OT Sat Nav

[dennis]

Why do you think tyres wear more when loaded?

[Robin]

But what about the reduction in the radius accompanying the increase in width as the tyres get "squashed"?

It is true that the static rolling radius is often smaller than the dynamic rolling radius. IIRC from once sharing an office with a petrol head that is taken into account in the ETRTO etc specifications of the dynamic rolling circumference at 60 kph. They even give correction factors for use at higher speeds.

I also asked the guys fitting tyres at the end of the road this morning about variations between manufacturers. They reckon the specifications allow for a manufacturing tolerance of a total of 4% in the circumference. Most no doubt are within a much narrower range already. But a statutory requirement for better speedos than in the Directive wd surely mean tighter tyre specs given the cumulative effect of the tolerance in the initial tyre circumference, tyre wear, load, pressure, speed factor and slip (and whatever else I've forgotten) ..........

[The Natural Philosopher]

That doesn't hold water, because the circumference doesn't change.

Only the tyre slipping on the wheel or the road can make RPM->speed go wrong. And actual changes to the circumference due to wear. Or tyre pressures which can slightly stretch the tyre, though mnot by much these days with circuferential steel belting.

No car I have ever ownned bar Jaguars has ever shown commercial traffic

- physically limited to 56mph - to be doing anything less than 60mph a

10% over read'

I guess in a 150mph plus jag, you don't need to market the vehicle by making the speedo run high :-)

That implies a tyre that is stretched circumferentially. That may be an issue on a drag racer, but not on yer average steel belted radial on a shopping trolley

You can get a fair difference changing to e.g. an M&S tyre. Many cars will in fact take different wheel and tyre sizes. I know my Defender could be fitted with smaller steel wheels and be a few inches lower as a result.

I am sure they didnt adjust the speedo!

[Ret.]

You have responded to me - but I am not arguing that! I am saying that the diameter of the wheel remains the same no matter what weight you put in the car!

[Ret.]

I didn't know that Marks and Sparks made tyres...!

[Robin]

Fair point; my brainfart; sorry.

Well the specifications for pretty ordinary tyres seem to allow a tolerance for "centrifugal" forces but I can't find actual manufacturers' figures.

Yep. I forgot that the tyre guys said winter tyres can easily add 1%.

Yes but what they had in mind was that even tyres with *exactly* the same specification have a tolerance of +1.5%/-2.5% in the rolling circumference.

And the tyre guys have also now pointed out to me that it is (as usual) all set out on Google:

[Ret.]

I can't for the life of me understand why the EU doesn't have the same rules as the States - ie + or - 5%. That would allow for a very accurate speedo to be made, and any manufacturing tolerances that resulted in a slight under read would be catered for via ACPOs prosecution recommendations of 10% + 2 mph before prosecution...

[Ret.]

Which is something I've often commented upon. You have to be seriously speeding (by your speedo) to be booked. I regularly drive past cameras at an indicated 5 mph over the limit and I've never triggered one yet. That's because my true speed at an indicated 35 is actually 32 mph - and below the speed that would trigger a camera.

[ARWadsworth]

By "mine" I assume you mean the short bus that takes you to the care home?

[Ret.]

Although I have heard posters on other usenet groups argue that the speedo in their car is 100% accurate, and they've checked it against a satnav, every car I've ever checked (and I've asked friends and relatives to carry out the check as well), has an over-reading speedo, and they are all remarkably similar in their over-read. With the over-read increasing with speed.

I notice this as well when approaching a speed camera in a line of cars. The line slows down to the speed limit (or the speed limit as shown on their speedo) and my speedo invariably reads precisely 30 or 40, depending upon the limit of course. This tends to confirm that the built-in over-read is virtually identical across the vast majority of EU produced cars.

The ACPO recommendation for the prosecution level is 10% + 2mph which means

77mph on a motorway - although it is extremely unlikely that you would be booked on a motorway for doing 80 or less. The speedo on my last three cars has over-read by 5-6 mph at 80 and so this means that you are certainly safe driving at a speedo indicated 80 mph on motorways because your true speed will be 74 - 75 mph and way below a speed you would be prosecuted for. You are also fairly safe driving at an indicated 85 because your true speed will be 79 - 80 mph - but I agree that you should always check your speedo against a satnav before relying on an over-read.

[The Natural Philosopher]

Don't be silly.

M & S means more than just Marx and Spender. aka the Labour Party.

[ARWadsworth]

There is a big difference between someone causing you inconvenience and someone incorrectly trying to police your actions.

[dennis]

But it doesn't. The diameter gets less, the circumference stays the same. However the tyre doesn't slip around the fixed diameter of the wheel so it must slip at the road surface. This is what wears tyres and it is worse the lower the pressure or the higher the load. It also explains why low pressure in tyres has a higher rolling resistance than higher pressures.

[dennis]

You do know that some forces will do you for a couple of mph over the limit, none of this 10% lark.

[John Williamson]

Yes and no. The overall diameter remains similar, but the effective rolling radius reduces with load.

The difference isn't great, but the hub height reduces as load inceases, with the increased surface area of the tyre in contact with the ground leading to slight scrubbing of the rubber. Older cars had different tyre pressures specified for loaded and empty running to reduce this problem.

It is still advised for commercial vehicles with a small tare weight relative to their gross loaded weight. Not that many people actually

*do* it.

[Mike Barnes]

John Williamson :

Older cars? I've never had a car that didn't.

[Andy Champ]

Time's pretty accurate actually. It's the others that drift off. My hand held has had me on walks at 25000ft before now, and last weekend was taking mile long hops under clear skies.

Andy

[Andy Champ]

Andy

[Andy Champ]

I'm not sure that's it.

You squash the tyre at the bottom, and the rolling radius decreases. The surface of the tyre slows down when it gets to the bottom, and speed up elsewhere - most probably the rest of the tyre gets marginally bigger to maintain the same (nearly) circumference. But it's the radius that matters.

Andy

[The Natural Philosopher]

Originally Matsch und Schnee I think.