Road & gravel hookless rims: truth twisted by manufacturers

Hookless rims are a controversial topic. They have some “pros”, in accordance with the manufacturers, and cons — in test data and real-world experience — that you may want to be aware of before you buy.

PRO 1. Hookless rims “are lighter”

There are two schools of thought for hookless rim design.

First — the Zipp way — is to “cut off” the hooks, while preserving the width of the rim sidewalls. This makes for greater internal width at the same external width of the rim — as is shown on the picture below.

In this case, clearly, the rims will indeed be lighter — by the mass of the “cut” hooks, which looks to be really marginal in comparison to the overall amount of material in the rim. Anyway, let’s fix for now — true, the Zipp way allows saving some grams.

Second — the Enve way — is to “add” material underneath the hooks, which results in thicker bead walls at the same internal and external rim widths. Again you can see the demonstration on the following picture.

Apparently, in this case the overall weight will be increased by the amount of “added” material. Again, this hardly has any real-world significance, as the increase in weight must be very marginal. But the fact is, these hookless are, vice versa, heavier.

The theoretical weight benefit — as is promoted by the manufacturers — contradicts the fact that there are many hooked rims that are lighter than comparable hookless (although that could be explained by various small differences).

Light Bicycle, a manufacturer of high-quality carbon rims, has model WR35 that can be hooked or hookless (made the Enve way — while both are identical in any other aspect). The claimed weight, which has a margin of +/− 15 grams is the same for the hooked and hookless variants.

Takeaway: no, in real life the hookless are not lighter than normal rims. (Well, at least if not to try and fit a three-gram peg into a hole of measurement error).

PRO 2. Hookless rims “are stronger”

In the Zipp way — I don’t see how “removing” material from the bead wall would make it stronger. It must either be the same, or even weaker, depending on the materials and/or layup. Less material — means less strength, da.

In the Enve way — yes, the rim should be stronger. This matters if you ride an mtb or hardcore gravel (think more drop-bar mtb), where you run very low pressure and tend to bottom out and hit the rim hard against rocks, roots, and such.

That said, to damage a carbon rim one should smash it so hard that the tyre would get a double-puncture (yeah, the thick tyre, not an inner tube). I had that a couple of times in many years of riding, but okay, things happen. (And aluminium rims bend quite easily — even before the tyre is punctured against them.)

If you ride gravel roads, not trails (basically, where you would drive a sedan), or asphalt of any quality, it doesn’t really matter. If you bottom out to the extent that the rim cracks in such conditions, that only means your tyre pressure was way too low.

But please note the first manipulation in the disseminated manufacturers’ claims — hookless are:
— either insignificantly lighter (in the Zipp way),
— or stronger (in the Enve way).

Never both, as they try to convince us.

Takaway: the hookless can be stronger, depending on the particular implementation (even if that is only valid for mtb, but not gravel or, all the more so, road).

PRO 3. Hookless rims “are more aero”

Here we’ll have to dig a bit deeper. If that feels too complicated to you, jump ahead to the takeaway right before the next section, where it gets simpler again.

In the Zipp way — the difference between the internal and external width is smaller than with both hooked rims and Enve way. For example:
• the traditional rims with 21 mm internal width are normally around 28 mm externally;
• Zipp hookless wheels of the same external width — 27 or 28 mm, depending on the model — have internal width of 23 mm.

Zipp recommends 28 mm (nominal size) tyres for such rims — and per the latest 2023 ETRTO standard that is the narrowest tyre that could be put on a 23 int rim.

IMPORTANT UPDATE:

In 2024, ETRTO updated it’s standard — prescribing minimum 29 mm tyre for a rim of 23 mm internal width. (It should be noted that that happened shortly after a widely covered series of crashes at races due to tyre blowouts from Zipp wheels.)

Shortly after, Zipp has updated its own recommendations respectively — i.e. they omitted their marketing narrative of optimising the wheels for 28 mm tyres — and now just suggest that “non-optimally wide” tyres would be used instead.

/Q.E.D.

Factual tyre width differs from the nominal depending on the internal rim width — the wider the rim, the wider the tyre will in fact measure on it.

Take bicyclerollingresistance.com (BRR) where factual tyre widths are precisely measured on a rim with the same internal width — historically that’s 17.8 mm for them, which is quite narrow by today’s standards. Out of twenty tested tyres labeled 28 mm, only five measured slightly narrower — and the average factual width has been 28.55 mm across the label.

