Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  


Go Back   > >

General Cycling Discussion Have a cycling related question or comment that doesn't fit in one of the other specialty forums? Drop on in and post in here! When possible, please select the forum above that most fits your post!

User Tag List

Reply
 
Thread Tools Search this Thread
Old 12-15-09, 04:43 AM   #1
ausGeoff
EóBike Newbie
Thread Starter
 
Join Date: Jun 2009
Bikes:
Posts: 42
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Bike Computer Mileage?

Okay guys... this might be for the mathematicians amongst us LOL.

I've just fitted a Sigma BC 506 bike computer to my treadly. It's working OK (nominally) except that the odometer is measuring significantly less mileage than it should be. Over a measured distance (and confirmed via Google maps to within around 100m) it's reading 18% less than the true distance.

I've got 26 x 1.75 smooth tread tyre at 35 PSI.

As per the Sigma's instruction manual, I've used their 2070 setting on the computer.

...so, have I got a 'lemon' with the Sigma, or can I simply choose another size setting ó say for a 26 x 1.9 tyre?

Thanks in advance.
ausGeoff is offline   Reply With Quote
Old 12-15-09, 05:14 AM   #2
Esteban32696
Senior Member
 
Join Date: Feb 2007
Bikes:
Posts: 1,131
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
I would just change to another size setting. Tires can be quite different in OVERALL diameter , rather than rim size. If you have access to a GPS, use it to set up a planned route,, such as 1.2 miles, set bike computer , ride & adjust it til you get it close enough. It probably won't perfectly match the GPS.
Esteban32696 is offline   Reply With Quote
Old 12-15-09, 06:49 AM   #3
Comtour
Junior Member
 
Join Date: Jan 2009
Bikes: Surly LHT, Cannondale Hybrid, Raleigh Supercourse
Posts: 16
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
See the Sheldon Brown site for information on calibrating your cyclecomputer. http://sheldonbrown.com/cyclecomputer-calibration.html
Comtour is offline   Reply With Quote
Old 12-15-09, 08:36 AM   #4
BarracksSi
Bike ≠ Car ≠ Ped.
 
BarracksSi's Avatar
 
Join Date: Jul 2007
Location: Washington, DC
Bikes: Some bikes. Hell, they're all the same, ain't they?
Posts: 13,858
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
I don't have any bike tires inflated as low as 35 psi...

Anyway, what you should be able to do is measure "rollout", as it's called. Mark a point on the tire, ride it for ten revolutions of the wheel, measure the distance, then divide that distance by ten. That gives you your actual tire circumference while riding, taking into account the change in its shape due to rider weight.

I mentioned low PSI because it could be that the tires are deforming enough to change the riding circumference and induce error in the odometer. Imagine riding on a bare rim and how much smaller that is than a tire, then think of the difference between that and a tire inflated so hard that it doesn't deform at all -- a squishy-soft tire would be somewhere in between.
BarracksSi is offline   Reply With Quote
Old 12-15-09, 11:52 AM   #5
BarracksSi
Bike ≠ Car ≠ Ped.
 
BarracksSi's Avatar
 
Join Date: Jul 2007
Location: Washington, DC
Bikes: Some bikes. Hell, they're all the same, ain't they?
Posts: 13,858
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
On second thought, if your tires were deforming and throwing off the odometer, it would read longer than the actual distance. So, never mind that idea.
BarracksSi is offline   Reply With Quote
Old 12-15-09, 02:38 PM   #6
Velo Dog
Senior Member
 
Join Date: Jun 2004
Location: Northern Nevada
Bikes:
Posts: 3,802
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by BarracksSi View Post
Anyway, what you should be able to do is measure "rollout", as it's called. Mark a point on the tire, ride it for ten revolutions of the wheel, measure the distance, then divide that distance by ten. That gives you your actual tire circumference while riding, taking into account the change in its shape due to rider weight.
Don't know if this is the case for all bike computers, but every one I've owned for the last 20 years, at least, has used the tire circumference in millimeters as the setting. Roll out as above (I only use one revolution, because 10 takes 75 feet or so, but 10 would theoretically be more accurate) and set the puter.
In practice, close enough is close enough. I used to reset when I swapped the computer from a bike with 25mm tires to one with 32mm tires, but on the road you don't ride a straight line anyway--you wobble, go around potholes, change lanes, all kinds of things. What's it matter if you're off 10 feet per mile?
If you do your rollout next to a wall you can use for support, you can sit on the bike while you roll and allow for tire deformation. I usually do it that way, but I've never compared to an unloaded bike to see if it's significant.
Velo Dog is offline   Reply With Quote
Old 12-15-09, 05:20 PM   #7
RonH
Life is good
 
