tech
GO BIG!
Nobody gives much thought
to rear axle (or wheel) studs
and lug nuts. If the parts meet
the rules and the wheels don’t
fall off when you’re knee-
deep into the loud pedal, all
is well, so the reasoning goes.
But there are plenty of reasons to give
these components some extra thought,
not the least of which is safety.
Keep in mind the axle stud is the
component that actually transfers the
load from the powertrain to the wheel. The
wheel and tyre are what transfer that load
to the pavement.
The loads at the axle can be much larger
than you might think. Check out this basic
formula for load (and it is basic with a
capital B—it does not take tyre “hook”,
track conditions, or overall tyre dimensions
into consideration):
Engine Torque x Torque Converter
Multiplication x Transmission 1st Gear
Ratio x Rear Axle Ratio = Load
For a typical small block-powered hot rod,
the loads at the axles can actually exceed
10,000 foot-pounds of torque. Arguably,
there are two axles and ten studs to
distribute this load, but that’s still a bunch
of load to handle.
torque
Why rear axle studs and lug
nuts are more important
than you think
When it comes to rear axle studs, bigger is
definitely better.
A stock 7/16 inch (or equivalent metric size)
GM stud is inadequate for competition
use. These press-fit wheel studs have a
knurl to secure them to the axle shaft’s
wheel flange.
Rear axle studs for racing are threaded
all the way to the head, allowing the
studs to fully engage the backside of the
wheel flange. Axle studs for racing should
measure at least 1/2 inch at the drive
shoulder.
Really serious, high-horsepower cars
that use aftermarket axle shafts should
use axle studs made specifically for drag
racing.
Called drive studs, their shoulders’
outside diameter is the same as the
inside diameter of the lug holes in most
aluminium racing wheels. This ensures
the stud actually does the driving of the
wheel, not the lug nut.
You can get studs with drive shoulders up
to 11/16 inch in diameter with an equally
huge 5/8-18 inch axle thread (the portion
of the stud that screws into the axle). The
wheels are secured to the axles with open-
end flanged lug nuts with aluminium
washers.
STORY AND PHOTOS BY WAYNE SCRABA
The most important factor when selecting
the proper axle/drive stud is that the
driving portion is fully engaged into the
wheel.
The smooth “drive” segment dimension
of the stud needs to be slightly greater
than the combined thickness of the brake
adapter/drum and the thickness of the
wheel.
Washer thickness should be greater than
the stud shoulder extending past the
wheel. That means you must measure
the thickness of the wheel center, the
thickness of the brake hat (or drum), and
the thickness of a wheel spacer if one is
used to determine the stud length and
washer thickness.
Several manufacturers offer axle studs
of various lengths (the length of the
stud’s drive shoulder is what varies) and
washer thicknesses. For example, the
Summit Racing website lists more than 90
different 5/8 inch axle stud combinations,
along with hundreds of other types for
various drag race applications. Consider
these components overkill if you like, but
if bent or broken axle studs are plaguing
your car, you absolutely need them.