As I mentioned earlier in the thread, this BRR article doesn't provide any data or reasoning to back its assumption that spring rate against a flat surface is a good proxy for comfort. This post from Josh Poertner seems to call that assumption into question:

https://silca.cc/blogs/silca/part-2-tire-stiffness-wider-is-stiffer-harsher


I'm not saying that the BRR post is for sure incorrect, but I'm confused by the lack of scrutiny.

Sorry.. I thought the rest was self explanatory.

If you lower pressure on the wider tire, that will shift you to the left on the theoretical/casing losses curve which raises crr. We know from roller testing that a wider tire will roll faster than a narrower tire at the same pressure (casing losses strictly). So if you have to lower pressure on the wider tire to achieve the break point pressure, crr increases to a value very close to the narrower tire.



I am very interested to see your 25 mm vs 28 mm data. It would be nice to see a real world curve for both the 25 mm tire and 28 mm tire for a given road that shows the break point for each tire and all else equal (rider, bike, wheel, road). Similar to the below:

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I did a lot of field testing a long time ago and was never able to repeat those on road data:

https://drive.google.com/...Qn5/view?usp=sharing

I reckon things depend on tire, speed, mass, bump spacing/height.

Reliable testing of your own setup matters.

I'll just respond here... it turns out that Tom and Josh's and a bunch of other people's assumption that tires of a particular width will have the same vibration absorbing/deflection characteristics is WRONG.. this is shown quite convincingly to be not true in VeloNews/Wheel Energy analysis of 15 Paris Roubaix tires... I don't won't to break their pay wall.. the info is only in the paid version of article... they use an air shock to simulate the hysteresis of a human body on the bike i.e. impedance loses... it turns out that optimum pressure is NOT correlated with tire width.. it's not actually mentioned in the article, but if you look at the data it CLEALY the case..


I had this discussion with Tom A in post (link below) on Silca Tire pressure calculator.. where he acknowledges this... it was odd because I acknowledged that this assumption is likely correct, but had suggested that it should tested.. the odd part is that they guy that had gone out and tested a universally accepted assumption (higher pressure = less rr) and shown it to be false was poo pooing testing his assumption that had likewise not been tested... and it turns out that when tested this assumption is false as well!


https://forum.slowtwitch.com/...tor_beta_P7083498-6/

Every tire has it's own inherent vibration absorbing characteristics.. some 28mm tire will have higher optimum pressure than some 30mm tire.. other 28mm tires will have a lower optimum pressure that that same 30mm tire.. this is shown by real, analytical tests.. not theory.. if your theory doesn't match reality.. then your theory is wrong.. reality/nature is not wrong...

Edit: Just a note... Tom A was a consultant on the VeloNew article so familiar with the data and testing..

You do realize that when I’m talking a narrow tire vs a wide tire I am talking about the same exact tire but just different widths right? Eg: a Conti gp5000 25 mm vs a Conti gp5000 28 mm. Sorry if that wasn’t clear.

It’s a completely different story if I’m comparing a 25 mm Conti gp5000 25mm tire vs Schwable pro one tt 28 mm.

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Yes... still not correlated..

I don’t see anywhere in the other thread you linked that debunks this. Would love the read the vn article but I’m not about pay for it.

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Well the other thread was about Silca pressure calculator.. which Tom A acknowledges is shown to be essentially just a guess... imo, useless actually because the input of the calculator are not actually correlated with optimum pressure.. do you see Tom arguing that my assertion that tire width or crr are not correlated with optimal pressure is incorrect..?

Examples..
Pirelli Zero Race TLR (TL) - 28 and 30mm had same breakpoint pressure
Vittoria Corsa Control 2.0 graphene (tubes) 30mm break point was lower than 28mm

Another claim you make is also appears to be incorrect.. in EVERY case where the same tire is tested in two sizes at optimum/breakpoint pressure the 30mm aways rolled better (less watts) on a textured drum.. same pressure or lower...

