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Re: Tyres for the wet

Posted: 11 Nov 2009, 11:21am
by hamster
I think that there's a difference between aquaplaning (where the tyre floats on a cushion of water) and a slight water film between tyre and tarmac. Definitely there is less grip from the water film, but not as dramatic as aquaplaning.

I found that tyres seem very slithery in the wet when new, I put it down to the waxy release agent on the tyre from moulding. However, my Conti Contact seems noticeably worse on my tourer than the Tioga or Hutchison tyres I used before.

Re: Tyres for the wet

Posted: 11 Nov 2009, 5:24pm
by niggle
Just returned home from a 5 mile commute in a cloud burst, with standing water running across the road in streams several inches deep in quite a few places not to mention the gravel banks washed onto the road from the fields by said streams (so an average winter evening in Cornwall then :roll: ). No issues with grip though I did avoid the gravel carefully: this was on the Carrera Subway 8 with 26"x1.6" Continental Sport Contacts.

Re: Tyres for the wet

Posted: 26 Nov 2009, 2:10pm
by thekelticfringe
Not wanting to tempt fate, but things have been looking up, with more grip seeming to be available, combined with more focus on cornering technique. Maybe just greasy roads after the 'summer' and a bit of wayward enthusiasm on my part. Thanks for the advice and suggestions.

Re: Tyres for the wet

Posted: 27 Nov 2009, 9:22pm
by SilverBadge
thekelticfringe wrote:I got a new fast commuter this summer, which came with Bontrager Select K 700x25 tyres. These were fine across dry weather but now that autumn is here I have had a number of pretty scary slides, both on the road and on Sustrans type surfaces. I'm running the tyres at their recommended pressure (about 100psi) and am reasonably experienced rider on road and MTB.

Thing is; I can't decide if it is the tyres, the new bike, simply that I'm being too much of a hoodlum, or whether I've just had a spell of bad luck.

Has anyone else been taken by surprise in the wet using these tyres? What do people prefer to use for fast commuting. (I do about 15 miles each way on road and hard trails.

Hmmm - interesting. When I sought recommendations for successors to Michelin Performer (700x25, 300g, folding bead, long wear, approximately never puncture and come off and on rim easily if you do, plus decent wet grip - what more do you need from a winter tyre?), someone swore by Select K and my experiences have been pretty good. Conti 4seasons are good but a bit extravagant, Vittoria Evo KX were dreadful (dumped myself on the road twice with those and the second ride I was being specifically cautious) particularly for a wet specific tyre with directional tread and dual compound.

Whilst I agree about aquaplaning comments, slicks do feel skittish in the wet. Can suggest several reasons.
1) Sideways, a narrow tyre is a great shape for a hydrodynamic bearing, so
2) Any tread edges can bite into road surface roughness rather than just wrap over irregularities
3) Any tread allows rubber to squirm and thus effectively softens the compound.

Rain after a dry spell is always bad for greasy roads. I also find that headset issues are far more apparent on wet roads.

Re: Tyres for the wet

Posted: 28 Nov 2009, 12:18pm
by thekelticfringe
Yes - I think aquaplaning isn't an issue - if you think about it, my weight (C 75kgs) plus bike and baggage must come to about 85-90Kgs. Contact area is about "a postage stamp" per tyre. If you call that a contact patch of 1 cm wide by 2 cm long (or 2cmsq) per tyre we're up to anywhere between 20 and 30 Kgs/cm2 which is a fairly astonishing 450 psi.

What I have done is reduced pressure a bit in the front tyre, now running at about 85psi. I have to be more careful over rough stuff, curb edges and potholes to avoid pinching, but makes the front feel more secure.

Re: Tyres for the wet

Posted: 28 Nov 2009, 12:28pm
by meic
A few flaws in the assumptions made for your calculations there.

Firstly front and rear tyres are in greatly differing states of load so can not be treated as the same, you need seperate calculations for each.
These calculations need rewriting as soon as you touch the brakes.

The figure of 450psi means you must have made an error somewhere as the contact pressure has to be very close to the actual air pressure in that tyre.

Re: Tyres for the wet

Posted: 28 Nov 2009, 12:37pm
by thekelticfringe
Hmm - interesting.

Totally accept yr point about front and rear being different depending on geometry, riding style and dynamic loading etc. Only aiming at a very general calculation.

Maybe the conversion ratio is wrong then? Google offered me a factor of 0.07 to convert between Kg/cm2 and psi. So I took the kg/cm2 and divided by .07 to get my (admittedly surprisingly high) psi ??

Re: Tyres for the wet

Posted: 28 Nov 2009, 12:57pm
by meic
I had never thought to question the maths behind the assumption that cycles cant aquaplane.