On a 23 int rim, which is 5.2 mm wider than above, there is no way a nominal 28 mm tyre would sit narrower than 28 mm — in fact, by my experience, it would measure not less than 30 mm.

In front of me right now are wheels with int 19, 21, 23 & 25 mm that I’ve all ridden extensively with various width tyres, and took measurements with calipers. So trust me on that. Or, take a look at the picture above — a concrete example of how a tyre labeled “28 mm” can sit on a rim with internal width of 23 mm (even though currently you can’t do that :)

Thus, what Zipp suggests is the presumably optimised combination of a 30+ mm tyre on a 27 or 28 mm ext rim — as it directly follows from the above. And this is even without the consideration of the 2024 update, which even exaggerates what I’m showing.

This contradicts the 105 % rule — that the rim ext width shall be wider than the tyre factual width by 5 or more % for optimal aero performance. Guess who invented that rule… Zipp themselves — back when Josh Poertner worked for them (their better period, for sure).

Okay, one thing is that they contradict themselves, but said 105 % rule both has been confirmed by numerous independent and other manufacturers’ wind tunnel tests, and has a coherent engineering explanation.

If the tyre is even slightly wider than the rim, that breaks the laminar air flow, which never reattaches to the deep aero-shaped rim defeating its purpose and dramatically decreasing the aerodynamic efficiency.

Let’s get to the Enve way — both internal and external rim measurements are about the same as with the hooked rims. While I don’t see how what happens inside the rim could affect the aerodynamics, let’s give the manufacturer the benefit of the doubt and assume that there may be a slight difference in the corner between the rim and the tyre due to how the tyre bends around the rim. After all, Enve says a difference exists — based on their own (non-published) tests.

Again, here is a scheme of the same-dimension hooked and hookless Enve-style rims with the tyres on. Decide for yourself if you think there can be a significant difference in that corner. What I’m sure of — even if there is a difference, it cannot cause or prevent the detachment of the laminar flow, so the difference in aero-efficiency (if any) must be non-comparable with the instance of tyre-wider-than-rim against an aerodynamically optimised setup.

From all that we know from wind-tunnel tests, the real-world difference (at 30 km/h) between shallow box-section aluminium wheels (where tyres are much wider than the rim) and the latest & greatest 90-mm deep carbon aero-wheels in an optimised setup is less than 20 watts. The difference between more comparable mid-section wheels is around just several watts. So, that “corner” difference that is even hard to perceive (if it even exists) would save how much — several tenths of a watt, or even hundredths?

The conclusion is — no, hookless rims are not more aero, really. If anything, they are potentially less aero — depending on the particular implementation — if to base the analysis on the existing data and (incoherent) claims of the same leading manufacturer Zipp.

As far as Enve, we don’t know on what speed, etc. a difference was measured by them, nor even what was that difference. But what we have here is their official picture comparing hooked and hookless rims.

Here Enve highlights two things: tyres much wider on hooked rims (bad), and those corners significantly tighter on the hookless (good). Both are manipulatively incorrect.

1) The hookless rims on the picture are much wider — which allows to demonstrate the “lightbulb” effect (that breaks the 105 % rule) on the hooked rims. There would be no difference in that regard if the comparison was of apples to apples, not apples to watermelons.

2) The demonstrated rims are in fact Zipp-style, not even the way Enve actually produces their wheels. As shown, if the hookless bead walls are thinner, on the one hand, yes, that allows shrinking the undesired empty space in that corner between the rim and the tyre. But then the insufficient difference between the external and internal widths will inevitably create the lightbulb effect, as extensively argued above in the Zipp case. It’s either one or the other — no lightbulb effect OR tighter corners. But Enve choses to compare incorrectly sized rims to try and prove a false idea that you can have both.

So, even in the lack of published test protocols and the results — I think now we understand where Enve may have taken the idea that hookless “are more aero” — no wonder the test protocols and results have never been published. I’m sorry to say that this is not the only inconsistency I noted in Enve’s attempts to explain hookless — but digging into such intricacies is beyond the scope of this post. What is clear, both Zipp’s and Enve’s arguments are meaningfully flawed resulting in wrong conclusions.

