So about that 73 PSI hookless limit

Triathlon Forum
1

I have a pair of Zipp 303S hookless rims on my road bike that I run with 28mm tires. I’m about 182 lbs (82 kg) so my ideal pressure is around the 70 PSI mark depending on the calculator. That’s below the 73 PSI “cutoff” so all good.

I’m now in the market for a set of race wheels for the TT rig. I’m strongly considering the new Zipp 404 Firecrest, which are also hookless, with a 23mm internal width and ~28mm external width. Since the main point here is aero, I’d want to run 25mm tires. Zipp recommends 25s unless you’re over 180 lbs, so it’s clearly the best aero solution.

Now my conundrum is that for a 25mm hookless tire, at my weight (plus some extra frame and water baggage) my optimal pressure is closer to 80 PSI than 70. This puts me over the 73 PSI limit. So the real question is, how much is that 73 PSI number just “covering their ass”? I’ve surely pumped my 303S over that 73 PSI limit a bit when in a rush with no issues. Obviously it’s not a binary answer, but at slightly higher pressures, is there a real danger here? I’m guessing this is where the tire becomes an important piece of the puzzle. I would only consider new, hookless-compatible, tubeless tires. My likely choice would be a 25mm Conti 5000 S TR.

So, ST, is it completely irresponsible to run anything hookless in the 80 PSI range?

Side note, it would be nice if aero wheel manufacturers optimized for 28mm tires if they go hookless.

2

Doesn’t directly address your question, but the 25mm Conti 5000 S TR is not on Zipp’s compatible tire list for their hookless rims. 28mm is, though.

https://www.sram.com/en/zipp/campaigns/hookless-tire-compatibility

3

Doesn’t directly address your question, but the 25mm Conti 5000 S TR is not on Zipp’s compatible tire list for their hookless rims. 28mm is, though.

https://www.sram.com/en/zipp/campaigns/hookless-tire-compatibility

Oof. Thanks for pointing that out! Not a good look to say you recommend a 25mm tire and then not support one of the new gold standards…

4

couple things. first, we’re in the early stages of a compatibility chart. as noted, zipp has its own, and that takes precedence over ours. the way that works, zipp doesn’t do blow off tests (as does CADEX or ENVE), they simply make their wheels to ETRTO spec for hookless beads, and then verify from a tire brand that that brand does the same. if so, that tire gets listed. for example, veloflex just made it onto that chart, and you can read veloflex’s posture toward hookless, which led zipp and veloflex to correspond.

what we’re going to do with our chart is:

  1. make it brand agnostic, that is, it’ll be a compendium of what all the hookless road/tri wheel makers are doing.
  2. keep it up to date as best we can.
  3. if you look on our chart you see an OK in every cell where we think there’s a compatibility match. we’re going to eventually replace that OK with a pressure number, in PSI. that number assumes certain things. for example, there’s a sample road surface, use case, rider weight. the point is to show you all that the optimal pressure might be 65psi rather than the 95psi you’re getting ready to shove in it. we’ll refer you to pressure calculators, like those from silca and schwalbe, for a deeper dive into the right pressure for that wheel/tire combo for you. anyway…

what you eventually discover is what you discovered: that there’s not enough volume in a 25mm tire to justify a pressure lower than 72.5psi (and that’s the actual number) in all cases for all riders. for this reason, i really see hookless road as being a 28mm tire size tech. that size and up. when i ride a 28mm tire on a wheel like the zipp 303 S i ride it at, oh, 65psi. that’s a good number for that combo. that’s a pretty hard tire. right now i’m riding a 32mm tire on a 23mm hookless rim (internal bead width) and i’m riding it at 50psi.

i think what’s going to get explained during 2022 is that a lot of the rolling resistance rigs are incomplete. not wholly real world. for example, that 28mm tire on a 23mm rim, that keeps exhibiting yet lower rolling resistance at 80psi, 100psi, 120psi, most folks in wheel/tire manufacturing don’t believe that’s real world. i think when more testing rigs and protocols start to express their data you’ll find out a few things: lower pressure is actually faster; wider tires are a lot faster than you suspect. i think a lot of folks who keep their ears to the tracks are going to start moving to wider tires, for both road and tri. 28mm is the new 25mm. and that means hookless is back in the game for tri.

as to your last point. that is exactly what is happening. hookless offers some aero advantages. the point of the wider internal bead widths (zipp’s 353 is 25mm) is that a 28mm or 30mm tire on that rim can be quite aero. look for that to start showing up this spring and summer, both in road and in tri.