Yes, small bump compliance is dominated by pressure alone... for the same tire. 23mm at 80 psi and 28mm at 80 psi will have nearly the same small bump compliance. So the proxy for comfort is simple... pressure.

I won’t argue the silca tire pressure calculator is perfect. Josh has stated on podcasts that they are actually trying to solve for the breakpoint pressure but it is also just a starting point. People want to argue with them over the validity of the calculator and I honestly don’t know why. If you have data that says there’s a better pressure for your setup, great then go with that. They are providing free information to the masses that is based upon various testing. No one else is doing that and they should be commended for that. Most people don’t have the knowledge or capability to do their own testing.

Just because someone didn’t argue with you on a point you made doesn’t necessarily mean your point is accurate. I can’t speak for Tom so I have no idea if he actually agreed with you.

I would like to see the data on the pirelli and vittoria tires you mentioned. The vittoria tire data would seem to support my theory.

What are you considering break point pressure on a textured drum? I never said a 28 mm would roll faster than a 30 mm at the same pressure. I think we all know the 30 mm will roll faster. But would you not agree that if you continually lower pressure on the 30 mm tire, at some point the crr will increase to roughly the same as the 28 mm?

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• Have learned so much from listening and reading everything I could get my hands on that Josh and Tom have written and said in podcasts etc.. find this really interesting.. they are awesome and trail blazers for sure.. but when you're wrong, you're wrong.. thing I like about these guys is that they are not concerned with tradition, doctrine or being right.. only truth!
• Not arguing that "Silca pressure calculators" is not perfect.. arguing that ALL pressure calculators are inherently useless because the inputs are not correlated with the output, so the output HAS to be wrong.. suspect.. ballpark at best.. Joshes other methods of optimizing are FAR more accurate.. i.e. just filling your tires at different pressures riding on the typical surface you intend to ride on and if you feel road buzz, bumps, vibration.. lower pressure until feels “smooth” or handling is affected.. then add some pressure back.. 'you are looking for smooth'.. I would add... if at the point where handling is affected, you still feel road buzz, bumps, vibration.. try a wider tire and go through process again.. especially for heavier riders! Oh.. and Chung analysis if you are a real hard case... sucker for punishment 😀
• Not concerned with finding a non-optimal pressure where a wider tire and thinner tire will roll the same.. why?? Who cares?? the point to consider is that at it's optimal/breakpoint pressure on any given surface there is a pressure where the wider tire will always roll faster than every pressure of the thinner tire (excluding aero)... so from a practical point of view choose the widest tire that doesn't trade off too much aerodynamically with your setup... or may want to consider a different setup where you can go wider (rims, frame, discs...)

Do you have hard data that supports this? If so, please share it and your testing method/setup.

You don’t post here often and 10% of your posts have been here in this thread. You are also anonymous so if you want to make some bold claims like saying Josh and Tom are wrong, you may want to put your name to it if you want people to believe you.

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yup, i've seen that. but again, here is my hurdle. if drum testing is good at a general assessment of fast versus slow elements of tires (it can parse between tread compounds, let us say), but is bad at knowing what the right pressure is, i don't know how you can develop a chart that shows optimal pressures based on data from a testing paradigm that isn't reliable when it comes to pressures. you just can't squeeze that juice out of that orange. or the dog won't hunt. choose your analogy.

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Dan Empfield
aka Slowman

I was referring to Tom A's real world data/curve. It would be good to replicate his real world testing (I'm assuming he did the chung method) on a 25 mm vs 28 mm tire at varying pressures. I know it may not be the easiest thing to do but I if the data is repeatable it would test a lot of theories in this thread and answer various questions.

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that's what i'll be showing in about a month. that field testing to which you refer.

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Dan Empfield
aka Slowman

I have attempted Chung analysis myself (can see other thread).. to confirm that silca pressure calc was reading high for me.. does work.. is error prone.. but did statistically over runs gets you to answer.. did observe U-shaped!

