Torque and Horsepower - A Primer

[alelanza]

Well it can't be any clearer, not much else left to discuss I would say.
Anyways, big props to xCrisx and especially to Machin, this forum needs you guys. Also great contributions from Reca, it's great to see where sounds analysis can take you.

A question that's always bugged me and never got a good answer for, is wheel slip a function of wheel power or wheel torque?

[DumHed]

well you can't have wheel spin without rotation, so there is power there.

Wheel slip in general is due to exceeding the tyre's grip capability, so the rotational speed is not really important - hence torque is probably closer than power when describing the situation, but you would still need to know the rolling diameter to know how much force was involved.
On a rolling road dyno, the term is "tractive effort" - which is the force the tyre is exerting on the roller.
The power is calculated from that, knowing the rpm and diameter (and inertia) of the dyno roller.



One of the points a few people have been trying to make here is that, in a race car situation at least, you can't really have torque without power - or vice versa.
They are both present and directly connected, but for most calculations or comparisons, power is more useful because it contains more information.

When people talk about torque, in relation to and engine's performance, it's more about the torque characteristics - ie generates torque down low, or up high. Even then it can just as easily expressed in terms of power output, power curve, etc since they are directly linked.

It's just another area of confusion when you get people going around saying torque is more important than power.

"power sells cars, but torque wins races" would be better expressed as "peak power sells cars, but a wide power band wins races"

[n smikle]

I was banned for a week and now I am back! (Somebody has beeb reporting me in rapid succession for simple "witty" comments! but I forgive them).

I wish I was here when engineer roy posted this:

http://www.f1technical.net/features/16900

My arguments were very inline with Roy's article. Props to engineer Roy. It seems I already have a bad name on this site so when I post facts I get verbally stoned like a scorned woman.

Maybe my article PLAN 2 will help,I wrote it especially to move the debate concerning Torque and Power further ahead, and I trust you will find it useful in that respect. Get right to the end for the power curve (probably start there if you wish). You will find it in articles on this site, just search "PLAN 2".
I look forward to further discussion with you all, machin, chris, beelsebob, strad and uncle Tom too. Hopefully it will encourage some new contributors to this great Forum for I have written in simple terms. Best wishes Roy

Thanks to Roy, I think this primer is over. I am glad that you all have read Roy's article and moved on.

BUT.....Just in case there are still some staunch disbelievers, there are engines that produce no Torque, but there is no engine that does not produce power. If that is not enough then there is no convincing.

[DumHed]

http://i24.photobucket.com/albums/c36/v ... ics010.jpg

[machin]

A question that's always bugged me and never got a good answer for, is wheel slip a function of wheel power or wheel torque?

Now it gets messy....

If you look at this chart below (I made this for another post, so ignore the pink arrows!) it shows force at the wheels for a car with 4 gears. The shape of the force curves is the same shape as the torque curve, so taking 4th gear as an example, the force at the wheels is highest at about 90mph, and therefore it is most likely to spin its wheels at that point (highest torque), rather than any other point in the speed range in that gear.

So now you'll ask, "if the force is greater, surely the acceleration is higher than in the same gear at maximum power (higher up the rev range)?" And the answer is "yes" -in that gear, however you can also see from the graph that for the same speed (assuming that there IS enough grip) it is better to drop down to a lower gear (gear 3) and increase the engine speed where the torque x speed (power)is greater, because it generates more force (and hence more acceleration) at that road speed....

[alelanza]

Many thanks for that Machin, your response clears it up perfectly!