Bike speedo v GPS

General cycling advice ( NOT technical ! )
rmurphy195
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Re: Bike speedo v GPS

Postby rmurphy195 » 2 Jul 2019, 7:24pm

PH wrote:Use them both: Claim the highest and round it up.
Well don on your 26 mile ride :wink:


This one I like .....
Brompton, Condor Heritage, creaky joints and thinning white (formerly grey) hair
""You know you're getting old when it's easier to ride a bike than to get on and off it" - quote from observant jogger !

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RickH
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Re: Bike speedo v GPS

Postby RickH » 2 Jul 2019, 11:42pm

Mick F wrote:The speed part of it had a wheel magnet, and providing you had the settings programmed for Auto, the GPS and the wheel magnet conspired to make the speed and distance bang on. The 705 took speed and distance off the magnet primarily, but it was belt and braces to check on it using GPS. Tyre pressure variations, or tyre circumference as it wore, all were catered for. You did nothing. You left it to do it.

I'm reminded of back in the day (1990s) when I had a computer with a cadence sensor &, consequently, rear wheel speed sensor. IIRC there was someone I rode with who had the same computer but with a front sensor (& no cadence). I could set my wheel circumference to the millimetres (& measured using a weighted wheel roll).

I used to find that I always registered a slightly lower mileage. I figured that the rear wheel does actually travel a shorter distance on a ride! Round corners the rear wheel travels a a smaller radius curve. Even on the straight the slight weaving of travel is a series of slight curves one way then the other.

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Mick F
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Re: Bike speedo v GPS

Postby Mick F » 3 Jul 2019, 7:00am

Yes, there's all sorts of issues with rear wheel magnet systems. The tyre squirms, it deforms under power, bumps depress the tyre, and as you say, the corners effect the distance travelled.

On a front wheel, the bike weaves, so the front wheel does more distance than the rear.

Best thing perhaps, it to use two magnet systems monitored by a GPS unit that computes the true distance?
Mick F. Cornwall

Witterings
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Re: Bike speedo v GPS

Postby Witterings » 3 Jul 2019, 8:49am

Mick F wrote:Best thing perhaps, it to use two magnet systems monitored by a GPS unit that computes the true distance?


All that just to tell you if you'd done 35 or 35.19 miles ... does it really matter ???????

Especially as they're all going to have different readings so which are you going to choose at that stage as being correct?

If you're prepared to put your faith in one and as you suggest... "monitored by a GPS unit that computes the true distance" ..... why even bother with the other 2 ?

Sorry I'm not meaning to pick holes but if there's always going to be a slight variance between what you use and whilst I get it if your speedo's out by 3 miles over a 15 mile ride but most of the differences are going to be fairly nominal ... unless somethings really not set up right but that's not what the OP was talking about.

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Mick F
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Re: Bike speedo v GPS

Postby Mick F » 3 Jul 2019, 8:59am

I agree.
It doesn't matter.

Some years ago, I rode up to the A30 and came down the long hill heading west. 12miles of downhill.
I reset my GPS next to a marker post and counted them all the way to the next junction. 200mtr per post = Five per Km = Eight per Mile.

My Garmin was spot on.
I have a thread all about this somewhere on here.

Have an issue with our speedo in our Yaris. It reads too high, though at the top end of legallity. 70mph is reading 65mph for instance, and the real 30mph reads 34mph.
I did the same trial on the A30 with it and the Satnav plus my Garmin Montana .............. and the odometer is bang on. All three agreed.

Does it all matter?
No, not at all, but it interests me greatly. :D
Mick F. Cornwall

Bmblbzzz
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Re: Bike speedo v GPS

Postby Bmblbzzz » 3 Jul 2019, 9:14am

Mick F wrote:Yes, there's all sorts of issues with rear wheel magnet systems. The tyre squirms, it deforms under power, bumps depress the tyre, and as you say, the corners effect the distance travelled.

On a front wheel, the bike weaves, so the front wheel does more distance than the rear.

Best thing perhaps, it to use two magnet systems monitored by a GPS unit that computes the true distance?

So this is why front brake pads wear out before rears - they've done more distance! :lol:

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andrew_s
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Re: Bike speedo v GPS

Postby andrew_s » 3 Jul 2019, 9:53am

NATURAL ANKLING wrote:I think you will find that that's about right in comparison between the two.

Agreed.

In principle...
A speedo will give slightly high distances, as the front wheel weaves left and right as you balance the bike, and takes an outside line compared with the bike as a whole on corners.
A GPS will give slightly low distances, as it uses straight line distances between points takes a second apart, and takes an inside line on corners as the bike banks over (assuming handlebar mounting). Note also that not all of these one-second points get recorded in the track log (unless you've set it to do so), so a distance taken from a GPX will be less than that shown on the screen at the end of the ride.
(one point per second blows the 10,000 trackpoint limit in about 4 hours 20)

When I used to take precision seriously, I measured the first 4 km of my standard route to the start of the club rides using OS digital mapping (1:1250 scale, nominal), and then tweaked the computer calibration until it read 4 km at the point along the route I'd determined from the map.
Note that OS 1:25,000 maps aren't precise enough to do this - roads are standard rather than actual widths, and the map may be deliberately distorted in order to show close together features without overlap.
Last edited by andrew_s on 3 Jul 2019, 10:14am, edited 1 time in total.

