Garmin - wheel circumference

[Mick F]

I've recently bought a Speed/Cadence unit for my Garmin 305 so I can keep tabs on my roller riding and also see my cadence on the road. I works well but I'm a trifle confused about the readings.

The Garmin instruction manual states that my size of tyre - 700c 20mm - has a circumference of 2086mm - that was the figure I inputted for my bike's profile when I first set up my Garmin before I bought the S/C unit. There's a comprehensive table of wheel/tyre sizes. My earlier computer was a Cateye Mity8, and I measured the wheels by "chalk on the floor" method and found I could only input whole cms - 209cm.

I've checked again this morning, and found it to be the same as before as somewhere between 208 and 209cms. This agrees with Garmin's instruction manual.

Now I have the S/C unit, the Garmin calculates the circumference of the wheel by the spoke magnet rpm and GPS measured distance. Very accurate you would think.

My Garmin 305 now says I have a wheel circumference of 2061mm. Patently wrong!

Why is this?

[minkie]

Sorry, I don't know the answer to your problem, but I have another issue with the spoke magnet.
I think I've read that when the GPS signal is lost the spoke magnet is used to calculate speed instead.
I cycled through a long tunnel at the weekend and it reported a maximum speed of 255 mph. I'm pretty sure I wasn't going that fast.

I do like the cadence measurer though.

Maybe there's a real garmin guru out there that can help on both points.

[geocycle]

Now I have the S/C unit, the Garmin calculates the circumference of the wheel by the spoke magnet rpm and GPS measured distance. Very accurate you would think.

My Garmin 305 now says I have a wheel circumference of 2061mm. Patently wrong!

Isn't this due to the accuracy of the GPS receiver? Garmin units are typically accurate to within 15m so the computer will average the distance and by the time it has processed and rounded you could easily lose or gain a couple of cm. The empirical piece of chalk method must be most accurate as it takes into consideration your tyres, inflation etc.

[hubgearfreak]

as far as i can see, you're looking at difference under 1.5%.
do you expect these things to agree more closely than that? i certainly wouldn't

[Mick F]

Thanks guys.

This "problem" doesn't exactly rock the world, but I would think that as GPS is very accurate over a given distance, the wheel magnet information should be continually averaged out and the final figure should be accurate too.

Or is it "not yet"? Maybe over my 30 mile ride yesterday it was still calculating? Or was it only calculating for the first few revs of the wheel? The instruction manual isn't clear on this point.

If it's the latter, I could understand the error, as we live on a steep, high-banked little lane with poor satellite reception. Maybe I shouldn't switch on until I get in the clear after about a quarter of mile.

[NUKe]

I've recently bought a Speed/Cadence unit for my Garmin 305 so I can keep tabs on my roller riding and also see my cadence on the road. I works well but I'm a trifle confused about the readings.

The Garmin instruction manual states that my size of tyre - 700c 20mm - has a circumference of 2086mm - that was the figure I inputted for my bike's profile when I first set up my Garmin before I bought the S/C unit. There's a comprehensive table of wheel/tyre sizes. My earlier computer was a Cateye Mity8, and I measured the wheels by "chalk on the floor" method and found I could only input whole cms - 209cm.

I've checked again this morning, and found it to be the same as before as somewhere between 208 and 209cms. This agrees with Garmin's instruction manual.

Now I have the S/C unit, the Garmin calculates the circumference of the wheel by the spoke magnet rpm and GPS measured distance. Very accurate you would think.

My Garmin 305 now says I have a wheel circumference of 2061mm. Patently wrong!

Why is this?

Mike have you measured the wheel circumference. The difference you point out is minimal. the amount of air in your tyres, changes in air pressure, temperature will give some variation in wheel size, as will the amount of rubber left on the tyre. GPS is not 100% accurate often seems very close, particualarly if your route involves climbs. Sorry but the scientist in me says if you take into account all the variables, then an inccaruacy of 1.5% is very low.

rough calculating there is a 4mm discrepancy in the measured and the claimed radius. tyre wear and pressure more than account for this

If you want to get an accurate measurement of real wheel circumference use the method as you describe with chalk, but Paint a wet line on the tyre. ride the bike through one full revolution leaving 2 marks onthe ground . measure between the lines.

