Selecting the right front tire

[js]

I've decided on a new wheel build with a 23mm IW hooked rim on the front wheel. The external rim width on this set-up is 30mm, and one of the goals of the new wheelset is to boost aerodynamics versus my current wheelset.

With aero in mind, the rule of 105% says that I should be looking for a tire with a measured width of 28.5mm on the i23 rim. The problem is that my preferred Conti GP 5000s TR come in a narrow 25c and a wide 28c size. From what I've gathered, the 25c size would be 27.0mm and the 28c would likely be 30.0mm - both give or take 0.5mm for tolerance and variability.

The Michelin Power Cup is a bit wider and is one of the very few tires that deliver equal rolling resistance, cornering grip and puncture protection to the Conti, but I also love to corner hard, so the tread size of the Michelin is a concern. While the 25c Michelin actually measures notably wider overall (4mm bead to bead) the tread is actually 1-2mm narrower than the 25c Conti.

It seems a compromise is needed, and I don't feel like it should be in the quality of the tires. My questions then are:

1) Is it safe to mount a 25c Conti to a i23 hooked rim? I wouldn't consider this one on hookless, but even with hooks it feels a bit stretched. ERTRO is uselessly behind the times, so I guess I'm wondering if anyone has experience with real-world limits on internal rim widths vs tire size.

2) If the 28c Conti is the better choice, how much are the aerodynamics likely to be impacted by a 30mm tire on a 30mm rim? I remember the graph below showing the Zipp being equal vs the Bontrager being 105%, but I've never seen anything that equates watts to the amount of blue in those graphs.


3) Should I just stop being a baby and get over concerns that my other-worldly abilites will out-corner the Michelins?

[Hexsense]

Yes I used 25mm on Light-bicycle AR56 (23mm internal) for a while.
It works. No performance or reliability issue. But you don't get tall tire which better resist against pinch flat like a real 28mm tires though. Stretched 25mm is still not tall.

[K4m1k4z3]

S-Works Turbo Rapidair 26?

[toxin]

You don't need to target 105% exactly, that was for tubulars. As long as your tire isn't wider than the rim the only real difference in aero will be how smooth is the transition between tire and rim

[Jaisen]

#1 Josh from Silca recently clarified on a podcast that 105% applies to the max rim width, not necessarily at the brake track or right at the tire bead. So is your wheel rim 30mm at the tire bead or at its maximum? Usually the specs give the width at the brake track and at its widest point it is a little wider.

#2 Josh also clarified the most important thing for the aerodynamics is for the tire to be narrower than the rim track, it doesn't matter a great deal by how much

#3 A 700x28 GP 5000S TR on a 21mm ID came out to 30mm for me, on a 25mm ID it came out to 31.5mm, so you are likely on a 23mm ID to be just around the 31mm mark. But other tests from flow have shown that being a tiny bit bigger than the rim and violating the 105% rule doesn't have a big impact. Personally I'd just run the 700x28s.

[Mocs123]

Interesting.....I'm looking into the same issue with a new set of wheels that are 24mm internal and 30mm external at the "brake track" but 31.5mm max width. I've been looking at 28mm GP5000's (with latex tubes) but worried they may be too wide. I've also looked at the 27mm Challenge Criterium RS.

BTW - to the OP - I run 25c GP5000's on Light Bicycle AR56 (23mm internal, 30 external) and have about 11K km on them with no issues.

[js]

Thanks all for the responses thus far.

Glad to hear there have been some success stories running 25mm Contis on i23 rims.

Jaisen - those updates on the 105 rule make sense. Do you happen to have a link or idea of where / when he shared these updates, so I could read (or likely listen to) Josh's explanations directly?

[PeytonM]

Vittoria Corsa Pro 28c measure 29.1mm @70psi on my LB56 (23iw/30ow) after 1500km.

Corsa pro 30c measure 31.5mm @68psi on same wheels.

[Jaisen]

Thanks all for the responses thus far.

Glad to hear there have been some success stories running 25mm Contis on i23 rims.

Jaisen - those updates on the 105 rule make sense. Do you happen to have a link or idea of where / when he shared these updates, so I could read (or likely listen to) Josh's explanations directly?

I think it was briefly mentioned in the latest marginal gain podcast, not sure where exactly. Check the YouTube channel for Silca. AJA#32

[tiz92]

My 25 mm conti gp 5000s come out as 27mm wide on a 21 mm farsports rim. So on a 23 it will be most likely wider, also around 28. I would give it a try.

[js]

I think it was briefly mentioned in the latest marginal gain podcast, not sure where exactly. Check the YouTube channel for Silca. AJA#32

Thank you - they do indeed speak about the rule of 105 in AJA #32. For others and future refernece, it happens at 32:30 in the video linked below:
https://www.youtube.com/watch?v=v-w_CG_EUDw

[parcourswheels]

Hopefully we can add a couple of useful details to help on this one. Firstly, it's worth noting that whilst the rule of 105 is a great rule of thumb, it is exactly that. It works as a guide in the absence of any other data, but unfortunately if you want a definitive answer, the only option is to test.

