Tech Mailbag #5: Does hookless road tubeless have a weight limit?

Published March 4, 2022 02:11AM

Welcome to the CyclingTips Mailbag column, where you send us your tech questions, and our team of nerds gives you answers. Got a question about wheel and tyre standards? Want to know how to diagnose that weird shifting issue? Wondering where that darn ticking sound is coming from?! Post your question on the CyclingTips Forum and use #askanerd, or alternatively send your questions to tech@cyclingtips.com to be featured in an upcoming CT Mailbag column.

In this week’s Mailbag, we discuss the 72 psi maximum on hookless road rims, how to read tyre wear indicators and whether there’s anything that can be done about a painfully tight rim and tyre combo.

This is a truly great question and one that I honestly hadn’t given enough thought to before you raised it. I reached out to Zipp’s product manager, Bastien Donzé, for insight into this. Donzé is an expert on Tubeless Straight Side rims (aka hookless) and has previously been a guest on the Nerd Alert podcast to discuss this topic. I also added the question as to whether hookless road tubeless has a weight limit. His response is below.

“I can’t really speak to his choice of wheels, for obvious reasons. Hunt and Zipp have a different approach to rim width: while they tend to inflate their outside width for aerodynamic considerations; we like a large inner width, to create better support for the tyre. This is the key to explaining why we suggest such low-pressure recommendations.

I agree that on a wheel with 21 mm internal, 72 psi for a 25 mm tyre will be too low for bigger riders, suggesting this wheel is limited to smaller riders. Rims with a narrower profile tend to pinch the base of the tyre, creating a weakness that is usually compensated by higher pressures. This explains the pushback when we suggest low pressures: people claim they have tried and found that tyres become squirmy, unpredictable. This is due to narrow rims that do not support the tyre.

Wide rim profiles like 303 Firecrest, or my favourite 353 NSW, create a more stable platform for tyres. The wide stance does not pinch tyres at their base, so the sidewalls and casing do not require as much support from air pressure. Moreover, the spring rate is different: if you consider air in your tyre as a spring, then more air volume will change the rate of this spring, just like in suspension. The takeaway is that tyres on wider rims require significantly less air to be supported properly. The demonstration is easy: a tyre at 70 psi will feel harder, harsher on a 25mm rim than the same tyre at the same pressure on a 17mm rim.

My suggestion is to consider wider rims if you’re attached to this specific tyre size. More realistically, you should probably go for larger tyres – this is the recommendation we gave to all our pro teams for the Classics this spring. We have enough evidence now to prove that the small loss of aero on bigger tyres will be more than offset by the gains in vibration loss on any surface. This goes against all we think we know as long-time cyclists, but the evidence is there.

As for whether Tubeless Straight Side has a weight limit, this question is interesting. From a standard standpoint, there is no requirement to check for weight limits on wheel and tyre combinations. With that said, the testing mandated by ISO 4210 for wheel safety are pretty basic, so typically manufacturers have all developed more stringent tests on their own.

Maybe a way to answer is to check the SRAM tyre pressure app. The algorithm there was developed with the best performance in mind (rather than safety/durability) but it gives a good indication of what max weights are tolerable for a specific setup.

If for example, you use 28c tyres on 25mm hookless rims, you can get to 270 lbs. / 122.5 kg on this setup before you reach the limit (72 psi). If your bike is 16 lbs, this brings the max system weight to 286 lbs. For reference, our max system weight on those wheels is 250 lbs. / 115 kg – showing there’s good buffer even for large riders, with wide hookless rims and tyres.

Out of curiosity, I ran a few projections based on running 25c tyres on 21mm rims and found you would need 72 psi in the front and 77 psi in the rear (again with a 16 lbs. bike). As you have pointed out, this is over the limit. However, these recommended pressures become 62 / 66 psi with 28c tyres. I really think the best option is to use larger tyres – hopefully, people realize soon that comfort is not a bad word in cycling anymore. There is more to large tyres than additional plush – they are simply faster.

It’s worth noting that SRAM’s tyre pressure tends to recommend lower figures versus what Silca’s tyre pressure tool does. I’ve also reached out to Hunt for comment on this matter. Consider this a developing story.

Unfortunately you’re experiencing the joys of tubeless rims from a time before agreed upon tubeless road standards existed (and while those standards now exist, it’s still early days in terms of adoption).

That 17 mm internal width points to these wheels being when Giant’s rims were hooked and they made them slightly oversized in diameter to ensure easy tubeless tyre inflation and a secure setup. The downside is that not all tyres feature the same effective diameters or the same amount of bead stretch, and so you quickly run into fitment problems when an undersized or stiff bead tyre is used. I have certainly experienced just this on these very rims.

With the technique sorted, I’d then look to a tyre that fits a little more loosely and offers a bit of stretch in the bead. When seeking looser fitting tyres, I’ve had pretty good success with clincher (tubed) models from Pirelli, Hutchison, Michelin, Specialized, and most recently, American Classic. Meanwhile the likes of Schwalbe and Continental can be quite tough! However it’s worth noting that tyre tolerances can vary and so even within the same brand and model you can experience different levels of fit tightness.

Lastly it’s worth considering what rim tape is in place and whether a thinner tape could be used. The stock Giant tubeless tape is actually pretty thin, but if it’s wrapped twice around or has been replaced with something else then that may be partly to blame. Something like 3M’s 8992 (often marketed as a powder coat masking tape) is a nice thin option to consider.

Failing all that, it sounds like a good excuse for New Wheel Day. Not only will a wider internal rim width improve your ride, there’s the simple fact that life is too short to bleed from your thumbs while trying to fix a flat on the side of the road.

Those tyre indicators are designed to provide a visual cue to the depth of useable tread available. It’s possible that one of those indicators wasn’t made to the full correct depth in manufacturing and so has prematurely worn away, hence the uneven wear. Or maybe you only turn in one direction.

Either way, once all the indicators are no longer visible then it’s time to replace the tyre. That said, I’ve never managed to get that much life out of a performance road tyres and normally multiple deep cuts have me replacing the tyre well before those indicators are called into (or out of?) action.

If a tyre doesn’t have an indicator (or perhaps it did and is now worn away) then I look to the profile of the tyre. A road tyre with a noticeably flat centre will negatively impact the bike’s handling and should be replaced. This is also (partly) why I don’t recommend rotating tyres from rear-to-front, rather I tend to always put the fresh tyre on the front and then move the old front tyre to the rear.