|
11 July 2001 |
The steady decline over the past several years
of readily available hydrolastic displacer units has prompted an escalation in the
number of folk asking how to go about swapping from hydrolastic (‘wet’)
suspension systems to the far more wide spread ‘dry’ set-up. As in all things,
with enough skill, enterprise, facilities, and money – anything is possible.
Being all too ready to oblige in anyway I can, here’s the relevant details of
the most practical way of going about it for the majority. It’s not the
‘be-all-and-end-all’ definitive article on all the possible permutations, just
the simplest way out. And it’s nowhere near as difficult as most folk seem to
think it is.
Paddling about
in the past
Introduced to production Minis in 1964, the
system had actually been under development by the wizard Issigonis and gifted
engineer Alex Moulton for some years before hand. In fact Issigonis had wanted
this fitted to the Mini right from the word go. The powers that be denied him
this on the grounds of cost and the fact that they hadn’t managed to cure
persistent production problems culminating in a high percentage of the items
produced being faulty/failing in use and therefore rejected/scrapped. Alex
Moulton came to the rescue by coming up with the design for the rubber springs in a big hurry to ‘fill in’ for the
missing hydro units on the early production cars, then ended up being used on the
vast majority of Minis! The man being the man however, Issigonis bludgeoned the
hierarchy until he finally got his way. His primary reasons for applying the
hydro system to the Mini (at further/considerable expense – much to the
hierarchy’s chagrin) was to eliminate the choppy ride given by the rubber
springs and pitch under acceleration/braking. Searching for the ever elusive
‘Rolls Royce’ ride quality.
Unfortunately the pitching problem worsened, particularly
under severe use of the go/stop pedals. Fully laden, rear end sag caused
complaints by low flying aircraft – German authorities taking the severest line
by banning hydro cars from sale in
Main frame
Without a doubt, the easiest way to do the
conversion is using the complete under-pinnings from a dry suspended Min that’s
had it’s usefulness terminated - MOT failure, excessively dissolved bodywork,
maybe crash damaged/insurance write off. Anything’ll do. Van, estate, saloon.
Preferably the pre rubber mounted type - details ‘why’ below. New front
subframes are a fortune, so obtain ‘used’ unless excessively rich! Make sure
you carry out a careful inspection to make sure it’s in good nick. Particularly
around the tie rod mounting lugs and rear ‘legs’. Especially if it’s from a
crash damaged car. Measure the thing up for 'square' too.
Starting at the sharp end, whichever way you go
engine removal will make things a whole lot easier. I know some folk advocate
removing the entire engine/subframe/suspension assembly in one go by lifting
the body over the top of it all - having disconnected everything first,
naturally. I did this once, never again. You need an army of companions to man
handle everything about. A right pain.
Now, it’s entirely possible to modify the hydro
front subframe to accept the rubber springs and other necessary bits and
pieces. The displacer unit is seated in the tower in the same way as the cone,
the difference being the locating ring's size/shape. The displacer sits inside
its seating ring, the cone outside. The diameters of these differ
too, the hydro being bigger, and have lugs pressed into it. To get the rubber
spring to fit the hydro ‘frame, these lugs need to be bent outwards. The rubber
spring then locates inside the ring. Sounds easy, eh? Achieving this is
accompanied by all kinds of 'blue' air and skinned knuckles. More often than
not, eventual submission and fitment of a dry ‘frame!! It’s all down to access
- or lack of it (as in ‘severe’). There are ways round it, but we’re supposed
to be doing this the easy way, remember? Then you need to drill holes in the
tower outer ‘elbows’ to take bump stops. Use the later single stud fixing as
this eases the whole drilling/fitting thing.
So it’s a dry subframe then. The easiest is to
use the pre 1976, non-rubber mounted type, as this bolts straight in using the
fixings from the donor car except it uses bolts in the towers rather than the
studs of the hydro one. Floor pan and front panel mounting holes are all in the
same places. Using the later rubber mounted one has complications.
The post 1976 shells have a stiffening tube
welded into the bulkhead to prevent it from being deformed/crushed by the bolt.
This doesn’t happen when doing the big bolt up. When using the standard rubber
tower/bolt mounts, the design has the bolt bottoming out on its thread to
ensure ‘space’ for the rubber mounts to work in. The deformation occurs in use,
as the support tubes for the original twin bolt/stud fittings aren’t in the
right places to be effective. Using solid mounts actually slightly improves
things a bit - although causing slight deformation when doing the bolts up, the
reduction in movement in operation reduces further deformation markedly. See
separate article for bush and mounting options.
When removing the ‘frame assembly from the
donor, make sure you unbolt and keep the top damper mounting brackets. They’re
held to the body by four 7/16”-AF bolts wound into captive threads. These
captive threads are on every shell, including the hydro
cars - you may have to dig through the cack and poop to find them though!
Bringing
up the rear.
Again, as with the front, the hydro rear ‘frame
can be modded to take the rubber springs. Same mods, but very, very much
easier. However I don’t advise this unless the Min’s used solely for normal
road use, carrying two people in the front, minimal luggage, and totally
standard wheels/tyres and standard ride height only. Why? Coz that curious
metal cone shape on the dry type acts as a variable spring platform - progressively
increasing the spring rate as more load is applied. Carrying more load than the
minimal amount mentioned above will have the tyres bashing the wheel arches.
Trust me in this, that’s all the explanation you’re going to get, as it’s a bit
complicated (not enough room here for full dissertation).
I’ve never taken a rear ‘frame out of a car and
found it useable, so I don’t see why any of you should be any different!
