rbent Forum - Wheel Size Code for Computer

Mdicke

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Posts: 207

Wheel Size Code for Computer
Mar 15^th, 2016, 9:36pm

I installed a new Sigma computer last night on the trike and found the code for my 35-406 is supposed to be 1460. On my ride this evening Strava gave me 27.1 and MMR gave me 27.5 and the computer gave me 26.3. I am use to the numbers being different but rarely are they a off by a mile. Any thought on what the correct number should be. The tire I am using is a Marathon+.

Thanks

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jayg

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Re: Wheel Size Code for Computer
Reply #1 - Mar 15^th, 2016, 11:46pm

Quote from Mdicke on Mar 15^th, 2016, 9:36pm:

I installed a new Sigma computer last night on the trike and found the code for my 35-406 is supposed to be 1460. On my ride this evening Strava gave me 27.1 and MMR gave me 27.5 and the computer gave me 26.3. I am use to the numbers being different but rarely are they a off by a mile. Any thought on what the correct number should be. The tire I am using is a Marathon+.

Thanks

Pump the tires up to riding pressure, then do a roll-out on a flat surface, while sitting on the trike. Put a mark on a piece of tape and apply it to the wheel to be measured. Get an assistant to put a mark on the surface coinciding with the mark on the tape. Move the trike in a straight line, until the wheel makes a complete rotation. Mark the surface to coincide with the mark on the tape. Measure the distance between marks in millimeters, and that will be the value you enter into the computer. This is the most accurate method for setting the computer.

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MrWizard

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Re: Wheel Size Code for Computer
Reply #2 - Mar 16^th, 2016, 10:23am

ditto what jay said .. don't trust the manufacturing numbers, i've found 5mm difference between the stated and actual (measured) numbers.

Also don't trust the garmin "auto calibration" function on the EDGE devices unless you have a couple of mile of absolutely straight and flat road to use it on .. better to just measure

Quote from jayg on Mar 15^th, 2016, 11:46pm:

Quote from Mdicke on Mar 15^th, 2016, 9:36pm:

I installed a new Sigma computer last night on the trike and found the code for my 35-406 is supposed to be 1460. On my ride this evening Strava gave me 27.1 and MMR gave me 27.5 and the computer gave me 26.3. I am use to the numbers being different but rarely are they a off by a mile. Any thought on what the correct number should be. The tire I am using is a Marathon+.

Thanks

Pump the tires up to riding pressure, then do a roll-out on a flat surface, while sitting on the trike. Put a mark on a piece of tape and apply it to the wheel to be measured. Get an assistant to put a mark on the surface coinciding with the mark on the tape. Move the trike in a straight line, until the wheel makes a complete rotation. Mark the surface to coincide with the mark on the tape. Measure the distance between marks in millimeters, and that will be the value you enter into the computer. This is the most accurate method for setting the computer.

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FlyingLaZBoy

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Re: Wheel Size Code for Computer
Reply #3 - Mar 16^th, 2016, 2:59pm

Forget the tape -- find a crack, and put your valve stem right at it. Roll out straight for 4 or 5 rotations till the valve stem is at the bottom again, and mark/measure that distance using a 25' tape.

If you measure with a standard tape in inches, multiply your inches by 25.4 to get mm... Then divide the distance by the number of rotations. There's your number.

Four rotations would be about 20 feet, or 240 inches

240" x 25.4 = 6096 mm
6096 / 4 rotations = 1524

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As an interesting variation, do this with tires fully pressurized, then repeat with pressure ~25 pounds lower. The difference between the two, divided by the first one, times 100, is the percent change in speed/distance readings due to tire pressure. A lower tire pressure will give you a "false" higher speed/distance, since the tire is moving you less far per rotation.

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rmillay

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Re: Wheel Size Code for Computer
Reply #4 - Mar 16^th, 2016, 3:59pm

I've done this indoors, on a bare floor, in a 16' square room, but only with one revolution. If you are careful, you can do it alone. Use masking tape, and put a piece on the floor at the start with a line on it. Align the tird mark and get on the trike and roll until the tire makes on revolution. Allow two or three trials until you can go straight without bumping into the furniture or a wall. Put tape down at the end point and mark it. You can get off the trike to do that. The trike shouldn't slide. Check your marks by rolling backward on it to the first mark. It should be within a millimeter of your tire mark. When I was satisfied, I checked my 700 rear measurement (I calibrated both my rear tire computer and my front tire computer) rolling the trike unweighted. The difference in diameter was 18 mm more. Paul's method may be a bit more accurate, but you can't use fractions of a millimeter anyway.

Just a millimeter difference in tread thickness between tires will make 6 mm difference in circumference, so even tire wear will make a noticeable difference on a long ride. For my three laps on the trike at the lake today, my two computers differed by two tenths of a mile. My Garmin split the difference.

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Mdicke

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Re: Wheel Size Code for Computer
Reply #5 - Mar 17^th, 2016, 9:36pm

Thanks guys. This measurement method thing is kind of like opinions, everybody has one. Grin

I came across a nice book from Shwable (sp) , anyway they gave 1510 for my specific tire. Tried that yesterday and it was much closer to the riding apps.

With all the help information I will be doing a couple of different measuring activities this weekend if I have the time to dial this in.

Again thanks for all the help.

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MrWizard

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Re: Wheel Size Code for Computer
Reply #6 - Mar 18^th, 2016, 9:49am

Its not the size of your measurement, its how you use it.

Quote from Mdicke on Mar 17^th, 2016, 9:37pm:

Thanks guys. This measurement method thing is kind of like opinions, everybody has one. Grin

I came across a nice book from Shwable (sp) , anyway they gave 1510 for my specific tire. Tried that yesterday and it was much closer to the riding apps.

With all the help information I will be doing a couple of different measuring activities this weekend if I have the time to dial this in.

Again thanks for all the help.

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jrob_bent Five Star Member rbent member Posts: 953	Re: Wheel Size Code for Computer Reply #7 - Mar 18^th, 2016, 7:20pm I just turn the bike upside down and tape my metric tape measure to the wheel and wind it around until it meets. Of course, I am a bit on the lazy side.
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jayg Five Star Member rbent member Posts: 4133	Re: Wheel Size Code for Computer Reply #8 - Mar 18^th, 2016, 8:55pm See paragraph titled "Roll-out test for high accuracy" here: http://www.sheldonbrown.com/cyclecomputer-calibration.html. Also, see the two paragraphs following the above referenced paragraph.
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Mdicke

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Posts: 207

Re: Wheel Size Code for Computer
Reply #9 - Mar 19^th, 2016, 5:07pm

Ended up playing with the computer calibration, but will have to wait until tomorrow to see the lower effect. I used the 5 rotation roll out and ended up with 1492. On the same roll out I stop at the one rotation point and marked as well. that ended up with a 1489. Either is close enough for me. It will be interesting to how they compare to MMR and Strava. I have an old ETREX Garmin I may throw in the bag just for fun and really confuse the picture.

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Rans F5
Catrike 700
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