My new Nimbus 29" from UDC USA is exploding tires?

Hey guys, as per my last thread asking for recommendations, I ordered a blue 29" Nimbus from UDC USA with 125/150 KH cranks and it only took 7 days for it to get to the west coast. I spent the whole week stressing, since this is the most expensive “toy” I have purchased yet and the anxiety from the wait for it was excruciating! I was even LOSING SLEEP. Clearly, I am crazy.

But! (And I warn, I write a lot, but I figure it’s best to describe exactly what happened so I can get a better opinion…)

I got the uni today and it’s pretty. I put it together at work and then used my trusty footpump (which I’ve used on my car tires, bike tires, friend’s car’s tires, my other unicycle) to pump it up to the MAX that the tire indicated: 65 PSI. It took a lot of work with the foot pump but I got it there and tried to mount it in my office… which was tough and after two attempts I decided I’d just wait until I got off work so I could practice it on my street, so I lean the uni against the wall and sit down at my desk and about ten minutes later


Dust blew all around the office and the explosion was so loud that my co-worker was complaining about inner ear pain for two hours after, I got a headache, and the guys in the warehouse came running in expecting to find me standing over my co-worker with a discharged shotgun.

About 1/4th of the tube was split open and it had knocked the tread off the rim. Here’s a picture:

At this point I assumed a number of things: perhaps the pressure gauge on my pump was wrong, maybe the tire was faulty, or… well that’s really it.

So I called a place near my house that said they had 29" tubes so I went by and they checked out my new Nimbus, felt the walls of the tread, and examined the rim. They commented that the set of four holes near the valve stem hole could perhaps be a cause of the tire exploding like that, but after observing that the four tiny (and sharp!) holes are pretty much covered by the tread, that it was probably my fault for over-inflating the tire which I was willing to except.

Since they only had the basic screw valve tubes (whatever those are called), the guy very nicely called four other stores he was aware of and I ended up driving 20 minutes to La Habra to pay $7 for a new tube, which was obviously thinner than the other one but I figured as long as I used a quality pump I probably wouldn’t be in any danger of popping this one.

I get home and struggle through my first attempt ever at putting a tube on a bicycle tire. I get it on just fine, and I get the tread seated just fine–as far as I can tell I did nothing wrong and I start to inflate the tire.

When I get to about 42 PSI (the tire says min 45 max 65) there’s another POP!!! and, sure enough, this tube exploded just the same. At this point I just sat down on a curb: 6 hours since receiving my $400 unicycle, I’ve exploded two tubes, driven a good 30 miles, poured a bunch of sweat in this CA heat, and still have yet to ride it.

I feel really defeated.

Do you guys have any perspective to share with me? Is there something I could be doing wrong? When I got the uni, I just inflated it, I didn’t check it at all. As I said, when I got the second tube, I put it on just fine and got the tire into place just fine… why’d it explode at 40 PSI? :thinking:

Should I take the second tube back to where I got it and see what they say, to get a second opinion?

I emailed UDC USA after the first tube popped just to see what they say, and I figure now I will call them on Monday and see how their support works out. I, somewhat stupidly assuming there couldn’t be something wrong with the uni itself, already got rid of the shipping box (for the boys in the warehouse to use).

In short: Le sigh, I guess I’ll go ride my 20" CX around while fantasizing about my KH Moment cranks… :frowning:

The first blowout could have been due to a number of things including:

First, the tube may have been pinched when the tire was installed. It didn’t blow off the rim with low pressure, but when you brought it up to pressure… Boom!

Second, there could be a problem with the tire. Sometimes bead problems aren’t evident until a tire blows off the rim, and even then you can’t tell that there is anything wrong with the tire. If there is a sidewall weakness you will know because you will have a blowout through the sidewall. These blowouts are usually small, and look like a bunch of radial cuts going from a common point.

Third, a blowout from a spoke, or the rim on the inside. This is not likely, and if you get one of these it’s usually of the small variety listed above (still like a gunshot though).

