11/6/14 8:17 PM

Just a comment/data point.

People talk about how wider rims result in a different tire profile (wider than for a standard rim). I now have mounted Conti GP4000S 25mm tires on Velocity Aerohead and Velocity A23 rims. The tires come out to exactly the same width and exactly 25 mm. This is in complete disagreement with those who state that the wider rims gives a wider tire.

Also worth noting that the tire on the Aerohead is the same width as it was when it was first mounted (2300 miles on it now) and so I don't think you can attribute any of this to casing stretch, of which there appears to be none whatsoever. I do find tires easier to mount and remove after they've been on the wheel a while, but no change in width.

Comments?

11/6/14 8:57 PM

Swap the tires and rims and see if the older casing is stretched??


Or put 125lb in the tire on the A23 and measure in the morning.


Did I understand it correctly the new tire is on the A23?

My Conti GP4 23&5 and 28 All seasons well blathered about in previous threads/post regarding how they sit/width etc.

11/6/14 9:51 PM

For me, it wasn't the width and much as the height of the tire. A 25mm Conti 4KII on a Velocity Chukker rim had no clearance issues to the rear brake hood but when I went to the 25mm HED Ardennes CL+, it didn't rub, but the clearance was so reduced that any ride through wet and grit would make contact and spray road stubble on the following rider - OTOH, that'll teach 'em to suck my wheel....the tiny flecks of pavement filling the gaps in their teeth...alas, I digress.

Anyway, overall volume increased in height where the sidewalls, instead of tapering inward, were more flat which I read leads to higher potential for blow offs at rated max pressure.

When I talked to a tech at HED, he said "no more than 100 psi even on a 23c Conti 4KII to which I asked "even though the tire is rated at 120 psi" to which he said "they weren't thinking of wide rims when they printed that.

So, with the mad push to box (wider) rims, me thinks the tire companies need to rethink max p.s.i. ratings else their lawyers will, assuming the blow off risk is in fact greater.

11/6/14 10:00 PM

If I measure the 650b x 42 Hetre tyres on my tourer when new and first mounted, and then measure again a couple of weeks later, the width of the tyres increases by about 1.5mm over that time. This is with 45psi in the tyre. However, this is a fat tyre with a pretty lightweight nylon casing, whereas I believe the Conti has Vectran in the casing, and Vectran is a form of Kevlar with very low stretch.

11/7/14 6:59 AM

...as wider tires, increased air volume. Therefore, you have to lower the tire pressure to compensate. The max pressures on tires are generally much higher than anyone needs (unless you're really heavy) and there is no advantage to riding excessively high pressure on typical roads. Unless you're on billiard-table smooth surfaces like a track, lower pressure are more efficient, as well as being more comfortable.

FWIW, with 25C Conti 4000S tires on standard width road rims at my weight of 175#, I run 72-73 in the front and 82-83 in the rear. The ride is smooth, I've had no pinch flats (or any flats, for that matter) and the tires don't feel mushy when I get out of the saddle to climb or sprint.

11/8/14 7:53 PM

Per Sparky's suggestion, I swapped front tires on the two rims. Here are the width measurements:

old GP 4000S on Velocity Aerohead = 25.0 mm
new GP 4000S on Velocity A23 = 25.0 mm
old GP 4000S on Velocity A23 = 26.3 mm

So yes indeed the old tire (2300 miles on a front wheel) is wider on the A23 than on the Aerohead. There is an obvious difference in ease of mounting/removing a new and old tire, so I wonder how much of it is actually casing stretch and how much is bead stretch (which would allow the tire bead to ride a little higher in the rim and therefore be a little wider).

11/8/14 10:13 PM

I've noted significantly greater maximum width in tires mounted on wider rims.

Same tires also showed a slight increase in height at the tread when mounted on rims that were closer to the width of the tire itself.
At some point though, increased rim width would start to reduce a mounted tire's height.

Narrower rims are more highly stressed by wider tires whose outward-canted sidewalls pull the rim's brake tracks outward with more force than do narrower tires.
Perhaps this was the main reason for HED's concern? I note also that some of today's wider rims have really, really thin sides, which would lose strength over a shorter service life and blow outward at lower-than-expected pressure. The Ardennes/Belgium rim has very thin sides.

Road tires typically expand by a half-millimeter or so during the first hours of inflation.

11/9/14 6:51 AM

...by ~1/3 the difference in the rim width, since what you're effectively doing is added to the circumference of the more or less circular shape of the tire/rim.

11/9/14 7:06 AM

These days, most wider rims are for a minuscule aero advantage.

I have yet to try 25mm tires because the added weight is so much unless I get high dollar tires. I may in the future but for now, I will roll on my good ole 23mm tires.

11/9/14 11:57 AM

I have said it before, but will repeat for the sake of the thread. I have noticed, and I only refer to tires size, 35+ mm tires don't change hardly at all as compared to the 'road' range of size tires. The Road Race range in the 23-5mm seeming to yield the most profile change.

11/11/14 10:30 AM

True, the tire profile of the bigger tire doesn't change noticeably, but the stability (lateral stiffness) of the big tire should increase by the percentage of increase of the bead-seating width, I'd think.

Narrower rims add unwanted flex to any tire that is significantly wider than the rim imo.