199482-largest_353nsw.jpg

5

Doesn’t directly address your question, but the 25mm Conti 5000 S TR is not on Zipp’s compatible tire list for their hookless rims. 28mm is, though.

https://www.sram.com/en/zipp/campaigns/hookless-tire-compatibility

Oof. Thanks for pointing that out! Not a good look to say you recommend a 25mm tire and then not support one of the new gold standards…

alrighty. i posted to this, and then pulled the post, because when i went back and looked i see now what is going on.

all the tire and wheel brands involved in road hookless are following the ETRTO’s dicta, per the annual manual it produces. the ETRTO says the following:

  1. the wheels have to follow particular specs (radii at various direction changes, the well, the rim wall, etc.)
  2. tire pressure can’t exceed 500 kilopascals (5 bar).
  3. there’s a chart in the ETRTO manual that shows tire sizes that can go on rims with given internal bead widths. in general, the tire must be 2mm or 3mm larger than the rim’s internal bead width. so, a 25mm tire is the smallest tire that can go on a rim with a 23mm bead width. a 28mm tire is the smallest that can go on a wheel with a 25mm bead width. and so on.

accordingly, nobody’s 25mm tire can go on a hookless beaded rim with a 25mm bead width. if you’ll note, you’ll see that conti’s 5000 S TR is perfectly compatible with every conforming zipp hookless beaded wheel. you just have to follow the ETRTO’s guidelines. the only incompatibility you see is a 25mm conti on the 2 zipp wheels that have 25mm internal bead widths.

6

Thanks Dan! What I suppose I’m wondering is, how hard of a line is that 72.5 psi (5 bar)? As an engineer I assume they’re throwing a LOT of margin on this number to avoid any dangerous situations.

Playing devil’s advocate here I guess I’m just wondering if running at, say, 80 PSI is “very dangerous” or simply not recommended. For example maybe 73 psi is set at a failure rate of 1/1million, and that’s how they set the standard, and 80 PSI is 1/100K, if you know what I mean. In other words I’m guessing there’s no discrete event where 72 psi = fine and 74 psi = death

7

Thanks Dan! What I suppose I’m wondering is, how hard of a line is that 72.5 psi (5 bar)? As an engineer I assume they’re throwing a LOT of margin on this number to avoid any dangerous situations.

Playing devil’s advocate here I guess I’m just wondering if running at, say, 80 PSI is “very dangerous” or simply not recommended. For example maybe 73 psi is set at a failure rate of 1/1million, and that’s how they set the standard, and 80 PSI is 1/100K, if you know what I mean. In other words I’m guessing there’s no discrete event where 72 psi = fine and 74 psi = death

i would not play around with this. apparently - what i heard - is that ISO is recommending a blow-off test that’s 110% of the max pressure. in the case of conti, maybe it’s the 28mm, i don’t remember, there are 2 max pressures: 94psi and 73psi, depending on hooked or hookless. i don’t know what’s printed on the tire. but stipulating that 72.5psi is the max pressure, 110 percent of that is 80%. that would mean that if the tire blows off at 81 percent… 'tsall good!

you… do… not… want… that tire to blow off while you’re riding. so, me? i kind of err in the opposite direction as you. rather than considering some percentage above 72.5psi, i won’t put more than about 65psi in the tire, and more like 60psi. which is fine, because, i don’t ride anything smaller than a 28mm tire. we have partners here, like zipp, like CADEX, and they’re not going to necessarily be 100 percent happy with my writing this, but i just think hookless is a 28mm tech. and up. i just don’t see its utility if you use a 25mm tire, and you pretty much nailed in your original post why that is.

