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Discussion starter · #21 ·
Updated the table in the first post with the measurements for the Innova Gravity Vidar 29+ on a 35mm rim (Velocity Blunt 35).

Overall mounted outside diameter: 767mm
Width knob-to-knob: 74mm
Width casing: 71mm

This means the tire is pretty much the same diameter as the Surly Knard, Bontrager Chupacabra & Vee Fatty Trax. I have a pair of Maxxis Chronicles on the way and will post the measurements for those eventually.

I started this thread to figure out which tire will have the most clearance under the fork's arch when fitted on my Fox 32 F29 fork. That tire is still the Surly Dirt Wizard by far, but alas far from ideal tire for my riding conditions. The Panaracer Fat B Nimble has about 2-3mm more clearance than most of the others.

Btw, I added a blog post about my unexpectedly successful attempt to mount the Gravity Vidars tubeless on my Blunt 35s.

Innova Gravity Vidar 29+ tubeless setup on Velocity Blunt 35
 
#24 · (Edited)
can you prove that tyres, when inflated, tend to circular cross section (as in your diagram) ? (albeit with a varied flat section that is the rim)

given the variance in thickness and materials at different circumferential points, i am not convinced that it would be the case. i dont know though, so would love to see your evidence.

i see it a bit like blowing up a ballon with a strip of tape on it.

in addition, and perhaps making the above a non-issue, the effective point of contact of the tyre is when it hits the ground. what happens at any other point is irrelevant. the ground clearly deforms the tyre and this varies with pressure as well as tyre structure and size, right? so how does this affect your calculations?

apologies if you have, elsewhere, already dealt with this, but could you point me to it?

craigsj said:
Here's an old diagram I did of the same size casing varying over a wide range of rim widths. You can see that height does vary but so does the center of curvature. Since the rim on the wheel has a constant radius, the change of center affects the tire height and neutralizes any benefit of effective tire growth.

I also included a range of real tires with measured b2b dimensions and showed effective casing heights over a broad range of internal widths. The change in height is 1-2% over interesting rims. Big deal.

Using the Chronicle's b2b number advertised here, the tire's height would vary 0.4mm between the rims LyNx has mentioned. The width, though, would vary 5mm, or about 10 times as much. I guess that qualifies as "mainly height" in LyNx's eyes. :lol:

Incidentally, the Chronicle's height peaks at 40mm internal rim width. Another reason to think that 50mm rims are too large for plus sized tires.
Click to expand...
 
#25 · (Edited)
so i got to thinking about this a bit more. i dont know too much about the physics, but i do know i like the combination of a higher volume tyre at lower pressure for my relatively low speed, reasonable tech riding on a hardtail or rigid bike.

why is that?

considerations for tyres (and or wheels):

Comfort
Control/traction
Rolling resistance
Puncture resistance
Rim durability
Weight


my thoughts:

Lower pressure = more comfort given same casing construction

A bigger volume tyre feels similar to a small volume tyre at less pressure (pounds per square inch…I guess it is self explanatory) given similar casing

Less aggressive tread gives less rolling resistance given same casing construction and size (I don’t know how true, but for truck tyres, figures of 60-70% of the rolling resistance is associated with the tread design)

increased hysteresis can lead to increased rolling resistance ('tacky' DH tyres anyone?)

increased hysteresis gives more traction ('tacky' DH tyres anyone? :)~)

less tyre pressure is unlikely to make you slower (increase rolling reistance) given the range that is realistic for bikes. it is also probably unlikely to make you faster if considered in isolation

bigger volume tyres create a bigger foot print in general, though pressure and casing design affects this

bigger foot print is associated with more traction

lower pressure tyres can lead to pinch flats with tubes, and rim strikes

casing design can be beefed up (specifically on the sidewall) to mitigate this, but then the tyre becomes less supple – see hysteresis

casing design can reduce or promote sidewall wear and piercing type puncture risk

So: a bigger tyre volume, with a lower pressure, and less aggressive tread might provide similar or better traction than a smaller, narrower tyre
it might roll at the same speed (or perhaps faster if the tread allows) and it might be more comfortable.
If it is made well - with good quality casing - it might not wear fast or be at risk from punctures and it might not be too heavy. Given the whole light strong cheap triumvirate, it is unlikely to be inexpensive.

Depending on how you load the tyre this may be a *good thing*. If it is more likely to flop around on the rim (too narrow a rim, or just high bulbosity) and is used in a high speed/direction changing terrain, it might feel more vague. This is probably why enduro racers/DHers are using no more than 2.5” tyres I suspect?

i guess that is why i like em?

now - i am no expert - this was purely typed as a vague self assessment. id love to hear if anyone else has any thoughts on all this. yes, even craigsj! :)~
 
#26 ·
dRjOn said:
can you prove that tyres, when inflated, tend to circular cross section (as in your diagram) ? (albeit with a varied flat section that is the rim)

given the variance in thickness and materials at different circumferential points, i am not convinced that it would be the case. i dont know though, so would love to see your evidence.

i see it a bit like blowing up a ballon with a strip of tape on it.
Click to expand...
No, that can't be done generally, especially considering that tire construction could change. It would be true for tires that had uniform casing construction as they largely do, but variations could cause some deviation from round. The balloon analogy is a good one, though casings aren't that stretchy of course.

More important than a tire's variance, though, is how much tires vary from one another. They will all deviate from round in the same ways because they have the same trends in construction. To me, what's important is how tire compare to one another, not how precisely one tire is modeled.

