1974 Unibody Triumph Spit6 (with some detours inbetween)

Looking forward to more of this! Bookmarked

Thanks guys and yeah, the Jag doesn't meet a pretty end I'm afraid. Still a bit gutted about that, but you'll see soon enough...

That I have, in the past at least. I've been meaning to update my thread over there with all the updates since I last posted (quite a while ago). It's on the list now I've got a job that's more amenable to forums at lunch!

Thanks man we'll get to it soon.

Yeah the mk1 gearbox mount turned out to be a terrible idea. Went through a few iterations before I got it right. Yours looks much better (different gearbox notwithstanding).

Is yours a 4 or a 6?

Ah I've only just made the connection. Steve Attenborough right? Read through your whole project as I was starting mine. Big inspiration

Initially I was planning to try and get stock Spitfire distribution without shifting the engine back like you'd done (seemed like a lot of work).

Turns out I really should have done that to begin with

Yup, that'll be me, just realised that I might have jumped a few step ahead





Or you can fit a Spitfire front engine plate to the six cylinder block instead to get a similar result

Looks like a very promising project. As a fellow Spitfire owner who's potentially considering a 6 pot swap, I look forward to reading more!

Great thread! 🍿🍿🍿

Off we go! First patch:



Up on the jig again to box in the underside (with a demonstration of why my Spit's called 'Patchwork')...




...prop tunnel off and on again so I can weld up the inside of that...






...and roll hoop stitched into the hardtop:



It was about this point that I discovered the issue with wooden jigs for suspension work. Turns out nothing's straight. curse word.

Ah well, I'd been having a play about with the suspension design in my head anyway and think I've come up with a better mounting location. To try and test out my ideas, I made a scale model of the rear suspension of a GT6 with various different options for lower wishbone location (and a mock leaf spring to try and account for the flex in that as the suspension compresses, shortening the effective upper arm).

May I introduce you to the Spitfire-Suspension-o-Matic!





You can see the angle of the lower wishbone, and how it produces good negative camber change on bump, but a lot of positive on droop.

I made up lower wishbones for a number of different aftermarket suspension options (including lowering blocks that fit under the spring), GT6M's wishbones, and a few other potential locations.



From all of that, this is what I came up with:



What we learn from this is pretty interesting actually to geeks like me anyway. For instance, the much-maligned stock setup actually seems to have quite a good camber setup for cornering (for a car with a decent amount of roll), but would likely suffer under braking or acceleration.

The aftermarket lowering blocks (represented by the -13/4" and -1" versions) reduce the high positive camber on droop, but replace it with negative camber so would likely be more stable under braking, but less grip in corners unless the added 0.5 degree negative camber on bump offsets that.

The aftermarket relocation brackets (Canleys) also reduces the sharpness of positive camber on droop, but replaces it all with negative camber. When you start lowering it, things start getting really weird. -3/4" is pretty much zero camber change, aside from full droop. -1" is all over the place.

Racing -1" is a location derived by a very clever individual to produce zero camber change over the whole of the suspension travel, so it’s reassuring that the same result is found using my model

The suspension design pioneered by GT6M produces very slight camber change (less than a degree either way), and in the right direction. Would work very nicely on a car with very little roll.

3 choice picks of my dozen or so experimental locations are interesting too. I liked option 2 and 9, although again would probably need to have very little roll if my understanding of optimal camber is correct. Option 4 is even more interesting for racing as you can have dead-flat wheels on droop meaning nice straight braking, but still have a little negative camber on corners.

Now I just had to take the favourites and work out where the roll centres are...

In the end I think I plumped for number 2. Stuck that into an online model and measured how stuff like the roll centre and camber changes in roll and compression. All of the compression stuff is near as damnit the same as my wooden mockup The dynamic roll centre looks pretty good too, as does the camber change in roll (a little in excess of the body roll right up to 2.5 degrees where it levels off to match body roll, roughly calculated via weight, roll stiffness and COG height).



At this point, it's probably useful to do a bit of a layman's guide to what I learnt about suspension geometry. Above is what my suspension looks like at rest (-1deg static camber). On bump, it gains a moderate amount of negative camber.



On droop, it gains a smaller amount of positive camber.

On droop, it gains a smaller amount of positive camber.[/img]

So far so good. However, when subjected to roll, the loaded wheel (on the right) gains positive camber. This is not so great for cornering.



However, it’s only during a sweeping corner (and a fleeting moment in tighter corners) that you get roll in isolation. For the rear suspension on corner entry it’s accompanied by droop, and on corner exit it’s accompanied by bump.

Combining droop and roll basically combines the two effects, meaning for me I get more positive camber, which is also not so good of a thing, but pretty unavoidable with the existing compromises.



A similar combination happens for bump, but I gain less positive camber, which is a good thing.



So, what does all of this mean?

The easiest way is to describe the process of accelerating towards a corner from a standstill, braking and then going through the corner, roughly comparing my suspension design to another chap who’s very cleverly made a suspension design that remains at ~0.75 degrees of negative camber throughout the suspension travel.

So, we both nail it at the start. The rear ends of both cars squat down. His car gets more traction off the line due to the fact that his wheels stay more level with the surface of the road (-0.75 deg vs -2.3 deg).

Approaching the corner at full tilt, we both slam on the brakes. At this point, I can’t quite work out whose will be more stable. Mine transitions through neutral camber to slightly positive at full droop. His remains static at -0.75 degrees the whole way through. I want to say mine will end up with more grip overall so better braking from the rears, but I’m not certain if camber thrust will mean his is more stable throughout than mine which ends up twitchier. Lets call it a dead heat.

Entering the corner, we both tip it in. The rear end is still up in the air from braking, and the car starts to roll. Mine is definitely twitchier and has less overall grip at this point. The positive camber from droop combines with the positive camber from roll and the rear end starts to slip and slide a little. His rear end is a little more planted as his consistent negative camber on droop works to combat the positive camber from roll, allowing him to carry more speed through the turn.

Mid-corner, both cars level out in a steady state of roll. Here, mine starts to claw back some speed. Although it’s not really modelled above, my design winds up with less positive camber gain during roll. The roles are reversed and his rear wheels start to slip slightly.

Exiting the corner, we both plant it. Both cars squat again, but are still in a state of roll. Here, his car tends to positive as his slight negative camber isn’t enough to offset the positive camber from rolling. He’s forced to countersteer as he comes out of the corner slightly sideways, scrubbing speed for the straight ahead. My more aggressive camber curve does offset the positive camber from roll and I put the power down nicely for a good clean corner exit.

Who winds up ahead at the end of that I have no idea, but hopefully that makes a little sense for how all of these camber curves actually affect real-world driving dynamics

Seemed good enough for me anyway!

Sketched up a mock wishbone:



Took a leaf out of Project Binky's book and made my jigs out of metal this time...





...and welded up some wishbones




A little heavier than the previous iteration, but still in roughly the same ballpark as the alloy rotoflex wishbone



Chopped up the chassis again. Luckily for me with the unibody idea it didn't matter overly that the wishbones poked through the chassis a bit as that area's well braced.