|
Gearbox
- Up-rating diffs, FDs and ancillaries |
24 October 2000 |
Terminology
-
|
FD |
Final
Drive/diff ratio |
|
LSD |
Limited
Slip Differential |
|
Drop
gears |
transfer
gears (primary, idler, and input gears) |
There’s a
good selection of straight-cut final FDs available. Examine the FD table, and
using information from 'Gearbox
- Final dives, standard' and 'Gearbox - Formulae for car speed,
etc.' you can assess which would best suit your usage. Bear in mind
that they’re noisy, make sure you select one that’ll fit your diff unit, and
also consider that using drop gears will allow fine-tuning of the ratio where
necessary. See 'Gearbox -
Up-rating drop gears' for more info.
STRAIGHT
|
NUMBER
OF TEETH ON GEARS |
RATIO |
FITMENT
AVAILABLE |
|
|
|
CROWN WHEEL |
PINION |
|
(LSD is
specific fitment) |
|
|
52 |
15 |
3.44 |
Std and twin pin, NOT LSD |
|
|
64 |
17 |
3.76 |
Std and
twin pin, NOT LSD |
|
|
55 |
14 |
3.9 |
Std and
twin pin, NOT LSD |
|
|
53 |
13 |
4.08 |
Std,
twin pin, and LSD |
|
|
55 |
13 |
4.23 |
Std,
twin pin, and LSD |
|
|
56 |
13 |
4.31 |
Std,
twin pin, and LSD |
|
|
64 |
14 |
4.57* |
Std and
twin pin, NOT LSD |
|
|
56 |
12 |
4.67 |
Std,
twin pin, and LSD |
|
NOTE; All above are actually ‘semi-helical’ with the exception of ‘*’.
The
standard diff unit’s componentry falls well short in the performance stakes. As
an absolute minimum you should fit an up-rated diff-pin - whether this is
because your racing regs don’t allow alternatives, or merely for the road -
along with new planet-wheels and thrust washers. If at all possible, use the
later diff-cage that has cut-outs next to the planet-wheel seats that take the
later 'locating tab'-type thrust washers. This massively reduces diff wear of
all types by stopping the thrust washers spinning. In the old-style diff these
used to wear away very quickly, further exacerbating the planet-wheel/diff-pin
wear problems and substantial irrevocable damage to the diff-cage. It is
entirely possible to modify an early-type diff-cage to use the later thrust
washers, but really only practical if there's a later one to hand to take dimensions
from, and the diff-cage isn't already completely goosed. The wear is obvious to
the eye - the planet-wheel seat should be smooth and not at all recessed.
Unfortunately
this doesn’t guarantee complete reliability - even in a road car. The
planet-wheels have a seemingly very broad manufacturing tolerance not really
suitable for spirited driving, irrespective of power output. To counter this
problem, Mini Spares/Mania also supply a special 'bushed' planet-wheel/diff-pin
kit that massively reduces these tolerance/wear problems by the fitment of
bronze bushes into modified planet-wheels. The bushes are precision reamed to
give a proper fit on the diff-pin, greatly reducing twisting of the
planet-wheels in use. The bushes also improve lubricity/reduced friction. The
kit includes the competition diff-pin and new thrust washers.
Twin
cross-pin diffs are the only real solution to long-term reliability for an
'open' diff. As the title infers, the unit has two diff-pins. Consequently it
also has four planet-wheels. These two factors combined with proper, exact
machining during manufacture make them practically indestructible - assuming
tightly controlled tolerances, material spec, and heat treatments are adhered
to as is the case for the superlative 'Trannex' unit. An absolute must for any
Mini with anything like a whiff of increased power output, or/and where
spirited driving is envisaged. Not particularly cheap, but a sight cheaper than
an exploded diff and the consequent damage it’ll inflict!!
The
ultimate is an ‘LSD’. Its title says it all. It limits the amount of slip
allowed before it engages, providing positive drive to enable progress! They
are generally referred to by the mis-informed as ‘
Ancillary components
It's
every-bit as important to up-rate the collection of components that combine
with the main gears to make up a whole and reliable gearbox. Mini Spares have
poured substantial resources in to researching what needs doing and developing
suitable components to make you life easier, and your Mini happier.
Genuine
Rover balk rings are quite satisfactory for road use, and limited weekend,
short distance racing. Circuit racing soon eats them up. Main problem is the
material and manufacturing spec. They’re made by a process called ‘sintering’
that essentially means metal powder is pressed and bonded then baked in extreme
temperature. An unfortunate by-product is brittleness. Wear rate is OK, but
they break when used in anger by those less-than-sympathetic to the cars
mechanicals. When used in sports other than the like of sprints/hill-climbs,
it’s necessary to replace these every couple of events - approximately 100
racing miles - dependent on mechanically sympathetic level of user! To wit
there are special manganese-bronze replacements. These need replacing at least
once a year as they still wear, BUT are far less prone to breakage - thus
preserving gears/dog-teeth/selector forks for longer periods.
Standard
or cheap replacement layshafts are simply not up to the punishment of
protracted high rpm use. The extremely hard chromed finish of the needle roller
bearings used soon eats into the cheap/low-spec/heat-treated/poorly
surface-ground materials used. Again, good, high quality versions are available
from Mini Spares/Mania at sensible prices for all gearbox types. Essentially
they are manufactured using higher-grade materials, far closer tolerances, more
exacting heat-treatments, and finer ground finish.
For those
struggling to fit LSD units or Hardy-Spicer-type couplings to rod-change
gearboxes, help is at hand! The problems are two-fold. The diff output shafts
are larger in outside diameter than pot-joints, and there’s the selector detent
plunger/spring to cater for. Specifically manufactured side plates are
available from Mini Spares/Mania to sort both problems in one go. They are
basically the original S side plate cast with the necessary detent lug on. Consequently
this means the original S side plates are also available.
And last
but not least - whatever you use your Mini for, ALWAYS fit a centre-oil pick-up
pipe to maintain oil supply when cornering hard!
Part
numbers:
|
CBTA166 |
Competition, higrade diff pin for standard
diff |
|
C-BTA167 |
Bushed planet wheel kit |
|
DAM6624 |
Standard planet wheels - 2 needed |
|
BTA101 |
Diff output shaft thrust washers - 2 needed |
|
2A7062 |
Early, non-locating tab-type planet wheel thrust
washers - 2 needed |
|
DAM5071 |
Later, locating tab-type planet wheel
thrust washers - 2 needed |
|
DAM6027 |
Later, locating tab-type planet wheel
thrust washer diff cage |
|
RPS1418 |
Diff-pin locating roll-pin |
|
C-AJJ3385 |
Original and best Trannex cross-pin diff
unit |
|
C-22A1731 |
Hi-grade competition spec layshaft for
3-syncro gearboxes |
|
C-22A1738 |
Hi-grade competition spec layshaft for
4-syncro pre-A+ gearboxes |
|
C-22A1739 |
Hi-grade competition spec layshaft for 4-syncro
A+ gearboxes |
|
C-22A1740 |
Competition manganese-bronze balk rings |
|
22G419 |
S-type diff side plates |
|
22G420 |
Rod-change diff side plates to take
Hardy-Spicer-type output shafts |
|
AHU1082 |
Output shaft seal for 22G419 and 22G420 side
plates |
|
C-AHT54 |
Centre-oil pick-up pipe |