EtekChopper: Drivetrain Design and Parts Selection

Time to get down to the nitty gritty and make some drivetrain decisions. I need to shuffle around the basic variables of my system in order to finalize the specifications and start buying parts. Decisions will be bold and underlined.

So far all I know is I am using a Sevcon Gen 4 motor controller  but I still need to choose a battery voltage, design or pick a battery pack, choose the motor, choose the gear ratio, etc. The constraints I'm working with are those of the Sevcon Gen4 controller, the space in my motorcycle, and the tug-of-war between performance and my wallet, among other things. 

The Sevcon Gen 4 wants a 72-80 Volt system. 72 Volts seems to be a standard voltage for things, so I will run a 72-Volt system. 
What does this mean? My battery will be a 72V  nominal. Assuming I'm going with 3.3 volt LiFePO4 cells, I am running a "24S" system, meaning 24 3.3V cells in series. Battery systems made up of smaller cells are usually defined by "XsYp", or X in series, Y in parallel. Parallel batteries allow for increased charge capacity and greater allowable output current. The more cells your have in parallel, the less electrical stress there is on one cell. 

Here is a CB750 frame solidworks model I found online, a motor for show, and four  A123 12S8P battery packs with included Battery Management Systems (BMS) that my friend Dane has access to and may give me. The BMS balances the charges of the many small 3.3V batteries to ensure the entire pack is healthy. Issue is, they are HUGE. I don't think I can fit more than two of them (for 12S8P) comfortably. I can probably fit the other two (12S16P) on sides, or as saddlebags. Maybe? Hmm. 

Or I can make my own custom 24S battery pack with some of the infinite 3.3V A123 cells, maybe 10P? That's 240 cells. Adding more cells in parallel will increase my maximum range, but how far exactly will that take me? 

ElectricMotion.org, the website build log for Lennon Rodgers' eMoto has a nifty spreadsheet available for download in the Specifications section where you can modify characteristics such as weight, battery size, voltage, gear ratio, etc and see projected top speed, power dissipation, range, etc. 

I took the liberty of uploading my own version of the Spreadsheet onto Google Docs. The most important parts of the document (to me) are the top speed and range. By changing the cruise speed (I usually have it set to 55mph, which is not energy efficient), drag coefficient, and other parameters, the spreadsheet spits out a maximum range estimate. I should be getting 15 to 30 miles of range on 24S8P of 2.5Ah cells, double that if I can fit 24S16P. By fiddling with the max voltage, the max motor RPM at 72 volts, and the gear ratio, I can find an achieve a max velocity that can keep me comfortable on a highway, so around 65mph. 

But before I can finalize all that, I need to find a motor that is happy running at 72 Volts (electric motor speed is proportional to input voltage).  For Cruscooter's Kv=190RPM/Volt motor to be running at 72 Volts, it will be running over 13000 RPM, which is REALLY FREAKING FAST. A quick hop over to McMaster shows that only their highest quality bearings have a maximum RPM that high. And they each cost half as much as Cruscooter's motor. 

So I need a low-Kv motor (Which translates to high torque constant) or one which can deal with being driven at many thousand RPM. I can go the way of Bayley and get myself a hybrid vehicle motor, like his Ford one. The issue is I would have to design my own case for it, like his, and would need to find one, which I can't seem to be able to do in five minutes of scanning Ebay. 

There's a HUGE Baldor Induction Motor Bayley was going to use in his motorcycle before he decided to use the Ford hybrid motor. Apparently Adam Bercu, another MITERS-goer, had gotten it to spin with the Sevcon Gen4 controller, albeit not efficiently. It also weighs like 100 lbs. 

Everyone around MITERS keeps praising the Etek motor, and there's apparently a few lying around up for grabs. 

Here's one I dug off a shelf at MITERS, though Charles tells me (and there's a sharpie scrawl on it) that the built-in hall sensors are dead. A quick check with an oscilloscope confirms this. I'm going to have to replace them. While this particular Etek-style motor was a custom order, not one that was ever available for retail, I am told it is almost identical to the ME0907, though a little bit more powerful

Looking at the datasheet of the ME0907 (Remember, mine is more powerful), it seems to be a decent-enough motor, but not quite taking full advantage of the Sevcon controller's 25kW possible power output. It has a Kv of less than 70 rpm/volt, so at 72 Volts it will spin at about 5000 RPM. It's 90% efficient at voltages from 24-48 (But I'm running at 72). It has a peak current of 250A, but the Sevcon can deliver another hundred.

