I'm looking at buying and electric bike - at 70+ I need a bit of assistance !
I was an electronics engineer so am reasonably aware of electric motor powe r, and also what torque is. But can someone care to give me this further b it of mental assistance on the motors that are used in bikes. My problem i s that there is a power limit in the UK of 250 watts - I believe also that they are meant to cut out above 15mph but that's a different matter. Where I am struggling is that I am being offered bikes with 40 NM motor torque an d up to 80 NM. Mechanics were never my strong point, so there is probably some simple answer to this, but I am seeing radial force as being related t o power in, so how can there be a 2 to 1 ration between different motors?
I can't really answer your question technically but assuming they are all correctly rated at 250W then could the differences be partly down to marketing and also efficiency / design?
If a motor was say driving the crank it would need to be slow revving and therefore produce more torque? A motor in the wheel would be higher revving (especially a 16" wheel) and for the same 15mph upper limit, offer less torque (measured at the motor itself)?
I know how for the same set limits (24v, 25kg of battery and 1 hour duration race) how a motor design can make a vast difference to the final outcome. This was especially so when racing against someone who developed their own motor then went on to bigger things ... ;-)
formatting link
Cheers, T i m
p.s.
formatting link
I think this was the one and only time anyone beat Cedric (No1) .
Power is torque multiplied by angular speed, but perhaps confusingly the latter isn't measured in RPM but radians per second. So a 40 Newton-metre motor giving 250 watts must be going at 6.25 radians per second, or 375 radians per minute, or near enough 60 rpm.
Similarly, a 30 rpm motor would give 80 NM torque for 250 watts.
ower, and also what torque is. But can someone care to give me this furthe r bit of mental assistance on the motors that are used in bikes. My proble m is that there is a power limit in the UK of 250 watts - I believe also th at they are meant to cut out above 15mph but that's a different matter. Whe re I am struggling is that I am being offered bikes with 40 NM motor torque and up to 80 NM. Mechanics were never my strong point, so there is probab ly some simple answer to this, but I am seeing radial force as being relate d to power in, so how can there be a 2 to 1 ration between different motors ?
Thanks Tim - I did wonder about the low revving torque but if you look at s ome electric bikes now you will see that the chain wheel is only something like 16 teeth and so revs much higher. It is actually concentric on the cra nkshaft with the pedals, which I imagine drive down through an enclosed epi cyclic gear. I asked the cycle shop about this and didn't really take the guy's line about this being a measure to overcome chain drop on rear suspen sion systems - that may be a bonus but I suspect higher motor revs is the t arget.
Interesting to look up Lynch's website. That must have all such fun to be involved in. Rob
Taking it a bit further, for a 27 inch wheel, 15 mph is about 90 rpm. I assume we are talking about hub motors here, rather than ones driving through a belt or chain drive. So your 40 NM motor would be legal if it drove you (unaided) at about 10 mph.
In practice, I think I would expect the motor to be governed electronically to limit the power to the legal limit of 250 watts. They
*may* be quoting you peak torque figures; the higher the torque the better the acceleration.
Ok. However, I was thinking anything driving the crank might be doing so at a lower revs / higher torque than anything that wasn't geared up.
I know on my EV I had to down gear quite a bit, even with top speeds of only 40 mph.
formatting link
You might just imagine a light chain drive (under that ally cover with the hole) where the motor had a very small gear (from the cam chain drive / crankshaft of a small Honda motorbike) to a much larger gear I made by hand (you can see the bottom of it under the cover). From their a std cycle sprocket from a 7 speed block driving a fairly large range block on the rear wheel.
Ours has a powered wheel and I've not played with anything else (production anyway).
Ok.
It was indeed. ;-)
I felt quite 'normal' (by comparison and for a change ) as most of the others were (more) 'eccentric' to some degree, as is often the case with 'boffin / inventive' types. Great bunch of guys though and we raced at quite a few places that I might never have visited otherwise (like MIRA and Oulton Park).
