If you’ve ever installed a tube or tubeless tyre on your mountain bike you probably noticed the ridge of material at the top of the rim that juts several millimeters inward from the rim’s sidewall. It’s called the bead hook, and a growing number of carbon rim manufacturers are doing away with this feature in favor of rims designed with straight inner walls. Performance gains, durability and lowers manufacturing cost are all reasons company’s cite for moving away from this long-time rim feature.
Why? Let's delve into the tech behind this trend.
What does a bead hook actually do?
The hooks on the sidewalls of bicycle rims came about as a way to center the tyre around the rim while the tyre was being inflated. They were initially developed as a way to hold road tyres on the rims when inflating them to higher pressures.
Several things have happened since the bead hook was introduced more than 50 years ago that make it a vestige of our cycling past. The interface between tyres and rims has become much more standardized. Rims now have central drop channels that aids inflation and in centering the tyre on the rim. Many rims also have small ridges on either side of this drop channel, called the bead lock, that prevent the tyre from shifting inwards while cornering. Last but certainly not least, the aramid or Kevlar material used for tyre beads in significantly stronger and less prone to stretching than tyres of the past.
ENVE's original rim design above used bead hooks; the company has recently moved away from them, citing higher impact strength and performance gains
What the bead hook doesn’t do is to prevent the tyre from blowing off the rim or keep tubeless tyres from burping.
Benefits of hookless mountain bike rims
Rims without bead hooks are nothing new, in fact they are standard for most automotive and motorcycle applications.
Specialized’s wheel brand, Roval, introduced carbon-rimmed wheelsets with hookless rim profiles last year. Within the last week ENVE and Ibis have also debuted new wheelset built with hookless carbon rims. The ENVE M-Series rims do away with the hooked bead design of their previous XC, AM and DH-branded rims. Ibis surprised many with the introduction of its ultra-wide carbon rims, which were co-developed with Derby, who introduced wide carbon rims without bead hooks in 2013.
The new ENVE M Series rims cover everything from cross-country to downhill (left to right) and do away with the bead hook while adding a bead lock feature down the center of the rims
Specialized product manager Joe Buckley said the design team made the transition to hookless carbon rims as a way to improve impact resistance and cut manufacturing costs.
“In carbon rim production, there are a couple ways to make a bead hook. You can mold it into the rim, which is pretty challenging because you are trying to pack a lot of carbon material into a very small space. This can lead to high reject rates at the factory and drive up cost of the rim. Or you can mold the rim and then machine the bead hooks into the sidewall, which creates a nice bead hook, but adds steps to production, which costs more, and also when you machine the rim, the carbon fibers are being cut, which weakens the rim a bit. By creating rims without bead hooks, we were able to get the impact resistance of the rim to improve by a lot, and bring the cost of the rim down as well,” said Buckley.
Scott Nielson, director of R&D for ENVE Composites, notes that performance gains were also part of the transition to a hookless design.
“Through our development program we discovered that by removing the hook we could produce a rim and tyre combination that performed better that rims with hook beads. We feel that the hookless system gives us a performance advantage. The tyre shape that results is more stable and gives the rider a wheel that corners better and will not burp,” said Nielson.
Of burps and blow-offs
The primary concerns these companies hear about the transition to hookless rim profiles are fears of tyres blowing off or of an increased likelihood of burping air from tubeless tyres.
In both cases, the bead lock (the small ridges on either side of the drop channel) is the primary component in retaining the tyre and preventing blowoffs.
These cutaways of Derby rims show the drop channel, used to aid in mounting tyres, and the raised bead locks on either side of this central channel that prevent the tyre from unseating and burping air
EVNE states that the company has been prototyping M-series rims for a year and found greater burp resistance through laboratory loading and months of field-testing with a variety of tyres.
Roval rims have been on the market for more than a year and customers appear pleased with the design
“When we introduced our first hookless rim a couple years ago, we did a lot of blow-off testing with our tyres as well as our competitors’. We were pretty confident that tyres coming off were not going to be an issue. We also logged a lot of ride hours on them. We never had a problem with tyre retention in development, and in the couple years they’ve been on customers’ bikes, tyres have been staying on just fine, no matter tubed or tubeless. To sum up, existing standards that companies are using today have been fine,” said Buckley.
The way forward?
So if the benefits are lower manufacturing costs, improved durability and better trail performance, will hookless rims become the norm for all mountain bike rims in the future? Buckley thinks it is possible.
“If you’d asked me that a few months ago, I would have probably answered with ‘maybe.’ With the product releases from our competitors in the past week or so though, it’s looking like what the Roval team discovered might be quickly becoming the norm for carbon," Buckley said. "As for alloy, that remains to be seen. The manufacturing method is very different, as well as the material, so it’s a bit of a different animal. There is definitely something to it, though.”
Ibis' Scot Nicol agrees. "We feel that hookless is the future. Enough companies have done it successfully that others are starting to get convinced hooks are not needed for low pressure tubeless tires."