chaadster

Of course you're confused, because there have been several pointlessly obfuscating posts by people who don't really know what they're talking about. Maybe I can help you understand the issues, here:

First, it's important to understand rim width as having two components inner and outer, because each has a different impact. There is no meaningful thing as "rim width" alone; it means nothing without knowing which dimension we're talking about.

Outer width, or overall outside rim width (hereafter, OW) is important for two main reasons. First, it impacts brake function, as at some OW, the brakes are too spread and maximum leverage is lost, or at extremes, the pads don't even hit the rim wall squarely. Secondly, OW is a factor in the tire/rim transition, with a smooth, relatively flat transition prized for aero benefits. A bulbous tire profile that tapers down to the rim wall is less aerodynamically efficient. Lastly, a tire sidewall that exits the rim wall more-or-less parallel to it will be less resistant to "rim slip" across the contact patch than a sidewall that exits the rim wall at an angle, creating that bulbous shape where the rim is narrower than the widest part of the tire casing. In other words, better, more affirmative feel through a tire/rim match-up that creates a U-shape tire profile than a bulbous one.

Inner width, or bead seat width (hereafter, BSW) is important because this defines whether a tire will actually fit and stay connected to the rim in use. Too wide a BSW, and the tire bead does not engage fully, and could come unseated under cornering. BSW also describes the shape and size of the tire, as the chart Rm-rf posted above shows. BSW is really the critical component for fit, because if that's matched properly to the tire size, the OW falls in line automatically (as a practical matter).

To answer your question about a 25c tire on a 25mm rim, then, we really do need to know whether 25mm refers to OW or BSW. In general, and assuming it's OW, there is nothing extreme about that at all, as SpeshulEd demonstrated by showing 26mm OW rims with BSW of 19mm; no problems with a 25c tire there at all. A 25mm BSW, however, would be extremely wide for a road rim, and though they exist (for example, Velocity's Cliffhanger rim), they're not at all common offerings. In such a case, which has a 30mm OW, putting a 25c tire on there would probably be ill-advised, but I'm not sure. I personally run 28c tires on 26.8 BSW/30 OW rims handily, and have for years without incident (precisely, Vittoria Randos on Velocity Blunt SS), but still, the casing width is greater than BSW, so it may be a different matter.

Knowing what will work at the extremes is hard to define; the ERTRO chart is conservative to the point of verging on dumb (as Jiggle noted), as lots of people run outside the parameters at very high levels of performance, all the time and have done so for many years without issue. But, before we go into how to figure out whether a 25c tire would make sense on a 25mm BSW rim, let's see if that's what you're talking about.

Carbonfiberboy 

Not that you'll notice any difference in drag unless you're an elite. Couldn't tell you about cornering - probably depends on the tire. IME recreational riders don't push it very hard in the corners compared to the possible. IOW you probably won't notice any difference unless you're an elite or ride a lot of crits.

TimothyH

This is a great post.

chaadster

Thanks, @TimothyH!

StanSeven

Except for possibly one post, there was nothing wrong with what's posted. Agreed for someone unfamiliar with wheels, inside and outside widths might be confusing. But saying insulting things about other posters when you don't agree is just poor taste.

chaadster

Yes, I see you are right. I was chafing a bit earlier because of something else, and my fuse was shorter than it should have been. Sometimes my efforts to supress my inner d*ckish tendencies fail. My apologies to everyone.

StanSeven

We all get that way sometimes. Thanks for saying that.

chaadster

Thanks for calling me on it. I don't want to be seen as a jerk.


Usually.

grolby

As previously noted, that chart is not just conservative. It's outdated and, not to put too fine a point on it, dumb (credit to Jiggle). As for width, yeah, without knowing the actual inner width it's hard to make a recommendation. 21mm inner width is rare, but given that 23mm tires are perfectly fine on the 20.5mm Pacenti SL23, I wouldn't sweat it. I agree with whoever posted earlier that they don't run lower pressures on wide rims. I don't know if damage or flats are more likely, but I already run my tires at very low pressures relative to most people.

