Diagnosing Tire Wear

[wae]

The tires on my coach are only about 4 years old and have approximately 11-15k miles on them (they were put on by the PO not long before I bought the coach.  I've put 11k miles on it, but I don't know how many miles they went on these tires).  As far as I am aware, there has never been any tire rotation done.  The steer tires are exhibiting wear on the inside of the tire - one of them is actually showing the steel belting in a small spot on the inside.  Let's get this out of the way:  Yes, I am absolutely aware that the tire that's worn through to the belting must be replaced right away and I plan to do just that.  Further, I intend to do a more thorough inspection of the other tire and I will replace it if it is too worn to be safe.  Yes, I have verified the date codes on the tires and they are not aged out.  Other than this inside wear, the tires do not exhibit any significant treadwear.  There's no other damage to the tires (cuts, cracking, etc).

Before I just slap on new tires and go, I would like to try to understand what has caused that wear so that I can correct that and don't have to replace tires every 10k miles.  I've had bad ball joints and bad wheel bearings on cars and light trucks before, but when I get the front wheels in the air, I don't feel any play in any direction.  The alignment seems to be good; while I haven't had a shop check it, it tracks fine down the road with no steering input required to keep it straight.  When I bought it, it was missing all of the front swaybar bushings and the airbags inside the coil springs were split open and totally deflated.  It wasn't until about 1,500 miles ago that I put the new airbags in, but I did the swaybar bushings about 2,500 miles after I bought the coach.  I've never heard of swaybars/lack of swaybars having any effect on tire wear, but could the lack of airbags have let the coach come down too much and too often on the front suspension causing excessive negative camber?  Is that just the way these things wear tires and I need to be doing frequent tire rotations?  Since it's a medium-duty truck suspension, does it take so much more back-and-forth and up-and-down force to deflect the wheel to diagnose wheel bearings/ball joints as opposed to a car or light truck?  I'm not afraid of getting in there and pulling things apart, but I'd rather not just fire the parts cannon at it since I'm already in to this for at least a tire!

 

[56safari]

Excessive negative toe and negative caster both will cause the inner tire wear.  Could well be that the inop air bags were behind the excessive negative caster.  It's not normal tire wear.  Negative toe and negative caster will still track and handle OK.  If your comfortable the steering components are all in good shape, a good alignment may be all you need.

 

[Gary RV_Wizard]

Negative camber will also cause wear on the inner edges. It's not an unusual problem with the solid front axles used on most gas chassis and many diesel pushers.

You can learn about wheel alignment terms and how they affect tires in this helpful article:  http://www.tirereview.com/diagnose-tire-wear-problems/And this one too: https://www.bridgestonetire.com/tread-and-trend/drivers-ed/tire-tread-wear-causes

Basically, though, you need to have a competent truck alignment shop go over the front and and get everything aligned properly.  There might be some worn components, but most likely its just out of adjustment. 

 

[darsben]

Seems to me that you need to have a competent person (can be you if you are knowledgeable) look over the front end parts and if no bad components found then do an alignment.
I do not know the proper alignment specs for your rig but someone will point you in the right direction for best handling versus tire wear.

If it were me I would do 2 new steer tires after all else on front end proves good and relegate the good front tire to spare

 

[SeilerBird]

My guess is the tire pressure is set to the max rating on the tire and not set by weighing the RV and looking up the correct pressure on the manufacturers tire chart.

 

[wae]

Thanks for the guidance!  This is very helpful!

- I know that I've never weighed the coach, but an over-pressure should manifest itself as abnormal wear down the center, I would think.  Regardless, getting good weights is on my to-do list.

- Looking at the guides posted, and my own experience with how my race car eats tires when I street-drive it, it does look very much like wear indicative of excessive negative camber.  I don't feel any feathering at all and apart from that one rib, you'd think these were almost brand-new tires.

- About 2,000 miles ago, I spent a significant amount of time under the front-end putting the new airbags in.  While I thought I took some pictures of that work that might happen to include the tires, apparently that was not the case.  I do know, however, that I had the suspension at full-drop and my face right next to the tires.  It isn't completely impossible, but it seems very improbable that if there was any odd wear on the tire, I wouldn't have noticed it.  So, I'm wondering if instead of the repaired airbags fixing the condition if they didn't cause it.

Looks like my next step is to take my gauges, plates, strings, and tape measures out there, find a perfectly level patch of asphalt, and check camber, caster, and toe.  Then I can dump the air from the bags and re-measure to see what kind of difference that makes.  Then I can get the front end up in the air and give the suspension a good looking-over.  I actually found a pretty good alignment info/guide while I was researching this that kind of decodes and adds some flavor to the P30 service manual's information:  http://users.sisna.com/cebula/P-Chassis-AlignmentProcess.pdf.

 

[SeilerBird]

I know that I've never weighed the coach, but an over-pressure should manifest itself as abnormal wear down the center, I would think.  Regardless, getting good weights is on my to-do list.

There is your problem. Get the coach weighed and set the proper tire pressure. Over inflation should result in center wear but if it is combined with a bad alignment or defective front end parts it can wear like you describe.

 

[Gary RV_Wizard]

You may or may not have over or under pressure, but I doubt if that is what has caused the inner edge wear you have observed.

I just noticed that the coach is a 1993 on a Chevy chassis. Therefore you have front air bags inside of coil springs and an independent front suspension, right?  Not a solid axle, so camber is a more complex affair (each wheel camber is set separately).  Presumably the new bags you out in are holding air ok, but what bag pressure are you using?  That psi has an effect on the front end geometry.  That chassis is also notorious for a sloppy steering bell crank (the Henderson Super Steer bell crank is the highly recommended replacement).

Do you have a chassis manual for it?  You can find old P30 chassis service manuals online.

 

[WILDEBILL308]

I would listen to Gary, and the outher thing is to have it loaded like you were going on a trip when they do the alignment. Somewhere in your owner's manual it will say " it is the owners responsibility to have the alignment checked after delivery". They do a basic alignment when building the chasey but it can be off quite a bit based on how it is loaded when finished.
Bill