25 mm tubular tire on a narrow 20 mm rim?

[ak47]

Will it work? I have a set of Fulcrum Racing Light XLR and I would like to move to 25 mm tires. The rim is narrow at 20 mm measured externally.

[joejack951]

Wasn't this done all the time in the days before wide rims became popular? So long as you have plenty of width for adhesion (and I can't see how you wouldn't) it should be fine.

[joejack951]

Recent thread on the same topic: viewtopic.php?f=4&t=148249

[Calnago]

Yes, it works, and I've run 25mm tubulars on my 20.5mm external rim width boras. But it's not ideal. It's the radius of the rim bed that is a little tighter which kinds of pushes the tubular "up" on the sides, with less adhesion in the center channel. Never had a problem with adhesion so much as it pushes the tire up enough that for my C59 as an example, the clearance was a smidge too tight under the fork. Same tire on the new 24.2mm Boras fits the rim bed much better and sits just that much lower (heightwise) that it's perfect for my C59 and no clearance issues at all. I consider a minimum of 2mm of clearance necessary for a road bike. Anything less, and maybe it won't actually rub, but any little debris etc that the tire picks up from the road creates what I call "chatter" under the fork while riding and is really annoying. The other thing is that if you're using rim brakes, the width of the tire is often large enough that even when looseing off the brakes to remove the wheel, the tire will get caught up in the brakes since it's a lot larger than the 20.5mm width of the rims. With the 25mm tires on the newer rims, you don't even have to "release" the brake to remove the tire. And the aero guys like that it is a better interface between the rim and the tire. Of course, a tubular still doesn't have as good an "aero" rim/tire interface as a clincher can, but they more than make up for it in every other way when it comes to handling and ride quality. I could never tell if there was an aero difference, but I can for sure tell the handling and ride difference.

[ak47]

Thanks guys, I really appreciate your help. I found a picture of Racing Light XLRs fit with 27 mm Vittoria tubs for Roubaix, so I guess I should be fine. I'll use more glue than usually.

[joejack951]

Never had a problem with adhesion so much as it pushes the tire up enough that for my C59 as an example, the clearance was a smidge too tight under the fork.

I hadn't considered this as a potential source of an issue but it definitely could be a consideration on frames where clearances are tight. Thanks for mentioning it.

[Calnago]

Yes, particularly on a nice frame like the C59 or EPQ... where I really wanted to run the 25's, and did, but you can see the issue here. Not a big deal... it was just paint, so nothing structural, and not all the time either... just annoying. But with the new rims, same tires... all is good. Perfect in fact. I just needed that extra 1-1.5mm that was afforded by mounting the same exact tire on the slightly wider tubular and all was good... in fact, I had one of the first sets of the new wider 2015 Boras that came out, just so I could test out my theory. Once I saw it did in fact sit lower, I immediately ordered the second set of Boras in the new width... to have both the 35mm and 50mm profiles. 25mm tubulars fit these rims perfectly.