At any rate, the fact that you got a second blowout with a new tube suggests that your tire is no longer going to seat properly. In my oppinion it is most probable that it wasn’t installed correctly to begin with, and that UDC should send you a replacement tire, and tube (maybe a couple of tubes). It’s unfortunate, but even one blowout can render a tire useless.

have you filed/sanded these down, and looked for any others? use your fingers, not just your eyes. the 45-65psi is the pressure for the tyre not the tube, although at those low pressures it should not have made a difference…
good luck!

stop inflating it to the max?

your tire should be at 1lb air per 10lb body weight + 10lb in end.

that works extremely well for me.

but the sharp things in it, i’d say that’d be the first culprit, but i also know that if your tube has a defect or something in the seem (like one of mine did, i luckily have my hearing still), inflating it to a lot does not equal good.

On a road going uni you definately want more than 1lb/10lbs of body weight. I run my Big Apple at 65psi (the max psi), and it works better than when I run it lower. When I ride trails on this same tire I run it a 35-40psi.

When you say that the sharp holes are covered by the tread are you talking about the rim strip? Or are the holes on the side of the rim at the bead, and covered by the tire? Either way unless they are in contact with the tube they shouldn’t cause a blowout.

Tire too Small?

I had a similar problem with my KH29 because I tried to fit a tire that was too small so it blew off the rim. It would have been ok according to others as long as the tire was perfectly seated.

The “issue” tire was a Kenda Klaw 29 x 2.1
The exploding tire a 38-622, I don’t recall the make
Now I have a Schwalbe Marathon tire 47-622 = 28 x 1.75. It provides a smooth ride and no explosions.

Your tire might be a poor choice for the rim.

1/10 + 10.

wait… frick, this doesnt add up to what i thought it did,.

well, in actuality, i run my trials stuff at about 28-32 psi, i weigh 135. so its more like 1/10 + 20.

I second this - with the same tire, I run 65 for street (not street street, but urban riding), 30s for offroad.

Are they not shipping the nimbus 29 with the Big Apple anymore?

If I’d had two blowouts, particularly one as low as 40psi, I’d be looking at the tire and the rim for defects.

That’s completely wrong for a road tire. On roads, you want the tire to be well-inflated; my Big Apple performs poorly under 50psi. This isn’t a trials uni.

Definitely make sure the rim strip covers all the spoke holes; that’s a common cause of a blowout. And make sure the tube is installed correctly; it’s possible that it was installed incorrectly by UDC (twisted or pinched), which will result in a blowout.

Once you’ve had a big blowout on a high-volume tire, often the tire bead itself gets deformed and it will no longer properly hold the rim; it’ll just keep blowing off in the same place.

Replace the rim liner with a cloth one, this will keep the spoke heads/holes from popping your tube. The tire is probably fine, some tires, esp larger ones, can be loose on the rim if the rim is narrower as it probably is if you bought the Nimbus 29er. A small hole, esp one in a lightweight tube, will cause the tube to self-destruct in a big way when at high pressure. 50 PSI doesn’t sound like much at first, but keep in mind that car tires typically go to 32 PSI.

I remember getting a really nice 29er MTB, kept having tube blow outs, couldn’t find the culprit until I’d repaired one of the tubes a couple times, then I noticed a linear cut, very fine, that followed the circumference of the rim. So then I look at the rim and examine the plastic rim liner. It was sharp, but certainly not enough top cut the tubes; or so I thought. I replaced the rim liner with cloth, no flats since.

If the tire has a wire bead, then it’s probably fine. You can check the tire by running your hand along the bead, looking for irregularities. Think of the pressure riding puts on a tire, a tube blowout when pumping it up is not likely to cause a tire defect.

BTW, don’t run a presta valve tube in a shrader rim, it’ll rub a hole at the base of the valve.

These symptoms could be caused by:

Absence of rim tape, or rim tape incorrectly seated. The rim tape is the strip of cloth or rubber which lines the inside of the rim, and covers the ends of the spokes.

One or more rogue spokes - not filed off short enough, and poking through the rim tape. Unlikely.

The tube being installed incorrectly. This is quite likely.

A foreign body loose inside the tyre - possibly a bit of swarf (sharp metal from when the rim was drilled or the spokes were filed down).

A sharp object embedded in the carcass of the tyre. Check with fingers (carefully) not by eye, flexing the tyre as you do so. Run your fingers all the way round the inside of the tyre in both directions (in case the sharp object is at an angle).

Faulty tubes: unlikely as it’s happened twice.

It is very unlikely to be caused by the pressure.

To fit a tube:

Make sure the rim tape is properly positioned.

Makes sure the inside of the tyre is clean.

Put one side of the tyre on the rim.

Slip the valve inside the tyre and through the hole int he rim.

