PDA

View Full Version : Brammo Shifts Gears (article)



Brammofan
15 June 2011, 1316
I highly recommend reading:
Brammo Shifts Gears (http://www.motorcycle-usa.com/750/10144/Motorcycle-Article/Brammo-Shifts-Gears.aspx) by Mark Gardiner which was recently posted on the Motorcycle-USA.com site.
An excerpt:

So what gives? Do EVs need gearboxes or not? And if they do, is it logical that we go from a single speed to six? To get up to speed on the engineering case for a gearbox, I called two old friends: James Parker, an iconoclastic freelance motorcycle designer who I happen to know has put a lot of thought into gearboxes for EVs; and Lennon Rodgers, a Massachusetts Institute of Technology PhD. candidate who's leading the school's TT Zero project. Lennon then referred me to Charles Guan, another MIT brainiac for a crash course in electric motor theory. Finally, I called Brammo CEO (and fellow Kansas Citian) Craig Bramscher.

It's not your usual tripe. Gardiner does his research.

frodus
15 June 2011, 1358
I like the "I suppose if you were the kind of guy who sees the battery as half-drained" versus "If you're inclined to see the battery as half-charged" thing, that was a nice touch.

Allen_okc
16 June 2011, 1015
im pretty sure all of us here see the battery as half full... but i get what they are saying...

Thank You Brammofan, very interesting article...

jazclrint
16 June 2011, 1235
That was a very enlightning article. My question is, they say that a properly geared 2 speed transmission was really hard to make. So what were the 2/3 speed gearboxes that folks on this forum came up with geared like?

Nuts & Volts
16 June 2011, 1253
Great article!

If I made a two speed I would have first gear in the range of, 1:1.5 to 1:1.9. Then second gear would be 1:1.

frodus
16 June 2011, 1424
That was a very enlightning article. My question is, they say that a properly geared 2 speed transmission was really hard to make. So what were the 2/3 speed gearboxes that folks on this forum came up with geared like?
who made a 2/3 speed?
I know brutus made a 6-speed right?

The rest are more concept than anything, IIRC.

podolefsky
16 June 2011, 1638
Great piece. I totally agree with the "need" vs "want". Anti-transmission people have been using the "don't need" argument to say there's no justification for "wanting" a transmission.

There doesn't have to be just one answer to the transmission question. I like that this article actually explains both sides, and does it very clearly.

GUFF
16 June 2011, 1826
Really good article! Thanks so much for posting this Brammofan!

However, I after reading it, my mind is still on the no tranny side of things. I would much rather have a bigger motor instead of a smaller one + transmission. The more I dig deeper into electric motor theory and controller theory the more I realize that we haven't even touched the tip of the ice berg yet with those components. I think it is even more pronounced when you talk about batteries.

That said, during a few of my earlier rides I found myself wishing I had a clutch during certain traffic situations. I really have to focus sometimes to keep myself out of trouble and not do anything stupid because of the low end power available. I am getting better with that part of the mental game so no big deal, yet....

podolefsky
16 June 2011, 1854
What about a bigger motor and transmission? It's not an either-or is my point.

mpipes
16 June 2011, 1921
What about a bigger motor and transmission? It's not an either-or is my point.

That's my plan as of right now, using the AC-18 I snagged from Travis before you could get to it :) Though I'll probably start with a single speed to get things on the road faster then work out another drivetrain later.

GUFF
16 June 2011, 2005
What about a bigger motor and transmission? It's not an either-or is my point.

I would rather have an even bigger electric motor! :)

I think this conversation could go on for ever. I guess it boils down to personal preference.

cycleguy
16 June 2011, 2027
Good article, I'm with GUFF though, I don't buy the transmission argument, and believe that electric motors are far from fully optimized. More refined designs like the Motoczysz, Lightning or Mission R have great performance and don't use a transmission, there is a reason why they don't, and it's not because it never crossed their minds, or that they don't have the capability of producing one.

A transmission on an electric motor simply solves a problem that doesn't exist.

The perceived problem is that the motor doesn't produce enough torque for adequate acceleration when geared for the desired top speed, and the solution is to add a transmission to boost acceleration at low speeds.

The real problem is that the motor doesn't produce enough HP to achieve the desired top speed when geared for adequate acceleration, the solution is to increase the motors HP to achieve higher top speeds.

Both solutions may seem like they achieve the same thing, that's why the article, and many here somewhat accept both as viable options, but only one is the correct solution.

Electric motors are torque rich and Horsepower poor, an ICE is HP rich and torque poor, so adding a transmission to an ICE is a perfect engineering solution to improve it's weakness. Adding a transmission to an electric motor does nothing to improve it's weakness. We need to work towards improving electric motor HP, once that's done, it will always produce enough torque to make a transmission unnecessary.

