a word about gearing

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rpmarco
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*This is a somewhat lengthy post, but for those who might benefit from it it might be worth the read:

I've always thought one of the problems with electric bicycles (besides their dorky look) on the market today is their lack of some kind of variable transmission. You car doesn't have just one gear--why should your e-bike?

When a bike has only a single gear, the rider must choose his gear with the knowledge that he will not have the most efficient gear at certain speeds. Going fast down hills he will begin to spin out; if he tries to up the gearing to address this, he'll quickly notice how much more effort is required just to spin the crank one revolution.

The thing is, there is no reason for this...not since the advent of the dérailleur at least. Being able to change gear ratios means being able to adjust to varying amounts of torque at a given rpm and allows for higher top speeds by turning the wheel at a higher rpm.

It's simple physics. A motor typically reaches its point of maximum power at 50% of max rpm; At 80% of max rpm, a motor is operating at its most efficient speed. My most recent project is a street cycle built from bicycle parts with a 900w motor with a rated speed of 2600rpms. So for the sake of this argument, let us assume that this is the motor being used and 2600rpms is the top speed rather than the rated speed. Assuming 26" wheels, multiply 201 by gear ratio (chainring/sprocket) to give you top speed. This is how fast your bike (with 26" wheels)" will be traveling when the motor is spinning at its maximum rpms. *note: this is my own modified formula for determining mph of 26" wheels.

So if you have a 10t sprocket on the motor and 60t sprocket on the rear, your top speed at maximum motor rpm (2600rpm) would be:

201*(10/60)= 33.5mph

If your bike was a typical e-bike with one gear, you would top out at 33.5mph. The thing is, the higher you gear the bike for speed, the less torque you have for quick starts and tackling hills. With a multi-speed transmission, you can retain your high gear for those times when you need it while having other gears to use if conditions change (stops, hills, headwinds, etc.)

A bicycle's 'transmission' is relatively simple. It consists of 2-3 rings in the front and 5-10 cogs in the back. Using a shifting mechanism (a derailleur, adjusted with a shift lever), the chain hops up and down these two sets of sprockets to create different gear combinations (ratios). Contrast this with a car or motorcycle which uses a 5 (sometimes 6) speed internal gearbox in the front and a stationary cog afixed to the rear wheel (note that you do not have the ability to 'freewheel' on a motorcycle the way you can on a bicycle, since the sprocket is bolted to the wheel). This method uses planetary gearing to change the gear ratio, but also requires the use of a clutch to engage/disengage the flywheel. A clutch is not required on a bicycle; one advantage of this is that no power loss is experienced as in a car (try shifting from first to second in a car without putting in the clutch and you'll see what I'm talking about). Another is that it makes shifting that much simpler.

Bicycles are not restricted to external gearing, however. The old '3-speed' English bikes are an example of a bike with variable gearing which takes place inside the rear hub using planetary gearing. Recent developments in this area by companies like Shimano, SRAM, and Rohloff have put 5,7, and even 14 speed internal hubs on the market. Although they have not gained a huge following, they do have the advantage of being completely sealed and relatively maintenance free, having a cleaner look, and being able to shift while at a stop. New companies like Fallbrook Technologies are springing up as well with creative solutions to the shifting 'problem,' including a CVT hub called the NuVinci.

Now, since all of us here are DIY folk, we know that we have the advantage of being able to build stuff for ourself that mainstream manufacturers might not be able to get away with. Top end speed and motor size restrictions legally tie the hands of companies that would otherwise like to put out a faster, more powerful e-bike. We don't have that problem. But even for those companies, the use of gearing would allow manufacturers to get more bang out of the relatively small motors they are using.

It is (again) one of the laws of physics that a vehicle will only be able to go as fast as the horsepower propelling it. Wind resistance is the primary force working against propulsion. If you hunch lower on your bike and maybe put some fairings on, you will notice that you either don't have to work as hard to maintain a certain speed or you can go faster at a given level of exersion. But even with these techniques, you can only go so fast before the force of the wind stops you from going any faster.

You want your top-end gearing to coincide with the maximum hp that your motor is able to produce at max rpm. There is a somewhat involved formula used to determine this, but Walter Zorn has made it easy with his interactive webpage used to determine bicycle power and speed:

http://www.kreuzotter.de/english/espeed.htm

If you find that the gearing you have chosen allows you to travel 33.5mph, but you are only using a 250w engine, then you will find that you only have the horsepower to reach a top speed of 22mph. In this case, gearing down your bike allows for a wider range in gearing, which results in more torque at low speeds--something no one would complain about having.

The more gears you have, the wider a range you can have. Take a mountain bike for instance with a 42/32/22 up front and a 11-34 in the back. Low gear (22/34) produces a (gear inch)GI value of 17; High gear (42/11) would be 99. That's a pretty descent spread. Road racers who are more concerned with speed than low-end torque will typically run a 52-53/39-42 up front to increase GI and, thus, top speed.

So what possibilities does this create for DIY e-bike builders? Well, lots. Like I mentioned earlier, I'm working on a streetcycle made from bicycle parts and a 900w motor. I'm making it a simple 3-speed using an old sturmey-archer hub as a variable-jackshaft, but really i could run 2 or 3 of these for a 6 or 9 speed. if i wanted to I could combine that with a 6,7, or 8 speed rear freewheel and derailleur, or a triple chainring in the front. Too much gearing can get unnecessarily complicated, but having enough to maximize torque and top speed can really help you squeeze as much performance out of your little motor as possible so you can smoke those Harley dudes off the line...with a fraction of the horsepower.

