For me it's good roads but pinch flat concerns. You're going along forever on pretty good tarmac, then you miss the one big pothole with a 1" lip on it at 25mph and you're toast. So a higher pressure makes sense.

For an all out race, I totally get what you mentioned there.

I suspect if he reads this thread then Tom A. can answer your question definitively. I believe it was he who came up with the impedance break point initially and has done a lot of testing around this idea. Josh at Silca also noted Tom as a pioneer in this area and they have further validated the info. The take away I got (maybe incorrectly) was it is usually better to err on the slightly low side of the optimal pressure than the high side because of the slope after the break point being so steep vs the slope before the break point.

that was my understanding too, but then I started looking at BRR cuz I need new tires, and saw the chart, then I was all like WTF??

There are other segments of that blog from Silca that you may also want to look at. Depending on your wheel, the actual MEASURED width of the rim and tire are important especially for racing. People are going to 25s when their older rims have 17mm internal width and the tires end up pumped and wider than the rim, which hurts aero. Josh goes into detail about the 105% rule, which further shows the issues in tire choice. One need to balance tire width, rim width and pressures when finding the best case for what they have. I'd suggest reading every one of his blogs in that series and also his "Marginal Gains" podcast is really interesting and informative.

I always thought it was because of the drum they use. That is, they use a completely smooth drum so the higher pressure results in a smaller contact patch -> less friction.

In the real world, the roads aren't close to that smooth, so the lower pressure helps you stay "on the road" so to speak and moves the mighty watts you're throwing down through the wheels more effectively.

that was my understanding too, but then I started looking at BRR cuz I need new tires, and saw the chart, then I was all like WTF??

It's not about the smoothness, or roughness, of the drum per se....it's because there's a part of the bike+rider "system" that's missing from the test setups, namely the vibration losses in the touch points of the rider to the bicycle. Without that, the measured Crr on the drum will decrease with increasing pressure. A rougher surface just adds an offset from the smooth drum data, since the losses being captured in the test all related to tire casing flex losses (hysteresis).

Is that purely a function of body weight, or is there some rider technique or equipment that can influence the amount of energy lost to touch points?

eg, if someone rides with locked arms and a lot of weight on the saddle, vs another rider who unweights the saddle and grips the bars lightly. or more suspension under the elbows of a TT bike? would that make any difference, or not really?

Rider mass is a part of that, but the biggest factor will be just the inherent damping of body tissue. I don't think there's really much you can do about that...and if you think about it, the BEST thing to do would be to make sure most of that energy input caused by road surface roughness doesn't make it into the bike and rider in the first place. Pneumatic tires can be VERY efficient at returning most of the energy back into the road surface, so the best overall approach is going to be selecting the appropriate tire and pressure, rather than in trying to "suspend the rider".

That above all applies to road bikes...once the terrain starts getting significantly rougher (such as MTB tracks) then bike suspension starts becoming a more attractive proposition, but that's more a function of traction and control.

https://www.bicyclerollingresistance.com/road-bike-reviews?maxweight=401&max=24&min=6&minpr=24

I thought lower pressure was better? Why do tires on this site consistently, maybe universally, test better at 120psi vs 100? methodology?

One important takeaway on the bigger tire trend from most of these sites is that the CRR gain lies in still running the larger tire pumped up to a level that is less comfortable.

There are multiple contributions to a tire's rolling performance.

One is energy that is spent deforming the tire.
When a part of the tire enters the contact patch, it gets deformed against the ground: this deformation consumes energy from your forward movement. When this part of the tire leaves the contact patch, it springs back to round, and in doing so it returns some energy to your forward movement. However, materials have internal friction, or "hysteresis": when you deform them, some of the energy spent doing so ends up as heat. So the amount of energy that is returned is always less than the amount of energy that was spent, resulting in rolling resistance.
You can minimize this effect by using higher pressure (since this reduces the amount of deformation), or by using a faster tire that loses less energy to hysteresis for a given amount of deformation.
This is the "rolling resistance" that gets measured on the drum.

When people talk about lower pressures performing better, they're talking about how the tire performs as suspension.
Think about what happens when a tire rolls over a surface irregularity. If the tire is pumped too stiff to deform around the irregularity, and instead deflects vertically off that irregularity, you're creating a vertical motion in the entire bike+rider system. This steals energy from your forward motion.
If the tire instead smoothly deforms around the irregularity, this energy loss doesn't happen. So you can prevent this by reducing tire pressure.
Reducing tire pressure does result in greater loss from hysteresis. But even on fairly smooth roads, the energy wasted vibrating a bicycle tends to be far worse than the added hysteresis loss of using an appropriate tire pressure. So there tends to be a pressure sweet spot: you want to be low enough that your tires are working properly as suspension, but not pointlessly low.

Suspension affects depend on the particular system being suspended. They don't show up on BRR because, even though the drum is rough, there's nothing being suspended: the tire is just locked in place against the drum. A rough drum can give higher rolling resistance values than a smooth drum, but this is just because it's creating additional deformation in the tire tread.


Maybe, but I'm not convinced that this is generally the case. BRR's width comparison seems to claim this in their "same comfort" test, but their way of determining "same comfort" seems to be based on a simplistic assumption of how tire spring rate works. At least in static testing, tire deflection against small deflectors is far less width-dependent than it is for flat surfaces. Which would seem to imply that, if a 23mm tire and a 32mm tire are deforming by the same amount against the floor, the 32mm tire will deform more for finer surface irregularities and would thus likely be "more comfortable."

What might be useful is a precise survey of breakpoints for tires of different widths of the same bike+rider and road.

that was my understanding too, but then I started looking at BRR cuz I need new tires, and saw the chart, then I was all like WTF??