Sy Reene

As far as I can tell, riding with such is just great. A tire's nominal width generally matches what it actually inflates to, and these are/were practically universally perfectly fine for decades. Now it seems that we're way behind the times if our 23mm tires don't inflate to 26mm because our rims are at least 25mmm wide.

HTupolev

The widening of rims is primarily for aerodynamic reasons. Ideally, air flows smoothly from the tire sidewalls onto the rim... problematic when the tire is bulbous and much wider than the rim.

There's nothing wrong with that sort of wheelset, but it's unlikely to see top-notch performance in the wind tunnel, especially with the tire widths that are popular right now.

Sy Reene

So is just minor bulbousness (is that a word?) or less, better though for holding a corner? Why does this seem like a thing that coulda been figured out maybe 50 years ago? I mean, yeah, isn't it obvious that if your tire comes off the rim straight up and over, vs 'bulbous', that it's a bit more aerodynamic?

redlude97

a matched tire/rim holds a corner better IME, less squirm for a given pressure and measured width. Just feels predictable.

HTupolev

I'm not sure there's much agreement on what precise combinations are best.

Very few people were putting much thought into bicycle component aerodynamics 50 years ago.

Actually, very few people in the bicycle industry were doing any real performance testing of much of anything 50 years ago. Keep in mind, for example, that just two decades ago, the experts claimed that performance consequences of suspension was an irrelevant factor for tire pressure choice in road bicycles; Brandt's insistence that higher pressures are always faster even on cobblestones would have taken all of a few minutes to disprove, if he had only bothered to put such a basic aspect of bicycle setup to the test.

Keep in mind also that aluminum isn't as good of a fairing material a carbon. Even with the aerodynamics much better-understood than in the past, aluminum rims usually still aren't designed to be as aero as high-end carbon, because the weight of the resulting wheels would put people off.

Sy Reene

There wouldn't necessarily have had to even be a great understanding of aerodynamics though; just a guy (or gal) who thought to themselves.. "let me try making a wheel rim that's 5mm wider than what everyone else is selling."

HTupolev

I think you're asking for several leaps in one step.

Firstly, in the old days, nearly every single bicycle rim had a basically rectangular cross-section. If you're capturing the airflow from the tire sidewall to the rim brake track, but then promptly dumping the airflow off a cliff anyway, is it obvious that you've gained anything?

Secondly, the resulting rims would tend to be heavier, and you'd need to convince prospective buyers that the aerodynamic benefits are worth it. Would be pretty tricky to do with no data 50 years ago...

bruce19

Maybe this will be helpful.......

rubiksoval

It's not just aerodynamics. At the end of the day, a narrow wheel with a matching skinny tire is still faster aerodynamically.

Rather, it's the drop in air pressure and CRR and the corresponding improvement that results from that. A 25mm tire at the same (correct) pressure as a 23mm tire will have a little better rolling resistance.

In addition, if you want to lower it even more, the lower air pressure results in a more comfortable ride.

rubiksoval

Agreed. Feels like you can just lay the bike over.

Abe_Froman

redlude97

Great way to describe it, I can never explain that nuance to others who haven't ridden a similar combo.
Also, with the advent of carbon, and even lighter weight aluminum extrusions, weight is no longer a huge penalty going wider.

Sy Reene

I'm not sure I understand this idea. To use an example wherein I want to achieve a net width of a 25mm tire, if I were to mount a 25mm tire on a 15c rim, and it measures the same as its nominal width (ie. 25mm), vs mounting say a 23mm tire on a 19c rim and by doing so its larger and comes to the same 25mm. Do I inflate differently for these two scenarios?

Sy Reene

Thanks for the insight. I guess I was imagining that wider rim would make a tire have a flatter profile that's hitting the pavement on a steep bank, vs more rounded profile (bulbous) with a narrower rim, yet had no idea which is better at holding pavement.

redlude97

Yes, the 23mm tire on the wider rim will measure the same width but have lower volume than the 25mm tire on the narrower rim, but more than a 23mm tire on a 15c rim. It ends up falling somewhere inbetween in terms of volume so inflation pressure should be inbetween the two.

redlude97

The profile in contact with the ground is rounder though, and the sidewall flexes less which results in less squirm so it ends up being more predictable. Look at sport bikes, still very round profile with matched tire/rim widths that allow them still to be leaned over quite far

Sy Reene

Ok, maybe.. but people pick tires to achieve the width they want. And, I don't see how a tire that measures eg. the same width and height as another tire has a different volume, eg. this oldie done by HED, shows sometimes actually a 23mm tire can be larger (overall) on really wide rims vs. a 25mm tire on narrower rims. My hypothetical was just that they'd net the same dimension.

redlude97

Each combo would vary slightly, but notice how the the 25mm on the 19c is both shorter and narrower then the 23mm on the 23c? That was the example you provided I would then run the 23/23 combo ~5psi less than the 25mm/19c combo. I'd run a 25mm/23c combo ~10psi lower than the 25mm/19c combo. Hope that makes sense

rubiksoval

Yeah, I would.

Abe_Froman

What he is getting at is that you want the width of the tire, and the outer width of the rim to match as closely as possible. A 25mm tire on a rim that measures less than 25mm on the outside will create unnecessary aerodynamic drag.

Bah Humbug

Said guy or gal would have to have the factory make something different from what was considered standard. This seems obvious now (even in terms of blind experimentation), but you can look back on forums and see the number of people who criticized HED for the C2 rim when it came out, even with data supporting it, even in an era of "things can change because science". Fifty years ago? Mavic would have fired the guy who suggested they make that rim.

Sy Reene

No, I understand the points made on aerodynamics (though is there an expense perhaps in less protection of the actual rim from road ruts/scrapes with less or no tire sidewalloverhang?).

I was responding to his part on how a wider rim somehow allows for less inflation and more comfort. I was under the assumption that you are supposed to inflate a tire to a PSI that relates to the tire's actual inflated width, so I'm not seeing how there's a comfort/CRR gain.

rubiksoval

Why? Do you think a 100 lb rider and a 200 lb rider should have the same air pressure in their tires?

redlude97

if you are comparing a 25mm/19c to a 23mm/23c combo, then yes you'd end up with roughly the same inflation pressure and crr but with a significantly more aerodynamic combo that also handles better. With the same tire, ie 25mm/23c you can lower the pressure, get better comfort and roughly the same crr(slightly better) or keep the same pressure and keep comfort the same while decreasing crr and still having better aero and handling. Go to 25mm/25c and all those gains are increased again.

Sy Reene

Of course not.. rider weight is a very important factor but we're not talking about that at all.

Ok then.. and to reiterate, I understand the aerodynamic advantage (what this translates to in real world would be interesting to know however), and have no reason to doubt there's a potential handling benefit.

I do question "with the same tire" argument though.. why would you use the same tire? Your 25mm tire might inflate to a 28mm on a wide rim, and the narrow rimmed user, if they wanted lower pressure and similar CRR, could use a 28mm tire to achieve a comparable result, so this argument is going in circles. A 25c rim would likely have a 31-32mm exterior width -- at this point I would probably advise caution putting a 25mm tire on such.. this seems a bit extreme.

I suppose if you're trekmogul and are sitting on an inventory of 2 dozen narrow tires, and have decided you want to go wider, a wider rim might be a cost effective upgrade so as to avoid having useless inventory.