v8kid
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| posted on 12/8/07 at 06:50 PM |
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Flipping the UN1
This is a short account of how to reverse the CWP in a Renault UN1 gearbox.
In its standard form the UN1 gearbox is used in a front wheel drive car with a longitudinal engine ahead of the gearbox. This makes the box ideal for
mid engined cars where its high torque capacity often mates it to V8 engines.
The UN1 comes from saloons or MPV’s where there is a need for medium to high ground clearance and as a consequence the output shafts to the wheels are
lower than the crankshaft centreline – to make sure the sump does not bottom out.
For a sports vehicle we want to go quickly and corner quickly too. In order to corner quickly the centre of gravity has to be as low as possible and
we need to do everything possible to lower it. That means that the engine has to be as low as can be, usually but not always dry sumped, and the
gearbox has to have its output shafts at or (at rest) slightly higher than the wheel centres.
So the gearbox output shafts have to be higher than the crank centre line – the exact opposite of the UN1. If in doubt check out serious sports cars
and sports racers – Hewland has a good site illustrating this.
Easy enough just rotate the gearbox 180 degrees around the crank centre line and hey presto! The output shafts are now above the crank centre line.
Only trouble is we now have 5 speeds in reverse and one very low forward one.
What we need to do is transfer the crown wheel to the other side of the pinion in the diff. Here is where the first bit of luck comes in Renault had
to do exactly the same with the GTA ‘cos they decided to hang the engine out the back for some reason (packaging I believe).
Despite all the stories you may read, from those trying to sell their particular wares, the UN1 box in all its versions uses a common set of main
casings and mixes and matches the internals with saloons and commercial vehicles. Even the gears from its predecessor the 369 are interchangeable.
First get a copy of the manual on line, those nice chaps at the alpine club host a version.
We have to ensure that there is enough backlash on the diff CWP or in a very short time indeed they will self destroy. When the diff is flipped the
backlash is adjusted by shims under the taper bearing opposite the crown wheel. Surprisingly the bearing can be removed effortlessly by easing it off
with a couple of tyre levers. I checked with Chris Cole of Mach 1 transmissions and this is the norm.
Chris tells me he usually starts with a 1mm shim, assembles the box, including torquing up the big bolts either side of the first motion shaft and
measures the backlash. It’s a matter of trial and error from then on until you get it within limits.
But first you have to get the shims! I had no luck getting any – not helped by the postal strikes at the time and ended up buying a pack of assorted
shims and cutting my own. The shim material cuts easily with aviation snips and can be filed nice and smooth so there are no protrusions.
I got it right first time with a 0.4mm shim which gave me 6 thou backlash (4.5 to 11 thou or .12 to .28mm).
If you don’t have a dial gauge and holder – get one from eBay. Chris Cole measures backlash by the sound it makes – a faint click he says, but he’s
been doing it for years and a tenner for a dial gauge is a hell of a lot cheaper than a CWP.
There are other issues with running the box upside down. The breather has to be blocked off and a new one drilled opposite it. The input shaft seal
has a drain back to the box which will now be ineffective, so make sure the seal is renewed. The 5th gear has a modification to prevent oil starvation
which will also be ineffective (it works by gravity) but the good news is that the GTA is exactly the same and they don’t appear to suffer from any
long term problems.
That’s it really it was as simple as it sounds. It took me a few weekends to suss out, clean, dismantle and rebuild I only wish the twin plate clutch
I used was as easy.
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zilspeed
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| posted on 12/8/07 at 09:23 PM |
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You'll be ready to hit the startline soon then David...
No ? 
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RazMan
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| posted on 12/8/07 at 09:46 PM |
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Always wondered what was involved - did you take any pictures during the flip?
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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v8kid
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| posted on 13/8/07 at 06:53 AM |
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Sorry I should have taken some pics - perhaps next time. Still not ready for the start line - SVA next maybe geta couple of sprints in at the tail end
of the year
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thomas4age
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| posted on 15/8/07 at 07:23 AM |
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Kid,
in one word, Thanx!
Grz Thomas
If Lucas made guns, Wars wouldn't start either.
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Simon S
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| posted on 28/8/07 at 06:26 PM |
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Flipping UN1
V8Kid,
The only thing that worries me is your statement '5th gear has a modification to prevent oil starvation which will also be ineffective (it works
by gravity) but the good news is that the GTA is exactly the same and they don’t appear to suffer from any long term problems. '
I was under the impression that the GTA installation reversed the diff direction but did not run the 'box casing up-side-down, and if this is
the case then we have a potential problem with 5th gear lube.
Are you sure the GTA casing is flipped over, or is it just the diff?
If I can get some idea of the internal lay-out then I can ask some gearbox design people I work with how this could be overcome. It may come down to
adding a small lube pump as a solution often employed on some production cars, or running with more oil and and adding a breather using the former
drain hole now located at the top of the casing.
Is the 5th gear lube mod. something found on UN1 or 369 as well?
Cheers,
Simon
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v8kid
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| posted on 31/8/07 at 08:38 AM |
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Ha! You spotted the deliberate mistake! To be honest I missed that - time will tell we have all got to take a risk in life and in my case its £2k of
Quaiffe gears.
Ah well.
The 5 th gear mod is exclusive to later UN1's and was not carried out to earlier boxes including 369's. It comptises a little plastic cup
which gathers the oil mist and feeds it into a hole drilled in the end of the first motion shaft which in turn is cross drilled to feed it into the
5th gear. There are no oil pumps in the UN1 it operates in a oil/air emulsion
I will do some phoning around and find out after how many miles the problem raises its head and wether it is power related. Ta for pointing it out
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v8kid
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| posted on 31/8/07 at 10:05 AM |
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On further investigation I've decided to change my name to Lucky Jim.
The problem with the 5th gear seems to be high milage related although it is aggrivated by power.
Chaps who have used the same quaiffe gearset in the normal orientation are transmitting 500bhp + (claimed). So there is not a issue with power for
most of us only lube.
The gear that was shearing was the one on the first motion shaft with all the synchroniser gubbins attached which as the box is now inverted is at the
bottom of the box. i.e. its immersed in oil.
A further note on interchancability the 5th gears from the UN5 box also fit the UN1 which is really handy as they make a set of 38x33 gears in one
version. I have ended up with a nicely spaced set of gears at 2.66-1, 1.75-1, 1.38-1, 0.96-1 and 0.87-1 coupled to a rather highish 9x35 diff for a
lazy v8 that gives me through the gears 37,60,92,131,146. Hopefully if I can make the engine torquey (is that a word?) enough 3rd and 4th will suffice
once its rolling.
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Simon S
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| posted on 31/8/07 at 06:43 PM |
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All Misty
Thanks V8Kid,
I am using a 369 with (ultimately when I've finished it) a straight 6 BMW M50 2 litre engine, so no silly BHP for me!
Given that it is just a splash lubrication feature if really bothered you could probably replicate something similar. It would mean drilling holes in
the casing (and then either helicoil or maybe just bolts & seal up the hole) to attach a small 'shelf' or similar to catch and drip
oil in the right spot.
Stil, probably not worth worrying about. After all, what could possibly go wrong!
I have nearly finished mounting my box to the chassis and then I'll have to start thinking about the linkage mechanism, so I may be back with
more questions.
Cheers,
Simon
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