The importance of tread pattern on cycle tyres.

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Why do tyres have a tread pattern?

If you go onto the website of any cycle tyre manufacturer you will see a bewildering range of tyres – all sizes, constructions and weight. But the most obvious difference between tyres is the tread pattern which seems to come in all shapes and sizes from massive tread blocks to completely slick, it's no wonder that cyclists looking at this get confused, so the purpose of this article is to try to uncover some of the mysteries of the subject and more importantly to expose some of the misconceptions.

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Mud glorious mud.

First off let's get one group of tyres out of the way. If you are a mountainbiker or cyclecross rider you need a heavy deep tread. What this tread does is provide blocks of rubber that cut into the soft ground and thus give grip on otherwise slippery mud. Of course there are many designs and the tyre manufacturer will sell tyres to come with soft deep mud, sticky clinging mud (so the tyre blocks need to clear the mud on every rotation) or much more solid turf. I've done a bit of mountainbiking and can confirm that the effect is massive and what works on one surface may not necessarily work on another – I guess the pro's have whole racks of tyres to choose for each condition they may race in.

 

But this isn't the subject of this article – here I'm dealing with road bikes – i.e. bikes travelling on an unyielding surface where tread blocks have no effect whatsoever on the surface – so what's going on?

Let's look at car tyres...

What strikes me most when I look at car tyres is that though the tread varies in different pretty patterns, they all have a similar amount of tread*. That's because all face a similar set of problems and so the compromises they make are going to be of a similar nature.

 

Everyone knows that racing cars, when racing in the dry use 'slick' tyres. What this means is that the tyre has no tread at all. And as anyone who has driven such tyres will tell you, such tyres give incredible levels of grip. In effect taking away all the grooves in the tyre (the 'tread') means more rubber** on the road. This rubber not only has a Coefficient of Friction that helps grip, but also will deform to 'cog' into the small imperfections on the tarmac. In the dry your cornering grip, stopping distance and economy will be far better than on a conventional tyre. The last point is important – because the tyre is smooth, very little energy is wasted in deforming big blocks of tread. If you listen to a big 4X4 driving past on spuddy tyres you will hear a whine coming from the tyres (or indeed on MTB tyres on tarmac). The sound is energy - lost energy that doesn't get used to drive the vehicle forward – like riding with your brakes on. What you can't hear is the heat generated by those blocks (wasted energy again) and that the flexing blocks are rapidly being worn away as their edges roll over the tarmac.

 

So why don't road cars use slicks? There is one reason and one reason only – Aquaplaning. You see, when it gets a little damp those slick tyres will actually still grip very well – watch a F1 race and you'll see that when it rains the cars stay out on slicks until the track shows wet patches – this is because although the water will act as a lubricant, the rubber is still deforming to cog into the road surface and thus offers decent grip.

 

But when the water actually forms a layer (or 'standing' water) the layer acts much more like a very slippery solid like ice, and a car on slicks is doomed. Once the water lifts the rubber away from the tarmac so it no longer 'cogs' the grip goes from 'quite-a-lot' to 'zero' instantly – this is called aquaplaning.

 

So what a car needs is a tyre that has grooves in it so that as the tyre presses onto the surface the water can move aside and up into those grooves leaving the rubber to once more 'cog' into the surface.

 

So on a car tyre the amount to tread to groove ration will show how resistant a tyre is to aquaplaning and therefore how good they are in the wet. However the more grooves the less like a slick a tyre will be and so the poorer the dry weather grip. You see now that all car tyres are a compromise between the rate at which they can clear water to prevent aquaplaning and the amount of tread on the tarmac to give dry grip.

 

The other thing we need to realise from this is that the rate of water clearing is also dependent on both the width of a tyre and the weight of the vehicle. A heavy car on narrow tyres will have a lot of weight pressing on a small area – a high pressure on the road – and so will cut through water much better than a very light car on wide tyres. Happily what we generally find is that light cars run skinny tyres and heavy ones wide tyres so the actual pressure is similar and therefore the ideal tread/groove ratio likewise similar. In my case I run one of the exceptions as my Lotus Elise weighs under 800 kgs and has pretty wide tyres = great dry-weather grip but watch out in the wet!

 

Lastly – speed. The faster we ask water to move out of the way the more it acts as a very slippery solid. Try a belly-flop from the side of a pool and then from the high-board to illustrate the point...

