Setting tire circumference for computers

[meb]

So true - it's just that the engineer/pilot mentality that I am stuck with cries out for accuracy and precision.

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.

I've had very good success using manufactures numbers, and have not even been off .1 over 10 mile stretches- and that includes one bike with a combined rider bike weight at 300 lbs- although only about 75 lbs were on the front sensor shod tire. Of course Skydive's .8% error would not show up on my distance.

But being off several mph from manufacturers nominal seems a stretch, even 1 mph seems extreme. With skydive's .8% error, at 25 mph you'd have a .2 mph error in speed.

As for the manufacturers numbers being flattering, maybe they are factoring in a nominal weight and persons lighter than the nominal are coming in with flattering averages.

 

[artmichalek]

Good point.

Anyone got a model calculation that adjusts for weight on tire with sensor, and tire width and diameter?

I'm sure it could be derived. (with factors for tire pressure, sidewall stiffness, center of gravity...) Of course, by the time you go through measuring all of the things that go into it that are specific to your bike, you could have done the roll out test and gone for a nice ride.

 

[mitosis]

I've had very good success using manufactures numbers, and have not even been off .1 over 10 mile stretches- and that includes one bike with a combined rider bike weight at 300 lbs- although only about 75 lbs were on the front sensor shod tire. Of course Skydive's .8% error would not show up on my distance.

But being off several mph from manufacturers nominal seems a stretch, even 1 mph seems extreme. With skydive's .8% error, at 25 mph you'd have a .2 mph error in speed.

As for the manufacturers numbers being flattering, maybe they are factoring in a nominal weight and persons lighter than the nominal are coming in with flattering averages.

And how are you checking your distances? How do you know you are only .1 mile off in 10?

 

[meb]

And how are you checking your distances? How do you know you are only .1 mile off in 10?

I have 1/10 mile precision in English mode. A 1% error (1/10 mile error per 10 miles) would show up as a discrepency from the mile markers on the marked bike trails. The absence of a discrepency means the error is less than that available for the precision checked over that distance.

The Mount Vernon Trail has wooden mile marker signs put up by the US Park Service and a multitude of markings have been painted on the asphalt-I presume either runners or cyclists or their organizations have painted those markers for training.

The Washington & Old Dominion Trail also has markers put up by the Northern Virginia Regional Park Authority similar to the those on the Mt. Vernon Trail.

 

[steve_wmn]

Problem with this method is you don't take into account the smaller diameter caused by your weight on the tyre. Sit on the bike and measure. There is no error to allow for with this method.

The tire deflects at the contact patch, but does the effective circumference really change? Picture the contact patch rolling along the ground with and without weight. My guess is that the thread rolling across the ground remains constant in length and travels the exact same distance regardless of weight applied to it, unless the tire tread contracts as a function of the weight on it during deflection against the ground.

 

[meb]

The tire deflects at the contact patch, but does the effective circumference really change? Picture the contact patch rolling along the ground with and without weight. My guess is that the thread rolling across the ground remains constant in length and travels the exact same distance regardless of weight applied to it, unless the tire tread contracts as a function of the weight on it during deflection against the ground.

Yes. The tread of a tire unflattened at the patch (i.e. an ultra hard tire) would continue it's arcuate path before reaching the surface at a single point. The tread would continue advancing relative the ground immediately below till it touched the ground. The tread on a tire flattened at the patch does not complete its arcuate path, the tread takes a shortcut path along the chord or the circle.

 

[mitosis]

The tire deflects at the contact patch, but does the effective circumference really change? Picture the contact patch rolling along the ground with and without weight. My guess is that the thread rolling across the ground remains constant in length and travels the exact same distance regardless of weight applied to it, unless the tire tread contracts as a function of the weight on it during deflection against the ground.

Easy way to check. Pump the tire up and measure distance travelled after a few turns with no weight on. Do the same sitting on it. Believe me now?