Cargo Conversion from Analog to e-bike: How Much Low Gear to Give Up?

[speedub.nate]

This is a gearing question, or hopefully more a discussion, but first there's a lead-in. For my upcoming project:

For the past 8 years I've been running a un-electric cargo bike consisting of an Xtracycle Leap cargo conversion and a Rohloff Speedhub.

I've used this bike to tow my kids' bikes up some not-insignificantly steep Oakland hills plus other cargo bike duties mainly relegated to the flatlands, but there's always a hill to get home. So I'm feeling like the I've got the gearing adequately dialed in for my normal uses.

My plan is to replace this bike with a Shimano EP8-equipped Yuba and a 3x3 Nine gear hub. Obviously, switching to a proper electric motor will negate the need for a low, low gear, but I'm contemplating exactly how much low gearing I'd want to give up if, say, I found myself with a battery gone kaput, or tackle some even steeper routes while carrying a load (which might include an adult-sized passenger).

Here are the numbers:
- current Speedhub cargo bike is equivalent to 24 x 36 derailleur gearing, pretty close to the max low gear on a mountain bike, and right at the recommended minimum that Rohloff recommends for their hubs.
- Yuba Mundo ships the EP8 equipped bikes with 38 x 36 derailleur low gearing

So just from the outset, it's like Yuba looked at an old-school mountain triple crank (22/32/44) and said "let's just get rid of the granny ring and replace it with a motor."

And then we've got the high end:
- current cargo bike is equivalent to 36 x 11 derailleur gearing, which has done me well on the flats, but of course I spin out on any descent, but that's fine, it's a cargo bike
- Yuba's stock setup they ship is similar, 38 x 11 derailleur gearing, I'm thinking I'd spin out on the flats before the motor could reach its 20 MPH cut-off speed

With 3x3's internally geared hub I'm looking at, I've got some options. I can more-or-less match what I've got currently with my analog Rohloff Speedhub setup, but paired with the EP8 that's probably too low. Or I can meet the Yuba Mondo's stock gearing about half-way, the equivalent of 24 x 24 derailleur gearing, or about halfway into a cassette while in the small ring of a mountain triple.

I know I'm overthinking this and I have a few ebikes at my disposal to compare to, but none of them are used to carry loads of any significance. Maybe I'll enjoy trading away a little bit of low gearing for something extra at the top end. Or maybe I'll regret giving up "too low" for some ridiculous load, insane pitch, or a dead battery situation that I'm just not aware of. I've read a comment or two that ebike Speedhub users don't even use the low half of their range and think all that capacity is wasted when there is a motor in the mix.

What do you all think?

 

[gromike]

Personally, I wouldn't give up the lowest gear. I have short steep hills, and even with my mid-drive (bbs02) I still like a super low in my kit. And you never know when your motor will conk out.

 

[Jeremy McCreary]

Maybe I'll enjoy trading away a little bit of low gearing for something extra at the top end. Or maybe I'll regret giving up "too low" for some ridiculous load, insane pitch, or a dead battery situation that I'm just not aware of. I've read a comment or two that ebike Speedhub users don't even use the low half of their range and think all that capacity is wasted when there is a motor in the mix.

Sorry, didn't read the whole OP, but I'd give cadence strong consideration in any gearing decision.

Your legs have an optimal cadence range for power output, and so will your new mid-drive motor. My legs and mid-drive motor both generally agree on 80-100 rpm, but my old knees demand a minimum of 70 rpm while climbing.

Find out what these cadence ranges are in your case, and pick a low gear that will let you maintain a motor-friendly cadence on your worst-case hills. Let your cadence go too low on your worst hills, and the climbing help you actually get from the motor will be compromised.

At the high end, find your spin-out cadence — where you stop producing useful torque — and pick a top gear that gives you an acceptable max ground speed at spin-out.

With those ideal top and bottom gears in mind, you'll be in a better position to pick the best external gear ratio given the internal ratios your IGH provides.

 

[speedub.nate]

That's close to the root of my thinking, @Jeremy McCreary , but I can't really know til I know, ya know? And 99% of the time this low gear consideration will be an academic exercise.

Being a heavy gear hub user, this is always my predicament: any consideration for low grading always compromises my high end.

The good thing is that if the choice I make proves to be lacking, it's fixable with a new chainring or cog. I'd just rather get the right out of the gate.

 

[Jeremy McCreary]

That's close to the root of my thinking, @Jeremy McCreary , but I can't really know til I know, ya know? And 99% of the time this low gear consideration will be an academic exercise.

It's not that hopeless if you apply the data available to you from your own cadence performance on previous bikes with known gearing and wheel diameters. Knowing your own power output from a bike with a power meter also helps.

Being a heavy gear hub user, this is always my predicament: any consideration for low grading always compromises my high end.

That's always the trade-off, isn't it? Having some idea of your ideal top and bottom gears would help you play the trade-off.

 

[speedub.nate]

None but one of my bikes have cadence sensors, I'm a high-cadence pedaler but this is largely seat of pants. My knees definitely have a preference for high cadence / low gear to tugging a loaded bike up the hill.

Actually, here's something else that helps inform my decision:

Yuba's non-motorized Mondo comes outfitted with 22/32/44 chainrings and an 11-36 cassette.
Their motorized version as I wrote is 38T front, and an 11-36 cassette.

So that means Yuba seems to be in agreement with my assessment that a motor is worthy of eliminating the 22T chainring.

I'm thinking I'll take @gromike 's advice and maybe bump down the equivalent of one cog size from my current low gear on the Speedhub, which would be like changing from a 11-34 cassette to an 11-32 cassette.

That's always the trade-off, isn't it? Having some idea of your ideal top and bottom gears would help you play the trade-off.

...and then I just found a Reddit thread of some owners of analog Mondos with electric conversions complaining they spin out too early. GAH! It's a cargo bike, not a fast bike, gotta keep telling myself that.

The 3x3 Nine hub IS slightly taller gearing than the Rohloff, so maybe that's my silver lining. Sleeping on it...

 

[speedub.nate]

@Jeremy McCreary I think a better answer to your cadence comments is that I tend to pedal my e bikes like I pedal my analog bikes, fast spin, so I'm evaluating these gearing options with that in mind.

If I tended to lug my e bike motors and relax my cadence, I didn't know where I'd be with this decision.