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  #21  
Old 09-29-2009, 04:24 PM
Drew Drew is offline
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Default Re: Torque is irrelevant or relevant?

Quote:
Originally Posted by PhantomPholly View Post
If you had a perfect CVT, only HP would be relevant. The CVT would keep the motor right at it's optimum rpm while using all of the available power. However, there is no such thing as a perfect CVT today...

Electric motors often out-perform ICE motors of similar hp because of the range of rpms over which they will deliver full hp. ICE motors peak at a very narrow rpm range, usually right at or near max rpm with a sharp falloff. Electric motors typically increase hp linearly (and quickly) until they produce max hp, then produce that same hp all the way up to max rated rpm. Usually they can produce 100% rated hp over 50% or more of their rpm range.

That's probably the same thing someone else said and I just didn't understand their explanation...

This is mostly what I was getting at when I was talking about torque being irrelivant in the other thread. My point was that torque, in context of any motor is actually only used to indicate power at a certain PRM, the example I gave in the other thread was ;

Quote:
Originally Posted by Drew View Post
Thats right, tractive effort is a force, not a torque, the force at the wheel for a given speed is related to tyre diameter, differential ratio, gearbox ratio as well as any transfer cases involved.

Take the previous example, if you use a CAT diesel engine with 280kW and approx 1500Nm at 1800RPM vs a small helicopter turbine with 280kW and approx 52Nm of torque at 52000RPM. Assuming that both of these engines are geared to produce maximum power at 50km/h then what would the tractive effort be in each case? Simple answer, the same.

More complex version is that if you assume that you're talking about a medium to large truck with a 295/80R22.5 and a rolling diameter of approximately 1.050m and rolling circumference of 3.3m making for a wheel RPM of 252.5RPM, so your total system ratio for the CAT diesel will be around 7.14:1 and the turbine will have a total system ratio of 206:1 but the wheel torque is the same for both, the torque might diverge at the differentials based on different ratios, or there might be portal hubs, or they might even be using the same driveline, but the turbine has a planetary reduction gearbox of 28.8:1 before it goes into the transmission... the point is that torque is totally irrelivant unless you're designing components in a driveline, but its certainly not relevant when talking about engine power outputs or vehicle performance.
I should also mention that the maths I used to demonstrate the calculation of tractive effort without use of torque was simply P=F/V where power is the engines mechanical power in Watts, F is the tractive effort in Newtons and V is the vehicles current velocity.

You can use that formula to calculate maximum available tractive effort if you know or can estimate the maximum theoretical Mu of the tyres and the mass over the drive wheel(s) when taking into account weight transfer. The result is something like this;



Which is a set of calculations based on using the Azure Dynamics AC24L as a drive motor in a motorbike that I've been designing up for a while. This is working with an estimated rider+bike combination of 150kg and a Mu of 1.1 from memory and I've simply taken the power curve of the AC24LS and mapped it directly against the theoretical power limit to determine an ideal performance set. My calcs indicated (using only the data that I've posted up) that I'd get something like a 4.5 sec 0-100 and a top speed of 145-150km/h, but I didn't go ahead with this plan because I'm pretty sure that I'd have problems with grades at 110km/h (top highway speed in Australia).

The reason the above was feasible was because of the fact that the motor has a significant run on after reaching peak power (major complained when I called it a constant power region ) but my point in the other thread, which still stands is that if I was using a DC motor then I'd still want to reach peak power (assuming the same peak value) at the same road speed of 60km/h which means I'd have different gearing but the same wheel torque and therefore the same tractive effort, but if I didn't have the available overrun then I'd have to either re-gear the motor diminishing the bikes performance or alternately install a gearbox.
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  #22  
Old 09-29-2009, 05:11 PM
Bowser330 Bowser330 is offline
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Default Re: Torque is irrelevant or relevant?

Setup#1
300ftlbs 0-2000rpm
200fltbs 2000-4000rpm
100ftlbs 4000-6000rpm

Setup#2
200ftlbs 0-6000rpm

Which setup would you pick and why...

Note: Horsepower Formula:
TQ * RPM / 5252 = HP

Last edited by Bowser330; 09-29-2009 at 05:13 PM.
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  #23  
Old 09-29-2009, 05:39 PM
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major major is offline
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Default Re: Torque is irrelevant or relevant?

Quote:
Originally Posted by Bowser330 View Post
Setup#1
300ftlbs 0-2000rpm
200fltbs 2000-4000rpm
100ftlbs 4000-6000rpm

Setup#2
200ftlbs 0-6000rpm

Which setup would you pick and why...
Well, first off, you're comparing a peak of 228 hp to a peak of 152.

Why don't we ask ZX-E to run the 1/4 mile with these two set-ups on equal vehicles and see which one wins? Ref: http://www.diyelectriccar.com/forums...547#post142547 Maybe adjust mass or gear ratio so exit speed on the winner is just below 6000 RPM.

Interesting

major
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  #24  
Old 09-29-2009, 09:30 PM
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CroDriver CroDriver is offline
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Default Re: Torque is irrelevant or relevant?

