Rohloff: torque limit issues

john61ct

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This 14-speed IGH has a hard limit spec'd of 130 N.m

There apparently are shear pins acting as "torque fuses" to protect the unit, but obviously it would be better to avoid depending on that, likely very inconvenient if you're miles from home when that happens.

I'd like this thread to serve as a reference for this specific topic.

Reference use case for me is: very low top speed is fine, all about torque at low wheel rpm

heavy cargo/tandem rig maybe 400-500#, gravel / fire roads,

mountain country, so very long steep climbs.

Assume a powerful mid-drive, at least 1000W rated, 3-5000W peak driving the pedal drivetrain at the BB, capable of shearing those pins at 130 N.m

 
My first question, sorry if too noobish

For a given motor+controller I'm told smaller O.D. wheels is the way to go to help with getting high torque.

But given the (at times very) rough ground, even with fatties, I can't go too small.

Since the torque is going into the Rohloff rather than the hub/wheel directly, does this "small wheel = more torque" rule even apply anymore?

Or is it only relevant to hub drives?

If it does apply to a right-hand mid-drive,

since the slowest gearing can be configured super-torquey anyway (top speed being irrelevant)

could going to a 26" wheel or even bigger, be a factor to help prevent going to **too high** a torque, staying under that 130 N.m ??
 
john61ct said:
Assume a powerful mid-drive, at least 1000W rated, 3-5000W peak driving the pedal drivetrain at the BB, capable of shearing those pins at 130 N.m

Absolutely. The BBS02 at 750 watt/25A setting is a stretch and with full throttle off the line along with hard pedaling will sheer the pins; maybe not the first time but eventually it will.

Rims size won't make a big difference as its a matter of torque input from the chain that matters most, unlike a hub motor.

The Pinion will take far more torque. I've been playing with some concepts using this on a mid drive trike.
 
Thanks for your response!

Triketech said:
The Pinion will take far more torque.
Interesting, but let's keep this thread to the Rohloff. I'm not actually looking for **more** torque here

just advice on **how to limit it**, in such a way as to allow getting the full 120-125 N.m when needed, but maybe with something enforcing a gentler ramp-up,

analogous to an electronic soft-start used for running an aircon compressor off a limited-current inverter, if that makes sense.

______
> Rim size won't make a big difference as its a matter of torque input from the chain that matters most, unlike a hub motor.

But not "no difference"?

Would going from, say 26 to a 27.5 or even 29" wheel,

assuming the Rohloff 1st gear is set up to deliver what's needed for those uphill standing starts

actually help with my goal of keeping the torque under that 130 N.m ??

______
Just how much of a pain **is** replacing those shear pin(s)? Something that could be done side of the road?


 
Would a belt drive help absorb some of the startup "surge shock"

as opposed to a chain drive?

Say a heavy-duty Gates belt? Some may have more "give" than others. . .
 
Relevant, CYC+ASI example

https://endless-sphere.com/forums/viewtopic.php?p=1556285#p1556285
 
I think you are mixing two things here:

1. Torque
2. Radial force

The thing you feel when accelerating is radial force transfered from the wheel to the ground. Torque on the other hand is the force acting in the middle of the wheel.

And as you know Torque is the product of Force times lever length.

So in order to have more force pushing you up a hill while maintaining your 130 Nm limit you have to reduce your wheel size.

For example:

A:
130Nm
26 inch wheel (d=0,669m)

= 194 N

B:
130Nm
27,5 inch wheel (d=0,700m)

=186 N
 
Program the Bafang to 18A rather the 25A peak and you should be fine.

Shear pins are not a roadside fix without a vise.
 
Amaxophobie said:
So in order to have more force pushing you up a hill while maintaining your 130 Nm limit you have to reduce your wheel size
So, just getting more confused.

A hub motor applies its torque, force, power of all kinds directly to turning the wheel.

The torque in the case being discussed here, is being applied to a chain or belt drive

which is then transferring the torque / force / power onto the gearing contained in the Rohloff hub.

The torque I need to cap is there, applied from the chain to the gearing **input**. The torque / force / power applied between the gearing output and the wheel / road is being easily and dynamically adjusted as needed, therefore freeing me up to select a bigger wheel O.D. including fattie tires for rolling over rocks & branches, plus suspension effect / comfort.

Assuming the hub is in 1st gear, the rpm of the chain is being reduced by 3/4 before being applied to the wheel.

Maybe I have no clue, but seems to me your post completely ignores that?


 
Triketech said:
Shear pins are not a roadside fix without a vise.
Fantastic, one nice firm data point!

> Program the Bafang to 18A rather the 25A peak

Well the actual motor is TBD, many possible candidates, best to avoid that issue for this thread I think.

But generally speaking, that programming is a function of the controller more than the motor itself, right?

I realize that "max amps" does have a relationship to torque and power

but my understanding is that, it is a very loose relationship, and dependent on rpm.

The shearing problem I understand is only most likely from a standstill up to say 7mph

accelerating between that and say 12mph, up a steep hill with 400+ lbs

I think I still need all the amps I can get?

Isn't it more the **ramp up speed** of amps rising, like "amps increase per second" that needs to be limited

rather than just static max amps?

People talk about sinusoid, and even better true FOC, controllers being "very smooth" in their acceleration, having a finer granulation of controll over the motor's internal components' rotation.

So assuming within the likes of Nucular Mobipus ASI Lebowski maybe even Sevcon

what type of features / setting variables should I be looking for rather than just a gross cap limit on amps?

Is "ramp up speed" the right term?

On the other hand, am I just crazy-overthinking, and just learning to apply throttle gently will avoid the problem, even with a motor capable of 200 N.m ?

If not, then is the solution necessarily electronic?

My preference would be a mechanical "torque fuse" to help me learn the limits that can be set to say 110 - 120 N.m, and is easily "reset" or fixed if it "blows" while out far from civilisation.



 
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