Wide wheel stressing tire sidewall?

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AW84
Posts: 87
Joined: Fri Aug 15, 2014 7:04 am

by AW84

I've never heard anything to suggest that wide wheels place any extra stresses on a tire, but I've had an interesting experience. I've ridden a whole pile of Conti GP4000S II tires in recent years, but this season got a set of Roval CL50's, with a 20.7mm internal width and put a set on them. Rather than run until the tread wears out without any other fuss, I noticed the external "thread" I guess you'd call it just above the bead starting to tear away in significant amounts, both front and rear. Basically nonstop...I'd cut a long piece off, and the next day another strand the full circumference of the tire would be hanging loose. Sometimes it would even get caught in the brakes and rip long sections loose. Tonight, the sidewall blew out on one of them, and upon closer inspection, you can see angled rows of the gold-colored cord through the outer rubber layer of the tire in the sidewalls all the way around...it's like the tire is being stretched to the point that you can visibly see the what's underneath the rubber. This is a 25mm tire, run at a max pressure of 100 psi, and there's close to half of the tread indicator still showing, so there hasn't been any funny business or overuse with them. But it's the first set that's come apart anywhere near this badly, and coincidentially, it's on the widest wheels I've ever owned. And it isn't just one tire, but both. Thoughts?

by Weenie


alcatraz
Posts: 970
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

How old are the tires? Is it possible that the thin rubber by the sidewall simply dried up?

On a wide rim I think we use more of the tire when we corner so it's also possible that you go too far on the sides of the tread when cornering.

Maybe your riding style has changed. Mostly cornering and fewer straights.

/a

pdlpsher1
Posts: 1543
Joined: Thu Jul 19, 2012 6:09 pm
Location: CO

by pdlpsher1

Your problem is due to the combination of a wide rim and too high of a pressure. The max pressure listed on the tire is based on an assumption of a standard rim, something like a 17C or 17mm internal width. When you mount the same tire on a wide rim, the tire’s air volume increases dramatically. The larger internal surface area of the tire means greater tire casing tension. In other words there’s more surface for the air to act on, so the overall tension in the tire’s casing increases. Let’s say the tire sees an increase of 25% in more internal surface area. At the same pressure the air is acting on an area that is 25% greater. So the casing is seeing a 25% increase in tension at the same air pressure. Because the tire manufacture doesn’t know what type of rim the tire is mounted on, they have to use an assumption. The 120psi maxi pressure of the tire is not based on a 20.7mm wide rim. In your case the pressure is way too high. For a Conti 4K 25mm tire you should run about 70-75psi. The same tire on a narrow rim will need a much higher pressure than 75psi to maintain the same casing tension.

I have the Hed Belgium + rims on my tandem. With Conti 4K 25mm tire I run 80 front and 90 rear. This is a tandem bike with TWO riders. The Hed rims have an internal width of 20.8mm. I used to run 110psi on the rear. After some time the tire started to show the sidewall fibers coming apart from the high casing tension. You won’t see it immediately on a new tire because the casing is tight when new. But over time the casing start to come apart, even on the side part of the tire’s tread. I lowered the pressure on the rear to 90 psi and all is OK now.


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alcatraz
Posts: 970
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

How does a wide rim increase the inside surface area of the tire?

To me it has more to do with the increased air volume. You need less vertical compression of the tire to reach the air displacement volume required to support the same weight.

Maybe I'm crazy, just started thinking about it.

/a

dim
Posts: 223
Joined: Fri Oct 13, 2017 11:25 am
Location: Cambridge UK

by dim

My Hed Belgium Plus wheels state that max pressure is 90psi .... I currently have Specilaized Turbo Cotton tyres (24mm wide) and run them at 75psi on the front and 80 psi on the rear (I weigh 75Kg)

previously, I had IRC tubeless tyres fitted (25mm wide) and ran them at 65psi front and 70psi rear
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AW84
Posts: 87
Joined: Fri Aug 15, 2014 7:04 am

by AW84

The wheel states max pressure of 130 psi and the tire 120 psi, so I'm well inside of range. I'm close to 180 lbs. (80kg) so pressures in the 80s make the tires noticeably sluggish under my weight.

by Weenie


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