Let's compare an ICE and an electric motor of equal HP, 130HP Ducati 848 with last years 130HP(ish) Motoczysz.
The 848 has 72lb/ft engine and 850 lb/ft to the rear wheel in 1st gear.
The Motoczysz has 250 lb/ft motor, and 1000 lb/ft rear wheel at a fixed 4:1 ratio.
Both bikes would achieve the same top speed at that ratio, but the Motoczysz has 150lb/ft more torque to the rear wheel despite having no transmission.

podolefsky
16 June 2011, 2032
I would rather have an even bigger electric motor! :)

I think this conversation could go on for ever. I guess it boils down to personal preference.

Yup - and it does, over and over...

I can't really state my personal preference, since I've only ever had a single speed bike. All I can really do is compare how it feels when I've geared it two different ways...but that's just the ol' butt dyno.

I have to admit, there is something kind of pure and enticing about the single speed. I have a single speed mountain bike too, and it's just fun to ride and not think about shifting (it's also really friggin exhausting).

Also, just to wax even more philosophical for a sec...even if in the end the transmission does win out on performance, there's this notion that ICE vehicles need a transmission to make up for deficiencies in the technology. It is a good challenge to try and take advantage of the things you can do with an electric drive, as if to say we don't need no stinkin' transmission.

podolefsky
16 June 2011, 2057
Good article, I'm with GUFF though, I don't buy the transmission argument, and believe that electric motors are far from fully optimized. More refined designs like the Motoczysz, Lightning or Mission R have great performance and don't use a transmission, there is a reason why they don't, and it's not because it never crossed their minds, or that they don't have the capability of producing one.

A transmission on an electric motor simply solves a problem that doesn't exist.

The perceived problem is that the motor doesn't produce enough torque for adequate acceleration when geared for the desired top speed, and the solution is to add a transmission to boost acceleration at low speeds.

The real problem is that the motor doesn't produce enough HP to achieve the desired top speed when geared for adequate acceleration, the solution is to increase the motors HP to achieve higher top speeds.

Both solutions may seem like they achieve the same thing, that's why the article, and many here somewhat accept both as viable options, but only one is the correct solution.

Electric motors are torque rich and Horsepower poor, an ICE is HP rich and torque poor, so adding a transmission to an ICE is a perfect engineering solution to improve it's weakness. Adding a transmission to an electric motor does nothing to improve it's weakness. We need to work towards improving electric motor HP, once that's done, it will always produce enough torque to make a transmission unnecessary.

Let's compare an ICE and an electric motor of equal HP, 130HP Ducati 848 with last years 130HP(ish) Motoczysz.
The 848 has 72lb/ft engine and 850 lb/ft to the rear wheel in 1st gear.
The Motoczysz has 250 lb/ft motor, and 1000 lb/ft rear wheel at a fixed 4:1 ratio.
Both bikes would achieve the same top speed at that ratio, but the Motoczysz has 150lb/ft more torque to the rear wheel despite having no transmission.


I basically agree, except I don't think there is only one solution. A top fuel dragster has one speed. A Porsche 911 has 6 speeds. A Honda Civic has 5 speeds. They're different solutions to different problems.

My cordless drill has a 2-speed transmission. :D

DaveAK
16 June 2011, 2118
Good article, I'm with GUFF though, I don't buy the transmission argument, and believe that electric motors are far from fully optimized. More refined designs like the Motoczysz, Lightning or Mission R have great performance and don't use a transmission, there is a reason why they don't, and it's not because it never crossed their minds, or that they don't have the capability of producing one.

A transmission on an electric motor simply solves a problem that doesn't exist.
You're thinking in the purest sense though. This is primarily a DIY forum with people on limited budgets. We're not developing new electric motors, and we can't afford (or even obtain) the motors (or batteries) that some of these guys are working with. All I'm interested in is whether motor A is better or worse with a transmission in my application, or whether motor B which represents a similar over all value would be a better solution. That's a problem that does exist. Ultimately whether MotoCzysz is right in not having a transmission or Brammo is right in pursuing a six speed box is a matter of design philosophy and application. I wouldn't say that if one was right the other was necessarily wrong.

markcycle
17 June 2011, 0531
Good article, I'm with GUFF though, I don't buy the transmission argument, and believe that electric motors are far from fully optimized. More refined designs like the Motoczysz, Lightning or Mission R have great performance and don't use a transmission, there is a reason why they don't, and it's not because it never crossed their minds, or that they don't have the capability of producing one.

A transmission on an electric motor simply solves a problem that doesn't exist.

The perceived problem is that the motor doesn't produce enough torque for adequate acceleration when geared for the desired top speed, and the solution is to add a transmission to boost acceleration at low speeds.

The real problem is that the motor doesn't produce enough HP to achieve the desired top speed when geared for adequate acceleration, the solution is to increase the motors HP to achieve higher top speeds.

Both solutions may seem like they achieve the same thing, that's why the article, and many here somewhat accept both as viable options, but only one is the correct solution.

Electric motors are torque rich and Horsepower poor, an ICE is HP rich and torque poor, so adding a transmission to an ICE is a perfect engineering solution to improve it's weakness. Adding a transmission to an electric motor does nothing to improve it's weakness. We need to work towards improving electric motor HP, once that's done, it will always produce enough torque to make a transmission unnecessary.

