Measuring Power Input

Firstly, thank you to all those who have shared information with us so far, it has been much appreciated and we are endevouring to follow up on all of the replies.

We are getting reading to test our old bike to validate the predicted results from the last project group.  However in order to do this fully we need some method of measuring the power input from the rider.

We have found some rather expensive methods of doing this, such as the "Pedal Crank" and a chain vibration sensor made by "Pulse" but at hundreds (and more) these are not really viable for us to buy at this stage.

Does anyone have any information or ideas on measuring the power input from the rider? Such as a cheaper method or someone who might lend us some equipement. If so we would be very greatfull to hear from you!

Many thanks

Southampton University HPV Team

Are you trying to measure your rider's power output directly, or would it be enough to determine the power required to sustain various speeds in a given vehicle?

Obviously the two values should be pretty close, but I can see why the distinction might be important.

Kit

Our original thought was to measure the rider's power output directly during the testing of the bike.  This seems however to be very difficult.

After some thought, i came to the conclusion that the rider has to overcome all the inefficiencies in order to move the bike so whether you measure the power input by the rider at the crank, or by using the polar chain sensor i previously described or off the back wheel the values should all in fact be the same.

However after considering one of the power calculation spreadsheets on the ihpva website, it suggests the pedal power and the wheel power, if could be measured seperately, would be different.  Which also makes sense, but contradicts the statement above.

So in conclusion we are still at the same point, either measure the power directly, or measure the power at the wheel and estimate the inefficiencies in the system to predict the input power.

How would you determine the power required to sustain various speeds in a given vehicle, and could it be used to log the power during a test of the bike?

On behalf of the SUHPV team, thanks,

Alex

SotonHPV wrote: "After some thought, i came to the conclusion that the rider has to overcome all the inefficiencies in order to move the bike so whether you measure the power input by the rider at the crank, or by using the polar chain sensor i previously described or off the back wheel the values should all in fact be the same."

The power at the rear tyre will be the rider output multiplied by the transmission efficiency, usually about 98%. I doubt that your estimates will be within 2% anyway, so I wouldn't worry about it.

You really need to know the rider's power output independent of a moving vehicle (It's notoriously difficult (inacurate) to extrapolate power from moving bike performance data). Then having found out the CdA (wind tunnel) and rolling resistance of your vehicle you can work out how fast it might go, assuming you can solve polynomials (I can't ). Have you seen John Lafford's stuff on Legs Larry's website?

In my very limited experience and that of others, successful record breaking is as much about logistics (and a bit of luck) as it is about vehicle technology.