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I have a question relating to rolling road dyno's. If anyone is knowledgeable on such.

I've come across a few examples where people have referred to their dyno curve showing a bikes rear wheel torque, when, to my eyes, it plainly doesn't. But in their defence, sometimes the data associated with the curve actually refers to the torque figure as being rear wheel, when it plainly isn't. The clue is in the speed. It is engine speed. Besides which, the torque figure in question is just too low.

The interesting thing is that the torque figure isn't crankshaft torque either. In esscence it seems to be a torque curve related to the rear wheel power, but plotted at engine speed.

Why is it expressed in that abstract way? The resulting information, torque wise, misleads a lot of people.

What? The question is far to difficult especially for a Friday.

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I have absolutely no experience of dynos so I may be talking out of my 4rse
but if torque is a force X a radius then rear wheel torque is the amount of force exerted by the rear wheel to turn the rolling road, it will be different for each wheel size and will vary through the rev range as maxium engine power is achieved in each gear.
Maybe?

How crank torque is calculated from that I don't know, never had cause to get involved with it.

Merecat wrote:I have absolutely no experience of dynos so I may be talking out of my 4rse
but if torque is a force X a radius then rear wheel torque is the amount of force exerted by the rear wheel to turn the rolling road, it will be different for each wheel size and will vary through the rev range as maxium engine power is achieved in each gear.
Maybe?

How crank torque is calculated from that I don't know, never had cause to get involved with it.

Thanks for reply. I've done some searching on tinternet and found what I think is the answer to my question. Bit busy at the moment but will post link and hopefully better explain my question, probably tomorrow.

Cheers

I'll have a go but probably need to understand your question better.

Rolling road dyno measures the torque at the rear wheel (obviously).
Calculations are then made for the losses in the drive train, gearbox and the tyre slippage at the contact patch. Also for the reduction in radius of the wheel and tyre where the contact patch is strapped down onto the rollers.
They can either measure direct at the rear wheel or do all the calcs to approximate the results at the crankshaft - which is then roughly equal to figures obtained on an engine dyno. Don't forget any gear ratio used other that 1:1 alters revs and torque figures.

Not sure why you say that the figures are not rear wheel torque or isn't crank torque either.

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

The above, or more precisely, a paragraph in the above helped make the answer to my question obvious.

I'm not a total stranger to the horsepower formula or to power transmission generally but I've no experience with engine tuning or rolling road dyno's.

What was puzzling me was that a few times I've encountered people making claims of the rear wheel torque their bike makes, as proven by dyno printouts. But the figure is always way too low to be true, actual transmitted rear wheel torque. And besides, you'd need to either have as many torque curves as you have gear ratios, or be specific as to which gear is being tested.

I've seen dyno curves which display rear wheel torque and yet the curve is at engine speed. Confusing.

After delving into this, and maybe sp250 will substantiate, I realize what they and the curves are referring to is an abstract torque figure. The best way I can think of to describe it is "effective crankshaft torque". That is crankshaft torque with all the transmission losses about to happen, already taken off. If that makes sense.

In actual fact it is crankshaft torque as measured at the rear wheel.

But then of course it's not crankshaft torque is it.

For inertial rolling roads, I believe the computer measures the power taken to accelerate a known mass? In most cases it would seem the actual transmitted torque is not displayed on the printout. Instead it seems the machine is given information as to the crankshaft speed and as it already knows its own roller speed it essentially corrects the torque curve to engine speed. So, if I've described that right, it is definitely not transmitted rear wheel torque.

My understanding is that losses are worked out on the run down as the computer knows the deceleration rate of the roller mass driving the crankshaft.

el-nicko wrote:.




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Ha ha. I like the third one in. The little guy going "no! Stop!"

Sorry mate. I'm a sucker for a good power transmission debate. If I'm causing you to vomit, I do apologise.

The Teutonic Tangerine wrote: What? The question is far to difficult especially for a Friday.

I asked it on Thursday. Where were ya!

Hee Hee.

I’m having a hard time understanding the question too.

But, if I’d be a rolling dyno owner offering people to dyno their bikes, how would I like to present the results?
Obviously, the most honest would be torque at the rear wheel. ‘cause that’s what the bike is putting on the tarmac.
Plotted to what? For the customer, I’d think engine rpm would be the best. It tells him something, as opposed to plot it to rear wheel rpm. What else relevant can you plot it at?

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To do it properly you really need to do a set of coastdown runs to determine parasitic losses in the transmission/wheel friction. In the main a 'cookbook' figure is used to correct the number you get.

Tapio wrote:I’m having a hard time understanding the question too.

But, if I’d be a rolling dyno owner offering people to dyno their bikes, how would I like to present the results?
Obviously, the most honest would be torque at the rear wheel. ‘cause that’s what the bike is putting on the tarmac.
Plotted to what? For the customer, I’d think engine rpm would be the best. It tells him something, as opposed to plot it to rear wheel rpm. What else relevant can you plot it at?

I think you have perfectly illustrated exactly what I'm talking about. If you happen to have a copy of a dyno sheet to show us, hopefully I'll be able to illustrate what I mean.

If you try to put some actual figures to the words you've just said, I believe that we won't be able to reconcile them with the hp formula. Try it on here and let's see. Unfortunately I don't have a dyno sheet to use.

Most dyno sheets don't seem to show actual rear wheel torque.

If nothing else, there's a chance I'll end up proving myself wrong and putting a proverbial custard pie in my face. It'll give the audience some entertainment.

conkerman wrote:To do it properly you really need to do a set of coastdown runs to determine parasitic losses in the transmission/wheel friction. In the main a 'cookbook' figure is used to correct the number you get.

What I'm trying to say is that I've had a few debates where persons have said "my bike puts out xx ftlbs at the rear wheel". The figure they give sounds low for rear wheel torque in any gear so I say "show me the graph". So they show me the graph and it turns out what they are referring to is "effective crankshaft torque, as measured at the rear wheel".

So therefore it's not actual crankshaft torque and it's definitely not actual rear wheel torque. But lots of people seem to believe it is rear wheel torque. Either they are wrong or I am. I'd like to satisfy myself either way. There's nothing worse than having that nagging doubt that something you believe might actually be wrong. Whether getting a casting vote at the forum is proof I'm not sure. But let's try.

They are getting confused with torque curves and tractive effort graphs.

Tractive effort = Force at rear wheel using gearbox to multiply torque.

'Torque curve' = 'Fudged' engine torque to remove gearbox effects.

Bike used to publish the TE graphs in reviews.