How might we test a tyre’s wet grip?

[Kim]

FWIW I agree with the idea that tall CoG means you have more time to correct things before you go down vs a lower CoG. I was going to make a similar comment myself.

Readily apparent to anyone who's ever fallen off ridden a lowracer.  Or, presumably, a penny farthing.

In the real world, at this time of year, I reckon fallen leaves and mud are big factors.

Absolutely - it's not just the rain, it's the contamination it brings with it.

I think the mountain bikers know the answers - soft compounds for wet rock, knobbles for mud.  Neither being conducive to efficient rolling on tarmac.

Which isn't to say an objective test is a bad idea.  Let's have one for skog, as well as just water.

[frankly frankie]

You say YOU THINK :--
I think it’s the other way about: tall riders are more stable. Bicycles must be balanced, unlike cars, and we all know it’s easier to balance a broomstick than a pencil on a fingertip. Riders with a lower centre of mass must make more frequent corrections.

Certainly a low centre of gravity is useful to Judo players.  It's not particularly useful to bicyclists. 

[vorsprung]

The roads are getting wetter and I’m again wondering which tyres would improve my odds of staying upright in these conditions.

tyres with 10 psi less in them

[Ham]

The idea that a road surface is smooth is entirely counter to the experience of falling and sliding on it.

There are places where the surface is smooth : Steel covers, paint, I certainly find slicks (Conti are what I use) are more dependable, grippy and predictable in those circumstances.

What I think is happening is that the rubber compound is maximising the friction coefficient of the contact patch, given the size of the contact patch it is possible that the loss of contact area can't be made up by the effect of the tread - whatever that is.

If contamination reduces the coefficient close down to zero, you are screwed anyhow.

Looking around, I was curious to see what the comparative weight and contact area is between bikes and cars. The data is probably findable more accurately, but if a car is 3000Kg with 1000 sq cm contact (seems close to normal), and a bike is 10 sq cm with 100 kg, you can see that the friction may count for more. Interesting that race cars all use slicks, extra wide, though.

[Kim]

Interesting that race cars all use slicks, extra wide, though.

They also eat tyres at a rate that's incompatible with maintaining a tread pattern.  If the tyres only have to last half a race, then you can optimise for different things.

[jsabine]

if a car is 3000Kg with 1000 sq cm contact (seems close to normal), and a bike is 10 sq cm with 100 kg, you can see that the friction may count for more.

More like 1000-1500kg, at least for European values of car, otherwise no-one would ever be able to cross a bridge with a 3 ton weight limit.

Fiat 500 a bit less, Mondeo estate a bit more, Fiesta, Focus, Golf, Qashqai right in that range. Merc S-Class, LR Discovery a good bit more, but still closer to 2 tons than 3.

[Ham]

Interesting that race cars all use slicks, extra wide, though.

They also eat tyres at a rate that's incompatible with maintaining a tread pattern.  If the tyres only have to last half a race, then you can optimise for different things.

If tread gave them more grip, they would do it.

if a car is 3000Kg with 1000 sq cm contact (seems close to normal), and a bike is 10 sq cm with 100 kg, you can see that the friction may count for more.

More like 1000-1500kg, at least for European values of car, otherwise no-one would ever be able to cross a bridge with a 3 ton weight limit.

Fiat 500 a bit less, Mondeo estate a bit more, Fiesta, Focus, Golf, Qashqai right in that range. Merc S-Class, LR Discovery a good bit more, but still closer to 2 tons than 3.

The data I had found was for a Crown Vic and ICNBA finding anything else. Added in a couple of US Cops+Twinkies for good measure and it still came up with a low kg/cm² number.

[Samuel D]

If you’re trying to estimate the tyre’s contact pressure, just assume it’s the same as the air pressure. A car tyre needs about 30 PSI; a bicycle tyre about 100 PSI. Therefore the contact pressure is about three times higher with the bicycle, ignoring subtleties like tread patterns, casing stiffness, and dynamic effects.

I didn’t understand your point in bringing this up, though. What do you mean by “the friction may count for more”?

If contamination reduces the coefficient close down to zero, you are screwed anyhow.

Hysteresis grip works even in the total absence of molecular bonding. It is the hysteresis part of grip that makes some tyres grip so much better than others in the wet – a difference, by the way, that I have observed to be far greater than any difference caused by adjusting tyre pressure by 10 PSI.

The difference in wet grip between a road car tyre and a racing motorcycle tyre is mindboggling. It would be nice if a manufacturer of bicycle tyres made an all-out wet-weather tyre with a high-performance casing and a short lifespan. I think the grip could be much higher than anything on the market. Consider that motor-racing tyres have rubber with hysteresis so high that they only sluggishly regain shape after you poke them with a fingernail. I’ve never come across a bicycle tyre with rubber like that.

[mrcharly-YHT]

It would be nice if a manufacturer of bicycle tyres made an all-out wet-weather tyre with a high-performance casing and a short lifespan. I think the grip could be much higher than anything on the market.

