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Tire Wear - Too Much Toe In?

lluciani

Active Member
I was hoping someone could help me reduce my tire bill. The outsides of my tires wear out after only about 40 hours. I don't think I am dragging the brakes and I keep the tires at around 30 PSI. Could it be that I have too much toe in?

It is not easy for me to check the alignment the way Vans says to since this means jacking up the whole plane to take the weight off the main gear. With the plane level it would seem that it wouldn't make any difference. The landing gear bows outward with weight but with the same alignment, right?

Luis Luciani
RV8, 140 hours
 
Most RV's are that way. What happens is that the wheels are cambered in at the bottom under lightly loaded conditions. If you fly at gross more often, they will wear more evenly, because the geometry of the gear changes as weight is added.

Most of us just reverse the tire on the wheel about 1/2 way through the tire's life. That allows you to use up all the tread on each side of the tire..
 
FWIW, my RV4 would go 100 hours or more on a good set of Flight Customs with 1/2 degree of toe-in on each wheel.

Allan Tolle gave me the scoop on how to set toe-in. He recommended 1/2 degree toe-in with the plane sitting on its tail, with no passengers.

Then, with the plane rolling under load the rolling resistance of the gear would bring it back into alignment. I did just as Allan suggested, and my tire wear was really good.

This was on an RV4 - so this data point won't translate to how to set the 8's gear since they're so different in terms of deflection characteristics under load. However, since the great majority of time spent rolling on pavement will be in the three-point attitude, I suggest you take a good look at how your gear sets up in that attitude. And that's easy to do. Sounds like you have too much toe in if wearing out tires in only 40 hours...
 
What I found:

On my RV-8, I shimmed to within 0.1 - 0.2 degrees with the fuselage level, and I found that I then have about 1/2 -3/4 degree of toe-in in 3-pt attitude.

this was done with the fuselage+engine weight, wings and tails off.

I measured by clamping 30" sticks to the tires and measuring at the front and back of the sticks. Some people suggest you should use a pair of metal plates with grease in between under each tire. The contact patch is so small with just the fuselage weight that I can't imagine having any torsional pre-load from the tire-ground contact.

Should know in about a month how they roll.
 
Hi Luis,

40 hours to wear the outside is pretty low - I get about 150 when using Desser Retreads. I forget what came with the kit, but I seem to remember them wearing out very, very fast. I am not sure how tire pressure affects wear, but I am using about 38 psi - 30 seems quite low to me, and could be making your tire to wheel pants clearances a bit on the tight side as well (compresion during touchdown). I have never tried adjusting the toe-in or camber, as the airplane handles just fine, and I didn't want to mess that up. If you like your handling, the simplest thing to try is higher tire pressure and a different tire compound.

Paul
 
If you really want to fix your tire wear problem, you need to re-align your gear. First, a little background. The Rocket guys have been messing with this problem for some time because the heavier weight of the IO-540 makes minor gear alignment issues more acute. Also, our gear legs tend to flex a little more so having the proper alignment is really critical to having a good ground handling machine, and as a result, getting good tire wear. Here's what we've learned:

Your goal is to have ZERO toe with the airplane in the three-point configuration with it fully loaded with your normal flying weight. To get this, you have to put some grease plates under your tires and add weight to simulate fuel, pilot, etc.

Since your gear sockets are already drilled, you can either try some axle shims or you can re-drill your sockets to your gear legs. I'm not sure on your particular model of RV how the axles are attached but if you try for zero toe in this configuration, you will not only fix your tire wear problem, but improve your ground handling and gear leg shake.

If you can't get it to zero, having a little toe OUT is preferable to toe IN. For the Rocket guys, toe in is BAD!!.

I know there are lots of opinions around regarding whether you want toe in or toe out. Well, most of that talk is based upon hangar flying. I've persoanlly flown my Rocket with different toe configurations and I speak from experience, at least from a Rocket perspective. As always, YMMV.
 
