Lotus Esprit gearbox, same as the DMC

[MichaelRPack1@xxxx]

Dave,

Have an 89 Lotus Esprit gearbox, same as the DMC. Renault called it the UN1 gearbox.

Here is a link that has some technical info about the Renault gearbox, from
Derek Bell: http://www.bell-performance.co.uk/. He uprated the gearbox and
has been able to run a 500hp car with this box, for 5-years, without any
problems.


Up-rating the Renault UN1/369 Transaxle 


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 was the problem I had with my Lola T70 with 500hp 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. 

It is difficult to find any hard facts out about the Renault transaxle, it has been used for many years in many vehicles, from the Master and Trafic vans to the Renault GTA and the 21 turbo (the best one if you can get it, with 3.44:1 final drive ratio). 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 Iâve 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 âtoughâ 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 âgiveâ 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.



As you can see, the new shaft (lower in photo) 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. The EN 36 shaft has given no trouble, and has withstood full power and torque in all gears including 5th at the Nurburgring (185 and still going). A further three kits have been used in GT 40 replicas actively used in sprinting and hill climbs, including some with tuned 351 Windsorâs and the only thing they have broken is drive shafts!



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 (911âs have mostly small engines), and the stronger boxes like the G50 are expensive.

Hewlands cost Â5k plus second-hand, and are noisy and baulky for a road car, ZF boxes are mostly ex Pantera with low final drive ratios and cost Â4K and up.



So I took the view that up-rating the Renault box was the easiest route, and I still think itâs 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(1â dia, 23 teeth) for which clutch driven plates are easier to obtain than the Renault 24 mm spline.

The cost of the upgrade kit is Â1750 inc VAT. If you are interested, please contact me.