They do. Several companies include such tyres in their linup and people here have written about them.

The black chilli compound used in Conti GP4 seasons is similar to that used in their winter wet weather car tyres. I've ridden these tyres on snow and ice and they get some grip, despite being slicks. They only fail on black ice.

Conti also make a super-soft compound winter tyre (heavy, not a performance tyre) - top contact winter.

[Brucey]

Interesting that race cars all use slicks, extra wide, though.

They also eat tyres at a rate that's incompatible with maintaining a tread pattern.  If the tyres only have to last half a race, then you can optimise for different things.

If tread gave them more grip, they would do it.

when it is at all wet they do use a treaded tyre of course.  The reasons for doing so are severalfold and not the same as those in a bicycle tyre though.

cheers

[Kim]

Interesting that race cars all use slicks, extra wide, though.

They also eat tyres at a rate that's incompatible with maintaining a tread pattern.  If the tyres only have to last half a race, then you can optimise for different things.

If tread gave them more grip, they would do it.

Which is exactly what they do when it rains.  If your tyres don't last a whole race, you need some means of quickly changing tyres, so you can choose the right ones for the exact conditions.

[Cudzoziemiec]

In any case, that^^ is racing. In other situations, we can't change tyres whenever the weather or surface change. A tyre that was totally specific for wet weather would be too much of a compromise in real life road cycling.

[trekker12]

A treaded tyre on a racing car used in the wet not only displaces water from the surface - it's main purpose - but it also allows the blocks of rubber between the tread to move and build up heat, which then aids a small amount of evaporation at the contact point between the tyre and road, thus enhancing grip.

There's far to much energy at work however to compare that to a bicycle tyre or even a road car tyre which has tread purely to displace water as the heat build up isn't really enough.

Rally cars used on public road would use a full slick in the dry if the law allowed it. Instead we fit a cut¹ slick which ensures the largest possible contact patch whilst still legal. Also, we are often up to two hours away from our service crew so some protection when rain falls is beneficial. A cut slick will work to some extent in damp conditions but will aquaplane in a fair sized puddle.

None of which has much to do with bicycle tyres.

¹ They aren't cut by hand, the manufacturer moulds a series of angled depressions to a road legal depth into the surface

[Samuel D]

mrcharly-YHT: Continental doesn’t use the BlackChili compound in the 4-Season. The 4-Season compound is reported to have better wet grip, but hard data is elusive. At any rate, that tyre lasts many thousands of kilometres. How about a tyre that doesn’t emphasise long life but better grip, like the high-performance motorcycle tyres available for street use?

I put less stress on my tyres than a 100 kg rugby player who runs 130 PSI and lives in a hilly region, so mine last longer than his. It would be good to have options with a shorter life and better wet grip. Maybe Pirelli will deliver something now that they’re back in the game.

I think the main reason we don’t have such tyres is lack of consumer knowledge, which is mostly the tyre manufacturers’ fault. It’s a bit like car tyres (especially before the EU Tyre Label): data is hard to come by, most people have no idea how tyres work or why they’re so important, and the industry hides behind irrelevant or made-up marketing claims for new products.

With rolling resistance this situation has recently changed, because some people (notably Bicycle Rolling Resistance) have popularised concern about this parameter. This has made Michelin, for example, concentrate on rolling resistance (in the Power Competition) to the likely exclusion of good wet grip. Most of the tyres that do seem to have better wet-weather grip have heavy puncture protection (such as the Continental Grand Prix 4-Season). Not my cup of tea.

trekker12: I have never heard of this evaporation theory and would have to be convinced! I know tyres have most hysteresis (and therefore grip) around the compound’s glass-transition temperature, and since racing tyres have high hysteresis and are driven hard, they’re designed with high glass-transition temperatures and therefore must be warmed up before they give good traction. A complicating factor is that the glass-transition temperature varies significantly for different stress frequencies. I thought that was about the extent of the temperature concerns.

[DuncanM]

Interesting that race cars all use slicks, extra wide, though.

They also eat tyres at a rate that's incompatible with maintaining a tread pattern.  If the tyres only have to last half a race, then you can optimise for different things.

If tread gave them more grip, they would do it.

Which is exactly what they do when it rains.  If your tyres don't last a whole race, you need some means of quickly changing tyres, so you can choose the right ones for the exact conditions.

There are a number of reasons why treaded (or even semi slick) tyres are bad news in the dry for single seat race cars. Less precise cornering and braking due to tread walk, and additional rolling losses are the smaller ones - the big one is bad heat management. If they were to use treaded tyres in the dry, the motion of the tread would cause so much extra heat build up the tyre would damage the rubber underneath, and the tyres would only last a couple of laps.
Clearly, the above reasons are no use for cyclists - we don't have the power to generate that much heat in our tyres.

