# Commuting - bike computer questions

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JPprivate
09-21-09, 08:06 AM
So I just bought a basic wireless bike computer (you know the one that counts the rotations of your tire) and tried it out on my route. I usually have morning route and an afternoon/evening route. The morning route is approx 60% on a MUP along a river (as in curved). I measured the distance using bikely.com and got 7.1 m, but the bike computer spit out 7.356?
Question: How accurate are the distance measurements on bike computers? Because a large part of my trip is on a MUP, exact measurements are not that easily made on bikely.com.
I'll try my afternoon route, which is more along streets to see if I get a more accurate reading.
thanks,

Hot Potato
09-21-09, 08:12 AM

They all mulitply tire revolutions by the circumference to calculate distance. Your computer has a default circumference, and likely a choice of circumference by tire size. But for true accuracy, you measure your tire circumference on the wheel that has the magnet. Mark a spot on the floor, roll one revolution, mark another spot, measure the distance between the mark. Use the valve stem, or some other identifying mark on the tire or rim to see one revolution. Then input your measurement to the computer.

sci_femme
09-21-09, 08:41 AM
...And the difference is grand 3.5%. Wow. Tire widths are all over the chart, there are 28s that are closer to 25s than some 25s, and vice versa. With MTB tires the difference is even more pronounced - pump it up from 50 to 65 PSI and watch it swell... Even using Hot Potato's method of computer calibration your ride distance recorded will be affected by the difference between pressure at calibration and pressure during ride.

This is precisely the reason I tell my distances as "about xx miles".

I guess, in your case the tire is a bit narrower than nominal size so the wheel rotates a bit faster to cover the same distance.

Geez, listen to me reveling in my geekitude....

Have fun and ride safe

SF

JPprivate
09-21-09, 08:42 AM
thank hotpotato, that makes sense. But it sounds like you're suggesting that the tire circumference provided with the bike computer (for my tire size) may not be 100% accurate. Is that the case, have others had the same experience?

MMACH 5
09-21-09, 08:57 AM
thank hotpotato, that makes sense. But it sounds like you're suggesting that the tire circumference provided with the bike computer (for my tire size) may not be 100% accurate. Is that the case, have others had the same experience?

Yes. For the most accurate measurements, use the "roll-out" method for determining the wheel circumference.

Hot Potato
09-21-09, 09:14 AM
I think my computers pre-set circumference based upon tire size was 5% off. I have an MUP with mile markers, and it consitently read 1.05 miles for each mile marker I passed. I was just as impressed with the computers consistency as I was with the paths markers. Then I made the circumference measurement and calculated what the circumference distance change should be based upon the mile makers, and they were within a few millimeters, giving me confidence that my measurement and the mile markers were accurate enough.

Is it a big deal? Not really, like sci femme alludes to. However, I wanted to answer your question, and it would be a bigger deal if you never set the circumference to the correct tire size.

You could always go GPS, then your tire circumference gets measured and set automatically, updated as you ride for tire wear and pressure changes, or not used at all. A mere 400 bucks will spare you the agony of circumference measurement. :)

JPprivate
09-21-09, 09:16 AM
I guess, in your case the tire is a bit narrower than nominal size so the wheel rotates a bit faster to cover the same distance.

scifemme: I think you're on to something, I did change my tires from the original 1.95 to 1.75. While width shouldn't directly influence the circumference I guess the thickness of the tire is of course different too.

JPprivate
09-21-09, 09:24 AM
I have an MUP with mile markers,

That's an excellent idea, I think I going to go this route. Adjust my computer until MUP mile markers / computer readouts are the same.
(I don't trust my ability to measure one tire rotations accurately to 1/10 of an inch) :-)

CCrew
09-21-09, 09:27 AM
You could always go GPS, then your tire circumference gets measured and set automatically, updated as you ride for tire wear and pressure changes, or not used at all. A mere 400 bucks will spare you the agony of circumference measurement. :)

Naa, not \$400, but look at what ya get :) This morning's commute:

http://connect.garmin.com/activity/13736848

But what can I say... I freely admit to my geekiness.

09-21-09, 09:58 AM
Wow that GPS thing looks really convenient! (A bit too pricey imo though, even if it's less than you say). That's a great route, it's a trail practically the whole way!

I just got a computer for my bike and it's <2% error which is more than acceptable on my own terms. At the end of the week it's only gonna make a difference of a couple kms.

CCrew
09-21-09, 10:36 AM
(A bit too pricey imo though, even if it's less than you say). That's a great route, it's a trail practically the whole way!

I paid \$169 for it on sale. And yeah, it's a great commute by bike! But I have 50 more miles of it in a car to get to that point :(

Hot Potato
09-21-09, 11:43 AM
I paid \$169 for it on sale. And yeah, it's a great commute by bike! But I have 50 more miles of it in a car to get to that point :(

I was thinking of the Edge 605. Are maps on a bike computer cool? Yes. Is it worth three or four hundred dollars more? I dunno. I presume you have an edge 305 for that price? Or some other garmin product? They are awesome bike computers, even without the map features of the expensive high end models. I use the on bike map stuff less than once a month, if that, so my edge 705 is a bit overkill. I use the heart rate monitor even less than maps.

Wanderer
09-21-09, 12:13 PM
Rollling the tire out, about 3 complete revolutions, using that average circumference, is the only way you will get accurate measurements. Make sure the tire is loaded like it would be in riding.

Those mileposts were probably put in by someone using a speedometer in a pickup truck......

Using the averaged rollout measurement, you will be as accurate as you can be.

CCrew
09-21-09, 01:07 PM
I was thinking of the Edge 605. Are maps on a bike computer cool? Yes. Is it worth three or four hundred dollars more? I dunno. I presume you have an edge 305 for that price? Or some other garmin product? They are awesome bike computers, even without the map features of the expensive high end models. I use the on bike map stuff less than once a month, if that, so my edge 705 is a bit overkill. I use the heart rate monitor even less than maps.

It's an Edge 305. I have the HR and Speed/cadence. I pretty much use the commute as training, so I use the HR fairly extensively and track it to the cadence. For what I use it for the 305 is all I need. Don't get me wrong, the 605/705 are nice units, but from owning several other Garmins I've found that the routing via itty screen doesn't work well for me.

cyclefreaksix
09-21-09, 01:20 PM
I put a drop of oil on my tire and roll my bike across my garage floor. Measure the "wet spots" and I've got a purty good idea of my tire circumference.

HardyWeinberg
09-21-09, 01:23 PM
thank hotpotato, that makes sense. But it sounds like you're suggesting that the tire circumference provided with the bike computer (for my tire size) may not be 100% accurate. Is that the case, have others had the same experience?

Absolutely. And it varies w/ pressure too.

Hot Potato
09-21-09, 07:13 PM
I put a drop of oil on my tire and roll my bike across my garage floor. Measure the "wet spots" and I've got a purty good idea of my tire circumference.

And that is the kind of pearl that sneaks into a BF thread every now and then to make it all worth while.:thumb:

Now what do I do with the pink sidewalk chalk I hid from my kids.

CaptainCool
09-22-09, 12:03 AM
Because a large part of my trip is on a MUP, exact measurements are not that easily made on bikely.com.
Try http://www.gmap-pedometer.com and select "Manually (straight lines)." Zoom in, use hybrid maps, then start laying out your exact route.