by "BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
Published 05/04/2017 at 11:43
by "BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
Published 05/04/2017 at 11:43
Tags: spit6
; Instructables
STARS: 7
I’ve done some more mucking about with the digital model of my Spit’s modified rear suspension design and it’s really rather interesting. I thought it might help though if I related it to how it actually effects the handling though. Here’s what I’ve found (bear with me on this).
Here’s what it looks like static (-1 degree of static camber).
On bump, it gains a moderate amount of negative camber.
On droop, it gains a smaller amount of positive camber.
So far so good. However, when subjected to roll, the loaded wheel (on the right) gains positive camber. This is not so great for cornering.
However, it’s only during a sweeping corner (and a fleeting moment in tighter corners) that you get roll in isolation. For the rear suspension on corner entry it’s accompanied by droop, and on corner exit it’s accompanied by bump.
Combining droop and roll basically combines the two effects, meaning for me I get more positive camber, which is not so good of a thing.
A similar combination happens for bump, where for me I gain less positive camber, which is a good thing.
So, what does all of this mean?
The easiest way is to describe the process of accelerating towards a corner from a standstill, braking and then going through the corner, roughly comparing my suspension design to another chap who’s very cleverly made a suspension design that remains at ~0.75 degrees of negative camber throughout the suspension travel.
So, we both nail it at the start. The rear ends of both cars squat down. His car gets more traction off the line due to the fact that his wheels stay more level with the surface of the road (-0.75 deg vs -2.3 deg).
Approaching the corner at full tilt, we both slam on the brakes. At this point, I can’t quite work out whose will be more stable. Mine transitions through neutral camber to slightly positive at full droop. His remains static at -0.75 degrees the whole way through. I want to say mine will end up with more grip overall so better braking from the rears, but I’m not certain if camber thrust will mean his is more stable throughout than mine which ends up twitchier. Lets call it a dead heat.
Entering the corner, we both tip it in. The rear end is still up in the air from braking, and the car starts to roll. Mine is definitely twitchier and has less overall grip at this point. The positive camber from droop combines with the positive camber from roll and the rear end starts to slip and slide a little. His rear end is a little more planted as his consistent negative camber on droop works to combat the positive camber from roll, allowing him to carry more speed through the turn.
Mid-corner, both cars level out in a steady state of roll. Here, mine starts to claw back some speed. Although it’s not really modelled above, my design winds up with less positive camber gain during roll. The roles are reversed and his rear wheels start to slip slightly.
Exiting the corner, we both plant it. Both cars squat again, but are still in a state of roll. Here, his car tends to positive as his slight negative camber isn’t enough to offset the positive camber from rolling. He’s forced to countersteer as he comes out of the corner slightly sideways, scrubbing speed for the straight ahead. My more aggressive camber curve does offset the positive camber from roll and I put the power down nicely for a good clean corner exit.
Who winds up ahead at the end of that I have no idea, but hopefully that makes a little sense for how all of these camber curves actually affect real-world driving dynamics :)
"AntiSpeed" (AntiSpeed)
05/04/2017 at 11:53, STARS: 0
I would expect that the increased stability on corner entry would allow the other driver to get back to power sooner, which combined with the higher mid-corner speed would pay dividends down the straight. Even if a little bit of momentum is scrubbed by corner exit oversteer, it wouldn’t completely negate those two advantages.
"Jake - Has Bad Luck So You Don't Have To" (murdersofa)
05/04/2017 at 11:53, STARS: 1
This is good oppo. What system are you using to model your suspension with?
"RamblinRover Luxury-Yacht" (ramblininexile)
05/04/2017 at 11:54, STARS: 1
Frankly, I think your setup might be more fun. Better for nailing the throttle on the way out, a little slippy in a controlled way on the way in (instead of hard grip until loss). Both the piling on of gas and slight rear tire squeak are fun, ergo, your setup is better. Plus which, you can iron out anything objectionable by merely dialing up negative camber.
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 11:56, STARS: 0
Yeah that is definitely a possibility, but he’s losing speed not gaining it mid-corner as his suspension is decambering. He can carry more speed initially into the corner, but when he’s actually in it his rear end will wash wide and continue to wash wide all the way through the exit.
