thewalrus

The instruction manual for my computer lists a chart of stock wheel+tire sizes to be programmed in when first setting it up, here's the chart:

700 x 20 2086
700 x 23 2096
700 x 25 2105
700 x 28 2136

(Instruction manual: https://www.filzer.com/images/dB/4L-instructions.pdf )

The manual also says that for custom sizing, you can multiply the wheel diameter in millimetres by pi for a custom number. Measuring my front wheel (Alex ALX-295 with Specialized Mondo Comp 23mm), the diameter is 680mm. Multiplied by 3.14159, this yields a number of 2136, or what would normally be used for 28mm tires.

Are the sidewalls on these tires higher than normal 23mm?

To account for tire squish while riding, I decided to call the wheel size 675mm. This gives a number of 2120, which is still weird, since it's higher than the computer instruction sheet lists for 25mm tires.

TheKillerPenguin

What PSI do you have them pumped up to? Maybe if you have a high psi, it will make your diamater slightly off. And are they those puncture resistant tires? If so, they may be thicker than regular ones (just a guess). Hope this helps.

thewalrus

The front is around 110-115, the rear is at 120-125 psi. They're ordinary mid-priced OEM road tires - not Armadillos or the lighter weight S-Works 23mm

fsor

I had been really anal about such things and always did a roll out with full weight and correct psi, and thought that it was useful.....then it occured to me.....the chart difference between 28 and 23 width is about 2%.......aww screwit
I guess there is no hope for me

fholt

I just replaced my front tire with a Specialized Mondo Pro (slick) a very similar tire to the Mondo Comp that came standard. I had never actually measured the distance travelled by one rev, and had the computer set at 2096 like the chart says. Did a metric century (my first) this weekend, which was measured at 62.6 mi, my buddy's computer came in at 62.7, but I had 61ish. I showed a 19.4 avg vs his 19.8 and we rode together. I measured one rotation in my garage, and measured it out at 2124mm. Reset the comp to 2120 to allow for a teensy bit of squish, and hopefully I'll be more accurate now.

FWIW - the specailized tire is supposed to have an extended crown on it, making it more "pointy" at the point of contact when the bike is vertical, and adding contact area when it's leaned. Perhaps this explains the difference.

WillW

I found that my 700 x 23 are best set at 2105 on all bikes in the house. It has more to do with where my LBS placed the magnet on the wheel. I checked a measured distance and made adjustment on computer to match distance. Later I ran against GPS to confirm.

johnny99

Just roll the wheel along the floor (with you sitting on the bike) and measure how far it goes in one revolution. That distance in milimeters is your calibration number. Rollout is much more accurate than trying to measure diameter. On the other hand, a bike computer does not need to be super accurate anyway. A 5% error is less than 1mph (unless you are a pro racer).

thewalrus

The computer manufacturer's FAQ says this about magnet placement:

2 Does it matter where I put the magnet on the wheel?

- Nope!
- The magnet will only go by the sensor once for every revolution of the wheel. So the computer counts revolutions of the wheel. And since you enter in the wheel factor (i.e. circumference) into the computer: - Distance is calculated by: distance = number of revolutions x circumference
- Velocity is calculated by: velocity = revolutions/minute x circumference

- So magnet placement is not important. However we do recommend that you put the magnet near the center of the spoke (i.e. midway between hub and rim) - while at the same time making sure the sensor is in a secure/stable location on the fork.

helmets save

really tho, i dont think these computers are that accurate anyway. Since it depends where you place teh sensor and how close it is to the accural circumference of the tire.

However what really matters is that the results are consistant. Computers are used to track progress made. So the delta values or the increase you have for speed, etc is what is important. Those delta measurements will hlep you gauge your improvement.

baxtefer

IT DOESN'T MATTER WHERE YOU PLACE THE SENSOR!

as long as it's close to the magnet you're fine. All a computer does it count how many times per second the magnet passes the sensor. This is the same number, regardless of where the magnet is mounted on the spoke.

helmets save

it really does matter tho, if you place the magnet closer to teh hub, the magnet will pass the sensor alot more times, than if the magnet was further out to the circumference.

Its like running in circles, if you run along a big radius you will complete a revolution in a longer time than if you in a shorter radius. Why do you think people cut close to the corners when racing.

TheKillerPenguin

Dear god man...

The rim does the same amount of rotations as the hub. They rotate and different speeds, but they do rotate at a 1:1 ratio. They are the same object. They are one. If I lay down on a top and get spun around, my torso doesn't rotate 5 times while my legs only rotate twice. If that happened, I'd be legless and wouldn't be able to ride

johnny99

They may travel a shorter distance, but they still only turn the corner once.

helmets save

NM sorry i just thought about it. I am wrong.