Removing the rear sway bar?
 

I recently attended the Mazda & SCC hosted Track Attack event at Button Willow and had a chance to drive extendedly on both East and West Loop and the cars handling felt great. Everything was very predictable and linear.

The only time I had a problem was approaching a right hander at high speed 110+. It was a right incline going into it and then begins to decline almost immediately after the apex leading into a left turn. I would begin steering input for the left turn too early while load was still light in the rear and spun.

I didn't think anything of it until recently driving on some roads not quite as smooth & compliant as Button Willows. The car didn't feel very confidence inspiring, as if the rear wanted to come out. A friend was watching from behind and noticed that as the car was turning in, the outside rear suspension looked as though it was under full compression, while the inside was close to being lifted off the ground. The front was completely planted. After hearing this, it made me think right back to the Button Willow event in that, the rear felt like it was having trouble making full contact.

Im using the JIC FLT-A2s with 8kgF/6kgR spring rates with stock T2 sway bars, torsen differential, PU bushings. The cars been cornerweighted with exactly, to the pound, 50/50 front to rear distribution with driver and 11 gallons of fuel, exactly 50/50 cross weights, and left to right ration of 51.5/48.5. Vehicles overall weight without driver and 11 gallons is at 2580. So from a static view point, at least the chassis is balanced exactly the way I want it. Alignment settings at -2.4F & -2.6R camber and 1/32F & 3/32R toe in for both.

The thing I dont understand is how the rear outside can be compressing so much with so little relative static load on it. I've thought of removing the rear sway bar to allow the inside wheel to move independent of the outside wheels load and using 8kg/mm rear springs to compensate for any body roll.

I hope my information the situation paints a good picture, if not, I'll try to be more clear if you have questions. What do you guys think about this?

I'd be surprised if that car didn't oversteer with that setup. Most ITS FCs run without a rear sway bar.

My springs are the same proportion as yours, but about 10% stiffer, if I did the conversion correctly (500F/375R). I've got a 1.125" front sway bar adjusted to about 50% stiffer than the factory bar. No rear bar. I've got a little understeer with equal tire pressures. With +2 PSI in the rear tires, it's balanced pretty well to my liking. Sometimes lifts a front wheel, but I don't know that that's a problem.

Did you disable the rear steering? If not, do it. Your rear toe-in might be marginal for the track. It tends to toe out slightly under compression, even with the rear steering disabled. I've been advised to run 1/8" to 3/16" rear toe-in. Most track FC's run 0 or slight toe-out in front.

Make sure that you don't lift while you've got significant cornering load - that'll bring it around. If you can't maintain some positive throttle in that second turn-in, then I think you'll have to slow down a little for the first corner.

And finally, if I understood you correctly, please don't be running HPDEs on the street. The life you save may be mine.

There is a lot of static load on it!!! 110 mph uphill into a right turn? The left rear is going to be very highly loaded! What you don't want to do is crest the hill and brake or change direction at the same time. As you crest the hill the rear unloads and you don't want to hit the brakes and unload it even more or turn the front wheels and ask it to change direction before it has a chance to settle. Let the rear end "land" and settle before you point the car the other way. The hill makes all the difference in the world to this corner, you can't drive it as if it were flat.

If this is the only turn in which you don't like the car I wouldn't be concerned. This is an interesting corner and it's not being driven correctly. I would get a ride with a fast local guy or discuss this corner in detail with him. Spinning the car is always a driver error unless something suddenly broke. Given that you feel the car is excellent elsewhere the problem isn't the car and there's no sense in changing the setup for one corner of a track when it's so good everywhere else.

Right, I disabled rear steering with the RB bushings. Unfortunetly I had to compromise with the rear toe settings because I do drive on the streets about 400-600 miles a month so I didnt want it to be too excessive.


DamonB- Im sorry, I kind of jumped points, the point of reference I was using for that statement was as I had someone observing my driving at a later time. It was a downhill winding left turn. I understand that the suspensions going to load up on the outside, but so much so that it would be compressing near full on the outside rear and trying to pull the inside rear up with it?

Like you said, im kind of iffy on changing my setup for that one turn but it brings up the functionality of my stab. bars and if they're limiting the rear tires independent travel through certain turns.

Does their function start to void when you start increasing spring rates? I guess I need to read up on their fundamentals. Thanks for all the pointers so far though. For that specific turn, I believe if I just modify my line a lttle more, to where I can keep steering input to a minimum while still salvaging most of the speed from the straight. Guess I'll find out next time.

Sort of. You already know that a higher spring rate will reduce chassis roll. The sole purpose of the sway bar is to reduce chassis roll as well so if you have stiffer springs you won't need as much bar to get the same roll stiffness.

I'm trying to picture the turn that you are describing-- can you go to "Track Maps" at the Buttonwillow website and tell us:
- the name of the turn
- which configuration you were running at the time
- whether you were running CW or CCW

See: http://www.buttonwillowraceway.com/

The configuration was Race #27 Lost Hill in a clockwise direction. The specific turn was "lost hill".

The track was even a bit more modified then what the map details because of the use of cones on both straights.

Here's some video from a guy (Trygve) that has instructed at some of the driving schools I have been to. Take a look at his configuration 1 video, which includes Lost Hill in the same direction you were running. Note how he's set up so that he's already going the correct direction before he crests the hill. Watch his steering input and listen to the throttle as he sets up for and goes over the hill-- he keeps the wheel steady and stays on the throttle (as Damon explained, that will keep your car more stable as you come down the hill). Since he's already pointed in the right direction, he doesn't have to steer left until he's basically come off of the hill. That's basically how I've always been instructed to handle the Lost Hill turn.

http://www.bombaydigital.com/boxster/tracks/

Thanks for the video. I'll just have to work on it next time im out and try to figure out the suspension tuning on a skid pad.

You're doing 100mph going into magic mountain????

that's the only uphill section I can think of, and it's no where near 100mph in my FC.

Two things jump out at me.

Front toe IN? 0 is fine, toe out would be good, but toe in is not.
Rear camber at 2.6? I'm running neg 3.5 front, and would like neg 2 in the rear, but can only get 1.3 right now.

I'd stay with the rear bar, I didn't like the handling with no rear bar. But it all depends on spring rates, just removing the rear bar means nothing unless your spring rates make sense for that configuration.

PaulC

Paul, what springs and bars do you run? I've been experimenting with some different setups and think I probably don't have it quite right yet.

http://www.buttonwillowraceway.com/race_27.htm

Its the turn right before Lost Hill on a clockwise rotation.

The alignment was set to spec with the help of Darrin at West End Alignment and I can't say i have too many complaints about it. Yea rear is at -2.6

As I stated before, if I were to remove the rear bar, id try out the higher rear spring rate to compensate. But after consulting others, I'll hold off on the stab. bars until after I get my new set of wider wheels and tires.

Eagle,

500/375, with Eibach SB front and rear, mazdatrix endlinks. I like that much better than my original 400/325, I'm not on the bumpstops in hard constant corners, but it's approaching the limit my Konis can handle.

Nismo,

Understood, yeah, you're flying leading into LH (or magic mt.) , but at the actual corner it's slower.

No idea who Darrin is, but 2.6 is a lot in the rear, I don't know of anyone running that much negative rear camber, and it may be affecting your ability to power out of a corner.

I've got vid from my last outing at BW in April, check here: http://para.noid.org/~silkworm/bw-040404-race.wmv. We ran config 1, clockwise, using the same turn you used.

Anyway, good luck.

PaulC

I see. I'll definetely check out the video. Thanks for the link.