Dyno mystery

[Propellor]

Not a specific bike problem this, but a technical question.

I have power transmission experience but not rolling road dyno's or engine tuning.

Why does an inertia rolling road dyno seem to spew out what I can only think of as an abstract torque curve?

They seem to print rear wheel power first and then plot a torque curve against that power, but at engine speed.

Assuming I've got that right, then the resulting torque value(s) will be abstract. Of use, yes, but abstract all the same. And also fictitious in the strict sense? Fictitious in that the torque value will not have actually occurred anywhere in the transmission.

Have I got that right?

Assuming I have, then I'm guessing there's a good reason (or even a bad reason!) for it. What is that reason?

Thanks in anticipation.

[Steve H]

Dog -------------------------------------------------------------------- Bone !

[Propellor]

Dog -------------------------------------------------------------------- Bone !

Ha ha ha.

Frivolity aside though, it's a genuine post! I would really like to know.

[Propellor]

http://www.nrhsperformance.com/tech_power.shtml

Extract: 
"One point of confusion, and I see it all the time, is when people look at torque on a dyno sheet and call it rear wheel torque. Seems to make sense, after all, the measurement was made at the rear wheel, it's rear wheel horsepower, must be rear wheel torque, right? I even see this mistake made by veteran motor guys as well as in magazine tech articles. Not unusual at all to see a glowing report of "100 ft-lbs at the rear wheel", for example.

   Well, let me tell you, if someone really only has 100 ft-lbs at his rear wheel, get a stock Blast and you'll blow him into the weeds. Even in top gear the little Blast has 4.97 of gear reduction between the crank and the back wheel: 1.676 primary times 1.0 top gear times 2.963 final. With 30ft-lbs or so at the crank, that comes out to nearly 150ft-lbs at the back wheel. When you're in first gear, you've got 13.35 of gear reduction between the crank and the rear wheel giving you a whopping 400 ft-lbs!
 
The confusion lies in interpreting the dyno's numbers. It's not showing rear wheel torque, it's showing engine torque as measured at the rear wheel, and that's an important distinction. A Dynojet dyno won't even show torque unless you use the tach pickup, ever wonder why? It's because it needs to understand the gear reduction that lives between the drum and the crankshaft in order to calculate the torque at the crankshaft, which is what it displays. Notice how it plots torque against engine rpm, not rear wheel rpm, and the torque crosses the power at 5252 engine rpm, not rear wheel rpm. That's because it's engine torque, i.e. upstream of the gearing."


http://www.bristoldyno.com/info/whatis.htm

Extract:
"The computer then back-calculates the torque using the formula mentioned previously.  Those of you with a knack for physics will realize that the torque produced in first gear at the tire-drum interface will be significantly greater than that produced in fifth gear.  Since the rpm of the engine is factored in, the different speeds developed by the different gears are negated - therefore, one can say that the computer reports "engine torque as measured at the wheels."


Back calculates infers that the power curve comes first. This is counterintuitive because you need to know torque before you get hp. But is the difference in what data the computer gathers and what it spews out?


Are the two sources I've linked respectable? I wouldn't know. Anyway, they are echoing my own thoughts.



 

[guest564]

That's pretty much it. If you want to get really technical then there is an SAE spec that defines the standards for dynos.

An interesting plot is one that shows the corrected power through the gears. Below is an old one of mine, you can see that although it doesn't make huge power the power curve is very flat with no drop off between gear changes. The blue plot is the power corrected to 1:1 gearing. The dyno runs were done from second gear.

[Propellor]

That's pretty much it. If you want to get really technical then there is an SAE spec that defines the standards for dynos.

An interesting plot is one that shows the corrected power through the gears. Below is an old one of mine, you can see that although it doesn't make huge power the power curve is very flat with no drop off between gear changes. The blue plot is the power corrected to 1:1 gearing. The dyno runs were done from second gear.

Thanks.

I think I understand the red curve. That looks natural, unless one rides marc Marquez's honda. Is the blue one simply a rear wheel power curve for the engine?

The 1:1 "correction" crops up. This is likely the crux of my puzzle.

With regard to most printouts, they seem to show a power curve and a torque curve, both plotted at engine speed. Is the power curve in such cases representing rear wheel absorbed power?

[guest564]

At 1:1 the engine would be turning at the same speed as the rear wheel so the gearing has no effect, there are however still losses from the transmission. The rear wheel size is also corrected for so it doesn't matter what size that is either.
The power curve is bhp at the rear wheel, calculated from corrected rear wheel torque.

[Propellor]

At 1:1 the engine would be turning at the same speed as the rear wheel so the gearing has no effect, there are however still losses from the transmission. The rear wheel size is also corrected for so it doesn't matter what size that is either.
The power curve is bhp at the rear wheel, calculated from corrected rear wheel torque.

Thanks for reply. It's appreciated.

I've not been letting up. Not just yet.

