Tyre pressure physics

[Tim Hall]

Watching the racing on tv just now, that nice David Millar said that in wet conditions the riders run their tyres at lower pressures than in the dry, to improve grip. Thus if riders set out with "dry" pressure tyres and it lashes down with rain they're more likely to crash as the tyres will be too hard and won't grip.

This sounds a bit odd to me.

What say the physics brains of YACF ?

[quixoticgeek]

Watching the racing on tv just now, that nice David Millar said that in wet conditions the riders run their tyres at lower pressures than in the dry, to improve grip. Thus if riders set out with "dry" pressure tyres and it lashes down with rain they're more likely to crash as the tyres will be too hard and won't grip.

This sounds a bit odd to me.

What say the physics brains of YACF ?

Lower pressure increases the size of the contact patch, bigger contact patch, more grip (possibly more rolling resistance too). Wet road equals less grip, so increase the contact patch by reducing pressure, can offset some of the loss of trip from the wet...

J

[Tim Hall]

Watching the racing on tv just now, that nice David Millar said that in wet conditions the riders run their tyres at lower pressures than in the dry, to improve grip. Thus if riders set out with "dry" pressure tyres and it lashes down with rain they're more likely to crash as the tyres will be too hard and won't grip.

This sounds a bit odd to me.

What say the physics brains of YACF ?

Lower pressure increases the size of the contact patch, bigger contact patch, more grip (possibly more rolling resistance too). Wet road equals less grip, so increase the contact patch by reducing pressure, can offset some of the loss of trip from the wet...

J

I think that frictional force is independent of area/ contact patch size though, just due to the coefficient of friction and the force normal to the surface. But I can't be sure.

[LittleWheelsandBig]

Theoretically friction is independent of contact area but the real world doesn't have infinitely flexible tyre casings and other simplifying assumptions. A little more pneumatic suspension can help keep the rubber side down in marginal conditions.

[drossall]

LWaB's argument makes sense to me. Physics is all about beams of infinite length and zero width and stuff. The real world can be different (and I did a physics degree). On a similar theme, I was once told that riders in the know would use slicks in the wet, for exactly the same reasons - maximise the contact patch. This is (obviously) quite different from what you'd do in a car, where the contact patch is massive and you're trying to use tread to get the water away. I believe that the best available estimates (albeit not necessarily that good) suggest that the small contact patches on bikes mean that we aren't likely to aquaplane below 200km/hr, which personally has never been more than a slight possibility.

[mattc]

When I die, I want to be aquaplaning on my bike at 200kph, with the flag of Texas on my back ... etc etc ... 

[Greenbank]

The speed required to aquaplaning is related to the pressure of the tyre, not the contact patch size.

There's a section on hydroplaning here: https://www.sheldonbrown.com/tires.html

Back to the original question, roads are imperfect surfaces as are tyres. A lower pressure tyre will have a larger contact area which, for a tyre with some tread (or even just an non-perfect slick) will increase the chance that the tyre snags on the imperfect road surface.

[Brucey]

Theoretically friction is independent of contact area but the real world doesn't have infinitely flexible tyre casings and other simplifying assumptions. A little more pneumatic suspension can help keep the rubber side down in marginal conditions.

I was just about to make a similar comment; loss of grip can simply  be caused by a tyre bouncing slightly on a less-than-perfect road surface. Obviously it can't grip the road if it is not touching it....  running softer tyres allows more of the tyre to be in contact with the road more of the time.

BTW because the lateral grip of the tyre is a non-linear function of contact pressure, it is possible that the grip (on a road that isn't bumpy) will sometimes be reduced when the tyre is run at low pressures and the contact patch size is increased.

Also, NB; you don't have to aquaplane in order to lose grip or to start skidding. It is almost irrelevant to fixate upon that particular phenomenon.

cheers

[sojournermike]

I suspect that wet grip may be improved as a consequence of dropping pressures slightly. However, more importantly, the feeling of security and the predictability of the tyre’s behaviour is improved, making the rider feel more secure and able to use the grip available. I like file treads because they seem nice to ride, but I try and avoid approaching grip limits on any bike at any time!

