The Cessna 120-140 Association Forum

Question about Valve Stems:

About a month ago, I went to an event in St. Louis with the 120. In the couple of days prior, I had double checked all of the tire pressures and set the mains at 16 and the 3200 tailwheel at 30.

As I was spinning around to get into my parking spot, the valve stem sheared off on the left main and tire went flat. I was able to get a IA on field to put in a new tube. He was from Alaska and suggested I might go a little higher PSI to help with side loading on pavement so we aired both tires to 18 PSI.

Wednesday, I am taxing out of my base airport which is all concrete and just stopped cold in the taxi way. Now it was the left main that went completely flat and again, a broken valve stem.

It looks like the tires and tubes were last replaced in 2012 using Air-Trac. Is this just likely an age issue of the tube or is my tire pressure or taxi technique somehow to blame here? I've put about 100 hours on the plane since I bought it in May of this year so it's been getting quite a workout compared to years prior. I'm guessing after 6 years, it's probably just wear and tear. Any other thoughts?

I would suspect that tire pressures were too low. The stems were keeping the tires from changing position on the wheel, until they gave up and broke.

Did you have the brake locked on the affected wheel each time? I personally have never had this happen to me, so I don't know how much help I can be. (I did have a nose wheel tube let go on a plane I was working on, but have no idea how I made it happen... )

6298 wrote: ↑Fri Nov 23, 2018 12:25 pm I would suspect that tire pressures were too low. The stems were keeping the tires from changing position on the wheel, until they gave up and broke.

What tire pressure would you suggest?

6643 wrote: ↑Fri Nov 23, 2018 12:32 pm Did you have the brake locked on the affected wheel each time? I personally have never had this happen to me, so I don't know how much help I can be. (I did have a nose wheel tube let go on a plane I was working on, but have no idea how I made it happen... )

I did on the first time as I was trying to spin around in a tight spot. Don't recall doing that the 2nd time. When the tubes and tires were replaced 2012, it spent the last 6 years in Oklahoma. I suppose that could have helped to dry out and crack the rubber. If it's technique, I certainly want to learn from that.

I looked at the logs and I have flown almost 94 hours in it since May and about 120 landings.

Paint slippage marks on your tires to see if they are moving. I have run my mains at 16 psi and never had an issue. Also check the hole in the rim where the stem goes thru and make sure that there isn't a burr on it causing damage to the tube. Just a thought.

I installed a set of Leakguard tubes with new tires on a Cessna I was restoring. It sat in the same spot and never moved for a year. No turns, hard landings or hard braking. One day I looked and noticed that one of the tires was flat. After taking the wheel apart I found that the valve stem had rotted and cracked. So much for the so called new and improved Leakguard tubes.
JMHO, Steve.

Hey, they held air for a year! "Old" tubes would leak down in a couple of months at most.

Don't know about the valve stem issue. What I do know is that if I am below 20 psi, I can barely push it out of the hangar with my tail dragger "dragger" I usually keep the tires (6.00-6 with Cleveland wheels and brakes at 25+) and have not found any issue with landings. Tire wear is minimal with most of my landings being on concrete. The tires wear on the outside on the pilot side cause I usually fly solo. I just flip them on the wheel to get some extra wear.

6277 wrote: ↑Mon Nov 26, 2018 2:11 pm Don't know about the valve stem issue. What I do know is that if I am below 20 psi, I can barely push it out of the hangar with my tail dragger "dragger" I usually keep the tires (6.00-6 with Cleveland wheels and brakes at 25+) and have not found any issue with landings. Tire wear is minimal with most of my landings being on concrete. The tires wear on the outside on the pilot side cause I usually fly solo. I just flip them on the wheel to get some extra wear.

Interesting. I know the Cessna guidance back in the day was 16 psi but wasn't that also at a time where more fields were grass instead of concrete? Perhaps because of the grip you get on concrete, higher pressure helps keep the side wall structure a bit more solid?