PRO 4. Hookless rims are cheaper (to make)

No doubt here. Hookless rims require much less complicated hardware and process due to their simplified shape. Why that should concern a buyer of Zipp NSW or Enve SES wheels with MSRP of 3–4k € is not particularly clear to me, but I’ll leave it at that. Hookless rims cost manufacturers less to produce, fact.

If you feel that you benefit from the lower production costs, consider the cons that manufacturers will never tell you about.

CON 1. Hookless rims are inherently less safe

Here’s another Enve’s official picture :)

Rims have different internal shapes, which includes:
• bead walls of various tallness
• hooks, if any, of differing dimensions
• beadlocks, if any, in the range of 0.3–0.5 mm
• narrower or wider (and shallower or deeper cut) beds
• and, interdependently, shelves of changing widths.

Tyres also differ in casing stiffness, bead thickness, etc. Depending on all that, the rim can hold the tyre better or worse in place when it is deflated, momentarily overinflated, deformed, or burping.

That’s where the traditional hooked rim profile adds a margin for error by locking the tyre bead from two opposite directions — holding it in place with the beadlocks on the shelves, as well as with the hooks on the bead walls. With this construction the tyre-rim coupling is inherently more secure than with hookless and leaves room for error in precise matching of components produced by different manufacturers.

With hookless, everything depends on the assumption that there is no mismatching dimensions (outside of the slim tolerances — or even within them as happened just recently with Extralight/Continental scandal, where the tyre blew off the officially compatible rim on camera — with the resolution that the rim size was deliberately made just under the spec to allow for easier tyre installation) and non-defective tyre beads (whether for a particular sample/batch — or by design of a whole model, as with the recent Pirelli P Zero Race recall, which were apparently not safe to use on hookless rims despite being certified for that). In case of an error, there is no extra safety mechanism in place, such as the case with hooked rims. And errors happen.

Maybe that’s the reason why we’ve seen so many hookless blowouts in races — where the bikes are looked after by highly professional mechanics, the riders are slim and skillful, the roads are pristine, and all other reasons why that should not happen at least in such setting. We know that pro teams are now using tyre inserts that push tyre beads outside towards the hookless bead walls & beds to minimize the risk — I guess that’s one way to solve the serious problem where you don’t have a choice of what wheels to ride. Even if those inserts eat up exactly those several watts that everybody is trying to save with the advanced wheel technologies.

CON 2. Hookless rims do not work with road-width tyres

There is the hard pressure limit for hookless rims — 5 bar / 72.5 psi. It is prohibited to have more in your tyres when running hookless. Let’s leave aside for now the fact that the tyre pressure increases in the heat (especially under direct sun) or when going up the mountains (because of atmospheric pressure drop) — and just work with said limit as is.

I’m using Zipp’s own tyre pressure calculator (https://axs.sram.com/guides/tire/pressure). Parameters:
• Bike weight — 12 kg (this includes 8 kg bike + 4 kg for water, shoes, helmet & kit — not heavy!)
• Tyre labeled width — 28 mm (again, what Zipp recommends for their road wheels)
• Inner rim width — 23 mm (what Zipp wheels actually are, from 303s to 454 NSW)
• Tyre casing — thin (this represents high-performance road tyres, such as Conti GP 5000 or such)
• Ride style — road; surface — dry; wheel diameter — 700c.

The maximum rider weight before we exceed the pressure limit is 73 kilograms. That’s less than Peter Sagan, Wout van Aert, or Mathieu van der Poel — some of the best professional cyclists in the world — and way less than an average male weight pretty much all around the world where such statistic exists.

But wait, I’ve forgotten to switch from hooked to hookless. I do that and immediately the pressure numbers drop by roughly 5 psi. But why? BRR has performed a couple of tests on how tyre rolling resistance is affected by (a) hooked vs hookless at the same width & (b) various rim widths with different tyres. In both scenarios the rolling resistance is practically not affected (varies close to the margin of error). Thus, there is no engineering reason to recommend lower pressure on hookless.

So, what Zipp suggests to their clients who buy hookless wheels is just to drop the optimal pressure by circa 5 psi — making it suboptimal — apparently with the only purpose to try and keep more people of different weights within the 5 bar limit. Even then, the maximum limit is exceeded again at the rider weight of 89 kg — which is still less than an average adult male in the US. So, Zipp wheels are no good for average people?

