Question for any suspension gurus...

"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228) 06/17/2015 at 09:47 • Filed to: Spit6

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I’m trying to work out spring/wheel rates and roll stiffness distribution for the Spitfire. The specs of the stock suspension sets it up quite heavily to understeer (much stiffer front wheel rate, much stiffer front roll).

Now that I’m sorting out the rear suspension geometry, I can aim for a more neutral suspension balance.

Here’s the question. When I’m looking at relative stiffness front/rear, am I looking for something that’s roughly equal to the weight distribution?

For instance, my weight distribution is sitting at ~54% front. Should I be looking at a wheel rate distribution and roll stiffness distribution of 54% apiece to get neutral handling (or a couple of percent higher for a tendency towards mild understeer for safety)?

Pic to illustrate level of envy for those with functioning spitfires...

RamblinRover Luxury-Yacht > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 10:05

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I’m no expert, as I’m sure you’re aware, but I suspect the stiffness might be best off reflecting the dynamic balance under braking and not the neutral one. In other words, having relative stiffness close to your weight balance would seem correct, but relative to the amount of weight shifted onto your front wheels in emergency cornering or trail-braked nosing into a curve, not that of even speed. Might be best off at 60% or higher still.

Goodness, this reminds me of how important it is I swap to XKE rear springs on the Ranchero’s rear. Beam-like rear end-sliding epic oversteer is probably not the best...

505 - morphine not found > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 10:06

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I have no idea at all about this, only came here to share in your envy - I have wanted a spitfire at least since my early teens, and, sadly, am not much closer....

One thing though, people who have built suspensions for themselves always tell that it’s mostly a trial-and-error thing, with only the third or fourth attempt working to their exact liking. Are you using elements that are open to later setting-changes (say, a coil-over piece), or are you looking at swapping parts, if it does not come off?

Hot Takes Salesman > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 10:19

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Hmm? What? Spitfire?

BiTurbo228 - Dr Frankenstein of Spitfires > 505 - morphine not found 06/17/2015 at 10:21

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The front suspension is already a coilover unit from stock, and there are a couple of options for different spring rates plus I’ll have adjustable dampers. No off-the-shelf options for a different anti-roll bar that I know of though, so any changes there would have to be custom.

The rear is sorted by a transverse leaf, and there’s a few options there between marks of Spitfire and marks of GT6, although some would require modification from a swing spring to a fixed spring to work properly on my car.

I’ve also got the option of fitting a front anti-roll bar to the rear of my car as I’ll have a spare hole in the vertical link, plus I’ll have adjustable dampers on the rear too.

So yeah, options for changing components out but it’s a ballache with things like the rear spring and money I’d rather spend on other bits and pieces for it :S

Bonus pics for the Spitfire love :)

Highlander-Datsuns are Forever > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 10:37

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I have redone the suspension on a couple of 240z’s and my experience is put all the new stuff on and see how it drives then mess with toe and camber on the front, on the Z you can’t adjust the camber on the rear unless you slot the strut towers. Since the anti-roll bars work completely separate from the springs/shocks so I like them to be pretty substantial. Eventually you are messing with spring and shock rates to get the handling balance. The springs on the race car Z are so stiff the suspension doesn’t even move untill you are seriously flogging it so be careful, it’s nice to have a compliant suspension on a road car. There are adjustable shocks (dapeners) that can give you different dampening rates and adjustable springs available for most cars and would be fun to mess with.

BiTurbo228 - Dr Frankenstein of Spitfires > RamblinRover Luxury-Yacht 06/17/2015 at 10:59

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Interesting. The setup I have at the moment will get me to 61% front combined roll and vertical stiffness (50% front vertical stiffness and 72% front roll stiffness), so that might not be too bad.

If I add a front ARB to the rear that gets me to 54% front combined roll and vertical stiffness (50% front vertical stiffness and 58% front roll stiffness).

I’m using combined roll and vertical stiffness to get an estimated % understeer bias. No idea if that’ll actually translate to anything meaningful as it’s a metric I’ve entirely made up, but it’s nice to look at :)

Snuze: Needs another Swede > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 11:00

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If the car for road use, track use?

