The never-ending rear hub vs. mid drive up hills question

richj8990

100 W
Joined
Jan 21, 2020
Messages
277
Hello everyone,

Here is my situation: I've had a geared (500W Bafang) front hub drive for almost 5 years now. Put several thousand miles on it, some pavement, some offroad. For the most part it does the job but it's not easy to transport in a car of course. It's not super heavy at 55 lbs, and I do have an old GMC Yukon I could take it around with but it's still a pain to have the motor on the front instead of the center or rear.



So the plan is to get either a 500W Bafang mid-drive or a 500W Bafang geared rear hub drive. This would be mostly for offroad including steep climbs. Now...before you immediately say "Buy a mid-drive for that, silly", let's talk about some issues with mid-drives:

CHAIN: Most mid-drive mountain bike motors are 250W, I assume so they can be sold everywhere including Europe and Asia. I don't think 250W is going to cut it for me when climbing grades over 7%. Yes, you can boost most of those to around 400W but I know some local dudes that boost their mid-drives all the time and they go through motors pretty fast. Like every 1000 miles. That just seems ridiculous. Yes, Bafang does make a 500W or 750W BSS02 but then you get the risk of the chain snapping. I plan on doing a 34t chainring and an 11-52t cassette. If I'm in the granny gear that means a gear ratio of 0.65, not super low but pretty dang low for e-power, and pretty high for torque. Will the 11 speed chain survive a 0.65 gear ratio and 400-800W up something steep? I know a geared hub drive can...and a rear hub drive should be able to handle a lot more watts than a front hub due to the extra weight on the back of the bike, and I can normally get about 300-400W up something steep with a front hub. So that means with a rear hub I'm hoping for more like 450-600W or more up steep hills, assuming of course I have the traction and can keep the handlebars straight.

TRACTION: A gazillion people out there say mid-drives can effortlessly climb stuff all day long, but I'm talking about specific 8-18% grades on either singletrack or fire roads with some loose dirt/gravel. That's not so easy with any setup, I know that, because any loose dirt or gravel will threaten tire traction. Loss of tire traction = bike stops. Doesn't matter if it's a mid-drive or a hub drive, no tire traction = no climb. If mid-drives are so great, why are many of them sold with large 44-52T chainrings? And why are there smaller aftermarket chainrings for climbing steeper stuff? It's obvious that a higher gear ratio will result in a poor climb even with a mid-drive. And more than one person has snapped a chain with a smaller chainring and increased torque setup. You don't have that issue with a hub drive. For a geared hub drive, the only things limiting success up a steep hill or mountain are tire traction, battery life, and making sure not to overheat the system including the controller. Is that incorrect? If so, how?

So here's the real question: are you 100% confident that a 500W mid-drive with an 850W or so peak power option can handle a long 9-10% climb of say 1000 feet on loose dirt singletrack? Will it shrug off tire slippage; will the chain survive large fluctuations of torque if and when the wheel slips on loose stuff up the hill/mountain? Will the mid-drive motor shed that extra heat fine? If so, then I'll take the mid-drive. If not, a geared rear hub drive seems the best option because the drivetrain is not a factor that could limit climbing success. Only traction, battery power, excessive heat (and of course my own pedaling stamina) could threaten the climb, not the drivetrain itself. Good or bad argument?
 
I think your choice depends mainly on speed, not steepness. Mid-drive is best if you need to climb a steep hill slowly, like single tracks with switchbacks, where you can't maintain speed. If that's the situation, then mid-drive is the answer, accepting any of the related issues. If you can maintain speed, then you can go either way, as long as you if you go with a hub, it's big enough and can shed heat.
 
richj8990 said:
If mid-drives are so great, why are many of them sold with large 44-52T chainrings? And why are there smaller aftermarket chainrings for climbing steeper stuff?

It's because only 42t and larger rings can wrap around the gearbox housing to line them up more or less with the rear sprockets. The small ones that sit outboard of the gearbox are for folks who fail to appreciate how bad the chainline becomes when you do that.

It sounds to me like your application would benefit most from a mid drive, but they are much more maintenance intensive than hub motor drives. If that's not a deal breaker for you, get a BBS02 or BBSHD with a 42t chainring and a big ol' cassette, and you'll be all set.
 
Will the 11 speed chain survive a 0.65 gear ratio and 400-800W up something steep?
Maybe? As a drivetrain nerd once told me, an 8-speed chain has approximately 20% more material than an 11-speed chain. He was trying [successfully] to persuade me against an 11- or 12-speed drivetrain on a 1500W bike. I did just shear the teeth in my traditional 3-pawl rear freewheel hub using 1500W on my M620 ultra through a 10-speed chain on a 30T chainring with a 34T cassette (0.88 gear ratio), but the chain didn't break. Yet.