No one is going to notice any difference in drag regardless, because it's minute and our perceptions of what we assume to be drag might be something totally different. That notwithstanding, the question of wide rims and aerodynamics is a source of lots of misunderstandings and false information. On a rim that has been aerodynamically designed for a wider tire, that tire will be faster. On anything else, it's a crapshoot, but chances are that the narrower tire will be faster cause that's usually how things work. People have become confused about this because some manufacturers (HED) have claimed that a 23mm tire is more aero on a wider rim than on a narrower rim, while others (Zipp) built a much wider aero rim than before, which required a wider tire to work best. And people got these things confused and decided that wider tires are more aero on a 23mm rim. The reality is that's usually not true.

As for corners, I don't find there's a noticeable difference between traditional 19mm rims and 23mm rims with normal road tires and pressures. Riders who are heavier than me (I'm <125 lbs at race weight) may disagree. I do find a very noticeable difference with wider tires at lower pressures, such as for gravel riding or cyclocross. The wider footprint really gives a tire at low pressure a lot more stability, in my experience.

Jiggle

Nah, it makes for good reading.

Danny01

Thanks for the informative post.

The 23mm and 25mm wide wheels I am looking at have an inner width of 16mm and 18mm respectively

chaadster

Those are not particulary wide BSWs, so tire fitment is a non-issue; you can mount 23c or 25c tires without concern for security.

I also believe that the 25mm rim will not be a problem for most modern brakes, once adjusted properly.

Tire clearance is the main issue, as the 25c/25mm combo will be bigger than the 23/23 combo. Exact dimensions will depend on the tire brand and model, and you'd probably need to have the new wheels and tires mounted to measure tire width, at which point you'd just put them on the bike and be certain.

So assuming you don't have access to the mounted tires on the wheels, take a look at the fork clearance and chainstay clearance you have now; if you've got 23s with a few millimeters all around, you're probably fine to fit the largest combo, and if you have 25s with that kind of clearance, you're almost certainly good. I'm thinking clerance demands wont increase by more than 2mm or 3mm (again depending on starting tire/wheel, and new tire choice), so if you could fit a stack of three quarters (coins), I'd say go for it.

As was mentioned upthread by Grolby, the degree to which the larger/wider, tire/rim will benefit you depends on your weight and how you ride. I'm talking contact patch and increased volume, so the more aggressive and heavier, the more noticeable the improvement is.

Decreased rolling resistance is always a benefit for the larger tire, but the gain is almost invisible, and is within the range of gain you can get simply by switching to a better tire in the same size. There can be aero and feel tradeoffs for the larger tire, but the aero penalty is hard to assess as it depends on rim shape, tire, and how fast you ride, but in the best case is tiny factor, approaching negligible. Feel, especially at the front, can get soft and less precise with a 25, which you may want (say for rough roads) or not notice, and is dependent on pressure somewhat. In both cases, fitting the wider rim with the bigger tire will mitigate any negatives in those regards.

Overall, the changes brought by upsizing rim width, tire width, or both together really can run the gamut from game-changing to not noticeable, depending primarily on the rider's sensitivity, sensibility, style, objectives, and riding conditions.

Personally, I've run 25s on old-standard 14.4mm BSW rims and hated the feel, but love 23c tires on 19.4mm BSW rims, finding the latter sufficiently responsive and comfy over most roads that upsizing tires hasn't crossed my mind.

hueyhoolihan

what goes around, comes around...

Jiggle

It seems to me, from looking at the rolling resistance data of the same model in different widths, that the gains are as follows:

13% gain going from butyl to latex
1.5W (average) gain per millimeter of increased width
<0.1W penalty adding a latex tube to a tubeless tire

So if you increase 2mm front and rear, that is 6W total. That's the difference you'll get switching to an aero helmet or going from 25mm to 55mm rims.

To get the full gain, you need a rim the same width or wider than your tire for aero purposes. That is why the new Spec. CLX 64 are 30mm wide. 25mm is so three years ago.

There's a 16W difference between the Conti 4000 and the Schwalbe Durano in the same size at 26mph.

So say you are riding 23mm Duranos. You switch to 28mm 4000s. You improved by 42W minus the aero penalty, which only really affects the front tire. If you use latex tubes it goes to 50W.

That's pretty good for $129.99.