[Closed] This wider tyres being quicker

then it is a deeply flawed test

We all know they are deeply flawed, just look at the results!

Incidentally, VW's specs for my Passat are quite a bit higher than the charts the tyre companies get sent out - 38psi vs 33psi. Car is more comfortable at 33 as you'd expect, but I do lose maybe one or two mpg although it's difficult to detect in the noise.

Too narrow and high pressure = greater suspension losses, and carcass deformation.

Too wide = greater losses to aerordynamics, weight and friction

In the middle is the variable zone where the optimum depends on the exact conditions in terms of surface, speed, pressure, and specific tyre casing construction.

The fastest tyres will be lightweight, supple casing tyres, of a size and pressure that allow you to minimise hysteresis losses and minimise suspension losses, while balanced against rolling friction losses and aerodynamic losses. It's that balance point in the middle, too far either way and you lose out.

I think you've made sense of it there.

What I have learnt thought is just pumping up a narrow mountain bike tyre doesn't necessarily make it faster because of all the variables to take into account.

I just assumed it was a given.

This is worth a listen:

https://cyclingtips.com/2016/08/cyclingtips-podcast-episode-9-rethinking-road-bike-tire-sizes-and-pressures/

That is a great listen and covers most of the stuff discussed.

I get it for comfort and grip off road or broken roads but on normal roads I dont think you'll reduce the rolling resistance over 23/25mm 100psi tyres by going wider or lower pressure.

And you are basing this on?

It was the early-mid 80s when 'aero' was all the rage for road bikes and the introduction of super-skinny 18-20 tyres and rims - not supported by any scientific data and it's taken a long time for folks to change their views - particularly believing a 1-2mm difference in tyre width has any aerodynamic impact. Another factor is the impact of riding skinny, hard tyres is increased rider fatigue. The advent of super-stiff, 'aero' carbon frames and deep-rim wheels don't help either - they often give you a complete battering.

mrblobby - Member
Measured rolling resistance of GP4000S in 23, 25 and 28 sizes...

http://www.bicyclerollingresistance.com/specials/conti-gp4000s-ii-23-25-28

And more importantly, that's on a smooth(ish) drum, so it's only influenced by the shape of the contact patch and they carcass deformation to achieve it. On a real road you get all those added benefits of vibration absorption.

Basicly you could go as wide as you like until you start to trade off against weight (impacting acceleration) and aero drag (influencing top speed). So track sprinters are unlikely to ever stray into bigger tyres, but everyone else will probably find their optimum with wider tyres.

Car tyres are a red herring.
1) Their contact patch is rectangular, not round. So you have to balance load against construction and pressure, you can't just swap to a wide tyre and low pressure and get similar performance.
3) A car tyre has very different construction to a bike tyre. A bike tyre looks very similar inside all the way bead to bead (additional reinforcements aside). A car tyre has completely different constructions to achieve vertical sidewalls, the tread area, and even under the different areas of the tread.
4) Car tyres have to prevent aquaplaning, a lightweight car needs narrow tyres to maintain grip in the wet. A 2ton Range Rover doesn't quite have the same problem when fitted with 280 section tyres! It also needs those big tyres to dissipate the heat generated keeping all that weight on the road in the dry!

And it's still a compromise, a wide tyre has more of every other type of 'drag', so you still have to compromise. A car that does 70mph obviously has a different optimum to a road cyclist on a 15-20mph club run.

And you are basing this on?

Its what 99.9% of the millions of people who want to go fast on a bicycle use.

http://www.bicyclerollingresistance.com/specials/conti-gp4000s-ii-23-25-28

Looking at that page. I run 23mm GP4000S at 100R/90F. If I go to 25mm I'm going to gain approx 0.2 watts per wheel (if it doubles up like that). The tyre will be heavier and less aero so I am guessing these will all just about equal out and there'll be no real world benefit.

BUT, I wouldn't run 25mm tyres at the same pressure, so now the 25mm tyres at a lower pressure are heavier, less aero and more draggy.

I accept there's better grip due to an improved contact patch shape, better comfort due to the larger volume tyre and lower pressure, but I'm still to be convinced you can have both of these AND lower rolling resistance / more speed.

wilburt - Member
And you are basing this on?
Its what 99.9% of the millions of people who want to go fast on a bicycle use.

So your argument is based on hearsay rather than maths and science?

Which way did you vote on Brexit?

BUT, I wouldn't run 25mm tyres at the same pressure, so now the 25mm tyres at a lower pressure are heavier, less aero and more draggy.

