# Why did both of my inner tubes blow out on a new bike I've only put 4 hours on?

• If you had the bike inside the car on sunny, 95 degree days, the pressure in the tires very likely exceeded specs. In addition, the uneven heating of the tires would tend to cause them to bulge and possibly pull off of the rims. May 17, 2018 at 1:28
• thanks for the response. i assumed that is what happened. im just new to biking and cant afford to blow out tubes constantly due to ignorance May 17, 2018 at 2:00
• I would replace them with larger tubes, as I have had the same sort of experience. Letting some air out when storing or leaving in a hot car is a good idea. bicycles.stackexchange.com/a/54721/37531 May 17, 2018 at 2:24
• Shop around for better tube pricing - There are online suppliers for 1/3 the cost. And perhaps, not using the car to store the bike? Or put the car in the garage ?
– Criggie
May 17, 2018 at 3:10
• Very likely just leaving the car windows open an inch or so would suffice to prevent this in the future. May 17, 2018 at 11:32

I have had a couple of times that an inner tube blew up while the bike was parked in bright sunlight. It seems that high temperatures are a contributing factor, but overpressure? I never pump my tires close to the maximum pressure, mostly the pressure is in the lower half of the min-max range.

The pressure of compressed air increases proportionally to the absolute temperature, i.e. 273+T ; T in degrees centigrade. Hence an increase from 95F (35°C) to, say, 140F (60°C) will result in an increase by only 8%. I don't believe this itself is causing the blow outs.

More likely the blow outs are due to an undersized inner tube in combination with a high temperature. Rubber is a material that behaves elastically in a limited range. Beyond a point it starts a plastic deformation. I guess that this point drops with higher temperatures. Most inner tubes are specified for a certain width range, e.g. 18<-->23C for road bike clinchers. The remedy is to buy an inner tube that matches closely with the tire width.

• this was a brand new bike. do manufacturers like diamondback usually put undersized tubes in the bikes they sell? May 18, 2018 at 20:57
• You wouldn't suppose so, but they might underestimate the extraordinary circumstances. A companion of mine had an Idworx bike, a brand known for its extreme care to details. He had already used this bike for about a thousand kms, when we started on the Great Divide route in Tucson, Arizona in about 100F. One inner tube popped the same day without any external cause. The other tube blew up a few days later. His spare tubes were a larger size and they survived the whole 2700 miles, with the usual punctures. May 19, 2018 at 12:47

Tires blowing up in hot cars is a common occurrence, especially for high pressure road tires which are often already inflated to the specified maximum pressure. Heat expands the air inside the inner tube, as the pressure builds eventually something gives resulting in a rupture like you experienced. (Typically the tire bead comes off the rim, the inner tube escapes out, and without the reinforcement of the tire casing it expands until it ruptures.)

Pro tip: if you are storing a bike in a hot car, reduce the pressure in the tires. Carry a floor pump to reinflate the tires before you ride.

I'll take the contrary opinion that high temperatures won't increase tire pressure enough to blow the tire.

Using the Ideal Gas Law PV=nRT and solving for pressure: P=nRT/V. Since n, R, and V are constant, if we increase T by some factor, the pressure increases by that same factor. (ok, the volume may increase a bit as the tire heats up, but not a lot)

So if the tire starts at 60 degrees F (288K) and it increases in temperature to 120F (322K), that's a factor of 322/288=1.12.

So if the starting pressure is 70PSI, it will end up at 78PSI. Which shouldn't be enough to make it blow out since there's a margin of safety in the tire pressure ratings. Even if the sun was beating down on the tire directly and raised it to 180F, that'd still only lead to a 23% rise in pressure, and I'd hope that there's a much better margin of error than 23% in tire pressure ratings. I used to ride with a biking friend that switched from 23mm tires with a high 160psi pressure rating to 25mm tires with a 90psi rating, and he didn't notice the difference and he spent months filling it to 160psi until someone pointed it out to him.

This is pretty close to the rule of thumb of 1 psi of pressure rise for every 10 degrees in temperature rise, which would predict that the 60 degree rise in temperature would lead to a final pressure of 76psi.

Also, tubes don't hold in pressure, tires and rims do, and the original poster said that his tube was split, not that the tire blew or the rim failed. I suspect that it's a bad tube that failed during use, poor installation that damaged the tube and eventually fail, or maybe no (or bad) rim tape that led to the tube being damaged.

