Heres a head scratcher for Mr Bornman or......

[porky]

..........anyone else.

 

When a tyre is new its firm and reasonably difficult to put onto the rim (clinchers we are talking here and road tyres) - now lets say a tyre could give you under perfect circumstances 4000 kms riding before the tread was finsihed (just an example - lets please not get into a distance argument) - I contend that the tyre is kaput long before the tread is finished.

 

I base my reasoning on the fact that a tyre becomes less firm the more its used, the wall and bead become "pliable",(for want of a better word) - if you take it off and put it back on say 5 / 6 times in say 2000 kms you will soon realise that it just "falls" onto the rim, and is no longer an effort to install, my experience is that at this stage, you get lots of "snake bite" punctures, because the tyre is too "floppy" and the bead is not held firmly onto the rim, even when your tyre pressure is correct.

 

So I am of the belief that tread is not important to puncture resistance or tyre life, bead and sidewall strength is MORE important, as it reaches the fail point long before the tread is worn down and one can seldom use a tyre to maximum tread capacity without suffering a huge number of "snake bites".

 

So whats the consensus Mr Bornman?? or anyone else.?

 

porky2009-10-30 00:10:27

[Andyr249]

Very interesting and good question, especially for a Friday morning! But is the bead not made of wire? Wire should last a long time so it is possibly more of a sidewall issue? Having said that I replaced a set of tyres awhile ago where the rubber had in some places, actually worn through so that the wire was exposed. Also before the tread had worn completely so I think you may have a good point!

[Giant]

The other spanner to thow into the works is that more pliable tyres have a lower rolling resistance, so when is the point reached that the increased risk of punturing outweighs the lower rolling resistance

[Flowta]

 

Very interesting and good question' date=' especially for a Friday morning! But is the bead not made of wire? Wire should last a long time so it is possibly more of a sidewall issue? Having said that I replaced a set of tyres awhile ago where the rubber had in some places, actually worn through so that the wire was exposed. Also before the tread had worn completely so I think you may have a good point![/quote']

 

Not all beads are made of wire, folding tyres use a kevlar or some other material for the beading

 

[JasonW]

i'm not sure that it really makes a difference as both an old and new tyre are still inflated to 120 psi and it is this pressure rather than the stiffness of the sidewall that gives the tyre its rigidity.

 

I do however suspect that an older tyre may perform differently under heavy cornering.

[Giant]

LA has his tubbies stored for several years in some old geasers cellar in france to let them mature (soften) als most pro's use cotton casing tubbies for their pliability. side wall pliability does affect rolling resisatance, even in high pressure tubbies.  (apparently, can't say that I've felt the difference on tar on mtb tyres at 1.8 bar i believe)

[GoLefty!!]

There a few things to consider:

 

1) rubber aging

2) surface texture

3) wearing of the rubber tread therefore a thinner cross section

 

 

1) A tyre can become more pliable as the rubber ages.

after moulding the rubber casing is pretty uniform with the thread reinforcing. we call this homogeneous.

After months of use, or simply standing around in the sun doing nothing, the rubber starts to harden and embrittles.

When the tyre i snow used the rubber starts to seperate from the thread plies allowing a little more movement between the two. This has the effect of making the tyre feel more pliable  or conforming.

 

 

2) also as the tyre wears the rubber gains a greater surface roughness and therefore a larger surface area over which it can stretch. Imagine a landscape with rolling hills and a flat desert plain. You are a giant with unlimited power.

If you wanted to stretch out the rolling hills you could pull it from either end and it will flatten out. However the flat desert landscape is already flat so know you have to stretch it if you want to move it. Same with a new tyre and a used tyre. if you could zoom into the surface texture of a rough used tyre it would look like a landscape will rolling hills whereas a new tyre would look more like the desert landscape.

 

3) as the rubber tread wears down the tyre cross section becomes thinner making it more supple.

 

[porky]

Go Lefty, I am with you on points 1 + 3 however point 2 is a bit beyond me.?

 

Actually, point three makes most sense to me, as the tread is worn away, the supporting structure weakens and becomes more pliable.?

 

However, for the sake of been mischievious, I could then argue, "why buy an expensive tyre"?

If the structure weakens as the tread wears (cut resistance aside) the technology in an expensive tyre is never fully utilised (or amortised) because you will toss the tyre when its still got 50% or more "tread" lifespan.

 

So, would it not make sense to just buy three cheap 150 buck tyres and get a longer "ride" duration by tossing them at say 1000km intervals (3000kms total) than buying one 500 buck expensive tyre and tossing it at the same 1000km point due to the weakening structure which happens in ALL tyres unavoidably?

