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 — say for a 26 x 1.9 tyre?

Thanks in advance.

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.

See the Sheldon Brown site for information on calibrating your cyclecomputer. https://sheldonbrown.com/cyclecomputer-calibration.html

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.

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.

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.

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.

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

What he said.

That's the ticket.

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.

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.

Close enough,, is good enough, I would say.

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.

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.

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.

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.