Subaru diff, CV and coil over conversion

[itiejim]

Thought I'd update the forum on the Subaru diff conversion that I've done to my '67 Spitfire.

There are various arguments over whether it's worth the hassle etc. However, for my car, fitted with a tweaked VVC 1800 K-Series engine and type 9 'box, I wanted to eliminate the weak drive shaft and get a stronger, cheaper, LSD in there. I've always been fairly happy with the swing spring suspension, however, whilst I was designing something it seemed sensible to see if I could reduce the camber change and provide some options for more easily changing ride height and spring rate.

I've been thinking about doing this for years and had been following Karl Dandridge's efforts quite closely. He has fitted a Subaru diff to the Herald Coupe he's got and had been promising a kit for ages.... I got bored with waiting. Karl has made some progress towards a kit and had some drive shafts made up with splines to suit MGF CVs on one end and Subaru CVs on the other. I was able to buy a pair of them of him which helped.

I didn't want to modify the chassis and chose to mount the diff in a 3mm steel three sided box, locating in in the standard mounting position. The diff bolts into this box on the sides by using the top three drive shaft seal retaining bolts. I have then welded a Z section from the front of the box forwards to meet the front mounting holes on the Subaru diff. From this I have used rectangular box section with the original Triumph diff "ear cups" welded on to mount the front part of the diff to the chassis. To hook the rear up I know that some have used a bespoke alloy casting. I see that Karl had one of these made up but I was not particularly convinced by the strength or quality. Therefore, I chose to have the original Subaru diff back plate machined down to the minimum possible to allow clearance for the crown wheel and welded a flat aluminium plate on the back. This allowed me to add a fourth side to my box (closing the rear part behind the diff back plate) and permitted me to make up some hangers from rectangular box section with tube welded in to take the standard rear diff mounts. This allows me to fit the diff in virtually the exact same position as the original one (it's probably 15mm further forward in terms of drive shaft position).

Vertical links are GT6 ones machined out to Nick's specification to take the MGF bearings. I've also ordered some of Nicks drum centering rings to ensure correct drum location.

I have used Canley Classics alloy Rotoflex wishbones and mounting brackets welded onto the chassis and am currently waiting for the arrival of a set of coil overs (12.5" extended length with 250lb 7" springs - 1.9" ID). The extended length ensures that the drive shafts don't hit the chassis on full droop whilst spring rate and length is a guesstimate at the moment. 1.9" ID springs are cheap enough to buy some more it I need them! With the type 9 'box and everything moved well back in the chassis I've ended up with a 50.8 cm prop shaft, which is on order from http://www.dandfltd.co.uk/

Upper arms are tubular steel with standard Triumph sized bushes fitted to tubes on the end. It's an arm with a single point on the inner and outer ends as the tie bar provides strong location and the whole thing is massively better located than the standard swing spring set up. The inner mount is welded to the box which gives a slightly shorter top arm compared to the transverse leaf, and it is also positioned lower down. I've yet to get a gauge on it, but there is no visible camber change between full droop and compression.

Now, I know that a photo says a thousand words, however, I've been busy in the garage and just realised that I haven't actually taken any of the kit out of the car. Ooops... Below (should be) a few photos of the diff and suspension as it sits in there now. I'll update further once the coil overs and prop get here and I can actually try it.

[GT6 M]

Nice work, there quite a few variations on the rear end noo.
as for rear camber change, Unless ye actually use longer lower links,
with lower mountings, then im afraid that ye will still get  camber change,
not to mention track change.
But hey, top marks for having a go,

M

[itiejim]

Yes, I saw the mod you've done with the revised lower links which looks good, but I wanted to keep the standard Triumph pick up points. I think that lowering the height of the top link (and shortening it) has helped, but I'll have to get a gauge on it to be sure.

[Spit131]

Hi , Not wanting to put a damper on things with all the effort you are putting in but I believe Karl decided to re-design his upper arms after consulting a suspension engineer . His intent was to change the inner half of each upper arm to a triangulated design which was to be double point mounted off the diff box in order to give the upper arm more stability as the single mounting point lower wishbone combined with the bushed tie arm allows movement of the vertical link . The leaf spring being a continuous horizontal bar held pretty firmly by the diff locating bolts did a reasonable job at giving the VL some stability but I think a single bushed bar will make worse the problem . I am assuming the inner link is as the outer one in the photo ?

This is why I decided to go for a very rigid double mounting point lower arm . This should give such stability that the upper VL link is only there to give distance from the diff all though it is fact a single leaf transverse spring . Matt .

[itiejim]

Hi Matt, yes, Karl did start with a single link before going to a triangulated one but, having chatted with him about it extensively I know that he then settled on producing a single arm for his (still pending) kit (though I don't know why)

I'm not aware that he took any professional advise on it, however I did!

I trialed this set up on the rear part of an old chassis before thinking about fitting it to my car and, as I wasn't sure how to go with the top arms, I took the whole lot to a local rally car preparation specialist who has experience designing bespoke suspensions for him to review. It was on his advice that I went for a single point inner and outer top link and he actually produced the top links and fitted the pick up points for me. I'm no expert on this, however, I understand that the re-located tie bar used on the rotoflex suspension (when compared to the non-rotoflex) is positioned in such a way that the who thing - lower arm and tie bar, works as a single wishbone (therefore with three inner pick up points). This should provide more than enough stability.

In any event, compared to the non rotoflex suspension, the stability of the vertical link is massively increased. Have you ever tried trying to rotate the the jacked up wheel of a non rotoflex car with the handbrake on? The amount of movement is incredible.

I can't say that I expect to bolt this up and for it to just work perfectly out of the box, however, having located the diff securely (which acts as the platform for the upper inner links) if I need to revise the wishbones then it's no great issue. I'll keep this thread updated with how it actually works on the car.

[itiejim]

Hi folks, thought it was about time I updated this thread with some feedback after having done a couple of thousand miles.

Firstly, driving impressions. In short, it's a huge improvement. Now, I know that, as the creator of this set up I'm likely to be biased, however, in defence of my objectivity, I only ever took this job on in order to get a robust axle with an LSD in as I was always happy enough with the swing spring rear suspension. I do have to say though, that the far improved axle location is immediately apparent, in particular in transition from power on to trailing throttle. The car is far more predictable and it's now easy to drift (if that's your thing) - something I always struggled to do smoothly before.

Problems: firstly, the springing was too soft, initially I fitted 7" 250lb springs, however, they were just too soft and needed winding up to get the ride height and compressed too much under load. I swapped them out for 8" 325 springs which gave the right spring rate but the increased length meant that the spring platforms interfered with the CV boots... In the end a 6.75" 325 lb spring seems to have done the job.

The only other issue I had was some flex in the diff box which I wasn't happy with. I've subsequently droped the diff out and reinforced the brackets which has resolved the matter.

All in all, well worth the effort. I'm booked in for a sprint next week and possibly a trip to the strip on Saturday - weather dependent, so I'll keep you posted.