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Cornering Optimizing your suspension system.

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  #1  
Old 06-19-2006, 02:50 AM
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Default My suspension setup...lets discuss

So i went to an autocross today and was noticing some understeer.


This is a post Brownmini made in another thread that would suit this one:

Quote:
Originally Posted by brownMINI
You can reduce understeer many ways.

The "right" way is by replacing just about everything.

A really good way to do it is with front camber plates.

The cheap and easy way is with a bigger rear swaybar. About half the price of camber plates. The downside is that you can wind up with a car that loops (spins / oversteers) fairly easily. I don't think that is as much an issue if you use something other than a rear swaybar.

If you've got the cash for fronta camber plates, I think I'd recommend going that route. Of course, I don't have them myself, so I could be completely wrong.

I'm not up on what your current setup is, so I don't know where to start.
My current setup:

h-sport 19mm rear sway (on full hard) (stock front non-sport+)
Koni Struts: both half turn from hard
H&R springs: same spring rate all around from what i can tell
205/50/15 Kumho SPT's
Rear seats removed
Battery is up front (cooper)
Spare tire removed
tire pressure: about 35 all around...30 was rolling over too much last weekend. This weekends 35 seemed perfect (and the temps seemed rather even)

So lots of things come to mind for a "cure" to my understeer.

Driving technique:

I will admit, i was probably going a little too hot into the corners. I'm still learning at this whole auto-x thing :o Braking a little earlier and then gently releasing the brake before making the turn may definitely help.

Struts:

I was unable run all the way back to the paddock to get the adjustment keys for my Konis which i had forgotten, so i was unable to make any adjustments on the grid. I think this one would definitely help. The settings i had found worked well (1/2 turn from full hard) came from a suggestion from Randy webb and then a little fine tuning by me at my local touge. But that was with rear seats and spare tire in. I think that lightening up the rear may need new tuning??

Sway bar?

I'm already running full stiff in the rear sway (stock front)...and 19mm is what is suggested for Coopers. Maybe a 22mm? I dont think this is necessary however.

Camber

I have coilovers on order as well as the camber plates....so i will be able to play with this a bit more. I'm just running stock camber up front as far as i know.
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Old 06-19-2006, 04:00 AM
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Ok... I am not an expert in any way but here is my opinion...

1) Run 30psi up front and 35 in the rear
2) Maybe try some trail braking (you are on an autocross track and not on the street)
3) More Negative camber
4) I got nothing else BUT driver technique

I watched a local autocrosser (not a mini) basically throw the car into the corner way to fast sliding the rear out mashing the gas and letting that pull him around... am i saying do it? no but it seemed to work for him. with what you have I would say play around with driving autocrosses and changing YOU first then you will figure out the limits of the suspension and adjust from there

just my $.01

-Josh
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  #3  
Old 06-19-2006, 04:11 AM
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Yeah, camber is absolutely crucial for autocross, since the corners are so tight. I've driven some MINIs with lots of suspension modifications but no front camber plates, and under canyon testing, they have a tendency to understeer pretty badly on tight corners, despite a stiff rear swaybar setting.
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Old 06-19-2006, 04:17 AM
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Talk to Dave as he autocrosses his Cooper somewhat frequently (I think). I'm sure he will have some helpful input. Also, don't know how this is affected by tire size but when I did my first (and so far only) autocross, it was recommended to start the tire pressure at 40 both front and rear. I am running 215/40/17s.

And that's my 1/2 cent.
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Old 06-19-2006, 06:34 AM
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Rally,

What tire pressures are you running F/R.

My autocross set-up
  • 19mm rear bar on full stiff
  • Koni FSD's with stock springs
  • Falken RT-615 205/50 R15 tires Front: 35-36 PSI Rear: 39-40 PSI

My car can actually be fairly neutral with some tire pressure tweeking (more in the back to get more rotation). One item I am strongly considering is front camber. If I do that I will most likely either drop the rear tire pressure to increase rear grip to match the increase in front grip OR experiment with backing of my rear sway to increase rear grip that way.

While it is possible to get more neutral handling with tire pressures, as brownmini stated, the best approach would be to pretty much replace everything so that the car isn't just more neutral, but that it you are also getting a good balance of maximum front and rear grip.

Personally, I still think the 19mm bar is a good choice for the MC, but that may be because it is well suited for my driving style.

Hope that helps some.

Dave
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  #6  
Old 06-19-2006, 06:43 AM
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Yah these all definitely helped

I really think that it was 3 things

Driver (huge part of it), Tire pressure (I was trying something new....last weekends set up was better), and maybe the strut settings. The first two are the ones I'm gonna look into first.
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Old 06-19-2006, 12:21 PM
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Now that I've seen what you're working with, I'm gonna guess that the understeer was largely from driver input. Sorry, but that's my guess. Your suspension sounds at least sorta similar to mine.

Changing tire pressures doesn't cost any money, so that's a good place to experiment. For autocross, I run around 37 psi all around. I'm not too worried about being hyper competitive, though.

Camber plates are the obvious next step for parts on your car. But seat time will have a bigger impact than any part.
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Old 06-19-2006, 01:57 PM
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Oooh! Another thought...

If you haven't gotten an alignment recently (after getting suspension part installed at least), that may be an easy way to improve the handling. I got my alignment done with everything within specs except I got the front set to zero toe. I don't understand all the intricacies of alignments well enough to tell you how to get yours set up, but it is something to think about.
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Old 06-19-2006, 04:27 PM
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Quote:
Originally Posted by brownMINI
Oooh! Another thought...

