20 with power cut out.. I'd like to understand more but can't see a direct link to this issue at the moment even though it's exactly the same symptom.

Running stock oset 20 with stock sla config.If the bikes been ridden after a while the power can cut suddenly when you have the throttle open. Due to the braking nature of the electric motor in free wheel it nearly sends you over the bars.

My daughter first reported it and I dismissed it as a one off or her bad throttle control(my bad). I had it happen to me today and it was unnerving / dangerous.

I'm pretty sure the batteries are operating normally. But the stock charger is only getting them up to 13.5v on full charge. They are all reading the same so no obvious duffers and throttle reading medium power lights on low when the cut happens.

Why is voltage so low when lead batteries are normally over 14v charged?

If this is the result of a current / voltage drop why has this controller got a hard limit where all my old oset bikes have died gracefully when running low on charge. It means I can't use these batteries safely on anything other than the first 30mins of riding.

Any thought welcome.

Rich

[betarambo]

Doing some battery testing today I learned more about how it reacts with SLA's. The bike will cut out if the voltage goes below 37 volts. This can happen pretty quickly after a full charge if you try to accelerate up a big hill with a heavy rider (like me). It happens less quickly with lithium for two reasons. First is that lithium can have a higher charged voltage depending on which chemistry you use. LiFePo4 which are found in most lead battery replacements use four cells to emulate SLA so they end up 55.6v for the whole pack hot off the charger (52.2V for SLA).The second reason (and the main one) is that lithium doesn't have near the voltage sag so even when a pack runs low it still doesn't trip the LVC as easy.

In my experiments today I ran all three battery types that I have through a circuit in my neighborhood that includes some trails, some road, and a bit of motocross track. I noted the readings from my meter when I first hit LVC and also when I felt the batteries were totally done. On the Lipos I never hit LVC as I don't want to run them that low so I stopped when cells were around 3.8v each. All battery packs were almost new (less than 10 cycles).

Stock Oset SLA:

1.6 aH first LVC / 3.8 aH when dead

69/160 watt hours

less than one lap

EarthX 24 aH equivilant LiFePo4:

6.5 aH

305 watt hours

3 laps +

Lipo (Zippy Flightmax 8000 maH 6s1p X4 wired as 12s2p for 16 aH):

6.2 aH

277 watt hours

This probably isn't much help to you, but the bottom line is that the best way to avoid the LVC is to get some sort of lithium battery so that you don't get the voltage sag until the batteries are closer to dead. It is not a cheap solution but you do get the beneift of faster charge times and shaving over 20 pounds too. There are quite a few options out there now that are less complex than the RC Lipo route if you are willing to spend a bit more.

[marky boy]

Wow, good project , i love it well done.

On the wheel side. When you get real into it you are actually spoilt for choice.

You can get 'fat' rims and tyres up to a 5" and also 'half fat' rims and tyres. These are all 36 hole also, and would build into the hubs

I am running 4" on one of my fat mtb's, you run real low pressure and would be ideal as a trials rim. Maybe a little wide for the front.

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Edited October 12, 2013 by marky boy

[gwhy]

wouldnt 5" and 4" tyres require a extra wide hub ? , are these on 24" rims? and last question... where can I get some