am8117

Assume one mounts the same 35mm wide tire on a 14-622 vs 19-622 rim. It is often discussed that the actual tire *width* when mounted on a wider rim grows by measurable millimetres. Has anyone tried comparing the profile height difference? Is it negligible or reaching beyond 2mm?

I am asking because of mudguard clearance issue in a fork which seems to attract sticky mud right around the cantis area and above, while there’s no problem with the rear wheel which seems to have perhaps only 2mm extra vertical clearance. I tried to move the mudguard as high as possible even filing off some bracket material to get where it is now but that’s very much where it touches the fork already.

AnkleWork

Baseline: change in diameter = change in circumference / pi

am8117

Fair enough, though the tires cross-section is not exactly circles and squeezing it in "diameter" does not cause the "excess" material to disappear but protrude. There are the classic bulb-shaped diagrams with the "too narrow", "just fine" and "too wide" scenarios, but they always focus on width only.

tomtomtom123

There are articles about this if you search on Google. There are also a few forum threads.
I imagine the tire width could maybe increase up to 2mm if you're increasing the rim width by 5mm.
The height, don't know, I think it depends on the circular cross section of the tire. I would guess that the height may decreased a little bit, but it could also increase. You could also cut some notches into your fenders, so that they can get around the fork welds and sit further up and away from the tire for more clearance.

shelbyfv

The "height" increases as does the width. This graphic has been posted many times. There is no reliable way to calculate as tires respond differently.

dsbrantjr

^This. Fender fitting is custom for pretty much every installation for best results. Just be sure to make/leave rounded corners on the slots so that a crack does not start there. I do this by punching holes with a Roper-Whitney punch clone at the end/corners of the planned slots and cutting between them. https://www.amazon.com/dp/B0002T87CW

Sy Reene

A 35mm tire on a 19c rim is probably closer to being standard per ETRTO standards and would be likely closer to nominal sizing than on a 14c rim. That said, I don't know enough to say whether possibly on the 14c rim, that tire might end up even be higher than on a 19c rim. The chart above is showing inflation sizes when on rims larger than ETRTO would have recommended back then.

am8117

Is it me or the chart actually shows that with the same nominal tire witdh at same pressure ... the height is the same whilst the width increases stepping up the rim sizing only?

Perplexing to me anyhow because I thought when I squeeze something like a tire it goes narrower and protrudes and when I relieve that pressure it goes wider and shallower.

am8117

This is exactly what I am confused about all along. I mean, when I look at current ETRTOs it would rule out putting 35mm tire on a 14C rim, but that’s how CX bikes were sold in 2008 en masse. Now the trend is to go the other way and I assumed when width of the rim increases the profile of the tire should “sink in”, but another way to look at it is that it is enlarging or shrinking a circle (on the cross-section) by adding/removing the rim portion of it.

am8117

I might just end up doing this. Thanks for the tip with the punching!

shelbyfv

Looks like 25mm@100PSI is both wider and taller on the Ardennes vs the 19mm. That's the only one that seems a direct comparison.

am8117

You are right even the 23mm at 100PSI ...

I wonder how representative it is what one would typically do with wider tires. I mean ... I am talking 35 on 21 vs on 14 whereas these tests are for combo where rim with is almost as wide as tire. I also can’t see having 100 PSI in a 35mm tire so it would be less of a balloon effect.

Interesting anyhow, just inconclusive I guess for my case still ...

AnkleWork

No idea what that means or how you might apply it.

In the case mentioned, the independent variable is 'change in circumference.' -- HTH

am8117

Circumference is “length around” the circle, I don’t think the tire on a rim can be taken as simplistically as a circle to which one adds some “length around” by the means of putting the tire on a wider rim.

1) I basically don’t care about how long it is around the cross section but how much of that extra protrudes which way.

2) One might argue it would protrude equally each direction but into the rim bed, however that’s not how tires behave, their walls are thicker on the contact patch and thinner on the sidewalls.

3) I don’t think adding 2-4mm to the rim width is same “extra circumference” effect as would be adding 25PSI, it’s not as even. All else equal, if you pinch a tire at the beads it would like to go narrower and gain more height for instance.

I am not arguing about the answer which I seek here but I can’t accept tires cross section behaving like perfect geometric shape of a circle.

ThermionicScott 

If you're moving to wider rims but are concerned about having enough clearance at the tire tread, I'd probably size down the tires by one step.

Jean_TX

The graphic has me puzzled - it appears that the tire casing increases as the rim width increases. For example, look at two of the curves for 25mm GP4000 – the “100psi on Ardennes” (light blue curve), and “100psi on 19mm” (white curve). The blue curve totally encompasses the white curve. So the increase in rim width appears to expand both the tire’s height and width … in other words, the tire casing itself appears to expand. Magic?

shelbyfv

It is hard to grasp, isn't it? I don't know if it's geometry, trig or some other such but it's beyond me as well. Maybe someone who understands these things can explain it.

am8117

The issue with the chart being representative of anything in general, at least for me, is that beyond the fact this is special case where rim width almost reaches tire width, in actual life no one runs 23mm, 25mm and 28mm tires - no matter on what rim - at the same pressure.