Overheating at slow speeds

Nirmala

Active Member
Today my wife and I went for a short relaxed ride around nearby neighborhoods on our brand new Magnum Ui5's. We were generally riding slowly today and when we came to a longer but still not very steep uphill, the motor on my bike cut out 3/4 of the way to the top.It clearly was overheating as it came back on after just a few seconds of resting. My wife's bike had no problems, but then she is about 50 pounds (23 kg) lighter than me. Once the motor cut out, then it continued to turn itself off and back on for the rest of the way up that hill and then a few more times near the top of the next hill we rode up which was also not very steep. Once I sat still for a moment or two, the motor would come back on.

The one thing I can say is that we were riding slowly just for a relaxed experience, and so even though I could have pedaled up the hills faster, I was just pedaling easily. Again these were not super steep hills and I was not even in the lowest gear on the rear cogs. Also, I was just in power assist levels 3 (out of 6). As a result of all of this, the motor was not spinning very fast going up the hills as the wheels were not going very fast. My speedometer was reading in the range of 4-6 mph.

I had some time this afternoon, so I went back out and rode the same route. Only this time, I was not going slow up the hills. I left the assist in levels 4-6 and pedaled harder so I was mostly going 12-14 mph up the same hills where my motor overheated earlier. This time there was no problem, even when I rode up an extra and steeper hill at the end of the more moderate hills I rode earlier in the day.

So I guess I need to keep my speed up whenever possible on the longer uphills to prevent overheating, even when the hill itself is not pushing my motor to the limit. The motor seems to not like turning at slower speeds.

There is quite a learning curve so far with these new ebikes.
 

Ann M.

Well-Known Member
Yeah, unfortunately, the cutouts were letting you know that the motor was having to do too much. Good detective work! Although the motor was not spinning very fast; another option is to do like non-ebike riders do and drop the gearing to an easier to pedal gear and Spin the Cranks more. This would put less stress on the motor and is better cardio for you!.

The size of your motor will also be part of the equation as to how much work it can do on steep hills, so keep that in mind.
 

Nirmala

Active Member
I could drop the gear, but that would still mean I was traveling more slowly up the hill, which seems to be part of the problem. Of course, sometimes you have no choice but to drop to a lower gear as that is what it takes to climb the hill. In this case, I was going slowly up the hills because my wife and I were chatting side by side and taking a more leisurely ride. I could easily double or triple my speed up those same hills when I went back later.

Of course, I could try dropping the gear and lowering the assist down to 1 or 2. That will be my next experiment: I will ride the same hills in assist level 2 and see if the overall limit on motor power keeps the motor cool enough.

I wish the Ui5 had a lower low gear, but I just read in another thread about a 7 speed freewheel that will possibly fit my bike with 11-32 teeth: http://electricbikereview.com/community/threads/radrover-gearing.2975/
This would also give me a higher high gear, which I could use also on the Ui5 as the current freewheel is 14-28.
 
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pxpaulx

Well-Known Member
I could drop the gear, but that would still mean I was traveling more slowly up the hill, which seems to be part of the problem. Of course, sometimes you have no choice but to drop to a lower gear as that is what it takes to climb the hill. In this case, I was going slowly up the hills because my wife and I were chatting side by side and taking a more leisurely ride. I could easily double or triple my speed up those same hills when I went back later.

Of course, I could try dropping the gear and lowering the assist down to 1 or 2. That will be my next experiment: I will ride the same hills in assist level 2 and see if the overall limit on motor power keeps the motor cool enough.

I wish the Ui5 had a lower low gear, but I just read in another thread about a 7 speed freewheel that will possibly fit my bike with 11-32 teeth: http://electricbikereview.com/community/threads/radrover-gearing.2975/
This would also give me a higher high gear, which I could use also on the Ui5 as the current freewheel is 14-28.
I'm going to do some ride tests with that new freewheel this weekend, so keep an eye on my thread for an update! The shimano freewheel I replaced was a 14-28t, which is pretty typical fare - the only other option you sometimes see is the shimano with a 'megarange' low (easy) gear, which is akin to the 11-32t sprocket I put on.
 

JoePah

Well-Known Member
I think the problem is you're in too high a gear, going slow up a hill, pedaling slow, and lugging the motor.. If you could keep your cadence over 60 rpm and use medium pedal assist you should be ok.. You may have to be in the smallest chain ring up front and the largest cog in the back
 

flymeaway

Well-Known Member
the motor on my bike cut out 3/4 of the way to the top
You have a geared rear hub motor. You can pedal as furiously as you want but it won't help at all in this instance if that pedaling doesn't translate into a faster moving bike.. The motor output shaft is turning at the same speed as your rear wheel. When you ride up an incline at slow speeds the motor will be under extreme load and will draw as much power in amperes as the controller will allow. It's trying to accelerate the bike but can't and even at the highest assist level it won't be able to overcome the load without a lot of assistance from you to increase the speed of the bike. The excess power is turned into heat instead of useful work. You really only have a few options; ride at a faster bike speed (so the rear wheel is turning faster), or either, turn off the motor and pedal your way up, or at least turn the assist level to the lowest setting and try to keep the bike moving at 8+ MPH. Rear hub geared motors are at their most efficient operation when the motor is running near it's maximum RPM.

Court J.
 

Nirmala

Active Member
You have a geared rear hub motor. You can pedal as furiously as you want but it won't help at all in this instance if that pedaling doesn't translate into a faster moving bike.. The motor output shaft is turning at the same speed as your rear wheel. When you ride up an incline at slow speeds the motor will be under extreme load and will draw as much power in amperes as the controller will allow. It's trying to accelerate the bike but can't and even at the highest assist level it won't be able to overcome the load without a lot of assistance from you to increase the speed of the bike. The excess power is turned into heat instead of useful work. You really only have a few options; ride at a faster bike speed (so the rear wheel is turning faster), or either, turn off the motor and pedal your way up, or at least turn the assist level to the lowest setting and try to keep the bike moving at 8+ MPH. Rear hub geared motors are at their most efficient operation when the motor is running near it's maximum RPM.Court J.
Yes, that seems to be the best explanation. It is almost never a problem because on most hills around here, I can easily exceed 8 mph with the electric assist. Question: if I do turn the assist down to the lowest level so that it never is giving full power to the motor even when bogging down like this on a slow climb, will that prevent overheating? Or will the motor still heat up as it tries to deal with the slower rpm's?
 

flymeaway

Well-Known Member
if I do turn the assist down to the lowest level so that it never is giving full power to the motor even when bogging down like this on a slow climb, will that prevent overheating?
If the control/HMI system on the hub motors is designed the same as the BBS02 system the amperage is increased with each level of assist, and the motor cut-off speed is also increased. Setting the control to level 1 will provide the least amount of power (amps) output. Will it prevent overheating? It will be the least likely setting to overheat the controller or motor compared to the highest assist level, but there's no guarantee it won't.

Court J.
 

Nirmala

Active Member
Thanks for the info. It is not a major problem as I can ride up hills more quickly and then wait at the top for my wife if need be.

I rode 22 miles yesterday at my more normal pace and never had a problem, so it definitely had to do with going more slowly.
 

Nirmala

Active Member
I even got our two bikes during the introductory pricing, so they were only $1500 each including shipping.
 

Nirmala

Active Member
I will add that since I figured out what the problem was, I have almost never experienced the overheating again in the last several months. I just make it a point of keeping my speed up on the uphills and then wait for my wife at the top if necessary. If there is a very long uphill (which is rare around here), I will make it a point of taking a short rest part way up to keep the motor from overheating. The 350 watt motor on the Magnum is plenty 99% of the time.