Huh? If I switched to a wide downhill rim, I'd still need the wide tire on a wide rim. The wider rim is significantly heavier and I have provide the power to move it.

That's because all those other tire users aren't limited in terms of power. Bicyclists need the traction but we don't need the weight.

But that's not even true for all applications. Many off-road vehicles run tires that are much wider than the rims...4x4 trucks, ATVs, side-by-sides, etc. Swamp tires, for example are 14" to 15" wide and are run on rims that are 8" wide. Maybe not as extreme as running 55mm tires on 17mm rims but pretty close in terms of tire width to rim width.

There are even good reasons for going with an "omega" shape to the tire for off-road used on both bicycles and OHVs. The shape protects the sidewalls from damage. Take a look at the "oversize gumbo mudders" that Iride01 talked about in the above post as well as many mountain bike tires. The tread wraps around the edge of the tire. That's partly for control in corners but it also protect the tire so that you don't end up with a cut sidewall where you don't want a flat. A more "U" shape of a wider rim would be more susceptible to sidewall damage

spend enough $$$ and the wider rims don't have to weigh more. My new Ardennes Plus (21mm internal) are noticeably lighter than my old Kyrium Elites (13mm internal). I know this isn't exactly fair because I don't know the specs on the new Mavics.

In addition, my 25mm tires gained 2-3mm width on the new rims, and I could run them 30psi less (based on HED recommendations). It tamed the harsh ride I was feeling on my Giant OCR iC1, which btw, was designed for comfort but shipped with the harsh Mavics.

All things being equal...i.e. the rims being of the same quality...wider rims are always heavier than narrower rims because wider rims use more metal than narrower rims.

Sorry but I'm not buying the "30psi less" because you are using wider rims. If you used 25mm tire before, nothing has changed to justify that kind of pressure reduction.

Hed removed some metal (on the brake track area?) to get these results, iirc. But of course, if everthing is the same a wider rim weighs more than an exactly equivalent narrow one. My example was an implied case of a wheel upgrade.

well, I'm not making this sh$t up. I'm taking Hed's advice for tire pressure on their rims. I weigh 138# and am running 70psi (the recommendation to run 100psi on my Mavics came from a cycling coach, not Mavic, so it's not an apples-to-apples comparison - maybe I could have run less). A local racer/mechanic weighs 155# and runs his Hed's at 80psi rear, 70psi front). And btw, something did change. My 25mm tire is measuring 28-29mm with the wider rims. That's always been their position, as I've understood it. Wider rim -> wider tire -> more air volume -> less air pressure.

For the same tire 30% less air pressure raises an eyebrow.

But with the rest of your stats it's obvious you were way overinflated in the first place and probably a little under now.

I'm not saying you are making anything up. I'm saying that HED is making **** up. A wider tire can be run with slightly less pressure but those claims are usually overrated in my opinion based on the differences in volume. Going from a 23mm tire to a 25mm increase the volume of the contact patch...the only place the volume change really matters...by about 10ml or about 2 teaspoons. That kind of volume change doesn't warrant a 30psi...or 30% in your case...reduction in air pressure.

Think about it this way, if you could drop 30 psi by going from a 23mm tire to a 25mm tire, then going from a 25mm tire to a 28mm tire would allow for another 30psi reduction. Going from a 28mm to a 32 would allow for another 30 psi reduction. Going from a 32 to a 35 would allow for another 30 psi reduction. If you do the math, you can now ride on tires that are running a 20 psi vacuum.

As for your tires now being 28-29mm wide, they aren't. Or, at least, the profile has changed. The volume hasn't changed. The tire isn't a 25mm wide circle but a oval that is 3mm wider at the rim but narrower where the tire hits the road. Doug5150 said that car and motorcycle tires try to avoid using a wider tire than the rim. They also try to avoid using a tire that is narrower than the rim. That's probably more important than using one that is too wide. A tire that is too narrow can be prone to popping off the rim.

Someone mentioned it here or another thread. But wide rims that most of us would consider too wide might reduce pinch flats from low PSI. Then the risk becomes rim damage if the rim hits the pavement. Maybe not a big deal in dirt.

I'd rather pinch a tube and buy a new one than risk damaging a rim.

I have tried running low pressures on 23 and 25 mm tires with my narrow 14 mm rims. 70 psi worked okay for the front, and 90 psi worked okay for the back. But I still feel less tired when I pump them up to 100 psi and 125 psi.

I weigh 165 lbs. and only ride pavement.

