Introduction
The Renault UN1 transaxle is very popular with
various kit cars and replicas because it is (was?) easy to find, light,
convenient in design for
a mid engine installation (e.g. Lotus Esprit) and able to take much higher powers than
normally intended. If you use the car mostly on the road, with no sprinting, and
not using full chat in 5th and
drive with empathy for the machinery, you will probably
have no problems.
However, all things have limits and as owners of mid engine V8
powered replicas push power outputs above a mild level of tune with a
302 or 351 V8, the UN1/369 is right on the limit. The problem comes when
you want to strengthen the transmission as
it is basically at the heart of the car, change the gearbox and you may well
have to change the bell housing /adapter, clutch, gearchange linkage,
drive shafts, CV joints, gearbox mounts etc etc. The alternative gearboxes are not cheap,
and then you have to spend a lot of money and /or effort getting it to work!
This is what a 500 HP
small block Chevy did to the 5th gear end of the first motion shaft, snapped
like a carrot!
This was
the problem I had with my Lola T70 with the Small Block Chevy, In view
of the above difficulties in
changing the gearbox, I resolved to try to strengthen the Renault as being the
easiest solution, if it was possible.
Background and
weaknesses of the box.
I was however told by someone in the gearbox trade that it was
originally designed as a racing/sports car gearbox by ZG. Certainly, as I got
into gearbox design myself, I was struck by the similarity of the internal design to
some Porsche (ZG) boxes for example.
It actually has a
reputation as being a tough gearbox, any box that can take what the average van
driver dishes out must have something going for it!
But the nemesis of a
gearbox is torque, not horsepower. With the weight of an iron block V8 pressing
sticky slicks firmly to the ground, and 420 lbs.ft. on the input shaft, there is
nowhere to hide, and the weakest link will fail.
In the UN1 (the 369
is only very slightly different in small details), there are two main weaknesses
and one lesser one.
1)
the overhung 5th gear (like many 5 speed boxes, the 5 speed
UN1 is the earlier 4 speed version with an extra gear hung on the end!)
2)
the small diameter splined link between the clutch/input shaft, and the
first motion shaft
3)
Flexing of the first motion shaft between the two ball bearings due to
the thrust pushing them apart.
The good news is, all
of these weaknesses are in the first motion shaft. The output shaft on the
gearbox is massive and I’ve not heard of it failing, usually it is protected
by the first motion shaft failing first! So the obvious path was to try to beef
up the first motion/input shaft assembly.
Up-rating Gearbox
shafts.
What makes life
difficult as far as putting high torques through a gearbox, is that if you are
stuck with modest shaft diameters (which we are, because the input shaft has to
pass very close to the differential, in a tube) then only high strength steels
will enable the required strength to be achieved in the space available.
A widely used steel
for shafts is EN24, this is commonly used for good quality drive shafts. This is
a “tough” steel in that it cannot be hardened to the levels that EN36 can,
but can take some elastic deformation, whereas EN 36 is more stiff , it will
break rather than bend, although it takes a lot of force to do either.
The point is, that to
obtain the required strength in a gearbox shaft, you need
1)
the right alloy
2)
the right heat treatment
3)
the right surface treatment.
One possible solution
which has been tried (and failed) is to eliminate the weak joint between the two
pieces of the input shaft by welding.
The process of
welding, however you do it involves generating high enough temperatures to melt
the steel locally, this undoes the local heat treatment of the steel, both
through the bulk and the surface. In the weld itself , the crystalline/grain
structure of the steel which is a critical factor in the strength of the steel
is completely changed, almost certainly for the worst. If the welding technique
(e.g. MIG, or TIG) deposits new metal, then the composition of the weld metal
will also define the strength of
the final shaft, general purpose MIG wire is pretty soft and is unlikely to have
the required strength.
I considered various ways of joining the existing Renault shaft
to the clutch extension shaft to improve its strength and get rid of the
necked down weak area, but could not think of a way that got round the above
difficulties. In fact, I came to the conclusion that the whole assembly, even
without the weakness, had little strength to spare, which is why I went for the
one piece shaft.
The one piece
shaft
I increased
diameters wherever possible, eliminated the spline joining of the two pieces and
all possible undercuts, and
upgraded the steel, and the resulting shaft has given no trouble. I originally
made it from EN24 so that the shaft had a little “give” so there was some
resilience to absorb torque transients, but had to make another from EN36
because the shaft flexed too much between the two main bearings in the gearbox
first motion shaft, which spoiled the mesh between the two shafts, and also the
case hardening was less durable.
As you can see, the
new shaft (upper shaft in photo above) eliminates the necking where the clutch and first
motion shafts join, is larger in diameter wherever possible, and particularly so
at the (overhung) 5th gear end which is substantially thicker , and has no
undercut. the clutch spline is bigger as is first gear. You can see the
5th gear end in detail below. Note no stress raisers in the form of step changes in diameter or undercuts, and
larger in diameter throughout.
As you can see below, it is the step changes where
the trouble starts. these are two broken 5th gear ends from two different
gearboxes which have failed in a very similar way,. starting at the undercut.
The EN 36 shaft has
given no trouble, and has withstood full power and torque in all gears including
5th at the Nurburgring (185 mph and still going). Further kits
have been used in GT 40 replicas actively used in sprinting and hill climbs,
including some with tuned 351 Windsor’s and the only thing they have broken is
drive shafts!
More recently, the
Lotus Esprit community has discovered the upgrade ( I didn't realise the
Esprit used a (unique) version of the UN1 and now it has been used in tuned
Esprits as well.
An upgraded Esprit gearbox
Although the new bits
(the complete mod includes a higher ratio first and second
gear and a host of smaller items including the seals needed to rebuild a
box)) are not cheap, neither is any sensible alternative. Opinions vary, but the
older Porsche 911 boxes are not regarded as being particularly strong as far as
high torques are concerned (911’s have mostly small engines), and the stronger
boxes like the G50 are expensive.
Hewlands cost £7k
plus second-hand, and are noisy and baulky (they are straight cut dog boxes!) for a road car, ZF boxes are mostly
ex Pantera with low final drive ratios and cost £5K and up.
So I took the view
that up-rating the Renault box was the easiest route, and I still
think it’s the still the cheapest way to a practical quiet synchromesh
gearbox which has taken in my case 420 lbs.ft. and 500hp for 5 years now.
The only other change required is the clutch spline which is now the
larger Ford standard spline(1” dia, 23 teeth) for which clutch driven plates
are easier to obtain than the Renault 24 mm spline, and cheaper for Esprit
owners
For further
information, contact me directly so I can email the full technical details to
you.
Please note , for
Esprit owners, I did NOT supply any gearbox parts used on the 5th gear Esprit 200
mph project!
©
Derek Bell 2005
|
