Chainring size advice.

[Adwb]

I have a trike with a front hub 250 watt motor. gearing supplied by Shimano internal 7 speed .what I am finding is that on the steepest hills I can run out of power if the hill is to long even with max PAS and the lowest gear.
I changed the chainring from 48 teeth to 44 with if I’m honest little effect.

By it’s self the motor will push me along at 15.5 or a little over with no pedalling (yes I know that illegal)

W hen going flat out on a flat surface my pedalling cadence is very very slow and a long way off ghost pedalling at the 15 mph speed which is fast enough for me.

Is there a way to calculate a lower number of teeth on a chain ring. I am wary of buying something to low but suspect I need to be in the 30 something range to get that extra bit of pedal power for the long steep ones.

Thought or suggestions appreciated.

 

[Gringo]

I’d say you’re close at 30 something. I
f it helps I have 3 manual bikes, 2 x mtb‘s and a cruiser, all have a single chainring (32,34 and 36t) All bikes are comfortable at 15mm and I only start to spin out around 20mph.
To help deciding on ratios I brought a few different sizes from Alie express, I got 3 snail chainrings for less than £20 posted (15 day delivery)

 

[Artstu]

Sheldon Brown's Bicycle Gear Calculator

A JavaScript calculator for analyzing bicycle gearing

www.sheldonbrown.com

 

[StuartsProjects]

You can use a gear calculator that tells you the gear inches you currently have.

If the prime requirement is more hill climbing for steep hills, then clearly you need a lower gear. However given that riding styles vary, weights vary and steepness of hills varies, I cannot see how you can calculate what would feel good for you.

I have been going through a similar issue on a conversion for a Brompton. The first setup had a 54T front chainring, too high for me in the area I live. I swapped to 47T, and thats still a bit high. However the chainset is a 130BCD, and the chainrings are cheap to buy.

I might try a 39T\40T, but that might be too low, buts its only going to cost me £7.80 delivered.

Apart from actually trying the different ratios, over my typical routes, I cannot see how I could somehow work out what I needed.

 

[WheezyRider]

Can you work it out from your preferred cadence, and then see what speed that is for a given gear and wheel size?

 

[guerney]

Sheldon Brown's Bicycle Gear Calculator

A JavaScript calculator for analyzing bicycle gearing

www.sheldonbrown.com

If it helps, my Bafang 250W 36W BBS01B mid-drive converted 20" wheeled folding bike regularly manages 17.9% and steeper hills, albeit slowly. My bike and I also do this while towing a 90kg+ bike trailer up that and other steeper hills, at the same low speed. Gear inches for 52T>32T below (I haven't measured my cranks, used the default 170mm). I weigh 69kg, my bike weighs 22.4kg. My ebike battery is 19.2ah, therefore voltage sag isn't a problem when tackling steep hills. My motor controller's "Keep current" is 15A. (Wheelbase is 102cm, 3cm shorter than Brompton's)

I started with a 42T chainwheel and progressively tried larger for more speed, and quite honestly was thinking of going 52T+, by a bit - I would have done so by now, if I didn't have to haul heavy trailers up steep hills.

 

[matthewslack]

If you are ghost pedalling on the flat then it almost doesn't matter how low you go. You are the power up the steepest hills, so maybe go all the way down to 28 or 30 or 32 depending on what is available in the right fitting.

Alternatively change the motor to one capable of managing the hills.

 

[Raboa]

I would say a 36 or 28.