1969 Triumph Spitfire Mk3: Long term re-build 4AGE

Wishbones are looking good dude!



It'll be a little worse than that in actuality as well.

Basically, the higher your roll centre the more cornering forces are fed directly into the wishbones rather than into the spring (it's actually to do with the slope of the n-lines that go through the tyre contact patches and the instant centres, but for a basic explanation the roll centre height suffices).

I haven't quite got in my head where the forces fed into the wishbones actually go (do they just stress the component, causing heat and deflection, or do they push the car upwards). However, I do know that the effect basically reduces the amount of force the spring sees, which in practical terms means the suspension doesn't compress as much as you'd expect in a roll (unless your roll centre is sitting on the floor, and stays on the floor during the corner), meaning you don't get as much camber gain from suspension compression as you'd expect. You don't get as much roll either (as the spring sees less force, so the car doesn't roll as much), but the effect of a high roll centre is generally net-negative for grip.

Now isn't that interesting...

I seem to remember reading at some point about the way leaf springs compress and it being a bit counter-intuitive in that they deflect all as one unit rather than progressively, but I'll have to double check that. It would certainly be quite 'interesting' to calculate if they did move progressively, and I'm not sure what that would do to stuff like roll centre heights and whatnot.

I do remember playing around with the real life model where I simulated a GT6 spring with layers of plastic oval tubing, cut to size and with cable ties where the shackles are. At full bump of the GT6 shocks the deflection is worth about 1.6mm in the total length of the upper arm. Not a great deal, but useful at any rate.

I haven't got comparable data on how much camber that's worth. Need to try a hard link on the real-life model and see what that comes to.

Intriguing. Do you know if they deflect in a smooth arc or whether the longest leaf bends more than the others (and so on and so forth)?

Bother I think you're right!

Way back when I made the first model, I used the chassis diagram on Canley Classics here.

However, looking now they point out that the factory dimensions are wrong! From centreline to the roto bracket should be 315.7mm (so 631.4mm between eyes). Nearly a inch different on each side from my model!

Will see about amending the model. Need to go and find out which are the dimensions that I've measured in real life, and which ones I've worked out from the model (I suspect it's kingpin angle that I've worked out, so might change).

Does that new measurement fit with what you're measuring on your chassis?

I have a real GT6 Roto chassis and I reckon the back end of it is ok. What measurements do you need? They are awkward to measure iirc, lack of decent datum points. We made a jig to go on the diff and used that to transfer the dimensions to a non-roto chassis .

Nick

Vitesse Mk2 should be exactly the same….. I have one of those too, but as a complete working car so trickier to measure….

Measuring from our jig (having checked it still fits!)

Measurement between the pivot point centres is 625mm (24.606”)

Width between outer edges of chassis rails at the front edges of the brackets is 574mm (22.60”)

Vertical face of chassis rail to hole centres is between 26 and 26.5mm with the front nearer 26 and the rear nearer 26.5.

Hope this helps

Nick

All of them or just the MK2 Roto?

Thanks Nick,
Those dimensions confirm the bad news that my brackets are indeed 1/4" too far out in terms of the pivot offset from the chassis rail.  The aftermarket brackets from Canley/Rimmer/Paddock are correct at approx 1" offset, as per your measurements.  I cannot understand how I went so wrong with my brackets.  I was loaned a set, cut from what I believed to be a genuine Vitesse 2L Mk2 chassis and I copied them onto graph paper before sending them back.  This was way back in 1988 and I was only 24 years old.  So you might think a youthful mistake, but I was just as fastidious an engineer back then as I am now.  All I can think is that the brackets I copied were not truly genuine and/or they had been modified and carefully tidied up so that the car was set to more -ve camber?  Anyway, it is too late in the day to weld up and re-drill my brackets as the car is almost complete and the chassis is painted and waxoyled.   The extra 1/4" offset would explain the more than normal -ve camber when using the standard hole height and also why it goes bonkers -ve if I use the 44mm lower Eickoff hole.  I now have adjustable wishbones, so I can compensate for the mistake in the offset by shortening the wishbone from standard.  Not ideal because the GT6 rear suspension is already compromised by having shorter lower arms than the upper  'arms' (spring).  The opposite of good practice for roll characteristics.  This year I want to get the car on the road and drive the flippin' thing, albeit in primer, so that I can check all systems are good.  Next year, the tub will come off the chassis for a paint job.  I may then take the opportunity to re-position the wishbone bracket holes , or maybe not...
Cheers,
Chris 

FWIW, my Vitesse, which is standard Triumph geometry-wise, other than a tired spring and telescopic dampers, is a sweet handling car even on the limit, without the traditionally claimed murderous tendencies. Most of the effort there has been put in up front to try and get it to turn in.

My GT6 has not been nearly so extensively tested, but seems ok so far. Having considerably less weight up front, as you do, should definitely make it easier to get it to turn in!

Nick