I recently received my CarBent HPV Raven dual 700c hiracer from BentUp Cycles. Beautiful bike, of course, very well executed. But I was surprised to find that, using the same method of measuring seat angle as I did in the earlier article, that it measures about 19 degrees (!). My bike is supposed to be approximately 23 degrees seat angle. So, I called Dana (owner of BentUp Cycles and CarBent HPV), and chatted about it.
It turns out Dana measures seat angle by putting a straightedge on the bottom of the seat, and measuring that angle, in order to factor out the ambiguity caused by the lumbar curve present in most of these 'potato-chip' style seats. I did the same on my CarBent, and indeed, it comes out at 22 degrees on my little iPhone app. I need to get a straightedge of my own to measure the TiCa, but the bottom line is that, using my somewhat less precise method on both the CarBent and the TiCa in question yields a 23 degree seat angle for the TiCa, and an 18-19 degree angle for the CarBent.
The TiCa in question came with a medium M5 seat. This is really a little small for me (I have a long torso), so I had replaced it (or rather, my LBS replaced it) with a large Velokraft seat I had lying about. The thing is, the original M5 seat has a V-shaped cutout at the front of the seat to accomodate the TiCa's main tube, and this allow the seat to be positioned further forward (where M5 intends it to be); when the Velokraft seat was installed, it got forced back a bit. It's not really a problem, I just shorten the boom, and all is well. But it has the effect of lessening the seat angle to 23 degrees, as measured on my iPhone. I speculate that using Dana's method, if it can be done accurately on the TiCa (I need to get a straightedge and see what if there is nothing obstructing measurement underneath the bike), we'd probably get 26 degrees, I reckon.
When using my method, it's really less sloppy than it sounds like. You get about a 2-3 degree variation when you push the iPhone to one extreme of the lumbar curve to the other. Putting the iPhone kinda in the middle and eyeballing it so there's the same amount of 'gap' (which is quite tiny) on one side or the other results in a number that's exactly between the two extremes. so....good enough for me. Or at least, it's my opinion that this is as close as is meaningful to measure, given that the seat shapes themselves differ somewhat, i.e. we're kind of down to the 'noise' level of measurement, I think.
I was kind of excited to pit the CarBent and TiCa against each other. However, it looks like the only way to get them to be more-or-less the same seat angle is to pop the M5 seat back on my TiCa. This I have done, and it now measures 18-19 degrees (using my iPhone top-of-seat method), just like the CarBent.
When we get a dry day, and I'm not too shot otherwise, I'll repeat the same tests, but with CarBent and TiCa. I expect to get somewhat faster speeds than the earlier test, given the lower seat angles, but we shall see. I'll do a general ride report on the CarBent Raven in another post (it's a super nice bike, of course).
Oh....Robert Chung (from BROL) has given me access to a build of his Golden Eagle software for measuring drag - a protocol using this potentially is (hopefully) more precise and (certainly) a lot less physical effort to implement than my method. However, since I seem to be struggling with the software a bit, and just for consistency with the earlier test, I'll probably duplicate my earlier protocol once more. At least that's my current plan. Hopefully, at some point, I'll get it together with the Golden Eagle software, and render these 225W runs unnecessary.
I will note that, among the many uncertainties my test was rife with, there was the issue of using a different PowerTap/rear wheel on each bike. We assume they read the same, but....it'd be nice to be able to remove that assumption. For that matter, it would'be been nice to have exactly the same seat on both bikes, measuring at exactly the same angle via whatever method is used. Unfortunately, I seem to be able to get closer to the same seat angle (between CarBent and TiCa) using the M5 seat on the TiCa, so....this is probably the closest I can get to minimizing this variable. I plan to explore getting my LBS to cut a notch in the Velokraft seat; if that works, perhaps I can revisit this. In any case, I'll have the benefit of being able to use exactly the same rear wheel and PowerTap on each bike, which would be nice.
When I did the Corsa vs. TiCa tests, perceived effort on the Corsa was greater for managing 225W. Possible explanations:
- It's all in my head.
- I'm more used to the TiCa, and more easily able to generate power.
- The EuroMesh seat/frame interface is not as stiff, and it takes more effort to get the same wattage delivered to the rear wheel.
- The 2 PowerTaps vary a little bit.
- Something else I've failed to conceive of.
Oh, and lastly....there was at least one person on BROL asking about tire pressures. All tires in the previous test, front and rear, were pumped to exactly 120 psi. The Zipp clinchers spec a max of 125 psi, and the Stelvio on the front of the TiCa is a max 120 psi spec. I'm not sure if 120 psi on the 23 psi 650c front tire of the Corsa is "equal" to the 120 psi of the Stelvio 28/451 front tire on the TiCa, but....that's the assumption I was operating under, for good or ill.
