Aha... thinking about it some more, of course a steel wheel will have lower rolling resistance: This is the reason the balls and rollers in bearings are made from steel and not rubber!

So you will get lower rolling resistance with a tyre which has a very hard rubber compound and/or is pumped up to a huge internal pressure.

(But if rubber gets harder with lower temperatures, I still don't understand why Unthrottled said that rolling resistance is higher at lower temperatures)

With a hyperinflated tyre, assuming the tread layer's height is still the same, individual tread blocks may deform by the same amount, but will be in contact with the road (longitude of contact patch) for a shorter period of time per revolution and therefore heat up less? So the tread blocks stay harder.

Plus the contact patch's overall area is less, but then again the same weight is acting downwards on it, so do individual tread blocks actually have to deform by a greater amount, because the weight is carried by fewer of them?

In other words, given the same tyre, it offer less grip (sliding friction) when it is hyperinflated; but is this due to a smaller contact patch, or cooler tread blocks?

Regards my bicyling experience, I could envisage that, even if an irregular film of water molecules remain between tyre and road, if you were freewheeling in a totally straight line, the rolling resistance might be changed very little from dry conditions. But without a speedometer, I couldn't verify that. But I was referring to pedalling, i.e. I was having to propel the bike forward, so A) the rear wheel would be undergoing some slip due to the driving force, and B) it's likely the front wheel would be far from constantly upright and straight, so in the wet it probably was indeed the reduced sliding friction I was experiencing, rather than reduced rolling-resistance.

Back to one of my original queries: It is acknowledged that the deformation of the tread block absorbs energy; this is why the tread heats up. The car's engine must be furnishing this energy in addition to that required to drive the car against all the other resisting forces. But how do the tread blocks on a well-worn tyre deform in comparison? In other words, what I'm asking again is: For an identical tyre, at the same pressure, and neglecting the reduced overall diameter, does a virtually-worn-out one give lower rolling resistance (& better fuel economy) than a brand new one?

Ah, now I think I get it. So when the rubber is cold, the tread mainly undergoes plastic deformation, and the engine uses part of its output on energy which is merely heating up the tyre rubber. But the tyre rubber becoming heated eventually leads to the rubber exhibiting more elasticity and less plasticity, so then there is less of the engine’s energy wasted.

So what we really need is the tyre to be filled with (or even made from) some kind of ferro-foam, under the control of 2-axis g-sensors, so that when the car is ‘coasting’ arrow-straight, the carcass effectively becomes very stiff, but it can be softened by varying amounts when cornering / braking / acceleration is required.

The suspension could be controlled in the opposite sense, to keep the passenger ride constant.

A late addition to this thread. With the b***** awful weather this week, I had the chance to evaluate how much rolling resistance increases on a thoroughly rain-soaked road.

I was on the M6 heading south in heavy rain at 60mph, windspeed less than 5mph, lots of standing water on the carriageway. At that speed, and on the flat, the instant mpg read-out was showing around 40 - 43mpg.

A few miles further on, I'd emerged from the rain and the roads were almost dry. At 60mph on the flat in the same gear, instant mpg read-out was showing 60 - 65mpg.

The only difference was that my tyres weren't having to try to push large amounts of standing water away from the contact patches. Even so, the increase in fuel consumption surprised me. They're not especially wide tyres either (Pilot Premacy 205 / 55 16").

Obviously this is a fairly crude test but it does highlight how much work the tyres do on a wet surface.

Edited by craig-pd130 on 12/07/2012 at 12:47

It's not all extra friction from the tyres - there's also extra aerodynamic drag because the "effective" air density is much higher - as well as normal air, the water in the spray/rain needs to be deflected.

I would have thought that was caused by a lack of friction and not resistance.