Bike tires and rim size

Question

I have a 26x2.4 mountain bike tire. I have an ancient Trek bike with a rim that had a 26x1.5 tire. The tire on the Trek is punctured and unusable. Can I use the mountain bike tire on the Trek?

Answer 1

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The 26x2.4 tire will fit on to the rim that holds the 26x1.5, but you'll run into some problems due to the width of the proposed tire. The major one that will derail this idea is when the wheel with a 2.4" width is fitted back into the frame (IF it will even go), you'll find that outer parts of the tread will contact the frame. If the tire is in the rear position, the space between the chain stays will be too narrow to accommodate the wide tire and it will rub. Consider that the bottom bracket shell (of an "ancient Trek") is 68mm wide and the chain stays, having some thickness themselves, originate from this shell, creating space between them which increases as they stretch to the dropouts. However, the wheel comes between the stays very close to the the back of the 68mm bottom bracket shell. A 2.4" tire is 61mm wide and given that the stays themselves have a thickness and are attached inside the edges of the BB shell, the space between them hasn't developed enough to accommodate the tire wanting to move thru it.

A tire that's wider is also a tire that has a larger overall diameter. In other words, the wider the tire, the more distance between the edge of the rim and the outer tread of the tire. What this infers is that the widest part of the tire will be even closer to the BB shell where the space between the chain stays is even narrower. All this to say: that wide of a tire in an older Trek frame (given a 68mm wide BB shell and 135mm rear dropout spacing) will likely have frame clearance issues. If it doesn't rub when the bike is in the stand (or upside down), it certainly will contact the frame at points along a ride where frame flex, rim flex, tire deformation (all normal dynamics of a moving bicycle) create changes in the clearance tolerance.

In front, where an older Trek frame will have a rigid fork and 100mm dropout spacing, a 2.4" tire will have clearance issues at the top. The wide tread may contact the blades of the fork and the increased height of the tire from the rim will cause it to contact the crown.

An additional issue with a wide tire on the old Trek is again a clearance issue, but this time involving the rim brakes. These are likely to be cantilever brakes. Wide tires may contact the fully retracted pads when those pads are set to contact the rim properly. The pads could also contact the tire sidewall during their travel to the rim on braking. Tire height may be a factor because of inadequate distance between the cable mechanism connecting the brake calipers and the upper tire.

Finally, it should be noted that today's mountain bike tires that commonly run 2.3 to 2.8" wide have the best profile and performance when placed on a rim with an internal diameter of 25 to 35mm. An older Trek will likely be outfitted with rims with an ID no more than 19mm. A general guidelines is the tire width should be 1.45 to 2.0 times the rim ID. These are not hard limits, but note by using this standard, a 1.5" (38.1mm) tire works best when rim ID is not over 19mm. Adding an inch of tire width on to the same rim makes the tire width to rim ID ratio 3.2, a fair deviation from the standard guidelines and isn't recommended despite such a combo being possible.

On the bright side, there are still many suitable 26" tires (for this case, up to 26 x 1.95 or even x2.0) on the market, both new and used. These are more often than not, heavily discounted. So are 26" wheels. Given that one of the most noticable upgrades in terms of ride quality is an upgrade of the wheelset or even just tires, it certainly makes sense to obtain the proper size tire, at least, or even a new wheelset, at best.