I dont know s***

Well on my computer manual has got a chart with wheel sizes in it.

Your wheels are 700 x 23, which are the same as mine. Can't remember exactly what the setting was, but think it was like 1500. Would need to look in manual to check it up.

You also might want to hit the company's webpage that makes your computer to see if they have an online manual.

Shadbolt said:

how do I determine my wheel diameter to set my computer? I lost the manual and now I don't have a clue.
The tire has a few numbers

23-622
700x23c
k-191-011

the rim says drx 2000

Help:o

23-622 is the ISO standard your tire fall into. For a 700 x 23 the recommended wheel diameter setting is 2097.

However, if you need to be more accurate depending on tire pressure, your weight etc., do a roll out test whereby you you measure wheeldiameter with a marker on the road with you on-board. My preferred roll out test procedure is to put chalk (or paint) on the road and ride over it keeping a straight line. From the original mark measure distance until you see the next marking your tire produces on the road when you ran over it. Use this number to calibrate your computer.

Yeah, 2097 is about right. I just installed a new Blackburn computer and the manual says 2096 for 700 x 23c. Funny because when I did a roll out test on my Conti Ultra 2000 in that size it measured 2115 mm.

Wino Ryder said:

Yeah, 2097 is about right. I just installed a new Blackburn computer and the manual says 2096 for 700 x 23c. Funny because when I did a roll out test on my Conti Ultra 2000 in that size it measured 2115 mm.

Do the math, and your roll-out is less than 1% off from the manual. That level of error is well within the accuracy I expect from any spedometer. However, did you do your roll-out with your weight on the bike? That could make a difference as well.

dgregory57 said:

Do the math, and your roll-out is less than 1% off from the manual. That level of error is well within the accuracy I expect from any spedometer. However, did you do your roll-out with your weight on the bike? That could make a difference as well.

No I did'nt, but it'll give me something to do while I'm screwing around at work.
I just taped a library index card to my front tire, and with the tire off the bike I marked an edge on a long table, rolled the tire one revolution and marked the same edge again. Then I measured the two marks in milimeters. As far as I'm concerned that's pretty damn accurate enough.

You dont have to do so much guy, just remove the wheel from the bike and mark a point, roll it in as straight of a line as you can and then measure the distance where the marked point comes back around. It does not matter if you are on the bike or not when you roll it, see the rim will travel the same distance regardless of the wheel having weight on it or not. The only thing that would change the distance is if the tire somehow slipped or slid in the rim or on the floor as you are doing it. Imagine the wheel turning in your head.

tyler1212 said:

It does not matter if you are on the bike or not when you roll it, see the rim will travel the same distance regardless of the wheel having weight on it or not. The only thing that would change the distance is if the tire somehow slipped or slid in the rim or on the floor as you are doing it. Imagine the wheel turning in your head.

No, you're wrong... the diameter of the wheel is reduced by the amount of bulge in the tire due to the weight. I'm not going to get into picky calibration details, but your explanation is wrong.

DiabloScott said:

No, you're wrong... the diameter of the wheel is reduced by the amount of bulge in the tire due to the weight. I'm not going to get into picky calibration details, but your explanation is wrong.

my bad, (pie x radius)^two, the radius does change when you sit on it.

tyler1212 said:

You dont have to do so much guy, just remove the wheel from the bike and mark a point, roll it in as straight of a line as you can and then measure the distance where the marked point comes back around. It does not matter if you are on the bike or not when you roll it, see the rim will travel the same distance regardless of the wheel having weight on it or not. The only thing that would change the distance is if the tire somehow slipped or slid in the rim or on the floor as you are doing it. Imagine the wheel turning in your head.

Dead wrong tyler. Its about tire circumference - the outer distance the tire makes with one revolution. A flat tire or even the just the rim will result in a shorter distance versus an inflated tire. The formula for circumference is C=Pi x D (d is the diameter).

hd reynolds said:

Dead wrong tyler. Its about tire circumference - the outer distance the tire makes with one revolution. A flat tire or even the just the rim will result in a shorter distance versus an inflated tire. The formula for circumference is C=Pi x D (d is the diameter).

HOw anal are we getting here guys??? Yes it is Pi*d, so a 2mm difflection is 3.14mm error, vs a distance of 2110mm....

fabiosav said:

HOw anal are we getting here guys??? Yes it is Pi*d, so a 2mm difflection is 3.14mm error, vs a distance of 2110mm....

I was replying to tylers post who gave the formula for CIRCLE AREA and not CIRCLE DIAMETER.

FWIW, a 3mm error (ie. 2110 vs. 2097) yields a 4.13mm difference in dia. At any rate, assuming there's a 3.14mm difference in circumference as you say. A wheel/tire doing a slow 60rpm (or about 7.8kph) for an hour results in a 10.8meters difference.

Or, the average bike computer is pre-set to 700c x 25mm tires.

Obviously, one could just remove the battery for a short while and then ride a marked course to see if it isn't already accurate within 1 mile per hour or so. Adjust from there. . ., ride the marked course again. That'll get it.

Now, put the scientific calculator down on the marked course and determine the maximum safe speed that one can run over the calculator. Optimum results are achieved when the display reads 87HLR*3. lol!

fabiosav said:

HOw anal are we getting here guys??? Yes it is Pi*d, so a 2mm difflection is 3.14mm error, vs a distance of 2110mm....

I'll try not to get "anal" as you say but, wouldn't a 2 mm deflection i.e. 2 mm reduction in tyre height due to rider weight = 4 mm less diameter. Then 3.14 X 4 = 12.56 mm.

I usually find with a 23 or 25 wide tyre @110 psi a reduction of, with my weight of 78 kgs, approximately 9 & 11 mm respectfully of decreased circumference. I need to ride straight or it can measure less than this. This is for the rear tyre, cause this is were my magnet is. I would expect a front tyre with say 100 psi to show maybe half this difference when seated, cause there is much less weight on the front.

I also find different brands of tyres of the same size vary in one case 10 mm. [25 mm armadillo / 25mm Conti 4000]

I have so many white paint dots on my driveway it's embarrassing.

The speedometer works on radius, not the entire distance across. But the tires certainly do affect the reading.

Armadillo? Specialized: The fastest bikes on the slowest tires. You'll not really need your speedometer very much with Specialized Armadillo. 😉

danielhaden said:

The speedometer works on radius, not the entire distance across. But the tires certainly do affect the reading. ......

😉

[font=Times New Roman]2 x radius = diameter. Don't know what you need to enter into your cyclocomputer but mine is one tyre revolution in mm. In mathematical terms that’s circumference.😎 [/font]

[quote="Albert 50"]I'll try not to get "anal" as you say but, wouldn't a 2 mm deflection i.e. 2 mm reduction in tyre height due to rider weight = 4 mm less diameter. Then 3.14 X 4 = 12.56 mm.

ummm, no: the tire only deflects on the bottom! Diameter is reduced by deflection, not 2X deflection...

[quote="fabiosav"]

Albert 50 said:

I'll try not to get "anal" as you say but, wouldn't a 2 mm deflection i.e. 2 mm reduction in tyre height due to rider weight = 4 mm less diameter. Then 3.14 X 4 = 12.56 mm.

ummm, no: the tire only deflects on the bottom! Diameter is reduced by deflection, not 2X deflection...