Saying that, there's a couple of points to consider:

- ETRTO guidelines state that for stated internal rim width, a minimum stated tyre section width of 28mm should be used. N=1 anecdotal input may say otherwise, but bear in mind these guidelines were updated for 2023, after input from tyre manufacturers

- You've not mentioned the rim depth on your new wheel. Our testing has shown that deeper rims are far less sensitive to increasing tyre width than a shallower design. All our #thinkwider front wheels have a max rim width of 32.0mm with an internal width of 22.5mm. Penalty from moving to a 28mm GP5KTR to a 30mm GP5KTR was 2.1W for the Ronde (35.6mm) front wheel, 1.5W for the Strade (49.0mm) front wheel & 0.2W for the Chrono (68.6mm) front wheel

CFD visuals suggest that the deeper foil section (rim depth) recaptures the turbulence created by the wider tyre more effectively than a shallower foil. Regardless of rim profile, I'd expect the same to hold true for a slightly narrower rim moving between a 25 & 28mm tyre width.

[js]

Parcours - thank you for taking the time to weigh in on this.

I wasn't aware that there had been an official update to ETRTO this year (nor that my mind decided to store the acronym as ERTRO?) Despite the update, it seems there's still a large gap between many companies on the ideal of a 5mm minimum overlap.

“Right now it feels like these recommendations are striving to be inclusive of the lowest common denominator versus driving performance."

Enve offered that quote in this article, which I thought did a great job of summarizing the continuing debate:
https://gb.readly.com/magazines/cycling ... 2cff33b107

While the debate in the article is mainly about 28c tires on i25 rims, it's still nice to see a company like Zipp releasing a statement "We know there have been enough wheelsets ridden in the past several years with the 28c tyres on 25mm rims to prove that the combination is safe and delivers many proven performance benefits.” That is certainly more than an N=1 anecdote, even if it should still be taken with a grain of salt (as Zipp aren't the lowest common denominator, etc).


With regards to the rim depth, I'm looking at 45mm deep, so fairly comparable to the Strade from your line. Good to know things won't be too far off, but I'm still hopeful I can avoid leaving watts on the table.

Once these rims are built, I'll mount both the 25c and 28c tires and make my determination from there. But I know the interpretation of which tire will be a better overall match for the wheelset will be informed by the feedback in this thread, so thank you again to all who shared their thoughts, experience and advice.

[Jaisen]

Parcours - thank you for taking the time to weigh in on this.

I wasn't aware that there had been an official update to ETRTO this year (nor that my mind decided to store the acronym as ERTRO?) Despite the update, it seems there's still a large gap between many companies on the ideal of a 5mm minimum overlap.

“Right now it feels like these recommendations are striving to be inclusive of the lowest common denominator versus driving performance."

Enve offered that quote in this article, which I thought did a great job of summarizing the continuing debate:
https://gb.readly.com/magazines/cycling ... 2cff33b107

While the debate in the article is mainly about 28c tires on i25 rims, it's still nice to see a company like Zipp releasing a statement "We know there have been enough wheelsets ridden in the past several years with the 28c tyres on 25mm rims to prove that the combination is safe and delivers many proven performance benefits.” That is certainly more than an N=1 anecdote, even if it should still be taken with a grain of salt (as Zipp aren't the lowest common denominator, etc).


With regards to the rim depth, I'm looking at 45mm deep, so fairly comparable to the Strade from your line. Good to know things won't be too far off, but I'm still hopeful I can avoid leaving watts on the table.

Once these rims are built, I'll mount both the 25c and 28c tires and make my determination from there. But I know the interpretation of which tire will be a better overall match for the wheelset will be informed by the feedback in this thread, so thank you again to all who shared their thoughts, experience and advice.

Another point to keep in mind, the 2023 revision of ETRTO standards were alledgedly due to some 28c tires blowing off of 25mm internal width hookless rims. It isn't clear to me there were any problems using 28c tires on 25mm internally wide hooked rims. I wish the standards distinguished more clearly between the max allowable tire size based on whether the rims are hooked or not.

There were some YouTubers that ran an experiment trying to blow tires off rims. Sure this is a n=1 but they managed tire blowouts on hookless rims at close to 100 psi (well exceeding the 72 max psi on the tire label), whereas they could not blow off the same tires from hooked rims even going up to 200 psi. Should this experiment be repeatable, and I see no reason to suspect it wouldn't be, that would clearly indicate a one size fits all standard for hooked/hookless is less than ideal.

[parcourswheels]

I wish the standards distinguished more clearly between the max allowable tire size based on whether the rims are hooked or not.

ETRTO standards don't distinguish between hooked & hookless rims. They are treated the same for tyre sizing purposes. However, the wider internal widths will invariably be seen on hookless rims given how the measurements are taken, i.e. measured between the bead hooks on a hooked rim, which will sit closer together than the rim sidewalls on an equivalent hookless rim.

There were some YouTubers that ran an experiment trying to blow tires off rims.

One of those videos was me doing it with Francis Cade. Sure, it wasn't exactly perfect science but it was aimed at showing that the safety margins are much lower for a hookless rim versus an otherwise identical hooked rim. Our "test" got to 110psi on the hookless rim, but that was with a brand new tyre and a setup that hadn't been ridden. It was also with a wheel that we know definitely complies with ETRTO tolerances, fitted with a tyre that we believe did too. The max pressure guidelines are in place to offer a sufficient safety margin for all setups and scenarios.