 

The milages again are just for example, not factual.

 

[Lotus]

I believe that bicycle tyres do leverage the technology that goes into them. I think they all have certain characteristics that make them suitable for a particular purpose. Some do have great rolling resistance, while others have puncture resistance or wet grip advantages. The ones that combine a number of characteristics at a higher level of capability than the average tyre will earn a premium selling price. You need to decide if those features matter to you.

 

I've been riding for a long time on Continental Gator Skins, as I liked the puncture resistance they offered (and believe me, in that department they are tough!). I had a pair of Continental GP4000s in the garage for ages, thinking I'd keep them for races. Recently the Gator Skins wore out so I put the 4000s on - no sense in wasting money in these tough times. Totally different tyre - quiet, positive steering response AND puncture resistant (well at least until last Sunday). The thing is, I can see why these tyres are expensive compared to the Gator Skins.

 

I don't think the tyre degrades before it's worn (unless you leave it in the sun all day). These are no longer old-fashioned rubber, but highly specialised polymer compounds. The structure of the tyre includes reinforcing like nylon or Kevlar, and this (as well as tyre pressure) determines side wall strength.

 

For me, when it develops a ridge it's time to go. I tried to stretch that once, and I got dumped on a wet corner. Never again!

 

 

 

 

[porky]

I believe that bicycle tyres do leverage the technology that goes into them. I think they all have certain characteristics that make them suitable for a particular purpose. Some do have great rolling resistance' date=' while others have puncture resistance or wet grip advantages. The ones that combine a number of characteristics at a higher level of capability than the average tyre will earn a premium selling price. You need to decide if those features matter to you.

I've been riding for a long time on Continental Gator Skins,

 

SNIP


[/quote']

 

Darn, you had me right up to that bit about Gator Skins.....................!

 

 

porky2009-11-01 04:56:49

[Turbobok]

Some interesting ideas. I just wanted to add my thoughts - and I'm basing this also on experience with MTB, car and motorcycle tyres.

 

 

 

Firstly, tyres do have a life span, in terms of age of the rubber. This life span is a result of a number of things ? oxidation by ozone and oxygen in the atmosphere (which affects the rubber to steel bond of the bead), ingress of water weakening the rubber to steel bond, and ongoing vulcanistion of the rubber by sunlight, heat and use of the tyre (compression and decompression with every rotation) which leads to embrittlement of the rubber.

 

 

 

However, these mechanisms of aging are slow, and with regular use tread depth is by far the easiest and most accurate method for gauging a tyre?s age. However, if the tyres are older, even in ideal conditions, this is a lot more complicated. I firmly believe though that there is very little to worry about if a tyre is used regularly, and generally only requires replacement when there is no longer any tread depth remaining.

 

 

 

A slight aside, I use Kenda Nevegals on my MTB, and from the very first to last time I mounted them, they remained a pain in the backside. I am not sure about how time and oxidation affects the Kevlar to rubber bond in the bead; perhaps someone else has some ideas?

 

Turbobok2009-11-01 06:00:53

[Lotus]

 

 

[

Darn' date=' you had me right up to that bit about Gator Skins.....................!

 

 

[/quote]

 

You're not a fan?...

 

@ Turbobok

Agree with you there - if you're a regular rider the tyre will probably wear out before the compound and structure degrade. The thing that poses the greatest risk to the tyre's integrity is physical damage - sidewalls cuts and impacts.

Lotus2009-11-01 07:10:54

[Johan Bornman]

cut cut cut cut

 

I do however suspect that an older tyre may perform differently under heavy cornering.

 

I am not so sure. Your statement was suitably vague to argue several ways after it was challenged but let me try anyway.

 

Theoretically two things can happen with age:

 

1) The rubber compound can change and its grip can be altered.

 

2) The tyre can become worn and its profile changed in a way that alters handling.

 

Neither of these scenarios are real.   Severe age could of course see the rubber outgas to a point where it is just crumbs but I suspect that is not what was meant.  A change in shape, thanks to the tyre's thin tread when new, is so insignificant that it doesn't impact handling.

 

Further, a bicycle tyre doesn't respond to cornering like a car or motorcycle tyre. Break-away is sudden, precipitous and without warning. It is always irrecoverable. Therefore there is no way of judging that a tyre has less or more grip than when it was younger.

 

 

 

Johan Bornman2009-11-01 13:30:29

[Johan Bornman]

Porky, I don't agree with your observations. I shall explain why in the text itself.