It was also a good opportunity to run the C5 round and many of us had them (as one of the C5 designers also raced. In fact, he leant me one of his earlier machines to help me get started). ;-)
wer, and also what torque is. But can someone care to give me this further bit of mental assistance on the motors that are used in bikes. My problem is that there is a power limit in the UK of 250 watts - I believe also tha t they are meant to cut out above 15mph but that's a different matter. Wher e I am struggling is that I am being offered bikes with 40 NM motor torque and up to 80 NM. Mechanics were never my strong point, so there is probabl y some simple answer to this, but I am seeing radial force as being related to power in, so how can there be a 2 to 1 ration between different motors?
Electric transport has three phase permanent magnet synchronous motors. They have few parts, the tricky stuff is in the electronics.
Mechanical power is derived from torque times rpm. The rpm is limited to give the top speed. Various torques (Nm)are offered for thin people and fat gits, also if you a re in a hilly area you need a higher torque to get you up the hills with le ss personal effort.
Most bikes only "power on" when you start pedalling. This is a a safety thi ng.
IFAIAA the 250 watts limit only applies if you have no motor cycle licence. I think there are tax and insurance issues over the 250watt limit as it's t hen considered to be a motor cycle.
Also ask about range and battery size. (And cost of new battery) Bear in mind the batteries gradually deteriorate over time. And they don't like lack of use, especially if left in a discharged state. Range is much reduced by hills.
Everyone I know with an electric bike has been very pleased with them.
The torque by itself is meaningless, you can get any amount of torque you like by gearing, it's the speed at which the wheel is rotating when a given torque is produced that matters.
Harry - that's good to know the user satisfaction. The bikes I was trying yesterday fortunately had a steep hill within a mile or so of the shop, and it was interesting to find that I was using the steps in the motor power m uch like I would use the selection of chain wheel; that did answer my quest ion as to how with ~9 gears the e-bike matched a bike with the low ratios o f a 27 gear set. The nice thing was that the added ratio effect came on in stantaneously.
Newshound - if you are saying that the effect of higher torque is only on a cceleration, then for me as an elderly cyclist there is no point in me payi ng the extra. I just want some assistance to get up the hills and combat t he wind.
And Chris - that's a good question and one that I suspect will trip up the salesman. It still does raise the question that for the same RPM, can any one motor produce twice to torque of another for the same power input? Rob
Clearly not! If those torque values applied to the same electrical power input, they *must* have been at different rotational speeds.
As a matter of interest, how far can you go on one charge of the battery? If you're producing 250 watts from a 12 volt battery (and you wouldn't be using full power all the time, of course) the battery would have to supply about 21 amps. I don't know what the battery capacity is, but I doubt whether it's more than 21 amp-hours - which would limit you to one hour's use at full power. Does that sound about right?
Generally, electric motors run too fast for direct drive. So have a gearbox to reduce that speed. So the chances are the difference is the gearbox. So the speed at the output shaft will be approximately half that of the one with the lower torque.
If I can pop back in again on the point of "worst to best", this introduces another interesting element; the two bikes referenced in the original post are from the same manufacturer using a Bosch system - it may well be that Bosch do have several 'systems' but there is something in the engineering t hen that is allowing Bosch to make this claim.
OK, I've worked it out, and your 4-5 mph is a bit optimistic.
According to my calculations, a person weighing 168 lb plus a (typical) bike weighing 62 lb would require 250 watts of mechanical power to get up a 15% slope at 3.6 mph - and that's ignoring rolling resistance and wind drag (not that there'll much of the latter at that speed!). If the bike is limited to 250 watts of *input* power, the mechanical power generated will be less than 250 - and the speed will be even lower.
Not these days with permanent magnet AC motors. You can have lots of magnets, so obviating gearboxes and belt/chain drives. Hence "Direct Drive" washing machines.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.