Fat vs Thin.... an accidental experiment

[Brucey]

For the last couple of years I have (almost by accident) been using a nominal 35mm rear tyre (actual size ~32mm on the rim) and a 25mm front tyre (actual size ~25mm on the rim) on my most used bike. Other than width, the tyres are ostensibly 'identical' tyres from the same manufacturer and have similar carcass construction, puncture protection and tread thickness, pattern etc.

I'm familiar with this tyre because I've done about 10K miles on them in total, mostly in the 35mm version (front and rear) during which time I've had the least trouble with punctures that I can remember. When the tyres have given trouble it has been daft stuff like rim tapes going bad or just plain old age; a rear tyre failed in the carcass recently (near the rim) and when I looked at it the tread was perished all over. Despite this, the tyre hadn't punctured in the normal way, even though at any one time there seemed to be plenty of small stones lodged in the cracks in the tread. I replaced this tyre with an identical unused one.

Normally I run the 25mm front tyre and the wider rear tyre at about the same pressure. It works pretty well; the fork is nice and springy so it isn't uncomfortable or anything like that. You can see that the rear tyre has a wider footprint on the road than the front, as you might expect bearing in mind the difference in the weight on each wheel. [Luggage-wise I might carry 5kg in a saddlebag on this bike.]

Now normally I'd have said that wider tyres are perhaps a little more likely to puncture, just because they (literally) pick up more crud off the road. But then again there is more to it than that; often the pressures are different, the way the tyre deforms against the road is different, and it is rare that the tyres are genuinely the same construction.

But here's the thing; I've removed several of these variables (as best I can) and the result has been that....

I've had two front wheel punctures this winter, both from small stones. The second of these was yesterday. When I checked the front tyre it was full of tiny stones and was pretty badly cut up (despite occasional 'brushing' by hand in use). By contrast the rear tyre was virtually unmarked and stone-free. Back when I ran a 35 on the front, that didn't seem to get cut up in the same way.

Had both tyres been the same width I might have -in part- put this down to the fact that the front tyre might 'clean the road' before the rear tyre runs over the same stuff but here the rear tyre must see a lot of stuff that the front tyre cannot have rolled over; between the footprint width differences and any slight weaving, the rear tyre must see ~50% or more 'fresh surface'.

So I'm left thinking that

- maybe it is just a coincidence (but the way the front tyre looks suggests otherwise)
- maybe the front and rear tyres are not constructed in the same way after all
- maybe the way the narrower tyre deforms against the road is very significant
- maybe when you run over a small stone the contact pressure on the stone varies with tyre width even at the same tyre pressure
- maybe the rear wheel (seeing traction forces) causes small stones to scrub out of the tread before they get pushed into it too far.

Of the list above I'd have to say that it is quite likely that the rubber compounds are not exactly the same hardness between tyres; they have all been stored different lengths of time and will therefore be slightly different hardnesses now even if they started out the same. But then again maybe I've covered that by using both very old and much newer rear tyres in this period.

Now I'm loathe to jump to conclusions or anything but IIRC when I've run the same width tyres front and rear (which has been most of my cycling life I guess), the usual outcome has been that the rear tyre has cut up pretty quickly vs the front; had the result of my accidental test been the other way around I'd have been happy to attribute it to that effect. As it is, I'm quite strongly inclined to the view that 'there is something different that happens' when you use wider tyres. I don't know what it is or how it works; maybe it is something that cannot be proven, but that is the way my mind is working at present.

Has anyone else noticed anything similar?

BTW I'm normally inclined to discount any one rider's tyre set A vs tyre set B comparisons, because the weather plays such a large part in how much crud there is in the road etc; if I got two punctures in a year on one set of tyres vs none with a different set the next year, that could easily be explained by a number of things; to get something statistically significant might require input from dozens of riders. But when they are two tyres on the same bike at the same pressure on the same roads with this outcome, it is less easy to ignore or explain away....

cheers

[Bmblbzzz]

I'd think a slight difference in construction might explain it. Or just luck. But one other possibility comes to mind: you say you run both tyres at the same pressure. I presume this is within the recommended range for both tyres, but it might nevertheless be less than ideal for the narrower front tyre. If the tyre pressure is slightly lower than ideal, small sharp stones might not be pushed out by the tyre's rotation but instead be dug into the tread. This could explain why the tread of the front tyre is cut up.

[Brucey]

I did check the tyre pressures using the 'drop method' and because the width difference matches the load difference on each wheel pretty well, the same pressure front and rear gives a comparable %age tyre drop.

Thus by that yardstick the tyres are working 'the same' as near as can be judged, when they are at the same pressure as one another.

If I had a front-heavy bike for similar use I'd be tempted to try the same experiment 'in reverse' if you see what I mean.

cheers

[reohn2]

Since going over to larger section tyres(at much lower TPs)I've had far less punctures,despite riding more rough tracks off road on them than I did on smaller section tyres and despite the wider tyres having less tread and tread pattern ie;slicks.
Also what I have noticed is they cut up less than 28s(not the same tyre),and cuts tend not to be as deep,I put this down to the tyre more readily deforming to small cutting road debris.
The last front wheel puncture I had was about four or five years ago in a 32mm (@ 85psi) tyre on the front of the tandem,it was a sharp flint like stone when a local stately park dressed the roads with the it,others at the time complained similarly,and was dead centre of the tyre,so a direct hit.