Golden Cheetah has fitting curve built into their software if you are looking for a short cut or are not an Excel ninja!

Yeah - there are so many variables that they did not address in the article about how they determined Crr and tire pressure that I think it is impossible to really draw any conclusions. There are ways they could have done it that would have favored either tire. It also sounds like they calculated it from road testing which brings in some serious sources of error that I think are probably larger than the differences they found.

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Ed O'Malley
www.VeloVetta.com
Founder of VeloVetta Cycling Shoes
Instagram • Facebook

🎯

😂 Guess it depends on what you think the definition of "RIGHT" is 🤔





Oh... ok..

I had the same suspicions as you state here..

1. does that air cylinder REALLY add hysteresis in remotely same range as a human body? thought, not likely, but didn't really have details on that.
2. The values surfaced really don't have any absolute value anyone should take and use to set up their actual tire pressure.. that those breakpoint pressures likely can't be taken at face values..

BUT... that being said... what I found it interesting is that it does let us do something that's hard to do otherwise.. set all variables constant and see if tire construction itself has inherent characteristics that can affect break point pressure and this testing would seem to indicate... YES, it does... and seems that it's NOT necessarily correlated directly with crr.

So it's not just a matter of rider weight, tire size, surface roughness... seems the specific tire construction also matter in picking optimal tire pressure.. that is something new... no? Not sure they recognizes that as something new in the article.. wasn't highlighted, but a question I was asking earlier and seems we have at least a preliminary answer.. and I think the popular assumption was that the tire construction didn't matter WAS just directly correlated with crr.. but this testing would seem to indicate that is not be the case.

Right...but are you trying to find the tire with the best Crr, or looking for which has the highest (or lowest) breakpoint pressure?

In other words, let's say you have 2 tires and one is 1W per tire faster, but the other has a breakpoint pressure 5 psi higher (even though BOTH breakpoint pressures are most likely well above typical pressures you'd run on surfaces where breakpoint is a factor)?

I don't know about you, but I'd still pick the one with the lower Crr...

To which I reminded him the thread was about setting optimum tire pressure NOT picking a tire.. so is response was kind irrelevant in the respect...

Several years ago, I spent an afternoon testing various tire pressures on my then current setup, in order to find the ideal pressure. This was after noticing that on rides where I didn't get a chance to pump up my tires to whatever the highest number on the sidewall was, I consistently hit higher top speeds on descents. When I went to wider tires a few years later, I repeated the tests. Shortly after that, the Silca calculator came out, and my ideal pressures for both sets of tests were at or within a few psi of what Silca was giving me...

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"I'm thinking of a number between 1 and 10, and I don't know why!"

Even a broken clock is correct twice a day.. mine was off by 15-20psi 🤷🏽‍♂️

My assertion isn’t that Silca tire pressure calculators are inaccurate or off in some way, but that ALL tire pressure calculators, INHERENTLY cannot be a used as an accurate guide.. they might, by chance get you in the ballpark.. they might, like in my case steer you 15-20psi too high.. [all] the inputs are simplify not correlated with the outputs.. it’s not that they don’t work.. it’s that they CANNOT work!

It’s a clickbait headline by FLO, for sure. The real conclusion seems be that difference in total (aero + .crr) performance between 25’s and 28’s on modern wide aero rims is in the noise. I’m inclined to believe this to be the case in the real world bike splits. No, I don’t really believe that 25s are slower. But I’m not convinced that they’re appreciably faster than a 28.

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"They're made of latex, not nitroglycerin"

Hambini's take on rule of 105.. though you don't want a tire that is bigger than width of rim, the rule of 105 is less important than you might think if you have a sufficiently deep, shallow sloping (i.e. not too v-shaped) rim since the air will tend reattach if this is the case...

Hambini recommends:

• 23mm 45mm+ rim depth
• 25mm 55mm+ rim depth
• 28mm 80mm+ rim depth
• disc.. tire width less important

Go to 59:13 for rule of 105 specific stuff... though entire video is a good watch...