Tom A. wrote:
Interesting stuff. I too would love to see more description of your protocol. Thanks for sharing!
Thanks Tom. That's interesting and all make sense. I was also surprised by the 0.01 CRR on chipseal, but the loop I use is very rough and broken. It's local and traffic-free though, hence why I use it. When I've used my TT bike on the same loop, with Conti GP5Ks and Conti GPTTs, I need to use a CRR of around 0.006. I do sets of fast and slow laps, to try to separate aero and rolling resistance effects as much as possible.
I was thinking more about it today. Road surfaces, whether smooth or rough, don't yield at all under the pressure of bike tyres, whereas even quite dry off-road surfaces will deform if enough pressure is applied. I'm thinking about if someone walks across grass with either flat-soled shoes, or alternatively with high heels. Even a dry grass surface has a kind or plasticity, with the deformation related to the pressure applied. Since any plastic deformation doesn't return any energy, high pressure applications should be subject to more losses through this mechanism. I wonder if low tyre pressures causes less deformation of the underlying surface and therefore experiences smaller losses, as well as reducing the rider suspension losses.
Here is a description of the protocol I used:
- Stages G2 LH power meter. I realise this is not ideal, but it's all I've got.
- Tyre pressures measured with a digital Topeak pressure gauge. Possibly not accurate, but hopefully consistent.
- Garmin speed sensor. I realised after posting the chart this morning that my protocol has a slight flaw, because I didn't change the wheel circumference when I dropped the tyre pressure. Today, I measured the circumference changes, and there is almost a 2% reduction, going from 45psi to 15psi, so I need to correct the measured speeds and see how much that changes the CRR results. A slower speed for the lower pressures should give a higher CRR values for the lower pressures I think. I'll re-post the corrected chart once I do it.
- Air density: Pressure from the weather forecast app. Temperature from the Garmin head unit.
- 0.97 transmission efficiency assumed throughout.
- Bike + rider weighing before and after the ride.
- No adjustment for wind. Sessions 1-3 in the chart were done with the forecast wind being 10-15mph. From what I've read, the ground level wind speed is less that forecast wind speed by about half. My hope is that it would be consistent enough to give good enough increments between the pressures, even if the absolute numbers might be affected by the wind.
Tarmac runs 1) Tyres inflated to 45 psi.
2) 6 x laps of a tarmac oval loop, each lap 1 mile in length. First 3 laps done at ~230W, next 3 laps done at ~130W. No traffic.
3) VE analysis done in Golden Cheetah and Excel to check they give the same result, to check I haven't made a mistake in my Excel equations.
4) CdA and CRR determined post-ride so there is no elevation loss or gain over the six laps, trying also to keep the same flatness for the sets of fast and slow laps. The CdA derived from this analysis, 0.393, was then used for the VE analysis for the grass runs.
Grass runs Straight from tarmac runs, I cycled about 1 mile to a local grass field, roughly the size of a football pitch.
1) I checked the pressure before starting at 45 psi.
2) Accelerated up to speed before the maximum elevation point of the lap. Pressed the start/stop on the Garmin about 5-10 seconds before the max elevation point.
3) 3 laps at ~230W, followed by 3 laps at ~130W. I changed gear only once at the transition between fast and slow sets of laps, to keep the cadence reasonable. The time to do six laps was 7-8 minutes. I tried to follow a consistent line, but I had to avoid an occasional dog walker, approximately once every 5 laps (and these days trying to give people at least 5 metres of space!)
4) After the max elevation point at the end of the 6th lap, I continued for ~10 seconds, and pressed start stop on the Garmin.
5) Stopped in the shade. I avoided direct sunlight when stopped to prevent the sun warming the tyres. I checked the pressures, before then reducing the pressure by 5psi ready for the next run. I could get it within 0.5psi of the target. If I overshot the reduction for the first tyre (e.g. 39.5psi), I compensated by leaving the other tyre half a psi higher (e.g. 40.5psi). I chose to reduce pressure, rather than increase it, because I was concerned that pumping the tyre up might increase the internal air temperature and possibly the tyre wall temperature. Letting pressure out was also easier.
6) Step 2 onwards was repeated for 45 psi down to 15 psi in steps of 5 psi.
7) I then went home (<1 mile away), pumped the tyres back to 40 psi with a track pump, returned and followed the same protocol, but in 10 psi steps (40, 30, 20 psi). I wanted to do these repeats because I hadn't done an ABAB type test, and also because the grass was becoming increasingly flattened, which I think might have affected the CRR.
8) I also did a few repeats two days later, on a less windy day (~5mph wind according to the forecast)
9) CRR values were obtained to give no elevation gain/loss over the 6 laps, using the fixed CdA of 0.393. I obviously tried to keep the same kit and position on the bike throughout.
There may be other things that I've forgotten to mention. If anything is unclear, please ask. Any suggestions or criticisms about the protocol also very welcome, of course.
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