Unravel the mystery that is rim widths and find out what's best for you.
25MM Internal rim width
With the XM 421 having many of the same traits and characteristics of its 5mm wider peer XM 481, was it to be expected that we would simply discover the same results, but simply be able to appreciate its slightly lighter package?
With the XM 421 being almost identical to the XM 481, except having a 5mm narrower internal and external width, we set it up with our baseline figures and hit the trails. After several runs down our two Stromlo Forest Park trails we could feel that modifications were required – quite simply, there wasn’t enough grip provided under the baseline settings.
Moving out onto the Flow Track at Thredbo we also moved our XM 421 air pressures to match the sweet spot air pressure of the XM 481, reducing them to 25psi rear and 22psi front. Right away we were off to a much better start with improved performance until suddenly… BANG ! A massive burping off the rear tyre in one of the tight berms in the final metres of the track. After this and a few more small changes per run, similar to the XM 481 30psi rear and 27psi front did not deliver any enjoyable rides, however, by our 8th turn again down the Flow Track, we settled on the baseline rear air pressure of 27psi but maintained 22psi in the front. While this did feel a little unbalanced at times when descending at high speeds, dropping to lower pressures again would not be an option. Under lower pressures, such as reducing the rear tyre to 25psi we experienced the same burping that had occurred when riding at Stromlo Forest Park using the XR 361.
Our testing of the XM 421 when paired with a 27.5x2.4” tyre proved that the 25mm internal rim was not particularly durable, nor did it perform as strongly as its 30mm internal width peer the XM 481. However, this is certainly not a deal breaker. Perhaps using the XM 421 with a 27.5x2.3” tyre might deliver the stability and grip at lower pressures, but this could potentially also offer a rougher ride – it’s all about compromise.
Note: this smaller tyre size was not included within this test.
Claimed rim weight: 440g
Internal width: 25mm
External Width: 30mm
Depth: 7mm centre
Tyre: Onza IBEX 27.5x2.4” skinwall
Tyre weight: 885g average
Front wheel weight: 850g
Rear wheel weight: 955g
Installation pressure: 25psi
Tyre tread edge to edge: 60mm at 25psi
Tyre casing edge to edge: 56 mm at 25psi