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> The seeds were planted when DuPont researchers found that thicker membranes were actually proving to be more permeable. This came as a surprise because the co

by rconti 6y ago

> The seeds were planted when DuPont researchers found that thicker membranes were actually proving to be more permeable. This came as a surprise because the conventional knowledge was that thickness reduces how much water could flow through the membranes. It's amazing how often "how it works" is taken for granted. This reminds me of the trend of racing bicycle tires. It used to be "obvious" that you wanted to run a skinny narrow tire at high pressure, for better aerodynamic and rolling efficiency. Even when they started wind tunnel (for tires and wheels) and rolling resistance testing, it was done on steel rollers, not on actual road surfaces. It was assumed that rough roads caused efficiency losses primarily in tire flex (heat), so it made sense to keep pressures high. In the past decade or so, there's been a radical shift towards wider tires and wheels. There are a lot of people riding/racing on 28-32mm wide tires where you would have been laughed at a decade ago, and told to go back to the "efficient" 23mm wide tires. 10-15 years ago, you'd want to be running 120psi on a 23mm wide tire and today you want to run your tire pressure as LOW as possible, and run a wider tire to compensate, to give more air volume to spread the load and bumps across. The rolling resistance is LOWER, because they've found that the energy losses from rough roads or from bumps is in the tremendous amount of energy expended to move the 150-200lbs of "unsuspended" rider and bicycle up and down rapidly. You're losing single-digit watts in rolling resistance on that lower pressure tire, but saving tens or hundreds of watts in energy that would be lost moving the rider's mass up and down. I'm not kidding about the "laughed at" part, either. I'm looking up how wide a tire I can fit on my 12 year old road bike, and there are plenty of downright ABUSIVE forum posts from just 6 or 8 years ago, telling people to go buy a mountain bike if they want to run wider than 25mm tires on my particular bicycle, where I'm trying to fit a 28 and would really like to run a 32 if possible. And a lot of these findings started off as "huh, that's weird" when testing newer wheel shapes in the real world. https://cyclingtips.com/2016/08/cyclingtips-podcast-episode-9-rethinking-road-bike-tire-sizes-and-pressures/ https://cyclingtips.com/2016/08/cyclingtips-podcast-episode-...

