Electric bike torque question

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ChrisF
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Joined: 22 Mar 2014, 7:34pm

Electric bike torque question

Postby ChrisF » 24 Aug 2015, 8:09pm

I'm doing some preliminary research into e-bikes with a view to getting (or modifying) one for my other half, so we can perhaps do tours together at a commensurate speed. The main discussion appears to be between hub or crank motors, with most (all?) sites I have visited suggesting that crank motors are better for hilly areas. Generally they say things like "crank drives can increase torque through the cassette. This makes it perfect for going up steep hills". Now, surely the torque required to turn the wheel is less at the hub than at the crank, since the chainwheel is larger than any sprocket (e-bikes with crank motors don't use triple chainrings so can't provide the lowest gears). From this I would expect the hub motor to be better at climbing, rather than at the crank.
I suspect the real reason for a crank motor being better at hills (if it's true) is because it's more efficient to design a motor than can run at a more constant speed. But I haven't found anywhere which explains this. I'd like to understand before taking my research any further.
Chris F, Cornwall

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Electric bike torque question

Postby Brucey » 24 Aug 2015, 8:22pm

the various systems differ wildly in both conception and execution. I think you need to try a few before you buy.

For example I recently rode a bakfiets with a torque sensing front motor system. The way it was set up (with a motor good enough for a 27" wheel in a 20" rim) it was a bit too sensitive; you could spin the front wheel and low speed manoeuvres at full lock were almost impossible. You would undoubtedly have trouble on a steep hill with traction, too. But in a straight line with a load on, slight gradients etc it was perfect.

As an ownership prospect, the cheapest systems are almost unfixable (you often can't get the spares, and when you can do they are too expensive in relation to the original cost of the system), so when they break (and they will break) it is often a bin job. Cheap hub motors are real junk; they break often and if they don't break they wreck the forks because the torque arms are badly thought out. The more expensive systems are better supported and are usually less likely to give trouble to start with, too. But you can pay for those, oh yeah....

cheers
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ChrisF
Posts: 470
Joined: 22 Mar 2014, 7:34pm

Re: Electric bike torque question

Postby ChrisF » 24 Aug 2015, 10:41pm

Brucey wrote:For example I recently rode a bakfiets with a torque sensing front motor system. The way it was set up (with a motor good enough for a 27" wheel in a 20" rim) it was a bit too sensitive; you could spin the front wheel and low speed manoeuvres at full lock were almost impossible. You would undoubtedly have trouble on a steep hill with traction, too. But in a straight line with a load on, slight gradients etc it was perfect.

Understood, but isn't this more to do with front wheel/back wheel drive than crank/hub motor? A hub motor can be in the rear wheel.
Brucey wrote:As an ownership prospect, the cheapest systems are almost unfixable (you often can't get the spares, and when you can do they are too expensive in relation to the original cost of the system), so when they break (and they will break) it is often a bin job. Cheap hub motors are real junk; they break often and if they don't break they wreck the forks because the torque arms are badly thought out. The more expensive systems are better supported and are usually less likely to give trouble to start with, too. But you can pay for those, oh yeah....

Yes, but cheap things being rubbish is true for much more than e-bikes! I wasn't suggesting saving money, just trying to understand the technology.
Chris F, Cornwall

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Electric bike torque question

Postby Brucey » 24 Aug 2015, 11:34pm

sure.... but there isn't a perfect system yet IMHO. Rear drive can be dynamically easier to live with, but the rear hub motors are either a lash-up (some bolt onto the spokes...) or tie you in to one gearing system, and sometimes the frame has to be special too.

The closest to a good system is perhaps the Bosch one but the whole bike is built round the motor; the frame is made for that and that only. I'd like to see a good bolt-on system that placed the drive in the same location but could go onto almost any bike, but it seems that doesn't exist yet.

There is a company that makes a system that lives inside the seat tube and drives the BB spindle via a gear; it is small, light, unobtrusive, and efficient. It is also fiercely expensive.

