Are there limits to rim width or should fit fat bike rims to our regular trail bikes? As I mentioned earlier, most vehicles run a 1.5:1 tyre to rim ratio. That’s like mounting your 55mm wide MTB tyres to rims that measure 37mm between the bead hooks. In other words, you’d need to fit an extraordinarily wide rim to do anything weird to the whole tyre/rim interface.
Some tread patterns will perform differently on wide rims. With particularly square topped tyres, it may put the cornering knobs in contact with the trail slightly earlier in the piece. There’s nothing to suggest that this is good or bad; it’s simply different and whether you like the handling with a particular rim/tyre combination will be subjective. There’s almost always a handful of extra grams in a wider rim and this may deter weight conscious riders. On this front, extra-wide carbon rims can be very strong and stiff whilst matching the weight of a narrower alloy model—you’ll just need very deep pockets to buy them. Be aware that carbon rims tend to have very thick bead hooks, so an impressive looking 30mm wide carbon rim may only offer a fairly traditional 23mm internal width, and it’s the internal measurement that really matters.
You may encounter alloy rims that are both wide and light, but a thin-walled alloy rim structure could be more dent-prone than a narrower rim of the same weight. This is something that needs to be judged on a case by case basis. Our Syntace W35 wheels came in at 1,890g for the pair and withstood an absolute pounding, but they use a 560g rim that’s clearly very solid.
If weight is an important consideration, you can also use a narrower tyre to save some grams. The overall package weight may be the same (or potentially lighter) than a fat tyre/narrow rim combination, but you’ll gain added traction and squirm-free cornering at lower tyre pressures—a better proposition overall.
While the wide-rim advantages are pretty clear, those who ride hard on rugged and rock-strewn trails need to be wary of going too low with their tyre pressures. Dropping below 18psi with a 30mm wide rim might create handling problems, but there’s less to stop a big rock bottoming the tyre on the rim. Even if you run a tubeless setup it’s still possible to ‘pinch flat’ the tyre casing if you hit something hard enough.
As a result, going super-wide can be a case of diminishing returns. Changing from a 17mm rim to 24mm will provide a distinct improvement in stability and allow you to drop the tyre pressure by a reasonable amount. Jumping the same amount to a 31mm rim will allow you to go even lower with your tyre pressures but there comes a point where you’ll start bottoming the tyre on the rocks. This might not be a consideration if your trails are soft and demand good flotation and traction, but rocky terrain may force you to keep a little extra pressure in your tyres.
My experience has made it pretty clear that wide rims are much more than a passing fad to extract cash from poor mountain bikers. It’s more a case of righting the wrongs of two decades ago that saw us all mounting balloon like off-road rubber on road bike rims. We’re not saying that you need to rush out and sell your skinny rims but given a choice, there’s clear merit in choosing the fat option when looking for a new wheelset.