harrier6

So the situation is I have a set of rim brake wheels with 15mm inner width rims that will need replacement soon. I run 28mm nominal tires that measure 28mm actual with. It's worth it for me to go a bit wider on rims, if for no other reason than it would make getting wheels out of the brake calipers easier. There are several more or less equally good options out there for 17mm, 19mm, and 21mm aluminum rims. Whatever the setup, the tires have to measure 28mm max.

Much of the discussion over wider rims seems to miss an important point, which is: controlling for tire width, do wider rims have any comfort/pinch flat benefits? Imagine a nominal 28mm tire on a 15mm rim and a nominal 25mm tire on a 21mm rim, both measure 28mm actual width. Will there be a difference? I suppose it comes down to whether or not the larger rim bed adds a significant amount of air volume to the tire system? I imagine it wouldn't take much to add 10%-20% air volume, which would be significant. The whole "sidewalls are better supported/stiffer, so you can decrease 2-3 psi to accommodate) thing seems like one step forward, one step back for no net gain.

When I see rim width talked about, it's mostly, "wider rims=wider tires=more comfort/flat resistance," which is like, yeah, we already knew wider tires would do that. I know there are supposed handling/aero benefits with wider rims, but I'm just stuck on this question at the moment.

datlas 

If measured tire width mounted on the rim is the same, I believe the comfort difference in rim size would not be significant.

Iride01 

Going to wider rims is probably more about getting the entire wheel more aero with the chosen size tire. For just ride comfort, I don't think it'd be much. At some point though, too wide a tire might feel squirrely under you at normal low pressures for them when you go down really twisty roads and trails. But that is usually when the rim size and tire size are beyond the point of ridiculous looks.

rm -rf

It appears that at least some tires are more true to size these days.

On my aluminum HED Ardennes+ rims, that are 20.5mm internal width (and 25.5mm external width at the brake track):

Old GP4000 tires, 25mm tires measured 28-29mm.
New GP5000 tires, 28mm measure 28.8mm to 29.2mm

~~~
reposted from an old thread:

See this profile chart from HED wheels. The outer red line is a 25mm tire on a 20.5mm inside width rim. The white line is the same tire on a 13.6mm inside width rim.
The wider rims are essentially increasing the size of the circle. It's surprising to me that the profiles don't flatten out as the rim widens.

( On a side note, that small increase in the tire size, just a few mm, is enough to increase the air volume quite a bit. GP4000S size 25 measure 29mm on my 20.5 ID rims. That's a 30% increase in the cross section area over the nominal 25mm diameter. )

rm -rf

I really like my wider rims.Much easier with rim brakes, and bigger air volume.
Wider "actual width" tires allow lower air pressure, and are still just as fast on smooth roads, and faster than narrower tires on rougher roads.
Wider tires seem more grippy in turns, and are also less likely to be thrown off line when hitting a bump in the middle of the turn.

Air volume for different tire widths.
The cross section is close enough to a circle to use the area of a circle to compare air volumes. The tires all fit on the same rims, so the volume difference is the cross section area. It still amazes me what a difference the bigger tires make.
~~~
The math: Area of a circle = pi * radius squared. So the difference in area of two circles is proportional to the diameter squared.
A 23mm circle= 3.14 * (23/2)^2.

