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RV7 Tire pressure

XLS+PAT

Active Member
Howdy VAF folks

I'm somewhat new to the RV7 and looking for opinons on tire pressures. I've been checking the tires at least once a month. It seems like 38psi is the sweet spot but I'm curious as to what other RV7 drivers prefer.

Thanks




RV7 slider 0-360
C-150 Tail dragger sold 10/17
KHAF
 
Please take this the right way..."the "search" button is your friend.

Short version, yeah, about 38 is common as a compromise between leg shake and tube life. Higher generally means more shake. Lower appears to come with anecdotal evidence of tube pinch and scuff problems. I run 50 in an 8, but I don't have to deal with leg shake.
 
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I run 45 in the mains and 50 in the nose wheel on my 6A. I have wood dampers on the legs and never experienced any shimmy on the mains. Nose dances a bit, but only observable to viewers as it can't be felt.

Larr
 
7A tire pressure

45 psi on all three wheels. Check every 2 months at which mains usually show about 38 psi. Surprisingly, nose wheel loses very little air.
Shimmy has not been an issue.
 
Howdy VAF folks

I'm somewhat new to the RV7 and looking for opinons on tire pressures. I've been checking the tires at least once a month. It seems like 38psi is the sweet spot but I'm curious as to what other RV7 drivers prefer.

Thanks




RV7 slider 0-360
C-150 Tail dragger sold 10/17
KHAF

The tail draggers often run less pressure than the nose wheel RVs in order to manage gear shimmy. I ran 28 in my RV-6 tires for many years but now start at 35 and add air when they get down to about 28. If you don't have any gear shimmy at 38 that will be a good number for you. If you have shimmy you might try a little less pressure. The shimmy thing will vary depending on tire wear patterns/roundness.

And yes....tons of info in the forum archives on this discussion. :)
 
i have ran 45 in my RV7 400 hrs ago. never a shimmy. never a flat. on 2nd set of recaps.
fred
 
VETE76 made a subtle comment on his post.

Recaps.
I use Dresser Monster Retreads and 40 PSI after 200 hours the tires look almost new.

Being a corporate pilot our mechanic would never use recaps but the airlines do all the time.. I am sold on their tires. they wear like iron.
 
The tail draggers often run less pressure than the nose wheel RVs in order to manage gear shimmy. I ran 28 in my RV-6 tires for many years but now start at 35 and add air when they get down to about 28. If you don't have any gear shimmy at 38 that will be a good number for you. If you have shimmy you might try a little less pressure. The shimmy thing will vary depending on tire wear patterns/roundness.

And yes....tons of info in the forum archives on this discussion. :)

I started with low pressure (25 psi) on my -6 and had a fair amount of shimmy. Since I upped it to 38 or so, the problem has all but disappeared.
 
I run 37 PSI in my 7 as I am fairly new to the airplane not to tail wheel time, it has been the best combo of forgiveness, tread and I have never had any shimmy.
 
30psi in my 6.
First set of tires went 450 hours and still had a little tread. I am at 770 hours now and am just ready to flip them.
So, lower pressure = tire wear?

If I go over 25psi in the Bucker, get ready for Mr. Toads Wild Ride!
 
I talked to the good folks from Desser at OSH. He gave me a copy of this chart:
https://www.desser.com/content/pressure_chart.asp

He was adamant to keep AT LEAST 50 psi in my 5.00 x 5, 6 ply monsters at all times.

I can understand why Desser would recommend max pressure in their tires, I suspect they want to make sure the tire can support the maximum rated load. However, our RVs are considerably lower weight than many certificated aircraft on which these tires might be used. Lower loads can be safely handled by lower tire pressure. Each RVer needs to use the pressure that works best with their particular aircraft.
 
In my -7A, I inflate the mains to 50 psi and the nose to 40. I have no issues with shimmy and shaking. I check them every couple of months or when the weather turns markedly colder and they're all usually down about 10 psi or so.
 
I can understand why Desser would recommend max pressure in their tires, I suspect they want to make sure the tire can support the maximum rated load. However, our RVs are considerably lower weight than many certificated aircraft on which these tires might be used. Lower loads can be safely handled by lower tire pressure. Each RVer needs to use the pressure that works best with their particular aircraft.

Sam,
The Desser guy said the reason for maintaining the pressure was to prevent breakdown of the structure under the tread (not the sidewalls). Not trying to argue, just passing along what I remember was said. I?m coming up on 300 hours on my monsters and just flipped them around for another year of flying. I?ll have another pair ordered for my next annual in October ?19.
 
Probably depends . . . . .

. . . . . on the individual airplane and it's particular leg damping. I have tried mine at many different pressures. 45 psi virtually always shimmied. 37 psi gave me shimmy under some conditions - 22 psi (yes I tried it) never gave any shimmy. I am currently at 32 psi and it seems to be in the "No Shimmy" zone. probably a winner for me.
 
I am currently at 32 psi and it seems to be in the "No Shimmy" zone. probably a winner for me.

