How might we test a tyre’s wet grip?

[Samuel D]

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

The trouble is, there is almost nothing on the web indicating the wet grip of various tyres, even though their rolling resistance is finally being measured by a few people. Online magazine reviews sometimes make claims about a tyre’s wet grip, but I know better than to believe any of those.

Since no-one is measuring this, perhaps it is hard to do. But why? Wouldn’t a rudimentary method involving a roughened steel plate with a tilting apparatus give at least some useful information? Load a vertical wheel with 40 kg and see the plate angle at which it slips in the presence of water at room temperature.

Based on my cycling experience with different tyres, especially the Michelin Pro4 Grip, I think there are large differences in wet grip from tyre to tyre, so a useful test may not need to split hairs.

Thoughts welcome.

[quixoticgeek]

The key part is the test of wet grip when cornering. No idea how to test it, but in my experience if a bike tyre is going to run out of grip in the wet, it's going to be on a corner.

J

[ACyclingRooster]

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

The trouble is, there is almost nothing on the web indicating the wet grip of various tyres, even though their rolling resistance is finally being measured by a few people. Online magazine reviews sometimes make claims about a tyre’s wet grip, but I know better than to believe any of those.

Since no-one is measuring this, perhaps it is hard to do. But why? Wouldn’t a rudimentary method involving a roughened steel plate with a tilting apparatus give at least some useful information? Load a vertical wheel with 40 kg and see the plate angle at which it slips in the presence of water at room temperature.



Based on my cycling experience with different tyres, especially the Michelin Pro4 Grip, I think there are large differences in wet grip from tyre to tyre, so a useful test may not need to split hairs.

Thoughts welcome.

Hi Samuel. I suppose that cycle tyres,motor-cycle tyres and indeed car tyres are all reliant upon the type of surface that is actually beneath the water and the relevant tread-depth in relation to the load that is being carried - in the case of a cycle - the weight and bulk of the rider. Tyre pressures have a great-deal to do with staying upright.
Smaller riders will have a much lower center of gravity whilst larger and particularly heavy riders with be TOP HEAVY and therefore much more unstable in any given situation.
For many many years I have ridden Vittoria Rubino Pro but have recently started using folders and never had a worrying moment with either - but there again I don't descend like some of the World Tour Riders nor generally ride like a budding Evel Knievel.

I normally only use 700cx23 tyres but have occasionally ridden the 25's. They have a smooth narrow friction-less center and a tread pattern that extends around the sides to the tyre walls.

They are not a hard compound and I would always settle for a little less mileage and better grip than higher mileage and regular road-rash.

[Cudzoziemiec]

Wouldn't the test have to take into account the fact that the tyre is rotating? A stationary tyre on an angled plate might not give the same results. And as Quixotic geek says, cornering is the key thing (well, actually I think braking is the moment it's most likely to lose grip, but cornering too) so we'd need to angle the tyre. I'm sure there must be official "Ncap-type" tests for car and more usefully for us motorcycle tyres, which could be applied.

[quixoticgeek]

Wouldn't the test have to take into account the fact that the tyre is rotating? A stationary tyre on an angled plate might not give the same results. And as Quixotic geek says, cornering is the key thing (well, actually I think braking is the moment it's most likely to lose grip, but cornering too) so we'd need to angle the tyre. I'm sure there must be official "Ncap-type" tests for car and more usefully for us motorcycle tyres, which could be applied.

There are 2 aspects here that can be combined to make a 3rd aspect. Braking, turning, and braking & turning.Generally tho if you're turning and braking at speed in the wet, then it's kinda darwinian...

I'd say that the risk of locking a brake and skidding when going straight is largely down to the user and not the tyre. Which does bring us back to the turning thing...

J

[Torslanda]

Continental will tell you a road tyre has no need of a tread pattern. For the contact patch the size of the average 23/25 to truly aquaplane i.e. break contact with the surface due to water, requires a speed in excess of 120mph. It's an argument they have with the pro teams every season because teams insist on a 'tread pattern'

Tyres losing adhesion is usually down to surface irregularities or contaminants e.g. diesel, mud etc. or temperature, under or over inflation. There are so many variables . . .

[mrcharly-YHT]

I used to be a firm believer in the 'slicks grip as well as treads' but I'm starting to doubt it (depending on conditions).

When my beloved paselas wore smooth in the centre, they would start to slip when riding uphill on very smooth bitumen. Vittoria Hypers are the same, despite being very new. These are relatively fat tyres at lowish pressures.

IMO, as soon as there are some contaminants on the road (silt, sand, dirt of any kind), then totally slick tyres get less grip than tyres with some tread. This is even more true if you add water to the mix.

[Samuel D]

I suppose that cycle tyres,motor-cycle tyres and indeed car tyres are all reliant upon the type of surface that is actually beneath the water and the relevant tread-depth in relation to the load that is being carried - in the case of a cycle - the weight and bulk of the rider.

True, but testing with a representative surface in controlled (limited) conditions would probably give information that could be usefully extrapolated to all surfaces.