I ask because messing with spring rates can open up a whole can of worms. Now if it’s for racing, that’s understandable because you want all that adjustability. For a street car it may be overkill. Im not sure if the Spitfire has a rear anti-roll bar. If it doesn’t, can you add one? If it does, is there a larger one available? Increasing rear roll stiffness relative to the front may help dial a lot of that out.

The advice I’ve always heard is this - if it’s a race car, figure out your spring rates first, then tune your dampers to suit. If you do it right you often times won’t need anti-roll bars. But they can be a good tuning tool to help dial in that weight transfer.

On the street, do the opposite. You want compliance for potholes, speed bumps, etc. So unless the car wallows really badly, and/or you don’t mind a washboard ride, leave the springs alone. Tune your dampers and add/increase anti-roll bars to get what you’re looking for.

If only EssExTee could be so grossly incandescent > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 11:03

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I don’t know about the distribution but I can tell you that if you pit a super stiff anti-roll bar on the rear, it will make the car more prone to oversteer, but that oversteer will be more sudden and violent. There’s a reason why rear bars are usually thinner than front bars. If you put a front bar back there like you mentioned in another comment, you’ll likely be in for a world of snap oversteer.

RamblinRover Luxury-Yacht > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 11:08

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Considering that the dynamic shift will be hard to calculate, and is a complex multivariable equation taking into account loaded center of gravity distance above and to suspension mount points, overall length of the vehicle, dynamic resistance under braking of your dampers, grip limits of your tires and effective contact patches from geometry, balance of front and rear brakes, body torsional forces resulting from the front brakes, and any number of other things, I don’t know there’s any better answer (other than maybe making the car over many hours in Rigs of Rods) than “make a good guess, and see what happens”.

uofime-2 > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 11:18

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Echoing Snuze, Try adjusting your ARBs if possible.

If you’re fine with making your ride more harsh, you’re going to be best served with iterative design most likely since you have an existing application. Which is to say figure out that you have now for springs and what stiffer ones are available, try the next stiffer springs available, there likely aren’t that many choices shoot for 10% stiffer springs and see how you like them, if those don’t suit you go up again, springs are cheap, hopefully changing them isn’t too much of a PITA

BiTurbo228 - Dr Frankenstein of Spitfires > Highlander-Datsuns are Forever 06/17/2015 at 11:39

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Yeah the springs I’m using are softer than most used by modified cars on the Triumph forum. I’d rather try and at least get things sort of right when I’m putting it together, especially with the rear as it’s a pain in the arse to change the transverse leaf spring on Spits when it’s all installed.

I will have camber adjustability in both the front and rear very easily, and toe adjustability in the rear as well. Plus I’ll have adjustable shocks at both ends so there will be a lot I can do to tweak it after it’s all together, but I’d like to try and get it close before I start :)

AthomSfere > Hot Takes Salesman 06/17/2015 at 11:39

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Dat air suspension!

BiTurbo228 - Dr Frankenstein of Spitfires > RamblinRover Luxury-Yacht 06/17/2015 at 11:45

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I like that plan :) I’m good at making good guesses and seeing what happens ;)

BiTurbo228 - Dr Frankenstein of Spitfires > If only EssExTee could be so grossly incandescent 06/17/2015 at 11:52

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I’ve done the maths, and it seems to make sense to put the same ARB on both ends.

I’ve got slightly uprated springs going on the front with a roll stiffness of 197 ft-lb/degree and the stock anti-roll bar has a roll stiffness of 102 ft-lb/degree. That gives a front roll stiffness of 299 ft-lb/degree.

On the rear, I’ve got a GT6 MkII rotoflex rear leaf spring with a roll stiffness of 114 ft-lb/degree. If I add in the 102 ft-lb/degree roll bar from the front that’ll get me to 216 ft-lb/degree rear roll stiffness.

So, the distribution will be 58% front roll stiffness, set up for understeer.

My theory on why I can do this is that Triumph set up all their swing axle cars with really low rear roll stiffness to try and prevent jacking, a problem I’ve solved with my modified rear suspension, so I can set about sorting out the understeer bias :)

BiTurbo228 - Dr Frankenstein of Spitfires > Snuze: Needs another Swede 06/17/2015 at 12:17

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Primarily road, but with the certainty of track days and the possibility of hillclimbs. Fast-road basically. Don’t mind a bit of a rough ride, it is a Spitfire after all, but I wouldn’t want it to be rock-solid...

