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[gittit]

Hello, all! 

I'm proud to say that I just completed my first electric motorcycle conversion two weeks ago!  By this point, I've had enough time to give it some good runs, and boy is it lame!

Predictably lame, I mean.  It's a six to one gear ratio with a motor who's peak load rpm is 3300.  The acceleration is fantastic, but a top speed of 40mph?

Don't get me wrong.  The bike's a blast to drive--especially living in downtown baltimore.  But all this time designing and building this conversion--and now riding it--there's one question that's been burning in the back of my mind:

Why not a transmission?  I know this isn't a revolutionary idea--many have already used it in their conversions--but I'm a little surprised to see that it isn't more popular.  I mean, I can understand the beginner wanting to keep it simple, which a direct drive system certainly is, but I'm a bit flabbergasted over how commercial electric motorcycle companies that are pioneering the industry refuse to design anything other than direct drive systems.

Am I missing something?  Sure, you can get 70-80mph with these Sepex/AC Motors with their high peak RPM, but it seems that running them that hard is just a waste of juice.  It seems to me that except for maybe the smallest of bikes, the trade-off of one battery for a tranny makes for an obvious choice.  Think about cruising at 70 while the motor is still in its efficiency "sweet spot."  And I'd think that with the torque curve of electric motors, you could get away with a 2 or three speed with still considerable performance enhancement while not adding much in weight and loss of space.  Let me know what you think.

If there's one reason it may not be so mainstream, it could be availability of such transmissions.  Google has failed me in finding anything relevant to a simple, self-contained 2/3 speed transmission.  Can anyone give me some direction or recommendations in this department?


Thanks

[Bogan]

you mention it has fantastic acceleration, but not a high enough top speed. Perhaps you should try changing the gear ratio to obtain the top speed you want, if this makes the acceleration too sluggish then you should investigate a transmision, as transmisions in electric bikes really only give better acceleration. With regard to why there are not many manufacturers making electric bikes with gearboxes, there are drivelin efficiency losses, added weight efficiency losses, and less space for batteries, and generally there is no great need for a gearbox anyway.
Hope this helps

[NewAge]

Because electrics have such broad powerbands all you really need to do is manage the motors power through a series of gears, automatically, let me briefly explain, say you're going to chain drive your project directly from the motor sprocket to the rear wheel sprocket and you calculated a ratio top speed of 70mph, it's going to be slow to get up to speed because your stretching all of the motors available torque over too broad of a range, on the other hand calculate a top speed of 30 and you have narrowed the torque band and the thing will be quick accelerating up to 30.

Are you with me? So now imagine that your motor gear ran to a jackshaft that had three gears of different sizes on the shaft, the motor would be geared directly to the largest of the three gears, the second gear slightly smaller and the third slightly smaller that the second. Now you have a second jackshaft running parallel to the first with the same gear sizes reversed on the second shaft and short chains linking the gears between the shafts with the smallest gear on the second shaft running to the rear wheel.

  ...  Don't worry, what you have essentially done is built an automatic transmission. Say you calculated your new three speed automatic trans to give you ratios of 30mph 45mph and a final drive of 70mph, in real life application it would work something like this, you accelerate from a standing start and your motor would be providing its torque to the lowest of the gear ratios, (the rest of the gears are freely spinning on the shafts) quickly reaching the ratios peak speed of 30mph it magically transitions the motors torque to the chain driving the second gear ratio which continues to deliver good torque up to 45mph upon which the torque load makes its last transition to the final drive and 70mph.

[mKlRivPwner]

Hey Newage.

I think I might need some clarification.  You're using one gear direct from the motor, but that leaves 2 gear ratios to run through.  Also, are these clutched gears?  Or are they "Magically' free-spinning too.

I'm not trying to be rude here, but I'm just a wee bit lost, and I'd really like to understand because it seems like a good way to incorporate a transmission into my own initial build without breaking the budget.

Maybe a rough visualization?

Thanks!

[frodus]

top speed is based on several things.