Using these parameters I can get a ballpark for the absolute ideal peak power output of the ME0907 motor. A fully-charged A123 cell will be at 3.6 Volts. 

3.6V * 24 * 250A = 21.6 kW or 29 horsepower

The original CB750 engine was 68 horsepower peak, so at absolute peak ideal conditions this Etek motor can put out over 42% of the original engine's power. But the original CB750 weighed 500lbs, while I'm looking to cut it down to about 350lbs after gutting the 250-lb engine and adding 100lbs worth of batteries, controller, and motor. 

Power to weight ratio (PWR) is a thing, right? Assuming I'm 185 lbs (Well, I am), the PWR of the original CB750 (my weight included) would be 0.0992 hp/lb. EtekChopper would be 0.054 hp/lb. While I'm not even sure those are the right units (They seem to be inverted according to Wikipedia), it appears the ME0907 motor has over half the normalized power of the original engine. 

Considering the CB750 was dubbed a superbike, half the normalized power of that is not too shabby! Because it's direct drive electric, the torque to the ground is instant. Plus the ME0907 is free, but costs about $500 online. All I have to do is replace the sensors. I am using the ME0907 Etek Motor. (But you already knew that, didn't you! I have been calling this thing EtekChopper after all...)

Now it's time to choose the single-stage gear reduction. After tuning the gear ratio from 3.5 to 6, I settled on 5.8 (10 teeth at motor, 58 teeth at rear wheel) for the following reasons. 

I want decent wheel torque so I'm not sad when an intersection turns green. Most vehicles are defined by a 0 to 60 mph time. My 2005 Mazda 3 sedan goes 0 to 60 in about 8 seconds. The original '79 CB750 went 0 to 60 in about 5 seconds. Given the peak current (Charles claims this etek motor ), Torque constant, gear ratio, wheel radius (13" tire radius! I almost messed this up), loaded vehicle weight, a rough 0-60 time for my system can be found: 

60mph/(300A*0.13 NewtonMeters/A *(58/10) /(13") /((350+185)lbs)) = 9.5 seconds

So realistically it'll be more like 12 to 20 seconds because the above calculation doesn't take into account wind drag. As for city driving I can get to 20 mph in an ideal 3.17 seconds, so I'm content with this. 

As for top speed, I want to be able to drive on a highway if necessary, which is usually 55 mph minimum. I also want some wiggle room to be able to pass/dodge cars if necessary, to I shot for a 65mph top speed. Plugging into Wolframalpha the Kv, nominal battery voltage, gear ratio and wheel circumference yields: 

(68 RPM/Volt)*3.3V*24*(10/58)/Revolution*2*pi*13" = 71.8 mph

Sweet. Now, I know what you're thinking. But this time my system can put out 100A continuous. Just check out the following calculation of max motor force minus wind force at 65mph: 

0.13 Newton meters / Amp * 100 Amps * (58/10) /(13")
- (.5 *(1.275 kg/(m^3))* (65 mph) ^2 * 0.8 * .5 m^2) = 13.04 N 

I also need to decide what size chain to use. Too small and I'll be breaking the chain like Straight Razer ate through #25 chain. The original CB750 had #530 chain, which is quite a bit bigger according to gizmology.net's page on sprocket and chain sizes. I've heard of people using #40 and its vehicle derivatives on electric motorcycles, but this post of a broken 420 sprocket convinced me to stick with the larger #50-series of sprocket. 

But which one? What is the difference between #50, 520, and 530 chain/sprockets? This post has the answer, as does the gizmology page. #50 and 530 are nearly identical. Let's go with #50 chain! 

For the small 10-tooth sprocket, I looked no further than Surplus Center for an inexpensive steel one whose keyway and bore diameter was compatible with the 7/8" output shaft of the Etek motor. 

