Continental GP5000 TR wear

rookybiker wrote:

And yet objective measurements seem to show that worn tyres have less rolling resistance (as expected, as there is less rubber to deform, being thus closer to the pneumatic ideal). Another instance of subjective perceptions being wrong.

I'm all for objective data, but if you're going to make such a claim please actually provide evidence. 

The main source of objective data for bicycle tyre rolling resistance that I'm aware of is the aptly named Bicycle Rolling Resistance website, who have done a long term test of the GP5000 (not the TR version that OP is using) and they clearly found lower rolling resistance with newer tyres:

https://www.bicyclerollingresistance.com/specials/grand-prix-5000-endura...

BRR had previously rested the GP4000 S II, the results of which were a bit more equivocal but still tend to indicate the new tyres have lower rolling resistance, when inflated to real world air pressure:

https://www.bicyclerollingresistance.com/specials/new-vs-worn-gp4000s-ii

(the test tyres were 28mm, so 60 or 80 PSI seems more realistic than 100 or 120 PSI).

Another consideration is aerodynamics - Silca have data to suggest that new tyres are more aero (presumably because they are still rounded):

https://silca.cc/en-gb/blogs/silca/part-5-tire-pressure-and-aerodynamics

(although this assumes you have an aero tyre-wheel combination to begin with - it's unclear how big the effect would be on a non-aero tyre/wheel system.)

This forum post has a graph that purports to support the claim that worn tyres do have lower rolling resistance, but can't find the original source to back it up:

https://forum.slowtwitch.com/t/rolling-resistance-crr-new-vs-used-tires/...

I would imagine that the age of the rubber also makes a difference - used tyres are subject to air exposure, heat cycles and UV light during its lifespan, which in turn degrades rubber over time. The compound on a used tyre will be slightly less elastic and more brittle with age, making it harder and less grippy.

I would imagine that the last claim may only be from tests carried out on a smooth steel drum, not a textured surface that BRR, Silca and other companies do for real-world simulation testing. This will offer different results as the tyre contact patch isn't deforming to imperfections - the biggest acid test for a tyre's grip and rolling resistance coefficients. On a smooth surface, low grip = low rolling resistance, hence the thought behind the last claim . However rougher surfaces, even reasonably smooth tarmac, throws the curveball that the tyre's rubber does not deform as well to the imperfections, leaving the tyre to bounce back ever so slightly. Not enough to lift the tyre off the ground, but enough to add drag to slow you down, just as running tyre pressures that are significantly higher than optimal. Another downside is that the tyre also has a lower point of breaking traction, although the grip loss is not usually significant enough until the tyre is worn well past the tread wear indicator, or the tyre is of an extreme age (over 10 years old).

The Silca data shows only slight difference in aero drag, but no doubt all the different factors combined make a more noticeable difference overall. It's not really a saving worth chasing unless you are racing.