How to interpret the rolling resistance results from bicyclesrollingresistance.com for tires with very different tread patterns?

Question

Bicycle Rolling Resistance is one of the most reputed sources online for rating tires, but some results are still a bit counter-intuitive, especially when tires have knobs.

I understand that the rolling resistance test is done on a hard surface (a steel metal plate with some safety), but how representative is this surface from surface in the real world? My impression is that the presence of knobs tend to underestimate the rolling resistance on hard surfaces on BRR. But are there other observations/measurement in their dataset that would allow to differentiate/nuance the rolling resistance number tires with very different tread patterns?

To give some context, I've seen that they added recently a semi-slick tire, that I'm using on a gravel bike for road rides (Michelin Power Adventure 48mm, but I'm using the 42mm). However, this tire has more rolling resistance than the "pure gravel tire" from Michelin in 40mm (Power Gravel). So if I accept the compromise of the lesser grip due to the little knobs, I may in fact be better off keeping the Power Gravel for road rides and gravel, and avoid having to swap wheels/tires. But other combinations can be found where the knobby tire performs similarly in BRR than a semi-slick tire, but probably not in real situation (for example: Specialized Pathfinder Pro vs Schwalbe Hans Dampf Pacestar).

EDIT: clarified the question to be closer to the original intent

Answer 1

For the effect of knobs on otherwise identical tires, see this comparison on BRR. Less than 10% RR and no grip penalty for 1.3mm knobs vs a slick tire on the exact same casing. Lower-end tires might show a bigger discrepancy, but not by much. I'm not surprised by these results.

1. I wouldn't expect ultimate grip to differ much on gravel-type knobbies. Although the knobby in the linked comparison, the Challenge Getaway, is particularly well-behaved on asphalt for an all-condition tire, since its lateral profile is essentially round. It feels pretty much like a slick to corner hard on asphalt.
2. When talking about gravel-type tires, the knobs are under 2mm and just don't flex that much, so they don't have a very large contribution to rolling resistance.

The knobs are just one factor, in the end. We often think they make the biggest difference because we are used to comparing cheap 26x2.1 MTBs with expensive road bikes flying past us from back before we seriously got into bikes, I think. I also fervently follow BRR, and my impression has been that the factors that affect RR are, in order:

1. Casing construction and thickness
2. Then tread thickness and compound
3. Then knobs

And if you can't or won't run tubeless, get latex tubes. It's the cheapest way to improve your rolling resistance and in fact the most speed per dollar you can buy short of a gym membership.

Answer 2

Rolling resistance is about measuring how many joules are lost per each revolution. In real conditions, if your tyre slides (as a road tyre would slide on loose terrain), your apparent resistance is 0, but you lost many joules to the air instead of putting them at use to propel the bicycle forward. So it is a bit useless to define rolling resistance without having full contact, and it is difficult to translate rolling resistance results outside of time-trial and race conditions.

As every physicist will tell you, you compare oranges with oranges. The first check is "Do the things under comparison have the same measurment units?"

And then you compare a ship with a rocket, because they carry some weight over similar distances ;) .

Regarding the tyres you mentioned: first, do not be fooled by the "sound". A tyre with knobs produce more sounds, that your brain identifies with drag (whether it is true or not, this is exactly your question).

The best way is to apply some coefficient on the results. You are trying to translate the laboratory measurment into real-life variable conditions. In atmospheric science, this would be equal to translanting wind speed to the perceived cold (i.e. wind chill effect).

Get the worst possible slick tyre, one that roll so heavy that it looks like it has knobs (dutch-bike tyres that never wear out come to my mind), then try a tyre with knobs that give you a similar feeling, compare their rolling resistance, do the proportion, patent it and sell it as proprietary technology to the next greater gravel fool.

Answer 3

The question in the title:

How to interpret rolling resistance results on BBR?

The two tires mentioned are:
Michelin Power Adventure 48mm, RR Low = 21.9 watts, Rating = 4.0
Power Gravel (I'm assuming you mean the Power Gravel (v2) 40), RR Low = 22.9 watts, Rating = 4.0

Therefore: Power Adventure - given the testing conditions - comes in at 1 watt higher than the Power Gravel

Looking at the rest of the post the real question is:
What should I do with rolling resistance results in BRR?

Use the information as one factor in a list of requirements you have for your tires.

What do you want from your tires?
Based on the post:

1. Low rolling resistance
2. Good traction on the dirt when riding off road
3. Good traction on the street when riding on road
4. All requirements met in one tire so tires or wheels don't have to be swapped out for different riding conditions.

Sometimes people care about comfort or durability or flat resistance or looks.

It's a values question - what are my requirements and which ones are more important?

So if I accept the compromise of the lesser grip due to the little knobs, I may in fact be better off keeping the Power Gravel for road rides and gravel, and avoid having to swap wheels/tires.

Renaud is looking for confirmation that rather than swap wheels it might be better to just stay on the Power Gravel tires and not swap wheels/tires.

Grip=accepted reduction
Rolling resistance=small benefit
Swap wheels/tires=a hassle
Therefore: Don't swap wheels

Given the parameters this conclusion is logical

It might be good to make a complete list of requirements, rank them and see if the conclusion changes.

Answer 4

Rolling Resistant site does good work... I question real world value. I ran tubeless for several yrs and had enough of the hassle which is the commodity being hawked mostly now. I ran extensive roll out tests off a hill under same non wind conditions. Any variance is essentially minor noise.. of no quantitative value.

Quality tubed vs tubeless ran down to 10mph off hill were within a few feet... often my tests showed results same within 1/2 foot. In short... RR can be scientifically diced varying ways... the out comes are negligible comparing same Q tire... translating to precious little distance traveled.

RR might well be a predictor of casing quality per roll out. Yet I prefer online real world reviews.. per wear, mounting fit, grip, ride Q etc.

Low RR figures = HIGH psi's. Rolling on 25mm's at 120 psi... might as well be running no rubber given the harsh ride Q.