Turtle Speed

I'm having problems with repeated flats wheel with an aero rim and a 700x38c tire. The tire is rated for 50-85 PSI. I can ride the bike at low pressure (40-50 PSI), but every time I try to go to 85 PSI it ends in disaster, because the tube goes flat overnight.

I keep getting repeated holes in the belly of the tube at around 3/4" away from the valve stem, like this:







The rim tape seems smooth to the touch around the area where I get those rips, and I don't notice any rough or spiky bits there.

Am I not installing the valve stem straight enough? Is the tube just failing at higher pressure? I'm so frustrated with the constant flats, and I hate riding at low pressure. Help a brotha out? Please?

mrrabbit 

Does that valve stem barely fit through that hole - or does it seem like the hole is too big allowing for a lot of wiggle and possible bulge-in by the tube?

=8-)

Also what's the inside edge to inside edge width of the rim, 14mm,15mm, 16mm, 17mm, 18mm, 20mm?

=8-)

FBinNY 

Your flat is caused by the tube ballooning down into below the constricted area at the bead into the well of the rim. It happens at the valve, or more precisely near the edge of the reinforced area.

If you look at mounted tire in cross section you'll see that the space necks down at the bead, then there's the area in the rim's well beyond that. On a narrow rim than necked-down width is very narrow, and if the valve cannot pull down enough, it keeps the base above (below in picture) allowing the inflated tube not only to expand down into the well, but to expand back to fill the gap at the valve. That's more than the rubber can tolerate so that little side bubble bursts.

The solution depends on the details. If possible, pull or wiggle the valve until it's out of the rim as far as it can go without breaking, as a last stem after mounting and seating the tire. If that doesn't work, because the rubber cone on the tube is too wide, consider using a tapered reamer to slightly enlarge the valve hole. (deburr the sharp edges when done)

Another option is to consider changing to Presta valves whose, narrower stems are more suited to narrow rims, and can slip between the beads more easily. That opens a few other issues, so if you opt to go that route, search PV in SV rim on this forum for more info (you might have to write out Presta and Schrader), but you'll cross that bridge if/when you come to it.

Turtle Speed

The stem barely fits through the hole. It's very snug. I have to push down and wiggle around to get it through.


I can't measure right now, but my old el cheapo rubbery liner that I replaced said 15mm. I'm not sure I trust that, though. The Zefas rim tape I put on is 13mm, and it even climbs the sides a little bit. Prior to that I had a 16mm rim strip and it completely prevented the tire beats from sitting in place properly.


I'm still trying to digest FBinNY's post, not sure I understand the why's yet.

mrrabbit 

Here I'll help:

1. Both FB and I are suggesting that you are running way too large of a tube and tire in a narrow rim. Perhaps you need to bring it down to 700 x 28c?
2. FB is suggesting going presta...narrows the base of the tube a little stem construction wise so the stem base is flush against the rim - not sitting on the side ramps.

...as he pointed out - it the stem is sitting high - the tube before and after can bulge into the gap under and BAM - blown tube.

Too make a long story short:

It's a compatibility thing - narrow rim + large tube + large tire - it's not working out as-is.

=8-)

mrrabbit 

https://www.sheldonbrown.com/tire-sizing.html

Bottom of this link has a conservative "safe" rim size versus tire size matrix that you might want to peruse when considering changes to make.

=8-)

Just Riding

FB is correct; your tube is not seating in the well of the rim at the valve. The tube stretches, and then splits at the edge of the valve reinforcement.

Many single wall rims have narrow wells that are only a couple of millimeters wider that the Schrader valve itself. The thick reinforcement around the valve sometimes has barely enough room to squeeze down into the well. Add a bit of rim tape on the wall of the rim well, and the valve is not going to be able to seat down where it should. The valve stem is higher in the rim than it should be, the tube stretches down, and ....pop goes the tube.

Cloth rim tape is punched for Presta valves. I take an exacto knife and cut it back at the valve hole to make room for Schrader valves. On rims with narrow wells, I'll also cut it away so that is is not on the walls of the well. This gives more room for the valve to seat.

Turtle Speed

That's what always had me scratching my head about this bike - it always seemed like it came with very slim rims (15mm? Less?) and unusually wide tires (38c) compared to what others had on their 700c wheels. Granted, it's a hybrid bike, but I always wondered why there was such a huge difference in those sizes. Stock, it comes way beyond the Sheldon Brown suggested guidelines.

I was thinking of replacing the tires at some point anyway, because I plan on this being a commuter bike. If I put some Marathon Pluses at, say, 28c and make a switch to Prestas with some sort of grommet, should I be OK?

Also am I understanding the problem right? Here's my side-view drawing...



Thank you so much for your replies, by the way. Here's hoping I can patch that hole in my tube in the meantime, but I doubt I can pull it off. I never seem to have much luck with patching those types of "stress rips", only punctures.

mrrabbit 

If I've interpreted FB correctly, you got the idea. I'm not someone who changes tires/tubes or troubleshoots them on a daily basis - so besides the chart I linked to for you - pay more attention to the others.

=8-)

FBinNY 

Please don't put words in my mouth, especially when I didn't say them.

I said the OPs problem was related to the base of the valve being too high, and the tube blowing back under the reinforcement. Rim width and valve hole size are factors, but at no time did I suggest that the tire width was.


