SRMs have this little feature that make them such great units: the possibility to calibrate your powermeter at home. Dont get the terms wrong here, people often confuse calibration and setting zero offset. This post is about slope calibration and therefore it is all about your powermeter accuracy and consistancy.
To calibrate your PM, you need your cranks attached to your bike, your PowerController head unit, an indoor trainer, a known weight (very important) and a pen and piece of paper. You need to install your bike on your indoor trainer so it remains leveled and does'nt move, it makes the job easier. Though, you can also calibrate your SRM while holding your bike still like I did with my MTB because it wouldnt fit on my trainer.
You need a known mass of around 20kg. It needs to be accurately weighted otherwise the calibration could be useless. Gym weights and such often vary by a significant amount of weight even though they are marked as 10kg, 15kg, 20kg etc. Any post office or private shipping carrier will have an accurate scale to weight your mass. Mine is a 20kg iron kettlebell that weighted in at 20kg bang on at the local post office (thanks to my post man brother).
All you need to calibrate your unit.
The protocol is pretty simple and takes about 5 to 10 minutes. You start by whichever side you like. Let's say you pick the left side. Turn on the powermeter by pedaling foward to trip the reed switch on. Measure your zero offset by pressing mode+set while the crank arm is in horizontal position. When the zero offset is stable, note the value. Load the weight onto your pedal while your crank arm is in the same horizontal position and wait for the loaded zero offset to stabilize; note the value. Repeat the procedure 3 times on each side noting all values. Little tip: if your bike is on the trainer, put the weight onto the pedal and rotate the rear wheel backward to lift the weight and level off the crank. Make sure you hold the rear wheel tight or squeeze the brake to keep the crank perfectly still. If your bike is on the ground without trainer, place your weight on the pedal and roll your bike backward untill the crank is horizontal, then squeeze the brake hard to make sure nothing is moving.
Once you have your values, you can do the old school maths or simply plot the values in an excel document to have it calculate your new slope for you. (Contact me if you need the document).
You will need to plot:
- crank lenght in mm
- your known mass to the gramm
- the right side unloaded zero offset (average from 3 values)
- all right side loaded values
- the left side unloaded zero offset (average from 3 values)
- all left side loaded values
- your current/old slope
The excel doc does the maths!
The excel sheet will calculate your new slope and the % difference compared with your old slope. Last time I calibrated my road SRM, my slope was off by 1,3%. I changed the slope to the new one. Yesterday I had a 0,58% difference. I decided not to change the slope as the factory calibration could be responsible for such a difference and I assume SRM factory calibration is more accurate then the home procedure.
Having an accurate powermeter is essential and some brands allow you to calibrate it yourself. SRM just has this little edge that make them such awesome units.