The suspension-o-matic bears fruit!

"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228) 04/18/2017 at 08:27 • Filed to: spit6

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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.

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 another GT6 owner (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.

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...

Sweet Trav > BiTurbo228 - Dr Frankenstein of Spitfires 04/18/2017 at 08:38

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I just ordered a book on muscle car suspension. I spent a lot of money on a full suspension kit with coil overs and I have no idea how to tune it.

BvdV - The Dutch Engineer > BiTurbo228 - Dr Frankenstein of Spitfires 04/18/2017 at 08:43

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So there is indeed method to your madness? Haha

I’m thinking you have to find a compromise between little roll and slight camber change in the corners to have good handling. I don’t think a very aggressive camber change or no camber change at all is going to work. I’m just going on the basis of my gut and vehicle dynamics courses that I somewhat understood, so don’t pin me down on this being a right thing to do, haha.

RamblinRover Luxury-Yacht > BiTurbo228 - Dr Frankenstein of Spitfires 04/18/2017 at 09:13

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DATA.

I think you might find that minimal camber change was actually quite advantageous as long as the start negative camber was high enough. Perhaps something akin to #4, but with higher initial negative camber. I think you’ve got enough rubber to put down your power, so aside from a little bit of accelerated wear going straight, that might be best. If it’s got camber change under compression that’s fairly stiff, having a starting position set to “bite” a bit would likely not hurt, and with a reduced *increase* in “bite” with increasing compression, would probably not build up to a snap loss of traction as easily. Probably a better compliment to a stiffer front.

DasWauto > Sweet Trav 04/18/2017 at 10:12

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Not muscle car specific but potentially helpful nonetheless: https://books.google.ca/books/about/Race_Car_Vehicle_Dynamics.html?id=ICmmGzwPyG8C&redir_esc=y&hl=en

RallyWrench > BiTurbo228 - Dr Frankenstein of Spitfires 04/18/2017 at 14:41

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Is your car a Mk1,2, or 3? I’m just looking at Canley’s diagrams to better visualize what you’re playing with, I’ve never been under a Spit. At -3/4 with their kit, would -2.5 camber make the car too unstable under braking? How low do you hope to go? Does option 4 produce enough negative camber for max grip and good traction under power when cornering? Seems you’d want a bit more than -.75.

I need to get smarter so I can better understand this stuff, this is the kind of development I want to do to my 2002 someday.

BiTurbo228 - Dr Frankenstein of Spitfires > RamblinRover Luxury-Yacht 04/18/2017 at 15:28

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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 really interesting indeed!

RamblinRover Luxury-Yacht > BiTurbo228 - Dr Frankenstein of Spitfires 04/18/2017 at 15:42

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Unless other aspects are daft enough to make more camber change necessary, we may be onto something. Getting away from positive camber in a trail braking situation and back to merely flat - no bad thing, I suspect. A little negative camber to resist oversteer under throttle squat, probably also a plus.

BiTurbo228 - Dr Frankenstein of Spitfires > RallyWrench 04/18/2017 at 16:14

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It’s actually a bit of a mishmash really. It’s a Mk4 Spitfire, but with a rotoflex rear spring, CV axles, rotoflex uprights, Spitfire shocks (which might result in a different travel to GT6 ones) and a custom lower wishbone (the design of which I’m working on here).

You’re right in thinking that -2.5deg on droop would result in less stable braking. As the front compresses and the rear droops the rear end will have less grip than if it was 0 degrees, and probably result in some twitchiness of the rear end. I don’t think -2.5 deg is too excessive though so it’s probably not too bad.

As ever with suspension, you trade off against two different things when you’re after camber. What works best in corners (camber change that follows your body roll to keep the wheels flat to the road) is not what you want for accelerating or braking in a straight line (camber that doesn’t change at all).

However, what you really want is negative camber gain on bump (so your outer wheel tracks the road when cornering), and dead level camber on droop (for stable braking). That’s a decent tradeoff as your inner wheel which droops in cornering isn’t as important to track the road as the outer as it’s unloaded, but it is important that the rear end remains stable during braking.

The tricky bit is trying to make that happen, and also working out how much negative you need on bump to match(ish) body roll.

BiTurbo228 - Dr Frankenstein of Spitfires > Sweet Trav 04/18/2017 at 16:24

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DasWauto’s suggestion is a good one :)

As a start, if it’s a coilover kit I’d see if anyone has experimented with it previously and found some settings that work nicely just to get you rolling, and then while you’re finding out what you want from the car you can be reading up :)

BiTurbo228 - Dr Frankenstein of Spitfires > RamblinRover Luxury-Yacht 04/18/2017 at 16:40

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Seems good to me! I wonder if I can get it so that it’s dead level (or near enough) throughout the whole droop but decently negative on bump. So far not, but 2 (-1deg) seems good :)

BiTurbo228 - Dr Frankenstein of Spitfires > BvdV - The Dutch Engineer 04/18/2017 at 17:04

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Only a little ;)

I think you’re right though :) ostensibly just for corner performance, it doesn’t really matter how much the car rolls as long as the camber matches it. However, with such an aggressive curve when it comes to squat under acceleration and dive under braking you’ll lose rear end grip.

I’m thinking aim for dead-level under droop and negative under compression (maybe a smidge negative when it’s at rest), so cornering is good while braking is stable. Getting it will be the tricky part!