Conkerman said: "The dyno measures rear wheel torque."
Well if it does, it doesn't print it out! The curves I've seen are always at engine speed. Rear wheels don't run at engine speed. Simple as.
Dyno mystery
Moderators: slparry, Gromit, Paul
Sorry but I am getting lost now.
Yes the dyno measures wheel torque, but it also knows RPM. So beep, bap, boop a plot of corrected torque/power vs engine speed.
Which at the end of the day is exactly what the punter wants.
The fact that it may not be particularly useful is a bit daft.
The fact it is not measured directly is meh IMHO.
Yes the dyno measures wheel torque, but it also knows RPM. So beep, bap, boop a plot of corrected torque/power vs engine speed.
Which at the end of the day is exactly what the punter wants.
The fact that it may not be particularly useful is a bit daft.
The fact it is not measured directly is meh IMHO.
Gary
conkerman wrote:Sorry but I am getting lost now.
Yes the dyno measures wheel torque, but it also knows RPM. So beep, bap, boop a plot of corrected torque/power vs engine speed.
Which at the end of the day is exactly what the punter wants.
The fact that it may not be particularly useful is a bit daft.
The fact it is not measured directly is meh IMHO.
Nooo no! Don't bale out on me.
I've done a bit of quick research. The convenient and therefore common dyno in use seems to indeed be the inertia type. If I've got it right then these measure rear wheel power. The power needed to accelerate the deliberately heavy drum.
So to take the rear wheel power curve first and then express a torque curve for that power, but at engine rpm, is abstract.
I can't think of any other way to say/ask it.
I've lifted this extract from the link I posted at the beginning.
"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."
I don't know whether this guy is a respected source or not, but he and I seem to singing off the same hymn sheet.
![[smilie=dont know.gif]](./images/smilies/dont%20know.gif "dont know"){kind=small}
wouldn't this post be better suited to "Geeks' Corner" section?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 the difference is in what data the computer gathers and what it spews out.
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 the difference is in what data the computer gathers and what it spews out.
After pondering till I can ponders no more, I'm satisfied in my own head that what I'm saying is right. I've backed it up with two sources in the field.
Summary: they show you a power curve representing absorbed power at your rear wheel. Then they plot a torque curve against that power curve at engine speed.
That is 100% abstract. It doesn't represent your vehicle at all in torque terms.
What a strange thing to do.
That leaves me wondering why? Why is it done like that.
Summary: they show you a power curve representing absorbed power at your rear wheel. Then they plot a torque curve against that power curve at engine speed.
That is 100% abstract. It doesn't represent your vehicle at all in torque terms.
What a strange thing to do.
That leaves me wondering why? Why is it done like that.
Been ferreting around and finding some confirmation of what I'm saying, although no one, as yet, seems prepared to go so far as to use the word "abstract", as I have. And still do.
It seems that on the "coastdown" the clutch is pulled in, so only the losses for the final drive and gearbox are "felt".
It seems that on the "coastdown" the clutch is pulled in, so only the losses for the final drive and gearbox are "felt".
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el-nicko
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- Location: Nick from HEREFORDSHIRE, The Oceanian province of Airstrip One.
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= 1000 words.
= 1000 words.
Last edited by el-nicko on Tue Nov 26, 2013 9:47 pm, edited 1 time in total.
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Mr. Spock is my role model so be advised; I possess no (discernible) sense of humour.
It's all VFR (DCT) round here now. STILL missing my 1100s tho.
Mr. Spock is my role model so be advised; I possess no (discernible) sense of humour.
It's all VFR (DCT) round here now. STILL missing my 1100s tho.
My last word on this. Maybe I need to work on explaining?
A dynamometer measures torque. Power is calculated from torque and engine speed.
Wikipedia, encyclopedia Britannica and how stuff works agree with me on this.
One more time...
Bike on dyno. vROoooooM
Dyno sees torque applied by and speed of rear wheel and RPM data.
Coast down data allows you to determine transmission losses.
Therefore you can calculate crank torque and subsequently power over the rev range.
Coast downs and other coefficients may be generic but will be consistent and are probably poor value for money to determine every time.
At the end of the process you get a nice set of curves as Nick showed.
If this doesn't make sense I suggest going to chat to a decent dyno operator.
A dynamometer measures torque. Power is calculated from torque and engine speed.
Wikipedia, encyclopedia Britannica and how stuff works agree with me on this.
One more time...
Bike on dyno. vROoooooM
Dyno sees torque applied by and speed of rear wheel and RPM data.
Coast down data allows you to determine transmission losses.
Therefore you can calculate crank torque and subsequently power over the rev range.
Coast downs and other coefficients may be generic but will be consistent and are probably poor value for money to determine every time.
At the end of the process you get a nice set of curves as Nick showed.
If this doesn't make sense I suggest going to chat to a decent dyno operator.
Gary
Going by your above explanation the above graph is displaying a calculated (as opposed to directly measured) CRANKSHAFT power and CRANKSHAFT torque curve? If that is the case then everything I've said evaporates.conkerman wrote:My last word on this. Maybe I need to work on explaining?
A dynamometer measures torque. Power is calculated from torque and engine speed.
Wikipedia, encyclopedia Britannica and how stuff works agree with me on this.
One more time...
Bike on dyno. vROoooooM
Dyno sees torque applied by and speed of rear wheel and RPM data.
Coast down data allows you to determine transmission losses.
Therefore you can calculate crank torque and subsequently power over the rev range.
Coast downs and other coefficients may be generic but will be consistent and are probably poor value for money to determine every time.
At the end of the process you get a nice set of curves as Nick showed.
If this doesn't make sense I suggest going to chat to a decent dyno operator.
I was under the impression that the power curve was measured rear wheel power.
This whole thing came about by someone claiming (not the first person to do so) "rear wheel torque", which clearly wasn't.
Going back to nick's curve. He hasn't answered my question. Is the graph showing rear wheel power? Just a yes or no please.
The data to the side says "wheel", written under where is shows the power. Mind you, it also says nearly 13,000 rpm as well!
Please, don't anyone ever tell me this isn't confusing.
Ps: the fire blade has one hell of a motor! (Not trying to be sarcastic)
There are different types of dynamometer. The inertia rolling road type is different from the traditional "industrial" type.conkerman wrote:My last word on this. Maybe I need to work on explaining?
A dynamometer measures torque. Power is calculated from torque and engine speed.
Wikipedia, encyclopedia Britannica and how stuff works agree with me on this.
The crux of my point, whether my point stands or not, is not what the rolling road measures, so much as what it spews out on the graph.
It measures the power absorbed in accelerating the inertia of the drum.
It's what the pesky computer does with the data that is the confusing part.