Welcome to Wheel Anatomy 101. Today we will be reviewing the major
structural aspects of automotive wheels, focusing on the outboard, or
structural face of the wheel. Students, if you will all take your seats,
we can begin the class.
The outboard face is the part of the wheel you can see when it is
bolted onto the car. We often refer to it as the “cosmetic face” but it
is also the structural face of the wheel, since the other side is
essentially required to be an open cylinder. This makes the outer face
less directly vulnerable to impact damage, since it is simply easier to
bend the open cylinder than the structure, but it can also make the
damage that does occur a lot worse.
To see standard wheel diagrams, click here, and here.
You may find it useful to right-click and open the links in new tabs in
order to refer to the diagrams while class is proceeding.
Center Bore
Structurally, the empty space inside the center bore is one of the
most important points on the wheel. This hole fits over the end of the
axle when the wheel is bolted on. It is this fit between the axle seat
and the center bore that truly holds the weight of the car, as lugnuts
only serve to keep the wheel on the axle. For this reason, OEM wheels
are made to fit closely on the axle seats of their designated cars. When
buying aftermarket rims, care must be taken to ensure that the center
bore is the same or larger than the OEM size - large enough to fit over
the axle. Most correct aftermarket wheels will have center bores that
are larger than the OEM size, and so the gap between must be filled by
“hub-centric spacers” to avoid damaging both wheels and lugnuts.
Plate
Around the center bore there is generally a substantial piece of
metal interrupted only by the bolt holes. We call this the plate. The
plate is the core of the wheel, the point of contact to the axle seat,
the lug bolts and the lateral surface of the rotor. Everything else on
the wheel is connected back to the plate.
Spokes
In essence, the spokes are the structures between the plate and the
outer edge of the wheel. They are designed to tie the wheel together,
support the outer edge and resist impacts. Spoke designs vary wildly,
from the classic 5-spoke patterns to intricately overlapping multiple
“Y”-spoke extravaganzas. It's important to note that the strength and
damage resistance of spoke designs also vary, because if a spoke gets
cracked by an impact the nature of the structural relationship is such
that attempting to repair it by welding would be unwise and possibly
dangerous.
Dish
Although it also refers to the outer part of a 3-piece wheel, the
dish is generally thought of as that portion of the wheel that comes out
beyond the spokes. A wheel where the spokes are sunk inches below the
lip is a “deep-dish wheel.”
Deep-dish wheels are mostly made for looks, with the extra space being
used to showcase a polish or other nice finish. However, the deeper the
dish, the more vulnerable the face of the wheel is to impact damage, as
that outer rim is hanging out in space. The more distance from the
spokes, the more leverage an impact has to bend that outer rim,
or in the worst-case fold the dish against a spoke and crack it. This
kind of crack is also not safe to repair, since the repair is inevitably
weaker than the original and can fail catastrophically.
Bolt Circle
The bolt circle is the circle described by the centers of the lug
bolts. It's diameter is inexplicably called the Bolt Circle Diameter, or
BCD. The number of bolts plus the BCD comprises the bolt pattern, so
that 5 lug bolts at a 4.5 inch BCD can be described as a 5x4.5” bolt
pattern. Bolt patterns vary between car makers, sometimes even between
model lines. For example, most BMW wheels are 5x120mm except some very
early 4x100mm models, while almost all Mercedes wheels are 5x112mm,
which is why you can't crossfit wheels from one to the other.
Valve Stem
Somewhere on the wheel a small hole must be drilled for a valve
stem, that universal mechanism by which we fill our tires with air. Just
that small hole will often make one side of the wheel lighter than the
other side – enough so that a good spin balancer will often have to
compensate for it. Valve stems range from the good old-fashioned snap-in
rubber stems to fancy metal stems with rubber gasket seals to the
current mandated explosion of TPMS modules with valve stems on them.
This completes our module on the structural aspects of automotive
wheels. Thank you for your attention, and please join us next time for
Wheel Anatomy 201, which will concentrate on the outer barrel and energy
transfer points of the wheel.
credit goes to:
Sean Phillips Tires and Wheels Guide
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