Weight Weenies

I've got the new Stans a340's (24/28 CXrays with alchemy hubs, 2 layers of Stan's yellow tape) and have been using Pro3's with Conti butyl tubes since late spring. I haven't had any problems at all, and actually found them no harder to mount (and perhaps a bit easier) than my Fulcrum 3's. I'm pretty light (59.5kg) and have been inflating them to only 85psi front and 90 rear, and they have been great.

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WMW wrote:

DJT21 wrote:Interesting stuff. I'd never considered that some tyres may not work as well as others.

What appears to be happening is that the tire bead stretches and maybe tears internally, leading to it eventually being loose enough to pop off the rim. Heat from long descents and braking might be particularly taxing on the beads... but so far I've never heard of anyone having the tire blow off *during* a descent. It always happens randomly... many days after mounting, usually. Of course different tires have different beads, so they will respond differently. The 340s seem to be less tolerant than most other rims. They stretch the tire bead more and rely more on this tension to keep the tire on.

Also, some people have a poor mounting technique and get the tube pinched, resulting in the tire blowing off... which is a completely different issue and isn't particular to these rims.

It makes sense. The tyre popped off the rim after a ride that involved a very steep descent, and I was hung over so I was braking lots! About 4 hours later, the tyre popped off and the tube exploded.

When I removed my tyres last night, the front one need levers to remove (it is relatively new) but that rear came off easily (quite a worn one)

Anyway, rode earlier on the pro race 3s with no issues!

I've been using Conti GP4000s for 1.5 years, original lighter rims, red veloplugs. No problems at all. But I never use over 6.5 bar, sometimes <6. The shelf on the rim under the bead hook can make it easy to catch the tube and not be all that obvious and I think that may be a factor.

mises wrote:The shelf on the rim under the bead hook can make it easy to catch the tube and not be all that obvious and I think that may be a factor.

Maybe... but that should blow off the tire pretty quickly... not after a month of riding.

I got my ultremo zx and did the same: gave them 8 bar and no pop off. I noticed that the diameter on the zx also seamed smaller than the durano s and a bit tighter. I have som veloflex master 22 that I tried to put on they are so hard to put on these wheels that I will not use in case I get a flat but they seamed to be sitting very well on the rim

WMW wrote:What appears to be happening is that the tire bead stretches and maybe tears internally, leading to it eventually being loose enough to pop off the rim. Heat from long descents and braking might be particularly taxing on the beads... but so far I've never heard of anyone having the tire blow off *during* a descent. It always happens randomly... many days after mounting, usually. Of course different tires have different beads, so they will respond differently. The 340s seem to be less tolerant than most other rims. They stretch the tire bead more and rely more on this tension to keep the tire on.

I'm no so sure about your theory about the tire bead stretching and tearing. Not just because Kevlar is very strong fiber, but also because tire retention does not depend on the circumferential strength of the bead - it relies on the strength of the 'clinch' between the bead and hooked rim sidewall. Even if the bead is completely broken, the clinch will keep the tire on the rim.

How do we know this? Two reasons. Firstly, here's an experiment that you can try yourself, involving purposely cutting a tire bead and installing the tire:

http://sheldonbrown.com/rinard/tirebead.htm

I've repeated this test myself, only I cut each tire bead in 8 equally spaced locations. After mounting the tire, I inflated it to 140 psi and let it is sit overnight. The next morning, the tire was still on the rim. I then lowered the pressure to a normal riding pressure (100 psi) and went for a 30 mile, with several hard corners and quick stops. The tire stayed on the rim perfectly fine, even though the bead was severed in many places and had zero circumference strength.

The other reason we know that tire bead strength isn't required is from the observation of changes in spoke tension when inflating a tire. As many have reported, spoke tension decreases with tire pressure. This is because the tire is is squeezing the rim circumferentially, compressing the rim and relieving spoke tension. Bead tensile strength would be important if the tire pulled outward under pressure, but instead the pressure causes the tire to squeeze inward, making the circumferential tensile strength of the bead superfluous.