[zigzag]

i have always wondered how we can tell that one tyre is slippery and another is grippy without falling off or skidding on the road. usually when the tyres loose grip on tarmac in a turn it's very difficult or impossible to recover, but somehow we are able to tell that some tyres have good grip and other have poor grip. there must be some micro skidding going on well before the skid limit, that the brain is able to detect quite well?

[Samuel D]

I think the brain’s power of imagination exceeds its power of observation here.

However, it may be that the brain detects the tyre’s lateral stiffness and slip angle while cornering. (These things aren’t simply equivalent to grip although slip angle is tenuously related.) I have seen studies showing that bicycle tyres have large variations in slip angle for a given lateral force and that narrower tyres of the same construction have greater slip angles.

Since slip angles are a big part of motorcycle handling, it seems reasonable that slip angles would also matter in bicycle tyres.

On the original question: lower pressures improve grip (in the dry as in the wet but often only relevant in wet conditions) because grip comes not only from molecular adhesion (the high-school physics model of friction) but also from hysteresis of the tread compound and wear. These sources of grip improve with the size of the contact patch, at least in the range of contact pressures we’re talking about in bicycles.

On rougher roads especially, pneumatic suspension becomes important to keep at least some of the tyre in contact with the road.

[zigzag]

I think the brain’s power of imagination exceeds its power of observation here.

if that is imagination, why is it different for different tyres? imagination must be based on sensations that tyres provide. how do we sense that some tyres are grippy and some slippery without actually skidding?

[Brucey]

that high pressure tyres bounce a little over surface irregularities is not in the slightest bit imaginary.

It is easy to assume that the tyres just stick or they don't, in which case you skid. It is actually more complicated than that. In other fields of tyre use, the matter of 'slip angles' is well established.  This is the difference between the direction the wheel is pointed, and the actual direction a wheel runs over the surface.

It comprises two main components;

1) carcass deformation (so the tread can be 'aimed' slightly differently from the wheel) and
2) apparent slippage between the tyre tread and the road as the wheel rolls. This can be real slippage or that induced by tread deformation as the tyre rolls.

The slip angle varies with the tyre loading (both vertical and lateral) and road conditions, so some care is taken (eg with car tyres) to make sure that the geometry  settings etc allow for the slip angle of the tyres. It sometimes the case that you can fit car tyres that wear badly (even though the geometry is 'correct') and a possible reason for the bad wear is that the tyres have a different slip angle.

On two wheelers, the rider might feel a difference in slip angle if it occurs suddenly but otherwise you just adapt and get on with it. But if you hit a wet patch mid-corner, you may well feel an unsettling feeling, without actually skidding badly; this could well be a change in the slip angle of the tyre. Without any intervention or steering movement on your part the line you are taking round the corner changes.

cheers

[Samuel D]

I think the brain’s power of imagination exceeds its power of observation here.

if that is imagination, why is it different for different tyres?

Odd question. The nature of imagination is that it supplies the reasons.

Tyres like other products develop reputations that are only partially reality-based. Years ago photographers widely believed Kodak films to be warmer-toned than Fujifilm. Why? Merely because Kodak came in a yellow-red box and Fujifilm came in a blue-green one.

Confirmation bias is the great plague of our age and we are all susceptible to it. Arguably those with the greatest accumulation of theoretical and experimental knowledge are the most vulnerable.

But going back to the specifics of tyres. If the tyre does not slip, clearly we cannot estimate its grip. And if it slips in a corner, most of us, most of the time, will fall. And even then we cannot say with useful accuracy whether it slipped earlier or later than another tyre would have.

Tyre hiss while cornering (unfortunately mostly in the dry when we don’t need this info and less so in the wet when we do) may be useful feedback. But even that isn’t much use for comparing different tyres that sound different for a variety of non-grip reasons.

I think our only vaguely reliable ways of gauging traction are to note wheel-spin while climbing a wet hill and to check how much braking it takes to lock up the rear wheel on a wet road and the subsequent deceleration. These crude tests can be surprisingly useful over a period of months on roads and bicycles we know well.

