A transmission is the solution ;-)

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Johnny J
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jdh2550_1
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Re: A transmission is the solution ;-)

But what's the question? ;-)

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Mik
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Re: A transmission is the solution ;-)

jdh2550_1 wrote:

But what's the question? ;-)

"How many road must a man walk down....." ;-)

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Johnny J
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Re: A transmission is the solution ;-)

Seriously John, how can "one of the most frequently quoted tech pundits in the world" believe what he writes about efficiency and transmissions?

jdh2550_1
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Re: A transmission is the solution ;-)

Johnny J wrote:

Seriously John, how can "one of the most frequently quoted tech pundits in the world" believe what he writes about efficiency and transmissions?

Don't ask me, I didn't write it!

Although I do think multi-speed transmissions will start to play an important part in EV packages. Either that, or more use of field weakening to improve the torque vs. speed trade-off (I think antiscab said the Vectrix uses field weakening?).

Both approaches are in a few solutions right now so it's not new. I just think we will see it in many more as things progress. Just my 2 cents worth.

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Johnny J
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Re: A transmission is the solution ;-)

I thought the whole article was a direct quote from you ;-)

The Sevcon in my scooter supposedly has field weakening, in a couple of weeks I will go to a company nearby that has a programming unit and see if we can tweak it a little. Would like a little bit higher top speed, right now it´s around 60 miles/h.

LeftieBiker
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Re: A transmission is the solution ;-)

I still think that EVs need some kind of actual transmission, either a CVT, or a simple mechanical two or three speed that's light and small. My first "bike" was a Heathkit "Boonie Bike" and it had a two speed "jackshaft" type transmission that was about the size of an empty paper towel roll. When the outer shaft was immobilized (in this case by a cable end coming out to catch and stop a simple tab) it was in Low. If the outer shaft was allowed to spin, it was in High. It handled the 5HP 4 stroke engine on the bike easily, and you could even upshift while moving at low speeds. Downshifting would have snapped the cable end, but that was hardly the only possible way to immobile the outer shaft! A simple braking band on the outer shaft should allow downshifting. Have you manufacturers or backyard mechanics ever seen one of these transmissions?

MikeB
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Re: A transmission is the solution ;-)

There's been some huge long discussions about the usefulness of transmissions for electric vehicles. The first observation is that the torque and efficiency curves of an electric motor are much flatter than a combustion engine, so a transmission isn't mandatory. The next observation is that those curves are not perfectly flat, so there's room for improvement, and a transmission might offer some small improvements, especially at the very bottom or very top of the motor's operating range. The final observation is that while there's room for improvement, a transmission adds weight and cost to the system, and you might be better off simply adding that same weight and cost back into the batteries/controller/motor.

It becomes a fine line of optimizations and tradeoffs, are you better off with a 10% of your weight in a transmission, or 10% larger everything else. What if you can find a transmission that only adds 9.5%? Can you reduce the size of your transmission slightly by offering only 2 or 3 gears? Or does offering fewer gears reduce the overall efficiency gain over your ride profile such that the transmission is no longer worth it's weight? There will never be a final answer, since the tradeoff effects are small and the components are changing efficiencies as technology improves.

Personally, right now, with the speed profile of my commute, and the technology in my Current bike, I think a small 2-speed transmission would be a net gain. The bike could use some extra torque when the motor is spinning slow, that's my biggest remaining complaint about the bike. (Actually, the motor could provide the torque, but the controller can't handle the higher current levels, so the tradeoff is between a transmission and a more expensive controller) Of course, using a hub motor makes a transmission much harder, since there's no drivetrain to speak of where the transmission could be inserted. But changing away from a hub motor would be a significant redesign, and also reduce the space available in the body for batteries, so it's no longer even the same bike anymore, and all the numbers would have to be reevaluated.

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Derby
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Re: A transmission is the solution ;-)

MikeB wrote:

It becomes a fine line of optimizations and tradeoffs, are you better off with a 10% of your weight in a transmission, or 10% larger everything else. What if you can find a transmission that only adds 9.5%? Can you reduce the size of your transmission slightly by offering only 2 or 3 gears? Or does offering fewer gears reduce the overall efficiency gain over your ride profile such that the transmission is no longer worth it's weight? There will never be a final answer, since the tradeoff effects are small and the components are changing efficiencies as technology improves.

Considering the weight of the battery pack, I can't imagine the small transmission required for a scooter would be close to 10% of the total bike weight. Having said that, I think some of the appeal would be lost to those bike novices who were raised with automatic transmissions in their autos.

MikeB wrote:

Personally, right now, with the speed profile of my commute, and the technology in my Current bike, I think a small 2-speed transmission would be a net gain. The bike could use some extra torque when the motor is spinning slow, that's my biggest remaining complaint about the bike. (Actually, the motor could provide the torque, but the controller can't handle the higher current levels, so the tradeoff is between a transmission and a more expensive controller) Of course, using a hub motor makes a transmission much harder, since there's no drivetrain to speak of where the transmission could be inserted. But changing away from a hub motor would be a significant redesign, and also reduce the space available in the body for batteries, so it's no longer even the same bike anymore, and all the numbers would have to be reevaluated.

