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Unibody Mk4 Spitfire 6


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  • 2 weeks later...
Well, thanks to the weather progress has been glacial, but there has been progress nonetheless! This is a freshly repaired leading edge of my hardtop. As with any big panel with complex pressed curves it's warped a little, but some persuasion with a hammer and a bit of forethought should minimise the amount of filler it will need.

 

kjyxynhyuc1vbwb6vbqx.jpg

 

I’ve also finished the cladding on most of my parts shed :) just needs some doors in place and i can finish it off!

 

nrsmohjrdw8sog5hhuog.jpg

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  • 2 weeks later...
Onto the spit with the Spit ;)

 

hjf8ux7ed1uxkvesyird.jpg

 

Finished the majority of the welding on the roof (or at least enough for it to keep its strength when lifted). Now I’ll box in the chassis to the floorpan, hopefully giving it quite a bit of structural rigidity in the process :) certainly feels very solid (you could lean on the windscreen surround and it'd bend before!)

 

Patched up the sides which looked good on the outside but were very crusty behind the seams.

 

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Lastly, patched up the little sections under the rear quarter windows and boxed it into the bodyshell :)

 

ixvagdsahgsvmkj3unsg.jpg

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  • 2 weeks later...
So, this is why my Spitfire is called 'Patchwork'...

 

cz9llqul5bcyjtjmbion.jpg

 

Finished boxing in one half of the body to the chassis. The two floor panels are my repairs, all the stuff at the top is one of the dozen or so previous owners. From my (incredibly) rough maths, this should increase chassis stiffness to ~140% of its original stiffness, 10% more than just solid-mounting :) might even be more as I did all of that before I welded the hardtop to it!

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  • 2 months later...
Long-time no post, but I am still alive! Crappy weather and other bits and pieces not really worthy of a post means it’s only come along at a slow pace over the winter. 

 

Still, boxed in the other half of the chassis today :) just a couple of little corners and bits and pieces to weld then it’s officially a complete unibody Spitfire :)

 

mpgu5o5dii9x8fthvhri.jpg

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  • 4 weeks later...
Just finished beefing up the body mounting bracket so I can brace the suspension tower against it without having a crash result in a weld-hardedned spar of steel skewering my legs :)

 

Bracket:

 

jyb0rwgkw4oje9sirxu9.jpg

 

First step is to chip off all of the underseal gunk and make sure the steel behind it is solid.

 

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So far so good. Then onto some Binky-esque CAD (Cardboard Aided Design) templates.

 

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First plate made from 3mm steel was easy enough. Simple cut, bend roughly into shape, tack one end, beat with a hammer until it’s the shape it needs to be and then weld in fully.

 

bzr4m5ovbqpr0eoi4bxm.jpg

 

The next one required a cup of tea and some thinking time. I could bend it roughly to shape with some muscle and a vice, but it wasn’t quite as square as I’d hoped. Luckily, a piece of spare box section I had seemed the right shape so I used the vice to press it into a better shape.

 

pdmagyklgkm9cjel3ujz.jpg

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A bit more hammering and a bit more pressing and voila!

 

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Very neat :)

 

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Drill some big holes for some beefy plug-welds and spray with some weld-through primer in an attempt to rust-protect the back slightly...

 

qdtcnncuf1i1aepyx7mp.jpg

 

...and tack in place.

 

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This as proof that every now and again I can lay down a weld that looks bang-on ;)

 

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More primer and a new plate over the top.

 

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Hopefully that should be a proper strong-point for the tubing to mount to, and the double-thickness of 3mm steel welded to the chassis extension should prevent it tearing off there and pivoting backwards. I’ve done the other side in an identical manner as well and one more job ticked off the list :)

 

Welding-wise, I’ve got one more plate tying the hardtop to the roll hoop, welding the roll hoop feet fully to the bodyshell, re-locating the rear suspension mounts inboard of the chassis for better geometry and ground clearance, and the brace structure explained at the start of this post.

 

Then I might be able to do something that’s not welding! God forbid!

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It's the Spitfire Suspension-o-matic Grommit!

 

kxbpcnhozjyeo5pdxtzy.jpg

 

After a solid day’s worth of googling chassis diagrams, measuring, rusty maths and general head-scratching I am done! In order to work out the best place to mount a custom lower wishbone to get a decent camber curve as the suspension moves this is a to-scale plan view of a rotoflex GT6’s rear suspension :)

 

The idea’s cribbed shamelessly from Marcus, but with a few improvements based on his experience. Primarily, he used another piece of wood for the upper link, but as the upper link on a Spitfire is a leaf spring it bends as it arcs upwards, pulling in the top of the wheel more than it otherwise would do with a solid wishbone. Worked positively for Marcus in the end, but I thought I'd see if I could account for it :)

 

I contemplated working out the maths to calculate the curve and quickly decided that my decade-old education is thoroughly inadequate for that sort of complication. So, I thought I’d make a leaf using some thin-wall PVC oval tube I found lying around :) works rather well!