(So, one might be wondering how a tire can blow off a rim if pneumatic pressure causes it squeeze onto the rim harder. When a tire blows off a rim, it is not because the tire is pulled out off the rim radially, it is because the tension in the tire casing pulling in the lateral direction causes a portion of the bead to be slid laterally out of the rim. In other words, the pneumatic pressure causes the tire to want to become fatter, pulling the tire beads apart. Well, it would, except that the clinch between the bead and the rim sidewall locks the bead in place, preventing it from being pulled out.)

As for the Alpha 340 rim, I'm wondering if the problem isn't the internal geometry of the rim prevents the tire bead from seating under the hook properly, providing an insufficient 'clinch' to keep the tire on. Maybe the shape of the bead area of certain tires doesn't fit well with the Alpha 340's internal sidewall/hook geometry, making these tires more likely to blow off the rim.

I'm having the same problem with my rims using vittoria diamanté pro light. Got my LBS to fit then first of all and both tyres popped off an the tubes blew. They eventually fitted them and I got 1000 miles out of then and punctured one. Tried to fit a new tyre and tube but the same thing happened to me. Took it back to the shop and they have said they've fitted hundreds of conti's with no problems on these rims so probably my choice of tyre.

They contacted stans after the first time and they did suggest using 2 layers of tape which seemed to work first time but they might try 3 and see if it helps now.

Conti GP4000's seem to work fine for me. Mine are 24mm. Never inflate above 100 PSI. Some have had them blow off the rims randomly. Hutchinson tubeless tires seem to be pretty secure for people. See this thread:
viewtopic.php?f=3&t=108048

My PR3's have been on since Dec '11, and have been fine.

http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=97281

duplicate

MarkMcM wrote:As for the Alpha 340 rim, I'm wondering if the problem isn't the internal geometry of the rim prevents the tire bead from seating under the hook properly, providing an insufficient 'clinch' to keep the tire on. Maybe the shape of the bead area of certain tires doesn't fit well with the Alpha 340's internal sidewall/hook geometry, making these tires more likely to blow off the rim.

Long time replying to this...

That is likely the issue. The Stan's rims *don't* have a hook bead. There is a tiny ridge that sticks out at the top-edge of the rim, but Stan has told me that bead tension on the rim shelf rather than that ridge is responsible for keeping the tire on.

Kevlar beads definitely stretch. I don't know what the mechanism is.

The bead does make a difference. The Hutchinson tubeless have a carbon bead that doesn not stretch and is uber tight over a 340 rim. It snaps into place. I don't understand why people buy tubeless rims and use regular clinchers, especially ones that are loose on the rim. Just get the right tires and it's bomb proof, and duh, no tubes.

I have the same wheelset as the OP: Tune 70/170 with Stans 340 rims. I've used 23mm Vittoria Open Corsa CX EVO @ 115 psi for 18 months with no blow-offs, no problems. (Continental Easy Tape and Race Light tube.) Emailed Stans to ask about replacement tires and they specifically recommended to avoid Continentals.

tinozee wrote:The bead does make a difference. The Hutchinson tubeless have a carbon bead that doesn not stretch and is uber tight over a 340 rim. It snaps into place. I don't understand why people buy tubeless rims and use regular clinchers, especially ones that are loose on the rim. Just get the right tires and it's bomb proof, and duh, no tubes.

For myself, the tubed vs tubeless decision was being made 2 years ago when things were a little different. I read everything I could find about the pros and cons and wasn't convinced to switch. There was almost no choice in tubeless tires, they were heavy, they were expensive, and there were the stories about mounting problems, spoke detensioning problems, and problems on the road if there was a serious puncture. The Stans rims were being pitched as being suitable for both tubed and tubeless, and mine were bought for the weight advantage. I have never seen any warning NOT to use conventional tubed tires on them. Plus, I had the tires already in orange, whereas the tubeless were all black.

Two years further on and the tubeless picture has improved. Perhaps when I wear out my no longer available, orange PR3's I will give tubeless a shot.

Fourthbook wrote:I've used 23mm Vittoria Open Corsa CX EVO @ 115 psi for 18 months with no blow-offs, no problems.

I had one of those blow off.

Yes, a greater number of GP4000 have blown off apparently, but I think that is just because so many people ride them.

Note that the great majority of riders never have a problem with any tire. I had one GP4000 blow-off and a bunch of others not. I don't think any kevlar bead tire is immune, but some may work better than others.

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