That I think is what will make the difference. He can carry more speed into the corner initially, but I can carry more speed through the corner and out the other side.
"DC3 LS, will be perpetually replacing cars until the end of time" (dc3ls-)
05/04/2017 at 11:57, STARS: 1
MOAR Suspension stuffs!
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 11:58, STARS: 2
Thanks man :)
It’s just a simple one online actually. Bit tricky to get set up right as they use an odd choice of measurements for some things, but quite detailed in what they give you :)
Here’s the link: https://www.racingaspirations.com/mods/1fhre8zt/
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 12:04, STARS: 0
Yeah that was my thinking too :) his would be better for banzai diving up the inside for an overtake, but mine would be (hopefully) more controllable and faster on corner exit.
I think his would be better for bumpy roads and jumps though too...
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 12:06, STARS: 0
As soon as i understand stuff well enough to explain it i shall! :)
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 12:07, STARS: 0
Oh, and squirrely diving into a corner and then a lairy controllable slide on the way out is definitely more fun ;)
"RamblinRover Luxury-Yacht" (ramblininexile)
05/04/2017 at 12:10, STARS: 0
I imagine a *little bit* more scampering on a bumpy road wouldn’t affect much, and it’s not as if the rate or damping will be different, so it’s not a quality or ride issue as such. Jumps - well, with yours carrying some extra weight for’ard, I don’t know if I’d worry too much about bite in the rear, either. Controlled grip loss might be preferred - you can have all the angle to bite with you want but if you don’t have the force to pin down on it...
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 14:39, STARS: 0
Yeah that makes sense :)
The other thought i had on my way home from work was about confidence level and safety for non-race drivers.
In the other chap’s, you come up to a corner feeling nice and planted (so fast), and then the grip tails away as you’re going through it which isn’t ideal.
Mine you feel a bit squirrelly on the entry which would probably make you slow down a bit, and then grip firms up during the corner which sounds safer to me :)
On the flip side of course, if you bottle it and lift/brake mid corner in the other chap’s it stays nice and stable. If you do the same thing in mine it spits you off...
I’d probably say that for the most part his car would be a bit more friendly and forgiving in its handling taking both effects into account...
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 14:39, STARS: 0
Yeah that makes sense :)
The other thought i had on my way home from work was about confidence level and safety for non-race drivers.
In the other chap’s, you come up to a corner feeling nice and planted (so fast), and then the grip tails away as you’re going through it which isn’t ideal.
Mine you feel a bit squirrelly on the entry which would probably make you slow down a bit, and then grip firms up during the corner which sounds safer to me :)
On the flip side of course, if you bottle it and lift/brake mid corner in the other chap’s it stays nice and stable. If you do the same thing in mine it spits you off...
I’d probably say that for the most part his car would be a bit more friendly and forgiving in its handling taking both effects into account...
"RamblinRover Luxury-Yacht" (ramblininexile)
05/04/2017 at 14:56, STARS: 0
I’d say split the baby, set yours for a little more negative camber, and have done with it.
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 15:02, STARS: 0
Yeah that’s the plan :) i’ve done some nodelling at -2 and -2.5 static camber and -2 seems like a decent compromise :) i’ll make my jig for -1 degree with enough adjustment to take it from 0 to -3 :)
"RamblinRover Luxury-Yacht" (ramblininexile)
05/04/2017 at 15:09, STARS: 0
I must admit your whole approach has been more scientific than mine, which consisted of “make everything on the back suspension - track , ride height, springing, etc. - like an SII/SIII E-type mix because it weighs about the same and more or less matches on weight distribution, wheelbase, and the rest. Should be alright.” Then again, an E has no arrant Triumph Herald genes to stamp out.
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 17:59, STARS: 0
Well if i had anything half-decent to aim for i’d probably have stopped there :) stuff for triumphs is usually either mitigating the worst of a swing axle, or ensuring that your bump-steering lower links have a static camber curve (rather than one that follows the roll).
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 17:59, STARS: 0
Well if i had anything half-decent to aim for i’d probably have stopped there :) stuff for triumphs is usually either mitigating the worst of a swing axle, or ensuring that your bump-steering lower links have a static camber curve (rather than one that follows the roll).
