Lowering tyre pressures when wet

HertsG

Science or snake oil?

I don't do it in my car or on my motorcycle. Why is my bicycle any different?

ugo.santalucia

Neither your car nor your motorcycle operates at 100 PSI though.
Lower pressure gives you more contact surface with he road...

drlodge

What Ugo said. On Sunday I reduced my pressures down by 5-10 psi as it was forecast wet and it gives the tyre a bit more "bulge" and hence more contact area with the road.

You don't do this on a road car, but take a car on snow with big tyres and they often run at very low/reduced pressures following the same principle.

HertsG

drlodge wrote:

What Ugo said. On Sunday I reduced my pressures down by 5-10 psi as it was forecast wet and it gives the tyre a bit more "bulge" and hence more contact area with the road.

That's kinda why I pose the question - because on the Ride London I ran my tyres at 110psi and had no issues whatsoever. So I'm thinking that if I'd lowered them, the tyres would be squidging about on their sidewalls and I'd more likely be off.

drlodge

HertsG wrote:

drlodge wrote:

What Ugo said. On Sunday I reduced my pressures down by 5-10 psi as it was forecast wet and it gives the tyre a bit more "bulge" and hence more contact area with the road.

That's kinda why I pose the question - because on the Ride London I ran my tyres at 110psi and had no issues whatsoever. So I'm thinking that if I'd lowered them, the tyres would be squidging about on their sidewalls and I'd more likely be off.

Not unless you reduced the pressures to a silly amount. We're talking about a slight lowering of the pressures say from 110 to 100 or possibly 90.

ai_1

HertsG wrote:

drlodge wrote:

What Ugo said. On Sunday I reduced my pressures down by 5-10 psi as it was forecast wet and it gives the tyre a bit more "bulge" and hence more contact area with the road.

That's kinda why I pose the question - because on the Ride London I ran my tyres at 110psi and had no issues whatsoever. So I'm thinking that if I'd lowered them, the tyres would be squidging about on their sidewalls and I'd more likely be off.

As with many things, tyre pressure isn't black and white. There's a broad usable range with a narrower range within which most people will find their preference. The fact that you're having no issues with 110psi doesn't mean all other pressures are wrong or that 110psi is the optimum. I now run my tyres significantly softer (10-15psi less) than I used to but still 10-15psi higher than pressures I've previously tried and found acceptable.
For example I briefly used 115-120psi on the rear with my 25mm tyres. Then I tried dropping the pressure a little at a time and ended up around 80-85psi still with no problems. The front tyre was initially around 100psi and I've tried it down to about 70psi. Comfort definitely improves with lower pressure. I've never had a pinch puncture on these but reduce the pressure too far and you'll likely start to get some. I like to sprint from time to time and I often spend some time out of the saddle on climbs. The bike moves around a bit more at lower pressures and I find the front tyre mushes a bit too much when I'm out of the saddle if the pressure is below about 80psi. I ride on both good and horrendous roads and weigh about 86kg.
So I've settled on about 85 front, 100 rear. I sometimes drop a little lower for long rides on bad roads or in the wet and I sometimes raise it a smidge if I expect to be on good roads and plan to do some sprints.

But back to the original point: You won't suddenly find your bike unroadworthy because you drop the pressure from 110psi to 100psi. If you dropped it to 50psi you might have some problems with mushy handling and pinch punctures but the fact that 110psi was fine on your recent ride doesn't mean anything lower will be a disaster, far from it. Try it.

meanredspider

It's always an interesting question. When I started car racing, I contacted Yokohama because, when it was wet, half the paddock lowered their tyre pressures and the other half increased them (we were all running the same tyre - albeit on different cars). The Yoko guy basically said that (and here's the mostly relevant bit) regardless of anything, you're looking to run the tyre at it's optimum pressure (about 24psi for the Yoko).

My question to those who would reduce the tyre pressure in the wet is why do you think a larger contact patch is an improvement? Equally, why is a smaller one (in the dry) an improvement? The contact load remains the same - so whilst you increase the contact patch, you reduce the pressure on the ground at each point. The reason for fat tyres in other situations (say mud or sand) is that the surface is unstable so you need to spread the load. The reason why cars have fatter tyres is often to do with heat management and the work the rubber is doing rather than grip. Snow tyres for rally cars are like pizza cutters (increased local pressure). None of this applies to a road bike. Personally I'd suggest that the ideal pressure is the ideal pressure regardless of water.

I'm interested, Ai_1, in your approach. Is comfort the key for you? Whenever I've run (25c) tyres at those pressures (usually unintentionally, I should add), I've noticed (and measured) the increased rolling resistance. I know the pros run their pave tyres at around 85psi but I think that's as much to do with reducing the physical stresses from riding on cobbles than anything.

chrisaonabike

HertsG wrote:

That's kinda why I pose the question - because on the Ride London I ran my tyres at 110psi and had no issues whatsoever. So I'm thinking that if I'd lowered them, the tyres would be squidging about on their sidewalls and I'd more likely be off.

