Thread: Ex-7 - Electric RX-7 conversion

I sugest blk02si's sugestion with a different ratio in the diff as well.
RX-7 is a tuner car, and there should be loads of used parts for it awailable if you search around. Perhaps at the scrap yard? (no phun intended).
I have salvaged lots of goodies from scrap yards over the years

BTW: You've made me all possessed about building a electric motorcycle... *shame on you* hehe...

Originally Posted by blk02si

I was thinking about this and wondering if you can get different gear ratios for the rear differential?

Great minds think alike! It's funny and amazing that even as you guys were thinking about this, last night I was putting together a model of engine RPM's vs speed for different gear ratios! But let me back up just a sec. With the wide torque range, an EV should be able to get by with two gears. Some make do with one, but they can be limited at the low & high ends.

In the 1950's and 60's General motors made a great little 2-speed automatic transmission called the Power Glide. It is very popular with drag racers. A guy on the DIY EV Forums has shown that the torque converter can be removed, making a nice little two-speed clutchless transmission. The final drive is 1:1 which is typical 4th gear in a 4 or 5 speed gearbox. GM also made a manual shift version called the torque glide. I think they're pretty rare.

Someday I'd like to make a 2-speed gearbox like that. Or adapt a power glide. Adjusting the rear differential gears is a really great possibility. So is adjusting the wheel / tire size. Going to a lower profile tire also lowers the final drive ratio.

For this motor, the torque begins to drop above around 4000 rpm.

So I drove it a good bit yesterday and got it up to 65 mph which may be close to the max on level roads. Starting out in first gear is obviously the best. It will start out in 2nd, but is weaker from a standing start. I can get to 50 in 2nd. I don't have a tach yet so I'm not sure of the revs there, it should be around 3800. In 3rd, I made it to 65, and held that speed in 4th. So it seemed to me that 1st & 3rd was the combination with the best performance for normal driving. And I wondered how to lower the total final ratio so that the 2nd / 4th combo would meet the performance of 1st / 3rd that I have now. (The key here is the 1:1 ratio of 4th. 1:1 is always one of the possibilities with a single planetary gear set.)

So I put this spreadsheet together with changes in Differential ratio and also tire sizes. The white area is well within the max torque range. Green highlights the gears of interest. Yellow is the rpm range where torque is diminishing. Red is at the limit of the motor (The actual limits for the motor is 11,000 rpm powered, and 12,000 rpm unpowered). I found 3 common rear end ratios on the web: My car has 4.1 RE ratio. I can get 4.33 and in Japan they sold cars with 4.77. I couldn't find anything lower than 4.77:1 I also looked at current tires - 24.7" diameter on 15" wheels, and a lower profile tire that's 21.9" (195 45 15's) diameter.



So it looks like going to a 4.77 rear end or to a 4.33 rear end and lower tires would give me a 2/4 final drive close to what I have now in 1/3. Again the key being that 4 -> 1:1 ratio or 'direct drive'. Making any changes would be way out in the future, but I like thinking about this stuff. Tires are easy enough to change!

@Crinos: I have a Taiwanese scooter in my garage. Believe me, I'm already thinking about converting it!

More on the PowerGlide transmission.

Most modern auto transmissions have 2 or 3 planetary gear sets and a set of electronic controls that would shame the space shuttle. Most importantly they have a fluid torque transfer unit - a torque converter - that acts as a clutch. And usually two hydraulic pumps. Modern AT's are far more efficient than the old beasts from the 60's and 70's and they actually increase the overall efficiency of the car by being so smart about when to shift.

The power glide has two forward gears and a reverse. It does this with a single planetary gear! Operating as an automatic transmission, it also has a torque converter and two hydraulic pumps; as well as a governor and a control unit to decide when to shift the gears and to actually do it. The actual shifting occurs by using clutches to selectively connect or disconnect different parts of the planetary gearset. Locking all three parts up results in a solid unit that is, in effect a straight through shaft - no gear reduction at all -> 1:1. This site has a great animation that shows how the different gear ratios can be selected by connecting & disconnecting different parts of the planetary gear.

