2 different size wheels, is it practical and energy efficien

[horizon9]

can you have a larger diameter front wheel and smaller diameter back wheel? would it make it more efficient since a larger wheel is better for cruising and smaller wheel is better for accelerating and hills and snow?
want to use a front dd hub motor and a mid geared. and only used the front dd when maintaining 20 mph and only use the mid geared along with pedalling to reach that speed or start stop riding or hills or towing or going through snow.

if my route is mainly cruising i may not even take off the mid drive. but if there is not much cruising i will take of the front dd hub and put a non motor wheel on (same large size) or maybe the same small size as the rear wheel? i don't know.

maybe for a mid drive start stop route, 16 or 20 inch would be ideal for the front of both wheels? or geared mid and a geared front hub so i wouldn't need to shift out of medium rpm and would manually pedal to reach medium rpm. and the front geared hub can take over for higher rpm.

is 36 inch front dd hub and a 9 inch back wheel too extreme? or 32 and 16 or something? what would the ideal wheel sizes be? will i need to know the right watt and volt for a front dd to efficiently cruise 200 lbs at 20 mph on a 36 inch wheel?

my goal is energy efficiency so maybe 36 inches is a little to big to be efficient for 20 mph? if so, what diameter is efficient for 20 mph cruising and at what speed does a 36 inch wheel become efficient? what is an efficient front cursing wheel diameter and watt and voltage of motor if the desired cursing speed is 50 mph?what would be a good size for the back wheel?

 

[Hillhater]

I doubt if anyone has experience of a hub drive 36" wheel.
For a geared mid drive, the wheel diameter is irrelavent.
Larger wheels are more "efficient" in theory, but probably hard to quantify with real data.
If efficiency is your real goal, then you will find huge efficiency gains by simply pedaling more !
The next biggest efficiency gain would be to go recumbent style to minimise the wind drag effect.

 

[dogman dan]

Front DD and mid driving the rear, any size wheel will work. I would guess 26" or 29" would work fine. Pick the wheel size that makes the frame fit your body best.

Why a DD in front? Nothing will be efficient about pushing an unenergized DD wheel when you use the mid drive. So you'd still have to tickle the DD with 50w, or use up 50w pushing it. If the DD always ran, but the mid drive just helped from time to time, that could make sense.

Bottom line, the mid drive by itself could be operated the most efficient, if you shift a lot.

I have been thinking it would be nice to put a front geared motor on my mixte. It's 20" rear DD motor is already crazy efficient. ( As a system, not the motor per se) Most of the efficiency is just that it's slow, a low rpm motor. So I don't burn up watt hours going fast. The front geared would just freewheel 95% of the time, but it would be there to help tow trailers up long steep grades when needed.

Likely it just makes more sense, to make the thing a mid drive though. Mid drive, it would need no help on the hills.

 

[rborger73]

DD in the rear for me. 26" on the rear, and 29er in the front. I actually love it. Heading towards 18,000 miles that way. Most people rarely notice the size difference due to how I built my bike.

 

[horizon9]

DD in the rear for me. 26" on the rear, and 29er in the front. I actually love it. Heading towards 18,000 miles that way. Most people rarely notice the size difference due to how I built my bike.

isnt it more energy efficient to have front wheel drive, or is that a misconception?

but i guess the rear would become more efficient the more the inclines or resistance like snow or gravel maybe. i never realised that. does having a larger front wheel help with energy efficiency even tho it is freewheeling? less rolling friction i guess?

 

[horizon9]

Front DD and mid driving the rear, any size wheel will work. I would guess 26" or 29" would work fine. Pick the wheel size that makes the frame fit your body best.

Why a DD in front? Nothing will be efficient about pushing an unenergized DD wheel when you use the mid drive. So you'd still have to tickle the DD with 50w, or use up 50w pushing it. If the DD always ran, but the mid drive just helped from time to time, that could make sense.

Bottom line, the mid drive by itself could be operated the most efficient, if you shift a lot.

I have been thinking it would be nice to put a front geared motor on my mixte. It's 20" rear DD motor is already crazy efficient. ( As a system, not the motor per se) Most of the efficiency is just that it's slow, a low rpm motor. So I don't burn up watt hours going fast. The front geared would just freewheel 95% of the time, but it would be there to help tow trailers up long steep grades when needed.

Likely it just makes more sense, to make the thing a mid drive though. Mid drive, it would need no help on the hills.

i have figured that a rear dd would be better on routes with inclines above 3-5% or something and front dd for routes inclines below that. just a randomish approximation for the threshold. and yea mid geared drive always for inclines above 10% or so.

 

[Punx0r]

isnt it more energy efficient to have front wheel drive, or is that a misconception?

It doesn't make any difference with a hubmotor. It's true of cars with the engine transversely mounted in the front, but only because the drive doesn't have to be turned through 90° and sent to the rear of the car.

 

[dogman dan]

You keep focusing on efficiency, which has more or less been engineered into 99% of all bike electric motor systems already.

There aint no magic sauce, that's going to make your motor run under a load at 99% efficiency. Think about how efficient gas motors are. Now does running at 70% efficiency sound pretty good? It should!!!

But there are a few things that stick out here, that can be affected by your choices.

Lets forget legal for the moment. Obeying the watt limits, even the typical US ones of 800w or so, means your bike motor system will always be overloaded at some point, unless you ride only on the flat, when there is no wind, and never carry more than your body, and pedal your ass off.

But if you choose to build a more powerful bike, you stop overloading it. VIOLA, it stops running so damn inefficient. What does this vehicle look like? Almost always, a very small rear wheel, and a hub motor rated for at least 1000w. Talking 35 or 40 mm magnets here, not 28 mm. It might be a scooter wheel, or just a 20" bike wheel. Then feed it enough voltage and power to go very fast. You can see many bikes like this here on ES,, always built by some of the very smartest members here. The solution is simple, affordable, FAST when you want it, and efficient when you want it. Spinning too big a magnet around at low power,, hogwash. A huge motor runs at low power beautifully, compared to tiny motors that struggle.

Yes,,,, it won't sip power when it goes fast. But if you ride it 20 mph, the amount of power wasted into heat on stops and starts will be so low, you almost can't measure it.

It need not be a huge motor though, if you keep the weight loads and or grade loads reasonable. Here is my version. At 18 mph, loaded with 40 ah of 48v battery, it gets 25 watt hours per mile, with only minimal pedaling from me. Range is at least 80 miles.

Its the single most efficient E bike I have owned, ( but I have not owned a mid drive) and can carry 400 pounds over the rocky mountains. A mere 28 mm dd motor, but the 20" wheel gives it great torque. It runs on 1000w,, a 48v battery and 22 amps controller. On the steep hills, it pulls only 800w, and keeps its cool. It's virtually an indestructible ebike, for loads under 400 pounds total weight. The bike weighs 90 pounds, and I weigh 190, so I can tow a trailer that weighs 100 pounds.