Here I introduce to you bikebandit.com. On this site you can specify the year, make, and model of your motorcycle and look through pages of the manual for the exact part you need. They also keep track of compatible aftermarket or universal parts and ofer those options as well. They SHOULD have the right rear sprocket, right? 

Wrong. Apparently 58 teeth is not available for the CB750. The highest rear sprocket available is 48 teeth, which simply wouldn't be sufficient for my torque/speed needs based on my rear wheel size. I guess I'm gonna have to make my own then! (Which I've done before for MelonChopper, I just have been too lazy to blog about it yet). 

Some quick searches on Google led me to Sprocketeer Online! It takes in the desired pitch, roller diameter, and teeth, and generates both a DXF file and generic CNC code for milling out a sprocket. I then simply imported the resultant DXF into Solidworks and extruded it to the right thickness (0.343"). 

However, In order to make the rear 58-tooth 530 sprocket compatible with the CB750's rear wheel mounting holes, I had to either measure the current one or find a drawing with the proper dimensions. I found a drawing at Dime City Cycles (third pic), and added it to the sprocket. I'll waterjet it out of steel or high-strength aluminum and file the edges until the chain fits. Trust me. It'll work. 

They also had 10 feet of #50 chain (Solidworks estimates I need about 5' 7") and master links/extender links for pretty cheap. I freakin' love SurplusCenter. 

Is that it? I think so. For now. Time to start buying stuff. 

Final Specifications:

• Overall Estimated Specs: 
• Weight: ~350 lbs (no driver)
• Maximum Speed: >65 mph
• Ideal Maximum Power: ~19.8 KW (~26.5 horsepower)
• 0 to 60 mph: >10 seconds
• Range: ~15 miles @55mph, ~30 miles @ 30mph 
• (double that if I can fit the other two battery packs)
• Motor controller: Sevcon Gen4
• Free, Estimated cost $925.00
• 350A peak, 140A continuous
• 72-80 nominal VDC, (39-116VDC Absolute limits)
• Motor: Custom Etek-style, Almost an ME0907. 
• Free, Estimated cost $500.00
• Happy at 24-48 Volts (I will drive it to 72)
• Over half the normalized power output of the original CB750 engine
• 300 Amp peak, 100 Amp continuous
• Batteries: 2x or 4x 12S8P A123 Packs, 
• Free (Thanks, Dane!), Estimated cost $936 each pack
• 72 Volts nominal
• Integrated Battery Management System
• 2.5Ah per cell, 2x or 4x (12 x 8) cells. 
• Charger: I'm liking this one so far: http://www.cloudelectric.com/product-p/bc-sco7220.htm
• $425.00, but I'm sure I can find a high current one for even cheaper. 
• Transmission: Single-Stage 5.8:1 gear ratio #50 (530) chain
• Motor Sprocket: 58 teeth, custom made
• Rear Sprocket: 10 teeth, from surpluscenter.com
• Chain: about 105 teeth, from surpluscenter.com

EtekChopper: Making a 250lb Engine Disappear

TL;DR: We tried to start it. We failed. Now the Engine is gone. 

One of the first steps in getting my CB750 ready for electrification is removing all unwanted crap related to the gas engine, and essentially be left with only wheels, suspension, brakes, lights, etc. However, I want to check whether or not the engine will even remotely start before I go ahead and throw away a perfectly good hunk of steel. I was assisted by MIT alum, former CB750 owner, and my supervisor at my summer job at Aurora Flight Sciences Terrence McKenna, by fellow intern and MIT student Thomas Villalon, and two housemates of mine looking to eventually build their own personal EVs Adam Rodriguez and Miranda Gavrin. 

Because most of the cables were straight-up cut off of the "spinal cord" of electrical wiring, it involved a lot of multimeter continuity checks and squinting at the manual's electrical diagram to figure out what went to what. There is both an electrical starter motor and an alternator (I thought the alternator was the starter for a while, but I was mistaken). The starter is attached to a relay with an integrated circuit tying a handlebar-mounted starter switch to the motor, as well as preventing the system from working unless the keyswitch in the bike is turned to "On". Because none of these switches were spared from the "spinal tap", we had to strip the wires and just electrical-tape them together. We attached a trusty A123 12v7 battery to the starter circuit. 

