# BMW clone/w sidecar conversion



## Geoff Keyes (Dec 14, 2007)

I have an existing bike, a Chang Jiang, sort of a BMW clone. It's actually one of the family of bikes, like the Ural, derived from the BMW '38 R71. It currently runs about 36HP, since it's all steel it weighs in around 700lb. So in this case it's more of a retro-fit (retro-bike).

What is a reasonable expectation for range? 
What kind of HP requirement would you expect? 
Is there some kind of rule-of-thumb for how much HP you need to recharge a given battery pack, in a series hybrid configuration?

For that matter, what are the questions that I'm not asking that I should be asking?

This looks like it's gonna be fun.

Geoff


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## Geoff Keyes (Dec 14, 2007)

> Geoff, I'm sure you will be able to match the performance of the original set up. Most likely you will run somewhere between 48, and 72 volts? This would be the range I would think would work well for you.
> 
> Since you are thinking Hybrid, range will not be a problem, but as you want as much as possible, without running a motor I assume, I would think 72 volts with six batteries in the 55AH range would be acceptable in weight (about 250lbs) , and performance. I can only guess, but I think with reasonable riding you could see 40-50mi. pretty easy.
> 
> ...


Performance, if you can call it that, is a top end of 65 (HA!) and a happy cruising speed of 35-45. It's more of a tractor than a motorcycle. OTOH, around the back roads where I live, that's just fine. I'd like to be able to keep the current speed capabilities and a range of 50 to 75 miles, in a pure electric mode.

Since this is a shaft drive machine, does that help me or hinder me? I'd like to do as few mods on the bike as I can, so if I can use the existing rear end and final drive, I'd like to go that route.

I'm sure that more questions will come up, but thanks for all the help so far.

Geoff


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## Geoff Keyes (Dec 14, 2007)

I think that I have a basic mis-understanding about electric systems. Is a 48v system 48 volts worth of batteries, or is it some mass of batteries configured to produce 48 volts? This would be a difference between voltage and amperage?

Where do the differences in range and top speed come from? Am I even speaking english, or does my question make no sense?

On a configuration note, the BMW style bikes use a uni-case transmission and a ring and pinion final drive. Can I do away with the tranny and connect the motor directly to the shaft to the final drive? If I can, that makes that part of the build pretty simple.

More questions later, I'm sure.

Geoff


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## rbgrn (Jul 24, 2007)

Look into series and parallel wiring. That will hopefully clarify.

48v means that somehow, someway, the battery pack is wired up to produce 48 volts. This can be 4 12v batts in series or 8 6v batts in series. You could take 2 48v packs, wire them in parallel and still have 48 volts but twice the amperage 

Amps, Volts, Watts, Watt-Hours 101:

I'm going to use a water pipe analogy. 

Volts are the speed of the water moving down the pipe. 5mph? 10mph? Those are both speeds. They do not specify "how much" water, just "how fast."

Amps are a little trickier to explain but think of it as a width of a pipe at any given moment. Amperage changes based on load, but in my analogy, it's like a pipe expanding to get the amount of water needed through. Amps also don't specify "how much" water, but "how wide"

So, you have "how fast" (volts) and "how wide" (amps). What's that give you if you multiply them? "how much" (watts). A watt is how much power is being used at a given moment. It does not, however, say how much power is used in total. It's just at a given moment. If you want to measure the "Reservoir" of water, you need watt-hours. Those are like gallons of water. 

So, I hope this clears up some basic electricity physics. 

On to series and parallel wiring applied to batteries.

Batteries in series add voltage but don't change amperage.

Batteries in parallel add amperage but don't change voltage.

That means:

2 6v 10ah batteries in series make for 12v 10ah.
2 6v 10ah batteries in parallel make for 6v 20ah.

It's actually the same amount of watt-hours (multiple volts times amp-hours for watt-hours).

Top speed is going to come from
1) The capabilities of your motor
2) The output capabilities of your controller
3) The current (amp) capabilities of your batteries
4) Gearing, any other tweaks.

Range is mostly how many amp-hours you have. For really simple math, think: If my car pulls 60 amps to maintain 70mph, and my batteries (all series) have 60 amp-hours, I can drive for 1 hour at 60mph. Of course, you'd never want to drain your batteries all the way down like that as the undervoltage would damage them but that's the gist of it anyway.

Configuration:

Whether to directly connect or not really depends on gear ratio. Each motor has an rpm range that's usually 0-10000 or somewhere around there. Take a look at the torque curve for your motor to find the max efficient rpm. That's going to be your top speed for your bike (as far as the motor knows) if you only have one gear, which is direct-connect.

What you need to do is calculate out the ratio of the motor output shaft to the final drive to the tire diameter to know the ratio you need. Sometimes you can get different sized gears for the final drive, sometimes you can make bigger/smaller output shaft gears. It depends.


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## Geoff Keyes (Dec 14, 2007)

The final drive ratio is 3.89:1, wheel size is 19 inches. Max rpm is about 5000, the spec sheet says 4700-5300 rpm.

So if I understand the battery issue correctly, I can have a single series battery pack of, say 48v with Xah, or several series packs in parallel, with 48v producing X + (the number of packs)ah. So 2 48v 10ah packs in parallel produce 48v 20ah, correct?

There must be a point of diminishing returns in here somewhere.

I'd like to get top speed of 50-60 mph (to be safe on the main roads) and a range of 50-75 miles. These are pure blue sky numbers at this point. The range could be adjusted down, top speed is a bit less flexible.

Is there a rule-of-thumb to get a ballpark spec for numbers of battery's, or is this a build it and see project?

I think for the prototyping I will remove the car from the sub-frame and place the battery's and the controller in a home-built box on the sub-frame, with the idea of doing a clean installation beneath the car (or a cargo box, which might be more usefully anyway) at the end.

What motor, controller, and battery's should I be looking for?

Thanks again,

Geoff


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## rbgrn (Jul 24, 2007)

Using the range estimation formula from the wiki (70 = 250 * kwh / 700^0.6)..

That gives me 70 = 250 * kwh / 50.93
which led to 3565.1 = 250 * kwh
which gave 14.26 = kwh, which sounds about right for a motorcycle (anyone disagree?)

If you choose a 48v pack, you will need 297ah total. That sounds like a lot to me... If you go to a higher voltage you can cut that number down quite a bit. It's all the same number of batteries though in the end. 

You were right about the calculations and the only diminishing returns are from wire resistance, which is negligible in most cases.


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## BenNelson (Jul 27, 2007)

You should try to do at least 48Volts for your system.

In general, more volts = more speed.

My motorcycle is currently 36 volts and has a top speed of about 38 MPH.

I plan on adding another battery in there still, but I am limited by the frame of the cycle.

If you have a large frame, can weld or otherwise modify the frame, or put batteries in a sidecar, go for a 72 volt system.

Guys who do 72 volt systems can typically go freeway speeds.

Here is some information on what I did for my motorcycle:
http://web.mac.com/benhdvideoguy/iWeb/cycle/Welcome.html

Hope it helps


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