RonH's Avatar
 
Join Date: Jan 2001
Location: Not far from the Withlacoochee Trail. 🚴🏻
Bikes: 2001 Litespeed Tuscany and 2014 Cannondale SuperSix EVO Hi-Mod
Posts: 16,302
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 85 Post(s)
Quote:
Originally Posted by ausGeoff View Post
I've got 26 x 1.75 smooth tread tyre at 35 PSI.
How often do you have a pinch flat?
Have you inflated your tires to something nearer the top end of what is shown on the tire sidewall. If you do you may find that the mileage will be very close to correct using the 2070 number. If you want to run your tires that soft (35 psi) maybe you should do a roll out measurement. That will give you a more accurate number to program into the computer.
__________________


The LORD is merciful and gracious, slow to anger, and plenteous in mercy. - Psalm 103:8
RonH is offline   Reply With Quote
Old 12-15-09, 05:34 PM   #8
CB HI
Cycle Year Round
 
CB HI's Avatar
 
Join Date: Mar 2005
Location: Honolulu, HI
Bikes:
Posts: 11,963
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 197 Post(s)
Like the other said, put more air in your tires. The sidewall should have a pressure range for the tire.

Try setting your computer to 2440.

2070 x 1.18 = 2443
CB HI is offline   Reply With Quote
Old 12-15-09, 06:23 PM   #9
KungPaoSchwinn
Senior Member
 
KungPaoSchwinn's Avatar
 
Join Date: Nov 2008
Location: Ms/Ca, USA,Earth.
Bikes: 2009 Trek FX 7.3 ( pimp moment )
Posts: 705
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by BarracksSi View Post
Anyway, what you should be able to do is measure "rollout", as it's called.
.
What he said.
KungPaoSchwinn is offline   Reply With Quote
Old 12-15-09, 07:06 PM   #10
JanMM
rebmeM roineS
 
JanMM's Avatar
 
Join Date: Jan 2006
Location: In Central IN
Bikes: RANS V3, RANS V-Rex, RANS Screamer
Posts: 13,339
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 16 Post(s)
Quote:
Originally Posted by KungPaoSchwinn View Post
What he said.
That's the ticket.
__________________
RANS V3 - Ti, RANS V-Rex - cromo, RANS Screamer - cromo
JanMM is online now   Reply With Quote
Old 12-19-09, 11:57 AM   #11
ausGeoff
EóBike Newbie
Thread Starter
 
Join Date: Jun 2009
Bikes:
Posts: 42
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Thanks for the suggestions guys. Much appreciated!

I did the rollout measurement, which came out at 2050mm, but was still giving an incorrect distance. Over the last few days I've been fine-tuning the value over the same measured distance, and now have it set at 2010 which is almost spot on — and close enough not to worry about any minor difference.

This setting is actually closer to a 26 x 1 tyre, so I dunno how Sigma programmed their defaults.

Anyway, problem solved.
ausGeoff is offline   Reply With Quote
Old 12-19-09, 01:26 PM   #12
BarracksSi
Bike ≠ Car ≠ Ped.
 
BarracksSi's Avatar
 
Join Date: Jul 2007
Location: Washington, DC
Bikes: Some bikes. Hell, they're all the same, ain't they?
Posts: 13,858
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by ausGeoff View Post
I've got 26 x 1.75 smooth tread tyre at 35 PSI.
Quote:
Originally Posted by ausGeoff View Post
This setting is actually closer to a 26 x 1 tyre, so I dunno how Sigma programmed their defaults.
Are you still running it at 35 psi, too? If so, it sounds like the 1.75 tire is deforming enough that its rolling circumference is closer to a fully-inflated 1" tire. But, the front tire wouldn't deform nearly as much as the rear.

Sigma might have also programmed its 26x1.75 setting as if the bike had knobby tires instead of smooth tread. The tire size is more about its carcass and the room it has inside, not how deep the tread is, so a smooth 26x1.75 slick tire would have a smaller "on-the-road" circumference than a 26x1.75 deep-lugged mud tire.
BarracksSi is offline   Reply With Quote
Old 12-20-09, 06:47 AM   #13
Esteban32696
Senior Member
 
Join Date: Feb 2007
Bikes:
Posts: 1,131
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Close enough,, is good enough, I would say.
Esteban32696 is offline   Reply With Quote
Old 12-20-09, 11:54 AM   #14
ausGeoff
EóBike Newbie
Thread Starter
 
Join Date: Jun 2009
Bikes:
Posts: 42
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by BarracksSi View Post
Sigma might have also programmed its 26x1.75 setting as if the bike had knobby tires instead of smooth tread. The tire size is more about its carcass and the room it has inside, not how deep the tread is, so a smooth 26x1.75 slick tire would have a smaller "on-the-road" circumference than a 26x1.75 deep-lugged mud tire.
Good point...

Sigma don't say what type of tread pattern they've calibrated their computers for. I guess a slick and a knobbly tyre could have maybe at least 2cm diference in OD (10mm lugs x 2) whilst having identical carcass diameters.

