# 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!!