[Bilgemaster]

I checked my speedo readings once or twice against that shown on my GPS when she was still wearing her well-worn OEM Avon "Skidmaster IIs", but just haven't bothered with it since switching to the fatter Dunlops a couple years back.

At the time I was fairly surprised at how accurate the speedo was, within a mile per hour or two of the GPS depending on speed, given the general usage on this Forum of the parenthetical "(indicated)" whenever referring to speeds. Maybe I should break out my little magnetic tank bag with its see-through pocket for the phone/GPS, which normally sits in my pocket, just to see how she's doing with my fairly fresh Dunlops. Fact is though, apart from being used as a sort of notional tachometer along with the overall "feel" of things to seek out those "sweet spots" where she's running happiest, I don't think I'm in any danger of getting snagged at speed traps. Even my old Norton bitsa "Desert Sled", capable of real mayhem, has NEVER had a working speedo--just one for show for inspections. I guess I just don't feel that "need...the need for speed!"

[Stanley]

My old eyes can't read my phone in daylight. My municipality considerately installs radar speed displays near schools to aid RE owners in checking our speedometer error.   

[tooseevee]

My old eyes can't read my phone in daylight. My municipality considerately installs radar speed displays near schools to aid RE owners in checking our speedometer error.   

            How do you know THEY are correct?  [/img]

[Stanley]

I've also checked against my Sigma speedo on four different bikes. It's withi 1mph.

[AzCal Retred]

Well - what about tire wear? That bad boy progressively shrinks a bit as it rolls farther & farther along grinding it's way thru the asphalts of the world. 

My last experience with bicycle computers was that you need to figure out & input the multiplier factor for the wheels you have, so it's all fairly approximate anyway. The real test is finding out the secret speed limit of the day the guy running the radar gun is looking for...  - ACR -

[Keef Sparrow]

I believe the semi-knobbly 'off road' tyres fitted new the the current Bullet Trials are slightly taller due to their increased tread depth than the standard road tyres fitted to other Bullets and Classic 500s - so this should increase the gearing slightly, and also increase speedo error.

[tooseevee]

My last experience with bicycle computers was that you need to figure out & input the multiplier factor for the wheels you have, so it's all fairly approximate anyway.

       To get it as good as possible I just marked a plumb line down from the center of the axle, roll it forward THREE tire revolutions, make another plumb mark. Measure the distance. Divide by three. Enter that number in millimeters. It's all very simple.

        Truth be known, I'm perfectly happy with what the archaic cable driven speedo tells me and the little digital one still looks out of place to me. 

[Stanley]

I use that same method, and the Sigma outperforms the stock clocks on 2 Ducatis, a BMW, Kawasaki Ninja and Bullet.
It even worked on my bicycles and lathe.

[cyrusb]

Well, regarding your first question:

I would say it depends on what you mean:

If you ask, whether the radius from the axle to the ground is changing with internal pressure - the answer is obviously yes, this measure is growing with increasing pressure from 0 bar to nominal or an even higher pressure.

This is by the way how modern Tire Pressure Monitoring Systems are detecting with the inverse method an air loosing wheel:
Revolution of each wheel is compared among all wheels, and if one has higher revolution it is losing air.

As you can see, the difference is significant...


Regards
Toni

Yeah but, Whats up with this rev counting thing? I was under the impression TPM's actually measure the pressure with a load cell or whatever.

[Toni59]

As far as I know both possibilities are used nowadays:
The direct pressure measuring and the rotation counter.

On cars, usually were you have to tell the control system when the air pressure in the wheels is correct, as in my Volkswagen for example, they use the rotation counting method. Advantage: no separate device needed on the wheels, when you change from summer to winter tires, just teach in new „pressure ok now“ after mounting and setting air pressure to correct value.

Regards
Toni

[cyrusb]

As far as I know both possibilities are used nowadays:
The direct pressure measuring and the rotation counter.

On cars, usually were you have to tell the control system when the air pressure in the wheels is correct, as in my Volkswagen for example, they use the rotation counting method. Advantage: no separate device needed on the wheels, when you change from summer to winter tires, just teach in new „pressure ok now“ after mounting and setting air pressure to correct value.

Regards
Toni

Advantages and disadvantages for both systems. The direct system aluminum valve stems on my MB rotted out and leaked air! But it could sense a low tire at 0 mph. Thanks for the info Cy

[Toni59]

You are right:
Advantages and disadvantages on both systems.

And there are also a lot of error influences for both variants.

Some devices have a fairly good resolution:
I remember a situation in a former Audi, where the pressure sensors were so accurate, that you could see on the display the higher pressure on the left side, because  this was the sunny side.

Or an increase in pressure by several tenths after a sharp freeway journey.

„ But it could sense a low tire at 0 mph.“

But this is a rather uncritical situation - you probably also recognize a flat tire at zero mph ;-)



Always enough air in your tires :-)
Toni

[cyrusb]

„ But it could sense a low tire at 0 mph.“

But this is a rather uncritical situation - you probably also recognize a flat tire at zero mph ;-)



Always enough air in your tires :-)
Toni

Not if they are run flats.    It would tend to be difficult to detect even a five pound difference in conventional tires by just casting an eye over them. Particularly low profile skins. Lots to factor in, angle, on hill, etc. "Recognize LOW tire "is what I stated.

[Keef Sparrow]

I remember a situation in a former Audi, where the pressure sensors were so accurate, that you could see on the display the higher pressure on the left side, because  this was the sunny side.

My Vauxhall can do that.