Gearing v dyno hp

[Bones]

I have a question for the more technical among us. Would changing your gearing have an effect on hp readings on a dyno??? The reason I ask is, just say your bike is fitted with 17/42 sprockets, and a roll on test in 4th gear from 2000rpm to 9000rpm took say 10 sec, then put 18/39 sprockets on and do the same test, and with it being higher geared acceleration is slower and it takes say 15 sec.

Now not sure how dyno's works, but seeing as how the lower gearing gets to redline and top speed quicker, and feels more powerful, would that equate to showing more horsepower on the readout.

                                                            Tony.

[Fj.itis]

Hp will be the same, but torque can be manipulated.

[Bill_Rockoff]

Short answer: No.

In theory, if the engine is making the same power, gearing shouldn't affect the power to the ground(a) or the power measured by the dyno (b).  These are two slightly different things.

(a) In reality, chains eat up less power as you put them on a larger-diameter sprocket, all other things being equal, and they eat up more power as you run them faster, all other things being equal.  Chain design theory says 19 or more teeth would be smoother and more efficient, and a larger sprocket set would be easier on your bearings (twice the diameter means the chain only has to pull half as hard.)  However, then you start to run into chain speed issues, and since all those engineers in Japan have been working 80 hour weeks trying to out-perform each other since the first CB750, I guess they have found that "chain speed" is more important than "size of countershaft sprocket" for a bike geared to run 150+ mph, which is why 15/16/17 teeth sprockets seem to be OEM on so many bikes.  So, basically, as far as we're concerned, gearing shouldn't matter much for putting power to the ground (or the dyno drum.)

(b) Yes, shorter gearing would get to redline quicker.  That doesn't equate to "showing more horsepower" though, what that equates to is "showing the same amount of horsepower, except it has less work to do this time, so it can do this new smaller amount of work faster."  For the purpose of changing gearing a tooth or two, it shouldn't matter on a dyno reading.  For the purposes of getting a repeatable(*) reading, you want a transmission gear that's close to direct drive (1:1 ratio) so things don't heat up and have more drag / worse lubrication from one run to the next.  For the purpose of getting an accurate reading(*) on an inertia dyno (which is most of them you'll find in speed shops) you want as tall a gear as possible without over-speeding the dyno mechanism, because that way you minimize the inertia effect of the drivetrain components.  (This is more of a factor for cars, which have 10 or 20 lb flywheels and a lot more rotating mass before the wheels.  Running a car in first gear gives a lower hp reading, because a lot more of the car's power goes to speeding up the crank and flywheel quickly.)

Most dynamometers in a motorcycle shop will be "inertia dynos."  (Remember high school physics, the equation "F=mA," Force = mass times Acceleration?  That works for spinning stuff too, "τ=iα"  Torque = moment-of-inertia times angular-acceleration.)  These dynos have a big heavy drum where the moment-of-inertia  "i" is known, plus a way to measure the drum's rotation speed accurately, plus a built-in clock.  Using the speed measurement and the clock, it can compare "speed now" and "speed a fraction of a second ago" and calculate how quickly it's accelerating.  Since the computer knows the constant value of "i" it can calculate torque.  Using that 

much torque is

.  The dyno measures its own speed, and as your bike

(Actually, it's not its weight/mass that's important, it's the rotational inertia "i" that's important.)  The dyno has a way to precisely measure its speed, and a way to measure time.   The equation for acceleration, F=mA (Force equals mass times Acceleration) also works for rotation (t=ia, or torque equals rotational inertia times angular acceleration.)  So, you can measure

As your bike accelerates it, they can measure the drum's speed at two different milliseconds and calculate "it takes X ft-lbs to

[andyb]

Depends on the dyno.  Eddy-current, notsomuch.  Inertia, probably.

In theory, yes in any case, but you'd have the devil's own job to measure the difference with something as crude as a dyno.

What the original question asks has a reasoanble answer:  You're adjusting for that on the dyno itself.  A pull from 3-9k on a 16/42 gearset will take very little time, but it also won't cover much ground on the speedometer.  The same pull on 18/39 gears will take greatly more time, but the speedo's needle will swing quite a lot more.  Proportionately so, in theory.

There's all sorts of parasitic drag things to watch for, but other than ensuring that things are grossly in good order, they're only for those chasing tiny improvements with large budgets.

[Bill_Rockoff]

Argh, used to be you could edit longer than 5 minutes.  I can't do ANYTHING in 5 minutes, and it takes me 10 to explain anything (ask anyone at an east coast rally!)

Basically, changing the gearing shouldn't matter on an inertia dyno on a motorcycle.  An inertia dyno measures speed and time, and uses those two things to calculate acceleration.  It uses the acceleration and the known drum inertia to calculate torque.  It uses the torque and the speed to calculate power.  

If the units you use are "Horsepower" and "foot-pounds" and "Revs per minute" then the equation is "HP = Ft-Lbs x RPM / 5252."

For a given amount of power, you can twist it twice as fast but you'd only be able to twist it half as hard.  In the OPs example, his taller gearing is 14% taller - at a given rpm in a given gear, you'd be going 1.14 times as fast, but only twisting the rear wheel 0.88 times as hard.  (0.88 is 1 / 1.14.)

[Flynt]

lower gearing gets to redline and top speed quicker, and feels more powerful, would that equate to showing more horsepower on the readout...

Nope...  inertia dynos measure (change in speed)/(change in time) = acceleration of the dyno drum.  You'll get to the higher RPM more quickly, but you'll be at a lower drum speed at that RPM.  I see Bill has offered a much more thorough explanation, but this simplified one may be easier to digest depending on your physics background...

Frank

[andyb]

The guys have it right.

The theoretical tiny changes from differences in the amount of chain friction are pretty ignorable.  Until your speedo is displaying plaid, they don't amount to enough to matter.  And even then, they really still don't, as they're the least of your worry by then

Frank might win the award for clarity on this one, but he's saying the same thing that Bill and I are.  We could break out arguments about levers and stopwatches, but the short answer is no, gearing doesn't matter for a dyno.

It does matter on the asphalt, though.  You can have more thrust at the ground, or you can have more outright speed.  Pick a compromise you like, and roll with it.

[Bones]

Thanks gentlemen, we do have some clever people among us. One of the reasons I asked the question was because I've read here and elsewhere ( magazines ) that people think the early FJ's had more power than the later one's, when in fact the early one's had a bigger rear sprocket than the later one's giving the "impression"they had more power. Early one's had I think 42 teeth, where later one's gradually got down to 39 teeth, as standard on my 93. This quote from Andyb

gearing doesn't matter for a dyno. It does matter on the asphalt, though.  You can have more thrust at the ground, or you can have more outright speed.

Coincides with a roll on test between a 90 model and my 93 where the 90 had the slight edge from what I assume having a standard 40 tooth rear sprocket making acceleration a bit quicker, and not from having more power.

They all run the same engine specs so therefor, ALL must have the same power.

Hopefully this clears the myth once and for all that later models are down on power compared to the early models, their NOT, it's just the gearing making them seem that way.
                                                            Tony.

[racerrad8]

One of the reasons I asked the question was because I've read here and elsewhere ( magazines ) that people think the early FJ's had more power than the later one's...
                                                            Tony.

One other thing to keep in mind regrading the 1100 Vs. the 1200, the lighter rotational mass of the 1100 with the smaller & lighter pistons also allows it to rev quicker.

Randy - RPM