So when all is said and done, would it really be worth it?

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LeftieBiker
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Re: A transmission is the solution ;-)

Some of the jackshaft-type transmissions like the Comet are automatic, using two or three centrifugal clutches to shift from Low to High and back.

LeThala
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Re: A transmission is the solution ;-)

MikeB wrote:

.

Personally, right now, with the speed profile of my commute, and the technology in my Current bike, I think a small 2-speed transmission would be a net gain. The bike could use some extra torque when the motor is spinning slow, that's my biggest remaining complaint about the bike. (Actually, the motor could provide the torque, but the controller can't handle the higher current levels, so the tradeoff is between a transmission and a more expensive controller) Of course, using a hub motor makes a transmission much harder, since there's no drivetrain to speak of where the transmission could be inserted. But changing away from a hub motor would be a significant redesign, and also reduce the space available in the body for batteries, so it's no longer even the same bike anymore, and all the numbers would have to be reevaluated.

I agree about the need for a little more torque at low rpms but I'd rather have bigger mosfets in the controller than the mechanical complexity of a transmission. I personnaly don't need any more speed on the high end. I top out at 60 MPH (a little less on a garage) which is plenty for the city driving I do. With traffic lights and traffic it takes me 45 minutes dor-to-door coming home and 35 minutes riding in so I'm rarely maxing out on speed. I see a scooter as primarily an urban means of transportation and therefore I think any tradeoffs should be in favor of stop and go environments.

strawhistle
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Re: A transmission is the solution ;-)

If I know physics the way I think I do , any complex of extra moving parts will mean extra friction "heet" that in itself will lower the amount of speed to distance ! for starting, The wheel uses the most amprage just to get rotation ,once the rotation = over 3mph the amprage drops rapidly till wind resistance balances out with the torque !
John,
I don't know all the facts , but the idea is good enough to spark a good bit of advertizing for your new super ebyke!!! Let them Ponder what they will !!! LaTeR

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jdh2550_1
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Re: A transmission is the solution ;-)

@Mike:

I don't think transmissions will be worth it strictly from an efficiency standpoint (the relative flatness of the curve demonstrates that). It's all about widening the operational range to give the riders what they want. Being able to separate motor speed and wheel speed also expands the choices of electric motors and associated control and winding patterns.

@LaThala & @Mike:

But if we *just* went with bigger mosfets we would hurt efficiency. Bigger controller = more current flowing at low RPM = more heat = less efficiency.

@Leftie & @Strawhistle:

In general automatic transmissions are inefficient (CVT etc.). (Modern automobile automatic trans are now as efficient - but they are also amazingly complex!) When we're already dealing with limited range we can't "afford" to waste energy in the transmission.

@Johnny:

Naw, I didn't write it! But I smiled when I read it!!

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LeftieBiker
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Re: A transmission is the solution ;-)

"@Leftie & @Strawhistle:

In general automatic transmissions are inefficient (CVT etc.). (Modern automobile automatic trans are now as efficient - but they are also amazingly complex!) When we're already dealing with limited range we can't "afford" to waste energy in the transmission."

I don't actually think an automatic shift transmission is necessary - I was just noting that automatic jackshaft two-speeds exist. I personally think a manually shifted two-speed jackshaft (for geared motors, not hubmotors) would be a good idea. For hubmotors, I wonder how hard it would be to add a small motor in front of the rear wheel, with a chain and sprockets, to help get the bike to 10-15MPH and then shut off and freewheel. You'd lose a bit of range to friction and weight, but if you could get 'jackrabbit starts' I think your customers would willingly make the tradeoff. As it is, hubmotors seem stuck with that moment or three of weakness right off the line...

MEroller
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Re: A transmission is the solution ;-)

I preferred the bigger Mosfets, HAVE jackrabbit capabilty from standstill, and efficieny seems basically unhampered by that as it appears the bigger MOSFETs are more effcient when the current drops at higher speeds :-)

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LeThala
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Re: A transmission is the solution ;-)

jdh2550_1 wrote:

But if we *just* went with bigger mosfets we would hurt efficiency. Bigger controller = more current flowing at low RPM = more heat = less efficiency.

Not more current all the time. Just a 3 second burst to get things going from a dead stop. Then the controller can lower the mqx current.

LCJUTILA
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Re: A transmission is the solution ;-)

Hi All,

I think a planetary gearset like on an old English Three-speed bike would work perfectly. It could be built directly into the hub and has no clutches. You shift by selecting the gear and then coasting briefly for an upshift into a higher gear and can down shift without coasting. It is operated by a thumb operated switch located on the handlebars.

I don't believe it would be that expensive since it has been used on a low cost bicycle.

It sure would be fun to run it up through the gears or you could chose to leave it in the gear that suits your riding best.

The only problem would be trying to keep the front wheel on the ground when taking off in first!

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