Traction:
I've been riding uphill snow (several feet deep) snowmobile tracks this spring on my bike, average 7-8%, peaking at 22% and climbing roughly 2000 vertical over the course of 3-4 miles. Granted, it's a 1000W (nominal) bike, but the bike and I weigh 140kg together. On snow it's hard to keep traction at assist level 3, and at level 5 or full throttle, it's a zero traction burnout. I've re-learned drift-style counter steering to keep things approximately straight when I'm hotrodding.

I don't believe I could make the same climbs with the default 40T chainring, much less the 44-52T options other models offer. When I have gotten onto non-snow surfaces, the bike is working a lot less hard, even at 15% and steeper grades. I expect that I'll need a gear or two less to make the same climbs after the snow melts.

My chainline is fine. In the middle of the cassette, it's correctly parallel to the bike's centerline. That's largely because I bought the correct 104BCD spider for my motor - the Bafang Ultra family uses an ISIS spline rather than the square-taper offsets the BBSxx family does. That said, there are plenty of probably-correct 30-34T chainring options for the BBSxx family - look at Luna's Mighty Mini as an example.

So here's the real question: are you 100% confident that a 500W mid-drive with an 850W or so peak power option can handle a long 9-10% climb of say 1000 feet on loose dirt singletrack? Will it shrug off tire slippage; will the chain survive large fluctuations of torque if and when the wheel slips on loose stuff up the hill/mountain? Will the mid-drive motor shed that extra heat fine?
In my limited experience, yes. I don't think I would want to try tackling the snow rides I've made with a 500W motor, but when I'm not on snow I limit my bike to 750W and honestly rarely hit that. I'm strongly considering a BBS02 for my next build.
 
richj8990 said:
Will the 11 speed chain survive a 0.65 gear ratio and 400-800W up something steep?

A small front chainring increases chain tension. The rear makes very little difference. In fact, the larger the rear sprocket, the easier it is on the chain because it articulates through a smaller angle and carries load on more teeth at a time. A low overall ratio can beat up your freehub ratchet, but doesn't reduce the chain's durability versus a higher ratio with the same size front ring.

It also doesn't matter from the chain's standpoint how steep the incline is. Full load is full load, which means accelerating from a stop at full throttle is the same as grinding up a steep hill at full throttle.

11 speed chains are purported to be very durable, but they can only accomplish this by having higher hardness and more aggressive riveting than say 8 speed chains. So they'll never be as cheap to replace as 8 speed, and neither will the cassettes.

I'd go for Microshift Acolyte 8 speed 12-46 cassette and KMC X8 chain over 11 speed every time, because you can replace them much more often for the same cost.
 
Why not a low geared rear hub? If you don't need high speed assist, but only hill climb assist, it should climb well with no chain wear.
 
Be great! But they don't come low geared. You can find ones wound slow, but not low geared.

So in extreme loads, big guy, long hill, steeper than 8%,, the motor melts. Big meaning 300 pound dude or more.

But NOT a problem if the guy is not so big, and pedals briskly, in low gear of the chain.

I've been wishing they would make a big diameter planetary geared motor that WAS a lot lower geared. Then you could fill up your panniers and go climb the rocky mountains.

You can though, gear down the motor on a street bike, by using a 20 inch wheel Finished cargo mixte..jpg6-1-2015 Re bike with dd hub..JPG a street bike,
 
There is actually a "low geared" hubmotor, in that it has a double gear reduction inside, for 11:1 reduction...lots of discussion on it here:
https://www.endless-sphere.com/forums/viewtopic.php?t=92124

I'm not sure if what I'm about to suggest is truly the case (it's discussed in the thread though), but: if it was wound for a slow motor speed, it would then be a slow-low-geared motor. (but I don't know that it comes wound that way, so you might have to fix that...or use a low-voltage system to spin the motor slower).

Dunno that it would be suitable for any of the things discussed in this thread; just pointing out it's existence, and the thread with the data on it.
 
Here’s an example of a DD hub-motor doing a slow technical hill climb with basically no run up speed. With the right motor winding and the right amount of phase amps and DC amps you can pretty much climb anything with a DD hub motor, it’s just the heat can be the issue. In my case I’m riding off road but if I wasn’t I would just put holes in the side plate covers and possibly install fans. Not such a great option when riding in the mud lol

But yeah a mid drive obviously has an advantage because you have the drive train/gears. Now if you have the right power and the right motor a direct drive can do the same thing with less maintenance. Just have to worry about heat.