And yeah... I'm still running the front at 35PSI, and the rear at 60PSI as it's an electric bike, with a rear-wheel hub motor, plus a steel frame, plus my 95kg body mass! I've based those pressures on a 45kg front loading v. an 85kg rear loading.

Cheers.
ausGeoff is offline   Reply With Quote
Old 12-20-09, 12:24 PM   #15
stapfam
Time for a change.
 
stapfam's Avatar
 
Join Date: Jan 2004
Location: 6 miles inland from the coast of Sussex, in the South East of England
Bikes: Dale MT2000. Bianchi FS920 Kona Explosif. Giant TCR C. Boreas Ignis. Pinarello Fp Uno.
Posts: 19,915
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Quote:
Originally Posted by ausGeoff View Post
Good point...

Sigma don't say what type of tread pattern they've calibrated their computers for. I guess a slick and a knobbly tyre could have maybe at least 2cm diference in OD (10mm lugs x 2) whilst having identical carcass diameters.

And yeah... I'm still running the front at 35PSI, and the rear at 60PSI as it's an electric bike, with a rear-wheel hub motor, plus a steel frame, plus my 95kg body mass! I've based those pressures on a 45kg front loading v. an 85kg rear loading.

Cheers.
35 psi in the front and touch a kerb- fall down a pothole ot even run over a pebble in the road. Hope you carry a spare tube- levers and a Pump.
__________________
How long was I in the army? Five foot seven.


Spike Milligan
stapfam is offline   Reply With Quote
Old 12-20-09, 05:25 PM   #16
sknhgy 
Dirt Bomb
 
sknhgy's Avatar
 
Join Date: Aug 2006
Location: Illinois
Bikes:
Posts: 2,638
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 147 Post(s)
I've thought about this but haven't gone to the trouble to try it: Put a dab of paint on the ground. Ride over it. Your tire should print a mark on the road the exact circumference of the tire. Measure the distance between paint marks on the road.
sknhgy is offline   Reply With Quote
Old 12-20-09, 06:07 PM   #17
Bander
Son of Fred
 
Bander's Avatar
 
Join Date: Apr 2008
Location: Chesapeake, VA
Bikes:
Posts: 122
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by sknhgy View Post
I've thought about this but haven't gone to the trouble to try it: Put a dab of paint on the ground. Ride over it. Your tire should print a mark on the road the exact circumference of the tire. Measure the distance between paint marks on the road.
Or just use a wet spot...same result without a lasting mark anywhere.
Bander is offline   Reply With Quote
Old 12-20-09, 09:44 PM   #18
StephenH
Uber Goober
 
StephenH's Avatar
 
Join Date: Sep 2007
Location: Dallas area, Texas
Bikes:
Posts: 11,250
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 8 Post(s)
The difference between 2010 and 2070 is about 3%, not 18%. Anyway, sounds like you're good there.

The main reason I know of to have an accurate measurement is if you follow cue sheets in randonneuring, which will typically have directions like "Turn left at (unmarked) CR 318 at mile 78.6". There it helps to be pretty much right on, and using the roll-out (with just a single rotation), that has worked for me. I did the roll-outs with me sitting on the bike, not empty.

I've been running the tires on my Worksman cruiser at 45 psi. They're 26x2.125. I bought an old Schwinn industrial bike a while back, and the 26x2.125" tires on it were only rated for 35 psi. So that's not necessarily unreasonable for your slightly narrower tires. If it works for you, no reason to change it.
__________________
"be careful this rando stuff is addictive and dan's the 'pusher'."
StephenH is offline   Reply With Quote
Old 12-20-09, 10:23 PM   #19
BlazingPedals
Senior Member
 
BlazingPedals's Avatar
 
Join Date: Dec 2004
Location: Middle of da Mitten
Bikes: Trek 7500, RANS V-Rex, Optima Baron, Velokraft NoCom, M-5 Carbon Highracer, homebuilt recumbent
Posts: 8,911
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 81 Post(s)
The numbers in the manual are never right. They're only there as a very rough ballpark estimate. Speaking of estimates, I converted 1.75" to metric (44.5mm), doubled it and added 559mm for the bead seat diameter, and came up with a circumference of 2034mm. That's also a ballpark because most tires are not labeled accurately. If you get a number by actual roll-out with weight on the bike, and it still doesn't match up with the GPS, then there's something going on with the GPS.

How I do it: set the valve stem on the bottom of the wheel, over a chalk mark or sidewalk crack. Sit on bike and roll it forward for 3 tire revolutions of the wheel which has the sensor magnet, coming to rest with the valve stem at the bottom of the wheel. Mark the pavement and measure with a tape. If you need to convert, 1 inch = 25.4 mm. Divide by 3 to get the circumference of the tire. Using this method, you should be able to get within 0.5% accuracy.
BlazingPedals is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -6. The time now is 12:01 PM.