Dyno mystery

[themoudie]

Aye Propeller,

Exactly! 

Toodle pip, Bill.

[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.

"Corrected rear wheel torque". Is that the industry definition for it? If so, "corrected" seems an odd choice of words, given that (in my eyes) there's nothing correct about it.

You say the rear wheel bhp is calculated from this. I'm surprised the computer doesn't already know the rear wheel bhp from the data at the drum. Surely it would know its own rpm? Why does it need this abstract torque value to know the power? It must already have the true data from the drum and have used it. The only way (that I can see) it would get this "corrected" torque is by proportioning it against the actual torque and the actual speed compared to our contrived speed (crankshaft). Or by back calculating it from the already derived actual rear wheel power and our contrived rpm (crank rpm). This is how I thought it was done?

I apologise for prattling on about this but I am very intrigued by this contrived torque value. I've been involved in power transmission applications in the past and everything was black and white. Give us the torque and rpm and we can get the power. But we dealt in actuals! None of this corrected mullarky!

It's doin me freakin head in!

Let me try and put it another way. A torque value must apply to a given shaft. It can't exist somewhere in between. In the gearbox aether! We pretty much have four shafts to choose from, to calculate what torque value applies there. The crank, the gearbox in, the gearbox out or the axle (rear wheel). That's it. So which one does our corrected torque belong on?

I can't put it any simpler.

[guest564]

You need to try and find a copy of SAE J1349 which is the primary standard for dyno measurements.

[Propellor]

You need to try and find a copy of SAE J1349 which is the primary standard for dyno measurements.

Happy new year rhinoman! Thank you. I will give that a try. Past experience suggests one has to pay for the privilege of accessing sae standards?

Meantime maybe this slant may explain my viewpoint better:-

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.


Edit: btw, I'm not questioning compliance with a standard. That's definitely not my point at all. I'm in no position to do that, even if I wanted to (which I don't). They all seem to do it that way, so I'm assuming they ARE meeting a standard. I'm just proposing that it is abstract.

[Propellor]

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?

[Propellor]

You need to try and find a copy of SAE J1349 which is the primary standard for dyno measurements.

Had a look about re SAE std above. Lots of info on the ambient air correction factor, but only a cursory mention that when in doubt, use an 85% mechanical efficiency assumption. We don't seem to see this mentioned on our dyno printouts so I assumed the power curve we get is rear wheel hp.

A bit of advice out there to use 4th gear as this results in a direct drive 1:1 ratio, but this is nonsense with respect to most, if not all, motorcycles and clearly is referring to cars.

[Propellor]

You need to try and find a copy of SAE J1349 which is the primary standard for dyno measurements.

A bit more perusing. SAE J1349 certified power seems to be aimed at getting manufacturers to portray realistic hp outputs from the engine. It doesn't seem concerned with rear wheel outputs or transmission loss measurement. Have I interpreted that wrongly?

[Propellor]

You need to try and find a copy of SAE J1349 which is the primary standard for dyno measurements.

Had a look about re SAE std above. Lots of info on the ambient air correction factor, but only a cursory mention that when in doubt, use an 85% mechanical efficiency assumption. We don't seem to see this mentioned on our dyno printouts so I assumed the power curve we get is rear wheel hp.

A bit of advice out there to use 4th gear as this results in a direct drive 1:1 ratio, but this is nonsense with respect to most, if not all, motorcycles and clearly is referring to cars.

The 85% mechanical efficiency figure seems to be an assumption relating to the engine internal friction losses? Doesn't seem to be a reference to transmission drivetrain efficiency.

One thing I'm finding out is that on all the sites  I've looked at on the web, mainly dyno testers' sites and forums, nobody bats an eyelid at producing a torque curve by referring rear wheel power to engine speed.