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Safe margin of tread wear

Alright, In my defense, lets analyze what happens when we flip-I'm not alleging its wrong, just setting up an understanding.

Do we agree that some wear happens across the face, with more on the outside? If so, then flipping the tire in my picture some wear will occur on the already bald side? Back to my original question-how far can it go?

If flipping is going to be the plan, than the tire should be flipped before the tread depth is gone. Ideally estimating how much it will take to get "Even" with the high side.

We agree so far?

If so, the tire in question, in my estimation, was too far gone to justify flipping.

The question of flipping demonstrates the old adage "How was copper wire invented"? Answer;2 pilots fighting over a penny. My choice as I'm sure many others would be a time vs. value decision.

Jeez, I just wondered how many landings I could squeeze out of a tire.....

Why not flip them and watch for yourself? Learn. The tire wear is trully assymetrical so the portion of tire that is missing tread may last in its current state until the fresh tread is worn down considerably. You could certainly look at the good tread that is there at the moment. How much do you think it has worn down from new? On my first set of tires, I kicked myself for flipping too early.

I think this is a matter of personal tolerance. Each of us will choose differently. If it bugs you, put new donuts on it and watch for a better time (in your own mind) to flip them.
 
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While taking the tire off the wheel and flipping will buy more time, for the time it takes to do all that work I'd rather just put a new tire.
Carl

Thank you. My thoughts exactly.

Brian, How many hours are on these tires?

Should be right at 60 hours now. And as I said before over 100+ landings, 7 hours of low and high speed taxi. I just went to Google Earth and measured from the hangar to the runup area 1.13 miles. So a lot of ground ops.
 
It seems to me that if you flip your tires and install new tubes (as recommended), the increased savings will be at least partially offset by the cost of your parts, materials and labor.

I don't know that I'll flip my tires (Flight Custom III). I may just leave them on until they have been used up, then buy new. They aren't that
expensive...

~Marc
 
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It seems to me that if you flip your tires and install new tubes (as recommended), the increased savings will be at least partially offset by the cost of your parts, materials and labor.

I don't know that I'll flip my tires (Flight Custom III). I may just leave them on until they have been used up, then buy knew. They aren't that
expensive...

~Marc
Flip tires and install new tubes? One of my current tubes has been on the plane since the day I bought it--13 years ago and over 1,000 fight hours.
 
I was surprised also! We were Flight Flutter Testing a new large commercial transport near the turn of the Century and using the Company T-38 for high speed chase. Between flights we were walking the flight line, admiring the sleek looking, well maintained T-38. We noticed that one of the mains had cord showing and were quite surprised by that. We talked to one of the mechanics on duty and he said that the tire didn't have to be replaced yet, as not enough cord was showing. We were quite surprised by that given the high TO and landing speeds of the T-38, but they are 14-ply tires, IIRC.

The T-38 tires have a red cord, when that shows it is time to replace them.

My project at Test Pilot School was to modify a T-38 with a larger speed brake and use it to decrease the L/D to simulate the X-24B lifting body approach ( about a 24 degree glide slope). When I briefed the Flight Safety Board one of their requirements was to use old tires instead of new. The thought was that old tires had been heat cycled many times and new tires had too much tread for the high speeds we were touching down, increasing the chance of tread separation due to centrifical force. We crossed the threshold at 230-250 knots as I recall.

After the last approach when at Bingo fuel the closed pattern and normal 3 degree approach seemed way too low:D
 
If you are doing circuits go to a grass strip, stay off the seal as much as possible, rotate the tires and there will be plenty of life in them yet
 
Since I was about to swap tires anyway, I sectioned one for the cause. These are common Aero Classics. Measurements may vary given a different tire.

I ran this set somewhat past the point where tread grooves had disappeared from the primary wear area. They were not flipped. I inflate to 50 psi nominal.

Need a wear indicator? At the bottom of the tread groove, you have 1/8" of wear rubber before you'll see cord. Total carcass thickness from the bottom of the groove is 3/16".



The section looks like this:



Below, 10x magnification, taken at the thinnest point. Wear rubber is 1/16" to cord. Looks like four cord plies, with three in the rubber and one merely lining the inside of the carcass. The cord layers total another 1/16". Might get a little more life by flipping them, but not a lot. Going forward, I'll run 'em until one tread groove remains (first photo), then just replace them.

 
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When 1 remains? Wouldn't that assume consistent wear pattern?

At the bottom of groove there will only be 3/16" between me and running off the side of the runway- if lucky. Think I'll pull them when tread disappears.

I would have thought the carcass to be a bit more substantial. Thanks for the visual, Dan.
 
When 1 (groove) remains? Wouldn't that assume consistent wear pattern?

Safe enough assumption for repeated sets on the same airplane, but the wear pattern shown here many not be true for another airplane.

At the bottom of groove there will only be 3/16" between me and running off the side of the runway

The "run 'em until you see cord" camp is down to 1/16". However, we have no direct correlation between tire carcass thickness and the chance of a flat tire. Offhand, it does not appear that carcass thickness alone is an issue given a clean running surface. When we worry about flats due to thin tires, I think we're really talking about foreign object damage...and then the size and shape of the object becomes an entirely random factor.

Same category as primer, really ;)
 
Got my answer the hard way

Seconds from taking the active after run up. Ship would not roll. Figured out I had a flat and forced it off the in use area. Seemed like the right thing to do. Only about 100'.

