I ride Specialized Tarmac. A while ago I had to send it for warranty replacement. It was gone for about 10 says. Feeling sorry for me my friend loaned me Cervelo P2SL TT bike. It was my size and felt comfy so I used it during those 10 days. Now comes big surprise, my average during this time was 2-2.5mph faster (note, due to road/traffic conditions I was not using aerobar position). It is not likely Tarmac being bad fit since it feels really comfortable and it was professionally fitted in 2 different places 2 month apart with the resulting geometry being virtually the same. So what is it? PCad factor? I'm thinking of buing this bike now.

a) What color were the two bikes?

2) What is the standard deviation of speed on the Specialized?

were the computers configured properly in both cases?

1) Both are black.
2) Infinity

Yes, I checked it. I have long legs and short torso, so I'm thinking that may be that particular geometry lets me to produce some extra power?

O.K. Stop. Just stop. You have an observation and you've jumped from that data (which may or may not be statistically significant) to a cause and effect conclusion. Did you check the tire inflation on these rides, the wind, the barometric pressure? Was the power equal or did you produce more power for the faster rides. If so, was that due to the bike or external factors? How do you know? The questions just keep piling up.

If you want a Cervelo, get it, but don't believe for a minute that you know you'll be faster on it.

Did you check the tire inflation on these rides

I put air in my tires before every ride. It is always the same (110 back, 105 front). The wheels and tires on bikes were different of course. Bike weights were virtually the same.

the wind, the barometric pressure?

Obviously not, but since we had pretty even weather during that time and because the speed was consistently higher I suspect those were not factors.

Was the power equal or did you produce more power for the faster rides.

That is what I suspect and if that is the case I assume that P2SL geometry is better for me.

If so, was that due to the bike or external factors?How do you know?

Well, I do not that is why I was asking a question.


If you want a Cervelo, get it, but don't believe for a minute that you know you'll be faster on it.

Well, it could be the case or could be not. That is why I keep thinking about it for 2 month and still can not make up my mind.

I want to repeat the experiment but now I'm most likely biased so it won't be proper.

Go to the wattage group on Google and read about regression analysis or "Chung Method" then you can do the right experiment correctly. As it is, you have controlled for nothing and reached a conclusion that might have seemed obvious but is totally unsupported.

If I'm biased and will subconsciously try to ride harder on P2SL (which is kinda likely now) statistics will not help. For test to be proper now I'd have to be blindfolded and given bike at random . The only realistic thing I could do now is to get access to special fitting bike and try to find a geometry that let's me to produce most power and then make sure that the bike I ride comes as close as possible to that geometry.

I'd measure the Cervelo setup down to the mm, compare it to the Tarmac. It may be that the position works better for you, but start with knowing whether it is indeed different. Then you can start controlling for the earth's rotation and everything. But I'd want to know that first.

It would seem you totally missed the meaning of regression testing. You don't seem to realize that you could measure the coefficient for force that goes as velocity squared (aero drag) and the coefficient for force independent of speed (rolling resistance and frictional losses) without keeping power, atmospheric conditions, or wind constant (as long as they're known), and then characterize the performance of each bike. You could then test your ability to produce power on the bikes to determine if either truly allows you to go harder. The trouble with that is it ignores the time adaptation to change in position that says that for all but the most extreme changes, power production is limited by physiologically not position. But that's another chapter.

It would seem you totally missed the meaning of regression testing. You don't seem to realize that you could measure the coefficient for force that goes as velocity squared (aero drag) and the coefficient for force independent of speed (rolling resistance and frictional losses) without keeping power, atmospheric conditions, or wind constant (as long as they're known), and then characterize the performance of each bike.

The precision of such indirect measurements would most likely invalidate any results.

... for all but the most extreme changes, power production is limited by physiologically not position

Well the change of position was pretty extreme considering how much steeper the equivalent seat tube angle was. And since I'm pretty new to cycling (my second season) it could affect my power much more then that of seasoned person with proper fit and technique.

It is very different, no question bout that.

If the Cervelo had you in a more aero position, you'd be faster. But not 2-2.5 mph. Perhaps you just rode faster, or the seating position is better for you.

Some days or weeks, you just feel better. I'm guessing you might find a 2.5 mph difference going from a hybrid to a road bike, or a road bike to a TT bike, but not in between different road bikes.

Great reasoning to buy a new bike...well done Watson, another mystery solved.

Tarmac is a road bike and Cervelo P2SL is TT bike

I think that is the case. Unfortunately I can not adjust Tarmac to the same position

Yah, but he was riding in the bullhorns, not the aerobars.