baxtr 6y ago
> there are plenty of downright ABUSIVE forum posts from just 6 or 8 years ago Reminds me of another post from today: https://news.ycombinator.com/item?id=25600274 https://news.ycombinator.com/item?id=25600274
kevin_thibedeau 6y ago
We're now seeing hookless carbon rims that top out at 75psi. The industry has unfortunately taken this trend by the reins and will be shoving substandard wheels down everyone's throat in the name of profit.
- fumar 6y ago
  Substandard in what way?
  - kevin_thibedeau 6y ago
    They will fail where a stronger rim won't
    - to11mtm 6y ago
      The botique wheel industry taking advantage of an excuse to cheapen their product? I'm absolutely shocked.
xorcist 6y ago
Sounds like the optimum size was a little bit off what was previously thought, but not by much. 28mm is still a very narrow tire, by any other standard than racing bikes. It's narrower than any utility bike and about half than you would run on a mountain bike, and describing the difference as radical seems a bit excessive to an outsider. Those tire pressures also probably excludes anything close to a flat tire. It's not like anyone thought harder tires were always better. Otherwise they would all be running metal or wooden tires. After all, that was what everyone did before rubber tires were invented, which they were because they were more efficient than bumping around on wood. If someone had guesstimated the optimum at 23mm and 120psi and it was measured at 28mm, was it really that bad?
- AnHonestComment 6y ago
  It’s a huge difference — you’re increasing by 20-40%! Which means you’re dropping to 75-100 PSI... which is also a big change from 120 PSI. 1.5 the width and 0.65 the pressure is a “big” change for a part like a tire. That it’s because we assumed something untrue about the mechanics is fascinating.
  - 0xfaded 6y ago
    wow, this is the first time it was pointed out to me that I shouldn't be running my new fat tires at 120 PSI anymore. (face palm).
    - rconti 6y ago
      It's worth giving the podcast I linked a listen. In fact, I just listened to it a 2nd time yesterday after posting this. And it's 4 years old already! It sounds like the primary reason you need(ed) high pressure, in addition to assumptions about rolling resistance, was to increase air volume to protect the rim. As the tire gets wider, the air volume goes up, so the required pressure to protect the rim goes down. Which is why a narrow road tire can have a rim impact at 40psi where a mountain bike tire would not have a rim impact on the same obstacle at 40psi. So, the wider tire ALLOWS you to run lower pressure and not damage the rim with impacts -- completely leaving rolling resistance aside.
mc32 6y ago
That's gotta depend on speed and surface type. I know when as a kid I used to purposely run tires half flat just to see (on a loose dirt road) if wider (low pressure) or narrower higher pressure was easier. Higher pressure won over underinflated in pedaling uphill, unless it was powdery dirt where a wider tire didn't bog down as much.
- AdrianB1 6y ago
  I think the surface is a major factor, but speed not so much. I have no test numbers to prove that, just some practical experience racing MTBs for some years and using various width and pressure combinations based on the surfaces in the race.
icedistilled 6y ago
I think you've misinterpreted the paper, while the bike conventional wisdom being wrong is interesting, the story about the membranes seems to be that there was an unknown additional difference in the membrane structure between the thinner and thicker membranes that caused the unexpected result. This doesn't mean that the thicker membranes were better due to being thicker, but because they happened to have a better structure. If the thinner membranes can be manufactured with that structure, they will most likely still be better. Article abstract says exactly this: "They found that variability in local density most affects the performance of the membranes. Better synthesis methods could thus improve performance without affecting selectivity."
nabilhat 6y ago
If you're not already familiar with bicyclerollingresistance.com (I'm not affiliated, just appreciative), that has been a fantastic resource for understanding what tire, tube, and rim characteristics really mean. There's so much rumor and trend informed butt dynamometer expertise in bicycling and far too little tediously measuring some actual numbers. Even if a rolling drum isn't perfect, it at least correllates to some extent with real world use and is far more consistent than possible by testing on a bike with a human rider. Anyway, there are a few test series on same make/model/year tires over a range of sizes, which back up your point and add more detail as well: https://www.bicyclerollingresistance.com/specials/conti-gp4000s-ii-23-25-28 https://www.bicyclerollingresistance.com/specials/conti-gp40... https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison https://www.bicyclerollingresistance.com/specials/grand-prix... https://www.bicyclerollingresistance.com/specials/schwalbe-marathon-32-37-40-47 https://www.bicyclerollingresistance.com/specials/schwalbe-m...
- rconti 6y ago
  That's funny. I was just riding my commuter bike yesterday and remarking to my wife how it was funny how much slower I am on it than on my road bike. I assumed it had to do mostly with my aerodynamic profile and to a lesser extent what muscle groups engage when pedaling from a more upright position. The commuter is running Marathons in 32c and my road bike has Continental gp5000s :) likely a 30w difference when multiplied out for 2 tires. At a cruising effort where I'm doing perhaps 120-140w, that's huge! And it reflects where you're doing 12mph instead of 17mph.

by rconti 6y ago

baxtr 6y ago

> there are plenty of downright ABUSIVE forum posts from just 6 or 8 years ago Reminds me of another post from today: https://news.ycombinator.com/item?id=25600274 https://news.ycombinator.com/item?id=25600274

kevin_thibedeau 6y ago

We're now seeing hookless carbon rims that top out at 75psi. The industry has unfortunately taken this trend by the reins and will be shoving substandard wheels down everyone's throat in the name of profit.

fumar 6y ago
Substandard in what way?
- kevin_thibedeau 6y ago
  They will fail where a stronger rim won't
  - to11mtm 6y ago
    The botique wheel industry taking advantage of an excuse to cheapen their product? I'm absolutely shocked.

xorcist 6y ago

Sounds like the optimum size was a little bit off what was previously thought, but not by much. 28mm is still a very narrow tire, by any other standard than racing bikes. It's narrower than any utility bike and about half than you would run on a mountain bike, and describing the difference as radical seems a bit excessive to an outsider. Those tire pressures also probably excludes anything close to a flat tire. It's not like anyone thought harder tires were always better. Otherwise they would all be running metal or wooden tires. After all, that was what everyone did before rubber tires were invented, which they were because they were more efficient than bumping around on wood. If someone had guesstimated the optimum at 23mm and 120psi and it was measured at 28mm, was it really that bad?

