Todays question... Torque & BHP

ok... I know the SV is torquey and I know how that feels in comparison to the CBR...

but what exactly is torque? what does it measure?
and does this correlate to the power band graphs that bike mags like printing? or are they just measurements of bhp? and what exactly does the bhp figure mean? are the bhp and torque of a bike related??

please can someone explain cos I really would like to know...

I'm not going to do the maths explanation, since it's been too long for me since my physics a-levels and I just can't remember it, but I will do the wordy bit.

Torque is the tendency to rotate. If you apply torque to something you are trying to rotate it.
The measurement is the amount of force at a distance from the centre of rotation (Force x Distance)

Power is energy per second (Energy/time). It's the same as effort, for example, if you're walking along a flat road at a certain speed, to walk up a hill at the same speed would take more effort, more power.

The graphs in mags will have a torque figure and a power figure.
In fact, the power can't even be measured, it has to be calculated from the torque produced and the revs.

Which brings us to how they're related.
power = (torque x revs) / k
where k is a constant that I can't remember.
(finishing my explanation)
So power is equal to the amount we are pushing, multiplied by how often we push, all divided by a constant.

Originally Posted by lynw

but what exactly is torque? what does it measure?
and does this correlate to the power band graphs that bike mags like printing? or are they just measurements of bhp? and what exactly does the bhp figure mean? are the bhp and torque of a bike related??

well in 30 seconds - torque is the strength of the push, hp is the rate at which the push is delivered. An analogy I have heard is having prop forwards pushing a Volvo. More forwards = more weight = more torque so more load (Volvo and contents) is accelerated. The prop forwards' little legs can however only run at say 15mph so this is where the power delivery collapses. Harness the same weight of horse as you had beefy blokes pushing, and you get more power, same weight = same torque but delivered more quickly, so with horses you get up to say 25 mph............ Put an engine that delivers the same torque but with a wider spread of power and a gearbox in and you can go up and down hills and much faster. Does that make sense?

remember someone explaining it along the lines of going up stairs...

torque was how many stairs you could go at a time... ie a higher torque bike was equivalent to going up stairs 2 at a time

then power was like saying that if you had a low torque, 1 stair at a time bike but it was higher revs it was like saying although you were doing 1 step at a time the power/revs meant you were doing it quicker so were doing it the same rate as the 2 step at a time bike...

and torque was the ease in which a bike would pull away...

is that about right? but what do those power graphs actually mean? they look pretty but Id like to actually understand what theyre saying...

the stair explanation is right, yup.

What do they actually mean?
They are simply the torque output (and the calculated power output) at the full rev range of the engine.

Engines not being perfect, we have to make compromises on where we choose to make the torque and where we choose to make more revs.
Remember, power is made with either extra torque or extra revs.
But what if a modification to increase the revs changes the torque output? or vice versa.
So every change you make could change that graph, thus changing where in the rev range the torque is produced or changing the amount that it can be revved.

so basically if I understand this right...

the sv makes the power with torque while the cbr does it with revs...

which is why its easy to pull away on idle speed on the sv but I have to have the revs around 2-3k to pull away on the cbr...

and is this why the SV isnt such a top end performance bike like the CBR?

Originally Posted by Carsick

Which brings us to how they're related.
power = (torque x revs) / k
where k is a constant that I can't remember.
(finishing my explanation)
So power is equal to the amount we are pushing, multiplied by how often we push, all divided by a constant.

Isn't there some crossover point at a certain number of revs, eg 4400rpm (not sure if that's the figure) where torque = bhp?

ive read and heard so many people descibe torque and power in different ways with different examples but tbh, its still completly a mistery when i put it in terms of an engine.

all i know is torgue means it'll pull away faster at low revs. whereas low torque will require alot of revving. hence the wan&er with an RS125 revving the nuts off his bike to make it go.

Lyn, yup.

Ceri, yes, there is, and that's the constant I referred to. I can't remember the number, but for metric I think it's 5252 or something.

Originally Posted by lynw

which is why its easy to pull away on idle speed on the sv but I have to have the revs around 2-3k to pull away on the cbr...

and is this why the SV isnt such a top end performance bike like the CBR?

Just something to add onto this.
When you rev the CBR to whatever to pull away, you're probably producing about the same torque at that point as the SV does at it's pull away revs.

So, just because the SV is torquey, doesn't mean bikes that aren't considered to be particularly torquey can't produce the same amount.

In fact, I just looked at the figures, peak torque for a CBR600F is 63Nm at 10k rpm and peak torque for an SV is 64Nm at 7200rpm.