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What I did after finding out that the auto setting is not accurate at all is to measure the tire by making a mark on the floor & on the tire, roll it around until the line on the tire is vertical again and measure it in inches and multiply it by 25.4, that will give you total centimeters. The chart in the back of the Garmin operation manual is off also. Plugging in your actual circumference is the most accurate. I have 26 x 2 and it was about 7/8” off, not a big deal but it does add up. Hope this helps. Oh yea, that only works with the cadence sensor, without it you will get a GPS reading which should be very accurate also.
 

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Derailleurless
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Take a gander at this thread: http://forums.mtbr.com/showthread.php?t=314928

The main steps to achieve accurate tire rollout measurements:

1. Make sure the tire is inflated to your typical riding pressure.

2. Weight the front end of the bike to simulate your typical weight distribution. If you skip this step, then you may as well skip step #1.

3. Average your result. I usually measure 3 or 4 continuosus revolutions -- whatever my tape measure can handle. And then do it again, to make sure it comes close to the first measurement.

If you don't want to bother with the roll-out, then stick with the book value knowing you'll be close enough -- probably better than 10% error (and what's one mile among ten?).
 

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Waypointer
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Dogdas said:
What I did after finding out that the auto setting is not accurate at all is to measure the tire by making a mark on the floor & on the tire, roll it around until the line on the tire is vertical again and measure it in inches and multiply it by 25.4, that will give you total centimeters. ...
Oops, maybe just a typo above. Multiplying by 25.4 would give millimeters. Multiply inches measured by 2.54 to get centimeters.
 

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What I do is go to google earth. Measure the distance of my favorite road route then travel that route and make note of the difference between google earth and my computer. From there, make the necessary computations to set the correct tire circumference.

Current tire settings = 2170 mm
Google Earth Distance = 22 km or 22000000 mm
Cyclo Distance = 23km or 23000000 mm
= 1000000 mm difference

Revolutions = 23000000/2170
Revolutions = 105990.78

Effective Circumference = 22000000/105990.78
Effective Circumference = 2075.65
 

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angelobryant said:
What I do is go to google earth. Measure the distance of my favorite road route then travel that route and make note of the difference between google earth and my computer. From there, make the necessary computations to set the correct tire circumference.
That, my friend, is the ultimate tire roll-out measurement technique!
 

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wouldnt you have to turn off GPS to make sure the measurement is working? I thought it used the speed sensor only when GPS was not available. So it will be on and off during any typical ride where GPS reception is shoddy.
 

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Im sure this gets asked a lot, and I believe it can be related to the current topic too, so I wont bather with starting a whole new thread for it.

When I buy a computer for my bike, strictly to record my distance (GPS units dont calculate elevation gain yet, do they?) how do I know where to mount it? Six inches from the center of the hub? Nine inches from the outer most knob? Right in the middle? It seems that the mounting point, or distance from the center of the hub, or whatever, would be rather relevant, and I dont want to trick myself into thinking that the "16 miles" I just rode was really only five and a half.

Anyways, I hope this doesnt get asked every 3rd day in here, but I skimmed the first page and didnt see it at least. Thanks guys!
 

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Enduroblood said:
Im sure this gets asked a lot, and I believe it can be related to the current topic too, so I wont bather with starting a whole new thread for it.

When I buy a computer for my bike, strictly to record my distance (GPS units dont calculate elevation gain yet, do they?) how do I know where to mount it? Six inches from the center of the hub? Nine inches from the outer most knob? Right in the middle? It seems that the mounting point, or distance from the center of the hub, or whatever, would be rather relevant, and I dont want to trick myself into thinking that the "16 miles" I just rode was really only five and a half.

Anyways, I hope this doesnt get asked every 3rd day in here, but I skimmed the first page and didnt see it at least. Thanks guys!
GPS units record elevation gain, some better than others. If you're shopping for one and interested in this feature, look for a unit with a barometric altimeter built in (Garmin Vista HCx, for example). The GPS serves to calibrate the elevation, but the barometer is primarily responsible for tracking changes during movement, eliminating the spikey readings the GPS unit alone would record.

For what it's worth, my VDO MC1.0+ bike computer provides much more trustworthy elevation figures. My GPS usually reads 10 to 20% high, and after I upload it to my PC and overlay it on a terrain map, that error is worsened. This error is reported by other GPS users, too.

The position of the magnet relative to the hub doesn't matter. The computer only counts wheel revolutions. Each revolution is multiplied by the tire circumference you plug in during setup. A revolution is a revolution, no matter if it's marked by each passing of the logo on the hub, or by the valve stem protruding from the rim. No different for the magnet.
 

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Enduroblood said:
Im sure this gets asked a lot, and I believe it can be related to the current topic too, so I wont bather with starting a whole new thread for it.

When I buy a computer for my bike, strictly to record my distance (GPS units dont calculate elevation gain yet, do they?) how do I know where to mount it? Six inches from the center of the hub? Nine inches from the outer most knob? Right in the middle? It seems that the mounting point, or distance from the center of the hub, or whatever, would be rather relevant, and I dont want to trick myself into thinking that the "16 miles" I just rode was really only five and a half.

Anyways, I hope this doesnt get asked every 3rd day in here, but I skimmed the first page and didnt see it at least. Thanks guys!
It makes no difference, the sending unit only measures how many revolutions your
wheel is making. This is "why" you need to tell the computer how big around
your tire is.

Think about it.
 

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Ahhhh, I got it now. I never thought about it that way. Thanks for opening my eyes and explaining how "simple" that is.

Im not really looking for a GPS unit though. I have a hand held that I take with me on hikes, hunts, road trips, and all that fun stuff. I sometimes take it on long mtb rides too, but mainly so I know right where I am, and so I can tell someone else right where I am if I need to. I just want a bike computer that will tell me speed, average speed, highest speed, and distance. Not too concerned about elevation gain right now, just distance traveled.

Thanks again.
 
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