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TIRE SIZE
In theory, tires installed by trailer manufacturers should be capable
of carrying at least the maximum load the trailer is rated to carry
(its gross vehicle weight rating, or gvwr). If you suspect that your
tires are overloaded - indicated by tire failures or improper wear
(which also may be caused by improper suspension alignment or a bent
axle) - and have decided to upgrade with higher-rated tires, you'll
need to know how much weight your tires are carrying in order to select
tires with appropriate maximum load capacity, and this requires a trip
to a public scale. Weighing the trailer is required to see if the manufacturer
has made the right choice, or if your collection of antique books has
pushed weight beyond what the manufacturer had intended. If so, selection
of larger tires may seem apparent, but the higher-capacity tires should
not lead to overloading of wheels (check ratings on inside of wheel
rims) or axles (ratings for which are posted on identification stickers
on trailer exteriors, usually toward the front). If an overload potential
exists, you're better off reducing weight in the trailer so you can
retain the original tire size/load rating. In a tire-size change, wheel
suitability must be checked, and clearance in wheel housings must be
adequate. Hitch adjustment may be necessary if the replacement tire
is larger in diameter than the original.
All tires should be the same size. Unmatched tire sizes guarantees uneven
tire loading, which may mean at least one overloaded tire. The trailer's
dynamics and stability can be adversely Affected by mismatched tires
as well. Likewise, mixing bias-ply with radial-- ply, for example, may
lead to handling problems. Of course, there are contingencies where the
only spare may be a mismatch. If at all possible, wait for the proper
replacement tire to be brought to the trailer. Failing that, use a mismatched
spare like the mini-spare in a car. Limit speed to 35 mph or less, and
keep the mileage as low as possible.
In most cases, replacement tires can be chosen based on the load rating
of the original tire (listed on the sidewall) if it provided good service
and if weight readings indicate that overload is not a factor. If you
suspect that your tires are overloaded — indicated by tire failures
or improper wear (which also may be caused by improper suspension alignment
or a bent axle) — and have decided to upgrade with higher-rated
tires, you'll need to know how much weight your tires are carrying in
order to select tires with appropriate maximum load capacity, and this
requires a trip to a public scale. It will not only reveal any possible
overload, but weight bias to one side can be measured. Weigh your fully
loaded trailer axle-by-axle, and if possible, side-to-side. This is easily
done on truck scales by simply rolling onto the platform one axle at
a time and then doing a little subtraction from the total weight. Make
sure the aprons of the scale are level with the scale. Side-to-side measurements
are possible when there is sufficient room on the side of the scales
to run one side of the truck and trailer on the scale at a time. If you're
lucky, you may encounter a segmented platform scale, where the scale
is divided to allow axle measurements at the same time. Going in the
other direction, a single-axle scale can only measure one axle at a time,
but a little addition can furnish any combination of axle or total weights
you'd like to see.
If it turns out that the trailer is overloaded, you can face the music
and remove some weight, or choose to increase tire capacity while taking
care not to overload wheels or axles.
Next - Tire Wear & Replacement
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