Past buying the thing you are in the hands of your dealer. I know a chap who sells machines with motors on of various different types, and without any vested interest in any one type (he just wants them to work and not come back again. broken, too often) he still doesn't know what system to spec on his machines; they are all flawed in one way or another. I agree with him, they are! The only ones that seem faintly reliable and powerful are also very expensive, but not all expensive systems are any good....

That a given system might be factory fitted to some machines is no guarantee that it is either fit for purpose or indeed any good at all. My chum has boxes and boxes full of failed e-bike parts and has often said that if he had a choice (he doesn't, his customers keep wanting them) he'd stop doing them; they are easily the least reliable part of the machines he sells and also the most difficult part to repair.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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[XAP]Bob
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Joined: 26 Sep 2008, 4:12pm

Re: Electric bike torque question

Postby [XAP]Bob » 25 Aug 2015, 7:22pm

The "increase torque" really means that the motor can be at a better cadence for a wider range of speeds, trading torque for speed using the already existing gearing on the bike.
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.

bobc
Posts: 495
Joined: 5 Apr 2012, 11:59am

Re: Electric bike torque question

Postby bobc » 26 Aug 2015, 1:05pm

The system that hides in the seat tube is called the "Gruber assist" if memory serves, and is the basis for some concerns about so-called "mechanical doping" in professional racing (wouldn't it be nice to have 50W of electric help....).

andrewk
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Location: SW London

Re: Electric bike torque question

Postby andrewk » 27 Aug 2015, 4:46am

Good crank systems are available from Bosch, Impulse and a new entrant without a track record yet Shimano. Bosch being widely held to be the best. Both Bosch and Impulse are well supported, I guess that in time Shimano will be too.
The 8Fun crank motor (Chinese) is popular for DIY conversions, I have no idea of its reliability but those conversions I have seen looked crude with a profusion of cables. I suspect that its reliability is inferior to the big brands since quality ebike manufacturers have avoided it. 8Fun is also a lot less sophisticated than the Bosch or Impulse systems with a reputation for crunching transmissions.
Hub gears (both conventional IHG and CVT (NuVinci)) work well with crank motors and have the added advantage of using either a stronger chain or belt drive.

steve browne
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Joined: 8 Oct 2009, 12:30pm

Re: Electric bike torque question

Postby steve browne » 27 Aug 2015, 10:38am

I recently travelled to Copenhagen to try out cycle rickshaws for use with the Cycling Without Age project (taking residents of care homes out for a spin). We ended up purchasing one with BionX electric assist. This was the smoothest in operation and appears to be more sophisticated than the alternatives we tried. It is controlled from a switch console on the handlebar. It uses a torque sensor to determine when the motor kicks in. Electric Bike Sales in Lower Redland Road Bristol stock this and can give advice on converting your own bike. There is a mountain setting which is designed to prevent the battery discharging too much on long ascents. It works with a standard derailleur system.

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: Electric bike torque question

Postby Brucey » 27 Aug 2015, 9:17pm

bobc wrote:The system that hides in the seat tube is called the "Gruber assist" if memory serves, and is the basis for some concerns about so-called "mechanical doping" in professional racing (wouldn't it be nice to have 50W of electric help....).


I think I'd need more than that these days if I wanted to keep up!

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

GeoffBrassn
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Re: Electric bike torque question

Postby GeoffBrassn » 28 Aug 2015, 6:04pm

I'm a hub drive fan and my rear hub drive bike with derailleur has been used and abused without major issue for many years in the Peak District. It's much less expensive than the crank driven bikes and doesn't suffer broken chains or require expensive belts / stronger chains. I can fiddle with the gearing if I need to (I don't) 'cos all the parts are standard and available at my local bike shop or on-line from the manufacturer. I've enjoyed it immensely and it keeps me out of the car and nicely fit in a very hilly region.
I've read that there is no significant difference in hill climbing ability between crank driven and hub driven electric bikes and having ridden both I agree.