Hi, I am new to road cycling and just bought a Specialized Roubaix Elite Triple (2007) last week and I am loving it. I bought a sigma sport computer (BC906) model and am actually going to give it to my boyfriend and upgrade to the one with cadence pretty soon. But I wanted to have something to play around with.
My question has to do with the wheel size. I think I have it right, but want to make sure. The wheel is a Specialized Roubaix Pro, 700x23/25c, aramid bead, 120TPI. What is throwing me off is what does the 25c mean in the description of the tire? Is the tire a 700X23 tire? If so, I set the computer to 2133 as stated in the instructions. Is that right?

Thanks for your help! Everyday I can't wait to leave work so I can go ride.

take physical measurement of your wheel would be more accurate...

cheers

Did you get a manual with the computer? They usually have a list of tire sizes so that you can get the right code.

That's odd that the tire says 700x23/25. I don't know if I've ever seen that before.

The new Roubaix tires have a 23c wide tread and a 25c casing

https://www.specialized.com/bc/SBCEqP...jsp?spid=26461

My cycle computer has a table of numbers as yours does. The numbers are actually the supposed circumference of the wheel in millimeters. Simply measuring the wheel does not account for the slight change in diameter of the wheel that happens when your weight rests upon the seat. Make a mark on the sidewalk. Align your valve stem with this mark. With your weight bearing down on the bicycle, move it forward one complete rotation of the wheels. Mark the spot below the valve stem and measure back to the first mark. Convert the measurement to millimeters and that is the figure to use for your cycle computer.

I have two bicycles with identical cycle computers on them. I set both as described above. In a 14 mile ride there is still about a tenth of a mile difference between their readings, but either one of them can vary a tenth of a mile on different days. I do not know why. Maybe the reed switch tripped by the spoke magnet does not always respond the same at different speeds. If you need anything more accurate, you will probably need a Garmin tracking you by GPS.

Thanks for your help, great advice! Junker, Thank you for finding that explanation on the specialized website. I was wondering what the difference was between 23 and 25.

I always measure my circumference by rolling out the bike when I put on new tires. It seems that the measurements I come up with are always significantly different from what the pre-set value for that size tire would be, or the values that most websites quote. I'd like to know what people come up with when they roll out their tires. For 700 x 25 I usually come up with something around 2131-2133; for 700 X23 I come out with something in the 2117-2119 range.

Is that consistent with what others get, or am I way off?

Bruce

1) tire inflation on the particular day.

2) your weight that day.

3) the exact path you took around turns.

4) rounding error.

Unless you do a rollout before each ride, there's always going to be a little bit of calibration error.

Given that we're not trying to hit a target on the Moon, or landing the bike by instruments on a dark runway, I find the numbers in the chart with the computer sufficiently accurate for my purposes.

And if you must be more accurate, something like an Edge 305 which automatically calibrates your speed sensor with the GPS satellites on-the-fly may be the way to go.

Greetings!

I have a Cateye Cordless. My instructions say that a standard 700X23C (what's on my bikes) is 2096 and that is the default size in the computer. For a 700X25C it says 2105 is the average size and for a 700X28C it is 2136. It further states that if that is not close enough you can set whatever you want and tells you to be sure the tires are inflated to the pressure you will ride at and that you sit on the bike and try to make it roll as straight as possible when checking the distance for one full revolution by the use of a piece of tape on the tire touching the pavement so that the marks on the pavement are placed as precisely as possible. This is because they assume that if the standard figure is not close enough for you, then you are pretty anal about accuracy! If you were really anal about it, you might ride the bike for a while to heat up the tires so that it is even more accurate when you measure it. Measure using a metric tape measure and you have the figure to set in your computer. Mine is set at 2098 currently (because I was curious how close the figures would be), but should be checked again because I bought new Gatorskin tires and have not checked it since then.

As others have said, there are many factors that will change or otherwise affect the readings for a given ride. Tire pressure, heat of the pavement affecting tire pressure, your weight, variables in how you make turns from one day to the next, how straight you ride, wear on your tires, wet pavement, the list is endless. What it boils down to is that it can't be that exact, but it is close enough for most purposes.

Regards,
Jefferito

I've come across a few riders that do the opposite - they deliberately increase the wheel size so that they can go more miles and at a higher speed (at least according to their computer).

Always a bit irritating that when you do a group ride and someone consistently finishes minutes behind you but claims a higher avg speed.

[I've come across a few riders that do the opposite - they deliberately increase the wheel size so that they can go more miles and at a higher speed (at least according to their computer).

Always a bit irritating that when you do a group ride and someone consistently finishes minutes behind you but claims a higher avg speed.]

They're really only fooling themselves...

Regards,
Jefferito

I've calibrated both bikes with 25c tires using roll out method (with me on bike and tires freshly topped off to my chosen psi)

Both bikes matched my commute distance to .01mi. 6mo later I started using a Garmin GPS and it matches 2-50mi rides to .02mi.

My 8.48mi commute never varies by more than .01 mi. When it does it is because I've done a different 'thing' such as a very wide turn, say for construction, where I normally do a narrow (that changes by .05mi) or if my tire pressure is going down, which adds .02mi. I've done the exact same commute route for several hundreds of times and these patterns hold up.

I weight 185lbs, my front Conti Gatorskin 25c at 110psi is set at 2089 circumference. The Cateye manual recommended higher if I remember right, but it was wrong.

Al