JDH: CB750 Conversion - Out with the old!

jdh2550_1's picture

A big thank you to Jason and Jennifer! I picked up Jennifer's 1973 CB-750 this weekend - after a decade of no use they decided it was time to say goodbye. I promised to ride it back under electric power - but not for a while yet!


This weekend was spent stripping down the bike. It went remarkably easily - I mean I didn't even skin any knuckles! Getting the motor out was the biggest challenge but as of 3pm this afternoon the bike lost the last of it's old, loud, smelly, inefficient, oil burning, carbon dioxide belching parts. Know anyone who needs any CB-750 engine parts? Craig's list here I come...


Now that the bike is stripped down I want to weigh it and start mocking up cardboard components (batteries, motor, controller) and figure out placement. My goal is 60-65mph top speed and a 50 mile range. Not sure how close I'll get - but that's the goal.

My current plan is a 72V system. I was thinking of going with a Perm 132 motor - but Andrew has got me considering an ADC motor. The ADC's aren't as efficient or as high powered as the Perm - but they are much more robust. If I go up to 96V I could drop the amperage further - but I just don't think I can squeeze on eight batteries of sufficient Ah.

So far so good - but of course this was the easy part...

Next up: Doing some math (should make Mum proud - she's a retired math teacher!)

before comments


reikiman's picture

Another issue with Perm versus ADC is the geometry. The Perm is rather wide and takes up space into which you could be squeezing batteries. But that wide stance contributes to efficiency (so I understand) so that's a tradeoff. Is the swingarm long enough to mount the motor on it?

echuckj5's picture

With that big windshield, I'll suggest a warp 9 motor. Little more torque. For those windy days.

Seriously, I don't understand, not using a perm. I hav'nt seen a decent chart on either an adc or a perm, both, from what I gathered, are about the same, high 80's on efficiency. The adc weighs more. To help you decide, KTA services, has the motor performance curves, $5.00. A lot better than what the factory puts out. Not sure if Ken has one for the perm. Ken is about to retire, has a lot of experience, might give him a call, he answered all my questions, sent me a chart, no charge, along with his catalog.

Oh Yea, before I forget, a guy in the North Texas Dallas electric vehicle club, has the same bike, I'll let him know that your getting started. I don't know exactly what year his is. My last 750, was a SS, had dual front disc, spokes front and rear. Never was impressed with the brakes. Might look into brake issues, on your down days. Probably have some good upgrades in the brake dept. Forks, I just never was quite satisfied, but, they never gave me any problems. Tried different weight oil, never upgraded the springs.

That's all I know, probably will just watch, from here on out.



jdh2550_1's picture

Dave - I think you have that the wrong way round. The Perm is a pancake motor the ADC I'm considering is 6.75" diameter and 11" long. The reason the ADC is more robust is because it is heavier, more thermal mass to dissipate the heat.

Chuck - that front screen sure is big - I was considering double glazing it... Brakes, who needs brakes? Just kidding - I'm going to look into uprating the anchors. Please do have the guy from the club contact me if he's willing - email is jdharding ...at... comcast ... dot ... net.

Regarding Perm vs. ADC - both put out enough continuous power to cruise at around 60 (I'm guesstimating 110A for 60 at the moment - will do some better math later (but before I buy anything!). Perm has a higher peak power so would give better accel - as long as the ADC gives "decent" accel it's (a) cheaper & (b) more robust. Of course it is harder to mount.

I'm thinking I won't be able to a swing arm mount. If I did it would have to be an internal Perm configuration and I'm not convinced I can make a sturdy enough bracket to bolt onto the swing arm (I have a 3"x9.5" platform to play with). If I go with a frame mount then I'll probably mount where the battery and air box used to be - the ADC would stick out a couple of inches - but that shouldn't be much of a problem.

Next up:
- doing some math to figure out power requirements
- mocking up components and placing on bike
- saving my pennies so that I can buy the requisite bits

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

echuckj5's picture

Thought ya was gonna do some math. I've been horsing around, trying to find the same information, found this,


At the top, says it is broke, but I played with it. Basically, says my bike, won't run. But, it does come very close to my calc. for horsepower, wind resistance, rolling resistance. Says I can go 19.5 miles, and I don't have rolling resistance (for the bike). But the wind resistance and horsepower are very close to what I calculated for my bike and my motorcycle (future motorcycle). Rolling resistance I measure with a spring, just get someone to pull ya. Load the bike down with dumbbells, read the scale. Check the scale, just hang some dumbbells on it, or a 10 pound sack of taters.

Dumbbells are those things you use for resistance weight training, not the people pulling you around. Those people are genetically challenged, not dumbbells.

Anyway, nice to have something to check your work, know that at least your in the ballpark.



jdh2550_1's picture

hold your horses there chuck - I AM gonna do some math - just haven't got a round tuit yet.

Yeah, I'd forgotten about that link. Also, buried further back on this board are a couple of other posts about HP requirements. I'll dig those up too.

I have a copy of Bob Brant's B.Y.O.E.V. - he's got all the calculations in there as well, I just need to substitute a couple of values for a typical m/c. Bidwell's treatment of this in El Ninja is a little skimpy (on a quick scan he just seems to present the curves for gradient and doesn't explain them).

So, on with the math!

BTW, my first idea on batteries (UB121100) are HUGE and weigh 70+lbs each. This gives a bike weight of around 650lbs BEFORE rider... Hmmm, I might need to scale back a bit there!

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

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