I've recently had a discussion with someone who reasoned that the "torque referred to engine speed" that I've been talking about has to be rear wheel torque because that is the torque put to the rollers by the rear wheel. Doh!

It's everywhere. My gast is well and truly flabbered that this thing is called "rear wheel torque".

It just causes so much confusion. It is misinterpreted. IMO.

Cheers.

[Propellor]

Came across this very good article.

http://www.fjr1300.info/misc/torque-power.html

From it, this extract:

"Nevertheless, any dynamometer chart that shows torque and that does not specify the gear is most definitely the engine torque, albeit adjusted for drive train losses."

So it's not the engine torque then. It's the engine torque, but with losses removed. But if we measured the engine torque, it wouldn't be this. On which shaft could we measure this figure? None.

It gives the illusion that you measure this figure at the rear wheel, but you don't.

IMO.

[guest564]

Many dynos will measure the losses and then apply that correction as well. otherwise when run off the bike the torque is usually measured at the gearbox sprocket. A manufacturer will measure off the crankshaft with the gearbox removed.

[Propellor]

Many dynos will measure the losses and then apply that correction as well..........

But in those cases, they won't refer to the resulting figure as "rear wheel torque" presumably? Also, I assume that such a torque curve, reflecting true crankshaft torque, will be plotted with a partnering "crankshaft power curve"?

Is it reasonable to expect such a power curve to have a value around 10% higher than the rear wheel power curve detected at the drums?

Cheers.

[Propellor]

http://www.thumperclub.com/smf/index.php?topic=7132.0

The dyno curve sheet, second post on link above, illustrates clearly what I'm saying, I believe.

The three power curves portray reality as does the top (red) torque curve. But the the two lower torque curves (blue and green) are abstract, in my view.


Edit:  In essence, when they apply the 1:1 correction factor, what they are doing is giving the illusion that all the gears and chains in the transmission are 1:1 ratio. In other words all the losses are present, but no reduction in speed. If all the transmission ratios were 1:1 the torque figure wouldn't be abstract. But the transmission ratios are not 1:1.

So it is abstract.

See my point?

[Propellor]

At 1:1 the engine would be turning at the same speed as the rear wheel so the gearing has no effect, there are however still losses from the transmission. The rear wheel size is also corrected for so it doesn't matter what size that is either.
The power curve is bhp at the rear wheel, calculated from corrected rear wheel torque.

I've been trying to press the point that the torque curve produced on the majority of rolling road dyno curves I see is abstract. The more I think about this the more apparent it becomes, in my eyes at least, that virtually everything about this procedure is abstract, to a greater or lesser degree.

First the 1:1 ratio. That's abstract because the overall ratios aren't 1:1. I can't really put it any other way.

The absorbed rear wheel power figure will be true, but they make the graph abstract by showing it against engine rpm. So that's a mildly abstract way of depicting rear wheel power.

Taking what rhinoman said above "At 1:1 the engine would be turning at the same speed as the rear wheel". Look at that the opposite way around. The rear wheel would be turning at the same speed as the engine. Now if we have a 600 sportsbike on there, that'd be 16500 rpm. How abstract do you want it! You'd have pieces of wheel embedded in your body, so that's very clearly abstract.

The final, most abstract thing, is the torque curve. The nearest description as to what this is, is given in the extracts I posted from two sources on the web, namely "engine torque as measured at the rear wheel". Except that doesn't seem right to me either. Because the engine torque which will ultimately produce the rear wheel power won't have suffered any driveline losses. The true engine torque doesn't seem to get shown with the majority of rolling road graphs that I see.

In my eyes the torque figure shown is exactly what you'd expect it to be the moment you "correct" the ratios to 1:1. It's an imaginary rear wheel torque figure. Existing in a world where bikes have an overall ratio of 1:1. Which, I guess is the crux of what rhinoman has said in the paragraph quoted above.

On reflection his paragraph above does give the answers. He won't come out and say its abstract though!

(Abstract as in "doesn't really exist").

Curiouser and curiouser!

[themoudie]

Aye Propeller,

Can't be abstract, as that doesen't have a physical or concrete existance. You can see the trace formed by the deflection of a pen, caused by an electrical current generated from a spinning drum.

Maybe it should be a construct, rather than an object!  Construct

I shall now 'duck' away to the garage and do a bit more constructing of Mini the Minx. 

Toodle pip, Bill.

[Propellor]

Aye Propeller,

Can't be abstract, as that doesen't have a physical or concrete existance. You can see the trace formed by the deflection of a pen, caused by an electrical current generated from a spinning drum.

Maybe it should be a construct, rather than an object!  Construct

I shall now 'duck' away to the garage and do a bit more constructing of Mini the Minx. 

Toodle pip, Bill.

I looked up the word to be sure it was the one I wanted to use. I think you'll find the torque value doesn't exist. The white rabbit in Alice in wonderland didn't exist but the words on the paper did. And the torque value in question is every bit as curious.

I don't think we're singing off the same hymn sheet.