[major]

I was engaged in discussion with a member on another thread and the OP politely suggested we take it elsewhere. If interested that can be found here: http://www.diyelectriccar.com/forums...250-36771.html

The other member claims

when talking about motors of any kind torque is totally irrelevant

I realize this quote is out of context, but I have pointed the way to the origin.

I am of the opinion that motor torque is of great relevance when it comes to vehicle performance. Anyway, being an opened minded type of guy, I'd like to get some other opinions without totally hijacking hppyfngy's thread.

Regards,

major

[paker]

Torque is very relevant. Take a 9" motor and a 13" motor. Putting the same amount of current to both which would move more weight in the same time period? The 13" motor.

[CroDriver]

Power is torque X RPM.

Formula1 engines have about 300-400 Nm torque. My BMW X6 has double that torque but guess who's faster

However, torque is important in real life when you cruse around at low RPM. More torque means less shifting (if we talk about ICEs)

[MalcolmB]

Since I started looking at EVs and series motors in particular I've come to think of power as the rate of torque delivery (in other words torque is the key parameter).

When I wanted to work out if the motor I'd found was suitable for a fixed ratio conversion I first estimated the torque at my maximum current and then worked out if that torque would be sufficient to pull away on a 1 in 3 gradient.

I'm not sure how I could do a similar calculation using power alone, since power is zero at zero rpm, isn't it?

[Qer]

Originally Posted by MalcolmB

I'm not sure how I could do a similar calculation using power alone, since power is zero at zero rpm, isn't it?

Power = RPM * Torque, so yes indeed.

As I see it, and since my background is mainly from electricity, I tend to see it as torque is related to current and rpm is related to voltage. Both torque and rpm are potentially very powerful, but rpm without load or torque without movement won't make anyone happy, current without voltage or voltage without current are equally pointless. Of course.

At that perfect time zero when you slam the throttle there's no movement and thus no rpm and thus there can't be any power released and no actual work is done. There's nothing but torque and that torque will force the car to move by simply overcome friction and in THAT very instant, when the car starts to move, there's rpm and therefore you get power! If you don't have the torque you can never overcome the friction and if there's no rpm there's no movement.

Now, power = rpm * torque, if we apply that to the wheel the ONLY thing that regulate the final power at a certain speed of the car is torque! Nothing else. The rpm is directly related to the speed of the vehicle (decided by the size of the wheels) and there's nothing you can do to change that (except by changing speed or tires, of course) so if you want to alter the power that propel the car you alter torque. That's how all cars work.

CroDriver mentioned that Formula 1 cars have engines that only have half the torque of his BMW. That might be true, but that's missing the fact that a Formula 1 car is very aerodynamic and can rev to up to 18000 rpm, thus it doesn't need as much power as the BMW to gain speed and there's a lot of potential power in the high top rpm. However, those 18000 rpm can't be applied to the tires directly so they have to be geared down, trading rpm for, yep, torque and therefore better acceleration.

So my personal view is that torque is the main key here. For an EV acceleration is all about how much motor current you can keep up (which is why the Zilla 2K is still unbeaten on the drag racing track as far as I know), the motor current turns to torque which then is transformed to acceleration. The gear ratio has to be optimized for maximum continumous torque without hitting the limits (max rpm, max motor voltage etc), a motor with less torque but higher rpm can be geared down to trade all those rpms to torque, but torque is, in the end, what accelerates cars.

[Batterypoweredtoad]

I think this is an interesting discussion. Heres my take: If you have unlimited flexibility in design of your vehicle, then power is the only thing you need to know. For example, 50hp is 50hp whether it comes from a motor that spins 50,0000 rpm or whether it spins 500 rpm. The problem is that you cant make a 50,000 rpm (or really a 500rpm) motor work because you cant gear your car appropriately. So I say torque is key, but what rpm range the motor can carry that torque through is more important.

[CroDriver]

I would rather have more RPM than torque. This would make a single speed setup much easier. I think that such a setup is almost impossible with DC motors (except for very short runs -> drag racers) because the motor would run at very low RPM most of the time and would probably overheat (and the efficiency would probably suck). I think that a single speed transmission is especially hard to achieve with a sports car that has to be geared for high speeds.

I bet that the Tesla Roadster has less torque than a good DC conversion

Originally Posted by Qer

CroDriver mentioned that Formula 1 cars have engines that only have half the torque of his BMW. That might be true, but that's missing the fact that a Formula 1 car is very aerodynamic and can rev to up to 18000 rpm

Yeah, I forgot to mention the 18.000RPM (limited by regulations, a few years ago the red line was at 22.000RPM)

They trade torque for RPM with a short piston rod

[Bowser330]

Originally Posted by Batterypoweredtoad

...So I say torque is key, but what rpm range the motor can carry that torque through is more important.

+1

Torque without considering speed(rpm) is irrelevant...

[samborambo]

Originally Posted by Batterypoweredtoad

I think this is an interesting discussion. Heres my take: If you have unlimited flexibility in design of your vehicle, then power is the only thing you need to know. For example, 50hp is 50hp whether it comes from a motor that spins 50,0000 rpm or whether it spins 500 rpm. The problem is that you cant make a 50,000 rpm (or really a 500rpm) motor work because you cant gear your car appropriately. So I say torque is key, but what rpm range the motor can carry that torque through is more important.

-1

You're irrelevant.

You might as well say that Amps are irrelevant too.

[Qer]

Originally Posted by CroDriver

I think that such a setup is almost impossible with DC motors (except for very short runs -> drag racers) because the motor would run at very low RPM most of the time and would probably overheat (and the efficiency would probably suck).

Yep, that's a limitation. But is that because of the RPM or the construction?