If we eliminate the torque converter - which is the part with the most most 'loss' in the transmission - we can make a PowerGlide much more efficient. Then if we eliminate the automatic shifting, we can get rid of the governor and most of the control unit and at least one hydraulic pump. We're left with a planetary gear set and a couple of clutches. The Torque Glide transmission I mention in my earlier post is exactly this, except it keeps the torque converter. There is no neutral - it's not necessary. Reverse can be through the planetary gearset, or can be by reversing the motor - so we can possibly eliminate controls & clutch relating to reverse. In low gear, the input is the sun gear, and the output is the planet carrier, and we hold the ring gear stationary. To shift to high, connect any other two parts together and the entire assembly will lock together, making a high gear at 1:1 ratio.
So in a PowerGlide, high gear has very low losses.

Here are the best parts for my project:

1. A Power Glide Transmission is SMALL! I can get rid of the big bell housing, and make an adapter to the motor. Then the transmission and motor will (nearly) fit into the transmission hump of the RX-7. This would let me have much better options for more & better distributed batteries.
2. It would be at least as efficient as the manual transmission - maybe better.
3. Shifing is easier - fewer choices. Do I want to be in low or high?
4. I get a parking brake pawl for free. Engine compression doesn't work too well with an electric motor. If the parking brake slips, then watch out! With PARK it's safer.

Does anyone know of an old '60s Chevy - maybe a Nova or Chevy II or similar car with a 2-speed PowerGlide in it they'd want to sell? Earlier ones were cast iron cases and later were aluminum. Or better yet with a torque glide?

Remember those old WV Beetles and 4-series with the 'automatic-manual' transmission. (i.e. a manual transmission with no clutch.) I think they were similar, although I wouldn't want to try to adapt that to the RX-7!

SAAB had a AT transmition verry similar to the powerglide, that had a "free gear". Everytime you went of the speed, the transmition went into a "neutral state"... much like a bicycle when youre just ideling along.
Unfortunately, they had loads of trouble with it. Constantly breaking down because of the sudden torque difference when throttle where applied again.

I think the aluminum PowerGlide transmition would be perfect for you, but what's the weight of it?... Never mind... Googled it and found it. With aluminum housing the weight is 95lbs, or about 43kg. Fairly light I would say.

Originally Posted by Crinos

With aluminum housing the weight is 95lbs, or about 43kg. Fairly light I would say.

Without the torque converter & all the fluid in it, and the bell housing I hope it would be lighter still.

But you still need the fluids for the AT to work, well, atleast keep the gears oily. But you are right... You shave off alot of weight from the bell house and the torque converter (heavy things these).

Little circuit near my house, about 7 miles around. (mind you I still haven't installed any meters to know what my batteries are doing, but that didn't stop me)

Lap one.
- No problem-o. Left the house with a full charge. Let's do another.

Lap two.
- 1/3 way around. Hmm, dogging down a bit. Best keep it slow the rest of the way home.
- 1/2mile further up hill. Barely!
- Another 1/2 mile. Another Hill! Ugh! Made a stop at the top after going 2mph for 200 yards. Rest. Think. About 2 miles to home. How many hills? I forget. There's a very long down hill right before the house, then a short, very steep up -less than 100 yards. (meters for my metric friends). Rest some more.
- Couple of small hills up - barely. Couple of small hills down - coast! Big downhill towards home: 10, 20, 30 mph. Kick on the regen braking to throw some juice into the batteries (In my couple previous trips out I kept regen turned off). WOW, a lot of slowdown. That means a lot of juice into the batteries. Flip it off, speed up. Flip it on, slow down. Hey this is kind of cool... Near the bottom of the hill, leave regen on to come to stop at the intersection.
- 150 yards from the driveway. Do I turn right up the steep hill? Or go left downhill to the next street. Can you tell I'm a risk taker? Up the hill - more juice than I expected, considering how slowly I went up the previous couple of hills. Up, slower, up, slower yet, almost to the crest! Driveway just 50 yards ahead. AN SUV glued to my *** as I crawl forward - where the &$#%# did he come from that G#%&^%$ D$&^% A-$&*$^*&$-ole. Yay! Oncoming traffic. I turn on the left blinker and pretend I'm just slowing down for the driveway turn. Stop for traffic. Then.....
- In the driveway! Home! Charger!!!!! I better get to work, I've been goofing off way too long!

So. New batteries? This is as good as it gets (about 11 or 12 miles)???? A bad battery or bad cell? Loose connection somewhere? Naw.

I just don't know.

Oof! I vote you install meters asap. You are flying blind without them and could be damaging stuff.

Originally Posted by cgalpin

Oof! I vote you install meters asap.

Um. Yup. But it drove really great up until the point where it started to die!