Weird idea about using only 6 gears

[hollus]

Yes, this was justa crazy idea, I like to think loud sometimes. Everybody is using 7-8 gears and the ones using 7 will probably now use 8th in China. The points you raise are some of the very obvious drawbacks of the idea. I expect it not to be worth it at all.

I also don't know enough about the internal construction of gearboxes, so basically I wanted to know whether this trick could in real life, with an ad-hoc design, achieve mostly two things:
a) Reduced rotational inertia in the gearbox.
b) Reduced dimensions of the gearbox, in particular shorter axes as mechanisms that before had to extend beyond all gearplates now can exists in the space that the fake gearplates, as they only need to reach 6 gear plates.

[Kozy]

If you had a perfectly flat power curve from 0 RPM up to the rev limit then there is literally no advantage of having more than one gear....

I don't think this is true. My Ford Fiesta gives much more acceleration at 3500RPM in first gear than at 3500RPM in fifth gear. Gears are stull useful for their torque multiplication effect.

At 3500rpm, you might produce 45kW. Now, 3500rpm in first gear might be 10m/s, and in 5th that might be 30m/s.

Since we know power = force x speed, and thus force = power/speed, then we can figure out the accelerative forces created by 45kW at 10m/s and 30m/s.

45x10^3 / 10 = 4500N
45x10^3 / 30 = 1500N

The engine power, divided by the wheel speed, is what matters. Thus, if an engine can produce 550kW for the entire rev range, there is no need for gears.

Electric vehicles work very closely to this principal.

[machin]

No arguments from me on that maths, but I would tend to say that it is worth noting that, contrary to popular belief, electric cars generally don't have flat power curves (see example below), and therefore would benefit from a multispeed gearbox.....

[Kozy]

No arguments from me on that maths, but I would tend to say that it is worth noting that, contrary to popular belief, electric cars generally don't have flat power curves (see example below), and therefore would benefit from a multispeed gearbox.....

http://www.electriccarpartscompany.com/ ... roller.jpg

You're absolutely correct, generally they have a parabolic power curve in the absence of any torque limiting, it was a poor example. Their freedom from needing gearboxes generally stems from being able to produce full torque at 0rpm and having an operating range 2 to 3 times larger than that of a petrol engine.

[andylaurence]

No arguments from me on that maths, but I would tend to say that it is worth noting that, contrary to popular belief, electric cars generally don't have flat power curves (see example below), and therefore would benefit from a multispeed gearbox.....

http://www.electriccarpartscompany.com/ ... roller.jpg

Electric motors fed a constant voltage do not have a flat power curve. They're initially limited by the maximum current (the straight line) and then by the voltage versus the back-EMF from the motor speed. What's important is that they have a flatter power curve than an internal combustion engine, which means that they need fewer gears. Allied to the fact that electric cars generally have a lower speed range in their design parameters and that the motor isn't as noisy at high speeds as an ICE, resulting in more revs being usable at motorway speeds, that is why many electric cars can compromise at 1-2 gears.