You are incorrect on two counts here. Looking at a cross section of a tire, if the beads are closer together than the overall width (25mm for example) of the tire, as they move farther apart, the circle expands, increasing volume. That's just a fact. The height does not reduce as the width increases. This is true until the beads move farther apart than the max width (25mm) of the circle.

The other thing that you are missing is that because inner tubes fill the cavity that is within the rim, that volume is a part of the overall air volume as well. As the rim grows wider, so does the cavity, and so does the tube, creating an even bigger air pillow.

I have 35mm Schwalbe Kojaks on some rims that measure 24mm outside width. The tires measure 32.5mm.
I have 35mm Schwalbe Kojaks on some rims that measure 32mm outside width. The tires measure 37.5mm.

While the profile has indeed changed, so has the volume. The pressure requirements, handling, and ride quality are quite different between the two sets.

Only one part of the circle expands. The rest of the circle flattens and the height of the tire is lower. Because the tire is basically nonelastic, the volume remains the same so the dimensions change but not the volume. You've taken a round tire and made it oblong but you haven't increase the volume.

Again, the width grows at the expense of the height. A wider rim with the same tire has a lower profile and hasa shallower air pillow.

What do they measure in height?

The profile changes but the volume doesn't. A bike tire is just a balloon that's extra tough. Squash a balloon and all that changes is the shape. The volume remains the same. The balloon doesn't suck air in from outside just because you've changed the shape.

Any differences in handling, ride quality and pressure are because of the changed profile, not because of a volume difference.

Completely incorrect.

Take a cir clip. View it as a tire cross section. The two holes represent where the beads are. Place you pliers in the holes. Spread the holes. The circle expands. The volume increases. The circle doesn't magically shrink at the top.



No it does not. All the air inside the LARGER rim cavity can' be ignored. It's there. The PSI reading doesn't just indicate what is inside the tire alone, it is a reading of the entire chamber, which the tube has GROWN to fill. More volume. Period.



35mm, on both sets. When the beads are pinched together by a narrow rim, they measure narrower. When they are are spread apart by a wide rim, they measure wider.



This is where you are missing the boat. Unlike a balloon, that is a circle with no ends, a tire has two ends (where the beads are). The balloon can ONLY deform under pressure. The tire, because the ends are being spread apart by the wider rim, grows. It's not confined to deformation alone. Unlike the balloon, you are literally opening it up, like the cir clip. you are increasing the cross section's circumference, and therefore the volume as well.

This is plain and simply false. With a wider rim, the air chamber grows. FACT. With that comes the ability to run less pressure to achieve the same amount of deformation at the contact patch. Reduced pressure that still achieves the same deformation provides greater shock and vibration absorbing without increasing rolling resistance.

Balloons are stretchy and don't allow their surfaces to tension up much much to contain the pressure. They change shape to keep their internal pressure very close to equalized with the ambient air. More air inside, bigger balloon. If you squish it somewhere, it'll get bigger elsewhere.

Tire casing is specifically chosen to resist elongation when you tension it. So instead of getting bigger and bigger as you put more air in, it just gets pulled tighter and tighter, more and more tension as you increase PSI. So if you compress part of the tire (i.e. at the contact patch), the air displacement will cause an increase in pressure elsewhere in the tire more than it'll cause the rest of the tire to get bigger.

Bead seat width obviously affects the seated shape of the tire, which actually does change volume because different shapes contain more area per their perimeter.
Consider two hypothetical extreme cases with a tire that has 2" of casing between the beads: beads totally together so that the tire inflates to a circle, and beads apart so that the tire inflates to a semi-circle:

https://i.imgur.com/7cAhfdt.png

In the first case, the circumference is 2", so the radius works out to 1/pi, so the area of the circle works out to 1/pi.

In the second case, the circumference of the circle (if we pretend the bottom half exists) is 4", so the radius works out to 2/pi, so the area of the complete circle is 4/pi... then we divide by two, because we only have a semicircle, and the contained area is 2/pi. Twice as much contained area as the first case. So about twice as much tire volume.

In the real world, the shapes are more omega-like and will have less variation, but the gist holds.

On the other hand, I'm not sure the focus on volume makes much sense to begin with. Tires don't bottom out on a bump when they run out of volume, they bottom out when they run out of height. A tire on a wider rim is braced differently and probably squirms less when cornering at low pressures, but it doesn't really have deeper cushion to play with.

I started to compose a post along the same lines as yours, [MENTION=406505]HTupolev[/MENTION], but had to get back to work and didn't finish it.

