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Torque versus Power - the easy explanation

Discussion in 'General Motorcycling Discussion' at netrider.net.au started by friction, Aug 5, 2010.

  1. I've never really quite "gotten it" until now, when I finally decided come hell or high water I was going to figure out why peak power is after peak torque, when one can be calculated directly from the other. Now that I understand, it's really not that hard.

    Torque is the force applied to something to make it turn, while power is an amount of work (such as moving a certain weight a certain distance) performed in a certain time - best described by examples:

    Imagine a steel rod, half an inch thick, with an infinitely strong metal wire wrapped around is so that pulling on the wire will unravel it and cause the rod to turn. An elephant pulling on the wire creates a huge amount of torque - you'd have no hope of holding the rod with your hand to stop it (slowly) spinning - but very little power. Since the maximum RPM of my elephant engine is very low, a massive gear will be needed to get the RPM up high enough to start generating reasonable power.

    As always, there's the formula: HP = Torque (ft-lb)*RPM / 5250. What this means is that low torque is fine providing you have high RPM, and low RPM is fine if have high torque. You can always use gearing to get more RPM (and therefore more power), but while a higher ratio gear accelerates quick, it's speed at the engine's redline is slow - you run out of gear fast! So more torque from the engine also means less gear changing.

    Chances are you already knew that, I did - but here's the kicker: Why is peak power output AFTER peak torque output? Well the answer is more obvious than I realised. Once you hit peak torque (say, 11,000RPM), going to 11,500 means you have a little less torque BUT FAR MORE RPM, so power continues to rise. At 12,000, RPM is even higher but now the quickly decreasing torque is starting to take it's toll and the power flattens off. Any more RPM and the torque losses exceed the RPM gains.

    Moral of the story: change after peak power, not peak torque.

  2. Or you could follow the rest of the world and use the metric system
    Depends what you're after. Straight line acceleration? then yes. But for belting through short sharp twisties? hmm...

    We know that torque is essentially force acting in a rotational direction. from year ten, Force = Mass * Accel. Now, the point where maximum instantaneous force occurs must be the same point where maximum instantaneous acceleration occurs since the mass of the bike does not change. So the point in the rev range where acceleration is greatest is at peak torque.

    If someone is going through bends (tight rather than sweeping), the chance that they can wind out the bike to redline is fairly low, and the amount of time they will be accelerating for will be minimal. So to keep max accel when going through some curves you want to be closer to the peak torque of the engine rather than the peak power.

    The occurrence of max instantaneous acceleration is quite evident on electric motors where the torque and power profiles are well out of sync in comparison to an I4 combustion.

    As a rule of thumb, keeping revs between peak torque and peak power will get you through.

    Now for a problem: The way a bike is geared can render everything I just said completely irrelevant. How does the measured value of torque and power change with regard to the gear a bike is in? enjoy...
  3. And it's mainly for the above reason that I will generally be sticking to twins and triples for road riding. Track bike may be a different story if that ever happens.

    Also worth reading this thread
  4. I dont see why its torque vs power since my bikes engine has both.

    In america they probably charge torque as a $500 dollar extra on some cars. Standard of education...sigh.

    Ive seen some whacky peaking dyno graphs, the best way is to look at the graph and determine the RPM range your gearbox gives you, then superimpose that range over the graph to determine the most area
  5. Torque is easy. 1 foot pound = 1 pound of force applied at a one foot offset from the axis of rotation.

    To make it even easier:

    kW is how fast you hit the wall
    Nm is how far you move it

  6. Zackly. No point changing at peak power if your next change is 1000 rpm away and the power curve falls away to nothing 500 rpm after peak power.

    similarly if there is only 500rpm between changes and there is 5000 rpm between peak torque and peak power, then you are not going to be going quickest by changing at peak torque.

    It's all really a pointless discussion however unless you are on a 250 or smaller.
  7. African or Asian elephant?
  8. Power/hp = How hard you hit the wall.
    Torque = How far you take the wall with you.
  9. HP = top end pull
    T = bottom end pull

    That's how I've always understood it.

  10. for people like me who lope around on the weekend, enjoying the windy roads and scenery in the hills, its torque that does the job.

    for those that get from point a to point b....ASAP, its power, all the way.
  11. Nooooooooooooo

    HP sells cars.
    Torque wins races.
    Physics is for luuuuzerzz.
  12. And on a related note, sports 250s have been described as a Japanese attempt to multiply zero by infinity in a desperate effort to get something :LOL:.
  13. That's the 5 second version, essentially.

    Power (as in, how much energy is being converted from petrol's chemical energy to kinetic energy every second) is what propels/accelerates/moves the vehicle. Torque is just a force. Power is useful transfer of energy.

    But if you have an engine which generates a lot of torque at low rpm, you'll be making a lot of power at low rpm as a result and therefore have a strong low-rpm pull.

    The two are directly related (Power at a certain rpm = torque at that rpm x engine rpm x constant). The confusion comes from colloquial abuse of the terms "torquey engine" vs "powerful engine".

    Silly example of how confusing colloquial terms can get: Take two 1000cc engines. Both make 100Nm peak torque at some point in the rev range.

    If one's tuned to make peak torque at low rpm, with plenty of low-end power as a result, at the cost of becoming breathless at high rpm, it'd be called a "torquey engine".

    Tune that same 100Nm engine to make peak torque at high rpm instead of low rpm (to get that 170hp superbike powahh), and people will claim it has no torque at all, but plenty of top-end power. :tantrum:

    They both make exactly the same peak torque, they just make useful power at different engine speeds. Stupid colloquialisms. :)
  14. That's the best quote in the history of physics. :D
  15. force is a linear vector. Moment (or torque) is angular, and the expression is Moment = moment of inertia*angular acceleration (or second derivative of angle).

    this is usually laughable since it has been known since the superbike days that litre fours produce more torque than twins. If you compare a honda fireblade with the sp2, once you hit 2500rpm, it's all over for the twin. The four produces more torque everywhere from there to the (higher) redline. And no-one rides a bike around at 1500-2000rpm anyway.
  16. for the same power output, the higher revving engine will use a higher reduction gearbox and the lower revving engine will use a lower reduction and both will get the same quarter mile time.

    THe reason the harley's needed a seperate category class was they don't rev hard - the other bikes of equivalent capacity revved much higher and produced the same torque, so they disappear into the weeds while the harleys are still changing gear.
  17. Note: Rules of thumb are not always correct. Otherwise, spot on.
  18. Those saying not to shift at peak power - have you forgotten the gearbox?

    Torque is multiplied, power is not. Rear wheel torque will not be max if you shift at engine peak torque.
  19. iEdd is exactly right. Shift up when, for your road speed, power will be higher in the next gear than it is in your current gear. As RPMs will drop when you shift, and (lets assume) road speed won't, this RPM must be above max power, and will vary (slightly) depending on whether you're going 1st->2nd or 2nd->3rd etc.
  20. Ahhh, just rev it 'til the valves bounce, then change up :twisted:.