Repost for the evening crowd: Down the rabbit hole with Spitfire IRS

"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228) 05/09/2015 at 14:34 • Filed to: None

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This is the standard GT6 rear suspension (with CVs instead of a rubber rotoflex joint). The lower wishbone and the radius arm pointing forward form a virtual lower A arm locating the lower end (giving something approximating a high-angle semi-trailing arm suspension). The leaf spring connects to the top of the vertical link and bolts to the top of the diff. The issue is that that makes the upper link far longer than the lower link, meaning you get positive camber as the wheel compresses under load, which is bad for traction out of corners.

Back in the ‘60s, Triumph was developing a GT6 LeMans car to follow in the footsteps of the Spitfire LeMans racers. They had this setup which replaces the lower wishbone and radius arms with two rods that bolt to the chassis and form a virtual A-arm in line with the top link/leaf spring (like a double wishbone setup):

This still has the issue of camber change as the suspension compresses due to having a longer upper link, but it sorts the toe changes when compresses/relaxed that are endemic to a semi-trailing arm-style rear suspension (part of the reason E30s are so tail-happy).

Now, this guy on my Triumph forum (the chap with the !!!error: Indecipherable SUB-paragraph formatting!!! ), has changed the lower rod mounts from the side of the chassis to underneath it, lengthening the lower A-arm and reducing camber change.

However, the top link is still longer than the bottom so what camber change there is is in the wrong direction (positive as it compresses). So, the idea I’ve had is this. A common diff upgrade for these cars is a Subaru/Nissan R160. The guys who do this often switch to coilovers mounting on the shock towers (or inner wing, but that’s not really strong enough), with a long rod running to the diff like this:

My idea is to weld a bracket onto the chassis above the driveshaft (in-line with where the stock inner wishbone mounts on the setup above) and run a short rod from there to the vertical link (about half the length of the upper link above).

And I thought I was done with my chassis modifications :S

DasWauto > BiTurbo228 - Dr Frankenstein of Spitfires 05/09/2015 at 15:12

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I wouldn’t recommend guessing (even an educated guess) at the length of that suspension arm and the location of that pickup point. Try to find yourself a copy of Race Car Vehicle Dynamics. The book goes into great detail for suspension design but you should be able to get some basic dimensions for your suspension layout relatively simply by focussing on the relevant sections.

DrJohannVegas > DasWauto 05/09/2015 at 15:30

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RTFRCVD.

RamblinRover Luxury-Yacht > BiTurbo228 - Dr Frankenstein of Spitfires 05/11/2015 at 12:20

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I missed this. Interesting there’s no clear existing way to get equal length or UEL in the right direction with the current platform without getting squirrely with extra brackets and the like. The only thing I wonder is whether with the existing weight balance a deliberate touch of camber-induced slip would be as bad as all that - weighed against some potential instability and attempts at jacking up with a more “proper” UEL setup.

BiTurbo228 - Dr Frankenstein of Spitfires > RamblinRover Luxury-Yacht 05/11/2015 at 12:28

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Those are all questions that are way beyond my ken to answer :S I definitely need to do some reading up about camber and toe curves and their effects on handling.

I think I’ve come to a conclusion though. I’m going to fabricate longer lower links like the chap with the red suspension above, but link the two rods together to better control toe change as the suspension loads up fore-aft.

The chap pictured above has overcome that by using solid nylon bushes, but I don’t think much of their longevity in such an application, nor their kindness to the chassis, nor their kindness to my posterior...

Oh, and I’ll keep the upper link as the leaf spring. I’ll do some reading and then experiment with shortened upper links for my next project (a Saab turbo-powered GT6). That’ll need a beefier rear suspension anyway so I might as well go full-custom with that :)

RamblinRover Luxury-Yacht > BiTurbo228 - Dr Frankenstein of Spitfires 05/11/2015 at 12:39

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I think that’s a solid plan. I was mostly just wondering whether to some extent the setup was like that on purpose - a lot of nose dive on a short wheelbase and high/forward CG combined with the rear digging in might lead to a Herald/Corvair-ish episode in going topsy-turvy around a bend. Then again, with a setup that changes toe a good bit, there might be a vote for “suspension engineer is lunatic or moron” going up. Remember, though, you can always make a stopper bar or bracket of some kind for your top leaf to change its flex point. You could even make such a thing in several sizes or adjustable - probably the easiest way to experiment with effective control arm length.

BiTurbo228 - Dr Frankenstein of Spitfires > RamblinRover Luxury-Yacht 05/11/2015 at 13:31

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Interesting idea about the stopper bar, but that might change the springing significantly :S

There’s a couple of reasons I can think of for the suspension setup being as it is. Firstly they were very cheap cars back in the day. In some of the period reviews they tell you how much it cost, and factoring in inflation my 1974 Spitfire MkIV was £10,813 new! The fewer components there are, and if those components are physically smaller, the better.

The other idea that I had, which I’d need to do some investigating into, is what happens when you slam on the brakes in an emergency stop.

If you’ve got a longer upper wishbone, as the suspension droops you’ll get negative camber, which means if the rear end locks slightly and starts to slide will mean that it’s more controllable.

However, if you’ve got a longer lower wishbone the same maneuver will result in positive camber, which would diminish grip if the car starts going sideways.

So, I’m thinking short upper wishbones means better grip in corners, better traction under power but a twitch rear under braking.