If you haven't gotten an alignment recently (after getting suspension part installed at least), that may be an easy way to improve the handling. I got my alignment done with everything within specs except I got the front set to zero toe. I don't understand all the intricacies of alignments well enough to tell you how to get yours set up, but it is something to think about.
Toe settings will help eliminate understeer, but there are some negative side-effects in reduced high-speed stability and increased tire wear, so I would advise using other methods before resorting to toe. If you are interested in improving turn-in with toe settings, try 1/16" of toe-out (total) in the front, and 1/16" of toe-in in the rear. This should improve turn-in considerably. If you need to be able to rotate the car more, try 0 toe in the rear, but proceed with caution!
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Old 06-19-2006, 04:53 PM
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what happens with 0 toe in the rear ?
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Old 06-19-2006, 05:09 PM
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Quote:
Originally Posted by Detlman
what happens with 0 toe in the rear ?
Well, the standard setup is to put a little bit of toe-in on the rear suspension, which gives it directional stability at high speeds. Removing that toe-in will reduce that stability, giving the car the ability to rotate more easily, especially at low speeds. Combine that with toe-out in the front, which makes the inside front tire basically scrub harder, forcing the car to rotate, and you get a car that is very nimble in tight corners. If you want to go even nuttier, you could use toe-out in the rear, but I wouldn't advise it, as it might make the car too unstable and twitchy, even at low cornering speeds.
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Old 06-19-2006, 05:27 PM
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thats not entirely what I was expecting you to say I have an issue with the ass coming around in sweepers at high speeds ( 80 + ) other than that is really stable and even at 130 or so going straight its really stable. I am running 1/16th toe out in the front, 0 in the rear and -2* camber front and rear. I am really happy with what the car will do with the exception of the ass coming around at high speeds on a sweeper turn, that could get ugly if it actually got away.

I was thinking to put the front bar on the stiffer setting and see how that affects it but now I may just go and have a little toe in put in the rear. Or do you have a better suggestion?
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Old 06-19-2006, 05:33 PM
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Quote:
Originally Posted by Detlman
thats not entirely what I was expecting you to say I have an issue with the ass coming around in sweepers at high speeds ( 80 + ) other than that is really stable and even at 130 or so going straight its really stable. I am running 1/16th toe out in the front, 0 in the rear and -2* camber front and rear. I am really happy with what the car will do with the exception of the ass coming around at high speeds on a sweeper turn, that could get ugly if it actually got away.

I was thinking to put the front bar on the stiffer setting and see how that affects it but now I may just go and have a little toe in put in the rear. Or do you have a better suggestion?
I would definitely introduce a bit of toe-in for the rear and see how it feels.

Another thing you didn't mention that would definitely contribute to your problem is the front splitter. See, producing downforce isn't the only consideration in tuning for aerodynamics. More important, in your case especially, is aero balance, front to rear. At high speeds, where aerodynamics come into play, you're likely producing considerably more downforce (or considerably less lift) in the front, which causes the front end of the car to grip a lot more than the rear. The result is a car that becomes increasingly less stable in the rear as speeds increase. I'm not sure how you have that splitter trimmed, but if you can trim it slightly less aggressively, it may help your cause. That, or add a functional rear spoiler to balance it out.

In the meantime though, a little bit of toe-in for the rear would do a lot of good for stability. Stiffening the front swaybar would help your problem, but it would also hurt turn-in and cornering speeds more than a minor rear toe adjustment will.
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Old 06-19-2006, 11:28 PM
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ok first off you dont have to change much to get a car to stop understeering....
first thing you need to change is your technique...make sure you are taking the best possible line through the corner at the right speed...when you cant go any faster with how your car is....then dial in one thing at a time, so you can see what changes, rear sway, front camber and toe, etc. until you find something that suits your driving style
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Old 06-20-2006, 01:06 AM
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Quote:
Originally Posted by iDiaz
I would definitely introduce a bit of toe-in for the rear and see how it feels.

Another thing you didn't mention that would definitely contribute to your problem is the front splitter. See, producing downforce isn't the only consideration in tuning for aerodynamics. More important, in your case especially, is aero balance, front to rear. At high speeds, where aerodynamics come into play, you're likely producing considerably more downforce (or considerably less lift) in the front, which causes the front end of the car to grip a lot more than the rear. The result is a car that becomes increasingly less stable in the rear as speeds increase. I'm not sure how you have that splitter trimmed, but if you can trim it slightly less aggressively, it may help your cause. That, or add a functional rear spoiler to balance it out.

In the meantime though, a little bit of toe-in for the rear would do a lot of good for stability. Stiffening the front swaybar would help your problem, but it would also hurt turn-in and cornering speeds more than a minor rear toe adjustment will.
Actually the splitter was on way before the problem started. It started when I stiffened the rear with different bushings. I am not saying the splitter has nothing to do with it. I think I will try a slight toe in since the problem came up right about the time we did the last alignment and I believe the car had a slight toe in before. I am glad this came up.

Brit, obviously technique is always first to be looked at but the way the car comes loose is like the pavement becomes a slick track, no warning just a smooth drifting sensation as the ass tries to come around. Its actually very smooth when it does it and only at relativly high speed. AND its only happened on a left sweeper. Other than that the car really feels planted, stable and secure.
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