I'd hazard that the incremental aero losses of the tyre are relatively small, especially when aero wheels are getting wider too. And the 25 is a whole 11g heavier, so ~1% of the wheel.

At some point it becomes self defeating going bigger, but I doubt it's 24mm.

Otherwise, why run 23mm? Why not 21, or 19?

What this thread needs is a conveyor belt.

Its what 99.9% of the millions of people who[i] want [/i][b]to go fast on a bicycle use.

Yes, but do they actually go faster...?

Do a bit of reading, some links already posted on here, plenty more out there.
The evidence shows that narrow and high pressure does not equal faster except under artificial conditions.

Just because millions of people are doing it, doesn't mean it's right 😉

Also, as a few of us have already mentioned, before everyone was running thin and hard to go fast, people used to go wide and soft to go fast*.

If you look at this in pure isolation it means one of those groups of people got it wrong. Or possibly they were both right but didn't fully understand the situation.

if you come back with a counter argument that the early group were wrong and revised/better understanding lead to a change (thin and narrow) then why can you not accept that same process is at work now as we further our understanding even more?

*on handmade cotton casing lightweight supple tyres, it's important to compare tyres of similar construction and not just size.

ie: a 38mm touring tyre (eg. marathon plus) won't be quicker than a 25mm racing tyre. But it will be quicker than 30mm marathon plus at the same pressure.

Now translate that back to your racing tyre... a 30mm tyre of eqaal construction can/will be quicker than it's 25mm counterpart.

(Until you reach a point at which aero drag dominates the losses.)

so now the 25mm tyres at a lower pressure are heavier, less aero and more draggy.

But aren't people saying that lower pressures isn't less draggy due to roughness in the road surface?

I accept there's better grip to due an improved contact patch shape, better comfort due to the larger volume tyre and lower pressure, but I'm still to be convinced you can have both of these AND lower rolling resistance / more speed.

You can't really. You can either run them both at same pressure and have less rolling resistance with the wider tyre. Or you can run the wider tyre at a lower pressure and have roughly the same rolling resistance but a bit more comfort (which may well allow you to ride harder.)

Basicly you could go as wide as you like until you start to trade off against weight (impacting acceleration) and aero drag (influencing top speed).

The higher the speeds the more significant small aero gains become. A quick bit of googling suggests that a 22mm turbo cotton has a CdA about 0.002 better than a 24mm turbo cotton on a HED Jet 6 rim (0.020 vs 0.018). Plug this into the drag/speed/power equations and it's easy enough to work out the tipping point.

The higher the speeds the more significant small aero gains become. A quick bit of googling suggests that a 22mm turbo cotton has a CdA about 0.002 better than a 24mm turbo cotton on a HED Jet 6 rim (0.020 vs 0.018). Plug this into the drag/speed/power equations and it's easy enough to work out the tipping point.

In isolation yes, but when the aero drag of the system as a whole is dominated by the lump on top of the bike, and then the whirly bits disturbing the flow, the small (but significant in isolation) change from tyre size alone isn't as important.

BUT, I wouldn't run 25mm tyres at the same pressure, so now the 25mm tyres at a lower pressure are heavier, less aero and more draggy.

I accept there's better grip due to an improved contact patch shape, better comfort due to the larger volume tyre and lower pressure, but I'm still to be convinced you can have both of these AND lower rolling resistance / more speed.

There is some data (limited but increasing, mostly because not well tested yet) out there about how changes in pressure have very low effect on RR for decent lightweight supple tyres. Certainly less variation than for heavier casing tyres.

ie: the penalty for a lower pressure on a decent supple tyre is a lot less than the penalty on a stiffer carcass tyre, in some cases to the point where the reduced RR from the increase in size can then be 'used' to lower the pressure and bring you back to a net zero change in RR at the road, but with increased grip and comfort.

This obviously isn't always the case, and can be very condition specific, but again the thing that repeatedly gets ignored is the effect of jiggling a 70kg human around on top of the bike.

So even in cases where you might have a small increase in actual RR at the road, you end up with an net overall speed gain due to reduced suspension losses and reduced fatigue/better ability to apply power. And the longer the ride the more relevant that becomes. A straight out track sprint is a different ball game to a few hundred kilometres on rough roads.

When you look at the system as a whole 'speed' is about more than just the RR.

It really is a fascinating area to look into and research, but does require leaving some preconceptions at the door (from both sides of the argument!)