what we need, imho, is something like a rockwell tester for tires. we don’t rely on the pump or pressure gauge anymore as the exclusive determiner of proper inflation. i believe spring rate is the best current term i have for what i’m looking for. how hard is the tire? just like testing the durometer in a shoe foam. if you tested the tires that way you’d see how low the pressures can be in these high-volume systems. the volume of air in a wheel, with a pretty deep well, with a 25mm inner bead width, and a 28mm tire, is huge (by road standards). it doesn’t need much air. and it wouldn’t need much air if it was a 25mm tire in that rim. except you can’t put a 25mm tire on a hookless rim with 25mm internal bead width.

internal bead widths between 22mm and 25mm is what you want in a hookless system, with a tire between 28mm and 34mm.

8

i think what’s going to get explained during 2022 is that a lot of the rolling resistance rigs are incomplete. not wholly real world. for example, that 28mm tire on a 23mm rim, that keeps exhibiting yet lower rolling resistance at 80psi, 100psi, 120psi, most folks in wheel/tire manufacturing don’t believe that’s real world. i think when more testing rigs and protocols start to express their data you’ll find out a few things: lower pressure is actually faster; wider tires are a lot faster than you suspect. i think a lot of folks who keep their ears to the tracks are going to start moving to wider tires, for both road and tri. 28mm is the new 25mm. and that means hookless is back in the game for tri.

This is a very interesting paragraph.

I am not a tire engineer, but, if one doesn’t like the data, just blaming a wide range of testers is, I think, sort of a lazy way out. Because, from my reading, many test rigs for rolling resistance have been verified as doing quite well for real world results because real people have verified the results as the tested tires have gotten similar rr rankings in the real world on real roads.

If most folks in wheel tire manufacturing don’t believe the testing is real world, is there actually any evidence to support that belief?

I think there is no doubt that wide tires and low pressures are awesome for comfort and traction (heck, I ride on 75mm wide tires for training–but not racing). But that is kinda where it ends. When a rider uses recommended real world tire pressures (this is critical–many rr tire tests that seem to favor wide tires have used unrealistic and not advised way high tire pressures) on decent real world roads, there is just no doubt that wide tires are, on the whole, slower. Unbiased test results on good test rigs have and will continue to confirm this. It is just physics.

9

Unfortunately, for someone of your size (and you are not even that big), in my opinion, there are not a lot of good hookless solutions that are fast and safe for high speed road applications.

Tire pressure is tricky. How big is that blow off safety margin? Does anybody even know? Does the company make that information public and in writing? How accurate is your pump? How long has your bike and tires been sitting in the sun? How warm is it outside? How much has the temperature increased since you pumped up your tires this morning? How hot is the pavement at 3pm in arizona (or wherever)? And so on.

A blow off while riding at high speed is not an event to laugh at. If you value your front teeth, I would be very careful. Go with hooked rims, or go with wider (and unfortunately a little bit slower) tires. Don’t fool around with safety.

10

Thanks Dan! What I suppose I’m wondering is, how hard of a line is that 72.5 psi (5 bar)? As an engineer I assume they’re throwing a LOT of margin on this number to avoid any dangerous situations.

Playing devil’s advocate here I guess I’m just wondering if running at, say, 80 PSI is “very dangerous” or simply not recommended. For example maybe 73 psi is set at a failure rate of 1/1million, and that’s how they set the standard, and 80 PSI is 1/100K, if you know what I mean. In other words I’m guessing there’s no discrete event where 72 psi = fine and 74 psi = death

i would not play around with this. apparently - what i heard - is that ISO is recommending a blow-off test that’s 110% of the max pressure. in the case of conti, maybe it’s the 28mm, i don’t remember, there are 2 max pressures: 94psi and 73psi, depending on hooked or hookless. i don’t know what’s printed on the tire. but stipulating that 72.5psi is the max pressure, 110 percent of that is 80%. that would mean that if the tire blows off at 81 percent… 'tsall good!

