dwhite50

Looking for an RX-8 body (no engine or trans). I teach electronics at the local community college, the instructors of the auto mechanics classes and I want to let the better students convert an RX-8 to all electric. The finished car would be auctioned off for charity.

The students actually figured out the RX-8 would be ideal, so I'm trying to find one. With the aluminum body and weight ratio we think it could do 80 for two hours. Project starts Fall of 2010, we prefer the most basic model (i.e. lightest with fewest gadgets)

Thanks

davewhite50@verizon.net

alnielsen

iTrader: (4)

This sounds a lot more interesting then the V8 conversions some are doing. I hope you can keep us updated on the progress.

tiltmode43

iTrader: (15)

That's pretty awesome!

May I ask what school?

dwhite50

A combination of High Schools and Colleges in the High Desert (Lancaster - Ridgecrest). Actually only one or two students from each school.

The plan is to use two (2HP) electric motors attached directly to each rear wheel. (No differential either). Using golf cart batteries, three go in the depression in the truck, six go hump for the drive shaft, and maybe a few more in the engine compartment or rear seat.
The very ambitious aim is 300 mile range at 80 miles per. Bad part, if it works, the recharge time off of 110VAC would be 22 hours.

When done, we want to drag race a Tesla.

ZumnRx8

I live in Palmdale! & own an RX8 ... would like to help if i could. THere is an RX8 on rolling chassis for somewhere near $2k on here... it was Galaxy Gray. Also a complete 8 for about $6k

rjon17469

This is a comment on the design rather than the car, so if this is too out-of-place, feel free to delete/whatever is appropriate.

4 HP of driving motors at 80 MPH will give you approximately 20 lbs of forward "thrust" at that speed. The car will require much more power than this to maintain that velocity, regardless of how much weight you are able to remove.

Assuming correct gearing, 4 HP might be able to get you to 10-20 MPH, but beyond that the friction of air/moving parts overcomes the available power.

For comparison, the motor in a Tesla Roadster is capable of delivering 248 HP roughly from 6,000 to 14,000 RPM. The Roadster is in the weight range you are likely looking to achieve.

I still encourage you to give the project a shot, as it is one of my dreams as well. Just be aware that the results you want may take more time/money than you are currently considering. Another factor: a motor on each rear wheel means one motor will be spinning in "reverse" for forward motion. Many electric motors are rated/perform differently in reverse.

dwhite50

I think I'll ask them to make a sticky subject of Electric RX-8.

I've never done an actual conversion from start to finish on any car, so this is all book learin'.

The 'horsepower' rating of an electric motor is a bit confusing, because of torque and other things. The Tesla's motor is rated at about 5hp, but the torque.....
(Do I deliver 5hp as 370 joules in ten seconds, 37000 joules in 0.1 seconds? Both are the same HP, so are talking instantaneous, average, maximum?) We all know about the rotary's torque, or lack of.

A high torque motor directly driving the wheels will pull A LOT of juice just to move the first inch. but should maintain speed on a trickle. (1000 amps to get started, but a Tesla is twice as fast to 60mph as a Porsche.)

Regenerative braking is easier also, but batteries just can't recharge as fast as they discharge. I've come to believe regen is a marketing ploy unless you're coasting down the Rocky Mountains

2 motors on two wheels vs one and a differential is one of the harder decisions. We can get identically rated CW and CCW motors, but in slippery conditions the different current draw means much beefier control circuits.

There is also the problem of 'reverse' gear'. Lots of motors have great ratings CW and in CCW couldn't move a Tonka toy. (Electric motor companies don't always admit that.)

It will be tough and I don't expect it to happen unless we get some kind of sponsorship.

Dave White
davewhite50@verizon.net

ps. And yes I do drive an RX-8 (So I pushed them toward the decision on what kind of car to convert).

rjon17469

I'm going to urge further caution on your part, as I believe you are confusing a few different concepts.

The horsepower rating on an electric motor is quite simple: the power the motor may deliver to the shaft under certain operating conditions. Notice we are talking about power rather than energy. 373 watts over ten seconds delivers the same energy as 37,300 watts over 100 millseconds. Both deliver 3,730 joules to the load. Joules is a measure of energy. Watts is a measure of power. They are not the same.

370 joules in ten seconds = 37 W = 0.0496 HP. 37,000 joules in 100 milliseconds = 370,000 W = 496 HP.

At 80 MPH, the standard tires (on 18 inch wheels) of an RX8 are spinning at 1,034 RPM, or 108 rad/second. Assuming gearing is correct to be able to deliver 4 HP at 80 MPH, you can find the available torque through T = P/w = (4 HP * 746 W/HP)/(108 rad/sec) = 27.6 N-m, or 20.4 ft-lbs of torque. The force applied to the ground is calculated by F = T/r = (20.4 ft-lbs)/(1.08 ft) = 18.9 lbs of forward "thrust". This is not sufficient to maintain that speed.

In my opinion, here are some questions you should be able to answer before proceeding:



Why do most electric motors offer their maximum available power over a wide RPM range and what are the associated consequences?

Why can a vehicle powered by an electric motor be so much faster than a gasoline-powered vehicle of the same power rating?

Should I use an AC or DC motor and why?

How much power do I need to overcome air friction at various speeds?

How much energy will the vehicle consume per mile at various speeds?

Do I need any form of gearing between the electric motors and the wheels?

Using a motor on each rear wheel will require the motors to spin at slightly different speeds when the vehicle turns or if one wheel rides over a bump. How can I best account for this?

Regenerative braking works surprisingly well in this application given modern batteries and motors. Why?



If you don't have an immediate answer to each of these questions, then you have much more work to do before you should even dream of buying a car to convert.

I'd love to help you with this project as this is something I am very interested in as well. PM me if you'd like to talk more.

WhiteSnowflake8

iTrader: (5)

how much you looking to spend?

rx8spiritR

^ that was some deeeeeep **** my friend

luv4eternity

built one with carbon fiber body...would be light and strong....

luv4eternity

probably AC motor

rjon17469

In rereading your original post, this caught my attention. In that case, I'll ask a few more questions:

Why 4 HP of drive motors? Where did that figure come from?

The Tesla's motor produces 5 HP at very low speed, but 248 HP at higher speed. This makes sense if you realize the relationship between power, speed, and torque.

Why 1000 A starting current? Where is the calculation to justify this?

What calculations have you done to believe you can reach 80 MPH? Similarly for a 300 mile range?

Why ~10 golf cart batteries? What energy storage level are you attempting to reach?

What voltage would the system operate on?

How would you load-balance multiple motors?

elysium19

iTrader: (1)

hey.....show some deference to our visiting professor here...

rjon17469

You are right, my original post was over the top. I apologize and edited to make more reasonable.