This was actually my first hill climb with this new motor I tested yesterday. I hit some bigger hills yesterday but I didn’t record. There’s this famous hill near my house they called a $600 Hillclimb and I actually made it yesterday for the first time which my 5.5 T could not make it up in a 19 inch rim. The motor winding really does make a huge difference. This QS205 6T with the 16 inch Moto rim will climb a wall. I know this motor is probably too heavy for most bicycles but the MXUS 3k is a great option for hillclimbing if you get the 5T winding and push it at 60amps. I was able to do some Enduro riding with the MXUS 3k 5T at 60amps 200 phase. I mean it didn’t have enough cake for some of the extreme hills but 90% of stuff off road it would smash it. Definitely could be an option for people considering less Maintenance E bike and just using a heavy direct drive hub motor. Hub sinks and ferrofluid and you’re good to go :thumb:

https://youtube.com/shorts/XiYNBJHWF8I?feature=share
 
Eastwood said:
Here’s an example of a DD hub-motor doing a slow technical hill climb with basically no run up speed. With the right motor winding and the right amount of phase amps and DC amps you can pretty much climb anything with a DD hub motor, it’s just the heat can be the issue. In my case I’m riding off road but if I wasn’t I would just put holes in the side plate covers and possibly install fans. Not such a great option when riding in the mud lol

That's still fast enough for a DD motor in the video, but going 5mph up a 20%+ grade switchback where you basically need to navigate a 180 degree turn, isn't possible. The motor wants to go faster, but you physically have to go slow. Do a couple of those, and barely escape flipping over backwards and down the hill while making the turn, then you know it won't end well to keep going. I don't think there's a hill I can't climb, as long as there's traction and the ability to keep the bike going above 10mph, but not those very slow ascents.
 
In the real world:
Riding on tight technical trails, with my wife on a 1500w BBSHD and me with a Mxus 3k at 5000w, my motor can overheat after about 1 hour. Granted, there is a weight difference of say 70 lbs, but with lots of start and stop, the BBSHD does much better
 
PRW said:
In the real world:
Riding on tight technical trails, with my wife on a 1500w BBSHD and me with a Mxus 3k at 5000w, my motor can overheat after about 1 hour. Granted, there is a weight difference of say 70 lbs, but with lots of start and stop, the BBSHD does much better

Yeah for sure I bet the BBSHD handles the inclines with less heat. That being said you’re MXUS 3k is it 3T winding? I wasn’t able to run 5 kW without overheating so that’s why I turned it down to 40 A but that’s with a 5T. I would say it’s near impossible to off road with the 3T winding if that’s what you had considering it’s meant to be held wide open, not stop and go.
 
E-HP said:
Eastwood said:
Here’s an example of a DD hub-motor doing a slow technical hill climb with basically no run up speed. With the right motor winding and the right amount of phase amps and DC amps you can pretty much climb anything with a DD hub motor, it’s just the heat can be the issue. In my case I’m riding off road but if I wasn’t I would just put holes in the side plate covers and possibly install fans. Not such a great option when riding in the mud lol

That's still fast enough for a DD motor in the video, but going 5mph up a 20%+ grade switchback where you basically need to navigate a 180 degree turn, isn't possible. The motor wants to go faster, but you physically have to go slow. Do a couple of those, and barely escape flipping over backwards and down the hill while making the turn, then you know it won't end well to keep going. I don't think there's a hill I can't climb, as long as there's traction and the ability to keep the bike going above 10mph, but not those very slow ascents.

Well this hill is a lot steeper than my iPhone displays. It’s impossible to walk up this hill I’ve tried even with holding the ground. With my 5.5 T winding it would not make it up this hill I could only get about halfway. Then with this 6T it powers right up it no problem. I tried this hill when I was using the MXUS 3K 5T and failed. I guess so my point is without the proper winding a hub motor off road is no Bueno. With the right winding and mods a hub motor does well off road. By the way I had the controller set at around 8 kW going up that hill. I wasn’t quite at full throttle as I try to keep the wheel spin to a minimum with electric.
I should have recorded the $600 hill climb as that hill is an absolute monster. I pretty much ride on the back wheel going up the hill because it’s so steep lol. I plan on riding today and I’ll try to record the famous $600 hill climb lol
 
Eastwood said:
PRW said:
In the real world:
Riding on tight technical trails, with my wife on a 1500w BBSHD and me with a Mxus 3k at 5000w, my motor can overheat after about 1 hour. Granted, there is a weight difference of say 70 lbs, but with lots of start and stop, the BBSHD does much better

Yeah for sure I bet the BBSHD handles the inclines with less heat. That being said you’re MXUS 3k is it 3T winding? I wasn’t able to run 5 kW without overheating so that’s why I turned it down to 40 A but that’s with a 5T. I would say it’s near impossible to off road with the 3T winding if that’s what you had considering it’s meant to be held wide open, not stop and go.

