Lazyass

I'm looking to build a new wheelset with some stiff modern Velocity Major Tom tubular's (decals removed) and VO high flange 126mm threaded hubs. They are 23mm wide outer so I don't want to run a 23mm tire. I may go with 25 but my frame/fork will clear a 28.

I'm not sure if 28's will clear my calipers. It's hard to measure but it looks like I have 7-8mm of clearance on both ends with my current 23's. Any way to estimate how much higher a 28 would be?

nlerner

Well, I would guess 5mm! But I think it’ll really depend on the tire and how it plays with the rim. The shape will vary for different tires marked nominal 28mm, unfortunately.

JohnDThompson 

Tires are approximately circular in cross-section, so a 5mm increase in diameter would be a 2.5mm increase in radius. All else equal, a wider rim will result in a shorter tire as the cross-section becomes more elliptical. I suspect as long as you have at least 3mm clearance with your 23mm tures between the stays/blades and tire, and similar clearance under the fork crown, a 28mm tire should clear. Allow for more clearance if you intend to mount mudguards.

ThermionicScott 

FWIW, Velocity recommends a minimum 25mm tire on that rim, and I've read of people having trouble getting a good glue job with smaller than 28mm.

thook

going from a 23mm tire to 28mm tire will be a 5mm increase. you're not measuring diameter or radius in this case. it's the width or height of the tire. but, you're right about changing the tire profile with a wide rim. especially the tire and rim width being so close. that should help

USAZorro

The bicycle will ride about 2.5mm higher, and clearance will diminish 2.5mm all around due to the larger tire, but no difference in brake caliper reach.

thook

i don't think so. a 700c wheel (622 bsd) with a 23mm tire will have about 338mm radius. increase to a 28mm tire and that gives you a radius of about 343mm. nowhere in this calculation do you divide the tire ratio unless you want to talk about tire to chain stay clearance where you have two sides in which to account for.
try it on this calculator....
Bicycle Trail Calculator | yojimg.net
the increase/difference in radius is 5mm or 10mm in diameter. of course, you can physically measure yourself. i played around with this a lot trying to get an idea of how big of a 700c tire i could fit (before buying) in a fork with 360mm of clearance under the fork crown. i can fit a surly knard 41c with about 5mm left of clearance. the radius of that wheel/tire is about 355mm.

btw, this has no relation to brake reach. just tire clearance to the underside of caliper

thook

oh...pfffth! you know ...if you're talking side to side of the caliper, yes. 2.5mm. but, from the center of tread up to the underside of the caliper, it's 5mm difference....which is what i'm on about and what i'd be concerning myself with here. particularly on the fork...which often has less clearance that the rear bridge on bikes

squirtdad

IME impossible to tell unless you know what the true inflated size of the tire is. some are spot on and others vary +/- 1-3mm

thook

of course.

Lazyass

Thanks.

Lazyass

I've had countless tubulars and unlike clinchers I think all of mine have been pretty spot on with the advertised size. Maybe off by about .5mm at the most.

squirtdad

granted have had way more variance with clincher. In tubies, my 30mm vittoria corsa control are precisely 30mm, but my 25mm challenge elite measured more like 26.5,

HTupolev

Yes, but caution needs to be taken when interpreting this in terms of tire height. Even if the tires in question inflate to a perfectly circular shape (they generally don't), the center of the larger tire's "circle" will sit higher relative to the rim, so you'd get more than 1mm of height for every 2mm in width.

Real-world tire heights do not necessarily lower with increasing rim width. It's geometrically-complicated and it just depends.

To illustrate the issue, consider a hypothetical tire that inflates to a perfectly-circular shape, mounted onto two rims: one which pinches the beads together, and another which holds them far apart so that the inflation is a semi-circle:



Doubling the radius of a circle also doubles its perimeter, so these two shapes have the same exact height and arc length.

Lazyass

I have 23mm Challenge Elites and they measure about 22.7. They're awesome tires for the price, though.