I believe this to be the case because grip comes from two sources: molecular adhesion and hysteresis grip, with the latter dominant in the wet and resulting exclusively from surface roughness. I crudely described this here. Note that maximum grip occurs around the glass transition temperature of the compound, so a perfect test would vary the ambient (and therefore tyre) temperature to find the optimum temperature for each tyre. In fact, testing winter tyres at room temperature as I casually proposed might be daft. Already a complication rears its head!

Smaller riders will have a much lower center of gravity whilst larger and particularly heavy riders with be TOP HEAVY and therefore much more unstable in any given situation.

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.

Wouldn't the test have to take into account the fact that the tyre is rotating? A stationary tyre on an angled plate might not give the same results.

Why, though? My guess is it doesn’t matter whether the tyre is spinning or not. Similarly, braking and cornering grip amount to the same thing.

I'd say that the risk of locking a brake and skidding when going straight is largely down to the user and not the tyre.

My Michelin Pro4 Grip tyres are greatly less likely to skid than my Schwalbe One tyres when I apply the rear brake in the wet, so the tyre does matter. Of course, slipping when cornering is usually far more hazardous than the rear wheel locking up (which is usually innocuous and sometimes deliberate).

[mrcharly-YHT]

Any test only has to provide comparative results.

I'd suggest a suitable test rig would have a reproducible surface - maybe using a specified grit of abrasive paper, something like 200grit. Glue to a board, incline board at 30 degrees, push wheel vertically down, record force at which tyre slides. Repeat with water pouring down board.

[Samuel D]

incline board at 30 degrees, push wheel vertically down, record force at which tyre slides.

I think it usually will or won’t slide at a given angle, regardless of force. Therefore the angle has to vary.

Sandpaper is an interesting idea because it can be reproduced with some accuracy, as you suggest. I thought of a steel plate with surface features, since steel doesn’t wear out quickly.

[zigzag]

there are too many varriables in real world to replicate them in a lab environment, although it would be possible to get a rough idea if one tyre is grippier than the other. simplest is to choose a tyre compound that's been proven to be most grippy (e.g. conti black chili) and have a bigger margin for error if riding on slippery surfaces.

[Brucey]

one of the German magazines tested 'road bike' tyres for grip by having riders (who were clad in motorcycle leathers) corner on wet tarmac, ever faster, until they fell off.

I can see many possible variables and ways in which such tests (any tests) are made moot; there is no such thing as a standard road surface.

IME it takes an unusually slippy surface to tip off an experienced rider.  If you use grippy tyres, and habitually ride like they are grippy, weirdly you may be more likely to come off when the grip is severely diminished.

For me the most important thing is how the tyre feels when the grip is diminishing substantially.  I like a tyre that gives plenty of warning before it lets go, and I'd happily trade away some absolute grip for that.  Such things cannot be measured, but they can be (subjectively) tested.

  To this day tyre manufacturers still employ tyre testers who test motorcycle and car tyres; they make sure that the tyres behave in such a way as they provide a chosen level/type of feedback as well as transition into slides in a predictable fashion, etc. I don't know that bicycle tyres are subject to the same sort of scrutiny.

cheers

[Brucey]

BTW the idea that slicks grip as well as treaded tyres is 100% reliant on the road surface being pretty rough (as rough as Shell delugrip, more or less) and that it is contaminated with pure water (not petrol, oil, diesel, slimy mould, decomposing leaves, frost, sludge from road salt, ice, sweet wrappers, fag butts, etc etc etc, like real winter roads often are... .)

 As soon as the surface is smooth and/or contaminated, you will have almost zero grip with a slick tyre and worse still the tyre will usually break away without any warning whatsoever.

IME  treaded tyres (on a like for like basis) will at least break away with more warning and may offer slightly more grip on certain kinds of iffy surface, because the contact pressure is locally higher and may better break through a slimy film of some kind.

cheers

[mrcharly-YHT]

incline board at 30 degrees, push wheel vertically down, record force at which tyre slides.

I think it usually will or won’t slide at a given angle, regardless of force. Therefore the angle has to vary.

You've just broken laws of friction and physics then.

Thanks Brucey, your statement is confirming my experience. I'm falling out of love with slicks for this reason.

[Samuel D]

choose a tyre compound that's been proven to be most grippy (e.g. conti black chili)

When was this proven and where is the proof recorded? I didn’t find anything but hearsay when I looked.

You've just broken laws of friction and physics then.

Which ones?

School textbooks say the force of friction is directly proportional to the applied load. Of course we know this not to be exactly true even in the dry with vehicle tyres, since grip is not just a matter of molecular adhesion. In the wet, I accept it’s materially not true. But if what you suggest is right, heavy riders would routinely slide out on corners that lighter riders successfully navigate. I don’t see that happening in practice.

[mrcharly-YHT]

The surface is at an angle to the load, so the downward force is split into a force perpendicular to the surface (yes, this increases the friction) and a component tangential to the surface.

