Crank drive advantages

[hoppy]

Some recent discussion on this forum prompts me to put forward the view that for a limited power and torque motor as we all have, a drive through variable gear ratios is inherently advantageous. It enables climbing the steepest hills ,reaching the highest speeds, and the most efficient use of limited stored energy.The best use of the available power is achieved with a simple throttle control.Sadly, too few crank drive bikes offer this.Only the Tonaro range and the Woosh CD I think.
Interesting that the newest and fastest electric car has a four speed gearbox.

 

[Deleted member 4366]

Correct. The crank drive has the potential to deliver good hill-climbing and good speed. The lower the torque of two motors (crank and hub) with equal torque, the more advantage the crank motor has; however, in ebikes, there's a large variety of motors, so you're never comparing like with like. Most hub-motors are larger and give higher torque than most crank motors, and the 15 mph speed limit takes away most of the speed advantage of crank drives, so hub-motors can get the advantage provided they have enough torque to get your weight up the hills that you need to go up. There's also loads of other advantages/disadvantages of each system that also need to be considered, which complicate the matter.

Electric motors have a wider spread of torque than an internal combustion engine, so don't need so many gears. How many gears does an electric train have?

 

[hoppy]

Don't know! If it's only one, perhaps because they are not too worried about efficiency having an unlimited supply of energy. Our electric road vehicles have to carry their own supply limited by weight and cost. An electric motor is at its most efficient at certain rotational speeds,which variable gears enable to be used more.

 

[trex]

Electric locomotives don't have a mechanical gearbox. The controller acts as a CVT including reversing.

 

[hoppy]

That's interesting,Trex. How does the motor drive the wheels? Is it a future possibility to have an electronic controller cvt on an ebike?

 

[trex]

Ebike controllers already acts as a CVT. d8veh can explain way better than I can.

 

[Deleted member 4366]

Trains use basically the same system as a hub-motored bike, i.e. no changeable gears between the motor and wheel. The motors can produce torque over a very wide range of speeds. Sometimes they use clever electronics to get a wider speed range, and most of those techniques can also be applied to ebikes and electric cars, but, like with trains, in most cases it's not necessary.

 

[hoppy]

But what about efficiency? At very low rpm surely our ebike motors produce their highest torque but are least efficient. We need to run them most efficiently to eke out our meagre ranges.Similarly for electric cars.Hence the advantage of variable gear ratios.

 

[Deleted member 4366]

But what about efficiency? At very low rpm surely our ebike motors produce their highest torque but are least efficient. We need to run them most efficiently to eke out our meagre ranges.Similarly for electric cars.Hence the advantage of variable gear ratios.

Your partially right. The efficiency improves if you reduce the throttle, so by purring a ramp on the throttle, efficiency can be improved, but you can't do much about the low efficiency when labouring up steep hills, although you can gain a bit back at 15mph, when the hub-motor power consumption has started to decrease, while as the crank-drive can be under high load at that speed so consuming maximum power, the same as any time you're in too high a gear. Basically it's complicated and depends on so many factors.

 

[Marctwo]

What I really like about my crank drive is that you can always select the appropriate gear to find the sweet spot for the motor rpm. Uphill, downhill, headwind, tailwind...

More efficient? Mine certainly isn't. lol. Partly maybe because it doesn't cut off at 15mph but mainly I think due to always selecting the appropriate gear to find the sweet spot.

 

[GaRRy]

What I really like about my crank drive is that you can always select the appropriate gear to find the sweet spot for the motor rpm. Uphill, downhill, headwind, tailwind...

More efficient? Mine certainly isn't. lol. Partly maybe because it doesn't cut off at 15mph but mainly I think due to always selecting the appropriate gear to find the sweet spot.

Surely a contradiction if you are using the sweet spot then the engine is at its most efficient ?.

Of course if it isnt then wheres the advantage over a Hub?

Do agree on paper a crank should be more efficient etc but in the real world for most riders I am not convinced. Not that im saying a Hub is better. More that a buyer should try and use both before deciding which they personally prefer.

 

[Geebee]

Steep hills and crank drive equal more efficient especially if similar wattage's.

 

[GaRRy]

Steep hills and crank drive equal more efficient especially if similar wattage's.

Yes but you have straight away made a assumption that I am pretty certain is rarely true in the real world. Also what constitutes a steep hill ?. Most of us rarely go up anything for a sustains distance of much more than 10% which a Hub drive has no issues with.

Personally not pro one or the other. It really does come down to use, the expectations of the rider and what feels right for the individual. Same as with frame type, tyres and all the other bits on a bike

 

[Deleted member 4366]

I made a crank drive out of a hub-motor, so I could get an exact comparison of the efficiency of each system with the same motor, controller and battery. The result was that I could climb steep hills more easily with the crank-drive, but it didn't go as far because it used high power more of the time and I didn't pedal so hard on the hills. The power algorithms in the controller have much more effect on efficiency than the drive system.

 

[trex]

..The result was that I could climb steep hills more easily with the crank-drive, but it didn't go as far because it used high power more of the time and I didn't pedal so hard on the hills...

crank drive riders are getting lazy because they can?

 

[Taff]

I made a crank drive out of a hub-motor, so I could get an exact comparison of the efficiency of each system with the same motor, controller and battery. The result was that I could climb steep hills more easily with the crank-drive, but it didn't go as far because it used high power more of the time and I didn't pedal so hard on the hills. The power algorithms in the controller have much more effect on efficiency than the drive system.

Fantastic. If it was April 1st I wouldn't have believed you. Why don't you offer to do conversions?

 

[Artstu]

Fantastic. If it was April 1st I wouldn't have believed you. Why don't you offer to do conversions?

Cyclone do Cyclone: Electric Bicycle Transmission

 

[Geebee]

I have done the same as well with a Tongxin, power usage was about the same but climbed better, could add power if I wished at higher speeds and much more versatile.
The problem with the hub becoming a crank drive is that you can still be adding power at 60 kph where as mine as a hub drive cut out around 25 kph, so with out self control power usage can skyrocket, but it can be a lot of fun as well

Garry, a steep hill doesn't have to be long to stall a hub, I live in one of those rare places in the real world you mentioned

 

[Deleted member 4366]

Fantastic. If it was April 1st I wouldn't have believed you. Why don't you offer to do conversions?

I've taken it off now, but it's all still there if anybody want's to make an offer. The bracket fits exactly to the Diamondback frame, but could be adapted to fit any bike with scaffold clamps like Marctwo's

 

[Marctwo]

Surely a contradiction if you are using the sweet spot then the engine is at its most efficient ?.

Of course if it isnt then wheres the advantage over a Hub?

The sweet spot I'm refering to is not necessarily the most efficient spot (although it may be for all I know). It's where the motor is giving good pulling power but is going fast enough not to over heat. When you hit that rpm range it feels sweet...

The advantage: In 4th gear the motor and wheel rpm are pretty much the same so the motor is doing roughly the same work as it would in the hub. In this gear it'll go past the sweet spot doing a comfortabe ~18mph without pulling many amps but it's not doing enough work to go any faster. Going into 5th will then find the sweet spot and get a bit more work out of the motor at the expense of a few more amps. Of course, it'll go past the sweet spot in 5th too but it won't go as far past... The faster you go, the harder the motor has to work to maintain that speed.