I’ll be messing with spring rates as it is with the uprated front springs I’ve bought, and the GT6 rear spring I have to use with the CV conversion at the rear so that can of worms is already well open. They’re still softer springs than most people use on proper fast-road Spits though, so they should still be comfortable :)

Spits don’t have a rear anti-roll bar, but I can add one in on my rear setup which will help a lot with rear roll (bit of a problem with the other chap who’s got something resembling my rear suspension setup).

RallyWrench > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 12:55

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Is is an early version with the swing axle jacking problems or the modified MkIV setup? How much adjustability will you have? You’ll need to stiffen the rear relative to the front to decrease the roll couple & thus bring it closer to neutral, but if it’s an earlier swing axle version that may be hard to do with the insane camber curve. My knowledge of Spits isn’t that great so I don’t know what would be involved. I’d say a rough balance based on the weight distribution is as good a place to start as any, as it’ll take some trial and error on the road. I’d suspect you’ll end up dropping the rear somewhat from that point, which is still likely significantly stiffer in the rear than stock. If you balance the suspension well enough, you can then dial in your desired mild understeer tendency with tire pressures alone.

Hot Takes Salesman > AthomSfere 06/17/2015 at 14:25

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Dem wings yo!

tromoly > BiTurbo228 - Dr Frankenstein of Spitfires 06/17/2015 at 15:46

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Unless you want to dive into the world of vehicle dynamics equations, I’d recommend getting an adjustable swaybar for the rear, as the swaybar gets stiffer the rear will “lose” grip and effectively give “more” front grip.

BiTurbo228 - Dr Frankenstein of Spitfires > tromoly 06/18/2015 at 06:14

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I’d love an adjustable ARB at the back actually. Would have to make one myself though.

Actually, can’t be that hard. I’d just need to find the right sort of torsion bar from another car’s suspension and fabricate some splined arms to fit on each end with holes along the length so I can change the roll stiffness...

BiTurbo228 - Dr Frankenstein of Spitfires > RallyWrench 06/18/2015 at 07:15

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It was a MkIV, but I’ve gone off the reservation with rear suspension changes. Out with the swing axle/swing spring setup entirely, in with a heavily modified version of the rotoflex rear suspension used on MkII GT6s to eliminate bump steer and positive camber gain as the suspension compresses.

Part of the reason the stock cars are set up with so much stiffness up front was to avoid jacking on the swing axles at the back (the rotoflex cars are a lot more even), so I’m thinking that with my setup I can increase the rear stiffness a lot and get a more even balance too :)

I’ll be able to adjust front camber easily, and toe with a bit of fiddling, and rear camber and toe easily. I’ll also have easily adjustable shocks on both ends. I’d much rather get the suspension set up right and leave tyre pressures even if possible. Not certain why, but I always have the feeling that adjusting tyre pressures is just sticking a bandage over the wider issue of improperly set up suspension...

RallyWrench > BiTurbo228 - Dr Frankenstein of Spitfires 06/18/2015 at 12:49

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If it was a racing car I’d agree with you completely. In that case, you’re dealing with very specific tire temperature and pressure targets, and if you’re making significant tire pressure changes in session you’ve got the setup wrong somewhere. Road cars have too many variables in suspension, tires, and surfaces, so making moderate pressure adjustments (say, 5psi) is, to me, perfectly acceptable to correct minor handling foibles or change the balance slightly (and quickly) without changing suspension settings. Most road cars aren’t adjustable beyond toe and a bit of camber anyway, so tire pressure is all the effective damper change you get. Say you find a perfect hot road setup, but want to go to an autocross and after 1 run you want it to rotate a little better without getting under the car or mucking with your otherwise ideal setup. Tire pressures can do that.

I love GT6’s, I’ll have to endeavor to learn more about the suspensions so I can justify owning one someday :)

BiTurbo228 - Dr Frankenstein of Spitfires > RallyWrench 06/18/2015 at 15:25

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If you even need a hand understanding small-chassis Triumph suspension just give me a shout, I’m elbows deep in them at the moment :)

Good point with the ease of adjustability, but I’ll have shocks that can be adjusted while they’re on the car so that should take care of that :) any minor imbalances I should be able to tune out with them on the fly :)