Anyway, the Zipp calculator is less reliable than Silca one (https://silca.cc/pages/sppc-form), so let’s try that. Parameters:
• Surface condition — new pavement
• Measured tyre width — 30 mm (again, assuming a 28 mm tyre will be wider on a 23 mm int rim)
• Tyre type — high-performance
• Average speed — fast group ride (this setting has almost no effect anyway)
• Weight distribution — road bikes; wheel diameter — 700c.

And the 5 bar limit is reached at the rider weight (system weight minus 12 kg, same as above) of 59 kg! That’s less than Annemiek van Vleuten, one of the greatest female riders. Silca calculator does not discriminate between hooked and hookless — which is in accordance with the test data — so there’s that. And remember, that is the upper limit, and ideally you wouldn’t want to be close to that but keep a certain margin for ambient temperature and pressure changes. And for a possible error in your pump gauge. And for a potential defect in either rim or a tyre...

Basically, if you are an averagely sized person, you cannot use 28 mm tyres — to which Zipp wheels are aerodynamically optimised — at an optimal pressure. Even if to trust Zipp themselves for the data. You will have to either run suboptimally low pressure, or throw the aerodynamic optimisation out of the window and install wider tyres. Yes, on your 4000 € aero wheels. (But nevermind that, we already know from above that their aerodynamics was compromised to begin with by braking the Zipp’s own 105 % rule.)

CON 3. Hookless rims do not hold tyres on the bead

When you completely deflate a tyre installed on a hooked rim, its bead is firmly held in place between the hooks and beadlocks and, thus, it does not require a booster/compressor/CO₂ to be reinflated again.

This comes handy for regular checks of the sealant — where you can just unscrew the valve core, check the sealant level with a stick, add more if required right through the valve, and screw the core back again and inflate with any pump. Moreover, if you have a big puncture, it may be that the tyre is completely deflated before the sealant can clog the hole. In such a case, if the bead is still locked by the hooks on the rim, you normally can just repair it with a regular rubber plug, dynaplug or dart — and keep riding.

However, in case of hookless rim the tyre is often unseated in such situation. Which means that you cannot put it back in tubeless configuration with a carry-on pump. You could try that with CO₂ — but some sealants, including the popular Silca, are not compatible with that. (I recommend Orange Seal, which is compatible.) Also, with the single-use canisters it may or may not work on the first try — but you don’t have a second one by definition.

So, the only way to keep riding would then be to put in a tube. From experience, if you are on a terrain where you’ve got a big hole in your tyre, riding on with a tube means that you’ll puncture it soon — obviously, that’s much easier to do than with tubeless. And some manufacturers of hookless state that you are not supposed to use tubes at all, and only a tubeless configuration can be used with their rims. Anecdotally, you can use tubes, but without an accurate manometer will have to cautiously balance between the tyre that is too soft and prone to snakebites — and dangerously overinflated over the hookless limit.

Summary

Out of the manufacturers’ claims — that they actively promote through those cycling media which are ready to promote whatever if it is paid for — that hookless rims “are lighter, stronger, more aero, and cheaper”, only the latter is true without huge reservations: while the top hookless wheels still cost more than some of the best available hooked ones, I admit that Zipp 303s or Enve Foundation seem to have emerged at that price point due to the use of hookless (although, again, you can buy good hooked wheels for that money as well — with better hubs than Zipps for sure, *in my opinion*, lol).

At that, as a part of package you get not only serious safety concerns, but also significant inconveniences: from not being able to use optimal tyre pressure and/or tyre width that the road wheels have been designed for — to being stranded if you have a serious puncture in the middle of nowhere and run out of tubes on a rough gravel. Why on earth one would accept such tradeoffs is beyond me — hence this post. Because I want you to have a full picture.

In my opinion, hookless rims are acceptable at pressures significantly lower than 5 bar / 72.5 psi: on gravel bikes — or, in a pinch, on road bikes with 32+ mm tyres (if you are lightweight). Even though I’d still choose hooked any day of the week — even if for mere convenience of the tyre staying seated on the rim when deflated. But the idea of designing hookless wheels around 28 mm tyre, let alone selling (and buying) those wheels at multiple-thousand-dollar price level would be ridiculous — if it wasn’t just dangerous. Ride safe!