Tyres and flint shards

[ChrisButch]

I think we need to understand what is meant by 'flint'. Flint, properly so called, is only found as nodules embedded in chalk and some kinds of marly limestone, so clearly is not found in the many parts of the country without these rocks. However, 'flint' tends to be very loosely used, particularly by cyclists, of any small, sharp fragment of stone, whatever its origin. As has been said, the local geology is key to this. Any rock type containing a high proportion of quartz in its minerals is likely to produce these fragments, and true flint is a form of quartz. There are quite large areas of the country without rocks of this type, of which perhaps the most obvious example is the peatlands of East Anglia, Somerset etc where the mineral content is very low, but there are other rock types, including some igneous rocks, which also have a low quartz content.
I believe that the proportion of quartz fragments in the local mud is also the main factor in riders in different parts of the country experiencing widely differing rates of chain wear, but that's another topic...

[yostumpy]

I think it could actually be the tyre. I had a pair of the 'all conditions' and couldnt believe how muck stuff actually stuck to the tyre 'tread'. after about 5km both tyres looked like I'd ridden thro tar, then thro a gravel pit. i took them off PDQ, and sold then on.

[Brucey]

on the basis of the local bedrock you might not expect flints to occur in much of the country. However quite a lot of the UK has substantial deposits of moraine material left after the last ice age. This stuff has often been partially sorted but not heavily worn (and therefore rounded) by the action of running water. It can be remarkably abrasive stuff; if you go MTBing in parts of Epping forest you can destroy a set of brake blocks (or pads) in a day, easy, and there are plenty of 'flints' (both true flints and flakes of other hard stones) which can wreak their own merry havoc.

A lot of paths (forestry, some NCN etc) are made up using crushed limestone. This soon wears down under heavy traffic but until it does there may be jaggy shards of limestone which will penetrate soft tyres just like flints do.

IME if you use a treaded tyre of some kind (rather than a slick), flints may get stuck in the tyre but they seem slightly less likely to work their way through and cause a puncture.

Following some heavy rain recently, there are lots of flints hereabouts at present. In the LBS today there was a bike in to have its rear tyre changed; coming off was a nearly new (still had the centre ridge and the pips on it, i.e. 50 miles old tops) Conti Gatorskin Hardshell; the owner had suffered three punctures in a matter of days, all from flints.

cheers

[foxyrider]

[quote="ChrisButch"]true flint is a form of quartz. quote]

New one on me and my geologist friends! In five years of studying geology at university i can't recall anyone proposing such a thing. Uneroded flint is found in chalk so you might expect the Wolds and Downs to be puncture magnets whereas areas like the Peak District have no naturally occuring flint anything found there has been imported by man (of course there is a bit of chert but that tends to be too friable to be a puncture issue).

But we digress, punctures. 'flint' or should we call them 'sharp stone fragment incidents' IME are usually a wet weather phenomenom - the P fairy seems to delight in the wettest, muddiest conditions possible when throwing this stuff in our path.

Curiously i can't recall a single 'sharp stone fragment incident' whilst cycling abroad, its usually glass that does for me or larger stones causing impact punctures.

[Brucey]

strictly speaking flint is a subset of all chert, flints being those cherts which are found in chalk and limestone. A common source of confusion is that 'common chert' is obviously a rather different material to good quality flint. Flint is essentially microcrystalline Si02 (i.e. quartz) with some impurities, but cherts are primarily SiO2, usually with a higher impurity content. Chert flakes can still be hard and pointy enough to puncture bike tyres!

It is not certain but the most commonly held belief (as explained to me recently by the curators of one of our better geology museums) is that flints form from a gelatinous or liquid phase that is partitioned during the formation of chalk and limestone. Under certain conditions the same liquid phase can infill any hollow spaces within the rock. This explains;

- the weird shapes of flint nodules
- the occasional fossils found entirely within flints
- the banded structures seen inside some flints
- the occasional 'fossil impression' flints that are found with the interior shape of living creatures.

I have found a couple of flint 'fossil impression' sea urchins; these are thought to be the shape of the inside of a sea urchin shell. Once the fossil with its flint infill is eroded from the chalk or limestone, the fossil shell itself is usually lost, leaving a 'flint cast' of the interior, often with excellent detail.

I have also found occasional flint nodules which look like they might be geodes, in that there is a pocket in the surface with larger crystals of some kind (Agate perhaps) showing within. However once broken open usually the interior is mostly just flint, to my disappointment.

cheers

[ChrisButch]

Foxyrider - possibly you may have misunderstood me. I did not mean to imply that quartz found in other rocks than as cherts in chalk and marly limestones was also flint - far from it. It is however, true, as Brucey has also said, that true flint is quartz. In other words all flint is quartz but not all quartz is flint. Most quartz, however, in most rocks containing it, does break down to the kind of fragment likely to cause punctures.
This is, of course, academic: but for the purposes of this thread quartz in most of its forms is a puncture risk. Unfortunately, quartz is one of the most abundant minerals, found to varying degrees in very many rock types, so you can rarely escape it entirely.
Also, of course, relating the risk to local geology in this way is primarily about rural roads, where the origin of most loose material on the surface is mud washed out of fields, verges and banks. In urban settings where the surroundings are entirely artificial the materials broken down on the road surface could originate from anywhere

[yostumpy]

In the LBS today there was a bike in to have its rear tyre changed; coming off was a nearly new (still had the centre ridge and the pips on it, i.e. 50 miles old tops) Conti Gatorskin Hardshell; the owner had suffered three punctures in a matter of days, all from flints.

cheers

You mean someone actually fixed 3 punctures ...THEN took the bike to the LBS , and have THEM fit a new tyre. why did the owner not fit it....?????

[Brucey]

In the LBS today there was a bike in to have its rear tyre changed; coming off was a nearly new (still had the centre ridge and the pips on it, i.e. 50 miles old tops) Conti Gatorskin Hardshell; the owner had suffered three punctures in a matter of days, all from flints.

cheers

You mean someone actually fixed 3 punctures ...THEN took the bike to the LBS , and have THEM fit a new tyre. why did the owner not fit it....?????

dunno.... I'm speculating here but probably sick of the sight of the bike ( fixed gear commuter) and had to go work for a living...?

cheers

[flat tyre]

Yes, almost all the punctures I get are tiny flint shards that have worked their way through the tyre compound. It seems to me that most occurrences are after heavy rain when flint stones have been washed in the road. Are there differences in rubber compounds used and would one type cause the flint shards to stick more readily than another?