HereNT

So I broke down and bought a real computer because my GPS just wasn't doing the trick for me, and I've got a quick question :

If I'm running 700x23c Conti Ultra 2000's, should I still be using the 23c setting in the computer? Or do these tires run fatter or slimmer than normal 23c? They look a little fatter.... Actually, when I went to check the specs on them just now, I noticed that my front fork has been wearing away the rubber on the front tire! Time to dremel again, I guess...

Thanks.

operator

23c uses 23 setting.... I don't get it.

baxtefer

not all 23's are 23's
do a roll out to measure the *real* circumference of your wheels and input that number.

tlupfer

1.) You could not fret over the ridiculously marginal difference
2.) You could measure your rollout distance on a daily basis before riding (this is especially applicable if you're riding brakeless). Don't forget to properly weight the bike at the intended psi so that you can take into account tire deformation.

simple312

***scratches head*** sorry im still a bit new (after reading/lurking for a while) but does this belong in a diffrent subsection? this is the first question i have heard about computers for a bike here (the ss/fixed section).

seems like there would another section that uses bike computes more.

80sMutRider

I'd say repost this one in the Road Cycling forum, you'll probably get more/better/less sarcastic) answers there, maybe.

jab

You could start out with the 23c setting in the computer (rolling diameter 2097mm, per Sheldon), and use your GPS to fine-tune it.

-JAB

HereNT

Yeah, but since the computer is on a track bike, I figured I'd post here... I guess I'll go ask them

My GPS told me I hit 147mph the other day. I'm not really trusting it anymore - that's why I got the computer. The chart that came with it said 2096mm - I doubt it makes much difference, but I just kinda want to know how slow I really am...

LóFarkas

If I were you, I'd be more worried about the "front fork wears the tire away" bit. That sounds bad.
Just set the computer according to the 23c value they give and fine-tune it later to the milestones (?) on a country road.

HereNT

I think a lot of that was from riding in the rain the other day - lots of sand and grit getting inbetween the fork and the tire. It doesn't look to be rubbing when I just spin the wheel, but I think the clearance is tight enough that I could only fit about four sheets of 20# paper between the wheel and the crown... Got the battery for my dremel tool on the charger as I type...

phidauex

Just do a rollout test... I always do them, since I don't trust the charts. Pump your tires up, put a mark on the front tire, and a mark on the ground on a long flat place. Line the marks up, and then roll the bike forward for 10 revolutions, stopping with the mark right at the ground. Make another mark on the ground there, and then measure the distance between the two marks, and divide by ten. The long rollout will reduce your error, and you'll get a veeery close circumference measure. It only takes a few minutes, and is easier than trying to line up to road markers (which aren't always accurate either).

peace,
sam

jab

Yes, I've been up to around 800mi/hr myself, the same way. In my experience, GPS units get sketchy when enough satellites are obstructed, or coming around a corner in a mountain pass when the unit is "switching" between two groups of satellites. It seems that the units will "fake it" for a few seconds, before either being able to properly re-acquiring signals or giving up and deciding that they don't know where they are anymore.

That doesn't render GPS totally untrustworthy; one just needs to keeps its limitations in mind.

Two ways come to mind to get good GPS data for calibration: 1) on a straight, flat course with a good signal, make waypoints (and some sort of visual markers) at the ends of the course. Then, ride that course a few times with the bike computer. The GPS distance between the waypoints should be really accurate for comparison against the bike computer. 2) set the GPS up on the bike so that it holds a good signal while riding, reset the GPS and bike computer odometers, and ride for a few miles. I used #2 over several short (3-10mi) trips to convince myself that my bike computer was giving reasonable values. I kept an eye on the satellite signal display during tests.

-JAB