@Phill P: I would say you are correct on all counts. One of my favorite wheelsets is currently 25mm Veloflex Arrenbergs on Campy Boras. They seem to stabilize the Boras in a noticeable way. I've been experimenting a lot with these and another set of exactly the same wheels but mounted with 23mm Veloflex Carbons. The difference in comfort is of course noticeable but it is the difference in stability at speed that I'm most impressed with. Not sure why that is, but I like it. I guess if Reynolds can claim that tiny little "lip" helps stabilize the wheel then perhaps the added roundness up front with the larger tire does the same thing.
I can't say which set is actually faster... that would be splitting hairs. But I'll keep some data for both sets and over time maybe I'll be able to tell, not that it really matters that much to me unless I know I'm going to be going for a ride with my favorite sprint section involved.
The wide rim technology really benefits clinchers and does nothing for tubulars as far as I can tell. I had a set of HED Belgium clinchers which were really nice rims. I mounted some 25mm Conti Four Seasons on them for winter and found that there wasn't enough clearance with the fenders I was running. This was odd because I was running them just fine on a set of Campy Neutron rims. So I removed the tires off the Belgiums and mounted them on the Neutrons. Sure enough, the clearance was fine. So, I had to use 23mm tires on the Belgiums which looked just as big as the 25s on the Neutrons, which is consistent with what you are saying. I think the wide rim technology does in fact enhance the ride of a clincher a bit. But not so with a tubular. I still love my tubulars on standard size rims, much more than a clincher on a wider rim. I sold the Belgiums last year, not because they were bad wheels at all, just because I hated always having to readjust the brakes to accommodate the wider rim. And it was more than just opening up the calipers a bit, because if that's all you do, then the pads will not hit the rim exactly flush anymore without some adjustment that way as well.
But as @WMW, most people do not have a lot of interest in 21mm tires these days but if you did run them on 22mm rims I think I'd be a little worried about bottoming out on anything but the smoothest of roads. I do know a guy, and not a light guy by any means, who does in fact ride 21mm tires on a set of Lightweights. That's gotta be harsh, but I guess it's what he's used to.
Are you actually going to try this combo, or were you asking primarily for interest? If you try it, report back with your thoughts.
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