In the interest of science we should repeat at varying tyre pressure and record the results. Other variables we might want to test the effect of heat, we would then need either tyre warmers or to do over a period of time. IN which case we need a way to keep the Wheel size accurate owing to tyre pressure and wear.

[CJ]

The figures of 2086mm and thereabouts are the result of simple calculations and measurements which don't take account of how much the tyre squashes against the road when you actually ride the bike. I'd assume the figure resulting from the GPS recorded distance is more accurate.

Let's look at that figure: 2061mm you say. Divide by PI, subtract the 622mm rim diameter, halve it and we have a height for the loaded tyre of 17mm. Performing the same calculation on your earlier roll out measurement gives a height of 21mm for your nominally 20mm tyre. Seems like it flattens by 4mm when you ride the bike. Sounds plausible.

After some more calculations of areas, pressures and likely loads, 2mm seems more plausible actually. So I think there probably is a small discrepancy between how far your wheel rolls and the distance recorded.

This discrepancy has two likely sources. Firstly, the front wheel weaves about a bit, necessarily to balance the bike, and hence will travel a bit further than you do, more so when you go slowly.

Secondly, the GPS probably calculates distance according to its tracklog. The tracklog is a series of straight lines between trackpoints. A straight line is by definition the shortest distance between two points, so the curve you actually travelled will have been longer.

So a front wheel bike computer (even when perfectly calibrated) slightly over-records how far you've gone whilst a GPS slighty under-records. These two small differences add up in such a way that this GPS will always compute the wheel to be smaller than it is.

Depending on how you've set up your GPS, it'll place a trackpoint at closer or longer intervals, based on distance or time or automatically, which also takes account of curviness. I think that if you change its settings so that it places trackpoints automatically at the highest available frequency, and go for a ride without many corners or slow uphill bits, the thing will compute a slightly bigger wheel circumference.

Of course it will also do that if you pump your tyre up harder.

It's been fun to think about all these factors, but normal variations in tyre pressure and load carried, plus the aforementioned waviness of the front wheel's path, mean that there is no point in trying to calibrate a bike computer better than the nearest circumferential centimetre anyway!

[Mick F]

Great!
Thanks CJ, yet again.

Two points:
The wheel magnet is on the rear wheel, as the S/C unit is a combined bit of kit with two detectors in the one lump. It fits on the LH chainstay with a magnets on both the LH crank and a rear wheel spoke.

The rear tyre on my bike is inflated to in excess of the max as described on the side-wall - (115psi) - I inflate the rear to 120psi. 4mm compression sounds excessive at that pressure.

I now find that I want to do some experimentation:
1. Switch on the Garmin in a clear area, and try again.
2. Set up the Garmin so it records every second - this function is available but the recording will fill up the memory in less than 3 hours. The usual setting is "Smart Recording", this only records when there's a substantial change in direction/speed. This method gives days-worth of recording ability.

After each trial, I'll record the results and delete the previous info, so each trial will be "clean".

How's that?

[CJ]

The wheel magnet is on the rear wheel ...

That is also likely to go further than expected, for a completely different reason than the front wheel.

Reason is the force of propulsion and a small amount of slippage that arises between tyre and road. Given that tyre tread is an elastic material like rubber, this is inevitable. The rubber in contact with the road is distorted by the force of propulsion in such a way that the tread is compressed in front of the contact zone and stretched behind it. As the tread rolls back through this zone it transits from a compressed to a stretched condition. This process can be accommodated only by a certain small amount of slippage. So some of the rubber on a driven tyre is always sliding on the road, even when it seems like the tyre as a whole is not.

It's this movement, after all, which wears out rear tyres a whole lot faster than front ones. And it's going to be greater when conditions call for more propulsive thrust. So you should still try to avoid steep hills (or walk up them) if you want your GPS to provide a more accurate, i.e. bigger idea of your tyre circumference.

[Mick F]

Thanks CJ.

Weather permitting, it's Testing Day tomorrow!