If there is a threaded ring around the valve stem, tighten it up, but not very tight.

Working away from the valve in each direction, carefully put the tube in, making sure that it is not twisted or folded.

Loosen the little ring on the valve stem, and then put the second bead of the tyre onto the rim, starting at this point. Do the ring up loosely again.

Working both ways round the tyre, starting at the valve, push the bead onto the rim.

Take care not to trap or twist the tube.

You may need levers (or your best dessert spoons) to put the last couple of inches of bead onto the rim.

Now, work around the rim all the way, squeezing the tyre between finger and thumb to make sure that it is properly seated.

Pump the tyre up to about 10 psi.

Undo the little ring on the valve and wiggle it and make sure that the base of the valve isn’t trapped between the rim and the beads.

Let the air out again.

Now pump it up to operating pressure, watching carefully in case any suspicious bulges appear.

Finally, the little ring on the valve should not be done up very tightly.

Nimbus 29er tube blowout issues

Amiantos, Same experience. The issue is likely that UDC has likely supplied you with a 700c tube, which is a 29 diameter. It’ll fit, but the tube circumfrence is too narrow to inflate the tire bead and seat to the single walled rim properly. By being a single wall rim, the rims they are using are slightly narrower dia., and smaller. So it seems to take a lot of pressure to seat the tire right,65 psi like you experienced. And the lower 45 psi, will keep you from popping tubes, but cornering while riding creates some fold over in the tire. Tell UDC to make sure the new tube you get is a 29 x 2.35, the same one’s they supply for the KH 29er. Just a theory, but if a lower grade tube is buried in a Nimbus 29er package deal, it’s a way to cut costs.

Hope this helps:)

I agree with most of Mike’s advices

I agree you can find sharp stuff poking through the tire that you cannot see by eye, by rubbing your hand around the inside of the tire. In a tire mounting sense, this is to be applauded.:slight_smile:

However, I once cut my hand on a piece of glass, and have heard of worse stories. :astonished: In general, it is best to wear gloves, and use a soft rag, to swipe the inside of the tire looking for it to snag on some sharp piece of dirty garbage. Don’t use a bare hand !:slight_smile:

Thanks to everyone who has replied. From all your advive and instructions I can safely say I’ve done everything suggested, even before the second blowout. I’m going to take pictures of said holes, and recheck the tire. On Monday I will call UDC and see what they say.

The first tube I can’t identify but my second one that exploded at 40 was a Specialized schraeder. Marked 29 x 1.9/2.3. You think I should I should call my local bike shop and tell them to order me some heavy duty 29x2.35 specific tubes?

I have had tubes explode on me from standing in the sunlight at a time when I used very high pressure. That never gave me a headace. Perhaps you should check the pressure gauge on your pump? Perhaps you did pump it up to high with out knowing…

Heavy duty tubes are more resistant to punctures, but you don’t need heavy duty tubes to run at high pressures. Road racing cyclists use very thin light tubes at pressures as high as 130 psi or even more. I run a lightweight tube in my 700c at this sort of pressure.

The tube is just a bag for the air. If the tube and tyre are the correct size, then the inner wall of the tyre will stop the tube expanding beyound its design limits. A tube doesn’t burst because it is under too much pressure, but because the pressure has caused it to stretch beyond the size it should be.

The first time it happened I was willing to accept that my pump’s pressure gauge was perhaps incorrect… but the second time I used a different, much higher quality pump with a gauge I know for sure is accurate, and it exploded at 40… so, I dunno.

Sorry unibikeling, I have to come back and make an example of your suggestions. Bad math, applied to a totally different sized wheel for a different type of riding does not a “tire pressure formula” make. The correct formula remains pretty non-specific:


  • Enough air to keep your rim from hitting the ground
  • High pressure for road riding, or general riding on pavement
  • A little less pressure for riding indoors
  • Less than that for beginner riders
  • And a little less than that for beginners riding indoors
  • Then adjust from there based on tire width [/LIST] Sound complicated? It isn't really, though it might sound like a lot if you're new to unicycling. What works for a 19" Trials wheel is probably not what you need for a 29" road wheel. Skinny tires need more air than fat tires. Heavy riders may need more air than light riders. So if your road wheel is skinny, you need to run pretty high pressure. If it's fat (like yours is) you can use less, but this is more a matter of taste. I still ran my Big Apple pretty hard for road riding. But on my Coker I found that high pressure made the tire very sensitive to camber, so I ran it a little lower. This is not something I've noticed on a 29" wheel, except with a Schlumpf hub in high gear.

