Newbie rear hub drive Q's- how not to fry my motor

Chuckles1

1 mW
Joined
Jan 3, 2021
Messages
19
Location
Maine, west central foothills
Have a new Bafang 500W kit with SWX02 geared hub motor in a 700c wheel I will be installing when it warms up a bit in my garage. I weigh 155lbs, and plan to ride mostly paved roads and some gravel roads. Will be using a 48V battery, as I'm aiming for reliability rather than speed.

I have some serious hills I can't avoid. Don't know how to discern % grade, but the biggest is about a half mile long, steep enough that I normally crawl up it at 3-4 mph in lowest gear (30t F, 34t R on my gravel bike). I'll be climbing hills of similar magnitude on a regular basis.

What's the best way to attack a hill like that to avoid overheating the motor? Lower PAS and slow ascent, or hit the hill at high speed and power up as fast as possible? I'm guessing, never having ridden an ebike, that the motor will be pulling as much power as controller will allow, no matter what The PAS setting; but any advice on how to go at it the first time would be welcome. I'd hate to fry my motor before I learn how to use it properly.

BTW, I've been reading about ATF cooling, and might fool around with that later when I need to get into the motor for some reason, but since I'm new to this, I don't want to go that route at this time.
 
Paved roads are rarely very steep. The overheating danger is coming mostly from stop and go while climbing. A continuous climbing for half a mile is nothing dangerous, but low power would make some crank work necessary.
 
If you are using the motor power to climb, then staying at the fastest speed you can go will be easier on the motor. If you have to slow down you must also roll back the throttle so you're not pouring way more power into the motor than it is outputting to the ground, or else the difference ends up as waste heat.

If it is helpful, you can go to the http://ebikes.ca/tools/simulator.html and read the entire page to learn what evertything is and how ot use it, then play with a system similar to yours under various conditions to learn how different things interact and find the "best" way to tackle the hills (or if it might be a real problem).

Chuckles1 said:
Don't know how to discern % grade, but the biggest is about a half mile long,
If you know how long it is, and how tall the top is, the average grade is the Rise (height / Run (length). If you go to google maps, or maybe "strava", you can use the elevation features to see slopes in some places.

If you have a celphone, many of them can run free apps that use the sensors in the phone to give slope, if you mount it perfectly flat on the handlebars. There are "level" apps, and multiuse apps like Physics Toolbox and the like.
 
+1^ as mentioned above to approach the hill with as much momentum as possible to maintain as much speed as possible in an attempt to keep the motor in its most efficient range.

If you are concerned the hill might fry the motor attempting to summit first try, why not test climbing 1/4 the way and feel the motor with your hand to gauge the temp (or better yet use a IR thermometer if you have one), then descend and try again 1/2 way, test, repeat for 3/4, then all the way?

If the road is little travelled, can you zigzag your way up it? That can help maintain a higher motor speed.
 
MadRhino said:
A continuous climbing for half a mile is nothing dangerous, but low power would make some crank work necessary.

That's what I'm hoping. I will pedal up hills, but just wondering how long you can climb before overheating is a concern. My motor isn't listed in the simulators I've looked at, so I guess I'll take my IR thermometer and check temp when I first climb serious hills. That's a good idea.

What is a conservative max temp you'd want to stay below to avoid motor damage?
 
amberwolf said:
If you know how long it is, and how tall the top is, the average grade is the Rise (height / Run (length). If you go to google maps, or maybe "strava", you can use the elevation features to see slopes in some places.

What I do: use google maps to get "directions", set the route type to "Cycling", and mark start and destination at either end of the hill. Change units to metric if they aren't already, because I am not confident that their fraction of mile figure doesn't throw away some accuracy. This will give you the run and rise values. Divide 100 * rise/ run, it's your percent grade. It's really the only way to talk about steep. You've got the "long" dimension (half mile), but no real way to communicate "steep" without that number. Unfortunately, works only in parts of the world where google maps supports bicycle routes.
 
A half mile at 10 mph will take 5 minutes to cover. Looking at Grin's simulator, a 10% grade will smoke a G310 in 6 minutes. You can estimate your motor's heat soak capability by looking at the flange distance, or the diameter.

A G30 is 123 mm for flange. An Sx02 is 145 mm. That's 15% more metal and at least 15% more mass, maybe more. But give it 7-8 minutes? In any case, you won't be going 10 mph, so watch out on steep long hills.
 
If the bike has disc brake, you could lace the motor in a smaller wheel to make a better climber.

Geared hub motors can stand 130°C temp safely and will melt before 180°C. Motor temp, not casing temp. DD hubs can give a pretty good idea of temperature by checking the casing, but geared hubs can’t because the small motor inside gets hot well before the hub cover.
 
donn said:
amberwolf said:
If you know how long it is, and how tall the top is, the average grade is the Rise (height / Run (length). If you go to google maps, or maybe "strava", you can use the elevation features to see slopes in some places.

What I do: use google maps to get "directions", set the route type to "Cycling", and mark start and destination at either end of the hill. Change units to metric if they aren't already, because I am not confident that their fraction of mile figure doesn't throw away some accuracy. This will give you the run and rise values. Divide 100 * rise/ run, it's your percent grade. It's really the only way to talk about steep. You've got the "long" dimension (half mile), but no real way to communicate "steep" without that number. Unfortunately, works only in parts of the world where google maps supports bicycle routes.