Short lower wishbones means lower grip in corners, a more wayward rear under hard throttle but a more planted rear under heavy braking.

The second option definitely sounds better for a street car is the compromise has to be made. More average people are going to panic-stop than plant it coming out of a corner on the limit...

RamblinRover Luxury-Yacht > BiTurbo228 - Dr Frankenstein of Spitfires 05/11/2015 at 14:00

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It’s long been very deliberate in passenger car practice to shoot for sliding wheels over “too much” grip, because cars letting loose at the limit causes housewives to die or something. The Ford Falcon’s suspension is the basis for quite decent handling... which Klaus Arning immediately sabotaged after designing to produce slippy understeer and softened the spring rates on. That sabotage then carried over directly to all Mustangs of the first generation other than the Shelbys. On my Benz, low-speed slip of the same kind is encouraged by running lower pressures in the front per spec. It’s more common to shoot for sliding in the front wheels than the rear, but I think the Spit might just be so short in wheelbase that they feared even worse the results of everything under braking if they didn’t design in some wayward-ness. While it’s also true that having the control arms shorter makes for lower stress on their brackets and for a lighter arm, I can’t imagine it’s that radically different than what it would have been *under* the frame like what your Triumph forum mate did.

You’re absolutely right that a bracing bar would change the springing for the firmer, and changing both variables at once in the “right” direction for performance muddies a bit which one is doing most of the helping. That being said, I note that a Spit’s rear spring does not leave it very difficult to drop a leaf... With a leaf suspension having no preload (unless on a series Rover - we’ll ignore that), a drop to the same ride height indicates nearly exactly the same effective rate, so it’s easy to drop one of the factors out. Sounds worth experiment to me.

4muddyfeet - bare knuckle with an EZ30 > BiTurbo228 - Dr Frankenstein of Spitfires 05/21/2015 at 10:01

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Posting here to keep away from Oppo’s lecherous gaze. My mobile 07891 633085, give me a shout when RR gets here and what the ‘up North’ plan is. Ta ta!

BiTurbo228 - Dr Frankenstein of Spitfires > 4muddyfeet - bare knuckle with an EZ30 05/21/2015 at 15:19

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Shall do :) I think we’re planning on Brands Hatch on the Saturday, perhaps drinks in Brighton on the Friday. Got to help my GF with an essay on Sunday, but maybe and evening Karting session...

What are your plans with him?

4muddyfeet - bare knuckle with an EZ30 > BiTurbo228 - Dr Frankenstein of Spitfires 05/21/2015 at 15:50

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I dont know when he’s coming up this way, so nothing as of yet. There’s Speedway on Monday, and the rest...? Don’t know, but my small village has 4 pubs and two breweries, which I’m OK with, and I’m sure he will be too.

If I’d known about this earlier (as in at least a couple of months) I’d have come down to Kent. Not for you, for the Lada ;)

Ramblin Rover - The Vivisector of Solihull > 4muddyfeet - bare knuckle with an EZ30 05/21/2015 at 18:47

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The genius of my plans is in their lack of intelligence and forethought. Wait... Anyway, other than “hit speedway, possibly get halfway to hammered, hit junkyard the morning after” my plans are “float about and do things”, probably based more in Simon’s vicinity than yours. Probably just the one night Oop North, in other words.

I ought to get a phone with a GSM mode - my current one works with shit signal just about anywhere in the US (CDMA system) but it’s not any good other than for WiFi and LTE data only here, no calls. (Typed over the Heathrow internet system).

Need help boosting a Lada?

4muddyfeet - bare knuckle with an EZ30 > Ramblin Rover - The Vivisector of Solihull 05/21/2015 at 19:06

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You can probably tell I’m no good without plans, it really doesn’t fit my MO. Floaty is fine then, and beer and bikes with no brakes it is.

Go to Carphone Warehouse Heathrow and get a throwaway PAYG. Most will at least give you basic internet (EDGE, 3G) on a dot matrix screen... Ha! But at least you can make calls and texts while here.

FYI It turns out that I’m away camping on Sunday night, but will be back early/mid afternoon on the Monday. Enjoy Si’s ridiculous car collection, bring me the Lada.

Edit: Welcome to England. Drive on the left, go straight over roundabouts, and feel free to antagonise our police officers.

4muddyfeet - bare knuckle with an EZ30 > BiTurbo228 - Dr Frankenstein of Spitfires 05/22/2015 at 07:11

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So how did you enjoy that earthquake last night?

BiTurbo228 - Dr Frankenstein of Spitfires > 4muddyfeet - bare knuckle with an EZ30 05/22/2015 at 08:43

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Well that entirely passed me by...

This quote from the BBC is hilarious: ‘Kent Police said a number of calls had been received but no injuries or structural damage had been reported.

But Herbert Smith tweeted that he and his wife Doreen were taking refuge in their caravan after being evacuated from their damaged home in Flete’

Ramblin Rover - The Vivisector of Solihull > 4muddyfeet - bare knuckle with an EZ30 05/24/2015 at 19:02

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I’d been wondering why no emails to you had elicited a response in the past fre days, then realized on rehecking this I’d forgotten you Sunday camping, like a moron.

I’m heading part of the way North tonight, partly to find a service spot to stay overnight (and one has both free WiFi and a way to actually pay for long term parking without using a phone - looking at you, Toddington). Fun so far.