    Based on your description and photo, your original tube probably blew due to a problem in seating. Maybe the tire was installed with a little bit of the tube under it. Not noticeable at the lower pressure it may have been shipped at, but adding more pressure caused the tube to fight its way under the tire and pop. Or, it could have been something sharp in there, but the huge tear in the tube suggests to me that it got under the tire and made a giant bubble before popping, while nobody was noticing.

    That’s happened to me. In my case, the problem had a few causes. First my rim was too narrow. I’ve since put on a much wider one. Second, I took my 29er, with skinny rim, on the Downieville Downhill. Oops. Bumping down all those irregular rocks caused the tire to deform to one side, then the other, and eventually enough to let some tube come out. I stopped to see a large cucumber-sized blob of tube next to my tire. Before I could reach the valve to let the air out, BOOM!

    Anyway, those kinds of blowouts, as described above, can deform the bead on the tire. If it’s deformed too much, it ruins the tire and it won’t stay on the rim anymore. That’s happened to me once or twice, and I only think it happens when the tube finds its way out under the tire somehow.

    So your second tube may have blown due to an already-damaged bead. But it’s hard to be sure, because you’re not an experienced tire-mounter. If you just pop a tire on and then pump it straight up to high pressue, it can cover up what may be a kinked tube, tube pinched under the tire, or badly-seated tire. So follow Mikefule’s instructions above.

    When I’m pumping up a tire from scratch, I usually start by putting in just enough air for it to take shape, and then bounce the tire gently on the ground while slowly rotating it, so it gets bounces all the way around. You can give the wheel a little spin to see if the tire is staying even with the rim as it goes around. Don’t look at the paint (a painted-on stripe can be misleading), look at any lines molded into the rubber. If it goes up and down as the wheel rotates, it needs some more bouncing or other prodding by hand. Always get the tire straight and well-seated before pressuring up the tire.

    Also don’t be too trusting of tire pressure valves on pumps. Some are fine, but generally they are very inaccurate, especially if the pump was made for automotive or other use (most foot pumps), rather than for bike tires, which usually go to higher pressures. With experience you’ll learn how much pressure you want and be able to just pinch the tire to know if you’re in the ballpark. Until then, it can’t hurt to invest in a standalone pressure gauge.

    Also, if you end up buying a new tube yourself, get two. Having a spare at home can be very handy when the bike shops are closed! But talk to UDC first. If your unicycle came with air already in the tire, you should assume it’s been assembled and seated correctly. You should have been able to pressure it up from there without problems.

  • Softer is always better

    Right up until John’s ^ #1 rule about not bottoming the rim.

    Soft tires have better traction, and don’t kick back from stones and roots like a harder tire does.

    Riders with the talent to do big moves, and hard drops, are forced to use higher tire pressure to avoid rule #1 about how getting pinch flats is a waste of time. That is a cross they must bear to avoid pinch flats from bottoming a to soft tire.

    However I, as a kinda not big drop rider, can run 25 psi on my ax29 and that works great for me. Maybe my gauge that says 25 psi is off by 10 % from yours. I like to stand on the uni holding a wall and twist from side to side to see if the tire is folding. I always run it as soft as I can.

    Sadly,:(, it seems that the better I get at uni riding,for the 19 and muni, the lower I have to set my seat and the higher tire pressure I must run to avoid rim bottoming.

    Soft tires are awesome, a 24x3 muni with a 20ish psi soft tire can run over a 2" root or sidewalk crack and you won’t feel a thing.

    Not sure what you mean there but I think I get it. You’re saying hardcore riders who do big drops & big moves have to balance super-low pressure with risk of pinch flats? This is true. They are also not the intended audience for that very basic set of tire pressure guidelines. When you get to that level, you’ve probably already popped a tire or two, and have some idea where you have to deviate, just as you’ve deviated from normal riding.

    So modify my list above to say “does not apply to big drops!”

    This is true, and I always find the correct pressure for a fat tire to be a compromise. I love how the soft tire rolls over stuff, but then when I get to the sections with the square-edged rocks, if I’m running it too low I take a risk (as I’ve learned more than once). So in general, the keeping-the-rim-off-the-ground rule still holds, and is the most fundamental one on the list.