Karl Vacek wrote:
> However, didn't you really mean to say that the center hole in a brake drum
> centers the drum, and the braking force is transmitted by a combination of
> friction between the drum, hub, and wheel and also the wheel studs, with a
> little help from the screws (which aren't even used on most American
> cars...)? Try leaving the screws off - it's no problem at all.
>
> Therefore, a large-hole drum on a small-center hub will probably stop the
> car, but will likely have some pulsation unless the mechanic gets really
> lucky in centering the drum on the hub. This is one place where the screws
> will actually help quite a bit - the countersunk chamfers will center the
> drum.
>
> Karl Vacek
> '66 TR4A - IRS
> '64 Amphicar
> '16 Ford T Touring
> '46 Piper J-3 Cub
Hi Karl,
I'm copying this to the list as it may be unclear for others as well.
If the center bore was mis-matched, and so were the wheel stud holes, it would
be
humanly impossible to properly center the drum upon reinstalling the wheel. As
you
say, even slight error would result in pedal pulsation, if not brake
overheating from
continually dragging for part of each rotation.
Actually, the center bore that fits over the axle nut is the same for both
drums, and
there is no problem with drums locating on center for either combination. The
difference is in the shank of the wheel stud. While both cars take the same
7/16 - 20
lug/stud, the shank on the solid axle cars is stepped to about 9/16 at the hub
and the
holes in the drum for the studs are correspondingly larger. With that in mind,
if you
were to install a TR-4 drum onto a TR-6, the drum would locate properly on the
hub's
center, but you'd have axial shift about the extra clearance in the stud holes.
In
this situation, the only positive location for the drum would be the retaining
screws. True, friction between the drum, hub, and wheel would and should
probably
keep it from shifting. But if it doesn't, the drum may slip, especially in a
panic
stop the forces involved are quite high. If the drum does slip, all the
panic-braking
force would be transmitted throught the locating screws and the they might
shear under
such stresses. This could result in potential damage to the drum or hub.
Of course, you could run the combo without the screws, and just allow the drum
to
shift on the clearance around the stud holes, but I sure wouldn't want to take
any
chances like that with my brakes.
Regards,
Brian Schlorff '61 TR-4 '64 TR-4 '72 TR-6 '79 Spit
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