you… do… not… want… that tire to blow off while you’re riding. so, me? i kind of err in the opposite direction as you. rather than considering some percentage above 72.5psi, i won’t put more than about 65psi in the tire, and more like 60psi. which is fine, because, i don’t ride anything smaller than a 28mm tire. we have partners here, like zipp, like CADEX, and they’re not going to necessarily be 100 percent happy with my writing this, but i just think hookless is a 28mm tech. and up. i just don’t see its utility if you use a 25mm tire, and you pretty much nailed in your original post why that is.

what we need, imho, is something like a rockwell tester for tires. we don’t rely on the pump or pressure gauge anymore as the exclusive determiner of proper inflation. i believe spring rate is the best current term i have for what i’m looking for. how hard is the tire? just like testing the durometer in a shoe foam. if you tested the tires that way you’d see how low the pressures can be in these high-volume systems. the volume of air in a wheel, with a pretty deep well, with a 25mm inner bead width, and a 28mm tire, is huge (by road standards). it doesn’t need much air. and it wouldn’t need much air if it was a 25mm tire in that rim. except you can’t put a 25mm tire on a hookless rim with 25mm internal bead width.

internal bead widths between 22mm and 25mm is what you want in a hookless system, with a tire between 28mm and 34mm.

Great post. Exactly the kind of insight I was hoping for. I don’t have a death wish so I think I’ll forget this notion and look at hooked rims for now.

11

i think what’s going to get explained during 2022 is that a lot of the rolling resistance rigs are incomplete. not wholly real world. for example, that 28mm tire on a 23mm rim, that keeps exhibiting yet lower rolling resistance at 80psi, 100psi, 120psi, most folks in wheel/tire manufacturing don’t believe that’s real world. i think when more testing rigs and protocols start to express their data you’ll find out a few things: lower pressure is actually faster; wider tires are a lot faster than you suspect. i think a lot of folks who keep their ears to the tracks are going to start moving to wider tires, for both road and tri. 28mm is the new 25mm. and that means hookless is back in the game for tri.

This is a very interesting paragraph.

I am not a tire engineer, but, if one doesn’t like the data, just blaming a wide range of testers is, I think, sort of a lazy way out. Because, from my reading, many test rigs for rolling resistance have been verified as doing quite well for real world results because real people have verified the results as the tested tires have gotten similar rr rankings in the real world on real roads.

If most folks in wheel tire manufacturing don’t believe the testing is real world, is there actually any evidence to support that belief?

I think there is no doubt that wide tires and low pressures are awesome for comfort and traction (heck, I ride on 75mm wide tires for training–but not racing). But that is kinda where it ends. When a rider uses recommended real world tire pressures (this is critical–many rr tire tests that seem to favor wide tires have used unrealistic and not advised way high tire pressures) on decent real world roads, there is just no doubt that wide tires are, on the whole, slower. Unbiased test results on good test rigs have and will continue to confirm this. It is just physics.

the tire manufacturers have their own test rigs, pretty sophisticated ones, and they’ve been coming up with some very different numbers. have you read anything from josh poertner on this subject?

12

I have, but not anything super recent. But l would be delighted to learn more.

The tire manufacturers, do any of them seem willing to share what they are learning (that runs counter to lots of other open source testing) with the riding public?

13

A blow off while riding at high speed is not an event to laugh at. If you value your front teeth, I would be very careful. Go with hooked rims, or go with wider (and unfortunately a little bit slower) tires. Don’t fool around with safety.

didn’t you just ask for evidence one post above? what’s your evidence for this statement? i’ve been riding on hookless road rigs for the last 2 years, and i value my front teeth as much as the next guy. knock on wood, you never know what is yet to happen, but i’ve not only never had a tire blow-off, i’ve never had a *flat, over 2 years, thousands of miles. * the thing about hookless, this is the first time that every rim has had to be manufactured to such a tightly circumscribed spec, and every tire has to work on a rim that adheres to that set of specs.

but here is what’s true about this tech: the bead hook lets you get away with a lot. it protects you against overinflation, or against putting a tire too small on a rim too wide. riding with hookless beads is perfectly safe, but you have to use conforming wheels, conforming tires, conforming tire sizes, conforming pressures. if you do, then you have a stronger, more precisely made rim, tires the absolutely, perfectly, fit, tires that mount easily, very little leakdown.