 

mv59y7ufpwa0yzasdfcg.jpgvzdsy1yi8c65211tucfs.jpg

 

First impressions are that the camber change seems quite large (although I’m yet to work out what the actual suspension travel is yet). Next is that surprisingly it’s in the right direction. From a brief look at suspension location, you’d know that having a much shorter lower link will cause positive camber under compression as it describes a tighter arc, pulling the bottom of the wheel inwards more.

 

That’s counteracted here by mounting it at an angle, meaning that it pushes the bottom of the wheel out under compression, resulting in negative camber. The trade-off for this is increased positive camber under droop.

 

Now the fun bit :) the image below is the main chassis rail (the red box) and 4 potential new wishbone locations :) (the F, R and M are how far the chassis drops down from the main plane at the rear)

 

v5leongtbmoi3lloykul.jpg

 

Using some new scraps of wood, I can do this!

 

nsc0iyrpktus1ilxtae5.jpgkvlxtwuliil1m0cwtnff.jpgsrev8mj2tzccogdxhlmf.jpg

 

Much, much reduced camber change for the same suspension travel :)

 

Next up is to read up a load and do some more head scratching about what the camber curve should be, and experiment to try and find a location that fits that best.

 

Then, I’ve got some more head-scratching to do to make sure that my chosen location has an acceptable roll centre.

 

Then then, I’ve got a couple of popular aftermarket wishbone locations (and a few homebrews) to test out as well.

 

Then then then, I get to do it all over again with a new diagram overlaid in blue to simulate a 3/4" lowering block under the spring.

 

Then then then then, I can actually make a jig for the brackets, make the brackets and weld them in place.

 

Then then then then then, I’ve got a cunning plan for making adjustable lower links... ;)

 

Not much to do then ;)

 

Edit: I'll post up all the dimensions I used to make this when it's not half 1 in the morning!

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As promised dimensions! Some will be repeated in different areas, but just making sure I've got everything down. I'll also see if I can make a drawing with dimensions as that might be easier to reproduce :)

 

-----------------------------------------------------------------------------------------------

 

Upper link

Rotoflex leaf spring (as fitted): 522.5mm

Rotoflex leaf spring (unfitted so beyond max droop really): 505.0mm

Top vertical link eye to hub face (not the actual hub itself, but the outer face of the VL): 94.361mm

Distance from chassis centreline to outer spring bolt (the point at which the eye would be if the leaf spring was a standard link): 38.1mm

 

Total upper link arm length fitted (leaf length minus distance to spring bolt): 484.4mm

Total upper link arm length beyond full droop (by an unknown amount as I can't find what the suspension travel is): 466.9mm

 

Total distance from centreline to outer VL face: 616.861mm

 

Lower link

Centreline to outer face of chassis: 285mm

Chassis to rotoflex bracket eye (horizontal, using Canley's figure of 315.7mm): 29.3mm

Chassis to rotoflex bracket eye (vertical): Need to measure this as I described an arc until it met 29.3mm away from the chassis while the VL is vertical

Rotoflex wishbone (straight): 236.474mm

Rotoflex wishbone (as fitted on a slight incline to achieve the factory 0 degrees camber): 227.25mm

Top vertical link eye to hub face: 75.311mm

 

Total lower link length fitted (just the rotoflex wishbone): 227.25mm

 

Vertical link

Top eye to lower eye (vertical): 233.362mm

Top eye to lower eye (horizontal): 19.05mm

Top eye to VL face (horizontal): 94.361mm

Lower eye to VL face (horizontal): 75.311mm

Shock mount hole to lower eye (horizontal): 34.138mm

Shock mount hole to lower eye (vertical): 45mm (edited for accuracy)

 

Chassis

Chassis outer face from centreline (horizontal): 285mm

Chassis width (inc. flanges): 70mm

Chassis width (exc. flanges): 50mm

Chassis height: 82mm

Chassis distance below datum line (the technical drawings use a datum line above the height of the main chassis rails, but it makes it much easier if you use the upper face of the chassis on the main plane as a datum point for measuring from. All my measurements below are from that level): 28.7mm

Spring mount above chassis: 109.72mm

Lowest point of chassis where it bends down to loop under the driveshaft: I've got this at home but not with me at the moment.