To the OP, the key is pulling the valve all the way out. Before even mounting the tire try pushing or wiggling it through until the base touches the rim. If the hole in the rim tape is too small, enlarge that, if it's the valve hole itself, ream it a bit bigger (just enough and no more) or let a bike shop do it if you don't have the tool.

Once you address the valve problem, you can stay with your tires if you wish, but I suggest buying the largest tube that fits the tire, so it stretches less, and has more reserve for the added stretch at the valve area.

There may be reasons to go with smaller tires, but that's a separate issue entirely.

FBinNY 

Yes, the tire width above the constriction of the bead is NOT a factor. What happens above that point has little or nothing with what happens below that point.

People concur or contradict, or elaborate on my posts all the time, and I never have a problem with it. But they don't directly put words I didn't say into my mouth. Also, you've had a recent pattern of badgering and belittling me, so special rules apply. As you know, I'd earlier PM'd you a request for mutual courtesy, and you basically told me where to go. So that's where I am.

rekmeyata

Can't we all just stop bickering at each other? I for one get so tired of it. Maybe it's fun for some of you. Regardless, bickering doesn't help anyone.

On to the rim+tube=flat problem. I think you need to go to a presta valve tube too (say those last 4 words 10 times as fast as you can). But before you do you need to buy a couple of very expensive things so the presta valve will fit snug enough in the schrader holed rim; please see: https://www.amazon.com/Schrader-Prest.../dp/B000XNZU1S I can hear you screaming now, yes, I know these things are expensive, but it's the best and most assured way of getting the presta valve to fit in the Schrader opening without causing problems.

Turtle Speed

Appreciate all the responses, will examine them more thoroughly when I can.

BTW, I measured really fast and got about 14 mm inner rim diameter, so that is slim indeed.

Thanks for the tip. Do you just shove those through the holes? They naturally just sit snugly into place and there's no chance of them falling out?

CbadRider

+1,000

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acrowder

what FBinNY said

acrowder

Wow, I should have scrolled further before posting that reply lol.

FBinNY 

Duly noted,

rekmeyata

You push them in from the underside (tire side) and they will snap in. Easy man.

dddd

While this problem clearly is caused by the valve reinforcement region of the tube not falling readily into the rim cavity, I think that there are many variables as to why.

Firstly, the width of the tire being much wider than the inside width of the rim. I seldom see this problem with narrow tires on narrow rims, even when Shraeder valve tubes are used.
The problem then is that the tube inflates with tension inside of the tire, followed by the tube then expanding into the rim cavity.
This causes the tube to be dragged around the inside corner of the rim cavity, which creates high frictional forces from the combined contact pressure and the sharp angle that the tube is being dragged past. Note that the inside corner angle is much sharper when a wider tire is used on a narrower rim.
Obviously, the localized stiffness of the tube reinforcement patch around the valve stem creates even more contact pressure and greater difficulty with this part of the tube dropping into the rim cavity from pressure alone, especially as the reinforced area is both reluctant to bend and reluctant to stretch. Thus, the adjacent, normal-thickness part of the tube simply ruptures as it is pushed toward the rim floor and pulled away from the stiff, elevated reinforced area.

While several of the above recommendations of others are pertinent, such as clearing the valve hole, tugging on the valve stem and using talc powder as a lubricant, I also always have seen this situation improve with the use of a larger inner tube, or, quoting FB: "... I suggest buying the largest tube that fits the tire, so it stretches less, and has more reserve for the added stretch at the valve area....).

FWIW, I usually tug on the valve stem before inflation, and once during inflation (at ~20psi), when airing up wide tires on narrow rims.

rekmeyata

The larger tube thing is BS, all tubes are designed to stretch and stretch a lot more then the size of the tire it's contained in. All you need to do is take a tube and start pumping air into outside the tire and you'll see it can grow to at least 4 times larger then the tire! In fact using a larger tube can cause the tube to bunch up inside resulting from failure due to the bunching up the tube getting creased once PSI in introduced.

It's actually better to use a tube one size LESS then the tire size because it eliminates the bunching effect, it allows the tube to get out of the way easier when mounting thus less of a chance for the tube to get caught between the bead and the rim, makes it easier to install tires, and weighs less.

I've used nothing but smaller tubes for 40 years, and have known a lot of people personally, and heard of people here on this forum that do the same thing with no ill effects.

dddd

You need to stop calling everybody else's ideas BS. Just stop.

rekmeyata

Stop? it is what is, and I call it what it is. Besides I only said BS to the one idea you had, not all of the ideas because everything else was not BS like the Talic powder use...not BS, great idea.

dprayvd

This type of puncture I've done often enough that I now do this:
...No rimtape over the valvehole,
.....Chamfer & polish said hole,
.......Place a reinforcing "dickie" fashioned from a bit of tube snugly onto the
.......valvestem base.


It's worked mostly, since.

cny-bikeman

It appears you are not taking your own advice....

Nothing wrong with disagreeing with another's idea or opinion, but there's no good reason to add an insult - just present your rebuttal and be done with it. Defending the insult just adds to the problem. We don't need more Rabbit vs. NY feuds.

Turtle Speed

Thanks for the awesome replies, everybody! You've given me some great ideas on trying to get this solved. High pressure, here I come. No longer will I be trying to dodge this problem by riding on 30 PSI and fearfully pinching the tire every time I come back to the bike to check if it went flat again.