Technical climbing is all about *torque*, not power. You cannot have more torque (either peak or sustained) from a motor going faster or slower winds, period, this is entirely property of a correct controller/battery. Ok, there might be some differences, but they are miniscule... ok, long and thin phase cables can be a bottleneck on truly fast winds combined with very high phase currents, having a controller basically built into the motor is a nice feature.

When it comes to torque, mechanical reduction is king. When it comes to climbing steep stuff at slow speed, middrives are a great. When comes to 'riding around' you have a schisophenic system that first reduces engine RPMs to human cranking RPMs, than INCREASES RPMs to wheel rpms, with extra losses and drivetrain wear along the way.
 
Eastwood said:
Well this hill is a lot steeper than my iPhone displays. It’s impossible to walk up this hill I’ve tried even with holding the ground. With my 5.5 T winding it would not make it up this hill I could only get about halfway. Then with this 6T it powers right up it no problem. I tried this hill when I was using the MXUS 3K 5T and failed. I guess so my point is without the proper winding a hub motor off road is no Bueno. With the right winding and mods a hub motor does well off road. By the way I had the controller set at around 8 kW going up that hill. I wasn’t quite at full throttle as I try to keep the wheel spin to a minimum with electric.
I should have recorded the $600 hill climb as that hill is an absolute monster. I pretty much ride on the back wheel going up the hill because it’s so steep lol. I plan on riding today and I’ll try to record the famous $600 hill climb lol

Do the hill at 5mph when you take the video, no faster, and you've proven your point. You can go straight, or zig zag up, your choice.
 
BalorNG said:
Technical climbing is all about *torque*, not power. You cannot have more torque (either peak or sustained) from a motor going faster or slower winds, period, this is entirely property of a correct controller/battery. Ok, there might be some differences, but they are miniscule... ok, long and thin phase cables can be a bottleneck on truly fast winds combined with very high phase currents, having a controller basically built into the motor is a nice feature.

When it comes to torque, mechanical reduction is king. When it comes to climbing steep stuff at slow speed, middrives are a great. When comes to 'riding around' you have a schisophenic system that first reduces engine RPMs to human cranking RPMs, than INCREASES RPMs to wheel rpms, with extra losses and drivetrain wear along the way.

Wait you’re trying to say you can’t have more torque from a slow wind motor?? That’s the entire point of getting a slow wind motor so you have higher torque output with less current. I hear this nonsense regurgitated over and over on this form and that’s why people discontinue hub motors off road because they don’t buy right winding. So if you buy a fast wind motor you have to use more current which would overheat your motor therefore would be useless off-road. You say The difference is miniscule, could not be further from the truth. That same hill with the QS205 but in the 5.5T winding cannot get up the hill at the same exact power settings. With the 6T at the same exact power settings, same battery same controller it flys up the hill. It’s all about the winding for off road use.

Edit: and to be fair my older motor which was a 5.5 T was laced and a 19” rim. This new motor which is a 6T is lace in a 16” rim so it has an mechanical advantage is well with the smaller wheel diameter.
 
Many geared hubmotors have been fried in the past. I wouldnt even try to re-verify the breaking point.

Ferro-fluid doesn't help with heat-shedding in a geared hubmotor, but some benefit can be acquired with adding "some" ATF to the insides.

It will leak, in spite of your best efforts, so consider putting any vent and/or fill-port on the chain side.
 
BalorNG said:
Technical climbing is all about *torque*, not power. You cannot have more torque (either peak or sustained) from a motor going faster or slower winds, period, this is entirely property of a correct controller/battery. Ok, there might be some differences, but they are miniscule... ok, long and thin phase cables can be a bottleneck on truly fast winds combined with very high phase currents, having a controller basically built into the motor is a nice feature.
Sort of, but an 8kW DD motor has tons of torque, but a technical climb is about controlled torque combined with traction, not just how much torque you have, and that's where the characteristics of electric motors is a con. I think even my smaller DD motor has too much torque and not enough control or traction for the job. Specifically, too much uncontrollable torque when going slow. Even a motocross bike would have difficulty on a steep narrow trail with switchbacks, but at least you can finnesse the clutch and throttle a little easier. Those trails are on grades too steep to climb up on any vehicle wthout flipping over, so the trails zig zag up; but you could climb them on foot, without 8kW. So steeper than this, but with loose dirt, rocks, and trees and shrubs to negotiate around.
https://www.youtube.com/watch?v=c7a8XjeDRGg
 
