HorsePower and Torque
An answer I gave on kiwibiker to a post about HP and Torque
I agree with the writer regarding “low end torque” “high end horsepower” and the way some road testers waffle on about “waves of torque” “massive low end torque” and other bs, especially when testing the latest offering from Harley. No offence to Harley, they actually make pretty good bikes, but considering a GSX1400 or ZZR1200 (not to mention ZX14 and Busa) all put out more “low end torque” than a 96” Harley, it says more about the testers/writers than about the bikes.
I don’t agree with his assertion that HP is what makes your bike go, accelerate etc. and “Torque does none of the above” You cant have one without the other. Even he says that.
Using a chassis dyno we measure roller speed and rear wheel torque and from those we derive HP, measure engine rpm, and derive engine torque with
T(engine)=HPx5252/rpm(engine) and this is what gets put on your graph
Rear wheel hp and engine hp are essentially the same (not withstanding transmission losses etc)
Rear wheel torque and engine torque are different, because we have a gearbox, primary etc between the engine and rear wheel.
Example
At the engine
10,000 rpm
76 HP
40 ft/lbs
4:1 reduction gearbox etc
At the rear wheel we get
76 HP
2500rpm
160 ftlbs
HP is the same but torque has increased.
And that’s why your bike accelerates better in lower gears
Why you change down when going up a hill (some of us)
Because it is rear wheel torque that actually pushes us along, determines how the bike accelerates etc.
To accelerate harder we need more rear wheel torque, increase the HP or change the gear ratio.
Not wanting to be too picky but, his idea about VE is more wrong than right, and the “torque figure for a twin……is likely to be higher due to the longer stroke” is just perpetuating a common myth that long stroke motors produce more torque because they have a “long” stroke
I guess the flowery hyperbole is a bit like chrome, sounds and looks flash but actually means sweet FA
rock on
Well that's actually true. Given a motor that produces the same force within the cylinder (i.e. the force pushing down the piston is the same), the motor with a longer stroke will produce more torque. For precisely the reason illustrated with diagrams in the article -- a longer stroke means a longer distance from crank centre to crankpin; so there's a bigger leverage. We all know the longer the lever, the more torque is applied.
But the side effect of that is you need more movement to produce that torque. So the piston has to move further... means trouble getting the engine to rev higher. So develops less peak power compared to how much torque it's producing, as while you might be able to put out more torque due to more leverage on the crank, you can't put out that torque as often (because redline is lower), so the rate of work is less.
Of course what you say is true in the sense that every engine is different, so many different variables. So you're right in that just because it's a long stroke motor doesn't mean it's going to produce wads of torque... which is the usual bullshit the magazines and know-it-alls at the pub like to spout.
Good article, thanks for scanning it in TWR. |
Well I will have to disagree with you. Its not actually true. The assumption that the downward force from the piston is the same for both long stroke and short stroke engines is wrong. For the same cylinder pressure a short stroke piston will provide more downward force. If you work it all out and you will see that the torque applied to the crank will be about the same. I am talking about 2 engines, same size, same cylinder pressure, one short stroke, one long stroke, both engines with their best rod ratio.
cheers
no worries
assuming 2 engines same size one long and one short stroke the short stroke engine will have a bigger piston, more surface area. eg a 100 x 63 and 79 x 101 both 500cc. The 100mm bore will have around 12 sqin, the 79 mm bore around 7.5sqin. If you had say 300psi in the cylinder the force on the pistons would be 3650 lb and 2280lb. Assume max mechanical advantage for both ie best rod ratio, and you will end up with 453 ftlbs of torque at the crank in both cases.
450 ft lbs is a peak figure, lucky if it lasts more than 10-15 degrees of the power stroke so once you average it out over 720 degrees its a heap less.
cheers
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