But aren't people saying that lower pressures isn't less draggy due to roughness in the road surface?

Yes, they're saying that. I'm saying I'm not convinced they're any faster.

If the numbers on that site are to be believed then the bigger tyre is draggier albeit on a smoothish drum. Lets say for the sake of argument the bigger tyre is less draggy on a rough surface than it is on the drum

So you're back where you started with your narrow tyre at a higher pressure, but with slightly better grip, and a little bit more comfort, but no faster.

In isolation yes, but when the aero drag of the system as a whole is dominated by the lump on top of the bike, and then the whirly bits disturbing the flow, the small (but significant in isolation) change from tyre size alone isn't as important.

Tell that to a tester who spends thousands on aero testing 🙂 Tiny gains wherever you can find them add up.

So you're back where you started with your narrow tyre at a higher pressure, but with slightly better grip, and a little bit more comfort, but no faster.

Or...your bigger tyre at the same pressure and faster. There are choices as to whether you jsut go bigger and run the same pressure, or go bigger and drop pressure.

Or as with my post above, "it's complicated" due to other factors, and the mixed terminology when talking about 'speed'

ie:
speed, as in on a drum/in a test due to lower measurable RR

vs

speed, as in actually faster over a given course with a human prioviding the power.

the two are not necessarily the same.

Tell that to a tester who spends thousands on aero testing Tiny gains wherever you can find them add up.

I don't disagree at all, I'm just saying that for the majority of riders the reduced drag of a thinner tyre is mostly irrelevant compared to other factors. And as we all know you attack the big factors first, before going after the marginal gains. And what is lacking is a proper evaluation of RR vs Aero and the point at which they inflect for any given tyre choice in [i]real world[/i] conditions.

For example, if it could be shown that a gain in reduced RR was larger than the aero loss of the increased size it would make sense to take the aero hit for an overall improvement, especially if it increased comfort and allowed you to power the bike for longer or at higher output. You'd obviously also have to know at what speeds the crossover happened, the faster you go the more the aero factors dominate.

Likewise if it could be proven that smaller tyres always netted an aero gain bigger than any increase in RR you'd be mad to run the bigger tyres for that kind of use, even if you had to put up with increased fatigue or reduced grip, for a tester it'd be worth it.

The problem is we don't actually have a reliable and accurate enough data set (yet) to prove that either way because we don't have decent real world RR data that takes the entire system into account.

[b]amedia[/b], I think we are saying the same thing, ie it's not as straight forward as a bigger tyre = faster and better grip and increased comfort and less fatigue. You can't have them all, you have to choose the ones that are important to you and your riding.

amedia, I think we are saying the same thing, ie it's not as straight forward as a bigger tyre = faster and better grip and increased comfort and less fatigue. You can't have them all, you have to choose the ones that are important to you and your riding.

Kind of, and absolutely it's not straight forward, that's why it's so poorly understood!

The thing to take away though is that you can have benefits and mostly it's an overall benefit either a big one in one area, or a balance in multiple areas.

The downsides are increased weight and increased aero drag, the point being that neither of those are big factors overall.

Even if as you say you end up with no net improvement in RR overall, you will end up with an increase in grip and comfort (which is no bad thing 😀 ) and that might lead to an increase in actual speed between point A and point B, even if that's not measurable on the drum.

Another piece of empirical information - try riding crits in the wet on a 23mm @ 100psi vs 25mm @ 80psi and see who can get around the corner faster?

So you're back where you started with your narrow tyre at a higher pressure, but with slightly better grip, and a little bit more comfort, but no faster.

Not sure I quite understand what you are saying but:

On a smooth road, narrow tyres are faster. On a rough road, BOTH tyres are slower, but the wider one slows down by less than the smooth one. So you are better off on wider tyres if it's less than perfectly smooth.

Rolling resistance does reduce with tyre width, but at ~15mph+ the aero drag of the tyre surface area and how well the tyre blends in with the wheel rim become increasingly important with increased speed and also in increasingly windy conditions.

My Wazoo's main roll is commuting, in "full fat" wheelset mode with 4" JJs the ride is very comfy and on still days or days with a tailwind they are surprisingly quick. I normally use my FatNotFat 29er wheelset, where the 2.25" G-Ones are faster but still quite comfy; the 700x38 Marathon Cross are probably quicker still with a little less comfort; but only recently using 700x28 Grand Sport Races have I done the ~4.5 mile journey to work in under 15 minutes.

