From my experience, 30mm rims for 2.4 tires help a little bit by providing a little extra volume and tire support at low pressures but it’s quite marginal. 25mm will not be an issue
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Just thinking this through: Wider rims should provide no extra volume. They’re not stretching the tire by any perceptible amount, they’re just changing the profile of the inflated tire. By having the beads expand wider, the tread isn’t going to bulge as much so you’re going to get marginally more support, and less compliance.
A tire total air volume is the result of an extrusion of it’s section area. Now think about the geometry of that section area cut of the tire+rim. It’s a polygon, and a good enough approximated way to think about it is something like a pentagon.
The tire is the 4 topmost sides of the pentagon, the rim width (the variable here) is the bottom side. For any given pentagon, considering that the topmost 4 sides stay constant (the tire), if you increase the bottom side (the rim) you increase the area. Extrude it and you end up with an increased volume
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Except that the topmost sides don’t stay constant. By expanding the IW, you are decreasing the sidewall height. You’re assuming the other variables stay constant when they don’t.
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You’re not decreasing the sidewall height. That would be the case is the rim was wider than the nominal tire width, but this is not the case for MTB tires, where the nominal tire width is much wider than the rim.
A while ago when BRR did a test comparing different rim widths, they measured the same tires in various rim widths. The effect I’m describing is visible there :
Simple math - it’s not more volume, it’s more surface area in contact with the ground and a wider contact patch (and less vertical compliance) - which also changes the ideal air pressure dynamics. You’re not changing the amount of material in the tire, you’re not stretching the tire. While it’s not a rectangle you can think of it that way - If you increase the base dimension(s), you decrease the side dimensions to keep the total circumference the same.
If there’s something else going on here that leads to an increase in volume, I’m all ears. But, it’s not the way it’s being explained here, and just because BRR says “More Volume” doesn’t make it right, especially when they’re known for running some wanky high pressures in some scenarios (like, all MTB width tires)
Personally - I think it’s the tire design, primarily compound and tread / sidewall casings that drives the ideal rim width and will result in the “Best” rolling resistance because you’re changing the profile of what’s in contact with the ground.
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You’re not increasing the volume because you “stretching the tire”, you increase it because you increase one of the sides of the polygon formed by the tire+rim system. This is simple math indeed.
What I’m describing is well known in the bike industry, not exactly rocket science, but maybe I’m failing at explaining it clearly enough. As for the BRR link, I don’t care about what they say, you can simply read the tire measurements (height and width) on different rims and take your own conclusions.
This will anyway be my last post on the topic, I tried my best to explain it. Feel free to do a Google search on the topic
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And in increasing one of the sides, you also decrease others. Your argument says you are changing total circumference of the tire.
You are certainly explaining it poorly and haven’t said anything that changes that.
Here’s the “something else” what I think happens:
You increase the base (rim) dimension, you increase the tread dimension, you decrease the sidewall height (vertical compliance of the tire). You’re probably getting a small total “circumference” gain purely because of the extra rim measurement, but the tire outer dimension itself (the other sides) stays the same. Any volume change wouldn’t be related to the tire, just the extra rim width (“rim volume”).
Now it would be curious to see if anyone’s actually measured or calculated it, other than a lot of the incorrect assumptions out there.
(Edit - reading what I wrote above, I think I worded it poorly too. You are not changing tire measurement and you are decreasing sidewall height when you increase the rim width and tread width so those variables are changing, but you’re adding more base measurement purely from the rim. I think the “volume” argument is overstated and while it’s cited EVERYWHERE, I don’t think I’ve ever seen the impact actually calculated or measured. I think the bigger issue at play is what you’re looking for from the tire)
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Getting back to a rim width “experience” take. The latest rage is definitely wider is better, but I think it depends on the specific ride quality that you’re looking for more. While I have a 30mm IW everyday wheelset, I’ve been running 25mm IW rims on my race wheels, and feel like the extra vertical compliance (at the expense of more sidewall, taller tire, and a little more “squirrely”) helps on rougher courses and faster descents where you’re dealing with puncture risk. But, that works for me because I’m not pushing the cornering limits of the tire like some do, so the cornering performance isn’t the deciding factor for me.
I do think going too wide can be a problem (e.g. new Zipp 32mm gravel wheels and all the flats) - but not remotely an issue when we’re talking about 2.2-2.5 XC tires and 25/30mm XC rims.
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Maybe I misunderstood it, but that’s what I thought he was trying to say. The outer measurement of the whole thing changes because one of the sides changes (rim) while the other 4 remain the same.
Probably too simplistic an understanding. What am I missing?