 

Not only have all these considerations meant that most car tyres fall within a reasonable range of tread to groove ratio, but that tyres below a certain ratio are illegal – no slicks on the road!

A Schwalbe marothon bike tyre

So why all this drivel about car tyres in an article about bike tyres???

Because if you read the above and sit down and get your head around it you'll realise that we've all been 'led up the garden path' when it comes to cycle tyres!

 

Lets consider this problem. As with a car in the dry we'd be better off ALWAYS riding an absolutely slick tyre. More grip, lower rolling resistance (very important on a bike), quieter and lower wear – it's obvious.

 

Until it rains;-) Remember, the only thing we need to worry about is aquaplaning. So do bike tyres aquaplane? Well the simple answer is NO! There are three reasons – the first is that like aircraft tyres, they have a round rather than flat profile and that profile clears water away very easily. Second is speed – we don't really go very fast so water has time to get-out-of-the-way. Thirdly is that ground pressure thing. A typical touring bike tyre has a contact area of about 4 cm3. Let's say that the tyre is supporting a mass of 40 kg (count the weight of yourself, your bike+luggage). So we have a pressure of 10 kg/cm2. Now look at a typical car – each tyre is supporting say 400 kg and the contact area is 240 cm2***– that gives a typical pressure of around 1.7 kg/cm2! So no amount of grooves are going to get the contact pressure of the road tyre up to that of the bike! That bike tyre is ALWAYS going to be more resistant to aquaplaning than any car tyre, and given the much lower speeds involved and that the long, oval shape of the contact patch it clears water perfectly. The existence of groove patterns on a cycle tyre will have no advantage whatsoever on wet-weather grip – in fact quite the opposite as you will have less rubber trying to 'cog' into the road surface.

 

Yes you read that correctly. 90%+ of the tyres sold to us cycle tourists have utterly useless and counterproductive tread moulded into them.

But there's more!

So having demonstrated that we should all ride slicks it's worth pointing out one other advantage – puncture resistance. To be fair, this is a little dependent on what causes the puncture, but it remains true that slicks have a major advantage. The rubber on a slick is uniformly thick at the contact area and so will resist penetrations uniformly. On a tyre with tread/grooves some parts of the tyre will be thick, some very thin – little thicker than the sidewall. If, say, a thorn tries to penetrate the thick tyre block then it'll have a harder job than on a slick, but on the other hand anything getting between the blocks will have little stopping it, and moreover will tend to stay inbetween the blocks and work its way through the thin casing where on a slick it'd probably fall off after one rotation – glass and flints in particular show this especially when mixed with a little mud on a road or more commonly a cyclepath.

Disadvantages?

There really aren't any. Slick tyres actually grip quite well on most surfaces, even sand, cyclepaths, snow (yes really!) etc – it's only when you need something to physically cut into the substrate that they lose out - and for most of us on our road bikes that will occur – well never...

So why do 90% of bike tyres still have tread?

Marketing... It's a simple answer and the only explanation. 100 years ago bike tyres were slick, but to distinguish between makes manufacturers began adding light tread as their 'signature'. By manipulating the ignorance of the public they could make the buyer thing – 'wow! Look at the tread on that tyre – must be really grippy!' That this madness extends to even the thinnest of race tyres says a lot...

It also gives a manufacturer an easy way of making a whole range of tyres all aimed at different markets and price points by the simple expedient of using the same carcass and a bucketful of moulds of different patterns. Granted, some manufacturers will use a range of more or less complex/expensive carcasses to support their tyres, but with each of those ranges there will be several tread patterns at different prices which will have no value to the rider whatsoever. It would be a nightmare for the manufacturers if everyone decided slicks were best. Imagine walking into a bike shop and looking at a rack of 50 different tyres – all of them slicks – which would you choose? It's the scenario that keeps marketing men awake at night, but by any logical measure it's exactly what we need.

What do I use?

Well slicks of course, both on our hire bikes - Schwalbe 'Spicer' slicks - and on my own tourer Schwalbe 'Marathon' Slicks - both can't resist at least some vestigial tread markings, but are superb, very few punctures, fast rolling and very grippy – I rest my case...

*Specialist winter and off-road tyres are exceptions and have more 'aggressive' i.e. knobblier – tyres.

**Synthetic rubber in modern tyres.

***http://vehicledynamics.com/the-tire-contact-patch/

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