Wheel size

[MrRidley]

Looking for help with the wheel size for my bike computer, i got my new bike yesterday (giant escape) but cant get the size of wheel right, the wheels are 26*1.5 and i have done the measurement i:e rolling the wheel along the ground and measuring the distance etc, also 26in divided into mm multiplied by 3.14 and i am getting two different answers any advice welcome.

thanks

 

[Steve Austin]

off the top of my head thats 201 isn't it?

 

[MrRidley]

off the top of my head thats 201 isn't it?

2072.4 and 2020 are the sizes i'm getting, i have to enter it in my computer in mm

 

[Chrisz]

Pump your tyres up to normal pressure, find a clear piece of concrete, make a chalk mark across the top of the tyre (nice and thick), ride the bike in a straight line until the chalk mark is uppermost again, get off and measure the distance between the two chalk marks on the pavement.

This method will give you the actual rolling diameter of your wheel and should be nice and accurate

 

[HLaB]

I've not often bothered with the faff of measuring for 26x1.5 tyres my raleigh computer recommends 2030mm and my cateye recommend 2010mm, sounds like Steve guess was a good one.

 

[Dave5N]

Just mark the floor with the valve dead bottom and roll it backwards. Measure.

 

[jimboalee]

1cm in 2m is 0.5% error. That's nearly 9 yards in every mile.

That wasn't good enough for me, so I found an A road with distance markers. I set up the computer as per instructions and rode the measured mile ( or 2 km ) along the A road. At the end, I measured out the distance difference between the marker and where I stopped when the computer ticked over the 2.00 km distance.
I did some maths and adjusted the wheel circ to suit.

Performed a second trial and got the computer ticking to 2.00km at 3m error. There was no better adjustment than this.


That was 15 years ago. I have Garmin eTrex and Edge now, which is GPS and LESS accurate !!

 

[byegad]

GPS accuracy varies. My Legend tells me how inaccurate it is at any one time. If I ride ten miles and the accuracy is +/- 30 feet at the start and the finish it can only be 60 feet out over ten miles. The same cam be said of 30 miles or 1000miles. I find it hard to believe that ANY bike computer can be more accurate.

 

[jimboalee]

What you will find with Garmin GPS is it records a position every now-and-again along the route. Each point has an accuracy of, as you say, +/- 30ft. Sometimes, when the signals are being interupted or deflected, the accuracy decays and the thing thinks its sitting on the fence and not on the road.

Study the Track and you might see the plotted Track zig-zags along the road and sometimes detours off the road. The true distance will be according to Pythagoras' theorum and 'finite element' construction.

The 'Trip distance' displayed is an accumulation of all the straight lines between data points at changes in direction, speed or elevation. If the data points are laterally inacurate, the errors will accumulate uncontrollably therefore lengthening the distance.

Also, you may ride round a slow sweeping curve. The two datapoints at each end of the curve are at the distance of the straight line, not the arc. Thus shortening the indicated distance.

One may say the law of averages will sort it out, but I wouldn't trust that. GPS distance determination is inherently inacurate.

 

[02GF74]

don't bother calculating. as ^^^ say, tape on tyre, place stick on ground next to mark, sit on saddle and wheel bike forwards until mark in on ground, put down second stick. measure distance between the 2 sticks. repeat until you get a consistn measurement, job done.

 

[jimboalee]

I've given up on distances.
The nearest 0.1 miles does for my nutritional calculator. 15 mph = 4 min mile = 32 kcals per mile or 3.2 kcals error. Less than 1 g of CHO. I always over eat by about 50 kcals anyway B).

For Audax, the whole route is measured on Mapsource to 0.1 miles.
Calcs are done and the Garmin only serves to guide me round the route.

After the ride, in interest, I look at the data to see how fast I was climbing the hills, which gauges my performance against the previous year.