Wide rims is one of the hottest trends happening in mtb, and to keep with my pragmatic stance of being grounded with facts, I wanted to eliminate any doubt in statements so this has been many months in the making, giving me more opportunities to actually experience riding a few different sets, such as the Ibis 941 and Roval Traverse Fattie SL.
The tire profile/shape for a typical 2.2″ mtb tire has its ideal shape on a rim that’s somewhere between 18-30mm ID (inner dimension) wide. The wider the ID, the more volume the tire gains and its shoulder knobs get taller very slightly. The overall height does not increase, nor does the tread get wider. Higher air volumes demand lower pressure to achieve the same level of compliance as a lower air volume spring system. As air pressure is lowered, tire contact patch increases. As tire contact patch increases, traction generally increases. A higher volume tire will have an “air spring curve” that starts firm in its early “stroke”, but bottoms out with less force. The raised shoulder knobs, wider tire bead seat, lateral rim stiffness, and firmer feel at a specific pressure, all gives the tire a better cornering feel, and the wider stance in general makes the ride a bit more stable and easier to maintain balance. Older marketing emphasized the OD (outer dimension) of a rim, which determined its lateral stiffness (wider = laterally stiff, and deeper/taller = vertically stiff). A tire that is bigger offers similar benefits, but must have a much more substantial casing reinforcement to combat squirm and buckling and which would increase weight greatly, while a wider rim can enable a comparable increase in performance with less weight penalty.
This is the scientific proof I started off with, and what I aimed to prove later with anecdotal evidence later:
– The red line shows “tire circumference”, a measurement from where the bead hooks touch the tire bead, measured across with the tire flat. The larger a tire “2.2 vs 2.35”, the longer this measurement is. This measurement (126mm) is how long each curved line is in the next image.
– Grey shaded area represents volume and shape of the tire above on a 20mm ID (inner dimension) rim.
– Turquoise area represents the change induced by a 30mm ID rim.
– Going from 30 to 40mm doesn’t seem to offer as much benefit as going from 20-30mm did.
– Magenta area represents the change induced going from 40 to a 50mm ID rim, which shows the first point in which the tire gets a tiny bit taller.
The differences that can be seen are millimeter sized. 10mm of rim width over a normal 20mm ID rim width gets you about 5mm of extra casing width (the part of the casing that is exposed to sidewall cuts), and shoulder knobs that stick up taller about 2mm. You do get a significant air volume increase, which is hard to measure (a lot more complex than finding the volume of a toroid/torus).
The differences a rider can feel is a tire that is much harder to the touch than expected. 20 psi feels like 30 psi, and they must use a gauge and relearn what pressures will work for them. A tire inflated to 20 psi on the wider rim gets a bit better traction than on a narrower rim at 20 psi, for many reasons. At such pressures, the tire on the narrower rim lacks the support to be ridden hard, feeling squirmy, while the tire’s firmness on the wider rim feels like it gives the support of a higher pressure tire, with the contact area of a lower pressure tire, on top of the control offered by the stability of a wider footprint, and responsiveness from increased lateral stiffness, all which improves confidence and makes the rider less likely to do a panic move that would compromise traction.
Carbon rims can increase stiffness without being wide, and still be light. Lighter bikes just simply ride better, as long as ride handling and other things are not compromised. This balance of weight and performance has led to this trend becoming fairly popular with bike brands who decide to research it supporting it with their own production. With this, bikes which don’t mind a little increase in weight will be able to enjoy an increase performance without much trade-off.
Myths and misinformation:
– “Wide rims will make your 2.2 tire ride like a 2.35, and a 2.35 ride like a 2.5, and even downsize your tires from what you’re currently running to something narrower.” Not even half true. Tires are made so differently, even ones with the same name and similar tread pattern, and you cannot begin to generalize how a 2.35 and 2.5 rides, as there are some well-designed 2.5 tires and absolutely terrible ones that seem like all they did was scale everything up without any extra thought put into the design. One thing that larger tires does that wide rim doesn’t do, is make the tire taller and overall wheel & tire diameter larger, making a 26″x2.5″ look almost as big as a 27.5″x2.2″ when seen side-by-side.
– “Wide rims will make your bike as smooth as magic carpet ride.” What this means is up for interpretation. Those made in the middle east are pretty hard, and magic flying carpets seem to undulate and flutter off the ground, requiring a bit more extra balance to ride. Joking aside, the tire is actually firm at low pressures and you would dare not risk going for a more “plush” feel out of fear of rim strikes doing damage. What I find is smooth is better control, which the extra traction, increased stability, and easy to balance nature that the wide rims offers.
Bonus trivia for those who may be interested in a new wheelset:
Short and hookless beads is a recent trend which was made possible by the adoption of tubeless tires needing stronger beads. Folding bead tires in the past were notorious for being rather flexible, able to stretch and blow off of sidewalls if used without a tube, Schwalbe being a brand that struggled with this, and bead hooks didn’t really help. Stan’s No Tubes popularized the shorter hook, which resisted denting since so little sticks up, there’s less leverage for forces to try and fold it. Stan’s No Tubes better ensured their rims’ success by putting a strict pressure limit on their rims, expecting it to be used with regular tires, which kept blow offs to a minimum, since they could not control tire manufacturing. With so many issues, other brands tried to come up with their own solutions, brands with the ability to make their own tires to match their rims having a clear advantage, and those with engineers who listen to live customer feedback also being ahead of the game. Beadless hooks are will be the way to go in the future, but hooks are not bad that you should pass up a killer deal on a rim with one.
Trek’s Boost 148 rear axle standard has been introduced as an open standard, similar to BB30/PF30, 142×12, ISIS, QR15, etc. It is only a matter of time before it is adopted at this rate. It solves problems such as rear wheel dish, poor tension balance between the DS/NDS, lack of strength and stiffness in entry level aluminum alloy wheelsets, and more, all without major compromises such as increasing the BB or crankarm spacing. Perhaps in 3-4 years… if you get a nice and expensive rim, be sure it can be paired with a hub of your choice in the future, if you do not want to buy another expensive rim then.