Voltage to the motor
Gear ratio
horsepower of the motor
weight and rolling resistance
frontal area


MOST of the reason we don't use transmissions, is because the HP of the motors used aren't enough to get much past 80/90 mph or so even WITH a tall gear ratio and high voltage. Frontal area/weight/rolling resistance are pretty much fixed, and as such, require a certain force to propel them to a certain speed.

So, no matter what voltage and current you give a motor, you're still up against the motor limitations. HP is HP, without enough of it, you'll never go any faster with different gearing.

Now, put a 60hp motor in there and you might get some benefits of a 2-speed, but its gonna be one heavy motor, and you'll need a lot of area in the bike for it..... at least with current motors.

[NewAge]

mKlRivPwner, sorry for the confusion, my fault, this thread was entitled "transmissions" and i kinda ran with it but essentually what i was proposing was more of a torque converter than a transmission. I totally agree with Frodus regarding his explanation of the use of transmissions on electric vehicles. That being said, i would like to add that i have had good results from incorporating a torque converter into my projects



The primitive picture above is based on a the application of a Perm motor running on 72 volts (producing around 3600 rpm max)
You can see the gear ratios displayed to the left and to the right is the max speed at the designated rpm.
This system was geared for a top speed of ~ 63 mph.
We know that the taller the gearing, the slower the acceleration (due to the torque band being stretched to thin over a wider spectrum)
We also know that lower gearing makes for quicker acceleration (at the expense of a lower top speed)

When i was using the term "magically", what i meant was that the transition of the torque load between gears is seamless and happens without you having to do anything other than twist the throttle. There is no clutch and the system is direct drive, just directed through channels (kinda like a automatic transmission but that is where the similarity ends).

Anyway, when you accelerate from a standing start the torque load is applied to the lowest point of resistance, which is the 1st gear sprockets at a ratio of 6.66:1 which should zip you up to ~ 35 mph or 2000 rpms pretty quickly before "magically transitioning the torque load to the 2nd gear with a ratio of 5:1, at this point the torque load is now being put entirely on the 2nd gear chain between the two 14 tooth sprockets (one on jackshaft 1 and the other on jackshaft 2), and now the chain connecting 1st gears 16 and 12 tooth sprockets are experiencing no real torque load and neither are the 3rd gear sprockets (that's why i said that the other gears are spinning freely, they are keyed to the shaft but they are experiencing no load).

Now you are zooming along in 2nd and then at ~ 2700 rpms the magic continues and the torque load makes its final transition to 3rd gear continuing to a top speed of ~ 63 mph at 3600 rpms.

It can be argued that the series of chains and gears present friction and drag that would nelegate any advantage  but i say no, it's way quicker 

I hope to have my final concept bike completed and displayed in the garage brew section of the forum by around March of next year. 

[frodus]

any pictures so far of your other CVT projects? of this project?

I'm interested in seeing back to back comparison of tranny and no tranny.....

[mKlRivPwner]

Do the gears have to be of the same sizes (in opposite orders), or can I run the same set up with different gears?

12    12     24     drive-shaft
       15     20
       18     15

for gearing of 6:1, 4:1  and 2.5:1

[NewAge]

Sorry frodus, no pictures at this time and everything is in storage because of my relocation, i figure by March next year i should be up and running again.

mKlRivPwner, you can put whatever gear combo you like on the shafts just as long as the number of teeth totals out the same for each gear (you will notice that in my example each gear combo had 28 teeth total). your example has 33, 35, and 36 which may require idler gears to take up slack where necessary.

man, march seems like a really long time 

[mKlRivPwner]

Thanks again!  Obviously you would recommend a shield over the torque converter.  What did you construct yours out of?

