Setting tire circumference for computers
- Thread starter skydive69
- Start date
Interestingly, that is not even treated universally. One company says to simply measure the circumference, and another says after measuring the circumference, subtract 4 mm to make up for the difference when the rider's weight is on the bicycle. I guess the engineer in me cries out for exact, universal information which apparently ain't going to happen!gruppo said:
I concur. I finally decided to do the tire measurement manually, and using the method described in the Polar manual (measure the circumference and then subtract 4 mm), my measurement came out to exactly their recommended setting for a 700x25 tire - 2105.drewjc said:
my measurement came out to exactly their recommended setting for a 700x25 tire - 2105.[/QUOTE]
I usually use the manufacturer's default setting for tire size. Doing rollout involves probably more human error than the minute differences in tire size between manufacturers. I've checked my mileage on rides against another rider with the same size tires but weighing 30 lbs more than me and the difference was .2 mile over about 25 mfiles. One of us could have zigged or zagged that much, so I wouldn't get too worked up over the alleged precision of manually entered rollouts.
I usually use the manufacturer's default setting for tire size. Doing rollout involves probably more human error than the minute differences in tire size between manufacturers. I've checked my mileage on rides against another rider with the same size tires but weighing 30 lbs more than me and the difference was .2 mile over about 25 mfiles. One of us could have zigged or zagged that much, so I wouldn't get too worked up over the alleged precision of manually entered rollouts.
Tesla* said:
I usually use the manufacturer's default setting for tire size. Doing rollout involves probably more human error than the minute differences in tire size between manufacturers. I've checked my mileage on rides against another rider with the same size tires but weighing 30 lbs more than me and the difference was .2 mile over about 25 mfiles. One of us could have zigged or zagged that much, so I wouldn't get too worked up over the alleged precision of manually entered rollouts.[/QUOTE]
So true - it's just that the engineer/pilot mentality that I am stuck with cries out for accuracy and precision.
Having the engineer mentality myself, I'd do the roll test a bunch of times and average the results.
What's with engineers and accuracy? I don't know, but I guess that makes me one with the mentality too.artmichalek said:Having the engineer mentality myself, I'd do the roll test a bunch of times and average the results.
I once bought a cheapo computer and got results way off from my CatEyes. What I did was do some controlled runs on a measured straight course (wanted to use a theodolite but only steel tape was available
) then adjusted the wheel size accordingly until I got a negligible enough error on the trip meter.Tedious? Yes. Fun? Hehehe
artmichalek said:Having the engineer mentality myself, I'd do the roll test a bunch of times and average the results.
Being really lazy myself, I simply measure the actual with of the tire and estimate the circumference with
circ ~= (bead dia. + 2*width)*3.1416
Subtract 2mm for better accuracy
circ for 700x25 ~= (622 + 2*25)*3.1416 - 2 = 2109mm
cheapie said:
Nominal formula for diameter is bead diameter + 2 X tire width.
26 X 1.95 would be 559mm + 2 X (1.95 X 25.4)= 658.1 mm
26 X 2.1 would be 559 + 2 X (2.1 X 25.4) = 665.7 mm.
So take your calibration for 26 X 1.95 and multiply it by (665.7 /658.1)=
1.0115.
Should be able to make the adjustment from that.
So true - it's just that the engineer/pilot mentality that I am stuck with cries out for accuracy and precision.[/QUOTE]skydive69 said:
Your experience is far different to mine. The manufactureres numbers are a guide - and usually result in fairly flattering speeds and distances.
Even different tyres make a difference. Unless you are lucky, you will be several kmph out setting your computer to the manufacturers circumference.
Its no good comparing your speed and distance to your riding mates because they are likely to be wrong too. Do it the way I mention above and you are not cheating yourself.