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Mick F
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Re: Bike speedo v GPS

Postby Mick F » 3 Jul 2019, 10:11am

............. and maps have a flat earth too.

Go a mile up a straight 25% hill (if one actually exists) and then come back back down again, and you will have done less total distance than the map would have said.

Some Triganometry required here! :wink:
Mick F. Cornwall

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andrew_s
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Re: Bike speedo v GPS

Postby andrew_s » 3 Jul 2019, 10:25am

Mick F wrote:............. and maps have a flat earth too.

Go a mile up a straight 25% hill (if one actually exists) and then come back back down again, and you will have done less total distance than the map would have said.

Some Triganometry required here! :wink:

The wheel distance is more than the map distance, not less.

For 25%, 4 along and one up gives a slope distance of sqrt(17) = 4.12, so a bike computer would read 3% more than a map measurement.
Pythagoras is simpler than trigonometry.

Similarly, 10% has a slope distance of sqrt(101), or 0.5% difference between slope and map.

Because of the small proportion of any normal ride that consists of such steep hills (eithet up or down), it's generally not worth bothering about.

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Mick F
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Re: Bike speedo v GPS

Postby Mick F » 3 Jul 2019, 10:40am

andrew_s wrote:
Mick F wrote:............. and maps have a flat earth too.
Go a mile up a straight 25% hill (if one actually exists) and then come back back down again, and you will have done less total distance than the map would have said.

Some Triganometry required here! :wink:

The wheel distance is more than the map distance, not less.
Yes, of course. Sorry, my mistake. I was typing faster than my brain was thinking. :lol:

I was referring to maps and using them as distance calibrators. Maps have shorter distances than reality when in hilly areas.
Mick F. Cornwall

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NATURAL ANKLING
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Re: Bike speedo v GPS

Postby NATURAL ANKLING » 3 Jul 2019, 10:46am

Hi,
Mick F wrote:............. and maps have a flat earth too.

Go a mile up a straight 25% hill (if one actually exists) and then come back back down again, and you will have done less total distance than the map would have said.

Some Triganometry required here! :wink:

Or Pythagoras, no need for tables :wink:
If You Don't Try You Don't Do.....Don't Do You Don't Get...I'm Still Trying....Well Very..
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simonhill
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Re: Bike speedo v GPS

Postby simonhill » 3 Jul 2019, 2:34pm

There have been plenty of explanations to explain the difference even if the devices are perfectly matched. Good stuff.

However, have you thought about doing a simple straight comparison? When setting up my speedo some years ago, I chose a long straight road, about 1km long. Then set GPS and speedo to 0 and rode it. Did this a few times until I got the speedo to match the GPS (which I trusted more).

One other thing, I saw a couple of public road measured kilometre in New Zealand. I was never quite sure what or who they were for, just a bit of straight road marked with a start and finish. Has anyone seen one of these (presumably measured mile) in the UK?

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Mick F
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Re: Bike speedo v GPS

Postby Mick F » 3 Jul 2019, 3:12pm

Yes, though not recently and nothing "official".

Story from the 1970s. :oops:
Me and my mate and our wives, went camping. We lived in Scotland, both of us in the RN, and we lived nearby in married quarters.
Off we went up into the highlands. He and his wife in his Ford Cortina, and me and my wife in our Hillman Hunter.

Over the few days we were away, we did the same distance on the same route, one behind the other.
It just so happened, that one of us mentioned the mileage we'd done and the mpg figures and we compared our results.
Can't remember the mpg, but his mileage was maybe 25% less than mine. It transpires that his Cortina had a Corsair back axle and differential. Same fitting and size, but a different final drive. His speedo would have been reading very very low too. Illegal?

I took a taxi some weeks later to the station, and asked the driver how they calibrate their taxi meters. I knew where their HQ in Inverkeithing was, and he explained that if you parked next to the first telegraph pole on the left as you exit their yard, it's exactly a mile down the road, turn left, to the farm gate on your left!

I tried it in my Hunter and was a matter of a few yards out. My mate tried it, and went WAY past the gate and much further along the road before he registered a mile. :lol: :lol: If I were to speak to him all these years later, he'd remember the holiday and the mileage issues.

Therefore, there could be routes up and down the country where taxis etc check their meters even in this day and age.
Mick F. Cornwall

rmurphy195
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Re: Bike speedo v GPS

Postby rmurphy195 » 3 Jul 2019, 4:31pm

simonhill wrote:There have been plenty of explanations to explain the difference even if the devices are perfectly matched. Good stuff.

However, have you thought about doing a simple straight comparison? When setting up my speedo some years ago, I chose a long straight road, about 1km long. Then set GPS and speedo to 0 and rode it. Did this a few times until I got the speedo to match the GPS (which I trusted more).

One other thing, I saw a couple of public road measured kilometre in New Zealand. I was never quite sure what or who they were for, just a bit of straight road marked with a start and finish. Has anyone seen one of these (presumably measured mile) in the UK?


They are - or used to be -more common than you might think, Gives reference points 1km apart for speed cameras on police helicopters at one time, or at least so I heard!
Brompton, Condor Heritage, creaky joints and thinning white (formerly grey) hair
""You know you're getting old when it's easier to ride a bike than to get on and off it" - quote from observant jogger !

Bmblbzzz
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Re: Bike speedo v GPS

Postby Bmblbzzz » 3 Jul 2019, 4:34pm

Not much use for cyclists, but poles on motorway verges.