On considering it a little, I think that the calculations are for an equally loaded, static bike.
Now any real ilfe situation will only make aquaplaning less likely.

I did think of occasions when the front end is so light that it has negligable contact pressure and could aquaplane, not a problem, this only occurs in situations that are well under control. The back wheel is fully loaded and stable and the front wheel often leaves the ground anyway with no bad consequences.

When you are heavy braking the same thing happens. At "perfect" braking all the weight is on the front wheel preventing it from aquaplaning and the back wheel can quite literally fly for all we care. :D

So when you crash it will just be good old fashioned slipping on non-grippy road surfaces, how reasuring. :D especially if you have slicks.

Re: Tyres for the wet

Posted: 28 Nov 2009, 1:36pm
by niggle
To convert kg/cm² to psi the factor is 14.2, (the previous poster must have had the conversion the wrong way round, 1/14.2=0.07). Now assuming equal load on both tyres the 90kg load will be split to 45kg per tyre, but that is over a 2cm² contact patch, so that will be 22.5kg/cm², which is 22.5x14.2= 319psi.

The discrepancy between front and back tyres will vary according to riding position (more upright will cause less load on front, low set drop bars will make body weight act almost equally), position of loads carried on bike and as said the loading due to braking forces which will put more weight onto the front at that time.

The pressure cannot be equal to the internal tyre pressure as that varies so much according to the size of the tyre, a big 2.2" MTb tyre will be optimally inflated at about 35psi (it should probably be less but you have to guard against pinch flats on off road terrain) whereas a 23mm road bike tyre should probably be run at something around 130psi, but none of them at 300psi...

EDIT: actually thinking about it the issue may be wrong estimation of contact patch size, if the patch was 5cm² then that would give a psi of 130 (= the 23mm tyre?) and if the contact patch was double on a tyre twice as wide, i.e. 10cm² then that would give a psi of 65 which is the recommended pressure on the 42mm wide Sport Contacts on my commuter.

Re: Tyres for the wet

Posted: 28 Nov 2009, 1:54pm
by niggle
meic wrote:When you are heavy braking the same thing happens. At "perfect" braking all the weight is on the front wheel preventing it from aquaplaning and the back wheel can quite literally fly for all we care. :D

at this point it will come off the ground and you will quickly start caring again :wink:

Re: Tyres for the wet

Posted: 28 Nov 2009, 2:34pm
by drossall
The legendary Sheldon Brown reports aircraft industry research, suggesting that bikes will only aquaplane* at speeds approaching 200km/hr.

* Hydroplane in left-pondian

Re: Tyres for the wet

Posted: 28 Nov 2009, 2:50pm
by meic
Yes that is the sort of thing that I had just accepted.

However when you stop to think about it, that aircraft industry formula MUST be for aircraft landing systems and aircraft tyres with assumptions about those tyres which DO NOT apply to cycle tyres.

Or if they are universally applicable then it appears that tyre tread has no effect whatsoever and the whole argument is based on a falsehood anyway!!!!

Re: Tyres for the wet

Posted: 28 Nov 2009, 4:11pm
by thekelticfringe
I think tread on a cycle tyre has a couple of purposes.

The first is to improve **mechanical** grip so that the knobs/grooves on the tyre can physically engage with the irregularies of the road surface - very approximately like the cogs of a rack railway. This has the benefit of grip, but also the disadvantage of drag, as the road surface is 'chaotic'. So slicks reduce rolling resistance, at the expense of some of this mechnaical grip.

The second is cosmetic.

In car tyres the tread has a clear purpose to move water out of the contact zone, but as discussed, this seems superfluous with the narrower cycle tyres.

Re: Tyres for the wet

Posted: 28 Nov 2009, 4:19pm
by meic
Going slightly off topic, the tread can be usefull for dealing with things other than water on the roads.

It is about this time of year when the residue of fallen leaves and farmer's spillages leave the road with a lot of slippy areas which my 28mm ultragators will just spin on.
So I have to swap to my bigger and treaded Marathons or Passelas. The tread seems to bite into the slime enough to give me a little bit of grip.
So you have knobblies for getting a bite in mud and I have "micro-knobblies" for country roads in autumn.

Re: Tyres for the wet

Posted: 28 Nov 2009, 4:20pm
by drossall
Well it doesn't really matter if the aircraft research is a long way out, because most bikes won't reach 50 km/hr, let alone 200.

No, I don't see how tread can lock with road surfaces. It's the deforming rubber that would do that, so the rubber compound and inflation pressure will matter more than the tread.

Bike tyres work differently from car ones. Bike tyres are hard and curved. Car tyres are wide and soft, which is why they need tread. If you think about it, on a bike, when you need grip most, you aren't on the middle bit of the tyre anyway.