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Posted: Thu Jan 02, 2014 7:27 pm
by el-nicko
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Well, I don't know about everybody-else, but I'm still stuck on what, 'If it ain't broke, .....it's too heavy! Get some weight off.' means.
Posted: Thu Jan 02, 2014 8:16 pm
by Al
I have figured that bit out Nicko, but I have to confess I fell asleep on page two as my brain started to hurt
Al.
Posted: Thu Jan 02, 2014 9:06 pm
by Corvus
Corvus wrote:.........
A torque value must apply to a given shaft. It can't exist somewhere in between. In the gearbox aether!
........I can't put it any simpler.
Posted: Thu Jan 02, 2014 10:30 pm
by el-nicko
Al wrote:I have figured that bit out Nicko, but I have to confess I fell asleep on page two as my brain started to hurt
Al.
Mine too mate.
Oh, hang on...... I see 'corvus' has changed his signature line and is now mocking mine.
Posted: Thu Jan 02, 2014 10:36 pm
by Corvus
el-nicko wrote:Al wrote:I have figured that bit out Nicko, but I have to confess I fell asleep on page two as my brain started to hurt
Al.
Mine too mate.
How about this:-
When you start with a power value, you can see what a torque value would be against it, for any rpm. But only the rpm which actually happened will yield the actual torque which happened. Anything else is abstract.
Further, since we are dealing with rear wheel power, simply plotting crankshaft rpm against it will not represent actually happening crankshaft torque either. It will yield an abstract torque value.
Posted: Fri Jan 03, 2014 6:44 am
by Corvus
Coming at it from a more philosophical level.
We do the dyno run. Doesn't matter what kind of dyno, as long as it is testing the rear wheel output. We get the rear wheel power. We plot a torque figure based on crankshaft rpm instead of the actual rear wheel rpm. We then interpose a brilliantly accurate torque limiter between the crankshaft and the rest of the drivetrain. We set the torque limiter to our "corrected" 1:1 torque figure. We try to do the same run again.
Will we achieve the same figures, or will the torque limiter kick in and prevent us?
Posted: Mon May 26, 2014 8:53 am
by Corvus
Corvus wrote:Ok, so we are back to dyno power curves being rear wheel. Fine, that's what I thought. Don't have a problem seeing why and how. But the moment you plot a torque curve against it, but at engine speed, you get an abstract torque value. The torque value, expressed thus, won't have actually occurred. I've never thought it wouldn't be of use or have some meaning. I can see how we are kind of forced into crunching the numbers this way. But that wasn't my point.
Actual crankshaft torque occurs before any losses, so it will be a higher figure than the one we get shown. The best description I can think to give the torque value in question is "effective crankshaft torque".
That is the crankshaft torque but with all the transmission losses about to happen pre removed. As the losses occur, unfortunately it is the gear meshes which cause them and of course this multiplies the torque. The torque value won't have actually occurred anywhere in the transmission. Doing this also plays havoc with the power curve. The power is accurate and "as measured" but unfortunately it wasn't measured at the speed shown on the curves. The speed shown on the curves is crankshaft speed. The rear wheel power will have occurred at rear wheel speed (to state the obvious), but that makes the speed abstract not the actual power value.
Think about the curve posted by nick. Pick any spot on the power curve and then add back an estimate of the losses. Use 10% for the sake of argument. Then use the hp formula to get a torque figure. This will be actual crankshaft torque. It will have occurred at that value on the crankshaft. The torque figure shown on the graph won't have. It won't have occurred anywhere.
I reckon.
Picked up a perfect copy of John robinson's "motorcycle tuning- 4 stroke" for a reasonable price. I didn't want to read it for anything to do with dynamometers but there is a brief description of the various types. More specifically JR says this; "because horsepower is a function of torque and speed, it is not altered by gear ratios (ignoring the frictional losses caused by the gears). However, torque is amplified by reduction gears in the same way that speed is reduced."
I'm nitpicking here, but it's not quite the same. There is no loss of speed caused by frictional losses with gear teeth but there is a loss of torque.
JR then says that the torque figure we derive by essentially back calculating is "effective crankshaft torque" (also my exact words above). Effective because it is representing crankshaft torque with all the losses pre removed. But still a version of crankshaft torque and nothing to do with rear wheel. Or gearbox output if the dyno is hooked up at this point. He says "actual crankshaft torque" won't have the losses removed.
This to me fully justifies the abstract tag I think it deserves. That's because the "actual" crankshaft torque could be "actually" measured at the crankshaft itself. Whereas the "effective" crankshaft torque can't be measured anywhere because it doesn't "actually" exist.
See?
So why do people insist on referring to it as "rear wheel torque"?
Posted: Mon May 26, 2014 11:20 pm
by boxerscott
Does it really matter
It`s a load of bollox, one rolling road will argue with another, its all about comparisons, nothing more.
Posted: Tue May 27, 2014 9:45 am
by el-nicko
boxerscott wrote:Does it really matter
It`s a load of bollox, one rolling road will argue with another, its all about comparisons, nothing more.
Amen to that.
Posted: Wed May 28, 2014 7:26 pm
by Corvus
boxerscott wrote:Does it really matter
It`s a load of bollox, one rolling road will argue with another, its all about comparisons, nothing more.
That's not my point.
My point is not about how accurate one dyno is, compared to the next.
You're questioning the comparative accuracy with which two, three or four (or however many) different dyno's will compile the data from which we/the dyno computer then calculate the abstract torque figure I like to think of as "effective crankshaft torque", but which a lot of people seem to call "rear wheel torque".
Whereas I'm questioning what we actually call this value, what this value represents and also that we should accept the value as being abstract.