AnHonestComment 6y ago
It’s a huge difference — you’re increasing by 20-40%! Which means you’re dropping to 75-100 PSI... which is also a big change from 120 PSI. 1.5 the width and 0.65 the pressure is a “big” change for a part like a tire. That it’s because we assumed something untrue about the mechanics is fascinating.
- 0xfaded 6y ago
  wow, this is the first time it was pointed out to me that I shouldn't be running my new fat tires at 120 PSI anymore. (face palm).
  - rconti 6y ago
    It's worth giving the podcast I linked a listen. In fact, I just listened to it a 2nd time yesterday after posting this. And it's 4 years old already! It sounds like the primary reason you need(ed) high pressure, in addition to assumptions about rolling resistance, was to increase air volume to protect the rim. As the tire gets wider, the air volume goes up, so the required pressure to protect the rim goes down. Which is why a narrow road tire can have a rim impact at 40psi where a mountain bike tire would not have a rim impact on the same obstacle at 40psi. So, the wider tire ALLOWS you to run lower pressure and not damage the rim with impacts -- completely leaving rolling resistance aside.

mc32 6y ago

That's gotta depend on speed and surface type. I know when as a kid I used to purposely run tires half flat just to see (on a loose dirt road) if wider (low pressure) or narrower higher pressure was easier. Higher pressure won over underinflated in pedaling uphill, unless it was powdery dirt where a wider tire didn't bog down as much.

AdrianB1 6y ago
I think the surface is a major factor, but speed not so much. I have no test numbers to prove that, just some practical experience racing MTBs for some years and using various width and pressure combinations based on the surfaces in the race.

icedistilled 6y ago

I think you've misinterpreted the paper, while the bike conventional wisdom being wrong is interesting, the story about the membranes seems to be that there was an unknown additional difference in the membrane structure between the thinner and thicker membranes that caused the unexpected result. This doesn't mean that the thicker membranes were better due to being thicker, but because they happened to have a better structure. If the thinner membranes can be manufactured with that structure, they will most likely still be better. Article abstract says exactly this: "They found that variability in local density most affects the performance of the membranes. Better synthesis methods could thus improve performance without affecting selectivity."

nabilhat 6y ago

If you're not already familiar with bicyclerollingresistance.com (I'm not affiliated, just appreciative), that has been a fantastic resource for understanding what tire, tube, and rim characteristics really mean. There's so much rumor and trend informed butt dynamometer expertise in bicycling and far too little tediously measuring some actual numbers. Even if a rolling drum isn't perfect, it at least correllates to some extent with real world use and is far more consistent than possible by testing on a bike with a human rider. Anyway, there are a few test series on same make/model/year tires over a range of sizes, which back up your point and add more detail as well: https://www.bicyclerollingresistance.com/specials/conti-gp4000s-ii-23-25-28 https://www.bicyclerollingresistance.com/specials/conti-gp40... https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison https://www.bicyclerollingresistance.com/specials/grand-prix... https://www.bicyclerollingresistance.com/specials/schwalbe-marathon-32-37-40-47 https://www.bicyclerollingresistance.com/specials/schwalbe-m...

rconti 6y ago
That's funny. I was just riding my commuter bike yesterday and remarking to my wife how it was funny how much slower I am on it than on my road bike. I assumed it had to do mostly with my aerodynamic profile and to a lesser extent what muscle groups engage when pedaling from a more upright position. The commuter is running Marathons in 32c and my road bike has Continental gp5000s :) likely a 30w difference when multiplied out for 2 tires. At a cruising effort where I'm doing perhaps 120-140w, that's huge! And it reflects where you're doing 12mph instead of 17mph.