Winfried

Hello,

I was wondering how torque is impacted when riding a mid-drive and a rear drive.

As a pratical exemple, the Radwagon 4 longtail has a seven-speed derailleur, and, per local regulations, is sold in Europe with a 250W motor (rear drive Bafang with a 80Nm torque.)

I read that torque on a mid-drive should be divided by the gear ratio (crank teeth/cog teeth).

Would that bike offer more/less/even torque if it had a 80Nm, 250W BBS01 Bafang mid-drive instead?

Thank you.

cat0020

With smaller (20") wheels, hub motor on the (non-EU spec) Radwagon has enough torque for its intended usage on pavement and hauling cargo/weight.

Mid-drive may be better suited for off-road riding with steep hills and large gear ratio for in rear cassette, which Radwagon is not meant to performe.

Depending what type of riding you plan on doing, the hub/mid-drive motor should be chosen according to your riding needs, not just by torque output.

Winfried

Thanks, but I was wondering about the math part.

Given a 80Nm torque motor, does it make a difference if it's a mid-drive (ie. with the derailleur in the mix), or a rear-drive?

late

Yes, of course, gearing gives you more range, and more power up a hill, because of the mechanical advantage it gets from the gears.

Doc_Wui

If you wish to believe manufacturer's claims on torque, then do your numbers. I happen to believe that many Euro mid drives underrate their mid drives to meet the 250W limits. At the same time, a few muckrakers have also demonstrated that Rad has over-inflated the torque on its hubmotors too,

Chasing torque spec's at this point is just a paper chase.Best dome, in my opinion, by seat-of-pants testing.

fooferdoggie 

not even the regular rads have 80nm of torque the 250 would be much less.

Leisesturm

Mid-drive vs hub motor is not about torque, its about range! Speed range. The torque of a hub motor can be low OR high but not both! So the speed range is fixed around a high torque/low speed or low torque/high speed orientation. Of course there is a gotcha: the low torque/high speed winding does not work well on hills and those high speeds take a long time to reach. The torque of the mid-drive is variable depending on the range of the drivetrain. A mid-drive is the best (only?) way to make the best use of a limited amount of input watts like 250W. Most of us couldn't tell 80nm from 40nm. We measure performance in miles per hour! MPH.

Winfried

At the factory? So with a rear drive, you either get a motor that can go fast on the flat, or slow but can climb, but not both, while a mid-drive does both thanks to the derailleur?

Because the gears in the back make it possible to keep the mid-drive motor in its optimum range?

late

I don't know. I do know gears are popular because of the mechanical advantage they offer. The downside, not that I would ever get anything else, is that the motor puts more strain on the drivetrain.

Leisesturm

Yes. At the factory. The way the hub motor is wound determines its performance character. Electric trains are wound at the factory to be switch engines working in the yard at low speed but very high torque pushing heavy things around. Overland freight motors are longer legged (wound for more speed) they don't have as much torque as switch engines but they aren't that weak either. Given level ground and enough room and they can get to 90mph. Passenger engines are even longer legged. Given enough time they can get to 110mph. A tractor-trailer 18 wheeler is the mid-drive. It does it all. If you need to climb any kind of hill with a freight engine you gang 2, 3 or more of them to get the torque you need. The semi has 12 or more forward speeds and 3 or 4 reverse speeds. At night when the troopers have gone to bed semi's hit 120mph. But they can chug around at walking speeds on maneuvers.

fooferdoggie 

yes some. I get 2500 to 3000 miles per chain.but I also got 12,000 miles on the cassette and chainring before I changed them out.

cat0020

Analog or ebikes, lifespan of chain, cassette/chain rings are highly dependent on the type of riding & how well you keep the drivetrain maintained through the lifetime.
Nowadays, I see 11-speed chains seem to wear out much quicker than old 8 or 9-speed chains.
From data collected among four local bike shops, they are replacing 11-speed chains at 4x the frequency of 8/9-speed chains.
10-speed chains are not as bad, but still more frequent than 8/9-speed chains.

chas58

Yes, gears multiply torque. If you have 36t chainring and 36t cog in the rear, you are 1:1 (like a hub motor). If you gear down lower, you are multiplying the torque. That is why mid drives are often chosen for hilly riding, and hub motors are better for light and inexpensive in flatter areas of the world.

Kinda - you get the point even if the details are a little off.

I have a low speed (200rpm) and high speed (328rpm) motor. Same motor, different windings/speed.

Torque is pretty much the same (within 10%) on both motors. I can't really feel an acceleration difference. So, I can't say the low speed is a high torque motor.

The difference is heat and top speed.

The low speed motor won't do much of anything over 17mph. But riding full throttle at 10mph isn't gonna kill it either (think climbing a hill).
The high speed motor pulls all the way up to ~28mph, but...
it will overheat if it spends much time below 10mph (it will probably overheat at a steady 15mph if its there long enough). But on a flat road at 20-25mph, it is a happy camper.

fooferdoggie 

I think the weather too I don't get as many miles in winter with the rain. I lube the chain more often and seldom does it get dry or two dirty since I use a dry lube. I think I lose about 500 miles. a shimano e chain may get an extra 500 miles over the cheaper KMC but it is twice the price.

Winfried

Thanks much. That's the explanation I was looking for.

ebikingtoday

If you want exact measurements, I believe there are apps for that you might check out!

P.S. I have a RadMIni... love love love it!

ClydeClydeson

I believe this is not so clear above - the ratio of F-R gear sizes does have an effect on the wheel tourque when using a mid drive hub, but most bikes are set up such that most of the gears multiply the torque from the crank by some number less than one, meaning there is a reduction in torque. Only the few easiest gears (smallest chainring on the front, largest cogs on the back) will provide an increase in wheel torque. Most of the gear combos will be a reduction in torque and an increase in speed at any given RPM of a mid drive motor.

However, and this was alluded to above, MD and RD are likely not exactly the same motor, but are probably wound differently to provide torque and speed characteristics suitable for their intended purpose.

klevin

But this is the whole point. A mid drive allows one to maintain the same pedal cadence range, therefore the same motor RPM range at widely varying speeds. That means a motor can be optimized for the rpm range of the pedaling, which is much narrower than what one finds at the rear wheel. It also means that a single motor design can be optimized for a wide range of riding conditions by varying the gearing and the wheel size.