Dynamometers

I have a simple question to ask about dynos. I've heard that when using a dyno it doesn't matter what gear the bike is in. I can understand how maximum horsepower is measured, however how does a motorcycle dyno measure torque when the gearbox acts as a torque converter ? As I understand it torque is the twisting power of the crank, and I believe that crankshaft mass has a major effect. I know that when I've raced two strokes sometimes turning into a head wind, it feels as though the bike has stalled. I never seem to get that effect using the commando based bike. I've also found that it is almost impossible to detect an increase in torque from riding the bike, unless the overall gearing is raised and the bike accelerates quicker. I've also noticed that the high geared commando motor is much better with a close ratio box behind it.

Hi acotrel.
Tourque is Force x Distance so the piston stroke and matching crank throw is the simple answer I reckon.
Ta.

Inertial dynos like the Dynojet use a massive flywheel ( the roller that the rear wheel sits on) of known mass and they measure the rate of change of rotation of that mass to calculate torque.

Since the dyno's computer is also connected to a rev counter, RPM is taken into account when making caculations and horsepower can be calculated since it's a function of torque and RPM.

Every time I've been to a dyno we've used 4th gear for the tests.

All the rolling road dynos I've seen in action, they accelerate the 'bike*' through the gears.
*As pommiejohn says, the computer calculates the torque and hp from how fast the heavy flywheel is accelerated.

Engines with big outputs get through the gears quicksmart on those things, smaller engines take a while.

HP is torque x rpm . The GB multiplies torque, but sacrifices rpm. In a perfect universe, the product torque x rpm from the GB is a constant, regardless of the gear, but as pommie john has stated, top gear is usually used.

Slick

What I notice on clunker 4sp Cdo and my 6sp Suvee ESPECIALLY on THE Gravel is if I keep throttle set same I can go up and down gears to change rpm/power band+flywheel gyro balancing+tire pulse grip w/o changing speed but a few mph, 3 at most, which is critical decision points for easing another efficient loose turn w/o instantly too spirited on snot slip out. What else I noticed is grearing up so tall engine can not pull red line in top but so can stay in lower gear advantage longer/faster yet tops out about the same as if gearing a bit lower to hit redline in top. A semi equivalent inertial dyno are the accelerometer kind ya plug in pilot and bike mass and hook up to speedo or it own GPS and rip up and dn somewhere enough times to make sense of the calculated tq * hp.
Water brake is considered the bench mark and can strain engine down to a dead stop like the garden tractor pullers do through rpm range as direct shaft power. Btw 37 cc Brigs twin can make 70 hp/7500 rpm on water brake dynos sized for that range. Our engines 750 cc should/could be nearly 100 hp compared. They startle recoil ya not attract and hurt up close not pleasant. Will have to ask TC how they measured his 150hp 850s.

I want-need someone to filter though the cycle mounted data loggers I should get-buy that we could pass around for fun education and dyno charts comparison.

hobot said:

Btw 37 cc Brigs twin can make 70 hp/7500 rpm on water brake dynos sized for that range.

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If your 37 cc Briggs and Rotten can show 70 bhp on an accurate dyno, it would need to be doing 140,000 rpm or more. Please explain.

X-file said:

hobot said:

Btw 37 cc Brigs twin can make 70 hp/7500 rpm on water brake dynos sized for that range.

Click to expand...

If your 37 cc Briggs and Rotten can show 70 bhp on an accurate dyno, it would need to be doing 140,000 rpm or more. Please explain.

Click to expand...

37 cu inch

opp typo - so 37 ci vs 45 ci then. I recoiled first time i heard a garden tractor fire up rev up mean as snarling Harley or Norton. Someone might take on task of taking their Commando to various dynos and data logging to get answers we can make use of. Might be fun profitable side line to trailer roller dyno to bike rallys as somewhat commonly done. If I find a decent data logger to pass around who would want to try it out?