Let's compare an ICE and an electric motor of equal HP, 130HP Ducati 848 with last years 130HP(ish) Motoczysz.
The 848 has 72lb/ft engine and 850 lb/ft to the rear wheel in 1st gear.
The Motoczysz has 250 lb/ft motor, and 1000 lb/ft rear wheel at a fixed 4:1 ratio.
Both bikes would achieve the same top speed at that ratio, but the Motoczysz has 150lb/ft more torque to the rear wheel despite having no transmission.

I agree with this and you said it very well. The problem with electric motors is HP It just takes a lot of HP to go fast and no gearbox is going to increase HP, given there is enough voltage available.

If you're voltage limited or at Max RPM but not thermally limited then a gearbox might help, six speeds from an engineering point of view is ridiculous, but maybe not from a marketing point of view.

One thing six speeds does do is ease the load on the controller and batteries as the motor will be at or near the voltage limit almost all the time.

DRZ400
17 June 2011, 0559
I like the idea of a gearbox & clutch on dirt bikes and super motards....if only for the wheelie factor!

sparky_mark
17 June 2011, 0755
Here's a question for everyone that has built their own single-speed electric bike : What's your top speed, and how much HP do you need to get there?

sentinel
17 June 2011, 0819
How do you calculate the HP on these EV bikes?

podolefsky
17 June 2011, 0840
Here's a question for everyone that has built their own single-speed electric bike : What's your top speed, and how much HP do you need to get there?

My top speed is about 70 mph. It takes around 9 HP. My bike will actually produce over 30 HP, but not at 4000 RPM. I'm RPM limited with a 4:1 gear ratio. I could go to 2:1 and go 100 mph, but acceleration would suck (I know, I've tried just going to 3.25:1 and it's no fun).


How do you calculate the HP on these EV bikes?

The simplest way is HP = voltage * current * efficiency / 746. 746 is to convert from Watts to HP.

Example: 68V * 450A * 70% efficiency / 746 = 32 HP.

(68V is because of sag at 450A load. 70% efficiency is also because efficiency drops with that much current...my motor is more like 80% typically).

frodus
17 June 2011, 0937
but since efficiency is on a curve, you need to have a torque curve showing the eff at that RPM.

Most reliable way is a Dyno, which none of us "currently" have.... lol... waiting for it Noah!

sparky_mark
17 June 2011, 0951
My top speed is about 70 mph. It takes around 9 HP. My bike will actually produce over 30 HP, but not at 4000 RPM.
Do you mean that it needs 9hp to get you to 70mph, or that it needs 9hp to keep you at 70mph?

podolefsky
17 June 2011, 0957
Do you mean that it needs 9hp to get you to 70mph, or that it needs 9hp to keep you at 70mph?

Technically, both. 9 HP will keep you at 70 mph, and if you're willing to wait a while it will get you there too.

I also have an electric scooter. It makes about 4 HP peak, it'll go 50 mph.

podolefsky
17 June 2011, 1005
but since efficiency is on a curve, you need to have a torque curve showing the eff at that RPM.

Most reliable way is a Dyno, which none of us "currently" have.... lol... waiting for it Noah!

True. I have that curve. Assuming the data is right, my D&D is 65% efficient at 480A and 1800 RPM, where it makes peak power of about 30 HP. This also depends on the fact that my battery pack sags to 68V at 500A. In D&D's data, their pack sags to 58V at 480A, so they only get 25 HP out of the motor.

Anyway, bottom line is - it's complicated, but volts*current*efficiency is in the ballpark. It's also bhp (at the motor), not wheel HP...

Speaking of that dyno...yeah...I'm working on it.

sparky_mark
17 June 2011, 1008
Technically, both. 9 HP will keep you at 70 mph, and if you're willing to wait a while it will get you there too.
I think I see what you mean.... Are you saying that your bike uses 9hp to overcome rolling and wind resistances at 70mph, and also makes 30hp peak but only at lower rpms?

podolefsky
17 June 2011, 1018
I think I see what you mean.... Are you saying that your bike uses 9hp to overcome rolling and wind resistances at 70mph, and also makes 30hp peak but only at lower rpms?

Exactly. HP increases to a peak at mid RPM, about 2000 on my motor, then starts to fall off again. Top speed is where the power required to overcome air and rolling drag equals the HP that the motor can put out. If you gear lower, you get back down into a higher HP band and can go faster (but get slower acceleration from a stop).

sparky_mark
17 June 2011, 1026
If you gear lower, you get back down into a higher HP band and can go faster (but get slower acceleration from a stop).
And if you had a transmission, you would have both good acceleration and good top speed :)

One of the things that has always annoyed me about the whole 'transmissions are good/bad' argument is that people would say things like "you pick your drive ratio to give good acceleration or good top speed. you can't have both". These same people would then go on to say that electric bikes don't need a transmission! The two statements clearly contradict each other to some degree. Obviously your top speed will always be governed by your peak hp.