I've done a lot of measurements while developing my model and I don't see a great deal of variance. There will always be some for plenty of reasons, one of which is this issue you bring up. The answer is to manage expectations, you can get within a few percent but should expect any more.

dRjOn said:
in addition, and perhaps making the above a non-issue, the effective point of contact of the tyre is when it hits the ground. what happens at any other point is irrelevant. the ground clearly deforms the tyre and this varies with pressure as well as tyre structure and size, right? so how does this affect your calculations?
Click to expand...
Yes, yes, yes! I've been saying this and certainly agree.

My model shows that tire diameter isn't really affected by rim width, so that doesn't change here.

Clearly a tire under load will have a different shape so my width calculations won't be valid. That would be an interesting challenge to show width under load for narrow vs. wide rims.

If a tire stretches, does it shrink back under load? To what extent is stretch even meaningful?

This point you make is what's really interesting, not anything else that's commonly talked about. A tire deforms based on load, pressure, and construction, for sure, but is the rim width involved? Also, how does the existence of tread impact this? A smooth tire's contact shape would definitely change with rim width but does a knobby tire? People think wide rims make tires wider but they do not for these reasons; they do make tires work better though.
 
#27 ·
dRjOn said:
now - i am no expert - this was purely typed as a vague self assessment. id love to hear if anyone else has any thoughts on all this. yes, even craigsj! :)~
Click to expand...
I think all of that is well thought out and I agree.

Where I object is in the use of the word "volume" as a substitute for casing size. It's true that tire volume and casing size are directly related, but a wide rim will increase volume without increasing casing size. We want big casings and proportional rims, not the same casings on oversized rims (IMO).
 
#28 ·
Recently I read or heard that a computer with the capabilities of the human brain/mind would require a space the size of a good sized warehouse.
I don't have a cite for that so let's just put it down as a supposition. I apply that to my evaluation of my bike's performance as it goes over varying trail terrain. I can use that to perceive differences in tire performance from pressure, rim width, tread wear level and profile. No graph or equation is anywhere near good enough to help communicate what goes on. Demoing is the way to go. Maybe a friend's setup or one of the 27.5+ or 29+ bikes at a demo day will become available. Use that opportunity to get the info you need. Your own perceptions will tell you what you need to know.
 
#29 ·
eb1888 said:
I can use that to perceive differences in tire performance from pressure, rim width, tread wear level and profile.
Click to expand...
Your brain is notoriously terrible at measuring differences objectively. Even the most primitive computers, even calculators, are far better at such things.

Without "graphs and equations" we wouldn't have bicycles to begin with.
 
#30 ·
I've been using the Bomboloni for about two months now. You should be able to get one just about anywhere. QBC has them I heard.

I've been running all my 29+ tires on 50mm OD rims. I have all the measurements on my garage wall. All measured after stretching and the Bomboloni is somewhere between the DW and Gravity. I'll post up the heights here soon, but what I wrote above is correct, and I measure with a level.
 
#31 ·
Haven't been riding for over 6 weeks due to a simple fracture (got lucky) of my C-1 vertebrae, so I have the spare time and extreme boredom needed to measure my tires and further waste some more of it here on the forums;-p

My Bombolonis only had about 60 miles on 'em before I was forbidden to ride and became sentenced to wearing this neck brace. They're on 50 mm Rabbit Holes.
They had aired down to around 8 or 9 lbs, so I pumped 'em up to 14, mainly to generate some excitement, and ensure that the bike is ready to ride when my doc says its OK.

Bombo casing is barely 72 mm. approx 71.8, maybe
Tread-to-tread measures approx 73.5 mm. I had to measure using a straightedge on each side, since the outer tread knobs aren't directly opposite from the ones on the other side of the tire, but are staggered.

Didn't measure the diameter.

My tires are fairly new, and may stretch with more use.
 
#34 ·
Thanks for catching that glaring error.
Yes, I can see by comparing my wheels that 29+ tires are larger in diameter than 26" fat bike tires.
My experience riding on the beach here with various 26" fat bike tires has left me with some opinions. My 26" Fat B Nimbles are less than 3 3/8" wide when inflated to beach riding pressures. As a result of this relative skinniness, they don't float nearly as well as a fatter fat tire on the soft sand. So I don't like them for beach riding. Every other fat tire that I have tried on the beach has been noticeably better on the various textures of the beach sand. My Halo Nanuks aren't much wider, at only 3.5" wide, but they roll much nicer on the sand than FBNs due to their tread design.
I swap over to the FBNs for riding logging roads and trails.
Knards, with their fatter casing, are better on the sand than the FBNs. They are mounted on my SS wheel set, which is "mothballed" and stored for now.
My fat rims are 65mm Marge Lites.
MY next set of fat tires will be selected from among the widest that will fit on my Pugs. That's way out in the future, at least a year from now.

So, for my 29+ wheels, I wouldn't mind a fatter tire. I really like my Bombolonis, but they don't like softer sand, so I generally stay off the beach when rolling on 29+. I will blitz downwind the 6+ miles to my house (from Westport to my house in Grayland) on the hard packed sand at low tide on my Bombo'ed Krampugs, if the tide is out and the wind is from the N or NW. Its a nice fast beach ride, then.
I'll roll on the Bombos for this year, but when they start showing some wear, I'll be ready to try something different.
I'd like to see a Vee Bulldozer 29 x 3.25" mounted on a Rabbit Hole, and measure the actual width.
 
#36 ·
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