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Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
Posted: Wed Jan 16, 2008 8:26 pm Post subject:
Tgold
Of course my reasons are suspect but no more suspect than those given for the tyre size and deformation.
I do appreciate that the geoid is not spherical, but over the distances that we are talking about (very small compared with a long way), the earth might as well be flat (let's start a society. Damn, it's already been done) as long as it is horizontal so that the height change does not come into the calculation. But we seem to be tending back to the bits of string again.
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Wed Jan 16, 2008 9:36 pm Post subject:
Robin2 wrote:
Gnomeface wrote:
..... So, something has to give!
In reality the tyre bead may be forced to slip round the wheel rim and the tyre's tread may be forced to skid somewhat on the road surface, depending on which has the greater grip. The conflicting forces are huge which explains why flat tyres overheat and destroy themselves very quickly.
and also explains why driving a 4x4 with different tread depth on either side will quickly wreck the transmission
Sorry folks, but pretty much all vehicles have a differential system on the driven axles which compensates for the difference in wheel speeds.
Tyres DO NOT slip in relation to the rim, if they did then the tyre would go out of balance within a few miles and the car would vibrate at speed.
Also, the tyre does not slip in relation to the road if one wheel is smaller than the other either. If this was so then the forces involved would be so great that the car would pull violently to the left or right.
The reason flat tyres overheat is that the carcass of the tyre is being flexed around so much that it generates far too much heat for the tyre to dissipate.
Furthermore, a 4x4 will not wreck it's transmission if the tyres are of different sizes either. Once again, if this was true then it would cause the vehicle to pull violently to the left or right.
The differential compensates....
What WILL damage the transmission of some (primitive) 4x4 vehicles is driving them on the road in 4 wheel drive mode when the road isn't slippery. This is because older 4x4 systems didn't have a centre differential and the front wheels would go slightly slower or faster than the rear wheels. This would put a lot of stress on the transmission and wear it out pretty quickly.
Modern 4x4 systems and vehicles with "full time 4 wheel drive" have a third centre differential which can split the power between the front and rear as required. _________________ Gone fishing!
Joined: Nov 30, 2007 Posts: 649 Location: Doncaster
Posted: Wed Jan 16, 2008 10:14 pm Post subject:
I like the way you worded that Skippy, top notch. Reminded me of when i was at Land Rover a while ago and was trying to explain to a gentleman why he was needing his 3rd Transfer Box. Well done. _________________ NEW Tomtom GO 500 (hope it is good)
iphone (running all day, doesn't like it)
XL LIVE IQ Routes (Got stolen and miss it)
GO 510 (in drawer now, time for upgrade)
Hey, don't quote me out of context - which was talking about one wheel with a flat tyre, and whether the wheel's rotational speed depended upon the circumference of the tyre or the rolling radius of the flat tyre+wheel. Nothing whatsoever to do with diffs. Did you understand anything of the concepts behind what I was saying? I gave you real reasons for the internal stresses that destroy tyres that are running flat.
Joined: Mar 09, 2006 Posts: 194 Location: Barnsley, South Yorkshire.
Posted: Thu Jan 17, 2008 9:15 am Post subject:
Lets see if I can explain my perspective on the 'flat tyre circumference'...
When there is less air in the tyre it sits closer to the ground, the radius from the the centre of the wheel to the ground is reduced.
The 'slack' from this is not pushed out to make the radius larger at other points (e.g. a tank track), but it is pushed out sideways.
If you were to look from the front of the car you would see a pear shaped tyre.
So, as the wheel rotates the radius that is being used is always smaller and the virtual circumference is smaller.
In conclusion; the wheel will travel a shorter distance per revolution and the speedo will read faster.
Joined: Aug 22, 2007 Posts: 211 Location: Peterborough
Posted: Thu Jan 17, 2008 11:36 am Post subject:
nivek22 wrote:
The 'slack' from this is not pushed out to make the radius larger at other points (e.g. a tank track), but it is pushed out sideways.
If you were to look from the front of the car you would see a pear shaped tyre.
All you are seeing is the tyre wall pushed out becasue the support from air pressure is lost. Circumference stays the same.
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
Posted: Thu Jan 17, 2008 12:36 pm Post subject:
Snudge wrote:
All you are seeing is the tyre wall pushed out becasue the support from air pressure is lost. Circumference stays the same.