Useable, ‘as new’ condition, second-hand ones are rare. Considering the
relatively low cost against the severe aggro of fitting a rear ‘frame - buy a
new one. Treat it with some decent rust proofing gear, and have no worries. The
fitting holes in the shell are all the same, so even the later post 1976 mounts
will fit OK. Just remember when buying service parts (i.e. rubber mounts) which
ones you’ve used.
Componentry
Pointy end first again. If you’ve persevered
with mod-ing the hydro ‘frame (give that man a medal!) or have used just a dry
‘frame and mounts with no bits, it is possible to use all the hydro stuff plus
the necessary additions - rubber springs, ally trumpets, dampers, top damper
mounts and lower damper mounting pins. New knuckles would be a good idea too
(those on the suspension, not your hand! Although…), as they are very cheap. Two
of each needed of course! The only flies in the ointment here are the top arms.
Using the hydro items will give a stiffer ride as they’ve got a slightly lower
working ratio (knuckle seat is a bit further out, nearer the swivel pin). Fine
for a competition car, not so funny for a road car. Plus you need to drill the
bump-stop mounting bolt hole out to 3/8” for the lower damper pin to fit. And
this is exceptionally difficult - buy a bucket load of drill bits,
those arms are real hard! The top damper mounts bolt straight to the shell
using the previously mentioned captive threads. The top pair are easily found
since the metal block with the captive threads in is visible in the corners
next to the tower mounting bolt heads. The other lower pair are hidden behind
the inner wing near the subframe tower top - so it would be prudent to uncover
these and clean/prepare the threads for use whilst the front 'frame is out of
the car if that's the way you are doing the conversion. Bump stops need to be
fitted to the subframe where you’ve drilled the necessary holes. I actually
prefer the hydro bump stops, so drill the holes out in the mounting bracket to
3/8” and use cap head (Allen) bolts to mount the dampers. These go through the
bump stop and the damper. Getting easier, isn't it?
Now the back end. As with the front you’ll need
rubber springs, ally trumpets, dampers, suggest new knuckles for what they
cost, new radius arm to wheel cylinder brake pipes (hydro’s are shorter as they
fit under the arm instead of on top), and last but not least - hand brake
cables. Two of each once more. Now, to avoid all sorts of hassle, it really is strongly
advisable to use dry-type radius arms. The hydro design will see your Min fall
flat on its arse unless you get some severely lengthened rear trumpets. Then
there’s nowhere to bolt the dampers to. The dry type have a pin sticking out to
take them, the hydro doesn’t. This damper mounting pin is part of the rear stub
axle, swapping these can be an absolute nightmare and they’re not cheap either.
It’s cheaper in the long run to buy a pair of completely refurbished dry arms.
Honestly.
Other
useful stuff.
Whilst you’re doing this, it’d be a good idea to
give the entire suspension a good overhaul. The components needed to do this
are very cheap, even if you go for up-rated stuff.
At the front, replace bottom arm bushes (slight
snag, keep reading), tie-rod bushes, and check the top-arm shafts and bearings.
These consist of two caged needle roller bearings, one pressed into each end of
the top arm. They’re subject to seizure/wear as most folk completely neglect
greasing them. Rebuild kits are reasonably priced. Once done, don’t forget to
grease them regularly in future!! Bottom arm bush snag thingy - before buying
parts for them, you need to check which type they are, The early arms used a
top hat type bush, later ones a tapered, metal-sleeved type. To identify, peer
into the holes in the bottom arm. If the walls are tapered, use the later style
bush. Parallel sides indicate the earlier type. Unfortunately you can only get
up-rated bushes for the later tapered type. Using up-rated bushes improves
stability - particularly the tie-rod ones. These reduce weaving under braking,
and fractionally help to reduce roll (long story!). For material options, see
separate article.
The radius arms at the rear are the biggest
problem. If you use recon ones, no worries, but if you use the existing ones,
overhaul the pins and bushes. Inexpensive to buy but a bit of a pain to fit
since the outer bronze bush needs reaming after fitment. Most engineering
companies can do this, but again - recon arms are really cheap when bought on
exchange. Otherwise it’s just the ‘frame mounting rubbers that need attention.
Lastly, the only specialist tool you’ll need to
do the job is the rubber spring compressor. Cheap enough to warrant the
investment as you’ll need it again in the future.
Useful part numbers:
|
GSV1118MS |
Complete
knuckle joint assembly - 4 per car |
|
FAM2764 |
Single stud
mounting front bump stop - 2 needed |
|
2A4332 |
Twin stud
mounting front bump stop - 2 needed |
|
2A4267 |
Front top arm
rebound buffer - 2 needed |
|
C-AJJ3361 |
Up-rated top-arm
damper mounting pin (use especially when up-rated dampers are fitted) - 2
needed |
|
2A4337 |
Standard
Rover top-arm damper mounting pin - 2 needed |
|
31G1155 |
Standard
tie-rod bushes - 4 needed |
|
2A4294 |
'Top hat' type
early bottom arm bush - 4 needed |
|
21A1882 |
Metal-insert
type later bottom arm bush - 4 needed |
|
TRA33 |
Recon 'dry'
rear radius arm - right hand |
|
TRA32 |
Recon 'dry'
rear radius arm - left hand |
|
GSV1125 |
Rear radius
arm repair kit - 2 needed |
|
2A7351 |
Radius arm
stub axle with damper mounting pin - R/H |
|
2A7364 |
Radius arm
stub axle with damper mounting pin - L/H |