Eastwood said:
BalorNG said:
Technical climbing is all about *torque*, not power. You cannot have more torque (either peak or sustained) from a motor going faster or slower winds, period, this is entirely property of a correct controller/battery. Ok, there might be some differences, but they are miniscule... ok, long and thin phase cables can be a bottleneck on truly fast winds combined with very high phase currents, having a controller basically built into the motor is a nice feature.

When it comes to torque, mechanical reduction is king. When it comes to climbing steep stuff at slow speed, middrives are a great. When comes to 'riding around' you have a schisophenic system that first reduces engine RPMs to human cranking RPMs, than INCREASES RPMs to wheel rpms, with extra losses and drivetrain wear along the way.

Wait you’re trying to say you can’t have more torque from a slow wind motor?? That’s the entire point of getting a slow wind motor so you have higher torque output with less current. I hear this nonsense regurgitated over and over on this form and that’s why people discontinue hub motors off road because they don’t buy right winding. So if you buy a fast wind motor you have to use more current which would overheat your motor therefore would be useless off-road. You say The difference is miniscule, could not be further from the truth. That same hill with the QS205 but in the 5.5T winding cannot get up the hill at the same exact power settings. With the 6T at the same exact power settings, same battery same controller it flys up the hill. It’s all about the winding for off road use.

Edit: and to be fair my older motor which was a 5.5 T was laced and a 19” rim. This new motor which is a 6T is lace in a 16” rim so it has an mechanical advantage is well with the smaller wheel diameter.

Yup, mechanical advantage, just like I said.
As for higher vs slower winds - yea, given SAME CONTROLLER, higher turn count will give more torque per amp up to a point of saturation.
The thing about low turn counts motor is that when fed 'higher amps' to produce same torque, they actually produce same amount of waste heat, because they have much less resistance (given same copper fill of course - which imply thicker wires allround).

If you want more torque out of the motor without cooking it - you want a bigger motor, with larger magnets and more copper, or more mechanical reduction.
 
E-HP said:
BalorNG said:
Technical climbing is all about *torque*, not power. You cannot have more torque (either peak or sustained) from a motor going faster or slower winds, period, this is entirely property of a correct controller/battery. Ok, there might be some differences, but they are miniscule... ok, long and thin phase cables can be a bottleneck on truly fast winds combined with very high phase currents, having a controller basically built into the motor is a nice feature.
Sort of, but an 8kW DD motor has tons of torque, but a technical climb is about controlled torque combined with traction, not just how much torque you have, and that's where the characteristics of electric motors is a con. I think even my smaller DD motor has too much torque and not enough control or traction for the job. Specifically, too much uncontrollable torque when going slow. Even a motocross bike would have difficulty on a steep narrow trail with switchbacks, but at least you can finnesse the clutch and throttle a little easier. Those trails are on grades too steep to climb up on any vehicle wthout flipping over, so the trails zig zag up; but you could climb them on foot, without 8kW. So steeper than this, but with loose dirt, rocks, and trees and shrubs to negotiate around.
https://www.youtube.com/watch?v=c7a8XjeDRGg

Given proper torque control of the motor (limiting phase amps, not just system voltage and preferably programmable throttle), whether you have middrive, DD or geared hub is irrelevant... you want to invest into a better controller.
 
BalorNG said:
Given proper torque control of the motor (limiting phase amps, not just system voltage and preferably programmable throttle), whether you have middrive, DD or geared hub is irrelevant... you want to invest into a better controller.
Hence the 5mph test.
Thats where the $8k factory ebikes have the advantage.
 
E-HP said:
BalorNG said:
Given proper torque control of the motor (limiting phase amps, not just system voltage and preferably programmable throttle), whether you have middrive, DD or geared hub is irrelevant... you want to invest into a better controller.
Hence the 5mph test.
Thats where the $8k factory ebikes have the advantage.

Nothing you cannot replicate on a much cheaper bike, you simply want to invest into a better controller - which is still a small fraction of that 8k$.
A proper torque *sensor* control (pedalling torque multiplication) is more complex, I give you that, and I'm not even sure that is desirable when you want to maximise traction - human power is highly pulsatile... likely there is some smoothing going on, but I've never ridden any of those 8K+ bikes to be fair.
 
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