How's Your Castor Rolling?
by Jim Lunson
got in on an interesting discussion last summer during a tech session
during the MG 2008 in Valley Forge, Pa. The session was about
different methods for lowering rubber bumper MGBs and installing roll
bars, but the talk ventured onto a subject about castor angle.
This was a subject I knew little about and had never paid any attention
to. This angle concerns the suspension and steering and works like
this. The front steering for MGs work around a large vertical pin
(called the king pin) that is the pivot for the turning of each front
tire and hub assembly as the steering wheel is rotated from left to
right. This king pin location is framed by the front cross member
and shock absorber. It is set not quite precisely vertical with
the top end sloping slightly toward the rear of the car. This
angle of slope is called the castor angle and on the B is about 8° from
This angle was
fixed during the design of the MGB (which copied the earlier A and TF
systems) and has no adjustment. It was set with the layout of the
front cross member. From the discussion we had at this session,
including several renowned MG experts, one of the major factors that
went into setting this angle on the car was the use of bias ply (non
radial) tires which were the norm of the day. Due to the physics
of the wheel assembly, the greater the castor angle, the more resistance
to turning there is in the steering system and a greater tendency of the
car to return to the straight ahead position.
According to the discussion, the old bias ply type
tires when put into a turn, tended to stay in that turned position.
This turned position set was much stronger than that exerted by the
steel belted radial tires used today. So the king pin castor angle
was tipped back to induce counter resistance so the force on the turned
steering wheel was able to overcome the tire tendency. Otherwise,
you would have hardly been able to straighten the steering wheel after
entering a turn. The fallout from this counter force was that the
steering was more difficult to put into a turn. This angle was
tried and tested during the MG design; the optimal angle was determined
and then fixed into all the parts that go with it, including the hub,
shock absorber, and cross member. End of story, no adjustment
provided, and handling was set forever.
Today, with the switch to radial tires that occurred
during the late 1970s to 80s, this design is no longer optimal.
Modern cars, designed around radial tire usage need a castor angle of
only about 2-3o. Our discussion during the tech session turned to
a new option that has been developed which allows for the reduction of
this angle on the MGs. This angle reduction is accomplished by the
use of two long shim plates under the four large bolts that connect the
front cross member to the frame. These shim plates are about 1/2"
thick on the front and taper to about 1/16" on the other end. When
inserted between the frame and the cross member on both sides of the
car, these shims tip the entire front cross member slightly rearward.
This tilt then reduces the castor angle of the king pin by about half in
relation to absolute vertical; a new approach to adjust the castor angle
to modern radial tires.
Two guys in this tech session had added these shims to
their cars and they both said you could not believe the difference in
steering effort required in the B. They said it was like power
steering, with the steering wheel able to be turned all the way to the
end using barely one finger. Handling and cornering were
tremendous. They really raved at the improvement. I was not
able to get up with either of them later to test drive and see for
myself, but they swore the difference was phenomenal. So, if you
are interested in upgrading the handling of the MG to take full
advantage of modern radial tire design, these shims may be the way to
go. Moss has them as well as several other shops.
When the MGRV-8 was made back around 1994, part of the
upgrade made to that car then was a completely redesigned front cross
member, including tube shocks, new steering rack, and also this reduced
castor angle. These assemblies are apparently available today in
the UK, fit right onto the old MGB frame, and also greatly improve the
steering, but are very pricy (over $4,000). The idea of
using shims under the front cross member is a new idea and will do the
job for less than $100. Interesting, and apparently they work.
Perhaps a nice upgrade to the MG.
One note of caution is critical if you add these
shims. For safety reasons, it is imperative that on all MGs, there
must be precise alignment between the upper steering column and the
pinion shaft from the steering rack unit. Failure to maintain this
alignment can cause stress in either the universal joint where the two
shafts are connected or to the brackets on the front cross member that
anchor the steering rack. Inducing this stress could lead to
fracture and failure of either of these parts. This would
result in total loss of steering control of the car, something you do
not wish to occur while driving along. You must pay attention to
this alignment as there is very little float in either piece. I
dont know the sequence of how the MG was assembled in the factory, but
somewhere during the process, every steering rack was adjusted with
small plate shims pop riveted to the cross member as necessary to
provide this proper alignment. There was even a special tool, (BL#
18G 1140) made to accomplish this alignment. Adding the castor
reducing shims noted above between the cross member and the frame to
change the castor angle alters this alignment so a readjustment of the
steering rack is required. Dont overlook this important safety
step if you add these shims; very critical.
Adding these shims is not for everyone as the steering
handling of the MG, especially the B is not bad as it is; part of its
longevity perhaps. And, in high speed racing applications, the
stiffer steering and strong tendency to return to the straight ahead
position are still desirable, but for the normal daily and recreational
driver where going around street corners is a common occurrence, this
modification is something to consider. Again, no first hand
experience yet, but according to the guys at the convention, it really
makes a difference in driving ease and pleasure. It brings the MG
design up to the standards used with the modern radial tires now found
on all our cars. Happy castoring.
(Editors note: The Castor Reduction Kit from
Brown & Gammons (B&G) includes six packing shims for the steering rack
in order to maintain proper column alignment. Also please note
that British Motor Heritage crossmembers already incorporate castor
reduction into their design and should NOT be fitted with a castor