Any IGHs with over 350% range, >20,000km service life & 300nM torque rating???

[mschwett]

Here's what I question. I have read that the gear reduction inside the M620 motor itself is something from 18:1 to 28:1. The design uses 3 levels of gear reduction ... it makes no sense to do that and then give 50% of the torque gained via gear reduction with the belt / chain ratio unless there was too much reduction to allow the top speed / cadence desired when using an IGH. I don't think the Kv of the Bafang motor is such that it would need this. You get some of that back via the underdrive gears on the IGHs but even then I just don't it makes sense unless not wanting the full torque from rider and motor to impact the IGH.

that’s an interesting tidbit, i don’t think i’ve seen the reduction ratio for mid drive motors published before. let me see if i understand this:

motor at 1,600 rpm, internally geared 20:1 ->
crank at 80 rpm, geared 1:2 via cog sizes ->
rear axle at 160 rpm, geared 2:1 to 1:2 via IGH ->
wheel at 80 to 320 rpm.

tire circumference is approx 7.1 feet, so that’s 568 to 2,272 feet per minute, or 6.5 to 26 mph, a normal and workable speed range for pedaling at 80rpm.

what would the advantage be to changing it up as you propose:

motor at 800 rpm, internally geared 10:1 ->
crank at 80 rpm, geared 1:1 via cog sizes ->
rear axle at 80 rpm, geared 1:1 to 1:4 via IGH ->
wheel at 80 to 320 rpm.

wouldn’t the motor be bigger and heavier to generate the same power at half the speed, and wouldn’t the IFH experience greater internal stresses and losses to go all the way to 1:4?

i’m probably missing something here but it seems like the way it’s done works very well for a system which has to take into account human pedaling speeds, standard tire sizes, small and lightweight motors, and speeds in the 5-30mph range.

 

[m@Robertson]

A rider on an electric cargo bike that is loaded will not 'stand' on the crankarms. I have two separate residences where one is in flat land and the other... well, nothing is flat. Everything is steep and oftentimes that means San-Francisco steep. You just don't ride a cargo bike like that when it has assist. And cargo bike riders in particular use their throttles for takeoff.

So, in terms of real world, it doesn't happen by the nature of the rider who - once they try a throttle - will never try and start under heavy load any other way. Cargo bikes are utility rides and people riding them aren't so caught up in the semantics of proper cycling... they have a job to do and they use the tools on the handlebars to do it. You might consider looking to Cargo Bike Republic on Facebook, where almost 14,000 total members reside and the knowledge there about riding habits and equipment is both broad and freely shared. Non electric startup? Sure I can see someone having to stand on the crankarms. I can say I never do it, although I did experiment with 165mm crankarms to allow exactly that kind of manual startup (to avoid drivetrain wear) and gave it up. The proper solution was drive programming.

What ordinarily would be a given would be programming the motor so 300 Nm is unquestionably impossible. In particular setting start-degree, start-current and slow-start mode on the pedal-assist screen, and start-current on the throttle screen. With the new CAN bus motors replacing the UARTS that were programmable, this is no longer the case. But as has been pointed out repeatedly above, WattWagons has been building bikes with Rohloff, Kindernay and Ultra motors - and sometimes 2600w controllers. Its not about the controller limit its about the programming to smooth out the power delivery.

I would not buy an M620 from anyone who is not offering an aftermarket controller for this reason. I just placed my order for my 7th BBSHD. Now that programming is largely off the table for the Ultra, there is little to no benefit to buy one over a BBSHD, which offers comparable output, complete customizability and fits a wide variety of frame designs.

I almost scored a Salsa Blackborow recently. Thats a cargo bike that is not a good fit for the BBSHD thanks to its downtube configuration at the bottom bracket. If I had been able to buy the bike, I would have put a Cyc X1 Pro inside the triangle - these days the Cycs are customizable as well. But motors like the Cyc and Cyclone are inherently less reliable in design than Bafang models.

 

[Ken M]

Found this table that provides the 1st and 14th sprocket equivalents with different belt sprockets and cogs. The Rolhoff clearly has a lot of negative gearing (meaning below 1:1 direct drive ratio).

 

[kmccune]

That was the trouble with some of the old Truck gearing systems,you had to know how to split gears in the correct sequence, some of the gears combos were not linear( then along came the High torque rise engines, you didn't need as many gears, so it goes with ebikes, more torque less gears). And yes its the same principle.

 

[Ken M]

That was the trouble with some of the old Truck gearing systems,you had to know how to split gears in the correct sequence, some of the gears combos were not linear( then along came the High torque rise engines, you didn't need as many gears, so it goes with ebikes, more torque less gears). And yes its the same principle.

The more torque / power resulting is the need for less gears seems to be accepted / understood by most of us on EBR. The interesting thing is that with the bike IGHs the fewer gears always is accompanied by a significantly reduced range (this will impact either the low gear climbing ability or a cadence that is just too fast for most riders in the high gears and it seems the torque ratings of most of the IGHs are not robust enough for rider + a motor like the Bafang Ultra. They can be OK but do have to be ridden with some careful considerations.

I did finally get around to doing some stress calculations on internal gears if subjected to 300nM of torque and it became clear of the design challenges. The constraints of size of IGHs leads to some significantly high shear and contract stresses. This is very much like what someone contributed about the hot rod auto world - the high powered motors can exceed the capability of the drive trains so you have to know what you are doing.

 

[Ken M]

that’s an interesting tidbit, i don’t think i’ve seen the reduction ratio for mid drive motors published before. let me see if i understand this:

motor at 1,600 rpm, internally geared 20:1 ->
crank at 80 rpm, geared 1:2 via cog sizes ->
rear axle at 160 rpm, geared 2:1 to 1:2 via IGH ->
wheel at 80 to 320 rpm.

tire circumference is approx 7.1 feet, so that’s 568 to 2,272 feet per minute, or 6.5 to 26 mph, a normal and workable speed range for pedaling at 80rpm.

what would the advantage be to changing it up as you propose:

motor at 800 rpm, internally geared 10:1 ->
crank at 80 rpm, geared 1:1 via cog sizes ->
rear axle at 80 rpm, geared 1:1 to 1:4 via IGH ->
wheel at 80 to 320 rpm.

wouldn’t the motor be bigger and heavier to generate the same power at half the speed, and wouldn’t the IFH experience greater internal stresses and losses to go all the way to 1:4?

i’m probably missing something here but it seems like the way it’s done works very well for a system which has to take into account human pedaling speeds, standard tire sizes, small and lightweight motors, and speeds in the 5-30mph range.

See the table on the Rohloff final gear ratios at the different chain ring / cog ratios. To have reasonable cadence at the higher speeds the chain ring / cog ratios have to be as they are (this does reduce the torque transfer to the rear axle). I actually think this is a very complex subject because it does involve both human / rider and motor efficiencies and when both share the total drive train there are compromises made i believe. A separate motor drive system would likely result in more optimized ebike solution.