Horsepower Vs. Torque

[Cabot]

Hey, my name is Cabot, from WI but currently living in GA working on a co-op. I go to school at Michigan Technological University and I'm an OK B rider in district 16 (which from what I hear is a pretty slow B rider everywhere else) I read this forum every once in a while - alot of good information on here. I don't post here all that much - but I thought you guys might find this interesting / add some more insight on this topic.

Me and my buddy/roomate have been trying to figure out this debate over Horsepower/torque for a good year now. It seems like everyone has a different opinion about which one is better.

Horsepower can't easily be measured, instead it is usually calculated:

HP = (Torque*RPM)/5252

Since 1HP = 33,000 ft-lb/min
divide by the circumference of a circle with a 1ft radius = 33,000/(2*pi) = 5252, to convert circular motion to linear motion.

(which is why on every dyno graph you look at, you will have more torque then HP under 5252 RPM, and more HP then torque above 5252RPM, having exactly the same HP as ft-lb of torque at 5252 RPM.)

So the ultimate question we wanted to answer, was would you rather run in your peak torque or peak hp range.

Since Torque = ft-lb, you can divide by the radius of whatever your applying the torque to get straight lb force.

and Force = mass*acceleration, and since mass is constant.. you would have your max acceleration at your max torque.

But then what does Horsepower mean then? This took us a while to figure out.

Power = Force * Velocity
HP = (lb-force * MPH) / 374 (374 is just a conversion factor)

well Rearrange to get

lb-force = (HP*374)/MPH

so if you maximize power, you would maximize force = maximize acceleration.

So what do you want to do?

Take a look at these graphs I made from my old bike (05 CRF250R) from looking at the dyno graph. *Note* these numbers are approximate since I didn't have exact HP/torque readings at the exact RPM.

Also, I got the gear ratios/wheel dia off the internet, accuracy may not be exact. Also assuming stock gearing. (the speeds seem a little fast(?)) - but gives a good enough idea of what is going on.

Since I want to maximize FORCE to the rear wheel (f=ma), lets do an example where I coming out of a corner in second gear. I want to know where I want to shift. Well.. if I shifted to maximize my torque curve, I would be shifting at around 10,000 rpm to put my bike in the peak torque curve for 3rd gear. looking at speeds (assume I wouldn't gain/loose speed while shifting), I'd be running roughly 8,500 rpm in 3rd gear. Well my force at the rear wheel will drop from ~ 345 lb to ~295 lb.

Now lets say I shift later.. even though my torque curve drops off, I'm still putting out more torque (& force) to the rear wheel then I would if I shifted to be in the max torque in my next gear.

So where do I want to shift?
well if I shift at 12,000 rpm going 43mph (putting out 290 lb-force), that would put me at ~10,000 rpm in third gear (putting out 286 lb-force). if I shifted later then that, torque starts to drop off more then it does in my next gear.

Now take a look at the HP curve.. I'm getting peak HP roughly between 10,000-12,000 hp!

Even though the maximum acceleration comes from maximum torque; for a certain velocity range - I want to run within my peak HP because that is where I am taking the most out of the mechanical advantages caused by gearing

So Horsepower can be explained by maximizing mechanical advantages within the system!!



Now my buddy is a circle track car guy. Last two years he was racing a Chevy Cavalier (4 cyl class) since he doesn't shift.. he would benefit most from gearing his car to run at an RPM that maximizes HP instead of running at an RPM that maximizes torque - because even though he is past his *peak torque* - he would still be putting out more Torque at the same velocity because of gear ratios giving him more mechanical advantage!

I don't know - me and my buddy could be wrong. But we've done a lot of thinking/math/other things trying to figure this out and understand it. If you think I'm wrong - let me know!

:ride:

 

[helio lucas]

no, your are not wrong.
rear wheel driving force is what really matters. there is a lot more important things in mx or other kinds of racing than peak Hp or peak torque.
i first became aware of this by tune some scooter engines (variable gear). the more rpm you make the motor turn (making the engine peakier) less Kph the bike would run. :nod:

john bradley´s book "the racing motorcycle" volume 1, deals with all this basic design principles including frame geometry and aerodinamics, suspension and less "mainstream" aspects but equally important, gearboxes and ratios...
one of the best money i ever had spend (the correct word is not spend, but invested). :cool:

 

[RM_guy]

I have to say I'm impressed with your analysis :nod: I have wondered the same thing (and just got a headache :))but since I can shift, I just put it in the gear that gives me the most power to the ground and twist the throttle accordingly. After all, what's important is what is getting to the ground and once I know, by feel, what RPM/gear selection works for a particular condition, that's what I go with.

Now, since your poor buddy can't afford a gearbox, this becomes much more interesting :laugh:

 

[Cabot]

And I'm not saying that its best to follow this guide on the racetrack. Knowing your bike/application is way more important than any numbers will ever be!
My buddy spends way more time trying to make his car turn left then he does worrying about peak torque/hp. Even in circle track, races are won in corners.
But I guess it gives a better meaning to the numbers that manufacturers give out to show off the performance of their machines (bike, car, lawnmower, whatever)

Helio - I just ordered the book! thanks!

 

[jsantapau]

From what I understand and I beleive is that if you are throttling the engine you want the engine rev range to be from peak torque to peak horsepower in a fixed gear kart coming out of the turns at peak torque and ending the straights and peak horsepower
in a gear box the spacing of the gear ratios of winding out one gear to peak horsepower to drop in the next gear at peak torque with wider spacing at the lower gears since the mechanical advantage of the ratios allows you to stretch each gear a bit to let you catch up to how quick the engine revs

In a fixed rpm engine you want it to run at peak torque since that is the most efficient the engine will run

 

[2strokerfun]

I'm way impressed with the work you put into this. You could have spent all this time perfecting your video game skills !! Maybe my doom-and-gloom predictions for the younger generation are all wrong.......... Naaahhh !!
Seriously, nice work.