VL top eye above chassis (assuming leaf is level when fitted): 109.72mm

Shock eye above chassis: 154.2mm

Shock eye from centreline (horizontal): 374.6mm

Front diff mount below top of chassis: 5mm

Rear diff mount above top of chassis: Forgot to write this one down!

 

Shock location

Chassis shock eye above chassis (vertical): 154.2mm

Chassis shock eye from centreline (horizontal): 374.6mm

VL shock mount hole to lower eye (horizontal): 34.138mm

VL shock mount hole to lower eye (vertical): 45mm (edited for accuracy)

 

-----------------------------------------------------------------------------------------------

 

I need to try and find dimensions for the outer face of the hub as fitted to the VL, as thinking about it that extra length will affect the camber change, and actually get some solid data on the shocks...

Edited by BiTurbo228
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I like that....... I like it alot.

 

You do need the damper dimensions and also to consider how you will mount the damper (direct to swing axle chassis bracket, using chassis bracket extensions or copying the GT6 Mk2 mounts on the wheel tubs).  The reason I say this is that it sets the range of travel and camber variations are most extreme at the ends of travel.

 

The hub dimensions won't affect the camber change......

 

Nick

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Thanks!

 

I've managed to measure my vertical link to get the damper locations (it's 45mm up from the lowest hole, not 40mm), and got an email from Rimmers confirming the shocks travel from 240-300mm. I thought that would be way too long for my Spitfire suspension bracket, but it actually sits quite nicely at 268.5mm with the spring dead horizontal.

 

Using that, I've measured that there's 43mm of bump and 50mm of droop at the hub face, and marked it on my model (including marks for inches of suspension travel for when I test stuff).

 

rwdzgzq7yme5rkdflsz2.jpg

 

Oh, and the hub is 13mm from the face of the VL, and the brake drum is 6mm thick, meaning I've extended the vertical link (and 0 degree line) by 19mm. It has affected the camber, but only imperceptibly. If I stick my metre rule along the VL face it's about 1mm away from the line at the far end so I'm not worried.

 

I've also drilled a load of holes for other popular wishbone mount locations.

 

Red is standard spring fitment, blue is with a 3/4" lowering block and green is with a 1" lowering block. 'Racing' is the racing bracket (25mm down, 5mm out) and 'Roto 2' is the lowest bracket hole in the Canley's rotoflex bracket.
 
This is the point where I realised I don't own a protractor :S drat.
 
Off to the shops tomorrow!
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Oh, and here are the new and improved dimensions:

 

Upper link

Rotoflex leaf spring (as fitted): 522.5mm

Rotoflex leaf spring (unfitted so beyond max droop really): 505.0mm

Top vertical link eye to hub face (not the actual hub itself, but the outer face of the VL): 94.361mm

Distance from chassis centreline to outer spring bolt (the point at which the eye would be if the leaf spring was a standard link): 38.1mm

 

Total upper link arm length fitted (leaf length minus distance to spring bolt): 484.4mm

Total upper link arm length beyond full droop (by an unknown amount as I can't find what the suspension travel is): 466.9mm

 

Total distance from centreline to outer VL face: 616.861mm

Total distance from centreline to outer hub face: 635.861mm

 

Lower link

Centreline to outer face of chassis: 285mm

Chassis to rotoflex bracket eye (horizontal, using Canley's figure of 315.7mm): 29.3mm

Chassis to rotoflex bracket eye (vertical): Need to measure this as I described an arc until it met 29.3mm away from the chassis while the VL is vertical

Rotoflex wishbone (straight): 236.474mm

Rotoflex wishbone (as fitted on a slight incline to achieve the factory 0 degrees camber): 227.25mm

Top vertical link eye to hub face: 75.311mm

 

Total lower link length fitted (just the rotoflex wishbone): 227.25mm

 

Vertical link

Top eye to lower eye (vertical): 233.362mm

Top eye to lower eye (horizontal): 19.05mm

Top eye to VL face (horizontal): 94.361mm

Lower eye to VL face (horizontal): 75.311mm

Shock mount hole to lower eye (horizontal): 34.138mm

Shock mount hole to lower eye (vertical): 45mm

Hub face from VL face: 13mm

Brake drum thickness: 6mm

 

Chassis

Chassis outer face from centreline (horizontal): 285mm

Chassis width (inc. flanges): 70mm

Chassis width (exc. flanges): 50mm

Chassis height: 82mm

Chassis distance below datum line (the technical drawings use a datum line above the height of the main chassis rails, but it makes it much easier if you use the upper face of the chassis on the main plane as a datum point for measuring from. All my measurements below are from that level): 28.7mm

Spring mount above chassis: 109.72mm

Lowest point of chassis where it bends down to loop under the driveshaft: I've got this at home but not with me at the moment.

