Monday, April 14, 2008

Neaves Road TT testing - the numbers

With some minor exceptions the TT iBike testing went very well on Saturday. First there was possibly just a little more wind than I would have liked. About 3-5kph. Second, there was more traffic on my test roads. Finally my scale misbehaved so I ended up with wrong weight entered (83kg should have been 85kg.)

I did testing with two wheelsets on my Cervelo P2C, all aero-gear in place (skinsuit, aero helmet, gloves, booties, bottle cage off):

  1. Bontrager Aeolus 5.5 - Bontrager XXX Lite Clinchers - Bontrager X Lite Tubes - 120PSI
  2. Bontrager Aeolus 6.5 - Veloflex Record Tubular - 120PSI
I did about ten coast downs and the "4 mile" ride with each wheelset. Once pulled into the iBike2 software, I was able to separate out and analyze each set without too much problem (although I'm still hoping I can reprocess with different weight.) There where a couple of obviously bad CD's in the first set (car's passing) so I removed them.

The results:

1. Aeolus 5.0 Cda 0.196 Crr 0.006
2. Aeolus 6.5 Cda 0.200 Crr 0.0044

I finished the day with a 10k TT drill on the second wheelet (Aeolus 6.5). The iBike reported Pavg of 262 watts. The ergomo a little higher at 288 Watts. The difference probably due to the lower (incorrect) weight I had used for iBike and the drive train losses (iBike measures effective watts, Ergomo measures applied watts which must get through the drive train before they are useful, using 3% for loss would bring Ergomo down to 280.)

I don't understand the math well enough to know what the error in weight will affect. Presumably it will cause the iBike to under-report watts (I'm moving more mass around than it thinks I am.) But how that affects the coast down numbers, and then the Cda results, I don't know.

I'm looking forward to some recently announced changes to the iBike2 software. It will use separately collected numbers from another power meter (e.g. ergomo) and then evaluate differences between what iBike sees and the other data as changes in Cda.

If that works as well as I expect it to, I suspect that I'll be spending some money on a Quarq CinQo and the new version of the iBike (see the last post in the above thread...) for the TT bike.