False Speedometer read-outs

dave atkinson wrote:

I've not had a great day, truth be told. But this has cheered me right up.

Which is often what I experience. Perhaps you should come here more often - you professionals write some lovely stuff but some of the comments are utterly inspired.

VTHEJK is  right in what he says

We have two common ways to calculate bicycle speed.  1) Use a relatively simple  electronic device to measure the rotation speed of a wheel of known size,  or 2) launch a series of satellites encircling the globe and do  timed triangulations between three or more of them.

The second method always seems like overkill to me, but there are one or two other benefits that can be obtained.

Regarding accuracy, bicycle computers rely on knowing the precise wheel circumference. I calibrate mine by pumping my tyres up to the standard pressure then riding round the local 10mile TT course. The local TT association have a device that measures the number of wheel revolutions along a set route. They calibrate this device by riding it along a Measured Mile. So the absolute accuracy depends on how well the local surveyors have marked out the measured mile, and how well the rider can ride in a straight line etc.

I find that I ride slower than I should do because I'm thinking about all this rubish instead of concentration to the job in hand.

Steve K wrote:

How does a head or tail wind lead to a false speed reading? 

It'll be those new-fangled airspeed metres that pros use these days. Can't imagine it'll catch on...

Times I've wished I had an airscrew speedometer! Headwinds are just not fair, as proven by the fact that they wheel around on the return leg.

Sriracha wrote:

Times I've wished I had an airscrew speedometer! Headwinds are just not fair, as proven by the fact that they wheel around on the return leg.

Quite, I've heard tell of the fabled "tailwind", 'tis a fireside tale for children and auld wiffies

Captain Badger wrote:

Sriracha wrote:

Times I've wished I had an airscrew speedometer! Headwinds are just not fair, as proven by the fact that they wheel around on the return leg.

Quite, I've heard tell of the fabled "tailwind", 'tis a fireside tale for children and auld wiffies

It's like the afterlife or the state pension

you're promisied that if you are good and do the right things you will be rewarded later. just leads to dissapointment

Elco wrote:

Like for example..if roads are wet ...the wheel wil turn..normal..you'll go faster less resistence between your tubes and the road ..so your momentum wil be higher ? Faster...not spin...faster distance ./ so .. 👨‍🔧🧰

Were you on illegal substances when you wrote this? Like the original post it makes no sense.

The bike wheel rolls along the road surface, the number of turns is exactly related to how far it has travelled. the front wheel does not slip against the road surface. They can be inaacurate if they are set up incorrectly with the wrong diameter used, they could arguably be wrong if tyres are changed or not inflated correctly chaging the diameter.

Air resistance and rolling resitance do not make them less accurate at measuring speed. These things can throw off power calculations, but if you want meaningful power data you need a power meter.

Elco wrote:

Like for example..if roads are wet ...the wheel wil turn..normal..you'll go faster less resistence between your tubes and the road ..so your momentum wil be higher ? Faster...not spin...faster distance ./ so .. 👨‍🔧🧰

The pedals turn: not just the left one but the right one too

Elco wrote:

World record run's happen indoor ... On a track if it's about distance ... Without wind ...on wood ... I'd use strava ..it has gps ..or connection..and it keeps track of training so ...at a desktop you coul'd see data of the training. _after training"

Strava's not infallible.  I hit 109 mph on Titsey Hill a few years ago, according to Strava 

Strava always claims I have gone higher and slower than Komoot, both recording the same ride on my phone. I guess it makes sense, more climbing = less speed, but even so.

Daveyraveygravey wrote:

Elco wrote:

World record run's happen indoor ... On a track if it's about distance ... Without wind ...on wood ... I'd use strava ..it has gps ..or connection..and it keeps track of training so ...at a desktop you coul'd see data of the training. _after training"

 

Strava's not infallible.  I hit 109 mph on Titsey Hill a few years ago, according to Strava 

gps can be a bit glitchy so max speeds are always questionable. Short segment times can also be distorted

Elco wrote:

World record run's happen indoor ...

Yes they do ... but you are failing to factor in that (a) runners feet are not continuously in contact with the floor, and (b) runners do not run with series of objectively identical length strides. 

Weather, wind, road conditions etc. make no difference to the circumference of your wheel, so you will travel the same distance with each revolution regardless, and measuring the number of revolutions per minute will allow calculation of speed. 

If a headwind caused your wheel to shrink (as it does the stride length of a runner) then you would have a point ... but it doesn't. 

Maybe cyclists should measure airspeed and groundspeed, like aircraft?

PRSboy wrote:

Maybe cyclists should measure airspeed and groundspeed, like aircraft?

But what happens when you start drafting someone? or a van?

wycombewheeler wrote:

PRSboy wrote:

Maybe cyclists should measure airspeed and groundspeed, like aircraft?

But what happens when you start drafting someone? or a van?

Your airspeed may drop, but your ground speed should increase!

You're welcome,

Pete, the Pilot

PRSboy wrote:

Maybe cyclists should measure airspeed and groundspeed, like aircraft?

I've switched mine to measure solspeed. It looks much more impressive. And I never have to mess around with autopause.