BTW the only reason I don't run 40mm Hypers on the tandems is due to clearances not preference.

[Vorpal]

Riding the same bike daily over the same route with 23 mm Conti GP 4000 tyres, I know I had more punctures than with 25 mm tyres. And when I switched back to 28 mm GP 4 Seasons, I had no punctures at all. I can't blame that solely on the width, as they are different tyres, but they are similar tyres, so I can't blame it all on different tyres, either

[fastpedaller]

I'd think Brucey may be along with his wisdom......OH

[Bez]

If we're chucking anecdata around then I've punctured a little more often with 25mm GP4000S than with 23mm (in fact, I'm not sure I can recall ever having a puncture with the 23mm ones).

I think the issue here is that the sample sizes are so far away from statistical significance that it's hard to justifiably see past random factors.

[Zanda]

- maybe the front and rear tyres are not constructed in the same way after all

...I'd have to say that it is quite likely that the rubber compounds are not exactly the same hardness between tyres...

Interesting observations and I agree with the possible explanations. In particular, differences in squashiness of the outer material would certainly explain one tyre picking up more shards.

1. I'm sure it's possible for a manufacturer to alter the formulation of the outer layer on a tyre, for whatever reason, after having sold a batch made to the old formula. Since the other aspects of the construction (including the profile, the thread count and tread pattern) remain the same, that manufacturer may then feel justified in applying the same model name. That may account for the outer layer being softer on one tyre than another and hanging on to more spikey stuff.

2. Then, as you say, they'd have to be stored under identical conditions, before reaching the end user, in order for the differences in the 'behaviour' of the outer material to be attributable to tyre width, or anything else.

[fastpedaller]

My belief is that a narrower tyre is less likely to hit what flints are there ............ Taken to it's conclusion, a tyre the width of the road will pick up everything!

[cycleruk]

As both tires are being run at similar pressures then, as noted, the front may be too low a pressure. Could this leave the "rubber" soft so allowing it to retain more "bits" than the rear. Does increasing the pressure alter the "softness" of the rubber? I suppose you could always increase the fronts pressure as a trial.
On the other hand I run the same size & type tyres with the rear a little higher pressure (~10 psi) than the front. Invariably it's the rear that suffers the most punctures but I get very few flints in either tyre.

[Bigdummysteve]

Hmm interesting. I did see a comparison recently which compared the foot print of various tyres from normal 29" tyres to 29+ through to full fat 4 and 5 inch monsters. Although it was comparing large off road tyres the surface area say from a 3" to 4" tyre went up by a much larger factor than I had imagined. Could the difference be down to the lighter road loading not being able to push objects into the tyres?

[Brucey]

My belief is that a narrower tyre is less likely to hit what flints are there ............ Taken to it's conclusion, a tyre the width of the road will pick up everything!

that is what I used to think, but this experience has lead me to question if that corresponds to the rate of punctures, especially if they are from flints, which don't go in immediately.

I went for a ride earlier on and I had a chance to mull over this. Whilst I was stopped admiring the scenery, (wondering what that bright yellow orb in the sky was, amongst other things), I idly prodded my thumbnail into both tyres and if anything the front does seem a little softer compound than the rear. It also occurred to me that the front tyre (having about half the volume of the rear) would tend to lose pressure a bit quicker than the rear, so whilst they might start out at the same pressure (about 75psi) that might not reflect the average state of affairs so well, because I might only pump them up once a week.

Nonetheless the difference in the state of the tyres is remarkable; I pulled about 15-20 small flints out of the front tyre vs 'couldn't find any' in the rear.

cheers

[bobc]

Like all the old fogeys on here apparently, I now (for several years now) buy the biggest tyres that will fit between my stays & pass under the brake calipers! That's basically 28s on the road bike & around 1.5" to 2" on others. You just seem to win win win, the greater air volume is important (I certainly only involve a pump it I suspect the tyre is particularly soft). I suspect the susceptibility to punctures is down to the amount of deflection at the road. With 90psi in and me on, a 23mm tyre would be squashed nearly to halfway & this involves a "sharp bend" in the rubber where the tyre meets the road of nearly 90degrees. With the 30mm tyre this "bend angle" would be down nearer 40degrees.
Dammit I have to go and calculate this properly now........
OK 700C wheels - 100psi & 200lbs load is contact area of 1290mm^2. On the 23mm tyre the "sharp bend is 45.9degrees and on a 30mm tyre it's 36.7degrees

[reohn2]

Could the answer lie in the fact that bigger tyres (that also roll better) because they deform to small imperfections in road surface,also deform more to sharps too?
So what pierces a smaller section HP tyre,a larger section LP tyre deforms around more readily,I know that sounds counter intuitive but so did wide LP tyres for speed.

[Vorpal]

Could the answer lie in the fact that bigger tyres (that also roll better) because they deform to small imperfections in road surface,also deform more to sharps too?
So what pierces a smaller section HP tyre,a larger section LP tyre deforms around more readily,I know that sounds counter intuitive but so did wide LP tyres for speed.

I think that is an explanation that makes some sense. I also think that a bigger tyre is likely to have more flexibility, even at the same pressure as a smaller tyre.