I've been wondering about this too. In the Ride London emails they were advising reducing pressure by 10-15 PSI because of the wet.

I figured I'd be taking corners very slow indeed, so why would I need extra grip? So I used my normal pressures (105 at the back, 95 at the front), and had no issues with grip, and no punctures.

Impossible to prove, of course, but I did wonder if the heeowge number of punctures was anything to do with people riding with softer tyres.

drlodge

Good and interesting points MRS. The question I am asking myself and haven't found an answer by googling is this - is the maxmium static friction force proportional to the pressure applied?

If yes, then it would not matter what the tyre pressure is since lowering the tyre pressure would increase the surface area and hence the two would cancel each other out (lower pressure and higher surface area to give the same force). But if the relationship is non-proportional then lowering the pressures would help i.e. a large surface area with low pressure would have a greater maximum static friction force than a small surface area with high pressure.

keef66

I too have experimented with different pressures in my 25mm GP4 seasons. Optimum for me seems to be 80 rear and 70 front. I'm looking for comfort first and foremost, then pinch flat protection, then grip / control. I don't lower them in the wet, just corner a bit slower.

monkimark

It never occurred to me to lower the pressures for wet riding. I still want the optimum amount of grip in the dry so they just stay at a happy medium all the time.

meanredspider

drlodge wrote:

Good and interesting points MRS. The question I am asking myself and haven't found an answer by googling is this - is the maxmium static friction force proportional to the pressure applied?

If yes, then it would not matter what the tyre pressure is since lowering the tyre pressure would increase the surface area and hence the two would cancel each other out (lower pressure and higher surface area to give the same force). But if the relationship is non-proportional then lowering the pressures would help i.e. a large surface area with low pressure would have a greater maximum static friction force than a small surface area with high pressure.

I think it's quite a bit more complex than static friction, DrL. Most of my knowledge comes from car racing but, in that situation, the tyre actually generates its peak cornering force whilst there's a degree of slip. The load also takes a discreet amount of time to generate (hence the technique of "hinting": applying a little steering first before applying full steering input).

I'm just skeptical of the lowering pressures bit. As has already been pointed out, I think, how does anybody proposing this advice know what you're starting at? Certainly, anybody running 70psi (wow) on a 25c tyre in the dry will end up with something very mushy dropping 10-15psi. Does anyone have any references to suggest lower pressures are better? And why isn't that better ALL the time?

Whilst it's unrelated, it wet conditions, we actually increase the pressure in the race tyres. That's because they won't generate heat in order to increase pressure themselves. In the dry, a tyre I might start at 18psi will be up to 24psi after a lap or two. The same tyre I'll start with 24psi in proper wet conditions. The point is, though, that the optimum pressure is the optimum pressure.

But I'd certainly be interested to learn more on the bike tyre situation

gloomyandy

One potential issue is that the road surface may not be the same when it is wet. Typically there is a lot more grit and other junk on the roads in wet conditions. I assume that a lower pressure tyre will deform more easily around/over such objects and will perhaps maintain better contact with the road. So maybe that is a reason to lower the pressure?

drlodge

Good point by gloomyandy - lower pressure in the tyre means the tyre is less likely to roll or slip over the cr@p on the road whereas a softer tyre stands more chance of deforming. This won't be the case for car tyres that are quite hard, but our bike tyres are pretty supple.

I also thought about car tyres on performance vehicles - they have wider tyres which means there is more surface area in contact at a lower pressure than a similar narrower tyre (assuming the force is the same, pressure will reduce as surface area is increased). Intuitively I can't help thinking that lower pressure and increasing surface area must increase grip.

lapavoni10

Car tyres are a different beast really. They sit square (or relatively square) on the road and have a flat contact surface. Optimum pressure is therefor when most of the tyre touches the road?

With bike tyres (and probably motorbike tyres), the tyre profile is very different. The contact patch is very small. Reducing the pressure increases the contact area and therefore reduces the chance of losing grip....not increasing friction as we know that friction has nothing really to do with area. All roads (concrete or ashpalt) are very irregular compared to the contact area of the tyre.
If the surface we are running on is completely flat with no irregularities you will be fine running at 200psi as long as the tyre can take it....but the only place you will find that is on a wooden track (although I would not try that in the wet )

drlodge

lapavoni10 wrote:

Reducing the pressure increases the contact area and therefore reduces the chance of losing grip....not increasing friction as we know that friction has nothing really to do with area.

