February 2012 Newsletter - KDA132

Twenty Topics by Dr David Davis
(Reprinted from Præclarvm 12-1 with the permission of the Author and Editor)
The NZ Rolls-Royce & Bentley Club Magazine 11-6 has an article
about fitting 4 wheel hydraulic brakes to an early 20hp GA69.
The story of how it was done is fascinating. A later front axle
has been used with the internals replaced with wheel cylinders
hydraulically linked to the brake pedal. Two separate master
cylinders with slave cylinders operate the rear brakes through
the existing levers. The installation required a lot of meeting
problems as they arose including the lubrication arrangements
for the front axle. I wonder if the hydraulics would lock the rear
brakes especially in the wet which is the main objection to 2 wheel
brakes. I suppose it would be a matter of balance between the front
and back. The modification passed compulsory certification.
There are two other front brake conversions that I know of being
76A5 which has had motor bike disc brakes fitted to the front
which involved machining new hubs. I understand that the pads
wear quickly due to the weight of the car. The other conversion is
Jim Kelso’s GRK81 which now has vacuum assistance applied to
the brakes including the RR front axle fitted many years ago by
Patrick Kane White .GF8, has vacuum assistance applied to its 2
wheel brakes which work well in dry weather but readily locks the
wheels in the wet.
Early in my ownership of 42G1, I asked Bert Ward should I fit
An excuse to publish photographs of three “Twenties” active here:
(clockwise from above) Philip Eilenberg’s Hooper bodied GEN39;
Mulliner Weymann Saloon with Division GFN49, cared for by
John Ferguson; and GTM28, James Young Drop-head Coupé
from 1934 replacing the 1928 Melhuish Tourer body, owned by
Vicky Ellis
front brakes to the car. Bert’s response was that, properly adjusted,
2 wheel brakes were more than adequate. After I had driven two
Silver Ghosts with 2 wheel brakes namely 45NA and 56 AG both
of which were great, I appreciated that he was right.
Early cars with A steering, such as Jim’s, have a special problem
as the forward seating position necessitates downward pressure
on the brake pedal rather than outward leg pressure as is the case
with D. The seating position on the latter is further away from the
pedal under the lower raked steering wheel. Clearly, nowhere near
as much comfortable pressure can be applied to the pedal in the
case of A steering as can be in the case of D. I can understand, then,
why there is a perception that A steering cars have a problem.
There is no doubt that the fitting of front brakes to a Twenty,
involves assistance as the pressure required to operate 4 brakes
effectively, is beyond leg strength which is a point raised by Bert.
I have always been wary of the RR servo system as reversing can
be unpredictable as can ahead, unless holding the wheel firmly.
There is a full discussion on the RR servo system in Fundamentals
of Car Performance by Hives etc including these issues which
relate to adjustment. That book also comments on the differences
between 2 and 4 wheel brakes noting that 60% of braking in the
servo system is taken up by the rear brakes the performance of
which in both systems is directly related to the proportion of the
car’s weight over the rear wheels. The book points out that, in
the end, it is all about tyre adherence to the road surface. The
book notes that the greatest retardation achieved in tests was
20ft/second, squared and that the limit for personal comfort of the
passengers in the rear of the car was 12ft/second, squared.
The vacuum assistance fitted to Jim’s car is effective and I would
say is more predictable than the RR servo. Of course the engine
has to be running for it to work which is not the case with the RR
system. It is interesting to note that the Springfield Factory retrofitted
vacuum brakes to Silver Ghosts, and a Canadian owner of
both Rolls-Royce and vacuum types, commented to me that he
preferred the vacuum.
The fitting of front brakes to an early car involves other
considerations besides assistance including whether king posts
and struts should be added to the bottom of the chassis frame to
counter braking stresses and whether changes need to be made to
the front springs to counter windup under braking which affects
steering. There is a discussion of the latter in Fundamentals of Car
Performance which explains why 4 wheel braked Twentys have
longer front springs.
Looking back over my ownership of 42G1, as Paul Samuels
commented when he owned the car, the brakes were hopeless. The
issue was the brakes seeming to not work at all followed by lock
up. Replacing the heavy body helped matters but a significant
improvement resulted from mounting the spare wheel at the back
which stopped the wheels from locking up especially in the wet.
Curing a differential oil leak followed by a complete overhaul
including softer linings which have a more progressive action, has
resulted in effective brakes in which I have every confidence.
David G Davis 42G1 NSW
NZRR&BC Issue 12-2 16