No dice. Comatose. Not one remote sign of movement, where a coughing engine would have been something. After double checking all the connections and verifying we weren't being idiots, we declared this engine dead, or too borked to warrant spending the time and money necessary to get it back running again. 

The first step in engine removal according to the manual was to unbolt and remove the carburetor from the frame. In order to remove that, though, the bulky air filter unit needed to be moved fully back. In order to do THAT, however, the battery holder framing had to be removed. Even after doing all that, the carbs would not budge from the rubber hose output connection the the engine combustion chambers. 

That's when we pulled out the Sawzall and tore that sucker out. With the hoses permanently cut, there was no going back on our decision to junk the engine. 

The carburetors weren't in bad shape, though. I'm going to take these apart, clean and oil it and maybe sell it on ebay for a half-decent amount.

Next was decoupling the rear wheel from the rest of the transmission. After removing the chain cover and loosening the rear wheel placement adjuster/chain tensioner, we were able to get the massive #530HT (High Tensile) chain off the rear sprocket.

Then came removing the chain from the small 15-tooth transmission output sprocket in order to free the transmission assembly from the rest of the bike.

This involved removing the sprocket itself. I decided to keep everything that I could remotely reuse, chain drive components included.

We then lifted the engine/transmission assembly with a jack and hammered out the attachment shoulderscrew/pins.

A hefty kick...

And the motor comes out! Note the lack of an oil pan in the transmission (we are now looking at the underside of the engine/transmission). Poor poor engine... I called the local scrapyard, Atlas Metals, to pick it up the next day free of charge. 

Here Adam gets a feel for the bike. It looks insanely high (like you can't touch the ground with your feet), but that's because it is up on the large kickstand. Still, I think it would be worth it to lower the front fork and possibly replace the rusty rear suspension with new, shorter shocks, so my stocky 5'8" build can comfortably and safely sit in the driver's seat.

Without the ~250lb engine, the ~500lb motorcycle was now SOO MUCH MORE MANEUVERABLE. Lowering the bike (and thus lowering the center of gravity) will help the bike feel lighter by decreasing its tendency to tip over. 

Goodnight, sweet prince. Soon you will live again, kissed by the power of electricity. 

Next step: drive and power train design!

So I Bought a Motorcycle... EV CONVERSION TIME!

A 1979 Honda CB750K to be exact. I'm gonna convert it to electric, but first the story of HOW I GOT THE THING: 

So fellow MITERS denzien Bayley decided to buy a for-parts 1982 CB750 for $250 off of Craigslist. His goal is to use a Prius motor controller to drive a Ford hybrid motor, and get an effective electric motorcycle. Often called the first superbike, or "King of Motorcycles", the CB750 (first produced in 1969) is a legendary motorcycle for being the original Universal Japanese Motorcycle (UJM). It was also one of the first commercial motorcycles to sport four separate carburetors (which mix gasoline and air in preparation for cumbustion), a straight 4-cylinder single overhead (later double-overhead) cam 75-horsepower (~50Kw) engine, an electric starter, turn signal blinkers, a 5-speed transmission, front disk brake, etc all in one ~$2000.00 (in 1969 dollars, which is like $12,000.00 today) package, which was unheard of at the time. With a top speed of 125 mph and plenty of motor torque, this bike can soar. 

Many people like the badassery associated with Kawasaki Ninja type motorcycles, but I've always wanted a classic-looking motorcycle. Not WWII classic, but one like the one above (from the US version of The Girl With The Dragon Tattoo). After researching the CB750 and hearing Bayley got his for $250, it also had my attention. 

The motorcycle was originally around 500lbs, but after helping him remove the non-functioning 250-lb engine, the remaining 250-lb frame felt both light and strong, certainly worthy of EV-ifying. 

Now, before Bayley found the Prius controller, he was going to use a Sevcon Gen 4  motor controller on it. This controller is found in commercial electric vehicles, and is a favorite among EV enthusiasts. I've been on his back lately about finishing the Melonchopper custom motor controller, but it seems the magic transistor he was going to use is still unavailable. Imagine my surprise when he just says "here, have a Sevcon 4!" and hands me a $925.00 motor controller!