During my childhood I was involved in testing rocket motors which were mounted on gimbals with a load cell on the front. We used to monitor the thrust. I can get my head around that way of assessing improvements in thrust. I find rolling road dynos a bit more difficult to understand. Perhaps if there was an easy way to get a pressure sensing device in the appropriate range and get the bike to push against it while the rear wheel is loaded up on the roller, the answers might be different ? How do they dissipate the energy in the brake on a bike dyno ?

acotrel said:

During my childhood I was involved in testing rocket motors which were mounted on gimbals with a load cell on the front. We used to monitor the thrust. I can get my head around that way of assessing improvements in thrust. I find rolling road dynos a bit more difficult to understand. Perhaps if there was an easy way to get a pressure sensing device in the appropriate range and get the bike to push against it while the rear wheel is loaded up on the roller, the answers might be different ? How do they dissipate the energy in the brake on a bike dyno ?

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Water brake dynos work like that. An hydraulic cylinder stops the brake from rotating, effectively becoming the load cell you used. The pressure in the cylinder is proportional to torque at the crank. ( Water brakes are normally mounted on the crank ).


One of the rolling road dynos I used was described as an " Eddy current dyno". I assume there is an electromagnetic brake on it. Sorry I don't know how heat is dissipated.

One thing I have learnt from using several different dynos is that air flow into the room is crucial. I was at one that was built into a sea container and we got a decent first run, but after that jetting changes didn't seem to make sense. We were getting poorer results no matter what we did. Finally we went back to our original setting and got nowhere near the same power. At that point we realised that our eyes were watering and we could hardly breathe. The engine had used up most of the oxygen in the room and there wasn't sufficient air flow to replenish it. We ended up running with the doors open and got good results.

Air flow is something many dyno operators overlook.

One thing I learned from my few years of road racing is that 'torque wins races'. I'm interested in using a dyno, however whether the torque graphs would be meaningful ? - I don't really want to find out by getting ripped off. My commando engined bike is the strangest I've ever ridden, you really know you'd got long stroke and heavy crank. With the standard box, it is really horrible.

acotrel said:

One thing I learned from my few years of road racing is that 'torque wins races'. I'm interested in using a dyno, however whether the torque graphs would be meaningful ? - I don't really want to find out by getting ripped off. My commando engined bike is the strangest I've ever ridden, you really know you'd got long stroke and heavy crank. With the standard box, it is really horrible.

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You can learn so much in a couple of hours on a dyno... if it's well set up and the operator knows what he's doing. So much of how a bike feels is subjective. EG a big hole in the mid range followed by a peaky power band can make a bike feel fast even though it's slower than a torquey engine.

I would ask other racers which dynos they go to.

acotrel said:

I have a simple question...

Click to expand...

Good luck with that!

acotrel said:

I've heard that when using a dyno it doesn't matter what gear the bike is in. I can understand how maximum horsepower is measured, however how does a motorcycle dyno measure torque when the gearbox acts as a torque converter ?

Click to expand...

The plain answer is; TORQUE X RPM / 5252 = HP. So, in anything other than high gear, even though you increase the torque, you reduce the rpm by equal amounts. Now, you would properly conclude that it would come out equal, and it would in a perfect world. HOWEVER, while it's true that you can use any gear when running on a chassis dyno, there's a few reasons for running high gear:
1) What usually ends up being the limiting factor is tire traction against the dyno's flywheel. As you point out, the gearbox acts as a torque multiplier in anything but high gear. Consequently, the higher the gear used, the less force (but higher RPM) is applied top the rear tire.
2) With an inertial dyno, the weight and rpm of the flywheel is figured into the dyno's software. The software takes into account RPM change that the engine goes through during a run, and compares that against how quickly the dyno flywheel accelerates. Using high gear forces the engine to labour longer against the dyno's flywheel, giving a better picture of RPM vs torque for the software to compile.
3) One of the things that makes our Nortons just a little bit faster than they should be is the fact that the design of the drivetrain effectively removes the transmission gears from the circuit whilst in high gear. Maybe only a couple of percent loss, but it adds up when racing. Consequently, high gear helps show a truer picture of actual engine horsepower without the parasitic loss incurred through the transmission. If you look at most "modern" motorcycles, the clutch is gear driven (1-2% loss), then, since the clutch is now spinning opposite direction of the engine, the final drive has to reverse that again in any and all gears (another 1-2% chopped off).

