• 4th May 2006 - By greg

    May 4

    May 4, 2006

    Rolling Resistance coast down tests, food bag holders, canopy bubble lifter

    One of the major changes with this version of CriticalPower are two new wheels. On the rear I was using a 700 Zipp deep rim carbon wheel with carbon disc covers and a Zipp latex high pressure tubular sew up tire. On the front I was running a 650 Renn carbon disc wheel with a Vittoria latex high pressure tubular. According to some data from Jobst Brandt at Analytic Cycling, high pressure tubulars were actually measured as having worse rolling resistance than clinchers due to the glue that is used to adhere the tire to the rim. He tested many different tubulars and clinchers and time and again, the clinchers out performed the tubulars.

    I spoke with Tom Compton at Analytic Cycling about this data and he assured me that this was true. A narrow, high pressure tubular would probably be faster than a clincher on a road bike, time trial bike or triathlon bike because the aerodynamic advantages of the narrow tubular would outweigh the Crr disadvantages. However, in my case almost the entire rear wheel and most of the front wheel are hidden from the wind behind fairings.

    This was the reason I wanted to change the tires I was using from glued tubulars to clinchers. I had to buy new wheels unfortunately. I am now running high quality latex clinchers on both wheels. I had conducted a Crr (rolling resistance) test previously with one rear tubular and one front clincher. I ran another series of slow speed coast down tests yesterday using both of the new clincher wheels, and the Crr was indeed better using two clinchers rather than just one. I never did run any tests using both of my tubulars, so I really can’t say how much worse off I was running with the two tubulars during the 24 hour event, but I have tried to estimate it using previous test results. Here are the results of 3 Crr tests along with expected speed at 150 watts of power:

    bike: front tire rear tire Crr speed fully faired (.3 CdA)
    at 150 watts
    M5 lowracer 20″ clincher
    120 psi
    700 clincher
    130 psi
    .0055 55.2 kph
    CriticalPower (old) 650 clincher
    130 psi
    700 tubular
    180 psi
    .0038 60.5 kph
    CriticalPower for previous 24 hour record attempt (hypothetical) 650 tubular
    130 psi
    700 tubular
    180 psi
    .0041 59.5 kph
    CriticalPower (new) 650 clincher
    130 psi
    700 clincher
    130 psi
    .0035 61.5 kph

    So – it appears that there could be as much as a 2 kph difference at 150 watts between the old tire configuration using tubulars and the new using clinchers. That’s worth as much as 48 km in 24 hours! Since the CriticalPower (old) Crr test was done on a much warmer day, the difference between the new set up could be even greater because Crr gets worse as temperature drops.

    It needs to be noted that running slow speed coast down tests is never very accurate. There is always some wind and knowing the exact CdA of the various bikes and wheels is difficult to quantify. It’s worse if the tests are conducted on different days, so this data should be considered approximate. During the test and tune day previous to the record attempt last November, I measured a Crr on CriticalPower running the two tubulars at .0053. That was using a different testing method using some different software, so I did not include those values in the above table.

    This is a lexan shape that I made to lift up the front edge of the canopy bubble to allow for more air flow through that canopy in case of fogging.

    The shape allows air to flow through it and a portion of the air flow is redirected up and back toward the inside of the windshield to de-fog if required. I also made another simpler lexan wedge that would allow more air through, but no necessarily redirect it upwards. The idea is to stow these wedges under my seat somewhere and secure them under the bubble nose with velcro if and when required.

    I decided to make something to hold the water bags tighter so they don’t sway back and forth. These are two Sintra plastic straps that were shaped with the heat gun. They are hold onto the frame using plastic tie-straps (my chosen form of attaching stuff to the frame)

    They work really well. Getting the water bags off and back on again is now very easy to do

    The bags are secured onto the aluminum threaded rod by a wing nut. I imagined someone unscrewing it in a hurry and dropping the wing nut down inside the fairing shell. So, I added a leash to prevent it from getting lost.

    Some results are coming in for the logo survey. Some of you don’t like any of them (that’s OK – I asked), but most of you like #4 and #5 and #2 in that order. Phil Evans thinks I should do WWII nose art:


    I really like that idea also, but I sort of want to tie the new logo in with the old one. What do y’all think?

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