Okay...I get it.
This explains.
Everyone-
This is Aaron Kadera from Kinetic again. I figured I'd drop some new information on what the power computer is and the procedure to calibrate our computers.
Tire pressure is irrelevent. Our computer measures power at the rubber, not at the hub. As long as your tire is not slipping, the Kinetic PC is accurate to +/- 3% of a PowerTap reading at all reasonable speeds (0-40 mph). To avoid tire slippage, use a cheap, hard-rubbered tire filled with proper pressure and make sure the roller is pressed against the tire firmly.
The following will eventually be posted on our website. I also have the calibration results and procedures for other trainers if anyone is interested.
The Kinetic Power Computer was designed to work with the Kinetic Road Machine. However, it is not limited to use only on our trainer. Here’s how it works:
The Kinetic PC operates on the principle that the Road Machine’s resistance is speed sensitive and ONLY speed sensitive. Some people don’t buy into that, but if you think about it, the only factor that has any impact on the resistance unit is how fast the roller is spinning – independent of rider weight, cadence, crank length, etc. After riding one of our trainers, you are, no doubt, aware of a relationship between speed and effort. The faster you go, the harder it is to pedal. The beauty of the Kinetic Road Machine is that we have tweaked and studied this relationship to a further extent than any other company.
We have used our test robot, Crank Armstrong, to analyze the forces experienced by a rider on our trainer. Specifically, we analyze the amount of wattage needed to sustain any given speed on our trainer. Once we get the data from Crank, we send it over to Tom Compton at Analytic Cycling. He uses a computer program called Mathematica to fit the data to a mathematical formula. The end result is a very accurate power curve and a mathematical formula that describes it. The best-fit formula for the Kinetic Road Machine is in the form P = Ax + Bx3, where P is power in watts, x is speed in miles per hour, and A and B are constants. With this formula, we can calculate how many watts a rider is producing on our trainer if we know how fast the bike’s wheel is turning.
The Kinetic PC has a rear wheel sensor that picks up speed off the rear wheel and then converts speed to power using the formula provided by Tom. The end result is an accurate estimation (+/- 3% of a PowerTap reading) using some very simple and inexpensive hardware.
Our computer is not a strain gauge and does not measure torque, so there are a few things it will not do. Unlike the much more expensive PowerTap and SRM systems, our computer will not register a spike in wattage prior to a sudden acceleration. With our computer, the wheel must turn faster before a higher wattage reading is observed. Likewise, our computer will register positive wattage while coasting even though no force is being applied to the pedals. Furthermore, wattage readings while not riding a trainer are obsolete because all those variables that are insignificant on a trainer (rider weight, cadence, crank length, etc.) are no longer controlled.
Trainer Compatibility
When we designed the computer, we set it up so that the user can input A and B just like you would input wheel circumference in a traditional bike computer.
A few things are required to make a trainer compatible with the Kinetic Power Computer:
The resistance must be repeatable. If a trainer has any sort of knob, dial, or lever that allows the user to notch up or notch down the resistance, the trainer does not have a repeatable workload. Every time a user changes the resistance, the force curve changes, making all previous calibration obsolete.
The force curve must be linear or cubic in shape, meaning it is in the form of P = Ax or P = Ax + Bx3. Our computer has the basic formula P = Ax + Bx3 built in. By changing A and B, one can alter the shape of the force curve to look more cubic or more linear. These A and B coefficients very closely related to the calibration numbers listed in the table linked above (calibration #A = A, calibration #B = B x 100, both are truncated to 4 digits). Most fluid and wind trainers fall under this category, but magnetic trainers rarely ever do. Fluid and wind trainers have nice cubic power curves, while magnetic trainers typically have bell or plateau shaped curves.
Every trainer has a unique power curve (it’s like a trainer’s fingerprint), so you might imagine that it’s near impossible to fit some – and you’d be right. Some power curves are too radically different from a cubic or linear shape (meaning a formula in the form P = Ax + Bx3 does not do a good job of describing the power curve). Attempting to fit our computer to some of these curves results in poor accuracy.
