Agreed. I've seen enough tests of the 20mm SS on Jet+ rims and other similarly narrow tires on wide rims to embrace that "narrow is aero".

It's something you can field test if you want to take the time to do it properly.


Good reason to orient your belief system around what really is faster.


Don't worry. You have an infinite amount of variation in position, posture, helmets, suits, shoes, etc to explore. Decent frames haven't had much drag in a long time.

I ride the same tyre & bodyweight at 70 psi on Reynolds Assault rims; and have seen as low as 55 psi without anything going wrong. I wonder if the issue is rim width for you?

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29 years and counting

And never mind upgrading the rider!

Yeah, i have a smooth, slightly downhill run or straight road near my home that i call the “wind tunnel” for such pseudo-scientific testing.

lots to play with in the aero world, but one could say that aside from materials advancements, “one solution” is arriving. Thing is, different conditions each have “one solution” so like wide tires or narrow, frames optimized for high or low yaw, is will all be choices of compramise.

I think tires had to get wider at a cost to aero simply because of impedance losses. Just like josh’s curve of casing losses vs impedance losses, there is a similar curve for crr vs aero, with an optimum tire width being a bit lower for lighter riders. I suspect 23-25c is the correct answer for most decient road surfaces, and rider weights.

Also funny, I spoke to a lbs owner about all of this. He saw none of the data, but said he has preached low pressures to all for a very long time. He said the mountain bike folks figured it out long long ago... it just took the roadies longer to come around to the concept that low pressures (to a point) are better. “When you walk out and actually look at our roads, they are not very smooth. Low pressures just make sense, for speed and comfort”

Like posted above though, wasn't it about bumps big enough to disrupt power transfer or the roll?

I see it as folks running lower pressure to gain something for a duration of maybe 1 to 2 seconds each minute of riding versus focusing on the other 58 seconds of that minute of riding.

If you're training or commuting, yeah, you focus on that shorter duration because that gets you comfortably home on time without issue.

If you're racing, you take the risk and focus on the majority...........which is normally a pretty smooth road.

The mountain bike analogy is nice, but their terrain is much more in the realm of disrupting the roll for at least a plurality if not a majority of that theoretical minute. Constant roots, drops, bumps, skips, slides, rocks, etc....

If you're riding a road that disrupts it that often to make that pay out, time to get off that road.

I think the power/comfort/low pressure road riding thing is probably most relevant for a commuter in an urban area with some crap streets. Frequent potholes, ruts, debris, grooves, etc..... But, that again gets into the idea that you're disrupting the roll/power output much more often than usual.

I'm still flabbergasted at how low a pressure some of the larger riders are using even on clinchers. That's gotta look pretty flat from behind to another rider.

Thats the thing... i thought it was about bigger bumps/cracks/rocks too. But the silca data shows the issue even with fresh tarmac. It is far rougher than a drum. Have a close look at “smooth” tarmac.

It makes more sense in my mind if we magnify the contact patch (quite small on a bike, especially as pressures go high). Now it starts to look really rough vs the area a tually in contact. Now double the contact patch size (larger tires and/or lower pressures) and those rough bits are putting on less force on the tire contact patch. Like laying on a bed of nails (vs a single nail), the force is distributed. The tire rolls better.

Kindof disagree. While there are lots of big disruptions on trails, much of the time dirt is quite smooth at least in context of the tyres used. By comparison chip seal is constantly bumpy so you are getting disruption all the time. The scale is smaller which is why 70 psi is low pressure and 25mm wide, compared to 15psi and 3inches. The point remains that a direct comparison is invalid but there are many aspects to why

Suspension/"impedence" effects aren't ansent from drums just because of roughness. Even when the drum is rough, it won't pick them up. BRR's diamond-plate drum is an obvious example.

Roughness causes losses at high tire pressure because the bicycle needs to move around surface irregularities. A sufficiently-squishy tire can deform around the irregularities. A too-stiff tire can't, and it deflects the rest of the bike/rider vertically. That vertical deflection sucks up energy that was being used to go forward, and there's usually not a good way to return it to forward motion.

Exactly.

Rocket_racing wrote:

Exactly.

Here's my on road data tire pressure and Crr/Power relationship (note that the magnitude of the pressure breakpoint effect - if any - is marginal when compared to other data available on the internet)





and some bumpy roller data that Al Morrison collected and shared:

http://www.biketechreview.com/...py_Data_BTR_rev1.pdf

FWIW, I was able to reproduce Al's roller work at the link above using a set up that did not rigidly constrain vertical displacement...my set up only rigidly constrained 1 DOF - as did Al's, IIRC.

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Kraig Willett
http://www.biketechreview.com - check out our reduced report pricing
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I weigh considerably more than you (typically 80-83kg), and I ride on very bad mountain roads much of the time.
My road bike has Mavic Ksyrium Elites from a few years ago with a 15mm internal rim width, so narrow. I typically use 23mm GP4000sII tyres in winter (needed for clearance with mudguards) at 85-95psi front and 95-110psi at the back depending on the route, conditions, and my mood. In summer I use 25mm GP4000sII on the same rims and drop the pressure to 75-80psi at the front and 85-90psi at the back. Last time I had a pinch puncture was about 6 years ago on horrible 28mm Gatorskins. All of the above is with Butyl tubes.
On my tri bike I use Swiss Side Hadrons with a 19mm internal rim width so fairly wide, but less so than many race wheels. I use latex tubes (no sealant) and 25mm GP4000sII. Typically I use 75-80psi at the front and 80-85psi at the back. The weight distribution is typicalyl further forward for tri thus the smaller difference front to back, and the fact that although I could go softer at the front (and have gone to 70psi) I prefer not to go much lower than this purely for feel when/if I get out of the saddle on steep hills. Don't often get out of the saddle in races but I do occasionally on some of my training routes. On my road bike I've occasionally used the Swiss Sides and 25mm tyres and dropped the front to 70psi (when not expecting to climb or sprint) and it feels great on bad roads. I've always used the Swiss Sides on 25mm GP4000 with latex tubes and these pressures and have never had a pinch puncture. Again - I weigh in excess of 80kg and often ride on very bad roads.

IMO, there is no need for 90psi on 25mm tyres for someone your weight unless you ride into potholes and kerbs all day for fun.
90psi certainly doesn't count as "low" around here for someone weighing 60kg and riding 25mm tyres.

Have you actually had any pinch punctures?

Yeah, i will be dropping my pressures 20psi. More comfort and speed seems too good to be true, but i will take the good news.

No pinch flats for me... or punctures. Well, except when i took my cx bike on mtb singletrack, and started some high speed hopping...

It may feel sluggish, but that doesn't actually mean it's slower. Maybe it is, maybe it isn't. Beware putting your trust in "feel".

Ever noticed how many people talk about test riding bikes before making a purchase and how they found one much faster, smoother, etc, than another? Or perhaps how reviewers often talk about light wheels or stiff bottom brackets making major performance differences that they can detect by feel? Most of the time it's absolutely clear, if you look at the physics, that such claims are most certainly nonsense. And yet they persist. I ride for enjoyment and fitness. Much of the enjoyment comes from competing so I care about performance, but much of it also comes from the sensation of riding itself. So I also value feel. But I think it's important not to fool yourself into thinking you can feel performance with any accuracy whatsoever.

Excellent point. As my buddies wife said... “was there a bike you did not enjoy riding?”

Another thing on the sluggish part, like racing cars, what feels fast is not always fast. Smooth is fast. And maybe “sluggish” is smooth. All hard to say, but i agree that feel is only useful if what you “feel” is actually faster.