Rolling resistance does reduce with tyre width, but at ~15mph+ the aero drag of the tyre surface area and how well the tyre blends in with the wheel rim become increasingly important with increased speed and also in increasingly windy conditions.

My Wazoo's main roll is commuting, in "full fat" wheelset mode with 4" JJs the ride is very comfy and on still days or days with a tailwind they are surprisingly quick. I normally use my FatNotFat 29er wheelset, where the 2.25" G-Ones are faster but still quite comfy; the 700x38 Marathon Cross are probably quicker still with a little less comfort; but only recently using 700x28 Grand Sport Races have I done the ~4.5 mile journey to work in under 15 minutes.

The downsides are increased weight and increased aero drag, the point being that neither of those are big factors overall.

To be fair the increases in grip, comfort or RR are going to be small too. Not many people are going to notice sub 1 watt gains.

Narrow tyres need to be run at high pressures so they kind of dictate the comfort and grip you'll get. Wider tyres give you some wiggle room with pressure allowing you to pick how your want your bike to ride. You can opt for grip, comfort or less RR, which is an advantage.

BOTH tyres are slower, but the wider one slows down by less than the smooth one

But if the narrower one had less RR to begin, could its RR increase more as a percentage and still end up the same RR as the wide tyre?

My Wazoo's main roll is commuting, in "full fat" wheelset mode with 4" JJs the ride is very comfy and on still days or days with a tailwind they are surprisingly quick. I normally use my FatNotFat 29er wheelset, where the 2.25" G-Ones are faster but still quite comfy; the 700x38 Marathon Cross are probably quicker still with a little less comfort; but only recently using 700x28 Grand Sport Races have I done the ~4.5 mile journey to work in under 15 minutes.

comparing 28mm with 4inch tyres isn;t really what this is about though is it!

And you've fallen right into the trap with comparing 38mm Marathon Cross with 28mm Grand Sport Races, they are not in any way comparable in terms of construction. You're comparing big tyres of one type with smaller tyres of another type.

Rolling resistance does reduce with tyre width, but at ~15mph+ the aero drag of the tyre surface area and how well the tyre blends in with the wheel rim become increasingly important

Yes, when compared in isolation, but the aero drag of the tyres is still small in comparison to the rest of the system. Even if the drag from 25mm to 28mm doubled that's still only a very small %age overall as the bulk of the thing in the way is the rider.

But if the narrower one had less RR to begin, could its RR increase more as a percentage and still end up the same RR as the wide tyre?

Possibly, possibly not, that's why more testing in real world conditions is needed, but current experimental data suggests that the wider tyre still gets you a net benefit overall.

And I know I keep banging this drum but it's important... NONE of the testing so far takes into account the effect on the human on top. It's all very well having the fastest tyres in the world on paper, but if the rider is impacted by vibration to the point where they either can't apply the same power or fatigue earlier it's an overall net loss.

I'm not saying that that definitely is the case, I'm saying that it's an unknown factor that needs consideration and further investigation, as there is plenty of anecdotal and situational (but not rigorous scientific) evidence that suspension losses at the rider/overall can outweigh RR changes.

ie: (purely mad up numbers but) if your tyres of size X are 8Watts faster than tyres of size Y, but impact the rider to the point where their sustained output is reduced by 10Watts then it's a net loss.

The elephant in the room is the tyre choice itself though...
The difference between the best tyres and worst tyres in any given type/size can be in the region of 10s of Watts.

It's silly arguing over 25mm vs 28mm if you're buying crap tyres in either size 😀

epicyclo - Member
For several years I've been running converted 29ers as road bikes using Big Apple 2.35" tyres. Freewheeling downhill beside other road bikes I have to trail my brakes to keep level.

That's F=MA, not rolling resistance. Your tyres have over 3lbs of additional rotating weight, once they've overcome intertia and are upto speed, their momentum is huge compared to road wheels and tyres.

But if the narrower one had less RR to begin, could its RR increase more as a percentage and still end up the same RR as the wide tyre?

No, the theory is that it increases much more, beyond that of the wider tyre.

It's hard to detect on the road but it's starkly obvious off-road, comparing 2.3s against 3.0s on a rough trail. You'd be getting the same effect on road but it would be a case of cumulative fatigue and lost minutes over longer rides.

I have two road bikes - a reasonably nice if old carbon racy bike on 23s and a cheap commuter on 32s. You know that feeling when you hit a rough crappy bit of road surface and it all gets harder work, like a headwind? That is far less apparent on the 32s.

