Quote:
Originally Posted by e*clipse
Here's what I don't get: Why do you need to shift gears at all?
Seriously - the rated speed with this motor is 10,500 rpm. Along with the stock 6.86:1 gear ratio and average sized tires, my calcs (and others) indicate that this is good for 115 mph. Now, 50kW may not be enough for that speed, but this is a power issue, not a transmission issue.
This is why I'm going for a two motor system initially. If spec'd with the correct battery, the car can easily handle normal driving conditions. Accelleration is reasonable, at about 8 or 9 seconds 0-60.
Would this be improved with a transmission? Not according to Tesla, who built the first Roadsters with 2 speed transmissions. Because of reliablity issues, they simplified to a single speed gearbox and re-tuning the motor.
I'm totally into the idea of a custom gearbox, similar to the MGR - just one that's a bit better suited for EV use.
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I have had this discussion with Dave Cloud on a number of occasions. I respect his ideas and have been involved in the building of a couple of his cars. If you do a google search for his name you will find a long string of EV activities in connection with electric cars. Not the gospel singer Dave Cloud but the other one with the wild looking hair. That is my next-door neighbor...
He asked me the same question. Why a transmission?
In the case of "go faster, faster" it is a common practice to use stacked or coupled motors. Ask John Wayland a.k.a. plasma boy or the builder of the white zombie. Massive amps more power bigger cables more torque get off the line in a flash and on down the track. In his case he did use gears... kinda. Series and parallel wiring of the Siamese 9's through contactors provides a lot of torque to get off the line and then a lot of top end for speed. Then he switched to the zilla controller with 2000 amps capability.
Why 2000 amps? Why not 250 and a first gear in a transmission? There would be less copper to lug around and a lot smaller controller. Longer battery life? I am not sure but everything I have learned from Dave the past 6 or so years kinda indicates that a transmission instead of a huge motor, controller and a sheep load of amps would be a good thing.
I cannot even count the times someone has told me "they only drive around in one gear". And then explain that when they go up a hill or get on the freeway and need a bit more pep they do what? Pour on the amps? No. Add more motor? No. They shift to a lower gear to keep the amps from climbing to high. Hmm. They use "only" one gear huh?
In most of the applications I have seen where the vehicle has the get up and go of an ICE or better there is a large motor or two with a sheep load of cables the size of my thumb and a controller that could be used as a power source for an arc furnace. Why not use a smaller motor and multiply the torque with two gear ranges?
Yes Tesla tried some two-speed gear thing but in my experience with transmissions he did not try very hard. Helicopters have been running gear boxes for years with gas turbine engines that turn at 200,000 RPM's. As far as reliability goes there are two speed gearboxes out there that are older than both of use combined that still work fine. I can walk in to my local transmission shop and walk out with a two speed tranny that will handle 1000HP @ around 450 foot pounds of torque easy. I am not going to pretend to know why Tesla went the way they did but history would indicate that it had little to do with unreliable gears.
But I can only go on what I know and I know you can get more torque out of an electric motor with gears, you do loose the top end but where in average daily driving traffic are you going to need top end? Yes you could go with a bigger motor... or two or four but I think that is a bit more complication than the average Joe wants to mess with for a simple conversion.
Less amps, smaller cables, smaller controller, less strain on the traction battery set all done with two planetary gear sets, two clutches and two bands in a transmission that has proven it's reliability for over twenty years.
I won't know for sure until I try.
Cyruscosmo