With some XC bikes coming with 25mm internal rims and some with 30mm internal rim widths... what's the right choice?
As mountain bikers a lot of us love numbers. From Strava times, suspension travel, reach, stack, trail, head angle, offset, BB drop, sag, bar width, stem length, dropper post drop and of course 27.5" vs 29"! Just about any trialhead, forum or group chat is a riddle of numbers, and rim width is no different.
The internal rim width, or distance from one inside edge of your rim to another, has seen a lot of focus over the past few years. With mountain bike rims originally being based on road rim designs, it is easy to see that we needed to catch up to running something suitable. Afterall, why should a 2.1" tyre be used on a rim width designed for a 19mm road tyre? Even with tyre width standards like that (which are over a decade old) it doesn't make sense.
Rim widths continued to get wider and have settled at a few popular options. Notably at 25mm, 30mm and 35mm. You could happily split these into XC, gravity and eMTB/plus bike but that kind of over simplifies it.
Two years ago we compared 22.5mm, 25mm and 30mm rim widths with some 2.4" trail tyres. The consensus was easy - 30mm internal widths gave the 2.4" tyres a better shape and more support at lower pressures. But it wasn't all clear cut, as tyre casing, rider weight and riding style all play an important role.
The big benefits of increased rim width when matched with the right tyres is being able to run a lower tyre pressure, without tyre squirm or flop. You can also have a slightly greater contact patch for better grip, and some say less rolling resistance as the tyre no longer deflects off so many small trail features. For more aggressive riders, a tyre liner like CushCore or a 'noodle' insert is used to add rim protection for heavy hits.
You'll see XC riders on 25 and 30mm width rims, and many gravity wheel sets are still sub 30mm as the benefit of lower pressure isn't always a huge benefit due to rim strikes, so 27-28mm are popular there. But in general, most rims are 25mm internal or 30mm internal.
With Nino Schurter being arguably the best current XCO and stage racer (although Pauline Ferrand-Prevot would be a good second choice) it is easy to look to Nino's spec to see what works for fast riding and racing. He would often be on the previous DT Swiss XMC 1200 spline 30 wheels, like we tested a couple of years ago. Other top DT Swiss sponsored riders opted for those wheels as well, like Florian Vogel and Jolanda Neff, despite a significant weight increase compared to the DT Swiss XRC wheels at the time, which had a narrower rim.
Having tested the new DT Swiss XRC 25mm with Ratchet EXP hubs in late 2019, I also had the opportunity to test the DT Swiss XRC 30mm wheels with the same Ratchet EXP hubs. Just about everything is the same with the wheels, the only difference being 25mm internal widths or 30mm. So with these two near identical wheel sets, I used late spring and early summer to test out the differences when fitted with the exact same tyres.
DT Swiss XRC 25mm vs DT Swiss XRC 30mm
As stated above, these two sets of wheels are just about identical. There is about 60g difference between each wheel set, so it's not really a choice you would make on weight. It really is about choosing which width suits you, your riding - and your tyres.
For the past 12 months I have predominantly raced on a Maxxis Rekon 2.25" front tyre and a Maxxis Aspen 2.25" tyre on the rear. No they aren't the most grippy combination, but the EXO casing is strong as an XC/marathon tyre, they're light and they roll fast. But with about a year on this combination, it would be pretty easy to tell what the differences were. You will need to extrapolate my experiences with these tyres across to your favourite treads. More on that later.
Setup widths on the 25mm rim
The newer Maxxis 2.25" casings work really well on 25mm rims, and inflate to a good size with the edge knobs right on the edge of the tyres. I'm measuring at the width of the casing, as opposed to the external reach of the tread. I have compared both widths before, but these photos should show how the tread placement changes. And yes, all tyres are showing their age!
This is the Maxxis Aspen 2.25" on a 25mm internal width rim.
Interestingly, the Maxxis Aspen does inflate a little wider, even though the casing is the same. I have found this on other Aspens as well, so it's not a one off.
For tyre pressures, I settled on 22.5psi in the back and 21psi in the front, without much variance. I don't jump like World Cup XCO riders do so that pressure worked fine for me. I did ride lower as well and it was ok, but these pressures were the go to unless I needed to vary them for conditions.
Setup widths on the 30mm rim
With a pair of the same tyre models inflated on the DT Swiss XRC 30mm rims, the measurements were just a little bit different.
Here's the Maxxis Rekon 2.25" on the 30mm internal rim.
That's the Maxxis Aspen 2.25" on the 30mm internal rim.
So, yeah they are a little wider. But that also translates to more volume as well, and a slightly larger footpront on the trail. With the wheels in hand, you do notice that the edge knobs have moved inboard a little. Not too far, but a little bit. The sidewall is a small amount more pronounced than the edge knob.
As for tyre pressures, I could run less. But not amazing amounts. I settled at 21psi on the back and 20psi in the front. I did run down to 19psi in the front but sometimes on hard corners it really felt too vague.
On the trail
So while the tyres measured up differently on different wheels, was there a difference?
Absolutely. The 2.25" tyres on the DT Swiss XRC 25mm felt great, and exactly what you would expect from an XC/marathon wheel and tyre combo. On the 30mm rims, everything felt just about the same if you weren't paying attention, and if you had the tyre pressures too high it made your bike feel like a rocket on the way to the trails and a pin ball on them. Wider rims really do need precision with pressures, and I used a digital pressure gauge to try to be consistent.
The slightly lower tyre pressures did help with traction though, although given the ability to control elements when testing small differences like this is near impossible, there was certainly greater confidence on loose and dry terrain, and in rougher conditions. Given the tyre model was the same, it really has to be a change to the volume and slightly lower tyre pressure that can be run without a loss of tyre stability. It's difficult to quantify, but it 'felt' like better grip.
This wasn't the case in all conditions, as in slightly wetter terrain with the edge knobs a little more inboard, the 30mm rims did not feel quite so secure with these tyres. This could well be different depending on the tyres you run. Which brings me to the next phase.
Running larger tyres on 30mm rims
So, 2.25" even for XC and marathon racing and riding is probably on the way out. 2.3", 2.35" and 2.4" tyres are all being used in marathons and World Cup XCO (take a closer look at Nino's bike at some races in 2019. Looks like a new 2.4" Aspen!). Having tested Specialized Fast Trak 2.3" tyres in the past, I already knew wider was good. With a Maxxis Rekon 2.4" on hand and an Ikon 2.35", I put them on the 30mm rims.
The Maxxis Ikon 2.35" on the 30mm rim.
Ok, so that Ikon is fat! I've had this tyre for a while and I suspect it has stretched a little. But the Ikon 2.35" has been said to be under-appreciated according to those who swear by it for its high volume, low tread height qualities.