[A/S] Radial vs Bias Ply Tires

[Garnett Horner <garnett@xxxxxxxxxxxxx>]

I ran across this article about radial versus bias ply tires.  I know the
subject has come up here before.  Although the comments are about larger
rigs, I think the general comments apply across the board.

Garnett Horner
WDCU, 2878, 60 Caravanner


10 Radial Tires On Older Rigs

W F Sill writes: 

This is NOT a "Frequently Asked Question" -but it oughta be: 

WHY CAN'T I PUT RADIAL TIRES ON MY OLD MOTORHOME WHEELS?


Your owner's manual for many early-70's GM and Ford chassis (possibly Dodge
but I dunno)will contain a warning: DO NOT USE RADIAL TIRES. So far, I have
never seen one that said WHY, and unfortunately many tire shops will tell
you emphatically they'll work just fine. WRONG, DUMBO! 


The problem occurs on 16.5 and 19.5 wheels fitted through about 1976. (Not
sure of the date - some chassis may have been in the pipeline into as late
as '77.) There are two aspects to the problem: First, motor homes are
almost fully loaded (if nor overloaded) ALL THE TIME, so wheels are highly
stressed. (this is the main reason the problem was never common on wreckers
and flatbed trucks using the same wheels) Second, a radial tire produces a
much sharper cyclic pressure against the sides of the rim than a bias ply
tire (that's why radials always look a little "soft" compared to bias ply
tires.) The combined effect of these forces causes the lighter, old-style
wheel rim to flex enough to crack over a period of time. 


We learned this the hard way with a '76 GM chassis motorhome. One had a
slow leak when we bought it (used) at around 50k miles, and four more
failed within the next few thousand miles. The original owner had
disregarded the manual and put on Michelin 8 x 19.5 radials at about 30,000
miles, so it took a while for the cracking to show up. Naturally we were on
a trip when the failures began in earnest, and we stopped at one tire shop
after another looking for wheels, welders, etc. We got a couple fixed by
welding (not a good long term fix and found a couple of used replacements.
This part was a hassle but not dangerous: when the rim cracked the air
escaped and the tire went flat. Whoopee. When we got home we sold the lot
to a farmer for a hay wagon and bought all new 
wheels.


HERE'S THE BIG DEAL: We ran into SEVERAL abysmally ignorant tire shop
people who actually recommended putting a tube in the tire to "solve" the
problem!! In addition to not knowing the dangers of putting radials on old
style wheels, they failed to realize that putting a tube in the tire would
almost guarantee the crack would grow until the outer half of the rim blew
off - very possible killing someone!!  If you have an older coach with 16.5
or 19.5 wheels, MAKE SURE they are rated for RADIAL TIRES before changing.
Radial rated wheels will be so marked, either in plain English or with a
circled R. DO NOT ALLOW SOME TIRE SHOP DUDE TO CONVINCE YOU IT AIN'T A
PROBLEM. If the dang thing blows apart in the desert or the worst happens
and it goes on a busy street, he is gonna be long gone and you'll be
holdin'the bag!