Rolling Resistance - Tyre Type and Pressure

[Colin Morgan]

On a recent outing I thought my late Ruby was down on power - it was reluctant to go over 40, but seemed to be pulling okay up to 30 in 3rd.  

So I suspected it might something other than the engine.  On checking, the brakes were free, but I found a front tyre down at 14 psi from 25 - the result of a slow puncture.  At that pressure, the tyre didn't look very different.  I was surprised how much this lower pressure affected the rolling resistance - even just pushing the car around was harder.

Having fitted a spare - another Longstone, I ran the car out again and it pulled away happily at over forty now all the tyres were at the right pressure.  But the difference made me think about the effect of rolling resistance. 

I have a couple of spare wheels with the heavier 4.50 x 17 Avon tyres on them, so I replaced the two Longstones on the rear.  The car then went a bit better than before.  But whether this was from reduced rolling resistance or better gearing from the larger tyre circumference on the rear, I am not sure.   

Has anyone else considered the effects of rolling resistance for different tyre types and pressures?

[Alan]

Yes, I used to do that sort of thing for a living, once. You can compare tyre rolling resistance by measuring time to decelerate from say 40 to 20 on a straight level road. Actually, rolling resistance is a complex subject influenced by all sorts of other things but as long as they don’t change you should be ok.

[Colin Morgan]

So, something like pumping up the tyres to a higher pressure could allow the car to go faster and use less fuel? But there would be a downside?

Is this something to be considered when choosing tyres for Sevens?

[Alan]

You never get anything for nothing in the world of tyres. Increasing pressure will improve rolling resistance, also load capacity and arguably things like steering precision. However you will pay for this with worse ride comfort and up to a point less grip.

[Tony Press]

"and up to a point less grip"

Isn't road friction independent if the area of contact

[Colin Morgan]

Presumably tyres have a set pressure for a given axle on a given car that puts the whole of the tread evenly on the road? Which helps with keeping the wear even too?

Where there isn't a lot of power to spare, it seems it is worth keeping the tyres pumped up?

[Alan]

There’s a set of tables all tyre manufacturers work to which define minimum pressure needed for a given tyre size and load
Tony, tyre grip is more complex than simple friction if this were not the case why would racing cars use wide tyres?

[Reckless Rat]

They use wide tyres because it allows them to use a softer compound. A narrow tyre with soft compound would wear out or overheat too quickly.

[Tony Press]

Mrs. Google tells me:
 
The relationship between frictional force and the area of contact is a nuanced topic in physics. Here's a breakdown of the key points:

1. Frictional Force Basics: The frictional force (Ff) between two surfaces is generally given by the equation:
   Ff=μFn
   
2. where μ is the coefficient of friction (which depends on the materials in contact) and Fn is the normal force (the perpendicular force pressing the two surfaces together).
   
3. Area of Contact: Intuitively, one might think that a larger area of contact would lead to a larger frictional force. However, in many cases, the frictional force is independent of the contact area when the normal force is constant. This is because while a larger area may distribute the force over a larger surface, it also reduces the pressure (force per unit area) on any given point of contact.
   
4. Pressure Consideration: The frictional force is more strongly influenced by the pressure between the surfaces. For a constant normal force, increasing the area decreases the pressure, which can lead to a decrease in the microscopic interactions that contribute to friction.
   
5. Exceptions: There are some scenarios, particularly in materials with very high surface roughness or in cases involving adhesives, where the area of contact can play a more significant role in determining the frictional force.
   
6. Conclusion: In general, for most practical purposes and under normal conditions, the frictional force does not depend on the contact area for rigid bodies in dry friction scenarios. The key factors are the nature of the surfaces and the normal force applied.
   

[Colin Morgan]

Wider profile tyres perhaps distort less when cornering so can hang on longer before slipping?