NONE of the testing so far takes into account the effect on the human on top.

I think some of the testing that Jan Heine/Bicycle Quarterly have done talks about how thin tyres can [i]feel[/i] faster, because of the extra vibration going into the rider. While having wide supple tyres feel slower but leave the rider feeling a lot fresher at the end of a long ride.
[url= https://janheine.wordpress.com/2016/08/08/the-missing-piece-suspension-losses/ ]The article is here[/url] if people haven't already checked it out. I do think the BQ testing is slightly more relevant to average riders than many others, because they concentrate on real world testing, rather than wind tunnels, rollers, etc.

The BQ stuff is the only testing I'm aware of that has attempted to include it, the problem is it doesn't (nor attempt to) [b]quantify [/b]it very well and although his tests are well executed they aren't as easy to repeat away form their own testing environment to actually start gathering meaningful data to compare with other test results.

I do think the BQ testing is slightly more relevant to average riders than many others, because they concentrate on real world testing, rather than wind tunnels, rollers, etc.

This is simultaneously both the best and worst thing about their tests. They are focused on the real world and testing things under normal conditions, which means when their results go against the accepted wisdom people are quick to dismiss them as non-scientific or inaccurate, when actually the opposite is true, they are very scientific (controlled, repeatable, in some cases double-blind) they're just not [i]idealised [/i]like many lab tests, which is exactly the downfall of the lab testing!

I like JH and have read most of his stuff, but his passion is easily mistaken for arrogance and I've been wary of pointing people towards his articles as when I've done it before most people have scoffed and not taken it seriously, or been put off by his particular quirks, which is a shame as he has probably done more for the wide tyre re-revolution on road bikes than most in the industry in recent years 🙁

But for anyone who does want to read and has an open mind I can wholeheartedly recommend spending a few hours reading through the last few years of his blogs, some very interesting stuff on there and not just about tyres.

..which is a shame as he has probably done more for the wide tyre re-revolution on road bikes than most in the industry in recent years

I'm not fully convinced by many of the points Jan makes about bikes, or at least don't prioritise the same things, but have a lot of time and respect for anyone that investigates the what and why of bikes to that extent.

NONE of the testing so far takes into account the effect on the human on top.

It takes into account the weight of the human, which is all that's really required if you want to eliminate variables from the testing process.

Its what 99.9% of the millions of people who want to [s]go[/s] [b]feel like they're going[/b] fast on a bicycle use.

How many riders do any useful testing beyond feel and maybe strava? [i]Feeling[/i] fast is why we have 120 psi in skinny tyres, 73.5 or 74 head angles and overly stiff frames. Feedback loop of feels fast - reacts fast - I go fast.

But

NONE of the testing so far takes into account the effect on the human on top.

It takes into account the weight of the human, which is all that's really required if you want to eliminate variables from the testing process.

Are these tyres on a conveyor belt?

It takes into account the weight of the human, which is all that's really required if you want to eliminate variables from the testing process.

It really isn't. It tells you the effect that that static* weight has on the tyre. It gets you 'a' metric, but not a meaningful one if you want to look at the effect on the system as a whole. It doesn't tell you what effect that tyre has on the human over a long ride, or it's impact on that humans ability to power** the bike over dodgy surfaces***.

I know that my fastest bike over 1 or 2hrs on decent roads is not my fastest bike over 8-10hours on mixed/crappy roads. The crappy roads one is 7lb heavier and has 32/35mm tyres vs 25mm, but it's quicker overall on the longer rides.

Same with MTB, for 1-1.5hr XCO races I can deal with getting beaten up a bit, but on 12/24hr races I need the comfort to stay fresh...

In extremis this is just like rigid/hardtail Vs suspension arguments, and most people have pretty much cottoned on to the fact that suspension, whether delivered by big floaty tyres, or springs, allows you to go faster for longer, it's the same principles at work on the road just the terrain is micro-bumps and the battering is vibrations rather than big hits, but overall it's the same principles at work in different levels of effect.

*this may or may not be important too, a static weight test doesn't tell you much about how a moving/bouncing lump affects the tyre. When hysteresis and carcass behaviour is so important to a tyres performance you also need to consider the loads form above as well as from the road, and a static weight doesn't replicate a human very well.

**same here, to my knowledge none of the testing attempts to replicate human power delivery, they just spin a wheel at constant speed with controlled power input, again, not a great approximation of real world.

***this really bugs me too, surely they can do a better job of actually replicating a road surface rather than just a poor approximation with a slightly bumpy roller (of a different material!)

