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Flight Deck Programing, Wheel Size?(4 posts)

Flight Deck Programing, Wheel Size?SSgt Jeremy in Germany
Jul 7, 2001 7:31 AM
Ok, I programed my Flight Deck computer today, and I have a question about what wheel size I should enter. The manual says to sit on the bike and measure the distance of one tire rotation. Did that and got 2081mm, but the manual then contradicts itself and shows a table with the different wheel and tire sizes and the distance in mm, so i measured my 700x23 tires without sitting on it and got 2108, so I ended up using something in between, 2105, what do you think, right thing to do? Do you use just the wheel and tire size? Also, the manual doesnt address where the magnet sensor should be on the fork, it obviously matters, what gives? Thanks for any advice.
re: Rollout beats manual every time...Akirasho
Jul 7, 2001 9:17 AM
... toss the table.

... as suggested, an actual rollout gives the most accurate measurment for a given wheel/tire/bike/rider combo.

As for me, after inflating my tires to the desired pressure, and getting on the bike... I do a four times rollout (as straight as possible) for better accuracy... that is... I do four complete rotations... measure the distance and divide by 4... this helps average out small discrepencies... usually helps a bit to have a spotter.

As far as sensor and magnet placement... it's more of a personal preference and application...

If the magnet is closer to the hub... it attributes less rotational mass (albeit small) to the wheel, however, you'll have a lot of sensor wire running down the fork (unless it's wireless)... or you could have wheels and/or forks with a more dynamic profile which mitigates placing the magnet closer to the rim (I run HED Stinger 90's with a Profile BDC fork... and it works better near the aero rim/spoke interface). Bottom line, place the sensor and magnet where it works best for you.

Be the bike.
re: Flight Deck Programing, Wheel Size?DrD
Jul 7, 2001 1:30 PM
A "sitting on the bike roll-out" is the most accurate way to measure - actual diameter will be influenced by tire pressure, too. That being said, 24mm is about 1% of the weighted diameter you measured - so worst case, you are off 1% - that means if you are travelling 20mph, your 'puter would read 19.8mph.

Why not use something close to your measured roll-out? 2080 would be off by only 0.04% and if you chose 2085 you would only be off by 0.2%...

You want the sensor positioned so that is is close enough to the magnet to get a good reading (usually 1-5mm is good). Vertical placement only matters if you have a short sensor wire, or a wireless system. It doesn't matter if it's on the front or back of the fork, though I generally put it on the front as I am less likely to bump it when I transport my bike in the car (I have to remove the front wheel and slide the bike into the car)
ThanksSSgt Jeremy in Germany
Jul 7, 2001 1:44 PM
Ok, I remeasured, again and again, kept getting an average of 2077, with tire pressurized and setting on it. So I am going with 2080, it will be plenty close to perfect. Never thought about how little the difference of say 20mm would make, thanks!