...in particular if the jack and spare tube are needed for ballast anyway ;)

Kidding aside, the numbers don't lie. At 1800 lbs and 8.1 ft per/sec with 90% on the mains, the dead minimum is 36 psi if the inside of the tread is not expected to mash against the bead. Even at 36, consider the tube abuse with the sidewalls folded double while rolling.

I realize shimmy is a serious issue for the round leg models, but it is best to be aware that tire pressure is not a random pick item. Everybody drops one in eventually, and there are also pavement edges to consider.
 
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there are four "A" types who had responded to this thread. This link was for them and any other non-responders who are "A" types.
 
...in particular if the jack and spare tube are needed for ballast anyway ;)

Kidding aside, the numbers don't lie. At 1800 lbs and 8.1 ft per/sec with 90% on the mains, the dead minimum is 36 psi if the inside of the tread is not expected to mash against the bead. Even at 36, consider the tube abuse with the sidewalls folded double while rolling.

I realize shimmy is a serious issue for the round leg models, but it is best to be aware that tire pressure is not a random pick item. Everybody drops one in eventually, and there are also pavement edges to consider.

Which tires?
Zero issues at 30 psi all these years with tires or tubes. Original Air Hawks (replaced at 460 hrs with tread to spare) and now Desser Retreads (passing 770 hours and just now ready to rotate with at least half tread remaining).
So, that's my experience with my 6.
Sidewalls folded double? There is little visual difference in tire profile between even 25 psi and 50psi. However, I do trust your smarts and I know better than to argue with you so I won't. I am having a hard time understanding why I have never had an issue when it sounds like my tires should be eating tubes and I should have flats all the time.
Perhaps I am just lucky, in all the airplanes I have owned as I have never ran any of them above 35psi.
 
Inquiring minds . . . .

.. . . . . At 1800 lbs and 8.1 ft per/sec with 90% on the mains, the dead minimum is 36 psi if the inside of the tread is not expected to mash against the bead.. . . . . .

Curious where this specific number comes from?
 
I hope the gear and prop aren?t folded too! ;)

Not at 8.1 ft/sec.

Curious where this specific number comes from?

One of Neal Willford's excellent design spreadsheets. The whole series was available for download, but I just searched EAA.org without luck. They were published circa 2003.

I should review the inputs for precision before swearing on a bible. The particular file I picked from my hard drive was named RV-7, and dated 2005.
 
there are four "A" types who had responded to this thread. This link was for them and any other non-responders who are "A" types.

I run 45-50 all around, and if your aiplane visits my shop that's what they go out with. Never had a flat or any shimmy issues.
 
Previously I said...

I should review the inputs for precision before swearing on a bible. The particular file I picked from my hard drive was named RV-7, and dated 2005.

...so I poured a cup of coffee this AM, then dug out the spreadsheet, an RV-7 leg drawing, and the Michelin Tire Databook. After making a few corrections, we have:


RV7%20Tire%20Deflection.jpg



BTW, the Michelin databook specifies 50 psi for a 6-ply 5.00 x 5. Lower pressure reduces the load rating.

I'm not trying to tell you lower pressures won't work. I am telling you tire pressure is an engineered value, not a random pick. The 36 psi value shown allows the tire to be mashed dead flat given a vertical velocity of 8.6 fps, about 500 fpm in pilot-speak. This is within the capability of the gear leg, although the margin is not huge.
 
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fill, check.

Dan has some very good data above and is always the place to start. I would only add that if you are new to tire care, there is an old habit you may wish to keep in mind. When you find that sweet spot that you like for you tires. Say 38 P.S.I.G., you may wish to think about rounding that up to 40 and add 3 P.S.I.G.
For most small aircraft tires the pressure will drift down a lot faster than a car tire. It is good to check them and not let them get to low as that is were they get the most damage, as in under pressure and not so much from bring a little high on the pressure. We always add 3-5 P.S.I.G. to our preferred setting and then inflate to that point. By the time most small aircraft tires set for a couple of week they will have drifted down to where you like them to be anyway. I know every tire and tube like to drift down at it own pace depending on what type and size it is. This is just an old trick we have found that keeps us from servicing tire more often than we have too and does not hurt the tires as well.
You will find that sweet spot that works for you and the type or brand of tire and tube you are using in do time. Change it around a few time till you like what you find. Yours, R.E.A. III # 80888
 
Thanks Dan for the thoughtful response and education. I am not too stubborn to make a change, even after hundreds of hours of doing something one way with no issues.
Perhaps all of the years of flying antiques has habits held over that where not based on anything concrete....
 
Tire Modeling

. . . . . . . . I'm not trying to tell you lower pressures won't work. I am telling you tire pressure is an engineered value, not a random pick. . . . . . .

I absolutely agree with this statement. Of course the engineered value is a function of the design criteria.

Do you happen to know how the tire deflection vs load at a given tire pressure is being calculated? I'd be interested in how that is being modeled in the spreadsheet. Or you could just send me the spreadsheet . . . . . ;)
 
I've never seen so much written about just tyre pressures! In near 40 yrs of driving planes (little ones) I've always run around 35 psi, it's not rocket science, tires are a flexible medium, very resilient and designed to take punishment, the only main variable effected by under or over inflation is the amount of wear. I consider tires a consumable item, I keep my RV8 at 35 psi but am not too concerned if it's 10 psi either side.
 