Whatever the result, I'll stick with what Garmin says, after all it's a machine - it must be right! The absolute figure is unimportant, because the Garmin must calculate the way it sees the physical world, not the way we see it.

[Mick F]

I think I've sussed it.

We've had discussions on here about whether GPS devices measure distance traveled or horizontal distance. I always maintained that GPS is a 3 dimensional science, so it must record the actual distance traveled.

I think I'm wrong.

My wheel circumference has been calculated using only horizontal distance with no allowance for the longer distance made by climbing and descending hills. I live in a hilly place, in fact the first 10 minutes of my ride was up Gunnislake Hill - 1 in 10 or so for a mile or so. Consequently, my wheel turned more times coz I went further than the horizontal measurement of GPS.

I hope you can see what I mean from this profile of our walk and cable-car ride in Gibraltar this summer. I took my 305 to track our cruise and recorded it all!
(sad, or what!)



We walked through the town, then up to the top of the Rock, then came down the cable-car. You can see the steep ride down. You can also see the horizontal scale across the bottom in Miles. Note how the scale is linear, ie not compressing for our 45degree descent.

The distance we traveled in Gib was FURTHER than the 7 miles recorded.

[Edwards]

What you say makes sense to me. I thought GPS was done using triangulation of 3 or more fixed satelites. That is how it gets the height.

As a EX RN why check on the cruise ship. If you did that now on a RN ship would it be a breach of security.

[meic]

Lots of good points here. To people who care about perfection

The contradiction in diameters which Mick F has bought to light explains why we have been wrong to calibrate our speedos based on the wheel circumference of an "unloaded" tyre.

I have written on previous posts about the search for the absolutely correct measure of distance.

It also explains why my 26x 1.75 tyres over read distances in comparison to my 28mm tyres.

It also partly explains why when changing from 28mm to 25mm tyres it was not a simple lowering of the circumference figure by the mathematicaly calculated accout. At the time I put it down to the 25 and 28mm measurements being nominal.

As a regular GPS user I can not stress enough that they are very good but imperfect.

If you look at your downloaded track after a ride you will see that it deviates from the road especially if you go fast, turn tightly or go under cover or tackle hills (linked to going fast).

As the GPS builds up its data bank of comparing these distances with the data from the wheel it will build up an error which will through averaging over long distance becomes quite constant.

So I would think the 4mm discrepancy is a mixture of compression and a long term average of GPS error which may possibly be plus or minus and a contribution from the straightening the curve by the GPS software.

Having switched to a new GPS with a better receiver I will have lessened the middle of those factors.
If you ride slower on flatter and straighter roads you will lessen the last factor.

So Mick stick to riding slow on 19mm tyres at 160psi on flat straight roads and buy a GPS 805 HCx++ 2010 model with external antenae.

Or just calibrate your computer on a measured mile.

[meic]

Looking at your trip to Gibralter, I think you may be jumping to conclusions.

Does the GPS have an altimeter?

I can not read the scale on the graph but the vertical scale is amplified hundreds? of times compared to the horizontal.

In my experience of GPS I would say the straight line on your descent down the rock is possibly the GPS filling a line between the last good fix at the top and the first good fix at the bottom.

[meic]

We need to ask someone who has taken a GPS without a fitted altimeter up in a balloon or a skyscraper. Or we could throw our GPSes off a tower. .

When I analyse my routes on the OS mapping it does give me two different figures for distance travelled. Which i believe take into account 2D and 3D distance. The discrepancy from the GPS is much greater.

Using "The square of the hypotenuse equals the sum of the squares."

A fairly difficult 200k Audax with 4,000metres of climbing and therfore 4000m of descent.

Will be a 3D distance of 200kilometres and 160metres. My GPS discrepency is an order of magnitude greater than this.

Edit I have realised the flaw in this is that I have taken the climb as consistent over the whole distance. So 160 metres is the minimum possible difference and 208km is the theoretical maximun 3D distance.

So we need to find if flat landers get the same degree of discrepancy as us mountain folk.
Another test is how does the GPS read if you do the same journey by foot or car?