Looking through the specifications of the controller, it can output 350 Amps peak at up to about 96 Volts (~35kW!!!!!!!). However, it seems the 80mm-class "melon" motor which powers Melonchopper is barely powerful enough to take advantage of the what the controller has to offer: a waste of a $1k controller Bayley handed me for free. 

What should I do with this Sevcon Gen4 controller? Should I perhaps build an electric motorcycle? Where can I get my hand on a good enough motor? As with most things, I consult my good friend and mentor Charles Guan, but he didn't have to do much convincing: next to him sat an electric motorcycle that was undergoing some maintenance. 

The motorcycle belonged to an IDC student Lennon Rodgers, who documented the entire build on his website, electricmotion.org. Charles said there was an ME0907 etek-style motor lying around MITERS up for grabs, which can draw 220 Amps peak, 80 Amps continuous at 72 Volts, for a peak power of about 18 kW (about 25 horsepower, a third the power of the CB750 motor), nearly 3 times as powerful as the "melon" motor.

And so, I decided I'm doing my own EV conversion of a motorcycle. I have the knowledge and experience, a decent (and free!) motor controller capable of putting out 35kW peak, a decent (and free!) etek-style motor capable of handling about 18 kW peak, access to a number of machine tools, and a rewarding summer job. When Bayley offered to lend me his craigslist-scoping skills, I took his offer. Within minutes, I was looking at a Craigslist post for a for-parts 1979 Honda CB750K for only $100. I took the offer. 

The next day after work I went to Revere, MA to pick up the motorcycle, UHaul motorcycle trailer in tow, accompanied by both Bayley and Thomas Villalon (interning with me at Aurora Flight Sciences this summer and a member of the MIT Solar Electric Vehicle Team). 

And here she is! Exactly as ordered, a CB750 worth $100. We tried to start the engine with the seller's handheld car starter, to no avail. It's either bad wiring, a faulty starter, or a seized motor. 
At least the rear drum brake works! The front disk brake needs to be tuned, but should be okay. The frame, tank, and body looks alright, surface rust is nothing a little sanding and repainting won't fix. I'm really digging the two-person leather seat, and the wheels and tires look to be in good shape as well. 

Here she is ratchet strapped and otherwise rigged to the motorcycle trailer. 

The seller also gave me a spare front kit from another Honda cycle (with a headlight and blinkers) as well as a saddlebag frame addition and two hard-case lockable saddlebags (In which I may store extra batteries for higher range). 

Arrival at MITERS

Thomas checking out the bike with the add-ons thrown on it. $100.00 was a pretty good deal for everything I got!

Here it is with just the seat on. Sexy. 

The next day I decided to run a random check of how things are working out. If it isn't difficult, I may go the Mikuvan way and at least attempt to get the engine running. First thing I noticed was there was no throttle, just a box with the steel cables. Also, the choke (which controls how much air goes into the fuel-air mixture) seemed to not want to budge. 

To check under the hood, you use your key to remove the seat and access the battery area. 

Part of the seat adapter was bent out of shape, so I'll need to make sure that gets back on correctly. I also need to clean out some of the rust on the underside of the seat. 

The fuel tank just... comes off. 0_0 I was expecting there to be some sort of cable I have to remove, or for the gas to start pouring out the moment I removed the tank, but nothing. 

And here's why: the fuel movement from the tank to the carburetors is controlled with this valve here, located under the tank, and it is currently turned shut. But how does fuel get from here to the engine?

Oh. Fuel line's cut.
Bummer. 

At least there's oil in the... OH WAIT. THERE IS NO OIL BECAUSE THERE IS NO OILPAN INSTALLED ON THE BIKE. The entire bottom of the transmission is simply... open. At least I don't need to drain it!

As for the electronics/lights on the bike, most of it looks like this. It's manageable, and all of the colors for the wires are detailed in the manual, but still kind of annoying. 

The front brake doesn't seem to do anything, so I need to drain and replace the old brake fluid. The manual seems to detail all. 

YAYYYY!!!!!!!! :D