Did a little experiment just out of interest and I was quite surprised at the findings. I use CompuTrainers quite a lot when I’m training and for these units to be accurate the calibration procedure needs to be followed meticulously. This is not an onerous procedure and if done properly the power data from these units is very consistent and accurate. A while back I produced a short video detailing the correct calibration procedure to follow, see below:

The main reasons for this calibration being necessary are that the temperatures of the tyre and resistance unit increase as they are used and therefore a proper system warmup is required before use and prior to calibration. I decided to measure the tyre temperatures before warming up, after warming up and after a short ride at around 200W just to see how much things really did change, here are the results:

Tyre temperature before warmup: 19.2 degrees centigrade.

Tyre temperature after warmup: 32.2 degrees centigrade.

Tyre temperature after 30 minutes at around 200W: 34.5 degrees centigrade.

I was surprised just how much the temperature rose, I was also surprised how quickly it came back down again after the session, the tyre returned to around 20 degrees centigrade within 2-3 minutes.

The tyre Crr reduces by about 0.6% for every 1 degree centigrade rise in tyre temperature, it can be seen therefore that the tyre Crr will change by a very substantial figure with a temperature change of +15.5 degrees centigrade.

The moral of this story is that if you use one of these devices it is vital that the system is properly warmed up and that it is properly calibrated before each use, this will apply to any trainer where the trainer and it’s software are the power data sources, if there is a tyre-roller interface.

I guess taken to it’s logical extreme a rider wanting to have the fastest possible tyres should arrive at the start of a short TT as close to his or her start time as possible, with hot tyres, electric tyre warmers anyone? :-)