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/04/2017 at 17:59, STARS: 0
Well if i had anything half-decent to aim for i’d probably have stopped there :) stuff for triumphs is usually either mitigating the worst of a swing axle, or ensuring that your bump-steering lower links have a static camber curve (rather than one that follows the roll).
"davedave1111" (davedave1111)
05/05/2017 at 04:54, STARS: 0
Are you setting this up for track/racing? If not, who’s theoretically faster doesn’t come into it. Who’ll actually be faster is the driver who’s going quicker at eight or nine tenths, because on the road you don’t pile into corners at top speed in case something unexpected happens.
I’d have thought that (unless it’ll make it way too slow-handling) you want to try and get the most predictable, controllable oversteer possible rather than a higher level of theoretical performance you’ll never dare to reach on the road. That is, if you know that you’ve a wide window to come in a little bit hot and still make the corner, you’ll be less risk-averse than if you know that coming in slightly too fast will lead to spinning backwards into the hedge.
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/05/2017 at 06:22, STARS: 0
Agreed, but what happens at 8 or 9 tenths is much the same as what happens at ten tenths when comparing between these setups. The actual numbers, speed and peak grip will be different, but how they relate to each other (as in, which part of the corner they will be more likely to start sliding in as grip drops away) will be the same.
I did put some thought into how confidence-inspiring they’d be, and how that relates to safety and driving psychology, but I can’t quite work out which would be best.
No doubt in a straight line on a rough and bumpy surface his car will be more stable, which was actually the key driver behind his suspension design as those are the sorts of roads that he drives on.
However, what I was thinking is that coming up to a corner, his will feel nice and planted and you’ll feel confident coming in hot, but then (comparatively mind you), his grip will drop away mid-corner and on corner exit, potentially causing the rear end to step out.
On mine, you’ll get a little warning from the rear-end squirrelliness as you come in, before it stabilises mid corner and through the exit, which I think would make for safer handling characteristics for the amateur driver as it’s less likely to spit you off on a corner you felt confident on the way into.
On the flip-side however, mine will be more likely to punish you for lifting or braking midcorner as it’ll decamber as the rear lifts, while his will maintain a steady camber.
Which of those two effects will be more important (or pronounced) I have absolutely no idea. Plus, the only conclusions I can reach are comparative between the two. I don’t have the knowledge or the data to be making assertions about whether either car will actually step out mid-corner, or be squirrelly on the way in. I’d need to actually test it for that, or at least compare it to camber curves and handling characteristics of other known cars (while controlling for a host of other stuff like weight, weight distribution, weight transfer, tyre grip, roll, bump steer and so on and so forth). Frankly that’s bloody complicated and I’d rather just drive the thing to find out what it’s like ;)
"davedave1111" (davedave1111)
05/05/2017 at 07:01, STARS: 0
I wasn’t so much thinking about differences in how the suspension works at nine tenths instead of ten, but about how hard it’ll be sensible to drive it in real world, on road conditions.
Take a normal FWD family car, and you can jam it into a corner at pretty much any speed, jump on the brakes late, and still probably make the corner. Take a twitchy RWD car, you won’t dare approach a corner fast for fear of spinning backwards into a tree.
So what you really, really want to have is the ability to bring it back when it starts to slide. Sounds like your setup will make it harder to control at that point, more prone to snapping round if you lift slightly.
But as you say, that’s just a slight difference compared to another very similar setup. The real question is where the balance is at overall, and like you say there’s no real way to know before you drive the car. You can always add some more static camber, right?
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/05/2017 at 07:44, STARS: 0
Yeah I get what you’re saying. FWD economy cars tend to bias towards handling characteristics that come naturally to inexperienced drivers. Things like biasing towards understeer so when people panic mid-corner and lift it’s the right thing to do to correct it. On a RWD it’s important to manage snappiness mid-corner so it doesn’t spit you off, if you’re selling to the general public.
I think you’re right to try and work through how it would handle when you’ve gone just beyond the limit. Two possible scenarios there: 1. We both start to slide but keep our foot in it slightly, or 2. We both start to slide and lift off.
In both scenarios the car will lose body roll, but in the latter we’ll also decompress the rear (how much by I have no idea).