Don't know yet what caused it but it went flat in about 5 minutes. Will autopsy tomorrow. In the meantime I have glass work to do.

BTW that is a jack point product with a modification.

2pr9oy1.jpg

4l2g40.jpg

essvux.jpg
 
The T-38 tires have a red cord, when that shows it is time to replace them.

My project at Test Pilot School was to modify a T-38 with a larger speed brake and use it to decrease the L/D to simulate the X-24B lifting body approach ( about a 24 degree glide slope). When I briefed the Flight Safety Board one of their requirements was to use old tires instead of new. The thought was that old tires had been heat cycled many times and new tires had too much tread for the high speeds we were touching down, increasing the chance of tread separation due to centrifical force. We crossed the threshold at 230-250 knots as I recall.

After the last approach when at Bingo fuel the closed pattern and normal 3 degree approach seemed way too low:D

Wow, X-24B, that takes me back a ways!
 
Why not fix the uneven tire wear?

On thing I find surprisingly lacking in this discussion is any suggestion of fixing the tire toe angles so as to have more even tire wear.

When I built my RV-8, I did very careful wheel alignment measurements, and shimmed the axles accordingly. It took a rather large shim on one side, a thin one on the other. But the result was that I measured essentially zero toe-in.

My tires have always worn predominantly in the middle, and the wear is symmetrical on the two edges, on both tires. I removed the original 500-5 tires supplied in the kit at 100 hrs, with still 1/2 tread depth, and replaced with 380-150-5 tires. Now at 450 hrs, the inner tread grooves are getting pretty shallow, but still there.

I guess its not easy to adjust toe-in on the other RV's with the round legs, but on the RV-8, why not just shim to zero toe-in and enjoy even, symmetrical tire wear, and 'normal' handling qualities?
 
Steve, did you do the alignment with fuselage level or tail down? Camber and toe, or just toe?

Positive or negative camber when level would add toe in or toe out with the tail down. The gear alignment we do during construction is equivalent to a longerons level, toe only measurement.
 
I ended up doing the toe adjustments with fuselage level. I made no adjustments to camber. I thought about this a long time, and there are two competing issues.

1. Wheel landing in more or less level attitude, misalignment causes a lot of scrubbing at wheel spin-up, even though there is fairly little weight on the wheels. Also this is the point where misalignment can cause more squirrelly behavior.

2. 3-pt attitude is where there is the most weight on the wheels, so presumably a lot of the tire wear occurs here.

In the end, I let point #1 win, and that's where I did the alignment. There wasn't much camber sitting in the hangar with weight on the wheels, but there would be at touchdown.

Ultimately, I either made the right choice, or got lucky, because the tires wear symmetrically. I wouldn't say 'evenly', since they wear more in the middle, which you have to expect with a more or less round tire profile.
 
I ended up doing the toe adjustments with fuselage level. I made no adjustments to camber. I thought about this a long time, and there are two competing issues.

1. Wheel landing in more or less level attitude, misalignment causes a lot of scrubbing at wheel spin-up, even though there is fairly little weight on the wheels. Also this is the point where misalignment can cause more squirrelly behavior.

2. 3-pt attitude is where there is the most weight on the wheels, so presumably a lot of the tire wear occurs here.

In the end, I let point #1 win, and that's where I did the alignment. There wasn't much camber sitting in the hangar with weight on the wheels, but there would be at touchdown.

Ultimately, I either made the right choice, or got lucky, because the tires wear symmetrically. I wouldn't say 'evenly', since they wear more in the middle, which you have to expect with a more or less round tire profile.

I know the community has beat up the alignment issue but now I'm addressing my wear pattern. Maybe Steve or Dan can weigh in. I aligned per Vans instructions using laser levels out 30'. The toe was -0-. I attribute my particular wear because of a large amount of taxi time both prior to phase 1 and due to the hangar is a mile from departure end of calm air runway.

Now I'm wondering if I could shim out some of the camber and would there be any adverse handling characteristics?
 
To be perfectly accurate, tread grooves and other molded patterns in the surface of a tire reduce paved surface traction. Maximum dry traction is found with a smooth tire, aka a slick....

Dan's quite right for friction in the rolling direction.

However, lateral friction on paved surfaces is HIGHER with tread grooves because they tend to dig in slightly. For nosewheel aircraft with the tread on the main tires, this promotes straightness down the runway, but for taildraggers, this increases, slightly, the tendency to depart from straightness in the direction of a ground loop.

I worked this out some decades earlier and ordered a pair of smooth tires for my Cessna 180. They proves so successful that I continued for well over 1,000 hours. I've landed with them on dry pavement, on wet pavement, on glare ice, on dirt, on grass and on gravel. And on a beach down in Baja. In every case they proved honest, reliable, and easier to control than treaded tires had been.

Smooth tires on pavement are a bit closer to what landing on grass is like, in terms of lateral control.

Tire wear was superb.

The tires were custom-ordered retreads from Wilkerson Tires in Crewe, VA. I discontinued doing this after they sent me two sets in a row with very poor sidewalls. Now the handling is back to normal (i.e., worse), and I've got to be a bit more careful.

I suspect that treaded nosewheel tires (that is, on the nosewheel rather than the mains) might tend to be a factor in nosewheel shimmy, but have nothing concrete to base that on.

On tailwheel airplanes, I do recommend that the tailwheel be treaded. Easier on the C-180, though than our RVs.

Dave
RV-3B, skinning the fuselage
 
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