Gearing up the front crank for higher speeds pedaling
- Thread starter kojack
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m@Robertson
Well-Known Member
- Region
- USA
Depends on the motor ecosystem. If you did the conversion yourself, no absolutely not. However, for all mid drives, they like to spin fast. Electrical energy that does not convert to motion (motor rpms) instead converts to heat. One of the classic mistakes to make on a mid drive motor is to bog it down with a big chainring. So, as is the case with many things... moderation is the key.
Unless you have a hub drive, in which case it doesn't matter one bit.
Unless you have a hub drive, in which case it doesn't matter one bit.
Jeremy McCreary
Well-Known Member
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- USA
- City
- Carlsbad, CA
You also have to worry about chainstay clearance when upsizing a chainring, and drivetrain efficiency and wear rate when downsizing your smallest rear cog.
You're looking at a 14% increase in ground speed from 35 to 40 kph. At constant cadence, that means a chainring with 14% more teeth and therefore 14% larger diameter. Or a 13% smaller rear cog.
As @m@Robertson noted, with a hub-drive, you have to keep up wheel speed — and therefore ground speed — to keep up motor efficiency. The more efficiency you lose, the less mechanical power out per watt of electrical power in.
So the more you let a hub-drive bog down on a hill, the less assist you'll actually get, the more the motor will heat itself instead, and the harder you'll have to pedal to keep from losing even more ground speed.
A larger chainring might help you mitigate that by hitting the bottom of the hill at the highest possible speed. But by reducing all your gear ratios at once, it might also compromise your ability to finish the climb with available leg power.
If your routes involve steep enough hills, that might push the trade-offs toward a cog-based solution.
I recommend getting your bike shop involved.
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Jeremy McCreary
Well-Known Member
- Region
- USA
- City
- Carlsbad, CA
Ah, then you're loaded for bear on the mechanical side. Looking forward to hearing your final solution.
I think a lot of ebike owners miss out by blindly accepting stock gearing. To deal with my hilly terrain without killing my old knees, my 500W, 65 Nm hub-drive eventually ended up with one more gear, a smaller chainring and smallest cog, and a bigger largest cog.
On my lightweight low-power mid-drive — which cares about cadence, not wheel speed — I reduced the chainring to keep my preferred cadence in the motor's sweet spot (80-90 RPM) on steep hills.
Lost some top end in both cases. Wrong direction for you, of course, but these were good trade-offs for my legs, knees, and topography.
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m@Robertson
Well-Known Member
- Region
- USA
You should be fine. @Jeremy McCreary makes smart note of the fact that you could be shortchanging yourself if you have hills to climb, since a hub motor is singlespeed since it powers thru the axle, and its life as a fixie sucks just like yours would grunting up a hill. Your hill climbing ability is all your legs once the hub motor bogs.
This hub motor bike lived on flat ground. Experiments worked me up eventually to a 60T front ring and a single-cog 16T Shimano rear freehub. The two hub motors meant I commuted at a fast cruise (despite very strong afternoon winds, which the motors eliminated from concern) and the big chainring meant I could work hard while doing it, taking the bike up and over the motor rpm limit for work/rest intervals.
Since you know bikes you can fart around similarly and just mix/match chainrings until you get what you want.
And even though the hub motor is independent of the drivetrain, you can still optimize that drivetrain just like you could any bike. This one has an 11-32T cassette (not a freewheel) that let me do just a 50T front ring for a working cadence at speed.
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