small wheel efficiency

im amazed at the power and efficiency of a 20 inch DD wheel. I hopped on my trike to go upto the store and the ca3 said I consumed 3ah for 34km travelled. im used to a ca2.3. so I was just kind of staring at it thinking no way maybe that's top speed but no. ive been upto 50kmh on it. so I get home and use grins motor simulator. in a 20 inch wheel, 52v 11ah battery,40 amp controller, 60% throttle. I can go 32km/h for 105km without pedaling. in a 26 wheel same set up, but 50% throttle, speed is 33km/h range 108km.

I don't know if efficiency is the right word because it takes the same amount of energy to move from A to B. I just had this notion that this little wheel would be slower top speed with less range but whats happened is I have a slower top speed but alot more usable power/torque at my usual riding speeds of 30 to 50km/h and off the line at lights with pretty much the same range.

I even think you can mount moped tires directly on the 20 inch bike rims

first kit I ever had was a 48v 10ah 500/1000 watt geared bafang, 20amp controller. topspeed 32km/h and range was maybe 25km. I liked it but it sucked.

Them leafmotors do well. Keep it 24-32 kph and get a lot of range.

Depending on the make and model, "some" 16-inch moped tires will mount to a 20-inch bicycle wheel...and vice versa

This is a rarely-seen option, with many benefits. John in CR has been doing the small drive-wheel with a fat motor for a long time, with great results...and he lives where it is definitely hilly...

If the motor you have is bogging down in a taller wheel, it will draw many more amps. A smaller wheel might use less total energy to cover the same distance, even though it had to spin more revolutions to get there.

Range 20 wheel 92 km at 45.8kph, Batt Power 640W.
Range 26 wheel 69 km at 54.3kph 974W.

goatman said:

I don't know if efficiency is the right word because it takes the same amount of energy to move from A to B. I just had this notion that this little wheel would be slower top speed with less range but whats happened is I have a slower top speed but alot more usable power/torque at my usual riding speeds of 30 to 50km/h and off the line at lights with pretty much the same range.

Click to expand...

Yep. You are seeing a side effect of the well known phenomena that it's easier to make a low torque, high speed electric motor than a high torque, low speed electric motor.

There are drawbacks on the small wheels in the rear, bumps and chain ring size.

I may start planning to put two of grins all axel V2 motors on the front of a trike in 24" rims @ 72V so I can have my 29er rear wheel and 11 speed cassette.

ZeroEm said:

There are drawbacks on the small wheels in the rear, bumps and chain ring size.

I may start planning to put two of grins all axel V2 motors on the front of a trike in 24" rims @ 72V so I can have my 29er rear wheel and 11 speed cassette.

Click to expand...

with double suspension it gets better,
I'm not young and I use the chains to turn symbolically, and for emergencies.

zeccato said:

Range 20 wheel 92 km at 45.8kph, Batt Power 640W.
Range 26 wheel 69 km at 54.3kph 974W.

Click to expand...

Not a good comparison, because aerodynamics affect the wattage per mph quite a bit at those higher speeds. You have to compare at the same speed, sadly.


Running a smaller wheel often gives you a small hit in efficiency due to higher eddy currents per RPM. This is more exaggerated on motors with higher pole counts like the 9C 212 or 215, and the magic pie also. A geared motor will also have more gear friction per RPM too and really take a hit in efficiency.

Motorcycle tires will give you a huge friction loss also, but the ability to roll over a goathead without a popped tire makes it totally worth it.. :thumb:

However even the worst case scenario still gives you the ability to push higher continuous watts, and also gives you a big torque gain too. That means you can get away with a smaller motor than one you'd spec out for a 26"+ wheel. A smaller motor means less eddy currents. Therefore a small wheel CAN actually be more efficient overall if you take time to think about all these factors whne choosing your motor

zeccato said:

with double suspension it gets better,
I'm not young and I use the chains to turn symbolically, and for emergencies.

Click to expand...

Yeah a 20" wheel with a moto tire and air/oil rear suspension will ride like a luxury car with the right tune... you'll only notice the small diameter of the wheel when you ride over a curb or something

I have rear suspension and when I switched to a rear 2.20 tire, very soft ride now, don't feel anything, I was reading about the leaf motors Toecutter and Zero Em have been discussing 0-30 mph in 6 seconds. so on the way to work pulling a bob yak trailer and 15lbs. 0 to 30mph was 6 seconds but I max out at 65 km/h and if I put about 1 amp of virtual freewheeling with the phaserunner the trike coasts and pedals like a normal bicycle. I love my trike and am thinking of doing a tesla truck look/velo body

neptronix said:

Yeah a 20" wheel with a moto tire and air/oil rear suspension will ride like a luxury car with the right tune... you'll only notice the small diameter of the wheel when you ride over a curb or something

Click to expand...

Or in goats' head country with no paving much less curbs

ZeroEm said:

There are drawbacks on the small wheels in the rear, bumps and chain ring size.

Click to expand...

Isn't the bump-absorption more critical at the front?

Maybe a 3" plus / balloon or even fattie could help if suspension isn't practical?

neptronix said:

zeccato said:

Range 20 wheel 92 km at 45.8kph, Batt Power 640W.
Range 26 wheel 69 km at 54.3kph 974W.

Click to expand...

Not a good comparison, because aerodynamics affect the wattage per mph quite a bit at those higher speeds. You have to compare at the same speed, sadly.

Click to expand...

you're right, thanks for the correction
I used an 8T for both wheels,
but for wheel 26 was 10T,
and the data is almost the same.https://www.ebikes.ca/tools/simulator.html?grade=&hp=0&frame=semi&wheel=20i&batt=cust_54_0.2_25&black=load&cont=C40&motor=MMAC8T&bopen=true&hp_b=0&wheel_b=26i&frame_b=semi&motor_b=MMAC10T&grade_b=&batt_b=cust_54_0.2_25&cont_b=C40&grade=0&grade_b=0&k=1&throt_b=100&throt=100&autothrot_b=false&autothrot=false

remains unchanged that on the power torque wins the 20.

by john61ct » Mar 09 2020 7:01pm

neptronix wrote:
Yeah a 20" wheel with a moto tire and air/oil rear suspension will ride like a luxury car with the right tune... you'll only notice the small diameter of the wheel when you ride over a curb or something
Or in goats' head country with no paving much less curbs
ZeroEm wrote:
There are drawbacks on the small wheels in the rear, bumps and chain ring size.
Isn't the bump-absorption more critical at the front?

Maybe a 3" plus / balloon or even fattie could help if suspension isn't practical?

Click to expand...

Well known facts: Rear suspension is about comfort unless you have a long bike that moves the tire out from under you, draw back it can take away from your peddling effort. Front suspension if more about speed and handling.

My issues: I have rear suspension with 26" 1.75 tire great for comfort and speed, bad when I peddle hard up a hill from the suspension squatting. No suspension up front 20" 1.35 tires, great for getting the most out of a battery at lower speeds and strong wheels and tires for high speed. Bad if over 32 kph and any bumps will jar so bad it flip switches and throws excitement into the ride. 60 psi rear and 90 psi in the fronts.
How will it be changed, next tires will be no less than 1.75 then if not good enough will go to a 24" wheel. My trike does not lean into corners so want a stiff high speed bike tire.

My next build I would go with 29er in the rear for peddling and a pair of grins V2 all axel motors @ 74V in 24" fronts. Cake and eat it too.