I do wonder if variation in slip angle can somehow be detected while riding, but even if it can be, it isn’t in proportion to grip so doesn’t tell us much about that. It may mislead us, though.

Reviewers who make proclamations of cornering grip from a 50 km test ride are making it up.

[rogerzilla]

I doubt it makes all that much difference (you probably can't tell whether you have 60psi or 100psi in a touring tyre) but pro riders and their mechanics cling to lots of myth and lore.  My favourite one is Anquetil (I think) moving his water bottle from bike to jersey pocket before a climb, because it made the bike lighter.

[zigzag]

it still does not answer my question why some tyres after a few fast corners encourage to push even harder next time while others give warning (without the actual skidding) to back off. imagination or not there are actual differences near grip limit between different tyres of the same size and pressure, so interested to know what makes us perceive those differences.

[rogerzilla]

Rubber compound is very important.  The suppleness of the casing is just as important.  Some of the early tyres with Kevlar puncture protection belts were very stiff and consequently skittish in the wet.

[sojournermike]

I doubt it makes all that much difference (you probably can't tell whether you have 60psi or 100psi in a touring tyre) but pro riders and their mechanics cling to lots of myth and lore.  My favourite one is Anquetil (I think) moving his water bottle from bike to jersey pocket before a climb, because it made the bike lighter.

I think there’s a fairly significant difference in how my 31mm wide 28s feel at 60 and 100 psi. Don’t know about grip, but at 100 I get fairly well beaten up.

[Brucey]

 .... If the tyre does not slip, clearly we cannot estimate its grip. And if it slips in a corner, most of us, most of the time, will fall.....

sorry, that is just not true. Good tyres develop fairly obvious changes in behaviour before they let go completely; as I mentioned earlier abrupt changes in slip angle are pretty obvious.

You will (with some practice and a good tyre)  feel and hear that the tyre is about to let go before it does. I prefer a rib and file tread design for this purpose. My idea of a bad arrangement is a typical slick tyre in the wet; IME there is very little warning before you are on your ear.

 When I was a nipper I rode in races that were like crits, but with corners that were only just possible to take at speed; I did fall off when I (just) grounded my pedals but the tyres gave enough feedback that I didn't push past their limit and fall that way.

Learning the absolute limits of tyres can be painful and expensive; but then education never came cheap...

cheers

[Samuel D]

If the changes in behaviour before a tyre lets go are obvious, it should be easy to put them in words.

Mine are these: as cornering force increases and the cause of slip angle changes from mainly elastic to include a significant scrubbing component, the tyre makes more noise. The volume of this hissing increases without abrupt change in character as more of the contact patch scrubs until without further warning the tyre slides.

This doesn’t tell me with useful precision when the tyre will slide, although of course I have my suspicions informed by past experiences of lean angles, the nature of the road surface, and all the other clues about safe cornering speed. These past experiences may give me false confidence that I can determine when the tyre will slip from the hissing volume.

In the wet, where this matters more because the traction limit is likely to be approached more often, cornering forces are lower and the hissing is reduced (and harder to hear over water noise) even though the elastic-to-scrubbing transition happens earlier.

You could treat any significant scrubbing (i.e. hissing) as the limit, but that’s harmfully conservative.

In all cases, steering angles are low (usually a degree or two in high-speed cornering) and variations in slip angle, although large from tyre to tyre, are small in terms of absolute steering angle. Since steering a bicycle is a feedback loop anyway, I can’t see how we can make use of this. Maybe we feel more than I think.

If abrupt changes in slip angle can be detected above the general noise of steering (balancing a bicycle around a corner is never a steady-state affair at the best of times), that only tells us the road changed. We might have our suspicions that a slide is more or less imminent after this abrupt change, but knowledge of the break-point remains as elusive as ever at any given instant.

[Brucey]

I think that most folk feel through the steering more than you suppose. You will tend to feel the front 'wash out' before it develops into a full-blown skid. The same thing happens on motorcycles.

When you are cornering, every bump, every pedal stroke, every input into the steering, every weight shift becomes an opportunity for the tyre to start moving around and this will feed back to you through the handlebars and through the saddle.

If you spend enough time riding offroad on slippy surfaces, you may start to 'listen better' with your hands/backside.