The other “3” don’t remain the same. Rim Increases, Tire Tread Increases, Tire Sidewalls decrease. You’re effectively taking sidewall and moving it to tread. You’re dealing with a fixed amount of tire so you can’t increase the tread without impacting the sidewalls. (Edit - you’d see the impact of this if you tried to mount a 2.2 Race King on a Gravel Bike with 50mm clearance. Mount it on a 25IW rim and you may have contact at the top of the fork. Go to 30IW rim and you’ll get more clearance at the top, less at the sides)
Tried to clarify this above as I think I actually worded it poorly too, but here’s what I was editing in:
Reading what I wrote above, I think I worded it poorly too. You are not changing tire measurement and you are decreasing sidewall height when you increase the rim width and tread width so those variables are changing, but you’re adding more base measurement purely from the rim. I think the “volume” argument is overstated and while it’s cited EVERYWHERE, I don’t think I’ve ever seen the impact actually calculated or measured. I think the bigger issue at play is what you’re looking for from the tire
Here you go:
The difference in volume for a perfect 2.4 tire compared to a perfect 2.25 tire is 113.6%, to give you a workable metric for interpreting the calculation.
So, hypothetically, going from an 18c rim to a 26c rim with the same nominal tire size gives a little less than half the volume increase as going from a 2.25 to a 2.4 tire. Tire width increases more than tire height. The smaller the tire the larger the marginal difference.
Note: The volume of the rim area contributes to the total volume of the tire+rim system and this effect decreases as the rim becomes wider, as wider rims generally have the same height rim sidewalls as narrower rims. However, rim shapes vary significantly and it appears outside of the scope of this exercise to include this variable, at this time.
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Of note many DH pros have been using narrower internal rims to reduce the chance of flats.
I just finished the 5-day Roc Azur CIC, which for those unfamiliar is basically five days of racing 120mm bikes down rocky enduro descents (and then racing up the climbs between them). I had a 2.3 CrossKing Front and a 2.2 RaceKing Rear (insert in rear) on DT 1200 25mm rims. 0 punctures; whereas all of my team on Vittoria Mezcals on 30mms flatted left and right.
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The primary disadvantage from running tires wider than the rim is ideally designed for is that you’ll feel the tire “rolling” off the rim when cornering hard.
I recently swapped my 2.4s for 2.6s on my 30mm rims - and this is the main difference I have noticed. I can still ride fast, but the deflection of the tire is noticeable during hard cornering.
On my previous bike, I ran 2.4s on 25mm rims, and noticed something similar- although not as noticeable.
All-in, I don’t think the above effect is much of a disadvantage. I’ve never rolled a tire enough where it has burped a significant amount of sealant, let alone come off the rim.
All in, I’d prefer to err on the side of a tire that is too wide than too narrow. If too narrow, it leaves the sidewalls exposed, increasing odds of sidewall tears/flats.
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Thanks for this. I’m thinking I’m going to take some measurements for some 2.2 and 2.4 tires I have on hand and do the calculation for 25mm vs 30mm IW rims.
Yeah - the 25mm will give you more travel / vertical compliance at the expense of slightly more volume. Would probably be like going to a larger diameter fork with slightly more internal volume but less travel. My guess is slightly better small bump compliance / rolling resistance with the wider (to a point) but when you really need it less deflection before you bottom out and rim strike.
I’m not exactly schralping turns on my xc bike so I probably won’t be held back by that small difference in tire stability. 2.4 sounds like the move.
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Anyone ridden Schwalbe Rick XC Pro?
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Couple reviews and comments from other users about 10 posts ago. I have a set coming and plan to try them out in my next XC race.
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I bought a set of Hutchinson Python Race 2.4" tyres about 2 months ago.
On my 30mm id rims they measure just over 2.4 inches. Weighing 615 and 620 grams, so extremely light for 2.4s.
Really fast rolling, good cornering grip, accepting low pressures with no squirming.
During the last World Cup racing weekend of the 2024 season in Mont-Sainte-Anne a week ago French rider Mathis Azzaro finished second both in XCC as well as in XCO on these tyres front and rear.
But there is more:
During the Roc d’Azur 2024 last weekend, Mathis Azzaro rode with these tyres front and rear again, gaining a one-minute lead over Jordan Sarrou. Unfortunately, he finished second (again) due to cramping in the final stage.
Joshua Dubau finished third, and Maxime Marotte came in sixth. Both were also riding on Hutchinson Python Race tyres front and rear.
In the women’s race, the winner was Emeline Detilleux, and, you guessed it, she was also using Hutchinson Python Race front and rear.
It’s a bit of a shame that the Python Race are largely unknown and don’t get more attention but I’m glad to see proof that these tyres are FAST.
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I like the look (similar to the Thunder Burt) and have a good experience with Hutchinson tires on the road.
Be nice to see BRR do a test on them.
I might try them out next!
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