Tyre pressure in the wet?

[disenfranchised]

It was a wet day in Auckland today, and it was coming up to the time that I'd normally pump up my tyres...but after a quick think about it, I wasn't sure if I should or not...so I'm asking all of you (particularly the more experienced riders)

In the wet, is it better to have tyres fully pumped to their recommended pressure? Or does having them a bit flat give you more grip.

Let me explain my thinking that got me to asking this question.

In a car, the tyre has a flat contact patch with the road, so regardless of how pumped they are, you always have the same surface area contacting the ground. If you let the tyres become flat, then the lateral forces when cornering can sheer the tyre wall away from perpendicular....allowing some momentum sideways to build in the car itself as it turns..which increases the likelihood of skidding.

In a bike, the tyre shape is curved, which should mean that a flatter tyre actually squashes giving you more surface contact with the road. Also as bikes don't get the same kind of lateral forces when cornering (or is that just the rider?) then the flatter tyre shouldn't be an issue in the same way as it is in a car.

I'm not talking about really flat....kind of in the order of 28PSI when 33 is the recommended amount.
Seeing as I've still got the Shinko's that everyone says is so dangerous, anything I can do to increase my safety during teh nasty wet weather is a good thing....and no, I can't afford new tyres

Any feedback would be much appreciated

[vifferman]

Your thinking's pretty good, but the tyre squashing itself onto the road with reduced pressure also means the stipes (rain grooves) close up a bit as the tyre deforms, so their ability to move surface water is reduced. However, most of the grip from the tyre has nothing to do with the stipes - they're just there to allow excess water to escape. So letting the tyres down a bit can mean that they get hotter and aren't cooled down as much by water so the grip can improve due to the stickiness of the tyre surface itself.

Running your tyres at reduced pressure can also make them feel like they're moving around, which can freak you out a bit and make you tense, so that's a negative. It will also decrease their resistance to punctures.

My advice is to keep the tyre pressures at close to recommended values.

[sAsLEX]

My advice is to keep the tyre pressures at close to recommended values.

Agreed road tires are designed to work across the board at the one pressure, maybe not Road Attacks or whatever the race rep tires are, but the majority of road tires would be the one pressure.

[sAsLEX]

In a car, the tyre has a flat contact patch with the road, so regardless of how pumped they are, you always have the same surface area contacting the ground.

Not true.

Under inflated tires will rest on the shoulders and over-inflated in the centre both conditions being less than nominal grip.

[codgyoleracer]

Good question & one that I have trialed on the racetrack.

For track applications, & with treaded tyres (particularily road type) its pretty tricky to get them up to their operating temperature in the wet, particularily in the winter (wetter) months. Increasing the pressure does in my experience "open"the grooves & allow water to be cleared from the road/tyre contact patch a little bit quicker. So in certain types of rain where there is more standing water this technique gave better "grip" compared to running the flatter. When water on the road is patchy of only a "damp" , then the warmer the rubber the better, so lower presuures seem to assist.
There are many other changes to suspension & riding style in reduced adhesion conditions that make quite a difference to traction at either end of the motorcycle for the track or for track days, but for the road run the recommended pressures.

[Masterchop]

Your thinking is spot on, I recently did a trackday at Phillip Island run by the California Super Bike School and the track was wet and cold early in the day.

They advised everyone there to drop their tyre pressures down to 30psi for exactly that reason of larger contact patch.
Although as a day to day idea on the road its probably not that practical.

[Crisis management]

Interesting question and a lot of even more interesting comments.
I have to admit to being of the "if it's round black and not flat it's ok" school with tyre pressures, in other words I check the pressures weekly and thats it.
My rationale is, that for road riding, the surface is so variable that being anal about tyre pressures seems a waste of time. I do put good rubber on the rims and then rely on my feel of the road and surface conditions to adjust my speed and style to suit.
Developing a feel for the amount of traction seems a better skill to have for road riding than adjusting pressures a few KPa each day.

[FROSTY:Me]

CM Has itin a nutshell. Run your tyres at normal pressures because of the increased unpredictability of the road surface/condition

[justsomeguy]

Just ride your bike, keep to your usual pressures and you'll be fine.

Take it easy until you feel more confident. How you ride in the rain I feel will be more dependant on how comfortable, warm and dry your motorcycle clothes are and how good your visibility is, than on your tyre pressures.

[TerminalAddict]

I was under the impression that a flatter than normal tyre will take longer to heat up .. fine on the track, as you actually have a chance to heat your tyres up, not so on the road.

The silica in the tyre need to be warm/hot to expel the water dunnit?

[sAsLEX]

I was under the impression that a flatter than normal tyre will take longer to heat up .. fine on the track, as you actually have a chance to heat your tyres up, not so on the road.

The silica in the tyre need to be warm/hot to expel the water dunnit?

No a flatter tire will flex and deform more creating more heat.


Silica is there to reduce rolling resistance...... I think?


The grooves in your tires are what expel the water.

[TerminalAddict]

hmmmm .. that's not what the sales guff says .. I'm sure

[sAsLEX]

hmmmm .. that's not what the sales guff says .. I'm sure

So who told you an under inflated tire will take longer to heat up?

[smoky]

I was under the impression that a flatter than normal tyre will take longer to heat up .. fine on the track, as you actually have a chance to heat your tyres up, not so on the road.

The lower tyre pressure generates more heat in the tyre, pretty much an accepted fact – unfortunately the heat is not where the tred is. And yes low tyre pressure hinders the tred from heating up. It’s the excessive flexing in the tyre wall that heats up the sides of the tyre and bead, and can damage it. I’ve removed a few tyres that have been run with low pressure – it stuffs the belting or structure of the tyre, weakening the tyre. Low pressure can result in a greater risk of tyre failure – not what you want in the wet.
Don't know about bike racing but in F1 they run the correct trye pressure to maintain good tyre shape and strength in all conditions - the temperature regulation is taken care off by using different gasses to inflate the tyre, different gasses have different heat dispersal rates

The silica in the tyre need to be warm/hot to expel the water dunnit?

No - heat in the tred softens the tred and makes it sticky, the Silicon facter is about the compound staying sticky in lower tempretures like on a wet road where water cools the tred, But has no advantage with expelling water, that's all about tred patterns.
On a road bike only - I think tread depth is more important in the wet, rather than a larger ‘foot print’ that may or may not be provided by an under inflated tyre. Tred is there to disperse water, or we would all be riding on slicks relying on sticky compound for grip. So I keep my tyre pressure up to maintain a good tyre shape, and keep the tread open.

To improve tyre grip in the wet stick to your body position, keeping the bike more upright in the corner, allowing greater tyre contact to the road and more grip.

I read somewhere that 85 per cent of ‘bins’ are a result of riders not using the brakes correctly
It would seem that very few motorcyclists practice emergency braking in the dry let alone the wet.
Going down hill in the wet at open road speeds, pillion on the back and having to stop in an emergency is a bit scary – I’m sure Terminal would’ve been history not so long ago but for his ability to stop, all that emergency stopping practice paid off dude.

[samwp]

Wow! What an interesting read. Would have been more interesting, less factual and more opinionated if some of the yoof got in here.

Oh yea and here's my spanner in the works:

Does grip increase with contact patch anyway, all things being equal?

i.e. a correctly inflated tyre on the right sized rim verse one an inch wider.

From my light research I have theorised that width increases stopping/accelerating grip and length increases cornering grip. But I feel there is alot more to it than that.