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reikiman
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Re: a word about gearing

?? I've never seen a single-gear electric bicycle. The ones I've owned or built all had multiple gears.

But for practical matters the gears aren't that useful because the motor quickly gets you in a speed range where pedaling doesn't help. I think the gearing ratios need to be adjusted on an electric bicycle.

__________________

- David Herron, Green Transportation Examiner, Green Transportation Info, The Long Tail Pipe, Electric Race News, davidherron.com, 7gen.com, What is Reiki
- Charger bike (rebuilt), Electrified Electra Townie, 1971 Karmann Ghia

rpmarco
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Re: a word about gearing

You are referring to the bicycle's drivetrain...the power transferred from your body to the pedal to the rear wheel. This is typically a supplement to the motor on an e-bike, and is usually the same gearing you would get on any stock mountain or road bike (ie, 18sp, 21sp, etc.)

But this is not what I'm talking about. You will notice on most electric bikes (like this Currie, for example: http://www.electrikmotion.com/uspdezcruiser.htm; or Eric Peltzer's DIY project: http://www.peltzer.net/ebike/) that the gearing is stationary...you might have a 10-12t sprocket on the motor and maybe a 50t in the rear. This results in ONE fixed gear ratio, ie, not variable. This is the case also with hub motors (though Crystalyte does have a "2-speed" motor that tries to account for this, though I am not sure about its effectiveness)

Combining the multiple gearing on a bike's drivetrain with a multi-gear electric drivetrain is the idea for efficiency and performance. My purpose in posting was to question why no making electric bikes are doing this?

reikiman
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Re: a word about gearing

Combining the multiple gearing on a bike's drivetrain with a multi-gear electric drivetrain is the idea for efficiency and performance. Oooohhh... To be honest your post was a little on the long side and I only skimmed it.

The Charger bicycle has a drivetrain like you say. It has a Shimano Nexus internally geared hub on the rear, and both the motor and pedal chain go to that rear hub. I have one of these bicycles.

I oughta post some pictures, as I've taken quite a few of it.

Anyway the original design was a "pedelec" where the motor and controller act only when you pedal, and it's meant as a pedal assist. But I messed up the controller and ended up converting it to a regular controller. The goofy thing about this bicycle is that since both chains go to the same hub the freewheeling doesn't work when the electric motor spins. This made the pedals spin while the motor was spinning. That wasn't any good.

It took some doing but I learned there was a Schwinn bicycle from the 60's where the pedals had freewheeling. This was called the Front Freewheeling System (FFS) and I found a bicycle shop which specializes in used parts and they had one of these on-hand. It was a bit of a journey to get it on the bicycle, but now when the motor spins the pedal chain does spin but the pedals themselves do not spin.

__________________

- David Herron, Green Transportation Examiner, Green Transportation Info, The Long Tail Pipe, Electric Race News, davidherron.com, 7gen.com, What is Reiki
- Charger bike (rebuilt), Electrified Electra Townie, 1971 Karmann Ghia

rpmarco
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Re: a word about gearing

You're right...the problem you describe, I think, plagues folks in the DIY HPV world. Panasonic also used the FF system back in the 70's, kind of hard to find. I have been experimenting with double-freewheel designs, but don't really trust the setup. Checked out the Charger...that is more along the lines of what I am talking about. Thanks for mentioning it.

spinningmagnets
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Joined: 12/12/2007
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Re: a word about gearing

I agree about running a motor through a gear set. I am willing to move slower on a steep hill by using a smaller motor that operates the bikes gears, in order to have more hill torque. One thing I noted in reading these posts is that using a direct drive motor that is large enough that it easily handles steep hills also means it draws a lot of current, shortening range.

Here's a guy with a 3-wheeler that uses an internally-geared bike hub as a motor transmission (halfway down the post)

Three wheeled bike

Here's a wacky expensive e-bike, but I like the motor mounting (uses the bikes gears) with the double freewheel. If anyone finds out the part numbers for a double freewheel that pops onto a common motor shaft please post the info!

http://visforvoltage.org/forum/2058-ride-sun%3A-electrobike-solar-electric-bicycle

Frying Pigeon
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Joined: 12/25/2007
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Re: a word about gearing

I believe the cyclone system takes the approach of using a relatively small motor in the same chainline with the pedals, so you can use the rear derailleur to shift gears for both motor and pedals.

http://www.cyclone-usa.com/index.php

I had a homebuilt setup on my bike involving a scooter motor mounted over the front wheel. The front hub was a sram spectro S-7 internal geared hub. I think the motor overpowered it, because it started slipping in and out of gear. I was going to try a derailleur and a multigear freewheel, but about that time I got a brushed hub motor instead, and started working on upping voltage. I think the idea of a small motor driving gears is quite viable, but you need a strong setup or the motor will chew it up.

marcus
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Joined: 09/16/2007
Points: 19
Re: a word about gearing

the tandum (2 person bike) uses a free wheel bottom bracket.
you can order it from your local bicycle shop.

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