:? Don't get this statement - if reducing pressure increases the contact area and improves grip, what is increased that improves the grip if its not friction (static friction that is)?

lapavoni10

Yeh...I struggled explaining that

You improve grip by reducing the possibility of sliding off the irregularities in the road. So if you look at an anti-skid surface, or a road that has been top-dressed and stable....there are lots of peaks and troughs...you want some of the tyre to either be in a trough, and pusing against a peak....having the tyre softer means it will mould better into these.

drlodge

lapavoni10 wrote:

Yeh...I struggled explaining that

You improve grip by reducing the possibility of sliding off the irregularities in the road. So if you look at an anti-skid surface, or a road that has been top-dressed and stable....there are lots of peaks and troughs...you want some of the tyre to either be in a trough, and pusing against a peak....having the tyre softer means it will mould better into these.

Thanks...sounds like increased static friction to me!

meanredspider

Still - no-one has adequately explained why you'd want a different pressure when wet as all the arguments made (whether they are correct or not and I'm not sure they're correct) apply equally to wet or dry roads. Are we saying we don't want the best grip when it's dry? I'm open-minded to hear a good reason but so far it doesn't stack up to me.

hypster

meanredspider wrote:

Still - no-one has adequately explained why you'd want a different pressure when wet as all the arguments made (whether they are correct or not and I'm not sure they're correct) apply equally to wet or dry roads. Are we saying we don't want the best grip when it's dry? I'm open-minded to hear a good reason but so far it doesn't stack up to me.

Could it be that running at lower pressure there is a trade-off with rolling resistance? Maybe less rolling resistance at say 90-100 psi with slightly less grip. More grip at 80-90 psi but more rolling resistance? Just a guess but wet or dry roads could be just irrelevant other than on a wet road you could be more interested in grip than rolling resistance.

meanredspider

hypster wrote:

meanredspider wrote:

Still - no-one has adequately explained why you'd want a different pressure when wet as all the arguments made (whether they are correct or not and I'm not sure they're correct) apply equally to wet or dry roads. Are we saying we don't want the best grip when it's dry? I'm open-minded to hear a good reason but so far it doesn't stack up to me.

Could it be that running at lower pressure there is a trade-off with rolling resistance? Maybe less rolling resistance at say 90-100 psi with slightly less grip. More grip at 80-90 psi but more rolling resistance? Just a guess but wet or dry roads could be just irrelevant.

Possibly something marginal there

hypster

meanredspider wrote:

Possibly something marginal there

I would agree, it's never occurred to me to run at lower pressure in the wet although I tend to run my tyres at lower pressures all the time these days anyway.

mitchgixer6

meanredspider wrote:

Still - no-one has adequately explained why you'd want a different pressure when wet as all the arguments made (whether they are correct or not and I'm not sure they're correct) apply equally to wet or dry roads. Are we saying we don't want the best grip when it's dry? I'm open-minded to hear a good reason but so far it doesn't stack up to me.

Surely the answer to this is self explanatory? Wet roads will offer up less grip than dry roads hence the requirement for increased grip in the wet.

monkimark

but why would you want less than optimal grip in the dry?

mitchgixer6

It's not less than optimal. If it's dry then there's more grip on the road surface, so ergo the grip is *more optimal* than it is in the wet for the same pressures.

keezx

That might be the point with cars and motorcycles, but I don't think aquaplaning plays a role in cycling , given the lower speeds.
There should be a formula that includes mass, tyre surface and speed.

Interesting question though...

davem399

I read somewhere that to aquaplane on a bike, your speed needs to be well over 100mph.

In the wet, my main concerns are avoiding things with less grip like white lines and manhole covers, and looking out for oil and diesel spills.

keezx

Yes, really dangerous are locations with unexpected less grip.
Some loose grit in a hairpin can kill you.

nferrar

It's a fair point that if it helps in the wet then why not do it in the dry to, I'd guess the main reason is grip is so drastically reduced in the wet that it's worth the extra risk of pinch flats to get more grip back for safety whereas in the dry it would just make you be able to corner slightly faster (rather than being a safety thing) so it's not worth the compromise.
I personally don't bother reducing my tyre pressures in the wet as I prefer to take it a bit easier when cornering (fixing a flat in the wet isn't something I want to increase the chances of having to do ). That said I don't think it's snake oil, it's a pretty standard thing for pros to do (even mid-race if it starts raining) and given the amount they test equipment these days I doubt they'd still be doing it for superstitious/we've always done it but don't know why type reasons.

nicklouse

DaveM399 wrote:

I read somewhere that to aquaplane on a bike, your speed needs to be well over 100mph.

In the wet, my main concerns are avoiding things with less grip like white lines and manhole covers, and looking out for oil and diesel spills.

it depends on the tyre pressure. if you lower the presure you also lower the Speed that a Bike could aquaplane.

so of you could do will doing some reading.

http://www.sheldonbrown.com/tires.html