Quote:
Originally Posted by paker View Post
This is about the best answer I've seen about torque.

Torque is what accelerates the vehicle and horse power keeps it there. Just about any car can travel at 100 kph or 60 mph but how fast it can get there depends on how much torque the engine produces.
I have a better one

"Understeer" is when you hit the fence with the front of the car.
"Oversteer" is when you hit the fence with the rear of the car.
"Horsepower"is how fast you hit the fence.
"Torque" is how far you take the fence with you.


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  #25  
Old 09-29-2009, 09:36 PM
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ZX-E ZX-E is offline
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Default Re: Torque is irrelevant or relevant?

In Reply to Major and Drew. I simulated two cars with the same shape and weight as Tesla Roadsters and chose the gear ratios based on when the cars hit 6000 RPM at the end of the quarter mile.

The car with the step function:
gear ratio 5.2
0-60 in 7.95 seconds
quarter mile in 16.5 @ 84.44 mph



The car with the constant 200 ft-lbs:
gear ratio 4.45
0-60 in 9.95 seconds
quarter mile in 17.3 seconds @ 100.4 mph




So you trade good acceleration for a high top speed. Not a big surprise.





Even if you have enough power to move your vehicle.Torque is relevant because it determines how you need to gear it. Also higher RPMs means more losses right?


-Rich

Last edited by ZX-E; 09-30-2009 at 01:15 PM. Reason: switched the order of the graphs to avoid confusion
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  #26  
Old 09-29-2009, 10:30 PM
Batterypoweredtoad Batterypoweredtoad is offline
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Default Re: Torque is irrelevant or relevant?

ZX-E, can you do the same calcs, but add a 2 speed trans with about a 1.7 first and a 1 to 1 second please? I'm curious how much it changes things.
Thanks!
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  #27  
Old 09-29-2009, 10:45 PM
tomofreno tomofreno is offline
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Default Re: Torque is irrelevant or relevant?

The forces acting on a vehicle to impede its forward motion apply a torque to the wheels. The vehicle moves at constant speed when the torque applied to the wheels from the motor through the drive train is equal to this torque. Call this the required torque. Acceleration from a given vehicle speed is proportional to the difference in this required wheel torque and available wheel torque, where available wheel torque is determined by the drive train and the max torque the motor/controller can deliver at that vehicle speed. This will be determined by the max current the controller can supply at lower motor rpm, and by the pack voltage and back emf at higher motor rpm.

Power is the product of motor torque and shaft angular velocity. Torque is the more fundamental parameter, just as force is the more fundamental parameter in rectilinear motion, where power is the product of force and velocity. Power matters because it describes how much torque can be delivered at a given rpm. A motor/controller that has high torque at low motor rpm but drops off rapidly at higher rpm, will have good acceleration at lower vehicle speeds, but poor at higher speeds. This of course will be affected by gear ratios which determine the mechanical advantage of the motor. I've calculated and graphed required and available wheel torques as a function of vehicle speed for a number of motor/controller combinations, and also plotted available power versus vehicle speed in a spreadsheet referred to as "ev calculator" at electricnevada.org.

Tom
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  #28  
Old 09-29-2009, 11:01 PM
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ZX-E ZX-E is offline
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Default Re: Torque is irrelevant or relevant?

Why would you want such a low gear ratio Batterypoweredtoad? It would take 30 seconds to reach 60 mph with the 1.7 to 1 gear ratio.

Make sure that's what you want because it'll take a while to set it up for more than one speed. Post it over here.
http://www.diyelectriccar.com/forums...567#post142567

-Rich

Last edited by ZX-E; 09-29-2009 at 11:45 PM.
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  #29  
Old 09-29-2009, 11:17 PM
neanderthal neanderthal is offline
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Default Re: Torque is irrelevant or relevant?

Well put tomofreno I studied physics in college. So its nice to see someone put it in terms like that. great stuff.

Anyhow, I think its best to put this stuff in terms of goals. I think we have fleshed out what torque and horsepower are. But what are your goals for your car?

If you want a drag car, especially one with direct drive, thats one thing. If you are into calculus, then you can see a graph showing this example. If you start off with mega acceleration, and it peeders off toward then end, you will still have finished the quarter mile faster than a car that starts off slower, but finishes with stronger acceleration. If you graph high initial acceleration vs time, you get more area under the curve in the first situation than in the second.

Sorry. In different terms, If you start fast you re already covering more ground faster and finishing faster and end up than someone who starts slower and finishes stronger. Even if they have the same overall power. So you win the quarter mile. Its about the curve.

However, most of us have street cars, so what matters is the torque capability in the rpm range we use it in.

It seems like some of us are talking about all out performance in a drag race, and others about driveablilty.
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  #30  
Old 09-29-2009, 11:43 PM
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ZX-E ZX-E is offline
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Default Re: Torque is irrelevant or relevant?

Would you agree that driveability has a lot to do with 0-60 acceleration? A faster 0-60 time at least for single speed drives would also mean that you are in the effective torque band at cruising speeds.
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