[MotoVolta]

Unless I am missing something major in your explanation, I don't think this is going to work.  Here's why.  The motor will determine the shaft speed of the shaft on the left and the speed of the right shaft will be determined by the speed of the left shaft and the gear ratio.  So hypothetically if the left shaft were traveling at 1000 rpm, the right shaft will want to spin at the following speeds based on the three gear ratios you have shown:

12:16, 750 rpm
14:14, 1000 rpm
16:12, 1333 rpm

Since all three sets of gears have chains connecting them they are all trying to drive the right shaft at the different speeds, which means there will be no movement at all, until two of the chains snap or one of the shafts break.  Chances are the motor doesn't have enough torque to cause either of those to happen so you'll just burn up the motor.  You need a way for the other two gear sets to slip while the one is active.

[NewAge]

Thanks for the input MotoVolta, you are absolutely correct. I took my drawing to John (the Shop Engineer), I explained the shafts and chains arrangement to him and he immediately said that it wouldn’t work for the exact same reasons that you sited. John told me that Steve (another Shop Engineer) had been working on a shifter transmission and that I should get him to explain it to me, I told John that Steve was the one that I had originally talked to before even posting to the forum, John and I both went over to see Steve and after a brief glance at my drawing Steve said that I had misrepresented the concept and that there was much more to it than that. Then Steve asked me if I could peel a banana with my feet… He and John both laughed. 

I learned a lot today, like when you’re the fabrication guy, stick to being the fabrication guy, and leave the engineering to the Engineers.

Oh, and if you’re going to post to the forum make sure that you have all of your facts straight first.

Thanks again MotoVolta, and apologies to anyone on the forum that I may have mislead.

Keep an eye out for my project in the Home Brew Section 3/10. 

[mKlRivPwner]

Would it work if the gears were all on one-way bearings similar to how a bicycle's rear wheel connects to the gears?

In first, all three gears on jackshaft one will spin at the same rate.  On the seoncd jackshaft, the largest gear (gear 1) is being driven, but the others, trying to spin faster, aren't 'caught' and so can spin freely.

In second, gear 2 on jackshaft 1 grabs the torque and so gear 2 on jackshaft 2 is driven.  So gear 1 on jackshaft 2 is now 'pushed' by gear 2, so gear 1 on jackshaft 1 is free spinning.  Gear 3 on jackshaft 1 is 'pushed' by gear 2, so gear 3 on jackshaft 2 is also freespining.

When the torque is shifted to third gear, on jackshaft 1, gears 1 and 2 will be free spinning (by virtue of the speeds of their counterparts), and all three gears on jackshaft 2 will be 'pushed' at the same rate.

Here's an example:

Motor     Jackshaft 1     Jackshaft 2      rear end
12           24                   NA (direct drive shaft)
              9 (1a)              27 (1b)
             12 (2a)             24 (2b)
             16 (3a)             20 (3b)

Gives three gear ratios of 6:1, 4:1 and 2.5:1

        | Motor speed
Gear  |1st @ 2000 |2nd @ 2700|3rd @ 3600
1a/1b| 1000/333 | 2025/675  | 4320/1440
2a/2b| 1000/500   | 1350/675 | 2880/1440
3a/3b| 1000/800   | 1350/1080 | 1800/1440

As you can see, each gear will be travelling at least as fast as the driven gear.  And so there will always be one set that is driven, while the higher gears are freespinning, and lower gears will be 'pushed' with their counterparts freespinning.
In fact, gears 1b and 3a do not need one-way bearings.

So as long as the torque does in fact transfer, the shifting will occur without lock-up.

[frodus]

except there needs to be some sort of clutch on gears 2 and 3 so as 1st gear tops out, it engages 2nd and spins faster than 1, then when 2 tops out, gear 3 engages and 3 spins faster than 2 and 1.

This is how RC Car 2-speed transmissions work, as well as some go-kart transmissions work.

Its a lot of hardware, and its actually a little more simple to get a CVT from Comet that can handle the torque, or if manual shifting is ok, modify a motorcycle transmission to work.

[MotoVolta]

No worries NewAge.  Keep us up to date on your progress.  I am definitely interested to see what you come up with for a transmission design.  Good luck with the build.