Anyone fitted a bigger chainring on Bosch Active Line Plus with Hubgears?

hankhill

Finding my (electric) wheels
Jul 15, 2016
12
6
108
London
I’m hoping someone can give me some advice based on experience, or wisdom. I have just bought a Raleigh Motus GT 2019 model, (bought on eBay, but unused as new) with a Bosch Active line Plus motor. It has a Shimano Nexus 8 speed hub gear, and 700C wheels (I’m 6 feet 2, 16 stone). My problem is that the bike feels under geared, seems a common complaint. Even in 8th gear (top) I seem to be pedalling way too fast at cruising speed (25kph), and I never use the lower 4 gears at all, not even on hills. I’m still getting used to the bike having now done 90 miles of purely leisure cycling. Sometimes I still accidentally start off in 8th gear and then try to change up, its so under geared.

Because it is a hub gear I can’t change the cassette or anything at the back, so I was thinking of fitting a larger chainring and I gather that’s quite a common mod. The current chainring has 38 teeth. There seem to be a few aftermarket chainrings going from 38T right up to 56T.

I have read this forum and there are lots of posts about this, but none are on bikes with hub gears, so with apologies I have posted this as a new thread.

Has anyone done a chainring swap, preferably on a Bosch ActiveLine Plus 2019 motor (Gen3 or 4?), and which size above 38T would you recommend for my bike, to raise the gearing for a better cruising cadence? Would 42/44T make a big enough difference to be worth it? Obviously I don’t want to overstrain the motor or hub. I haven’t de-restricted the bike by the way (although I might). Nice thing about the new Bosch is lack of drag when the motor stops over 15.5mph, I have got to around 22mph on the level but my 60 year old legs are going like the clappers.

What’s the size limit beyond which I’ll need a longer chain? This bike has hub gears and no chain tensioner (that came on the 2020 model), the rear axle seems to be midway along the dropout slot (if that’s the right term) so there is scope for some adjustment.
 

sjpt

Esteemed Pedelecer
Jun 8, 2018
3,686
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Winchester
How many teeth on your rear sprocket? You can go down to 16. I think you have a 22T, which gives you loads of options:
Bigger front sprocket may give problems with the chainguard; extra reason to consider smaller rear smaller rear sprocket.

(For my wife and I the older 7 speed Motus hub is geared about right; we don't go much over 16mph, just enough to ride past assistance speed. (70+ year old legs))
 

RossG

Esteemed Pedelecer
Feb 12, 2019
1,628
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I've just fitted a 56t chain set on my folder for the same reason as yourself, spinning out at top speed. I had no problems with the changeover you just have to make sure you have enough clearance by the chain stays because of the bigger chain ring.
If it's any help my 56t chain ring is just over 23cm in dia to the tooth tips.
 

soundwave

Esteemed Pedelecer
May 23, 2015
16,270
6,331
DSC_0673_01_01.JPG
:p
 

peter.c

Esteemed Pedelecer
Apr 24, 2018
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thurrock essex
Yes a simple job to change the rear cog but the circlip takes a knack and its a much cheaper option
a bit old but not rushed
 

hankhill

Finding my (electric) wheels
Jul 15, 2016
12
6
108
London
That’s really useful, thanks. Evans, from than link, look to be out of stock of 18T sprockets, would a 16 be too big a jump?
 

soundwave

Esteemed Pedelecer
May 23, 2015
16,270
6,331
Interesting, thanks. I’m a little reluctant to do something that extreme to a brand spanking new road bike though:)
its a 5min job to change a front sprocket if you have a lock ring tool and crank extractor id say it is less hassle than changing the rear one.
 

hankhill

Finding my (electric) wheels
Jul 15, 2016
12
6
108
London
Thanks again:). What I’m going to do is measure my cadence over the weekend and play with some of the online gear calculators, then hopefully decide whether to go 20/18/16T. I’m not going to be doing high mileages but I don’t want to be over stressing the hub gears or the motor. It’s a brand new bike, but I’m not so concerned about warranty as I bought the bike on eBay.
 

peter.c

Esteemed Pedelecer
Apr 24, 2018
1,611
492
thurrock essex
Use the right gear and back off the power when you change gear, and one gear lower to hill climb ,you will not cause undue stress 18t is the middle ground ,still a hill climber with a higher top speed top gear will end up overdrive on flat roads
 

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