MG Car Club of South Australia
Technical TopiCs
From..Bob Schapel
KNOCK-OFFS and WHEEL-NUTS
ever, it is more often a problem with
parallel shanked nuts as used in some
older mags, because they must have
slight side clearance which can lead to
slight movement. If there is any move-
ment, they could work loose. Knock-off
wheels tend to “work” slightly on their
taper set-up. A friend had a right-hand
hub and knock-off temporarily fitted to
the left side of his MGB. The knock-off
worked loose and the wheel fell off!
Another friend lost a left “mag” wheel on
his MGB. In the 70s, I borrowed a
trailer, checked the wheel-nuts and still
lost a left wheel. I later found that the
hubs had no spacers to compensate for
the absent brake drums. As a result,
the nuts tightening on the ends of the
threads instead of on the wheel, thus
allowing that critical slight movement
mentioned above. That incident initi-
ated my analysis of this issue and the
fitment of lock nuts on the left wheel of
my own trailer.
When I was a pit lane marshal at the
“Clipsal” and Grand Prix, I found the V8
Supercars interesting. Their single cen-
tre-lock nuts were male, so I assumed
their threads would be opposite to our
female knock-offs. I found that indeed,
many cars were set up that way. How-
ever, many others ran them the other
way round! I worked out that in their
case it did not matter, because the nuts
had very good safety clips to prevent
them from undoing. Without the clips, I
think ALL teams would have had prob-
lems. Some, because the nuts would
have worked loose, and others because
they would have self-tightened so much
that the rattle guns might not work at pit
stops. Because of the clips, I guess
W
hy is it so? Some older model
Chrysler cars, and many trucks,
have left-hand threads on the left wheel
nuts and right-hand threads on the right
wheel nuts. This is intended to prevent
the nuts from coming loose. If you have
ever seen a bolt-on wheel come loose
from a trailer or car, it was most likely a
left-hand wheel with right hand threads,
unless it came loose due to breakage.
Why then, do the knock-offs on our MGs
have the opposite threads? Knock-offs
have right-hand thread on the left side
and left-hand thread on the right side.
The reason is that conventional wheel
nuts have male tapers which fit inside
the taper on the wheel, whereas knock-
offs have female tapers which fit outside
the taper on the wheel centre. As the
wheel turns, and load is transferred
around the nut or knock-off, a type of
orbital action tries to turn the nut or
knockoff. If you roll an empty round jar,
with a smaller jar inside, along a bench,
the smaller jar turns faster than the lar-
ger jar. Similarly, when a wheel is turn-
ing, male tapered nuts are trying to turn
faster. If the wheel-nuts all have right
hand threads, that represents tightening
on the right hand side and loosening on
the left hand side Conversely, the larger
jar is turning slower, which represents
female knock-offs tending to self-tighten
irrespective of their side.
So, what about the safety of conven-
tional right-hand-thread nuts on the left
side of cars? It is not much of an issue
with tapered nuts which are done up
tightly, as long as there is no movement
between them and the wheel. (The two
jars described above will turn at the
same speed if they are a tight fit.) How-
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