Cycling and standards. Although the bicycle is a remarkably mature design with nearly two centuries of input and iteration, engineers and marketers always have a new idea. That’s why cycling has so many standards, from axles to seatpost diameters and, of course, wheels. 

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While wheel diameter has been a stable aspect of road cycling, off-road riders have experimented with and integrated several sizes. Mountain bikes evolved from their original 26-inch wheels to 29ers, with 650b appearing somewhere along that journey, but now only commonplace on tiny frame sizes. 

Gravel bikes have journeyed differently regarding wheel size. Wheel and tyre manufacturers took the existing 700c road bike wheel diameter and developed wider rims and larger volume tyres instead of attempting to create a ‘gravel-specific’ wheel diameter. For riders who wanted a more agile gravel bike, category planners simply refined their ultra-lightweight mountain bike 650b products. 

But now there is the potential of a gravel-specific wheel diameter, something larger than either 29er/700c. It’s called 750d, and WTB is responsible for the rims and tyres. The Californian supplier has partnered with Colorado titanium frame specialist Moots to showcase the first bike with 750d wheels. But why?

Is this gravel biking’s ‘29er’ moment?

Engineers and product developers never stop imagining or experimenting, but frame packaging constraints often limit what is viable for the market. In any debate about the merit or value of a new wheel size, one must reference the mountain biking experience with 29er wheels. 

mullet bike cover.jpg, by Liam Mercer

There were many sceptics when the earliest 29er mountain bikes appeared in the late 2000s. The superior rollover and larger contact patch, enhancing grip, were technically indisputable. But the application of 29er wheels was ungainly at first. 

The original 29er mountain bikes were ungainly to ride, suffering from several geometry and suspension design compromises. As geometry standards evolved, the potential of 29er wheels for mountain biking went from promise to purpose – and the rest is history, with 29er wheels being the dominant mountain biking wheel size. 

Will 750d be for gravel bikes, what 29er wheels were for mountain bikes? That’s a good question. 

Do gravel bikes need bigger wheels?

The margin between a 750d and 700c wheel is much smaller than the change from a 26- to 29-inch on a mountain bike – and therefore, the potential benefits are correspondingly minor. In metric measurement, the difference between a 700c/29er wheel and 750d is 38mm, the same as between a 650b and 29er mountain bike wheel. 

Moots-CRDD-prorotype-750d, by Suvi Loponen

Gravel bikes journey along much milder terrain than mountain bikes. When descending a technical black-rated mountain bike trail with severe rock gardens, large roots and sizeable drop-offs, the rolling stability of a 29er wheel, as opposed to a 27.5- or 26-inch wheel, is very real. But gravel bikes don’t ride black-graded singletrack. 

For gravel bikes, it’s a very different application. Gravel bikes have benefited from significant developments in tyre casing size, enhancing grip and ride quality by allowing riders to roll wider tyres at lower pressures. But some corrugated dirt roads are not the same as an enduro- or downhill race rock garden in mountain biking. 

When gravel bikes became popular in the mid-2010s, any 700 x4 0c tyre was considered huge – and often beyond the clearance limits of most frame designs. But rider demand has dramatically changed frame design and 700c gravel bike component sizes. Contemporary gravel bike frames are built with fork and rear triangle clearance for 700 x 47c tyres, with many riders choosing 700 x 45c as their default size. 

Narrow casings for 750d

The new ‘oversized’ gravel bike tyre from WTB measures 750 x 40c, narrower than a popular tyre from WTB’s current gravel bike portfolio, like the Riddler 700, which measures 700 x 45c in its largest size. If the air volume difference between 750 x 40c and 700 x 45c is minimal, why would riders opt for the larger diameter size? That’s the issue worth debating. 

The difference between 750d and 700c is so small, that it could hardly classify as a worthwhile benefit in rollover or small bump suppression. But the issues that arise with the 750d size shouldn't be insignificant. 

Even the smallest measurements significantly influence frame geometry. A few millimetres or half a degree make a big difference when rolling along at speed, or riding up or down steep terrain. A larger wheel raises the bottom bracket height, unless accounted for, making for a less agile bike through tight corners.

The other issue with 750d is frame compatibility. It’s the reason WTB partnered with Moots for its 750d wheel and tyre project. Moots is renowned as a fabricator and, because its frames are built by hand, welding and manipulating titanium tubes instead of creating carbon frames from set moulds, those welders involved with the 750d project could shape the additional clearances required for 750d tyres. If 750d was a standard coming in the very, very near future bikes will have to be custom-made to accommodate the larger wheel diameter, which means very premium pricing. That is unless brands are already secretly working on 750d-compatible moulds.

A small niche for big riders

Is there a market for 750d wheels and tyres? There certainly could be, but it won’t be the current middle spectrum of gravel bike riders. 

The cycling industry has developed many tyre tread patterns and casing sizes in response to the sustained growth of gravel biking. This has made the abundance of choice in tyres with 700c wheels more than most gravel bike riders ever need. 

Where 750d wheel and tyres, especially future casing sizes wider than WTB’s current 750 x 40c option could be valuable, are for large gravel bike riders. 

If you are a 6ft+ rider weighing over 200lbs, even a 700 x 47c tyre could be vulnerable to pinch flats when run at low pressure. For riders on XL, XXL and custom XXXL gravel bikes, 750d wheels rolling 45- or 47c tyres could be ideal, giving them a large enough casing size to experiment with ideal inflation pressures. 

For custom frame makers and clients requiring XXXL bikes, 750d could be the proportional wheel size solution for balancing extreme geometry requirements for larger riders. But it will remain a niche and is unlikely to become a standard in gravel bikes, as 29er wheels have become, for mountain bikers. 

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