[zigzag]

continuing the tangent about the slick racing car tyres - they are being used on totally clean race tracks (usually swept with the aid of jet engines just before the race), so no gravel or dust can reduce/compromise their grip. the rubber is soft enough so that it's possible to gouge it with your finger nail, yet hard enough to stay in shape and attached to a carcass.

real world conditions are very different - you don't want to pick up the debris from the road and carry it embedded in your tyres.

[Ham]

continuing the tangent about the slick racing car tyres - they are being used on totally clean race tracks (usually swept with the aid of jet engines just before the race), so no gravel or dust can reduce/compromise their grip. the rubber is soft enough so that it's possible to gouge it with your finger nail, yet hard enough to stay in shape and attached to a carcass.

real world conditions are very different - you don't want to pick up the debris from the road and carry it embedded in your tyres.

This is, indeed, the downside of Conti Sport Contacts, and a tradeoff I've accepted for years. I've not run Sport Contact 2 for long enough on a commute, but I have noticed that they are less prone to sucking up bits compared to the old.

[quixoticgeek]

In any case, that^^ is racing. In other situations, we can't change tyres whenever the weather or surface change. A tyre that was totally specific for wet weather would be too much of a compromise in real life road cycling.

You say that, but in much of the civilised world you have to change car tyres for the winter.

J

[Samuel D]

By law you don’t have to change car tyres for the winter in Paris, but I do so anyway because winter tyres have clearly better grip in wet, cool conditions. I change tyres on my bicycle for the same reason.

But what is the scale of the difference? This slide from a Michelin press conference may help answer that. It suggests Michelin’s Power All Season tyre has 27% more wet grip than the Power Competition. The All Season is much draggier, but that’s partly because it has coarser threads in the casing and heavier puncture protection.

As I’ve said before, it would be nice to see a cold-and-wet tyre with low rolling resistance. I don’t think this is impossible. It would require a high-performance (i.e. expensive) casing, minimal puncture protection, and a thin tread, since the tread compound would need to have high hysteresis. The thin tread would result in a short life, but so what? Would this be so hard to sell? If Michelin and the rest stopped spouting nonsense in their marketing materials and instead explained the issues simply but accurately, there might be a worthwhile market for something like this.

[Cudzoziemiec]

In any case, that^^ is racing. In other situations, we can't change tyres whenever the weather or surface change. A tyre that was totally specific for wet weather would be too much of a compromise in real life road cycling.

You say that, but in much of the civilised world you have to change car tyres for the winter.

J

A perfectly reasonable, twice a year change at predictable times, which many of us do on bikes anyway. Totally different from the motor racing situation in which teams can change tyres on a car several times in each race.

[DuncanM]

Winter tyres tend to have more puncture protection though. I don't know if that is due to convention, or whether the presence of water acts as an agent to cause more punctures.
Either way, I think it would be a hard sell to get cyclists to pay for expensive winter tyres with minimal puncture protection, on the basis that they grip better.

[Samuel D]

I think it’s both convention (based on an understandable objection to fixing punctures in the cold and wet) and because punctures are indeed more likely on wet roads.

However, don’t you think more grip would be a useful selling point if – and this has been the problem – people knew about it?

I wonder how many cycling accidents could have been avoided by a plausible (e.g. 30%) increase in wet grip. I suspect many, based on my own experiences.

And with the move to disc brakes that can offer more bite in the wet, you’d think the market would be ready for something that makes that increased braking power useful.

[Cudzoziemiec]

We're slipping (sorry) off topic here, but disc brakes are likely to reduce the likelihood of wet weather lock-ups, because they behave more predictably in wet weather and with minimal delay. But yes, of course more grip is always a selling point. So are puncture protection, reduced rolling resistance, longevity and of course price. Wet grip probably appeals most to commuters (and to racers and sporty riders, but they're precisely the ones who can change tyres each ride), who also value puncture resistance and longevity. So it's a question of finding that balance for a relatively small market.

[mzjo]

By law you don’t have to change car tyres for the winter in Paris, but I do so anyway because winter tyres have clearly better grip in wet, cool conditions. I change tyres on my bicycle for the same reason.

But what is the scale of the difference? This slide from a Michelin press conference may help answer that. It suggests Michelin’s Power All Season tyre has 27% more wet grip than the Power Competition. The All Season is much draggier, but that’s partly because it has coarser threads in the casing and heavier puncture protection.

As I’ve said before, it would be nice to see a cold-and-wet tyre with low rolling resistance. I don’t think this is impossible. It would require a high-performance (i.e. expensive) casing, minimal puncture protection, and a thin tread, since the tread compound would need to have high hysteresis. The thin tread would result in a short life, but so what? Would this be so hard to sell? If Michelin and the rest stopped spouting nonsense in their marketing materials and instead explained the issues simply but accurately, there might be a worthwhile market for something like this.

38€
Winter tyres, as any wintersport loving Parisien knows, are described as thermocontact and use a higher tread temperature to gain grip on winter roads - permitting said Parisien to use the motorway and then go up to the station without stopping to put on chains. Cheap winter tyres like mine are not thermocontact !