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.
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.
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.
Anyone ridden Schwalbe Rick XC Pro?
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.
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.
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!
this brings back memories. Pythons were big around early 2000s. Now, this tire shares only the name but still.
That’s not really why they do that.
Thinner ID gives the tire a more rounded shape and allows them to corner better. DH teams run a ton of sealant and roof tar inside their tires to prevent flats. Also a ton of PSI.
It’s really apples to oranges so it’s not really relevant to this discussion IMO.
Actually, it is relevant. By increasing the effective tread width, you’re shortening the sidewalls, which decreases “travel” of the tire.
Pressure and Volume does play a role, but the diagram you’re showing isn’t to scale and ignores that as you go from 22.5 → 40mm, the mounted tire is getting shorter (assuming using the same tire)
And actually, now that I think about it, compressing the tire could have the same effect to the point where going wider “could” make the tire taller too ( I think) - depends on the tire.
DH racers run 24-27 PSI so they have enough support to run those narrower rims without getting super floppy.
Yes, you are correct they are taller with 25mm ID but that’s also a helpful trait for DH racers who lean their bikes much further than XC racers tend to at much higher speed.
More power to people who can ride these types of tires with low 20’s/high teens PSI and 25mm ID. Not for me personally, hate the squirrely feeling.
Actually, if you ignore the labeled widths, the image is very close to scale and does a great job of illustrating the point of increasing volume as you increase rim width. In the image, the tire portion is the same for all 3 cases (you can measure the inside perimeter of the tire only and they’re within 1% of each other) and you can clearly see (and measure) that the cross sectional area (and therefore volume) grows as the rim width increases. I’m also not sure the sidewall height actually decreases all that much as the rim width increases. I understand your theory that as the tread spreads out it’s no longer contributing to sidewall height, however you’re also moving the bead outwards which allows for more of the sidewall to contribute to height and not ballooning out to the proper tire width (i.e. the sidewall gets “steeper” as viewed in a cross section). This is also illustrated pretty well in the images above.
You’re incorrect in assuming that the tread must get closer to the rim when increasing rim width. If you had a 1.5” rim with a 2” tire, then you are correct, wider shortens the tread height and reduces volume. But imagine the opposite. Take a 2” tire and mount it to a 10mm rim. It would almost be a complete circle. Now make that rim 0mm wide. The tire beads have to move in and touch, which makes the tread lower and decreases the volume. So clearly there’s a rim to tire ratio where the behavior reverses.
In the range we’re talking about (20-35 mm rim with 2”+ tire) increasing rim width has very little effect on tread height. This is because the tire sidewall is not vertical, it’s angled out. (See the red triangles below). So some of the sidewall is reaching horizontally and some is reaching vertically. As the rim width increases, the tire sidewall has less horizontal distance to reach, so the tread might move upwards. Then again, the tread can also get flatter. There’s no easy math to say which it will do (plus tread design and tire construction probably have a significant effect).
The below graphic can be representative of this situation. The tread height (distance from the rim) doesn’t have to change any significant distance with a realistic range of rim widths.
Now I hope that you can see that, given a tire width and a realistic range of rim widths for that tire, a wider rim is unquestionably going to increase the air volume contained by the tire and rim (assuming no inserts are used). What is uncertain is how much it affects the tread shape and ride characteristics for knobby tires.
No, you’re right, I posted this elsewhere too after thinking about it, sidewall height can actually go both ways depending on the combo but should change and shouldn’t be viewed as constant. I have seen fork clearance increase though with a rim width that went up (gravel fork 25-30 mm rim and gravel tires) - so I suspect tire design and casing plays a role too.
I’m going to measure some specific tires I have, I think at the end of the they day with the small changes we’re talking about, we’re mostly splitting hairs on the differences. I’m not sure most would even be able to tell the difference.
Looking at some new tires. I was thinking either Vittoria Barzo F/Mezcal R. Or Schwalbe Racing Ray F/Thunderburt R. Would there be a lot different in these sets? Or mostly just brand preference?
My very first set of tubeless tires where hutchinson pythons size 26 on mavic rims. Yeah, I am old and so is this model of hutchinsons. Off course it has evolved a lot, but just saying.
I was on Mezcal/Mezcal for 5 years. This year I went to Ray/Ralph. I’m never going back. They feel just as fast and I have more grip. I tried Barzo as a front for a few rides years ago and didn’t like it, but I don’t remember why.
