Technical FAQ: Could congealed tire sealant cause wheel wobbles?

Published February 18, 2020 08:24AM

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Dear Lennard,
I have been a tubeless road tire rider for many years and until recently, have never had an issue. However, the time I did have a problem has left me confused and more than just a little concerned. Wheels: Shimano Dura-Ace tubeless; tires: Hutchinson 3 tubeless. I was riding on a slight downslope with about 10 riders behind me, on a busy highway with a wide shoulder, suddenly my front fork started to violently shake, death wobble, and I lost complete control as I was thrown out into traffic, luckily no cars were coming, and I slowly managed to stop. I have been riding and racing for years and have never had this happen. I cannot shake the fear that happened to me that day and I currently descend much, much slower. Perhaps a good thing.

I took the tire off and noticed a large congealed amount of sealant had formed, and I think this might have caused the tire to contribute to start the wobble and then it just got worse. Sound plausible? How come it had not happened before? 24 hours earlier I had done a very high-speed descent down Washington Pass, in the North Cascades, and had no wobble issues. I cannot get that little voice in my head to shut up when I descend now.
— Dan

Dear Dan,
Until you get this resolved, that little voice in your head is probably doing you some good, so don’t be so eager to squelch it. Speed wobbles are often not easy to figure out, as they depend on the entire raft of environmental and mechanical conditions the bike is subject to at the moment.

The congealed sealant puddle in your tire, by throwing off the balance of your wheel, could have played a role in the inception and/or propagation of that speed wobble. As for why it had not happened before, there could be many reasons.

Though unlikely, it’s possible that your valve was left open overnight between the Cascades descent and the scary descent the following day, hardening what had been liquid sealant the day before. Another possibility could be that both tires have similar hardened globs in them, and on the descent 24 hours prior, those were randomly at different points on the rotation relative to each other than on your scary descent. For instance, I can imagine that on your scary descent, if the congealed blobs were in sync so that they were hitting bottom dead center simultaneously, that their effects could have been additive, setting off a wobble that the blobs did not set off when they were not in such a synchronicity. Another possibility is that the road surface and the wind conditions were just right on the scary descent to contribute enough vibration into the system at just the right frequency to set your bike off. I have ridden bikes that had nasty speed wobbles that only occurred under just the right conditions of road surface and wind, and that going the same speed on another road, and/or on another day, did not set off the wobble.

Once a wobble like that gets started, it is hard to decouple it, as you discovered. You’re generally not going to get it to stop simply by hitting the brakes; it will require very substantial slowing, while shaking like mad the whole time, for it to stop. If it happens again, clamping your knees against the top tube may be enough to damp a shimmy right when it gets started and could save you from crashing. That may not be enough, and there may be another way – a way that Rolf Dietrich, designer of Rolf Prima wheels, told me about but I’ve not tried (because I no longer have any bikes that shimmy!). Think of a kid’s telephone made by attaching two tin cans together with a string. The vibrations will only travel down the string when it is taught. Similarly, if the bike is only touching the road with one wheel, it can’t shimmy. While I don’t know many riders skilled enough to yank a shimmying bike into a wheelie, you may be able to stop a shimmy once it’s started by pushing your weight as far back as you can off of the back of the saddle to take as much weight as possible off of the front end. This quite clearly would take a leap of faith, and you would have to choose either it or clamping your knees against the top tube at the moment it happens, because you won’t be able to do both at the same time.
― Lennard

Dear Lennard,
Since 2012, my switch to road tubeless, the Hutchinson Fusion 3 was best. Can we have those back?

I switched from tubed to tubeless for reliability. I ride mid-day, have to work all day, I made it back to the office with tubeless every time on time. The ‘time’ cost has lost favorable balance. I spend non-riding hours fighting to get new-to-me vendors tubeless tireless to fit and seal. Not a quick process, made worse by having to do this every 4 weeks, either for my grinder, MTB and/or road tubeless bikes. Modern tires split sidewalls much easier, wear much sooner, and I do not feel the performance difference compared to years past.
— Mike

Dear Mike,
Because the market is flooded with them, I can only imagine that tire manufacturers thought it would be more profitable to make lighter TLR (tubeless ready) road tires (which require sealant, as they are not airtight) than to make UST (Universal System Tubeless) tires that hold air without sealant, like the Hutchinson Fusion 3.
― Lennard

Dear Lennard,
I recently purchased a Pirelli Cinturato Velo 700 X 26C tire that can be set up tubeless. I read several reviews on this tire before purchasing and the only consistent comment was that they tend to run oversized, i.e. the 700 X 26 is closer to a 700 X 28. I set the tire up on a DT RR440 rim with no problems and checked to see if it was indeed oversized. I measured the tire width at 25.7mm, which seems spot on for a 26.

Fast forward a bit, and I’ve done a few rides and put a little over 100 miles on the tire, when I notice it is starting to rub against the top of the wishbone brake bridge on my Time RXRS. The tire looks noticeably larger, so I measure the width, and it comes in a 27.6mm. The only two reasons I can think of for this is maybe the casing stretched or the bead stretched.

Anyways, being I can’t use it on the rear, I decide to try the front, so I mount it on a Stans Alpha ZTR 340, fill it up to 100psi, when the tire explodes off the rim. (Quick note: I have been running tubeless road almost since they first came out with no issues other than the occasion flat that sealant could not plug. I’ve put over 10,000 miles on this ZTR 340 rim, mostly tubeless, without an issue. However, I’ve always stuck with Hutchinson tires in the past, with the exception of one IRC (it worked fine, also).

The only reason I can think of for the Pirelli exploding off the rim is that the tire bead must have stretched, which I think would also explain the increase in width of the tire. If that is the case, and with the amount of comments on the Pirellis being oversize, it seems this could be an issue with all Pirelli Cinturato tires, maybe all Pirelli tires. In my opinion, this is not a safe tire to run tubeless, I’d appreciate your thoughts, and if you agree do think it would be wise to post a PSA?
— Tom

Dear Tom,
It sounds likely to me like that tire bead stretched in order for the tire width to grow that way. It could also be casing sidewall stretch, I suppose, but that would not tend to lead to a rim blowoff. It could have been the bead stretching enough that it climbed over the rim in one place.

That tire is not listed on the ENVE tire compatibility chart for its hookless road rims (click on “Tire Compatibility”). Just as with a car tire and rim, the bead diameter has to be perfect and non-stretchable in order to reliably seal and stay sealed on a hookless rim. The fact that it has not passed ENVE’s test must mean that either its bead diameter is not in spec, or that ENVE did not test it. Yes, I would be suspicious of this tire, given the two data points you took on tire width, followed by a blowoff.
― Lennard

Dear Lennard,
I just saw this article on Cyclingnews: How to set up tubeless road tyres on non-tubeless ready rims. The author states that it is possible to run tubeless road tires on any non-tubeless-specific rim. This would seem pretty dangerous to me, given the nature of road tires to hold such a small volume of air at such a high pressure. Due to the greater possibility of sidewall or bead failure, I’d never do it. What is your take on this? To me, it seems very irresponsible to publish such an article to a wide readership.
— Charlie

Dear Charlie,
I think this is a poor idea and potentially dangerous—and I have said so before. The required precision of the diameters of the rim bead seat and tire bead as well as of the rim’s bead lock ridge and of the rim valley for successful road tubeless use leads to the obvious conclusion that just because you can get a tubeless road tire to hold air in your garage on any random rim is not reason enough to go out and risk your neck riding on it.
― Lennard