Friday, September 25, 2015

Two Speed Gear-box's

Using a two speed hub-motor as a mid-mount to drive the left side rear with a larger chain could be the best way to motorize a large cargo bike. Then you would not need to worry about the chain wearing out. But my cycletruck's mid-drive powers a tripple crank with two heavy duty chainwheels [40t and 32t] that gives me 11:1 and 20:1 reduction ratios including the 6:1 planetary gear reduction in the motor's can.

The lowest gear is too slow for street crusing, and I have been using the 40t with the 34t rear sprocket for most hills [16:1 total ratio]. Hub motors do work at a slower speed than a “can” motor, but they are stuck in a high gear, unless you use a 5:1 “geared” hubmotor. I ususaly move about 400-450lbs total combined weight.

Two motors can replace physical gears

One way bearings on the motor shaft pulleys and the output sprockets, the direction is reversed by a polarity switching circuit, shifting gears.

keeping the central cogs lubricated would be too hard, so putting a ratcheting bearing on the big cog (second shaft) will at least keep it from moving at high speed. And it maybe OK if it were coated with Teflon of possibly a dry wax lube.

How ever this one will work and is reasonably easy to build if you have access to machine tools.

With gears you can move the "efficiency peak" around to match your mph. the only reason not to use a mid drive is the fragility of the drive chain and sprockets when using a large motor.

On a fixed hub motor you are permanently tied to your power-band and this presents a real problem at low speeds. Low mph/rpm on a hub motor places you into the low efficiency areas of the power-band. You can compensate a little if you have an ammeter and are really careful about using even less current than the current limit allows (so you are using almost no voltage down low in order to be more efficient) but you are still fighting a less than desirable situation

At 10 mph the efficiency is running about 50%. The good efficiency areas don't even exist until you are past 20 mph. So unlike with gears where you can "gas it" off the line and be in the good efficiency areas all the time, hub motors have a real "issue" with quick starting. Quick starts on a hub motor are really bad news for battery usage... (much worse than if you are geared right).

An inefficient system (like a small-diameter rear direct-drive hub motor) will convert much of the 1500W of input into heat instead of torque. A geared hub motor of the same diameter and width will have close to 5 times the wheel-torque per input watts. A mid drive that gives the motor the use of the bikes gears can have a dramatic torque-multiplication between the input watts and output torque at the wheel (especially in the lowest gear).

There are riders who are experiencing very accelerated drive-train wear on the Bafang 750W BBS02 mid-drive, when unrestricted and running 1,000W. Chains and sprockets breaking after a short amount of time that have been beaten into submission.

Large motors (like a 1500 watt out-put) used as a mid-drive can wear out you drive chain fast. Smaller motors like a 1000 watt out-put (Washington state maximum and still too much power for a bicycle drive chain) will drive 400lbs up an 8% grade at only about 13mpg. That is a very low gear, if you if you want good mileage, you don’t want to drive around in such a low gear all the time.

However if you drive your large motor mid-drive gently enough it will not pop your chain, and will last longer. Hard starts are very bad for your bicycle drive chain as well as the battery and tires.

The way around an efficiency robbing, reliability reducing, maintenance and weight increasing gearbox is to instead invest that money and weight, is the right size motor and gear it properly. Locomotives driven by electric motors don't have multi-speed gearboxes. Tesla engineers initially thought the same as you and tried to make the Roadster a 2 speed and learned better after wasting millions, so now Tesla's are single speeds. Brammo uses or used them on their consumer emoto's, but more to give potential customers something familiar than out of need as riders generally use just one gear, but their racing emoto is a single speed like all the other electric racers.

But if you can't use the right size motor due to laws, you need gears.

Retro-direct two speed

If you only need two speeds have you considered using a retro-direct type system on the motor output shaft.

First put a left hand threaded 1.37x24t freewheel shaft adapter mount fitted with a south paw 24t freewheel on the motor shaft followed by a 30mm right hand thread freewheel with a heavy duty 13t right hand smaller size right hand freewheel mounted.

Big 50+ tooth drive sprocket made using an over-sized chain wheel on the wheel or shaft to be driven. Run the chain around the one to be driven: big 50+ tooth, then over to the motor and around the 13t right hand outer sprocket and back to a bearing idler sprocket mounted a little ways away from the motor and around that idler sprocket and back to the motor this time wrapping around the larger south paw freewheel and then back to connect with the other end of the chain on the other side of the big 50+ tooth.

----- Motor spins CCW it engages the 13t small size freewheel and pulls the chain with it 13t/50+t reduction ratio (AKA=low gear) and the 24t south paw freewheel just freewheels going along with the chain.
----- Motor spins CW it engages the 24t south paw freewheel and pulls the chain with it 24t/50+t reduction ratio (AKA=high gear) and the 13t small size freewheel just freewheels going along with the chain.

And use a BMX  or #215 chain. Bicycle chain just will not last very long with a thousand watt motor. And use a drum brake if you're going to apply it to the left side.

 This is a gear box for robots,
I cannot swear that is it adaptable to a bicycle.

"We have run configurations of our shifting gearbox internals with 3 CIM motors in the past, which total up to 1011 Watts of power. The limiting factor inside these gearboxes is the #4-40 screw that holds the shifting dog in place. There's more of an issue with shock loading this via the output shaft, along with the shifting rod, rather than the motors attached."

a motorcycle transmission

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