WT160

I bought a spare wheelset a little while ago: Matrix IsoC rims on Specialized hubs.
I swapped out the the tires that were on the rims (25mm) with 28s. The went on fine, but I was a little surprised to find how narrow the rims were: 12.5mm.
Based on charts I've seen and rules of thumb that I have used before seem to suggest that 28mm is a little too wide for these rims. I don't want to go to 26mm or 25, though, as the terrain I ride would not be good for such widths.
Any opinions?

Iride01 

Like you said, they are rules of thumb that are attempting to be a guide for any situation. You particular situation might be okay for you, the particular tire, rim, type of terrain you ride, how hard you ride and all sorts of other things. One of those other things being... will it fit between the brake pads without deflating the tire?

You should be able to tell when you ride it enough if it's working for you. If it feels a little squirrely on you when you go through twisty turns that you have been use to on the other tires, and changing the pressures up on the tire doesn't make the turns feel better for your confidence, then maybe they are too big. None of us will really know if we don't try.

Not to burst your bubble. Ultra Sports were the worst riding and worst handling tire I ever rode. But that was almost 15 years or more ago. Maybe they've gotten better. The tire compounds and technology of the newer Ultra Sports is different than back then.

cyccommute 

More like “rules of thumb” by someone who has never seen a thumb

Every tire fit chart I’ve ever seen is ridiculously conservative. This bike has Velocity Deep Vs which are 14mm wide. Charts generally say to use 28mm tires, max. The ones in the picture are 37mm. No issues.



This one is running 17mm rims with 54mm tires. I’ve been using this combination without incidence for decades.

ThermionicScott 

Screw the charts, you'll be fine.

icemilkcoffee 

The tire fit is absolutely fine. Your biggest problem is the anodized braking surface. Prepare for long stopping distances.

FBinNY 

Looking at the photos, I'm convinced that you're fine. In fact I believe that you have some room to go wider.

From the photos I doubt that your 28mm tires are actually 28mm on those rims. Measure them, and if riding conditions warrant it, you might seek a slightly wider tire next time.

Note that tire width markings and specs need to be taken with more than a grain of salt. So one brands 32mm will be fine, another's 28mm may be wider.

Iride01 

I had trouble believing that they were that wide too. They don't seem anymore ballooned out than the 25's I still run on my 622x20 rims.

Crankycrank

I thought so too until I zoomed in 500% (yeah, my eyes suck) to see 700x28 on the tire.

FBinNY 

You might have missed my point.

I have no problem believing that the OP can read and the tire SAYS 28mm. However, based on the photo, I don't believe it's actually that wide. So, I'm suggesting that the OP follows Admiral Hopper's very sound advice.

Crankycrank

Yes, I did see that but just sayin' and I agree. Of the several versions of Conti tires I've used recently they all measured up about 1-2mm wider than stated on 15mm inner width rims but OP will have to measure his own.

WT160

Thanks for the replies. I'll measure with a caliper to see what the true width is.

Iride01 

If you got your calipers out, measure the outside rim width too. That might be part of why they look so normal to us. Though why the external would be that much more than the internal I don't know. Are you certain of the 12.5 mm internal bead seat? Usually new rims/wheels have a label with the ISO rim size on them. And the width on the ISO will be the internal width of the bead seat.

kcjc

I started run 28s starting in 1989/90 on my Mavic MA 40 that's 13.5mm and had no problems. Donated the wheel in 2014. I had a few Matrix IsoC rims back in the day and was running 28s before getting rid of them. Braking is not the best. I was using Ultra Sports back then and didn't find that bad. I'm running Ultra Sports II today. It's a bit tight getting the wheel out on all of my bikes with 28s.

ThermionicScott 

Velobase says they're about 19.4mm outer width, so ~12.5mm internal sounds plausible: https://velobase.com/ViewComponent.a...74a8e&Enum=107

WT160

I measured the internal width with calipers.
External is 19.5mm.
Tire width is around 26.3mm.

WT160

I checked some Sun CR18s I have with the same tires. There they measure 28mm. Strange.

Iride01 

My rims measure about 26.3 23.3 externally. The 25mm tires measure 28.1 mm externally. However tire width as I always understood it is given by the width of the tire casing. Which doesn't include the rubber on the outside. Plus, I have them aired up pretty high. 100 psi on that front tire.

So that somewhat explains the perception difference or lack thereof in the photo.

Still, I agree with everyone else, ride 'em.

If there is a issue, you'll figure it out and for other than a very high speed descent around a mountain switchback that has you going over the cliff, most anything else you'll have some indication if it's about to go somewhere else than where you want to go. And even that first one or two switchbacks on the mountain will probably give you notice if you heed it and just slow down.

FBinNY 

There's no true standard for tire width specs. You're right that it doesn't include tread, but sidewall rubber is thin enough not to matter.

OTOH the rim width does, by a factor of about 1/3. This is part of why specs can't be precise. When a tire maker publishes a spec, he has to make an assumption about the rim used. If the user opts for a different rim, the tire width will change accordingly.