14

I have, but not anything super recent. But l would be delighted to learn more.

The tire manufacturers, do any of them seem willing to share what they are learning (that runs counter to lots of other open source testing) with the riding public?

up 'til now the tire brands that i talk to have not wanted to share their own data. but they get frustrated by data that they don’t feel is reflective of the truth. i found this a pretty interesting article.

it’s not all about rolling resistance. you can have a discussion with exercise physiologists about the impact of muscle vibration on performance; and with mechanical engineers about how vibration slows down a wheel. how would you capture the entire cost of vibration in a system? it would take a polymath engineer to figure that out.

15

It requires a bit more work, but the pressure gauge is perfectly adequate. There is a pretty well established relationship between spring rate, measured tire width, weight, and tire pressure. While I wish they were a little more specific with some things, silca has this published in their pressure calculator. Even if up had a spring rate gauge, it would be dependant on weight and speed, so not exactly a huge savings from pressure.

16

Are you talking about measured tire width or published tire width? I run 25mm tires on 21mm internal rims and they inflate to 29mm at least, which means I run a tire pressure of 76 max (I’m 200 lbs). If I had a 23 or 25mm internal width rim I suspect I’d be a bit wider yet, so I’d be within that 73psi pressure limit (30mm measured puts me at 73psi). Just some food for thought.

17

This whole topic is all very interesting to me and I think I’m glad my wheels are still hooked 😀. At 220+ including the bike anything under 80 looks like it’s damaging the sidewall. Maybe it’s different with the wider hookless rims but with the two sets of wheels with 28’s I’m still riding at 90 with what looks like an appropriate bulge.

18

It requires a bit more work, but the pressure gauge is perfectly adequate. There is a pretty well established relationship between spring rate, measured tire width, weight, and tire pressure. While I wish they were a little more specific with some things, silca has this published in their pressure calculator. Even if up had a spring rate gauge, it would be dependant on weight and speed, so not exactly a huge savings from pressure.

The one area I see where these standards are still deficient would be differences in the rim bed aren’t accounted for in the volume calculation. I remember Andy T from HED chiming in on the difference between the C2 rim profile and the current profile on the Jet+. If my memory is correct, the deeper center channel on the + added an additional 7% to the volume. It would seem like the ETRO standard would need to specify the depth and curvature of the rim bed before you could really have a standard that gave equivalent spring rates at a specified pressure.

19

I measured a 9% increase in volume from a C2 rim (19mm internal) with a GP4000 at 80 psi to a Plus rim (21mm internal) at 70 psi with the same tire.
Keeping the pressure at 80 psi for both makes the volume difference 10.3%
Most of the difference is from the width, not so much internal well geometry.

20

It requires a bit more work, but the pressure gauge is perfectly adequate. There is a pretty well established relationship between spring rate, measured tire width, weight, and tire pressure. While I wish they were a little more specific with some things, silca has this published in their pressure calculator. Even if up had a spring rate gauge, it would be dependant on weight and speed, so not exactly a huge savings from pressure.

The one area I see where these standards are still deficient would be differences in the rim bed aren’t accounted for in the volume calculation. I remember Andy T from HED chiming in on the difference between the C2 rim profile and the current profile on the Jet+. If my memory is correct, the deeper center channel on the + added an additional 7% to the volume. It would seem like the ETRO standard would need to specify the depth and curvature of the rim bed before you could really have a standard that gave equivalent spring rates at a specified pressure.

The depth of the center channel does not make any difference in the ride or the spring rate of the tire mounted on the rim (unless the depth of the channel affects the structural stiffness or ride of the rim). For example, in this sketch of 3 completely fictional ‘nonsense’ rim profiles, all of these tires will ride exactly the same at the same psi. But there is an important caveat: as long as the tire wall does end up touching the rim surface during normal riding or during any impacts from rocks or road/gravel irregularities.

https://i.imgur.com/FJP35Wb.png