Does a tyre change its CIRCUMFERENCE when underinflated?

You should have used caps, Huge. For those whose physics are still at kindergarten level.

Ahh, so having a puncture causes the local space-time to become non-Euclidian? How does that work?

If they'll let me know where they are, I could go round (!) and try and pummel it into their heads?

Except there isn't a single distance from the centre of the axle to all parts of the tyre in contact with the road ... the average distance perhaps?

I've found that it triggers the alarm at a much smaller reduction than 25%. Our Honda developed a very slow puncture and when I checked the tyres, one was about 0.2 bar (3 psi) lower than it should have been.

How much does the radius of a bike tyre (at the point of contact) decrease when you sit on the bike. It's difficult to tell when I'm the one sitting on the bike so I don't get a side-on view. I think my speedo actually recommends measuring the (unloaded) circumference by marking a point on the tyre that is in contact with the ground and rolling the wheel along the ground until the point is next in contact. But I agree that *if possible* you should try to measure the radius under load and *assume* that the whole tyre is that radius.

I've heard is suggested that there is *significant* error between a brand new tyre and one with a worn tread, though I'd have thought that it was negligible.

Any sensor has to be able to distinguish between expected change in radius due to tyre wear and unexpected due to loss of pressure.

Oh, FFS. This is really, really simple. Why are you all making it so complicated? (It's the *shortest* distance, obviously. Just draw a bloody diagram.)

That isn't what I said, anyway.

Back to my 'real world' right brainer approach ...

Do you think we fully understand the inner workings of the human brain Robin?

Do you concede that the brain *is* made up of two physically separate hemispheres that are only joined together at one point?

Do you concede that is someone has a stroke, they typically lose different functions, depending on what side it occurs in?

Do you concede that most people are 'handed' and will have a dominant hand / leg / eye [1]?

Do you concede that there are many studies that have proven that certain things *are* either typically permanently centred or even focused in one hemisphere or the other.

Do you understand the concept of 'brain lateralisation / dominance' and what that actually means ITRW?

So, rather than reading (and believing) the opinions of someone who may well be a (closed mind) left brainer in denial , how about reading some *real* medical science on the matter?

"The notion of different hemispheric thinking styles is based on an erroneous premise: each brain hemisphere is specialised and therefore each must function independently with a different thinking style."

Bwhahahahaha! Whoosh! ;-)

Cheers, T i m

With both eyes open and arm outstretched, point with one finger and a small object in the distance.

Holding your arm still, close each eye in turn and note which one leaves the object in alignment with your finger and (therefore) which one doesn't. The one that stays aligned is your 'lead / dominant' eye.

Now, assuming a reasonably balanced quality of vision in both eyes, do you think it's the actual eye that is taking control over the other or that maybe that hemisphere of the brain that is being 'dominant'?

Quiz for you then.

Are our sight functions lateralised?

Are our smell functions lateralised?

Are our hearing functions lateralised?

Are our taste functions lateralised?

And if not, why not?

Because that's how the physics works. And that should be obvious from what happens with wheels with no tyre, with different diameter wheels.

Irrelevant to what determines the rotation rate of the wheel.

And it is very easy to actually measure if the rotation rate of the wheel does in fact vary with the distance between the axle and the road. when you vary the pressure in the tyre. Not clear if any OBD2 system does in fact report the rotation rate of each wheel using the ABS sensor on each wheel, but if they do, that would be very easy to do the experiment with.

Have you noticed any links between the inability to understand this concept and their known stance on Brexit?

I'd say that from my quick poll (and you know how reliable they are ) so far it's 100%. ;-)

I wonder if the reason they don't like the EU is because it's much closer to the edge of the world (we in Great Britain are in the middle of the flat world obviously)?

Maybe there is something to this whole l/r brain dominance thing after all. ;-)

Cheers, T i m

Quite. Not the average radius of the tyre, but the average of the part in contact with the road. Which will, of course squirm around a bit. One reason why an under inflated tyre wears out more quickly.

True, but the easiest way to visualise the rotation rate for a unit distance travelled is to divide the distance travelled by the circumference to give the number of wheel revolutions.

However... it all hinges on what the *effective* circumference is: it is the circumference of an imaginary circle with the same radius as the distance between centre of the axle and road surface, which will be less than the no-load radius because the tyre is slightly flattened where it comes in contact with the road.

I presume this means that if you took a piece of string and passed it round the circumference of a tyre this value would be smaller than the no-load circumference because the part that is in contact with the road will be a flat rather than a curved profile. This assumes that the radius of the rest of the tyre doesn't increase significantly when a load is applied to the tyre - presumably this is constrained by the steel reinforcing belts in the tyre.

I wonder how much smaller the in-contact radius is than the no-load radius: what sort of proportion is the reduction, typically? I'll have to measure the actual distance from the centre of the hubcap to the ground, and then jack the wheel up until it first touches the road and measure again. Note that you can't measure to a part of the car body because of compression of springs :-)

The diagram I linked to yesterday seems suitable

The radius of the orange arc showing effective rolling radius is inbetween the length of OQ and OP, what makes it "obvious" to you that the shortest distance applies?

Discos have been around for a long time.

Because it's wrong. In this context, the relevant dimension is O -> Q. How could it be otherwise? I'm afraid this seems so obvious to me that I cannot find ways to explain it.

I was just suggesting what I understood to be the maximum pressure drop threshold before such a system *must* indicate. It's good to see RW systems are much better than that. ;-)

I think that depends on the type of bike / tyre to some degree (balloon / racing skin etc).

Video from your phone?

Agreed.

You can do it yourself under load. Put the valve at the bottom and pus something on the ground to align with the valve (screwdriver, tape and pen mark etc). Then sit on the bike and walk it in a straight line whilst keeping as much weight off your feet as possible. Stop when you see the valve near the bottom, get off, adjust the wheel more accurately if required (by moving the bike forward or backward slightly) and make the second mark. Measure between said marks.

An extreme of that was replacing non-full profile 12" wheels on our Escort based kitcar with full (80) profile tyres on 14" wheels. I believe the rolling circumference increased by 30%.

(25% *maximum* pre triggering) ... On these iTPMS's I thought I heard mention of 'calibrating' the system to what would be considered 'normal' (tyre pressure, tyre size, wear status) and so the alarm would be a function of that? I know the prescribed pressures in the rears (particularly) of our vehicles can vary quite a bit because of load / speed etc.

Sure, but if it only triggers at say a 10% variation and tyre wear would only ever cause a 1% variation you should be good to go. ;-)

Cheers, T i m

I had a quick search and at least from Disco3 onwards they have a steering angle sensor, I couldn't see anything either way about earlier models.

One revolution of the wheel has to equal one passage of the tire's circumference along the road (since the tire does not slip around the rim and assuming no slip on the road). How could it be otherwise?

Talk of "effective radius" is not relevant.

Although I don't know this for certain, other than having owned several each of the first 4 generations of Disco, I believe the late Disco II's have it too, once they introduced roll stability control.