In the first scenario, mine will be probably be more controllable as it will tend towards negative camber much, much sooner pulling the rear back into line. His will likely keep spinning until the rear’s nearly level (maybe 1 degree of roll) at which point he’ll get back into negative camber and get some grip back.
In the second scenario, his will catch sooner as he’ll have a ~1.75 degree head start on getting back to negative camber (although mine will still be back to negative at ~0.5 degrees of roll, neutral at ~1 degree of roll).
Does sound like mine would be snappier if you bottle it, but more controllable if you don’t. Good little conclusion to reach before going out and testing it :)
As for adding static camber, I can offset all of the positive camber up to 3 degrees of roll if I go for -2.5 degrees of static camber. However, reading up on camber curves suggests that most manufacturers don’t tend to have suspension setups that do that. Whether that’s from a tyre wear perspective as -2.5deg of camber will definitely wear the inner edge quicker. However, it does have the added benefit of meaning the harder you brake the more stable the rear becomes (the camber tends towards levelling out at full droop, meaning your tyres get more and more stable the more you dive under braking).
A good compromise might be -2deg negative actually. From the model you get very nearly dead-level wheels at 3 degrees of roll (0.1deg positive) and less positive on droop for braking than -1deg static camber.
"davedave1111" (davedave1111)
05/05/2017 at 08:55, STARS: 0
“Does sound like mine would be snappier if you bottle it, but more controllable if you don’t.”
My concern there is that it’s not just if you bottle it, but also if you come round a corner near the limit and find some unexpected hazard that means you have to hit the brakes mid-corner. In the real world, on the road, that’s a thing that happens fairly often.
Thinking about it some more, for fun road driving rather than fast track use, what you might want is a car that’s very stable during corner entry and through the corner, and then controllably tail-happy under power as you exit the corner. That’s a much safer way to enjoy oversteer on public roads.
"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
05/05/2017 at 11:03, STARS: 0
You make a good point.
Alas, what you have with mine is a car that’s (comparatively) a bit loose during corner entry, stable through the corner and then controllably tail-happy under power, but loose if you have to brake mid-corner.
With the other chap’s you’ve got a car that’s stable during corner entry, less stable through the corner and less controllably tail-happy under power, but more stable if you have to brake mid-corner.
I shall do some testing when it’s on the road (in a safe area!) and see whether it is actually a bit snappy at -1deg static camber, or if it’s all just comparative.
Perhaps it might be helpful to compare it mine to some other suspensions. I’m thinking stock GT6 (rotoflex, not swing axle) and NA MX-5 as some good datum points for snappiness.
If I lift in mine, I end up with 1 deg of positive camber at full droop. If I lift in a GT6 I end up with 4 deg of positive camber at full droop. So, mine will be a hell of a lot more stable braking mid-corner than a stock GT6 (if not the other chap’s modified one).
If I lift in an NA MX-5, according to the camber curve I’ve found (surprisingly crap quality of information out there even for MX-5s), you end up with 2.2 deg of positive camber at full droop. So, compared to an MX-5 my car should be a bit more stable if I brake mid-corner, which is very interesting as MX-5s are hardly known for their snap-oversteer.
Of course, there’s a whole load more variables to actually consider when comparing the two. For instance, how much does the suspension of each car actually droop when you slam on the brakes mid-corner? That’ll be a function of spring stiffness, weight, weight distribution, polar moment of inertia, tyre grip and braking efficiency so again far too bloody complicated to work out, but just looking at the camber curves is interesting nonetheless :)
"davedave1111" (davedave1111)
05/06/2017 at 06:00, STARS: 0
“MX-5s are hardly known for their snap-oversteer.”
They are in my family (well, NBs, anyway). My little brother took my dad’s for a joyride, years ago now, and wrote it off backwards into a lamp-post. Shouldn’t have lifted...
As far as I know, MX-5s do have pretty benign handling traits, so that may speak more to his total lack of driving ability at age 13ish than anything else. But if you jam even a decent-handling car into a corner much too fast, lift, and then jump on the brakes, it’s not going to go well.
Are you going to have a setup that allows you to add/remove static camber once this is all built?