Large-ish (34t) nexus/alfine sprocket hack?

[woahdae]

My mom (~70yo) wants to get an ebike with a Shimano Nexus 5E IGH (this Electra Townie), but the lowest gear is 33.92 gear inches. The mid drive motor is 50nm, 250w, so pretty mild, which is great honestly to give her some extra range. There's a couple hills around her town that I think she still probably won't be able to do though.

Really, it's the perfect bike for her, except for maybe the gearing. So I'd like to do some preliminary planning on putting in a bigger sprocket on the IGH.

Sheldon Brown's site says this about Shimano IGH sprockets:

It is fairly easy to modify any Shimano cassette sprocket that doesn't have a built-in spacer. These are available from 14-34 teeth. Shimano cassette sprockets have the same internal diameter as those used with internal gear hubs, but have 9 splines instead of 3. With a suitable grinder, 6 of the splines need to be removed, and the corners of the remaining three rounded off

I see what he means about the 9 splines, and I can find some 3rd party 40-tooth single cogs, but anything in the 34t range is part of a cassette. He makes it sound like they're relatively available.

Anyone care to break down Sheldon's wisdom a bit more? I can dremel splines no prob, but I don't see where I'm supposed to buy anything suitable.

 

[Tommm]

If it works with a cassette spline you can buy or get one made on ebay to convert from spline to bcd 64 or 104.

But that is a big workaround, just get a smaller chainring.

 

[woahdae]

> get a smaller chainring

It's my impression the rear cog is /usually/ the easier option. This bike has a nice-looking shiny chainring+guard, it's not immediately obvious if the chainring can be changed independent of the guard. But if it's easy and only affects how it looks, that's a nice option to have.

 

[Balmorhea]

It's not a coincidence that Shimano doesn't offer sprockets bigger than 21t for that hub. If you over torque the gears, you'll break the hub. That's whether you do it with a small front chainring or a big rear sprocket. There is probably a specification somewhere for either minimum gear ratio of maximum input torque (which is a function of gear ratio).

Find out the gearing or torque limit before you spend time and money getting a custom sprocket.

 

[woahdae]

That's a great point, thanks. Note that this hub comes stock with either 24t or 30t, which makes sense b/c it's lowest gear is 1:1 so it needs a big sprocket to be practical. I know, 30t vs 34t isn't a huge jump, but if it was even as easy as ordering a sprocket and dremmeling some tabs it'd be worth the time.

I think the more practical (and expensive) option would be to replace it with an 8 speed Shimano hub, whose gear ratios start at something like 0.6. Now that you've pointed it out, I don't know whether the 8 speed could handle the torque of a larger sprocket anyways... something to research.

 

[Balmorhea]

That's not a hub I'm familiar with. I used the Nexus 4 on one of my bikes in the distant past, and I remember that it was all overdrive. The Sturmey Archer 8 is likewise all overdrive.

I don't think that lowering the gearing by 12% will make a critical difference for a relatively light duty application.