Why don't you try it and see? My experience of the world is that if you increase the force on an object on a sloping surface, at some point it will slide.

Conti black chili is a very grippy compound but I don't know if it is the *most* grippy.

[giropaul]

It's much more to do with pressure, and riding style.

A certain famous and highly successful criterium rider used only about 4 bars in his (tubular obviously) tyres for wet Kellogg's crits. Several well known continental racers just looked on in awe!

In the real world though, I reckon riding well within the capabilities of tyre and rider, especially in the wet, is he answer. Keeping an eye out for diesel, white lines etc is much more important than the breed of tyre.

[ACyclingRooster]

Hi again Samuel. You seem to be at variance to my statement :--  Smaller riders will have a much lower center of gravity whilst larger and particularly heavy riders with be TOP HEAVY and therefore much more unstable in any given situation.

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.

As the bike is being leaned into a bend or corner it is immediately being put into an unstable position by firstly not being upright and one the tyre begins to loose traction/grip the rest will be history.
The head is the heaviest part of the body and therefore once on the move and away fom the vertical it will just carry on going unless an opposite force is imposed.

This was the very basic lesson learned whilst I was actively doing competitive Judo many many years ago and the easiest opponents to overcome were the taller guys for two main reasons - they were always taller than me by a long way and there heads were further from the floor and they were not always falling victim of shoulder throws but often simply because they were off-balance because they were leaning forward and there heads were going first.

[Samuel D]

Why don't you try it and see? My experience of the world is that if you increase the force on an object on a sloping surface, at some point it will slide.

Just now, I tried manually loading (in plane) a bicycle wheel against my wooden floorboards, which are rough by floor standards but still pretty smooth. At about 30 degrees from the vertical, close to my full bodyweight couldn’t make it slide. As I increased the lean angle, there did come a point where the tyre slid only with high in-plane loading, but the effect was tiny. That is, the angle through which variation in force (from barely more than the weight of the wheel to almost my full bodyweight) determined slippage was extremely small – no more than a couple of degrees it seemed to me.

From this and my loose grasp of theory I don’t believe it’s sensible to design an experiment around varied load. The angle will have to change. Load should be around 30–50 kg to match typical real-world loads.

Conti black chili is a very grippy compound but I don't know if it is the *most* grippy.

Many such claims are made (including for my Schwalbe One tyres that I believe to be terrible in the wet), but experience has shown that people believe all sorts of things for reasons unconnected to fact. So I think even an approximate test would be useful.

It's much more to do with pressure, and riding style.

I suspect tyre compound matters more than you think, but hardly anyone has any real clue about the scale of the differences. Maybe Team Sky knows something – they changed to Continental tubulars after a spate of wet crashes on Veloflex rubber.

In any case, I think tyre pressure mainly affects grip by offering suspension to keep the tyre in contact with the road. If so, we don’t need tests to know that lower pressures help. The interesting story is in the compound side of things.

[Samuel D]

ACyclingRooster: I think (again!) that your Judo example is more akin to a car than a bicycle. Would you not agree that a broomstick is easier to balance on a fingertip than a pencil? If so, why would the same not be true for tall and short cyclists?

[Cudzoziemiec]

Wouldn't the test have to take into account the fact that the tyre is rotating? A stationary tyre on an angled plate might not give the same results.

Why, though? My guess is it doesn’t matter whether the tyre is spinning or not. Similarly, braking and cornering grip amount to the same thing.

Because if the tyre is rotating, it's already losing and remaking contact constantly. And because in practice, tyres don't wear evenly all round their circumference.

Braking and cornering grip are different because they use different parts of the tyre. Many tyres have different compounds or tread patterns on the centre and the shoulders. And the asymmetric load on the tyre might cause it to take on a slightly different cross section.

[Cudzoziemiec]

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

[zigzag]

from the myriad of different tyres i rode and by talking to other riders who consistently race in bunches and take their tyres to the limit, i don't think i need some dedicated (and most likely flawed) lab test to tell me which tyres grip best. my recent (i.e. last year) fall was on a steep damp descent approaching a hairpin. lots of tyre skidmarks on the road, the surface was smoothed out and covered by rubber particles - it was all masked by the rain water. not sure any knowledge of tyre grip would have saved me from 10m skid on my hip in those conditions.

[Samuel D]

Cudzoziemiec: you might be right about the rotating, in that tyres can ‘walk’ (slip angle) when cornering but do not in a static test, though I wonder if a static test would anyway rank tyres in the correct order. Good point about tyres with different compounds across the tread.

Zigzag: I maintain that we all (me included) believe subtle things with insufficient evidence and sometimes get it wrong. Besides, we can’t possibly notice small differences in grip that may nevertheless be the difference between falling off and not even knowing how close we were to catastrophe. It would be interesting to know those small differences.

Thanks for the discussion. I am not well-placed to conduct this experiment, but I might anyway try to rig up something crude. Might it even be possible to get some information by pressing an unmounted tyre against a surface with a known load and measuring the tug needed to cause it to break into motion?

[Brucey]

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.

cheers