I thought before getting too far into the next steps of the build, I’d quickly describe what I’m aiming for..
As mentioned in a previous post here, the v1 build has been pretty successful – I’ve been using it now for the best part of a year with no real issues. Still, the nagging feeling remains that I’d like something with sufficient accuracy that I don’t need to rely on an external power meter, which is where the v2 build comes in.
Looking at the unit as a whole, I think it’s helpful to break the overall resistance into two separate areas. Firstly, there is the resistance of the un-braked roller assembly (rolling resistance of the tire, bearing drag etc). Secondly, there is the deliberately induced resistance from the eddy current brake.
Modelling the un-braked rolling resistance
I am assuming that it won’t be sufficient to simply take a series of measurements from test rides and use them as a basis for the power requirements. The rolling resistance is liable to change with temperature, tyre pressure, and clamping pressure (or rider weight, depending on the design of the trainer frame). Therefore, I believe I’ll need to add an element of dynamic calibration through a spin-down process.
I think the following steps are required;
- Establish the moment of inertia for the roller assembly
- Instrument the roller for speed (using a hall effect sensor)
- Measure the deceleration through a series of spin down tests
From knowing the speed and the moment of inertia, I can derive the stored kinetic energy in the roller assembly. Measuring the rate of deceleration should then allow me to obtain the rolling resistance in watts, and calculate a speed/resistance curve.
Another limitation with the v1 trainer was that the resistance maps assumed a steady speed. By measuring the speed more accurately at the roller, I should also be able to factor any acceleration/deceleration into the power model.
Measuring the braking force
With the above (hopefully) taken care of, the next stage will be accurately measuring the braking force applied through the eddy current brake.
This would once again be susceptible to changes in temperature during operation. As well as affecting the electrical resistance & the current flow in the electromagnet coils, it would also alter the electrical resistance of the aluminium disk & therefore the strength of the induced eddy currents.
I think in this case, the best approach would be to actually measure the force applied through the use of a strain gauge on the electromagnet assembly – and yes, this would also be temperature dependant, but at least it’s only in one place.
That’s the plan anyway, as with all these things, it generally becomes clearer to me as I’m working through the process!
Next post will cover calculating the moment of inertia..