Been there and done that!

In the first hundred hours, I wore the outside half off both my original tires smooth. At the first conditional inspection, I replaced the tires. When the wheels were off, I removed the axels and installed a one degree shim on both sides to take the toe-in out. The next set of tires lasted three years.

I could also tell the toe-in was there in how the Doll behaved on a wheel landings. When either tire touched down first, the aircraft would attempt to turn away from that side until the other tire touched and then it would balance out and track strait as the outsides of the tires were being scuffed off.

Buy two one degree shims and remove the toe-in, and you will be fine. Trust me!
 
All you need to do is buy shims from Robby Grove, and set your alignment for zero toe, zero camber with full fuel. Its a lot easier to do on the RV-8 than it is on the Rocket. I have a friend with over 300+ hours on a set of tires on his F1 and I helped him set the alignment.

To set the toe, clamp two four foot angles to the brake disc's, parallel to the floor. Drop a plumb line from the CL of the firewall and another one at the tailwheel, and pull a string underneath both points. Measure from the front and back of the angles to each other and also make sure they are even with the center line.
 
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Toe OUT, not toe in

It's really counter-intuitive but on an airplane, you really want a small amount of toe OUT, not toe in like you'd have on a car. Somebody else will have a better explanation, but the reason is that if the plane is banked on the ground, toe in works against you for recovery. There was an article about this in Sport Aviation many moons ago.

When I bought my RV-4, the spec for toe out, measured across one axle, was 0 to .050". The builder had gotten a little enthusiastic, though, and had put in nearly 5/16" of toe out. After I fixed that, and it was an epic project, the RV-4 was so much easier to land... and the tires didn't need to be rotated in / out every 15 hours...
 
Ed isn't wrong, but he isn't completely correct either. Toe out might help if you are losing control with a wing up, but toe in will help self correct with wings level. So, there isn't really a clear answer here other than zero camber and zero toe.

Here's what a Ladislao Pazmany, author of Landing Gear Design for Light Aircraft and a man who is (was) much smarter than anyone on this forum, including me, says on the subject:

Dear Vince:

Unfortunately your question doesn't have a straight answer. There is a big controversy on the subject. Please review the attached document in order to understand it better. (the attached document was scanned from my book Landing Gear Design For Light Aircraft )
Regards Paz

toe-in.jpg


This is also posted at this URL if my photo insertion didn't work correctly.
http://www.vincesrocket.com/landing gear and related stuff.htm

Enjoy the mental wrestling while you debate this in your head. RocketBob and I agree that the optimum configuration is zero toe/zero camber at whatever weight you typically fly at. Full fuel works for most of us.

For you tapered spring gear leg RVdrivers, lotsa luck changing that alignment!
 
Follow Van's directions exactly

Your tire wear is really excessive, suggesting that your axles are seriously out of alignment. You should be able to fix the problem by shimming axles as suggested by Van.

With my RV-8, I followed Van's directions exactly. With the gear off the ground and fuselage level, I set the axles with no toe-in or toe-out. I never checked the axle alignment with the tail down. I have over 560 hours on my Condor tires and have not removed them from the wheels since the first flight. My tires still show little overall wear, with slightly more wear on the outside.

Dan Miller
RV-8 N3TU 560+ hours
 
It isn't necessary to raise the tail on the -8 to set the toe, since there is much less camber with the RV-8 gear. It doesn't flex as much as the Wittman-type gear. Even with that in mind I think its a better idea to set your gear up for zero-zero in the three-point attitude, since that is where the airplane spends 99.99999% of the time when the wheels spinning.
 
Another possible cause...

Do you do a lot of wheel landings?
If you fly at light weights, coupled with gental touchdowns, and wheel landings (which equates to higher speeds at wheel spin -up) it can cause excellerated tire wear on the outside edges. Because of the amount of camber on an 8 this is the area of the tire that will be touching first at tire spin up.
 
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