 

 

cut cut cut cut

 

the fact that a tyre becomes less firm the more its used' date=' the wall and bead become "pliable",(for want of a better word) - if you take it off and put it back on say 5 / 6 times in say 2000 kms you will soon realise that it just "falls" onto the rim, and is no longer an effort to install, my experience is that at this stage, you get lots of "snake bite" punctures, because the tyre is too "floppy" and the bead is not held firmly onto the rim, even when your tyre pressure is correct.

 

[/quote']

 

We know the bead firmly seats on the rim at all times. If not, even for a second, the tube will creep out and burst outside the tyre. It doesn't do that so we know the bead never unseats and is always held firmly onto the rim.

 

Snake bike punctures have nothing to do with the seating of the bead. It has to do with the rim sidewalls cutting the tube through the tyre - like one of those cruel castration devices that cuts the testicle's cords but leave the scrotum intact.

 

 

 

 

So I am of the belief that tread is not important to puncture resistance or tyre life' date=' bead and sidewall strength is MORE important, as it reaches the fail point long before the tread is worn down and one can seldom use a tyre to maximum tread capacity without suffering a huge number of "snake bites".

 

[/quote']

 

You have to distinguish between the two types of punctures namely penetrative and non-penetrative (snakebikes). The firmer is an absolute inverse function of thread thickness. Sidewall thickness, rim shape, tyre pressure etc have no effect on a piece of glass cutting through the tread.

 

 

The latter is a function of rim profile, tyre sidewall thickness, tube thickness and tyre pressure.

 

 

 

Just as an aside. The fact that a tyre becomes easier to mount after a few fitments has nothing to do with the bead stretching. It doesn't stretch. However, the rubber that covers the bead gets permanently squashed from prolonged pressure whist seated and even a small compression will make the tyre's circumference appear a lot larger when fitting it.

 

 

[Johan Bornman]

LA has his tubbies stored for several years in some old geasers cellar in france to let them mature (soften) als most pro's use cotton casing tubbies for their pliability. side wall pliability does affect rolling resisatance' date=' even in high pressure tubbies.  (apparently, can't say that I've felt the difference on tar on mtb tyres at 1.8 bar i believe)[/quote']

 

Not this old myth again. Tyres are not wine. Ageing doesn nothing good to them. The story used to go that the silk thread in the tyre casing got better with age. Now it is cotton. Tomorrow it will be organic cotton, then linen, then flax, then hemp then musk ox wool.

 

I doubt Lance is doff enough to still believe this nonsense. But then again, there is no-one as superstitious a a professional athlete. St Christopers, golden crosses, lucky sock, magic charms and salt over the shoulder can all be observed in the elite peloton.

 

 

 

 

 

 

[GoLefty!!]

Go Lefty' date=' I am with you on points 1 + 3 however point 2 is a bit beyond me.?

 

Actually, point three makes most sense to me, as the tread is worn away, the supporting structure weakens and becomes more pliable.?

 

However, for the sake of been mischievious, I could then argue, "why buy an expensive tyre"?

If the structure weakens as the tread wears (cut resistance aside) the technology in an expensive tyre is never fully utilised (or amortised) because you will toss the tyre when its still got 50% or more "tread" lifespan.

 

So, would it not make sense to just buy three cheap 150 buck tyres and get a longer "ride" duration by tossing them at say 1000km intervals (3000kms total) than buying one 500 buck expensive tyre and tossing it at the same 1000km point due to the weakening structure which happens in ALL tyres unavoidably?

 

The milages again are just for example, not factual.

 

[/quote']

 

 

Point number is  little difficult to visualise so I'll try another way.

 

If you look at your knuckles or any other joint on your body for that matter, you will notice the skin is folded and pretty loose in those areas. The texture is rough so that when you bend the joint the skin is able to move with it.

 

Now head down to Sandton mall and sit at one of the coffee shops and await the arrival of the often spotted SAndton Kugal. Notice how toight the skin over her boobs is. Also notive how toight the skin around her eyes and chin is.

Thats because the skin has been pulled back to smooth it out. Notice also how hard it is for Sandton Kugal to move her lips. this has numerous advantages for her husband or the pool boy but I digress...

 

Now link that to the bicycle tyres casing. If the rubber is smooth and tight it is harder to bend than if it was rough. Call it surface tension for want of a better description that does not involve a 20 page science thesis.

 

A new tyre will have higher surface tension than a worn one.

A new tyre will still have rubber that is fully vulcanised to the casing

A new tyre has thicker rubber.

 

All 3 add up in a small increments to provide you with a slightly firmer casing than when the the tyre is 2000km old.

 

Tyre pressure aside here is irrelevant here because it can be regarded as a constant