105mm howitzer is a great kit, but their choice of wheels and tires isn't
typical for a WW II weapon. (Not to mention that the tires are slightly undersized.)
Most photos of howitzers during the war show them mounting highway or civilian
tread tires on "six-hole" combat wheels or "two-hole" standard
wheels. These are actually two different tire/wheel sizes. The combat wheels are
9.00-20 while the standard wheels are 7.50-24.
is plenty of information out there on the 9.00-20 size but there's not much on
the pre-war 7.50-24. Only two carriages used the 7.50-24, and while 105mm howitzers
with this size are rare, the M2A3 carriage for the 75mm M1897 gun is relatively
common, with a number being preserved at veterans' halls across the U.S. From
these we can come up with a goodbut not definitiveset of dimensions
for this wheel and tire size.
problem with this technique is that the equipment is suffering from 60+ years
exposure to the elements. The tires are dry-rotted, deflated, and probably shrunken
somewhat form their new, inflated sizes. The wheels have corroded and swollen
which makes them somewhat out of round and distorted. Also, because it is impossible
to remove the wheel from the carriage, direct measurements across diameters are
blocked by the axle and brakes.
wheel is a two piece type, size 4.33R-24. The outer flange on the convex face
is a split lock ring that snaps into a groove on the disk/rim assembly. The rim
is roughly L-shaped in cross-section, forming the inside flange and base of the
rim. The disk is a somewhat bowl-shaped pressing riveted to the inside of the
rim. Its thickness tapers from about 5/16 in the center to about 3/32 at the outside.
The section contacting the carriage is flat and raised approximately ½
inch above the bowl contour. Two holes are punched in the disk; onewhich
is slit to the outsideis for the valve stem, the other for balance. The
holes are a combination of an elliptical shape and two straight sides, giving
something of a teardrop shape.
of tires have been seen with markedly different tread styles. Surprisingly, they
are both the brand and model! On the first, there is a pattern of lugs starting
at the maximum section width and ending at the shoulder with grooves crossing
to form an X, and three circumferential grooves at the center of the tread. There
are 71 identical lugs on each side. The second type has lugs start about a third
of the way up the sidewall but with a similar X groove configuration on the tread
face. The Xs reach farther in and there is no circumferential grooving. Also,
there are only 51 lugs on each side. Both tires are the same typeSeiberling
Special Serviceso the design must have changed during production.
7.50-24 tires are produced with a 40.3 inch nominal outside diameter. The six
surviving tires measured around 39.5 inches outside diameter. This difference
may be a result of the condition of the survivors or an indication that wartime
tires were slightly smaller. For modeling purposes, a 40 inch diameter seems to
be a reasonable value until better information becomes available.
on the Drawings
large section view was developed after some trial and error and measurements of
several wheels. I ended up using a contour gage to transfer the wheel shape to
coordinate paper and plotting the coordinates in CAD. I then did some smoothing
and curve fitting to get the best match using likely dimensions. It is still not
exact on the inside of the rim where the lockring fits into the main rim, but
this contour is practically invisible at 1/1, let alone at 1/35.
first sketch shows the two tread patterns, one on each side of the tire. Obviously
the real tire would have the same pattern on both sides.