Nathan

Fairly decent overall explanation of how dynos work, look here

http://en.wikipedia.org/wiki/Dynamometer

http://www.land-and-sea.com/dyno-dynamo ... rticle.htm

https://rideapart.com/articles/how-a-dy ... horsepower

http://skyshorz.com/university/resource ... basics.pdf

To answer your "simple" question, try this quote from the skyshorz article above:

"Typically the engine was operated at a given
throttle setting, and the dynamometer was controlled to maintain a constant shaft
speed. All measurements were taken from the transmissions final drive sprocket.
These numbers could be converted back to actual engine numbers by dividing the
torque by the gearing ratio, and multiplying the speed by the gearing ratio. Note,
however, that this does not account for the efficiency of the transmission, which for a
typical motorcycle can be expected to be on the order of 90%. All of the power and
torque numbers presented here are the raw numbers, uncorrected for transmission
losses."

There's an interesting article here that discusses the development history of the Dynojet intertial dyno, the one that pretty much established the inertial dyno market.

http://www.hotrod.com/how-to/engine/113 ... ssis-dyno/

Lots more info available if you search online and take the time to read.

Ken

I have had both my bikes on 2 x different Dynos in my local area, now have settled on the second of the two due to the closeness to home and the dyno operator.

Both are close to sea level and both have huge fans for ventilating and cooling in the booth.

Typically the operators here run the bikes in 3rd gear for a 4 speed box and 4 th gear for the 5 speed for power and torque, and in the case of the race bike, 5th gear for a speed run to check on my gearing at the time, just to give a bit of base data for that set up.

Going back to same dyno each time, makes it easier for set up plus they ave your bike profile in their data base.

Dont forget to go with a hand full of jets, slides , needles and spark plugs if you are trying to get optimum results for a high performance motor.
I went once with three different mufflers. Huge difference between all three sets.

Regards Mike IMHO

Ken, from the link you provided, I found this in the text :

Dynojet's final number-fudge was arbitrarily based on a number from the most powerful road-going motorcycle of the time, the '85 1,200cc Yamaha VMax. The VMax had 145 advertised factory horsepower, which was far above the raw 90hp number spit out by the formula. Meanwhile, existing aftermarket torque-cell engine dynamometers delivered numbers that clustered around 120. Always a pragmatist, Dobeck finally ordered his Chief Engineer to doctor the math so that the Dynojet 100 measured 120 hp for a stock VMax. And that was that: For once and forever, the power of everything else in the world would be relative to the '85 Yamaha VMax and a fudged imaginary number.

Seems the primary calibration is suspect. Of course what is really important are comparisons of performance 'improvements' and not the actual figure. The sensitivity of the torque measurement through the rev range is the thing which really interests me, not the max. horsepower.

What did Norton factory use that allowed Combat 65 hp claim? Otherwise might be able to adapt one of these on crank shaft of replace rear hub with
http://www.aemelectronics.com/files/pr/ ... ft_PR.html

acotrel said:

How do they dissipate the energy in the brake on a bike dyno ?

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On a rolling road dyno, they don't need to.

Its ACCELERATING the big heavy weight in the rollers that they calculate the hp from.
Powerful engines accelerate it radiply, less powerful ones more slowly.
Its reasonably accurate and consistent across quite a wide rate of bikes.

I once saw (and heard) a turboed ZZR1100 on a rolling road dyno.
The speed it went through the gears on that thing, you knew it was making some serious mumbo.
And it was....

They do have a brake, or sorts, to slow that heavy roller after a run.
Or it could take ages to come back to a halt.
They can also use the rear brake on the bike...

Rolling road dynos and braked dynos generally give reasonably consistent results too, although the braked dynos are rarer these days ?
Something strange has to happen to get a big reading on one, and not the other...

hobot said:

What did Norton factory use that allowed Combat 65 hp claim?

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Brit motorcycles factories often used Heenan and Froude brake dynos, although I don't know what they used for Commando era.
There is a photo of Joe Craig (Race Director) and staff in Nortons dyno shop, with a manx engine setup on the dyno.
It used a chain drive back to the jack shaft, and drove a big water pump (?)(I think, without finding the pic).

There was mention that when a crankcase burst, and the crank - still spiining at high rpm - went whizzing around the shop,
everyone ducked for cover !

There is mention somewhere that oiling the chain prior to a run was usually good for 2 or 3 bhp,
if the numbers didn't quite stack up.