Two Things: The Rule of 105 is No Longer Real, and 1 more...

[maxim809]

Hi everyone,

the tl;dr's are IN BOLD

1. We are rebranding the Racing forum to Watt Weenies. This is now a place to discuss all aspects of performance gains, without judgement. We may consider shadow-copying topics from other boards here to get inertia going.

2. In that spirit, I also want to use this board to keep up to date on latest learnings. This could be sharing brand new topics (novel cycling kit textures; new learnings about nutrition). It could also mean sunsetting old rules that may have once been true in the past, but no longer apply based on new learnings.

So with that, according to Escape Collective performance process podcast, it's looking like the age old adage of Rule of 105 no longer applies with wider rims and tires. Because their full podcast is behind a subscription, I want us to respect EC and be careful not to transcribe, and instead encourage those to maybe consider supporting EC to get the details.

But the tl;dr is those of us running road bikes with modern wide tires and modern rims, should move away from the Rule of 105, and consciously make an effort to stop perpetuating this to our friends, families, and dog.

https://escapecollective.com/podcast-ae ... p-ballard/

[kervelo]

From 2022:

https://blog.flocycling.com/aero-wheels ... ou-slower/

[warthog101]

From 2022:

https://blog.flocycling.com/aero-wheels ... ou-slower/

Read that. Can't hear the other link as I'm not paying.
The tyre widths were very close to the rim width and they did say not to discard the 105 rule.
The tyre still needs to be pretty close to the rim width if it is to be wider it seemed to indicate.

[CampagYOLO]

From 2022:

https://blog.flocycling.com/aero-wheels ... ou-slower/

Read that. Can't hear the other link as I'm not paying.
The tyre widths were very close to the rim width and they did say not to discard the 105 rule.
The tyre still needs to be pretty close to the rim width if it is to be wider it seemed to indicate.

That seems to be the consensus now that the tyre should be about the same width as the widest part of the rim. Significantly wider tyres than the rim will still lead to aerodynamic losses.

[CarlosFerreiro]

Giving up a rule of thumb is fine, now we need each manufacturer to step up with windtunnel data on a full range of WAM tyre widths?

[maxim809]

Thanks for the link from FLO. It's an interesting perspective.

2022 FLO's testing is only on their own set of wheels with two sizes of GP's. Small test matrix.
2024 Swiss Side's claims testing on a variety of wheels with a broad set of sizes and tire models. Larger test matrix.

Swiss Side claims they have not seen Rule of 105 hold across their matrix. In a follow-up they stated there isn't even a rule of thumb they could even provide when it comes to aero gains, because it is so system dependent. FLO's testing is looking at combined aero and rolling resistance, and concluding CdA+Crr can be better even when breaking 105. Swiss Side is claiming they have not observed correlation between 105 and aero gains in their sweeps.

I took that as either:
1. Rule of 105 either having suffered from Cause and Effect fallacy due to limited sample size
2. It could have been true in the past with flat V-shaped rims tubulars, but no longer applies with today's setups.
3. Fundamental differences in the testing protocols between 2002 and 2024

I like my chances with Swiss Side. The podcasts have a free section by the way. There's interesting stuff related to aero.

[warthog101]

From 2022:

https://blog.flocycling.com/aero-wheels ... ou-slower/

Read that. Can't hear the other link as I'm not paying.
The tyre widths were very close to the rim width and they did say not to discard the 105 rule.
The tyre still needs to be pretty close to the rim width if it is to be wider it seemed to indicate.

That seems to be the consensus now that the tyre should be about the same width as the widest part of the rim. Significantly wider tyres than the rim will still lead to aerodynamic losses.

That is my take too.
The podcast stopped before talking about the 105 rule for me.

[cheapvega]

Dylan Johnson just did a video on a gravel setup he rode where mountain bike tires outperformed narrower gravel tires on a deep wheelset in the wind tunnel. In the hierarchy of watt weenieism I think this might be pretty low. Especially considering rolling resistance (though I am annoyed that 2022 link makes no mention of pressure). There's also comfort weenieism that can make a sub 105% tire/wheel setup worth it if it enables less fatigue.

[eurostar]

Let the wheel manufacturers worry about rim width. They should agree a common wind tunnel testing protocol and tell us the drag numbers with a reference tyre, perhaps the bestselling 25mm race tyre. But they'll never do it because they don't want to be proven inferior to a rival. So someone independent should do it. A magazine? Silca?

[otnemem]

Josh from Silca mantains that 105 is a good rule of thumb or starting point.

Unless someone takes 20 popular wheelsets with the top 10 tires and respective widths to the wind tunnel or equally extensive Chung method testing, I'm pretty sure we'll never really know what's watt.

See what I did there?

[AJS914]

Is the Swissside data published anywhere else besides in that podcast?

I was looking at the Flo article. The 105% rule still holds true though they factored in rolling resistance gains/losses.

Wider tires have better rolling resistance at the same pressure as narrower tires but when pressures are lowered on wider tires, rolling resistance is about the same. We see this on brr:

https://www.bicyclerollingresistance.co ... comparison

Dylan Johnson just did a video on a gravel setup he rode where mountain bike tires outperformed narrower gravel tires on a deep wheelset in the wind tunnel.

The mountain bike tire was only 3mm wider than the 50mm "gravel" tire so it's not as dramatic as it sounds. Also, the difference was only .5 watts.

Looking at the two tires, the Race King looks rounder the Cinturato. And the Cinturato, flatter, seems to have more prounounced knobs on the outside edge. All that could easily account for a .5 watt difference.

Also, his most "interesting" finding about the 60mm wheels only showed a 1.4 to 2.3 watt savings. What is the margin of error? Can they even measure such a small differences? Me, being a slow amateur, I'm not going to go out of my way to source deep gravel wheels just to save 2 watts. I get it that for him at the pointy end of things, a theoretical 2 watt / 2 minute savings may be worth chasing.

[tdbayes]

One interesting thing I noticed about the JP Ballard podcast mentioned here: as we know, when considering the tyre width question there is very likely a trade-off between rolling resistance and aero drag. Yet in the Escape Collective podcast before this one JP Ballard said he didn't trust hookless because he'd seen tyres blow off hookless rims "even at 5 and a half bar" (i.e. 80 psi), as if that were some very low tyre pressure.

So whilst I'm sure he's absolutely on top of aero stuff, perhaps he's not that up to date with the wider picture (or indeed ETRTO guidelines, but please let's not start another hookless debate here...).

Perhaps something to bear in mind when listening to his advice.

[eurostar]

What assumptions are you making? What tyre width? The Silca calculator says 5.5 bar is at least 1 bar too low for 25mm measured width.

[AJS914]

What assumptions are you making? What tyre width? The Silca calculator says 5.5 bar is at least 1 bar too low for 25mm measured width.

The Silca calculator just gives a theoretical optimal pressure. I run 80-85psi all the time on 25mm tires. The calculator would want me to run 100/103psi which would be like riding on rocks.

As you can see from the chart, there isn't a much penalty from running less pressure. The largest penalty comes from running too much pressure.



https://silca.cc/blogs/silca/tire-press ... -explained

[eurostar]

The Silca calculator just gives a theoretical optimal pressure. I run 80-85psi all the time on 25mm tires. The calculator would want me to run 100/103psi which would be like riding on rocks.

So your measured width is what...27mm? What are you inputting to get 100 psi?