This works well and is reasonably accurate. Maximize the map, zoom in on the map to the street or route, click two or more points, check the box for "show grade" and it will show the grade between the points.
http://www.heywhatsthat.com/profiler.html
 
docw009 said:
A half mile at 10 mph will take 5 minutes to cover. Looking at Grin's simulator, a 10% grade will smoke a G310 in 6 minutes. You can estimate your motor's heat soak capability by looking at the flange distance, or the diameter.

A G30 is 123 mm for flange. An Sx02 is 145 mm. That's 15% more metal and at least 15% more mass, maybe more. But give it 7-8 minutes? In any case, you won't be going 10 mph, so watch out on steep long hills.

Wow, I'm glad I asked. I guess I'm going to be doing some climbing with the motor shut off. The hill I am concerned with turns out to be a mile long at 5% grade, with the steepest section being 9% for 0.5 mile.

The donor bike I'm using has a 28tF 34tR low gear, so I might be able to keep it upright pedaling unassisted.
 
You will do it twice as fast with the motor assistance, without any risk of overheating. Do it on motor power alone, it won’t be at risk as long as there is no trafic lights in the steepest section to make you stop and go.
 
Chuckles1 said:
Wow, I'm glad I asked. I guess I'm going to be doing some climbing with the motor shut off. The hill I am concerned with turns out to be a mile long at 5% grade, with the steepest section being 9% for 0.5 mile.

The donor bike I'm using has a 28tF 34tR low gear, so I might be able to keep it upright pedaling unassisted.

That's pretty mild. As long as after the climb the route is less steep or levels off to let the motor cools, I don't see a problem. You will never use the lower gears or smaller chain ring if you're riding assisted though, since the speeds you'll want to maintain are higher than what you'd use that lower gearing for.
 
Chuckles1 said:
MadRhino said:
A continuous climbing for half a mile is nothing dangerous, but low power would make some crank work necessary.

That's what I'm hoping. I will pedal up hills, but just wondering how long you can climb before overheating is a concern. My motor isn't listed in the simulators I've looked at, so I guess I'll take my IR thermometer and check temp when I first climb serious hills. That's a good idea.

What is a conservative max temp you'd want to stay below to avoid motor damage?

Follow the First Rule of climbing hills w/ a hub motor (or any electric motor I suspect), which states;
Never allow the motor rpm to fall below 1/2 max. rpm.
It's actually a little more involved when "no-load" rpms are considered, but to keep it simple, it can be stated this way;
Never try and maintain the climb when the climbing speed to falls to below 1/2 the bike's top speed.
It's at this 1/2 level when the system starts to make more heat than forward motion.
If you have to, jump off and push, although I doubt it would come to that.
I've climbed some rather steep hills in Mexico on a smaller geared motor than the Bafang, But they were not too long and I was able to get a gd. run at them.
As MR states, it's starting and stopping on hills that melts stuff, but in my experience, it's the phase wires that will melt before motor damage occurs.
Hopefully your kit came with at least a 20 Amp controller and for that motor, I would use one in the 20 Amp to 25 Amp range. As others have stated, the more momentum that can be maintained, the further you can make it up the hill before the rpm's drop into the danger zone. In this same vein, a 48V system would be better than a 36V system(and a 26" wheel would be better than a larger whl., just saying).
And don't put any oils in your motor. It won't help for the shortish hills we are talking about and it will slowly leak out and make a mess and render the rear disc brk useless.
 
Your original question has a really simple answer.

Weigh 155 pounds, is how you not fry the motor. Actually, less than 225 will do er, assuming you, plus your bike with motor, battery, and maybe a 12 pack of beer in a pannier still weighs less than 300 pounds. You should expect problems only if your total weight is over 300 pounds. Or the grade is extreme, and many miles long.

The hill you talk about is far from extreme grade, or even very long. Take it with full power though, full throttle or max pas setting. It will make the motor more efficient on the hill to go up it faster, and the time shorter. 15 mph up hills is the goal, and that will be easy as pie on that particular hill. You can expect to be fine on hills about that steep, even if 10 miles long.


But there are some general rules, most of which was said above. The general idea is to climb that hill at at least 15mph or so as much as possible. So use the throttle, and pedal hard to keep your speed up going up the hill, and then the motor runs more efficient. Even if your speed has to drop to 10 mph because its steep, as long as the hill is not miles long you will be fine. You can climb incredibly steep hills, pedaling hard in your lowest gear with no problems, as long as its very short, like less than a half mile.

I say all this shit based on my job 10 years ago, where I took motor kits from E-bikekit.com and deliberately melted them down. I have a mountain on my doorstep, and a wicked desert climate. I took the kits out on days above 105 f, at 4 pm when the tar is melting nicely on the roads. Then I'd climb that mountain and see if the motor melted. When it did not melt, I'd put another gallon of water in the trailer I was towing, and try again the next day, till it melted. 30 min of struggling along at 6 mph under full throttle would do the trick. That was about 400 pounds of load, and 5%.

On flatter terrain, its quite amazing how much weight you can pull, even going very slow. On the flat you can ride as fast, or as slow as you like, but still pedal moderately hard. I mean pedal, but still able to whistle a cheery tune as you go. On the flat, going 15 mph, when you could be doing 25 mph, will make your bike go an amazing distance compared to full speed.
 
I'm betting non-event. The 500w Bafangs are pretty peppy little motors. Especially when the rider is 155 lbs.

My advice would be to install a display that offers a decent watt meter that reads in real time. Keep that in check, and you'll have about the best assurance available that you aren't going to smoke your motor. When you see it creeping up over 5-600w, you know your pushing the upper limits of the motor. If it's reading just 2-300w, there's little chance of hurting anything....

To give you an idea, on level pavement with no headwind, it'll run 10mph on maybe 50w.......
 
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