I'm with nivek22 on this one and here is the absolute proof:
Archimedes states that "When a circle's diameter is 1, its circumference is pi".
Therefore, if you reduce the diameter of the wheel (ie, by deflating a pneumatic tyre), then the circumference will reduce. And you cannae change the laws of physics captain...
Now if you measured the diameter of a deflated tyre while it was on a car then it would be close to a fully inflated one and not the same as 2 x radius x pi because the tyre is only flat on the bottom and therefore the tyre is not round.
However, the effective rolling radius of the wheel is a circle and this is determined by the part of the tyre which contacts the road and this radius is reduced because of the deflated tyre.
Therefore, a deflated tyre has a lower rolling circumference than a fully inflated tyre therefore it will rotate more times per mile than a fully inflated one.
It was suggested that if this was a driven wheel then the torque of the engine would quickly destroy the smaller wheel trying to rotate it at the same speed as the other driven wheel. However, this is not true because car transmissions have a differential which allows the driven wheels to be driven at different speeds.
I hope that is all correct and makes sense. _________________ Gone fishing!
Joined: Dec 06, 2007 Posts: 30 Location: Brighton, MA, USA
Posted: Thu Jan 17, 2008 4:16 pm Post subject:
Skippy wrote:
Therefore, if you reduce the diameter of the wheel (ie, by deflating a pneumatic tyre), then the circumference will reduce.
That statement is true, but only if the diameter is reduced all the way around. If you have a flat the diameter has only been reduced in the vertical axis. The question you have not answered is whether the diameter increases in the horizontal axis to compensate for the distortion in the vertical axis. If it does, then your statement is meaningless.
After trawling through all the posts in this thread its obvious that some people are doing a wind up job or don't understand at all.
I've done a fair bit of car modification in my time and changing wheel/tyres was always an issue.
Rolling radius (perpendicular distance between the axle centre and point of contact on the road) is and will always be the starting point for calculating the gearing (or electronic adjustment) for the speedo to show the 'correct' speed.
Using the number of wheel rotations to indicate speed will always be suceptible to inaccuracies. A change in the rolling radius by what ever means (under/over inflation, change of wheel/tyre) will introduce an error in the indicated speed however small or large.
Type slippage on the rim is probably unheard of in almost any modern road car. Start trying to transfer large amounts of torque to the road and it might be an issue and thats when tyres are bolted to the rim (dragsters).
Under and over inflation is far more of a concern to tyre wear and reduced performance. Under and over inflated tyres will have less contact with the road. Under inflated tyres (and we're talking widish, low aspect here) will wear out the outer edges as the centre of the tyre tends to bow up. Over inflated tyres will wear out in the centre. In most cases this can be so bad that the type becomes unusable even though some parts of the tyre are OK.
Bottom line is, as others have said, TT indication is more likley to be an accurate indication of your true speed (notice I didn't say correct).
Joined: Jan 14, 2005 Posts: 19638 Location: Blackpool , Lancs
Posted: Thu Jan 17, 2008 5:45 pm Post subject:
Gnomeface wrote:
And it can't tell you if you've got a puncture.
As standard the TomTom will not do this, but it is possible to display fuel tank contents on the device, so it wouldn't take too much to display each tyre pressure, you fit absolute pressure transducers to the inside of the wheel and they bluetooth the data out as serial information (such transducers are already available for sale). If you collect that data and put it into the correct format you should be able to get the TomTom to read it from the serial port (RDS-TMC connector) it would also require some bespoke software, but its certainly possible.
Oh and by the way I have no intention of producing a summary of this thread, but it is entertaining like the last one - Mike
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
Posted: Thu Jan 17, 2008 10:32 pm Post subject:
Mike, You don't need all these fancy pressure devices and bluetooth. The TomTom will always give you the correct speed regardless of tyre pressure, rolling resistance, rolling radius or any of that baloney.
If anyone expects me to believe that a wheel running on its rim due to a flat tyre will travel the same distance per revolution as the same wheel with a fully inflated tyre, they will also probably aexpect me to believe that they can fly to the moon without the help of our American friends or elastic bands, the earth is flat and that there is no such thing as gravity, the earth sucks.
Well, now that you come to mention it, there isn't. It's just our 3-dimensional interpretation of the natural curvature of space, in a higher dimension, induced by the proximity of matter.
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but no more suspect than those given for the tyre size and deformation. 