All else being equal, I'd expect the extra volume provided by a wider rim to make the ride cushier at the same pressure, but I wouldn't lower the pressure dramatically from what I was already using. Tangentially(?), I sometimes wonder if old-school/cheap single-walled rims provide some of this benefit, too -- there is more air space available within the rim that adds to the overall volume. :thumb:

You are assuming in the case of the circlip and the tire that only one part of the circle changes. In both cases, the circle doesn't remain a circle. If you spread part of the circle, the rest of the circle has to change to accommodate the change. It does that by not remaining a circle. The area encompassed by the circle or by the oval shape you've created remains the same.

You are ignoring the fact that the tire profile flattens which decreases the volume of the tire in proportion to the increase in volume of the rim cavity. The tire doesn't "grow" because it can't "grow".


It is only increasing the cross sectional area in one dimension. Because the tire is a semi rigid structure, the other dimensions have to change to accommodate the wide change. The tire flattens out and reduces height accounting for the increase in width. Going from a circle to an ellipse means that the area encompassed remains the same.

The amount of air contained within the rim might be greater. But the decrease in the volume of the tire because of the lower profile doesn't provide anymore cushion. In fact, it provide less. The height of the tire is lower so the tire bottoms out on the rim sooner. Running a severely lower pressure like sunburst is opens the tire up to pinch flats and/or rim damage with the lower height.

You are missing my point. A ballon is a fairly easy system to visualize when it comes to changing dimensions. If you have a circular balloon and you pull it so that it is wider, the balloon maintains the same volume by flattening out. It's no longer circular but it maintains the same volume. Same with a tire.

Yes with the exception that a decrease in volume causes the pressure of the entire system to increase. It's not compartmentalized.

What you (and others) are missing is that the circle isn't a circle anymore when you pull the sides apart. The tire can't maintain the same height and circular shape. The top of the semicircle flattens out and becomes an ellipse and thus maintains the same volume. And ellipse has 2 radii as does a circle. In the case of a circle the radii are equal. Spreading apart the tire like in your illustration would result in an increase of the horizontal radius (a) but a decrease in the vertical radius (b). The volume contained remains the same.


That's mostly semantics. Volume is just the cross-sectional area times the height. In a tire's case, the "volume" of the contact patch is the area of the ellipse formed by the tire on the ground times the height.

I do happen to agree that the height of the tire is rather important and too many people make too much out of the differences between various tires and how they impact the ride. A 25mm tire is only 2mm taller than a 23mm tire but people will tell you that the ride is vastly different because of the much greater volume. It's 2mm. It's the thickness of a dime. It's not even that if you are going to decrease the pressure of the 25mm tire by 20psi.

Nope.

Never thought I'd say this, but those guys should pay road tax.

Anyway... I wonder if BF should start something called BFLabs to test all our controversies (tires, frames, pedals, etc.) Set up some protocols for testing and reporting and all that.

Not to an extreme degree, no. That the casing has to rise up out of a rigid rim wall will prevent the tire from being perfectly circular (hence the "omega" description), but widening the rim has an effect more akin to lopping off the bottom of the circle* than adding elliptical eccentricity.

There's also no obvious reason that the ellipticalness would be biased to maintain constant volume. In order for the wide-rim case to not have greater contained area than the pinched-to-a-circle case, it would have to flatten out dramatically. You're basically implying that this happens:

https://i.imgur.com/jTe5eiy.png

*While also making the remainder be of a bigger "circle", since the "lopped off" perimeter is now part of the remainder.

I think you both may be right, part of the time.

From what I've seen with my own tire-making experiment:

1) If a tire casing inflates to a width that is greater than the rim it is on, then the portion of the casing that is free of the rim always inflates to a circle. I've seen this myself, and the thread angle of the casing doesn't seem to matter. The tread layer of rubber may make the tire appear not-perfectly-round, but if you had a bare casing to use (like I did) then you'd see that the casing itself is going to settle into a circular cross-section, as much as it can.

2) If a tire casing inflated to a width that was narrower than the rim it was on, then it would have to inflate to an oval shape. This just makes sense because the edges would have tension pulling them together that would need to equalize.

I am guessing on #2 above because I don't have any suitable tires and rims to try it with. To try it, you would need a rim that was MUCH wider than the tire you were test-fitting on it.

If you wanted to conduct a casual experiment, you could try putting a narrow slick 559mm tire on a fatbike rim and putting a few lbs of air into it. To maintain a circular curve it would need to lie very flat against the rim.... and I don't think it's going to. The center will be a gentle curve but the edges are going to pull towards each other and form some smaller + tighter curves.
,,,,
I'd have to admit I've never seen anyone ride a wheel & tire combination like that however. If you had special tires made for fatbike rims it might work, but wouldn't work well IRL I think.