The tests are great at describing what is fastest in a lab, with a static rider and power delivered in a smooth constant way, not so great at describing what is fastest in real life and as we are seeing you can't necessarily extrapolate the lab behaviour.

How many riders do any useful testing beyond feel and maybe strava?

^ I guess Strava would work over the longer rides where the differences would show up. I have timed longer (5hr+) rides on the same route on a number of different bikes and tyres, but conditions and my own output vary a lot anyway.

an knew all along that when it came to randonneuring the French were right from the start and those expensive 650B tyres were worth it.

That's what's mildly infuriating isn't it, we've done this before, the evidence was already there from thousands and thousands of miles of testing in the real world accumulated over several decades, and yet we threw it all out the window for roller tests in artificial lab conditions!

When lab testing was showing that thinner was faster what should have happened is that people should have gone "hang on a minute, why does the lab result contradict our real world experience" but instead they went "thinner is faster, everyone go thinner!"

And then all the decent construction wide tyres started disappearing and that falsely reinforced the "thinner is faster mantra" because the only [i]decent[/i] tyres were thin ones, so thin became fast, and wide became slow, not because wide is slow but because the wide tyres were chunky stiff heavy duty tyres.

And now we're in a weird position of having to re-do the lab testing to try and prove that which was already known was correct, but we can't do it properly because our lab tests are still pants for replicating real life.

thisisnotaspoon - Member
wilburt - Member
And you are basing this on?
Its what 99.9% of the millions of people who want to go fast on a bicycle use.
So your argument is based on hearsay rather than maths and science?

Which way did you vote on Brexit?

Your not doing the credibility of your argument any favours by being a ****.

Its not hearsay, its choosen sloution of the mass majority of people who care a great deal about bicycle speed.

All the arguments I've so far have amateur speculation or junk science.

When team sky starts running 40mm tyres at 40psi I'll think you may have a point until then its just more mtb forum nonsense.

When team sky starts running 40mm tyres at 40psi I'll think you may have a point until then its just more mtb forum nonsense.

Don't forget though that bike manufacturers are only just making frames wide enough to experiment with wider road rim/tyre combos.
If you have a power meter you can try this stuff for yourself and no excuse not to really...

I've been running the biggest tyres in my choice I could find for twenty years, and the lowest pressures. Over that period tyres have got bigger and pressures lower.

I ride in a bike hash, and many riders are on narrower tyres, and higher pressures.

Anecdotal but these days the speed difference is such that on a gentle off-road downhill I'm constantly dragging the brakes to avoid running into them. Typically hardpack and sandy.

On the road I'm struggling to keep up.

These days I'm on 2.35" on the same narrow rims, and 2.5-2.6" on wide rims. Not sure how the latter works out.

My conclusion is that lower pressures are faster off-road, and I haven't found the limits yet.

As for those who've thought about it and think different, do what you think works.

I prefer to fit my theories to the evidence.

Its not hearsay, its choosen sloution of the mass majority of people who care a great deal about bicycle speed.

Based on what evidence? With 30+ years bike racing experience are you saying that it's not relevant? I started racing on 18-20mm tyres but never enjoyed the harsh ride and frequent punctures and we were riding shallow section rims that were more forgiving.

It really isn't. It tells you the effect that that static* weight has on the tyre. It gets you 'a' metric, but not a meaningful one if you want to look at the effect on the system as a whole. It doesn't tell you what effect that tyre has on the human over a long ride, or it's impact on that humans ability to power** the bike over dodgy surfaces***

It really is. They're testing the rolling resistance of [u]a tyre[/u]. A metric for each tyre on a slightly lumpen surface (it's not a smooth wheel), under the same load, at the same speed, on the same hardware means that comparisons can be drawn. Mechanical drag on the system is measured, translated and compared. The test tells you how much energy is lost as a result of normal rolling resistance. It doesn't try to compute all losses and their subjective feel, but accounts for a large percentage of energy lost directly as a result of tyre compound, construction etc.

When team sky starts running 40mm tyres at 40psi I'll think you may have a point until then its just more mtb forum nonsense.

They never will, because at the speed they're moving on the flat and downhill, aerodynamic drag on the trailing edge of the 40mm tyre will more than eliminate any gains from rolling resistance. I wouldn't be surprised to find that with the right tyre/rim combo that 28-32mm is about the sweet spot when considering rolling resistance, fatigue, grip, aero and weight.