Tyre temperatures

[TripleS]

I just was looking to see when it is recommended to change over

I keep an eye on the temperature for my normal (morning and evening) journeys and also the maximum daytime temperature. If the peak is below 7C then I reckon I should be on the winter tyres. If I see a temperature above 7C on my normal journeys then I suspect not.

Last year I fitted them at the end of November and took them off well into April, probably a little late for the conditions. This year my initial plan is to use them for the whole of December through February, but I'll extend that period based on the weather forecasts. Currently I'm waiting to see what is expected for next week.

Bear in mind that I live in Berkshire - I would expect that people in the north of the UK might initially plan to use them for a longer period.

I keep an eye on the temperature too. Yesterday Eileen and I had to go up to Whitby to visit our son and help with a spot of work. Gee whizz what a terrible journey!

The outside temperature was only between +5 and +8 C and it was very wet day. Old CUBby was wearing full wets on the front and intermediates on the back, so what with not being able to find a dry line anywhere and get some heat into the tyres, the grip was really terrible: and I couldn't go fast enough to get any aerodynamic grip, so it was all a bit grim. By 'eck, it must have taken me almost a full minute more than usual to do the 22 mile trip.

Best wishes all,
Dave - facetious (or foolish) on a grand scale.

[Russ_H]

I do concur that a better (more cost-effective and practical) choice for many people (including me) would be good All Season tyre.

There is an interesting article on the TyreReviews website which discusses the relative performance of summer, winter and all seasons tyres. It's particularly illuminating how a good all season tyre isn't far behind the performance of the summer tyre in summer conditions or the performance of the winter tyre in winter conditions.

Custom24 has recently driven an Italian car fitted with the all season tyre mentioned in the article ...

A few weeks ago, I bought a pair of Vredestein Quatrac 3 all season tyres, as mentioned in the article. I hadn't seen the article before I decided to buy them. They're not easy to get a hold of, and they were a special order. Being new, I had them fitted to the back wheels. When the current front tyres wear out, I'll get another pair of Quatracs, and have them fitted appropriately.

The only difference I've noticed is that they're a shade noisier than the old tyres.

Last night, the temperature in Newcastle fell to zero centigrade, according to my car thermometer. I'll let you know how the new tyres perform after a bit more cold weather.

[Gareth]

They're not easy to get a hold of [...] When the current front tyres wear out, I'll get another pair

Unless you expect to go all through the winter on your current tyres I'd suggest trying to source the second pair now as availability isn't going to get any easier over the next month or two.

[kfae8959]

The only difference I've noticed is that they're a shade noisier than the old tyres

Interesting: this afternoon I swapped to 14" wheels with Snowtrac 3s. On my short test drive, I was mostly looking out for pull, vibration, and other problems that might tell me I'd misfitted the wheels. On that front, there were no problems, but I did notice that there seems to be less road noise than with the summer ZV3s.

David

[Gareth]

what with not being able to find a dry line anywhere and get some heat into the tyres, the grip was really terrible: and I couldn't go fast enough to get any aerodynamic grip

Very amusing

I was thinking this morning about the relative grip levels between, say, summer and winter tyres, and then between the same (summer) tyre and different amounts of tread.

Tests have shown there is a significant drop off in (wet?) grip when the tread goes below 3mm so some people change their tyres at that point whereas others prefer to let them wear down further. Reasons might be cost or only being willing to conform to the legal requirement, and then at the same time some of those will accept the trade-off of less grip and drive accordingly while others (the majority?) won't even think about it.

It's a bit like this with winter tyres - if you're a person that changes their tyres at 3mm then there is almost no logically consistent basis for not using winter tyres during the colder months, however logical consistency has never been a hallmark of human behaviour. As far as the individual is concerned, doing either behaviour is an article of faith since the benefit of each is not readily apparent in normal driving and in the generally expected range of conditions at different times of the year.

Where I've seen winter tyres make a huge difference is when the road surface is slippery for an extended distance, such as might be the case where there is ice or a sharp frost. They also seem to make a huge difference in snow. All these are fairly obvious conditions but various sources say they are also better when it is cold and wet -- yesterday I saw a video clip demonstrating this with two otherwise identical BMWs.

For my car, I choose summer tyres exclusively on the basis of wet weather grip (across the lifetime of the tyre) so perhaps the point at which my winter tyres would be better than my summer tyres is different to those who select for ultimate warm weather grip or tyre longevity.

[TripleS]

Gareth - there's a lot of merit in what you say. Obviously you do much more travelling than I now need to do, so we make different judgements as to what seems appropriate.

I am aware that wet grip is reduced as the tread depth falls, especially below about 3 mm, but to me the biggest danger is aquaplaning on deeper water, rather than reduced grip on surfaces that are merely wet.

Having covered some large annual mileages across the North Yorkshire moors in very bad conditions in the early 1960s, I soon became accustomed to reading road surfaces quite carefully. This won't always guarantee freedom from trouble, but things could be worse, and I'm reasonably comfortable with my judgements at this time.

Best wishes all,
Dave.

[waremark]

Aquaplaning: apparently there is a precise numeric formula for the speed at which a tyre will aquaplane when it goes through standing water deeper than the tread depth. Key factors causing aquaplaning are speed, tread depth and tyre pressure. For a tyre at 36 psi the speed at which it will aquaplane is 62 mph. Hence maintenance of tyre pressures is critical for avoidance of aquaplaning.

