Any question answered...

I think it depends on road surface though. There's that chart that shows that as surfaces get rougher, the point at which increasing pressure stops making you faster and suddenly makes you much slower goes down.

some bro science below, but...

Presumably grip is needed for acceleration.
At one end you'd have an almost solid tyre that hardly deforms as it bounces over the imperfections on the road surface. For something like an indoor velodrome track then this works pretty well as you lose very little energy bumping over things. But you'd waste plenty by bouncing around on a gravel path, not to mention sliding out as the tyre fails to hold on.

Then as you get a wider tyre and a lower pressure then you can deform around/deal with the imperfections of the road surface. And the ideal tyre depends on the road surface.

And the added reassurance of a tyre feeling more sure footed will lead to a rider pushing a bit more than they might on a rock solid tyre on sketchy ground. So there'll be a crossover where a technically faster tyre probably ends up slower in the hands of 99% of riders/drivers who value the confidence inspired by the marginally slower tyre.

Yes, clearly the answer is going to be different in different scenarios.
So for the purposes of my original question, I'm talking all things equal, straight line speed on a smooth surface, a straight tarmac road for example. Same size tyre, same bike, wheels, rider, position.

Obviously a 24c turbo cotton is not going to be faster than a 1kg knobbly DH tyre down a world cup trail or faster than a 42c gravel tyre on singletrack and less grip of a harder tyre will clearly mean needing to slow down more for corners so I'm talking in terms of velocity rather than lap times.

Likewise for acceleration, real world use case, softer is better for grip and acceleration

But I wanna know about speeeeeed

a straight tarmac road for example

Tarmac isn't that smooth, if you look closely at it. Softer compound tyre could conform better to the very small but constant variations in the surface. At a guess.

I did think that.

Then again it would still answer the question more usefully than finding out why harder tyres are actually faster as long as you're riding on train rails

True but that's about pressure in the tyre rather than softness of the tyre compound

Straight line speed on a perfectly flat surface the harder compound will go faster, just take longer to accelerate due to less grip. I can't remember where it was but ages ago I was reading about how bikes actually go faster when the ground is wet

Right, cool. That's what logic told me but I could only find people saying softer tyres are faster and not any reasons why. (when not taking into account cornering speed etc)

Just to throw another half-remembered theory into the ring, aren't thinner tyres more aero, rather than having less rolling resistance?

I thought they were less aero, due to not channeling air across the rim as effectively or something.
(Also half-remembered!)

If you really need to worry about this, a chat to your director sportif will sort you out ;)

Also why is a thinner tyre faster than a thicker one

What do you mean by thinner and thicker?
Since we're on a bicycle forum, I'm assuming you're only talking about tyres of essentially circular cross section with very compliant carcasses, all most bets are off once you switch to square rigid tyres.
As other respondents have already intimated, there's a lot of difference between pure rolling resistance (mostly hysteresis, measured on a steel roller) and resistance to forward motion on a practical surface, where suspension losses become a significant or, in some situations, dominant factor.

Thinner, not narrower.

• I'm guessing that this is down to the energy loss over small bumps like everyone's mentioned and a thinner tyre filled with high pressure air, coming out as being harder on the surface than one with a couple mm more rubber.
  
  

I was talking about ~circular tyres yes.

Again, I'm not talking about a rocky trail or a gravel path but specifically road tyres so no point in talking about polished floors either.

If you really need to worry about this, a chat to your director sportif will sort you out ;)

This has no practical implications for me, I was just curious

I've just cycled my bike that lives outside for the first time probably 2ish weeks and the headset is now slightly notchy, if I take it apart, regrease it and make it so that the front is rotated around slightly and reposition the bearings, would it fix it or should I just get a new one? Or will cycling it again regularly wear it back in a bit? It's not awful but kind of noticeable.

New bike

^

New denim.

Just keep using it until it becomes the norm

So I got a alu frame with carbon forks just now and upon closer inspection, I think I have got myself a dud - am I right in thinking that these forks are for the bin given damage to dropouts and that there is a crack rather than a scratch on the frame? I know it is hard to tell for sure from photos but it seems an odd shape for a scratch- the location is on the underside of the top tube quite near the seat tube. Didnt notice either when I picked it up, but then the lighting wasnt great. FFS.

5 Attachments

Where can I pick up a decent 1/8 chain close to Fitzrovia at lunchtime?

Cloud 9 Cycles?

Repacking with loose, rather than caged, bearings is the recommended fix (short of a new headset) for notchiness. Since you can fit in more bearings most of them will no longer be aligned with the notches that have worn into the cups.

Thanks

Dunno if you'd regard it as decent, but Evans on Mortimer St have the SRAM PC1.