Should I do it?
#52
11-06-2006 | 04:46 PM
Senior Member
Joined: Oct 2006
Posts: 3,106
From: Misawa Japan
1st Gear Member
RE: Should I do it?
Goodness gracious, Nobrakes! I understand the principles you speak of, but why go through all of that (generator, load, various loss corrections and conversions)?
"A brake dyno incorporates some form of varying load, or brake, that the engine is forced to turn. The load is controlled to turn at a constant speed, and torque is determined by measuring the force on the brake. The drum of the dyno--spun by a motorcycle's rear wheel--is connected to the rotor of an eddy current brake; the actual brake looks something like a huge car alternator. Coils in the stator are controlled by a varying signal, and the resulting magnetic interaction (the eddy currents) places a rotating force on the stator. This force is measured with a strain gauge, and is representative of a motorcycle's torque. A typical step test run on a brake dyno begins with the motorcycle at idle and in a tall gear, turning the drum. The throttle is opened, and the engine revs to a set rpm, say, 2500. The brake is applied by the computer so that the rpm is held constant at 2500 with the throttle wide open. When the load and rpm reach equilibrium, strain on the brake is measured (and, hence, torque and horsepower), and the throttle and brake are released. This is repeated for each rpm increment--often 1000--up to redline. A step test can be conducted using any throttle position."
Now doesn't that sound a whole lot better?!
"A brake dyno incorporates some form of varying load, or brake, that the engine is forced to turn. The load is controlled to turn at a constant speed, and torque is determined by measuring the force on the brake. The drum of the dyno--spun by a motorcycle's rear wheel--is connected to the rotor of an eddy current brake; the actual brake looks something like a huge car alternator. Coils in the stator are controlled by a varying signal, and the resulting magnetic interaction (the eddy currents) places a rotating force on the stator. This force is measured with a strain gauge, and is representative of a motorcycle's torque. A typical step test run on a brake dyno begins with the motorcycle at idle and in a tall gear, turning the drum. The throttle is opened, and the engine revs to a set rpm, say, 2500. The brake is applied by the computer so that the rpm is held constant at 2500 with the throttle wide open. When the load and rpm reach equilibrium, strain on the brake is measured (and, hence, torque and horsepower), and the throttle and brake are released. This is repeated for each rpm increment--often 1000--up to redline. A step test can be conducted using any throttle position."
Now doesn't that sound a whole lot better?!
ORIGINAL: Nobrakes
I think he said is was a Dynojet 250.
It really depends on how well the machine can put a load on the rear wheel and how well it can determine what that load was. The load on the rear wheel and how fast it is spinning determines the power. And as Eddie said, if the rear wheel is slipping due to the knobbie tread, then the full power of the bike will not be delivered to the machine and you'll get a low reading.
If one hooked the rear wheel up to a generator and powered an electrical load - that would be one way of precisely measuring the output, i.e., just measure the voltage being produced and the current being pulled through the generator - power = current * voltage. But you'd also need to know what the efficiency of the generator was. For example, my pesimized 22 hp turns out to be 16 Kilowatts if you do the conversion. But the generator itself may only be 50% efficient, so the real output might be 32 KW - that's where it would get squirrely I think since generator efficiency is probably not a constant with its RPMs. Either way, that's a pretty big electrical load and would require a fairly hefty generator and some good sized heating elements to load it. But it would be one way of doing it.
But you should get the same reading for output power regardless of the drive train, rear wheel size, etc. Rear wheel torque is an entirely different matter and that does depend entirely on the drive train. Power is essentially a measure of how fast that torque can be delivered.
While I'm familiar with the physics of power and so forth, dynometer implementations are new to me so a lot depends on how they are actually implemented. Hence my questions. I'm sure Google has the answers.
I think he said is was a Dynojet 250.
It really depends on how well the machine can put a load on the rear wheel and how well it can determine what that load was. The load on the rear wheel and how fast it is spinning determines the power. And as Eddie said, if the rear wheel is slipping due to the knobbie tread, then the full power of the bike will not be delivered to the machine and you'll get a low reading.
If one hooked the rear wheel up to a generator and powered an electrical load - that would be one way of precisely measuring the output, i.e., just measure the voltage being produced and the current being pulled through the generator - power = current * voltage. But you'd also need to know what the efficiency of the generator was. For example, my pesimized 22 hp turns out to be 16 Kilowatts if you do the conversion. But the generator itself may only be 50% efficient, so the real output might be 32 KW - that's where it would get squirrely I think since generator efficiency is probably not a constant with its RPMs. Either way, that's a pretty big electrical load and would require a fairly hefty generator and some good sized heating elements to load it. But it would be one way of doing it.
But you should get the same reading for output power regardless of the drive train, rear wheel size, etc. Rear wheel torque is an entirely different matter and that does depend entirely on the drive train. Power is essentially a measure of how fast that torque can be delivered.
While I'm familiar with the physics of power and so forth, dynometer implementations are new to me so a lot depends on how they are actually implemented. Hence my questions. I'm sure Google has the answers.
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