23mm: 415
25mm: 490 18% more air volume than 23mm. Huh.
28mm: 615. 25% more air volume than 25mm!!
32mm: 803
40mm: 1256

~~~
My air pressures at 170 pounds rider:
23mm 110 psi
25mm 95-100 psi (for rough road comfort, we thought.)
28mm 65-75 psi (that's more like it!)
40mm: approx 35-38 psi front, 42-48 psi rear (floating over rough surfaces, yeah.)

choddo

I think it’s even more than that because the overall OD will also increase so the volume of the torus will go up by more (as a percentage) than the cross sectional area

however, I’m not sure that same tyres / wider rims as per the OP will be more comfortable because while you’ll gain a bit of volume, you’ll lose “height” and I’m not sure what effect that will have on handling or transmission of bumps or even risk of bottoming out the rim.

Carbonfiberboy 

My experience in running 28mm tires on 15mm rims on our tandem is that they seemed to roll a bit in a hard corner. I felt more secure with those tires on wider rims. The hooks on the wider rims, being further apart, will be more effective in countering the moment which is trying to roll the tire under the bike. That said, of course the force on the tires when cornering is much greater with a tandem. 15mm is the IW of Open Pro rims which were in common use back in the 23mm days.
https://efprocycling.com/tips-recipe...ke-you-faster/

Mtracer

I think if you end up running a narrower tire (like a 25 mm) on a wider rim, and get the same true tire width of 28 mm, the air volume is going to be very much the same. So, the air cushion would be very similar. But another part of the comfort is the expansion of the tire itself. The tires themselves give a little. Rather like a diaphragm. Since the 25 mm tire has a smaller periphery to stretch, I think it's possible it would be less comfortable. However, I'm just thinking out loud here. I don't know that it would work out this way.

Either way, I suspect there's just not going to be much difference and not enough to justify a new wheel set. I'd really take a close look at air pressure first. Even if you are already calculating an optimal pressure from something like the Silca calculator, you can still run lower than that if you want. The rolling resistance penalty for running lower than optimal is pretty small. It's running higher PSI than optimal were the losses increased rapidly. I.E., err in the direction of too low than too high of pressure. And the plus, you get more comfort.

PeteHski

I think this is your fundamental limitation. Whatever combination of rim width and tyre gets you to 28 mm is going to feel much the same. That’s why tyre pressure calculators usually focus on actual measured tyre width rather than nominal tyre width. They either require you to measure it directly or derive it from the tyre and rim width combination.

chaadster

Yeah…in terms of general comfort/compliance. However, in terms of handling quality (e.g. stability) and aerodynamics, I’d favor a 25mm tire on a 20mm IW rim over a 28mm tire on a 15mm IW rim.

PeteHski

I thought the OP was talking about the potential effect on tyre pressure and comfort. Neither combination is really optimised for the tyre ie most 25 mm tyres are designed to work with narrower rims and most 28 mm tyres are designed to work with wider rims. I agree the handling will feel “different” in those two cases. But hard to say which would be preferable. I think that is something you would have to try and see for yourself.

I have run 32c tyres on 17 mm internal rims and 30c tyres on 22 mm rims and the latter did feel more stable as you would expect. Comfort felt very similar and I ended up using the same pressure. But they were different tyres (Pirelli vs Conti) so not really a proper test.

harrier6

Thinking about this more, this might be it. Comfort + risk of bottoming out the tire are a function of 1. how much the tire can deflect in distance and 2. how much force it takes to get to maximum deflection. Without a change in distance, you can't make the tire softer.

Even if wider rim beds did add a significant amount of air volume, that is just going to make the tire stiffer for a given air volume without any benefit. Like how a 28mm tire at 90psi is stiffer than a 23mm tire at 90psi. FWIW this paper purports to have a scientific model for how tires deflect, but I'm not capable of understanding it.

Although maybe at the end of the day there isn't a whole lot to be discovered here

rm -rf

Wider rims: tire width and height changes
The HED tire profiles I included above show that both width and height increase for the same tire mounted on a wider rim. That wasn't obvious to me before I saw the profiles. At the extreme case, an extremely oversized rim would have to lower the tire height as the tire got very wide. But these more practical rim widths just make a larger circle that expands in every direction. Only 23mm and 25mm are shown, though. I'm guessing my 28mm work the same way, with a round cross section instead of a flattened profile.

rm -rf

Pinch flats vs tire size & air pressure
I've wondered about the air pressure protection against a bottoming out pinch flat. There appears to be no scientific studies or even systematic testing.

Would adding 10 psi to my 70 psi make a minor difference or an effective difference? How much risk in lowering 5 more psi?

And how to define the pinch flat types: I've flatted on a very sharp edged cast iron water valve cover on the edge of a pothole, or on square edged big gravel chunks, but not on deep, rounded potholes. And bike speed is a huge contributor to pinch flats. At walking speeds, none of these will cause a flat, but at 25-40 mph downhills, that much faster impact and higher energy doesn't give the bike time to harmlessly bounce over the obstruction.

Dave Mayer

Today I did a long MTB ride on 50mm tires mounted on 17mm rims. No problems, but this was standard for mountain bikes for what? 20 years?

The compromise in inflation pressure is pinch-flat prevention vs. rolling resistance vs. cushioning/rider comfort. I think the assumed effects of rim width is overthought.

choddo

How is that physically possible? I don’t believe that HED chart.

HTupolev

When the tire is a lot wider than the rim, a lot of the tire's "length" is spent curving inwards to the rim, so there's less left over to extend upwards.

Imagine a circle, and then imagine a semicircle but with double the radius:



The arc length and height is the same. Between those two extremes, the arc ends up a bit taller.

choddo

Ah yeah ok, I hadn’t considered the rim being notably narrower than the tyre. Although it’s pretty clear in the chart on the narrowest one, now you mention it. Sorry HED for doubting you (even if you have exaggerated things cos I’ll lay cash money that red line is longer than the white one )!