Flats

I've had two flats on roll out on my 6A in about 500 hours of flying. Both times it was on the left main, and I found a pin hole in the outer sidewall area of the tube.

The landings were no big deal, but getting the plane off the runway is a problem if you don't have some good help. Both times it had been a few months since pressure was checked and the tire was probably in the 30 lb area.

When I land at my home airport (Baton Rouge), I always request 4R/22L if available because the idea of shutting down one of the main runways due to a flat worries me. John
 
I've had two flats on roll out on my 6A in about 500 hours of flying. Both times it was on the left main, and I found a pin hole in the outer sidewall area of the tube.

The landings were no big deal, but getting the plane off the runway is a problem if you don't have some good help. Both times it had been a few months since pressure was checked and the tire was probably in the 30 lb area.

When I land at my home airport (Baton Rouge), I always request 4R/22L if available because the idea of shutting down one of the main runways due to a flat worries me. John

Make sure you use ample talc powder when servicing your tubes and tires. I cover the entire tube and inner walls of the tire. Talc sticks to any exposed rubber easily. Once I have a light dusting, any unreasonable excess is removed.
There have been many reports of tube sidewall pin holes. Not sure what the cause but it doesn't seem to be tire pressure related based on the reports I have read here.
 
My data point.

I was running 35 all around and tires were wearing well. Landed one day and the taxi back felt very odd. The front tire was flat; hole in the sidewalls of the tube and the tire (Michelin) was eaten up inside from sidewall flexing.

I now run 40-45.
 
-7 Whittman gear moves all over...

I have been running 50 psi in Good Year Flight Special II tires for a bit. These have been great tires so far. Wheel landing with them at these pressures works best when carrying enough speed in the flare to keep the tail up. If the tail is dropping in the flare there can be more rebound than desired if you don't catch the transition on the money. Three point landings need to be very close to stall to limit bounce back.
 
Talc

Jon jay, I?ve got a friend of mine who worked for Michelin tires for decades. He came round as i was changing a tire last week and saw me using a bit of talc. His expert opinion was that only the faintest bit of talc was needed because any more than that and with any moisture it clumps up into tiny balls that will rub pin holes in the inner tube. This may answer your question.

Seems plausible.
 
Jon jay, I’ve got a friend of mine who worked for Michelin tires for decades. He came round as i was changing a tire last week and saw me using a bit of talc. His expert opinion was that only the faintest bit of talc was needed because any more than that and with any moisture it clumps up into tiny balls that will rub pin holes in the inner tube. This may answer your question.

Seems plausible.

Interesting. I have never found any moisture or clumps, nor any pinholes.
I don’t leave excess talc, just enough dusting to where the rubber dulls. The talc sticks to the raw rubber in a fine coat.
At this point, I think I will keep doing what I have been doing and as I was taught by my dear departed AP/AI. I have had good luck in both tire and tube wear, but I am certainly no expert, just one dumb guy who only knows what he was taught, wrong or right.
Thanks for the input though.
 
46 years of using talc on many hundreds aircraft tires. I have never seen any adverse effects even though I did not use it sparingly!
 
All the best for 24 to everyone!

7 taildragger here with around 300 landings. from the beginning i pumped the tires to 42 psi . 42 is known to be the answer to the universe and everything and therefore easy to remember, so I took that. no shimmy. 200 or so landings later I rotated the tires and inflated to 42, again no shimmy.

over time the wheels were down to about 30 psi, so brought them back to 42 recently and now i have shimmy. Very noticeable at takeoff and landing and taxiing a little bit faster. any idea where this could come from? i didn't change anything. does tread wear has an impact maybe?

thank you
kay
 
7 taildragger here with around 300 landings. from the beginning i pumped the tires to 42 psi .... no shimmy. 200 or so landings later I rotated the tires and inflated to 42, again no shimmy.

over time the wheels were down to about 30 psi, so brought them back to 42 recently and now i have shimmy. Very noticeable at takeoff and landing and taxiing a little bit faster. any idea where this could come from? i didn't change anything. does tread wear has an impact maybe?

More likely tire ply separations due excessive flex with the low pressure... which would make it a classic example of natural frequency being matched by an exciting frequency.

Let's assume the gear leg assembly has a natural frequency of 12 hz. You can observe it if you wish; just jack up the airplane and rap the tire and wheel hard with your fist. It will vibrate at its natural frequency.

Assume on single lump in the tire plies. A new 5.00-5 has a diameter of about 14 inches, so the lump hits the asphalt every 3.66 feet. Each time, it generates a force in the assembly. This regular pulse need not be large.

12 times per second (hertz) x 3.66 feet = 43.92 feet per second. It would shimmy worst at 26 knots. Roughly 1.2x above and below that speed it would be become relatively smooth.

Do you know an accurate speed for your problem?
 
Dan: thanks for the great input. the airspeed indicator kicks in much later. but i estimate the shaking starts at 20 - 25 kts. always slowed down when it started to shimmy so i have no idea when it ends. will test it next time at the airport.
 
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