(This is all little understood by me, but was explained to me recently by top class driving coach John Cave).

[TripleS]

Aquaplaning: apparently there is a precise numeric formula for the speed at which a tyre will aquaplane when it goes through standing water deeper than the tread depth. Key factors causing aquaplaning are speed, tread depth and tyre pressure. For a tyre at 36 psi the speed at which it will aquaplane is 62 mph. Hence maintenance of tyre pressures is critical for avoidance of aquaplaning.

(This is all little understood by me, but was explained to me recently by top class driving coach John Cave).

OK, but are we sure that he understands it well enough to give us useful information?

ISTR the IAM (I think it was, and apologies to them if I'm wrong) saying something two or three years ago about being able to calculate the speed at which aquaplaning would take place, and the subject is revived again here; but I'm a bit sceptical about it.

My feeling is that there's probably more to it than the IAM and John Cave are saying. If you can predict the speed at which it will happen, I suspect it would involve more factors than they're considering. For example, there's no mention of tyre width and diameter, or the design of the tread pattern, or the load carried on the tyre. I can't believe it's as easy as they're suggesting.

I doubt if there is much to be gained by trying to calculate this. My money's on experience in reading the road configuration and judging the surface conditions as they change, and being alert to the grip variations as felt; and then working within whatever seems appropriate. I would have more faith in that, rather than a calculated figure that's probably completely wrong.

Best wishes all,
Dave.

[Mr Cholmondeley-Warner]

Dave, note that Mark's statement is "the speed at which a tyre will aquaplane", not "the speed at which you will lose control of the car". Once the water depth is greater than the tread depth, it's just a matter of building up enough momentum to ride up onto the bow wave of water.

This paper by NASA describes some of the research and suggests a formula. You do know who NASA are, right?

[TripleS]

Dave, note that Mark's statement is "the speed at which a tyre will aquaplane", not "the speed at which you will lose control of the car". Once the water depth is greater than the tread depth, it's just a matter of building up enough momentum to ride up onto the bow wave of water.

This paper by NASA describes some of the research and suggests a formula. You do know who NASA are, right?

Yes I know you're talking about the tyre aquaplaning, rather than loss of control by the driver, and yes I know who NASA are. OK, so they've got some clever people and they've gone into a lot of detail.

Even so, I don't think it is sensible for the rest of us to virtually ignore factors like tyre size (diameter and width, and therefore footprint) and the other factors I mentioned, as if they are of no significance.

In real world driving I think I'll stick to my method of deciding how to proceed. I'm certainly not going to be doing (possibly oversimplified) calculations and then using the speedometer to identify a safe maximum speed in hugely varying conditions.

There's already enough questionable wisdom about some aspects of driving, without distracting ourselves further with stuff that might not be as helpful as some may think. Sorry, but I'm still sceptical.

Best wishes all,
Dave.

[Mr Cholmondeley-Warner]

I think we probably both know Mark well enough not to suppose that he sets his cruise control at 61mph because any formula tells him his tyres will aquaplane at 62mph. However, being skeptical about formulae (which by the way included the size of contact patch, and considerations about tread design (the radius of the tyre is likely to have only a small bearing, because of the small depth of water which is necessary to induce aquaplaning, which is thus not affected very much by the tyre radius. Likewise width, which is generally offset by the tread clearing qualities, providing you have some tread on your tyres - effectively if you have enough tread depth to absorb the water, your tyre is like a group of solid tyres in parallel)) which have been tested experimentally, is just flying in the face of evidence. There is some evidence that a formula based principally upon tyre pressures (the other factors are there, but have only a small effect compared with the inflation pressure) can be used to predict the aquaplaning speed, and has been shown to apply in a range of conditions. Being skeptical doesn't change that. I don't think anybody is suggesting that you use the formula to determine your speed, although it might be useful to bear it in mind, while keeping well below the speed it predicts.

[Porker]

For once, I tend toward Triple's feeling here. While I fully accept that one could calculate the likely acquaplaning speed from a set of known values, there's one value which is constantly changing and which is difficult to estimate from the driver's seat, that being the water depth.

Empirically however, I can certainly certify that acquaplaning on a dark motorway (and doubtless a sunlit one) is quite an interesting experience, especially if there's any significant camber on the road.

P.

[Mr Cholmondeley-Warner]

Yes, from the driver's point of view, the water depth is the most significant variable. However the only change that has a bearing is whether or not it's deep enough to prevent the tyres from clearing it, and therefore for the formula to come into play. So, in pseudo-code:

Code: Select all

if (water depth > tyre effectivity threshold) then
    if (speed > predicted aquaplaning speed) then
        aquaplaning occurs

[TripleS]

I think the question here is the extent to which this is an interesting technical/theoretical subject for study, rather than something of real practical use when driving in wet conditions. Quite honestly I don't see it as being of any significant help, but if others do, then that's fine.

At the end of the day the real requirement is to be able to see the road far enough ahead, so that we can reliably evaluate its condition, and adjust our speed and steering inputs to maintain adequate control. When conditions can vary within a very wide range, perhaps even over quite short distances, I think this notion of aquaplaning speed may be a misleading oversimplification.

Best wishes all,
Dave.