VL top eye above chassis (assuming leaf is level when fitted): 109.72mm

Shock eye above chassis: 154.2mm

Shock eye from centreline (horizontal): 374.6mm

Front diff mount below top of chassis: 5mm

Rear diff mount above top of chassis: Forgot to write this one down!

 

Shock location

Chassis shock eye above chassis (vertical): 154.2mm

Chassis shock eye from centreline (horizontal): 374.6mm

VL shock mount hole to lower eye (horizontal): 34.138mm

VL shock mount hole to lower eye (vertical): 45mm

Shock fitted length as per above distances: 268.5mm

Shock full bump: 304mm

Shock full droop: 240mm

Suspension bump travel at hub face: 43mm

Suspension droop travel at hub face: 50mm

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Phoey - too slow!!!! Can confirm from a set of v links I have(one built up still - the other stripped) that your earlier missing dimensions are the same as I have measured on my set - give or take a few thou

 

Drive flange to spring eye bolt ctr = 106.5

Vlink face to spring eye bolt ctr. = 94.5

Drive flange to btm arm bolt car. = 87.35

 

Also Watching with great interest!!!!!! I've spent hours drawing this on CAD and farting around with pick up points till I've tied myself in knots hopefully you are about to confirm that I'm not as daft as I think I am

 

Interesting project - good luck

 

Dave

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Well, I've finally got a day of testing in over easter :) here are some choice results:

 

mgiuokpunnnvub6sfcnp.jpg

 

Now if only I knew what a good camber curve looked like...

 

Tested the stock setup, a number of aftermarket options, and a couple of custom locations. Came to some interesting conclusions. 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 due to the sharp positive camber at full droop.

 

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 from the inside wheel, unless the added 0.5 degree negative camber on the outside wheel offsets that.

 

The lowest hole on the Canleys rotoflex brackets also reduces the sharpness of positive camber on droop, but replaces it all with negative camber, likely having a similar effect to lowering the car but at standard ride height. When you start lowering that, 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 found previously 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 Marcus (Under) 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. That is provided I've got them in the right place as I did guess roughly where he had them from the pics and may have got it wrong.

 

3 choice picks of my dozen or so experimental locations are interesting too. I like 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 have to take the favourites and work out where the roll centres are...

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Well, after musing on this at work today (and also thinking about sidewall flex on initial cornering), I decided to try a few setups with just a smidge of initial negative camber. No.2 with -1deg of camber at rest seemed pretty damn promising.

 

From full bump to full droop it goes -2.5, -2(full bump), -1.5, -1(rest), -0.25, 1(full droop).

 

That would mean that you’d end up with a little more negative on the outside wheel when cornering, damn near level when braking, and still a little positive for inside wheel grip when cornering, but not quite as positive and previously under really hard braking. Seems rather promising :)

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You are certainly putting plenty of effort in....... and seem to  be getting some good results.  Not sure I'm understanding the description of the  pivot point locations though........ my little monkey brain needs pictures - simple stick-man sketches sufficient!

 

Nick

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Thanks! Yeah it's very interesting trying to fathom out what does what when you're moving stuff around. There's too much interplay between length, height and angle for it to be straight forward so lots of testing.

 

I've found it's fantastically tricky to explain anything more than very simple geometry using words so I think it's all of our monkey brains, not just yours ;)

 

I'll scribble something out to try and explain :)

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  • 2 weeks later...
Tedium at work today finally tipped me over to having a go at modelling my chosen suspension design online and I must say I'm pretty pleased with the results :)

 

k5gmb0yqvsitqglyzaq6.png

 

For reference, the stock Rotoflex wishbone brackets are 99.3mm outboard and a hitherto unmeasured distance higher (I'll measure that soon!).

 

From all this mucking about I've found that the roll centres don't move a great deal. In bump/droop they move by a maximum of 105mm vertically each way at full travel, and in roll they travel 259mm at 5 degrees of body roll (25mm of that vertical) which I gather is not bad :)

 

More importantly, the camber curves of this mirror my wooden model closely enough in bump and droop for me to use it for camber change in roll relatively accurately. Again, that's quite good. Up to 2.5 degrees of roll the negative camber of the compressed wheel slightly exceeds the degree of body roll. At 2.5 degrees it matches it, and then starts to drop off slightly (at 3 degrees of roll the camber gain is -2.8 degrees). From what I imagine that would result in good grip up until the body starts to approach 2.5 degrees. At that point, it'll either break away gradually or snap oversteer as it decambers. I can't quite figure that one out in the old headphysics model just yet...

 

Have a muck about with the model if you like :)http://racingasp.com/1fhre8zt

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