I just noticed that Batman gave an almost identical answer to mine in a different answer

• Agree with analysis, but both tubes blowing out suggests the time in the hot car had some effect. May 17, 2018 at 20:50
• When the tube is split it usually means that the tire bead popped out of the rim. This would be due to some combination of overpressure and uneven heating of the tire. May 17, 2018 at 21:47
• You argue why the effect of the heat on the pressure does not cut it as an explanation. Unfortunately, you do not give any alternative explanation. I'd suggest to think about the effects of heat on rubber itself: Rubber is a heat sensitive material, and tires are created to withstand the temperatures they get on the road, not the extreme temperatures that you get inside a sun-lit car. I guess, what kills the tires is the combination of slightly increased pressure and greatly reduced material strength. May 18, 2018 at 19:58

This happened to me today and I'll offer a possible explanation. I think I may have inflated my rear tyre on my Brompton too much. Anyway I cycled to work (this was a new tube by the way) and stored it under my desk. It was very warm in the office. After about 6 hours there was a loud hiss that took everyone by surprise - the tyre was flat. I took it to my local shop and, sure enough, it was not a puncture but an internal blow out. So IF a tyre is over-inflated AND the ambient temp causes the air in the tube to expand then it MAY have been the cause of the blow out. Who knows it may have just been a weak tube, a slight pinch in the replacement fitting - OR the universe simply wanted me to experience it!!

I would say that old or degraded inner tubes may split like banana peelings due to (over)heating.

This is something that happened to me once. I rode an old road bike with old tires and tubes and also old, dried up brake pads down a long and steep descent. The brake pads where too hard, forced me to squeeze the levers quite hard and continuously, which heated the rims quite a bit. Near the end of the descent both tubes popped about 5-10 seconds apart from each other.

They literally popped, making a sound like a small firecracker. Luckily I did not lose control and could stop safely. I checked both tires and none had unhooked from the rims. Both tubes had a longitudinal gash near the valve but not quite reaching right to the stem. Both rims where hot enough that I could not keep hold of them with bare fingers until I let the cool off a couple of minutes, just like a car's metal surface that has been parked under direct sunlight during noon.

The tires where 27" and about as wide as a 25mm road tire. The inner tubes where of an adequate size for the tire, they all where just old. I got the bike secondhand so i'm not sure how old. (The bike was an 1984 model, and this anecdote happened around 2010, the tires, tubes and one of the rims where not original, but the brake pads may almost surely had been the factory ones)

I replaced tires and tubes for new but low budget ones (only option available locally) which coincidentally, where the same brand as the failed ones, but did not have another instance of this happening. Of course I also changed the brake pads with fresh, modern ones.

The bike may have been new to the O.P., but little to nothing is there to know about the shelf life of the tubes fitted to it, and, unfortunately, they might have been in storage for way too long before being installed in the bike. Improper storage can also degrade inner tubes, which I have also experienced, storing old tubes (destined to recycle or reuse not inside tires) near paint solvents or near gasoline. Some tubes turned "dry" and gray and lost elasticity, others turned black and tar-like.

Coincidentally, there is a question on this site where the poster says some time after topping up a tire using a gas service station pump, the tube "dissolved" inside the tire, leaving just a sticky goo behind. The question does not have a definitive answer, as far as I remember, but the speculation was that the gas station pump may have had some oil or lubricant that was injected into the tube and somehow had a bad chemical reaction with it.

How can an inner tube literally *dissolve* itself inside a bicycle tire?

Another instance of chemical degradation I have experienced was with slime pre-filled tubes, on another bike I bought second hand, so I don't know how long had these tubes been installed, but the tires where well within service life. This time it was an MTB where "spontaneous" punctures would appear. The sealant would make the regular patches peel of when it seeped between the tube and the patch. Once I got tired of it, I just pulled the tube apart with my fingers and it tore up like paper, way less effort that it would normally take. I don't remember the brand name but it was a fairly known one, and they where stamped "{Barand}-slime" with the usual white lettering.

Blowouts are a mystery. For example, when riding rim brake bikes in mountainous terrain, when braking downhill, the tire can blow out, destroying the tube.

It is known that pressure inside the tire doesn't increase so much that pressure alone would be the cause. Yet, tires do blow out.

Here's some discussion about it, mainly from Jobst Brandt: https://yarchive.net/bike/blowouts.html

I think it has to be a combination of different effects, the most significant probably being that heat softens rubber which can then creep.