 When I was a kid I used to ride any old bike I could get my hands on, in a wilderness area near my home. I had a home-built bike that looked a bit like an MTB with riser bars etc, in 1970-something, except it usually had a three speed hub and road tyres; two-wheel drifts were de rigeur.... I carried on doing this, for fun, even when I had a proper road bike; buying a full blown MTB once I was able to  was just 'more of the same' for me.

 I thoroughly recommend riding offroad using tyres that basically don't have enough grip, if you want to improve your feel and your bike handling. There are certain things that you can't do when riding a dropped handlebar bike on the road, but a surprising amount of it is useful and does translate.

cheers

[Samuel D]

Motorcycles have some important differences to bicycles, though. Obviously you know this but the ones that seem relevant to me are:

• they have power while bicycles have none in corners steep enough to usually worry about traction
• the ratio of their tyre’s major radius to minor radius is much smaller
• they lean at steeper angles.

Consequently, a motorcycle tyre scrubs continuously while cornering because its contact patch is always rotating on itself, if you get what I’m saying. There is scrub quite aside from extreme slip-angle effects because the plane of the wheel is closer to horizontal than vertical and the contact patch is large. Since the tyre is always slipping, translational slip starts more gradually, in addition to the fact that the variable application of power itself enables control.

I rode motorcycles for years and saw how easy it was to control a rear-wheel spin coming out of a corner. I don’t find the same on a bicycle with high-pressure tyres. Perhaps an off-road bicycle is a lot closer to the motorcycle experience.

I take your point about constant small slips from surface irregularities and dynamic weight shifts. The more I think about it, the more I think those must be crucial to any relevant feedback about grip, the others seeming implausible to me. Some of my friends have claimed to recover from short slips in corners. They say this as if it was their merit. I think what happens is the wheel slips when provoked by local bad traction, unweighting, etc., and then almost instantly regains traction as the provoking condition passes. At that point the bicycle is off-balance and the rider may start a correction (although bicycles self-correct too). I don’t see how the rider does anything to regain traction while in the slip, since its entire duration is usually below average human reaction times (as can be determined from slip distance versus bike length versus estimated speed in slow-motion helicopter footage of pro racers slipping in corners and then regaining balance. In the recent Hammer Limburg series there was a slippery cycle path on one corner of the lap which racers repeatedly slipped on, giving us a chance to see this again and again).

[Brucey]

it has been shown that if you practice enough, it is possible to programme your brain to react without conscious thought, i.e. more like a reflex action. This has been demonstrated by taking a good racing driver and placing obstacles on blind corner apexes, then monitoring the speed of response in steering action vs when the obstacle was first visible. The reaction time can be less than 0.2s, IIRC.

When cornering and you feel the bike start to slide, you can often catch it by quickly shifting your weight sideways; this allows the tyres to regain traction (in a time period less that one second) but leaves you with a fairly violent imbalance to correct. The imbalance however needs to be sorted out in the next 1-2 seconds before you are in any real danger of falling, i.e. a longer period of time, during which you hopefully have got yourself into a situation where the tyre grip is not exceeded.

If you have enough practice, and you ride 'light in the saddle' the initial weight shift can be done is less time than you might expect, i.e. I think it can become almost reflexive in nature.

Motorbikes are different in many ways to bicycles, but you can usually feel the front end start to wash out on a greasy road, and do something about it, if you are lucky and you are not pushing too hard.

cheers

[sojournermike]

When I was teaching maths I had a poster of a motorcycle exiting a corner on a track, leaned at what looked like 30 degrees to the horizontal (although that may have been 35) with the front wheel airborne and the bike completing the turn in an apparently stable manner. I used to ask the brighter kids to think about how that worked and then come and tell me:)

[rogerzilla]

I've had a few front wheel slides on bikes.  They are usually recoverable on tarmac, less so on gravel.  The two main causes of recoverable slides on corners are white lines, which you can avoid, and diesel spills, which you can't always see.  Off-camber roundabouts where some previous muppet driver has overfilled his diesel tank can be terrifying.  I once had to take the exit before the one I wanted because the bike was running so wide.