A bit OT - calibrating a bike odometer

[robgraham]

The distance that my bike computer is measuring is not in agreement with what I plot out on Google Earth.

Yes, it's pretty obvious that that depends on how well the path is traced on GE, and on the setting of the wheel size into the computer. So taking those out of the equation, given a careful plotting of a calibration road distance of 4.0 miles on GE, what is the accuracy of their 'Path' measuring system?

The alternative question is how does one get a reliable distance to obtain a calibration.

I've just had an argument with myself as to whether I increase the wheel calibration figure or reduce it to compensate for the 5% high error I'm getting on the 4.0 miles, and have won the argument by realising that the entered circumference is the multiplier for each pulse from the sensor, so counter intuitively (initially to me anyway) to reduce an over-mileage, you reduce the computer wheel calibration figure.

Rob

[ARWadsworth]

SatNav?

[MuddyMike]

Ask a local policeman. They have markers at a set distance for calibrating on board speed recorders.

Mike

[Peter Crosland]

Using Google Earth is not going to give accurate figures. Probably the way to do it is to use a GPS and measure two marks on a length of road and then check to odometer against it.

Peter Crosland

[TheScullster]

Heath Robinson method

Get a pram wheel with rigid rubber "tyre". Measure diameter. Multiply by pi for circumference. Fix to handle and add revolution counter from bike shop. Mark pavement. Mark wheel and align with pavement mark. Walk a fair distance along a defined route ensuring constant contact between wheel and pavement. Stop at a full number of wheel rotations - mark pavement. Multiply number of rotations logged by circumference to give accurate distance between points. Travel between marks on bike and adjust your bike odometer to measure same.

Phil

[Ron Lowe]

THe way I do it is this:

In the cycle computer setup, there will be pre-sets for common wheel / tyre sizes, like 700x23. Those are usually close.

But there's usually a 'other' option where you can manually enter a number corresponding to the distance covered per revolution ( the wheel circumference ). To get an accurate measure of this, make a mark on the ground, set the wheel with the valve exactly down at the mark, and with your normal weight on the bike, make exactly 10 revolutions and make another mark. Measure the distance, and divide by 10.

THe reason I say to do it with your normal weight on is that you will squidge the tyre to some extent, so best to do the calb under normal load conditions.

[PeterC]

For my 700Cx28 tyres the 'book' says 214cm, carefully measuring 1 rev. gives

212.5cm, applying about 30kg on the bars gives 211.5cm so I use that. The tyres are about 80psi; the other bike has 25mm tyres at 100psi and the extra loading makes no discernable difference.

If you can find a measured mile[1] that far enough to eliminate most errors. Bear in mind that 1 pass of the magnet adds >2m, even if it moves only 1cm past the sensor.

[1] I've never managed to find a measured mile but did see in Leeds a sign "End of Measured Mile". A chap in our group wondered aloud where the start was - hilarious at first but sensible in that we hadn't seen it.

[Danny Colyer]

The Bristol-Bath Railway Path has markings at measured 100m intervals along the length of the Avon Valley Railway (it also has a measured mile between the Bitton and Riverside stations), which I used to calibrate my computer.

It's worth bearing in mind that tyre pressure makes a small but noticeable difference. My old commute used to be measured at 5.37 miles at 100psi, 5.38 miles at 90psi and 5.39 miles at 80psi. I use my trip distance for the ride to work to tell me when I need to to my tyres up.

[Grimly Curmudgeon]

The local traffic cops certainly used to know things like that - but I'd be surprised if you could find one nowadays.

[PeterC]

That's interesting. I've ridden it a couple of times but just didn't notice

- it's also about 100 miles from here :-(

There's a pot hole[1] at a nominal mile from home and that gives me some idea, although temperature also makes a difference.

[1] Usefully permanent nowadays.

[GB]

I'm confused by that, sorry. I can see that pressing down on the handlebars reduces the radius (but significantly distorts the tyre from a circle). However, the tyre must still lay down 212.5cms of rubber onto the road per revolution. So, where does the extra 1cm go when you only get 211.5 with the weight on? I'm assuming no slippage between the rubber and the floor.

[PeterC]

No slippage as it's on the concrete path and the bearings are good. The weight reduces the rolling radius, same a the tyre being under-pressure does.

[JTM]

Could ask at the local cop shop. If they 'do' a driver by following in a patrol (or other) vehicle, they then have to check 'their' speedo over a measured mile using a certified stopwatch

The markers are placed in metric measurements. OK if your speedo is set to kilometres, awkward if you're on miles per hour

John

[PeterC]

Good tip, thanks. ISTR a mention, many years ago, of a chained mile up the A5 going SE from Towcester.

Simple conversion, innit.

[Andy Champ]

They're every 100 metres. Within my limits 1600 metres will do for a mile. I don't mind being 9/1600 ~= 0.5% out.

Andy

[PeterC]

0.5% would have been about 130 miles out one year - that's a lot for nothing! The setting is to 1mm in ~2.14m; the measurement of the wheel is probably about twice that, so 0.1%. Makes 0.5% look a bit coarse.

[dennis]

The weight distribution/pressure and other factors will change the rolling radius by more than that so don't get the idea it actually matters.

[PeterC]

TFT! Had a front-tyre pucture on the way to Tescrot this morning. I'd realised that the pressure was down a bit and by 5 miles measured it was about 25 yards short of the usual place.

[larkim]

Mildly interesting fact, road races (running, marathons etc) are always measured to be longer than they need to account for the inherent 0.1% error when using a calibrated bicycle measurer. So in the worst case, a marathon would be 42 metres longer (could be 15-20 seconds for a world class marathon runner).

[PeterC]

That's changed! When I was riding time trials, mid-70s - mid-80s, our local courses of up to 50 miles were measure with a 'fifth wheel' behind a car. It was, I guess, the same as a road surveyor uses and the car would try to take the same line as would a bike. This meant that the car would sometimes have it's wheels just off the carriageway, so most measuring was done v. early on a Sunday morning. One local chap was measuring our club's 50 course on his bike with a cyclometer as mentioned on the linked site. He'd done about 45 miles and had a front-wheel puncture. Start again!