Pelican Parts Forums - Setting up for autocross - suspension (extreme tripod action shots here!)

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Cure for front inside lifting is more rear roll stiffness (ARB or ride springs)

Cure for jacking other than redesigning the suspension (or lowering the car to lower the roll center and CG) is to increase the ride spring rates so that the car jacks less for a given lateral load.

I don't think it is a jacking issue. The magnitude of the jacking force on the outside tire should only be about 50-100 pounds which will only extend the suspension about 3/4".

Flieger,
I had a local expert who was running a very fast 912 (on apparently stock sized rubber!) at the same autocross strongly reccomend stiffer rears - he was running 26mm, and he recommended 25mm for may car. I went with the 26's because they were available and I was impressed with the way the lighter 912 handled on the track. My car is a bit heavier in back, so I assume my ride will be less compromised than his.

They strongy recommended new shocks - and recommended setting them to the softest setting. He was running Ohlins shocks, I dont' have that kind of money. I have already ordered a fresh set of Koni classics to replace the factory installed Koni's that are on the car now.

Finally, he said I could pretty fearlessly drop the car 30mm. I may go a bit farther, but I'll use 30mm as a starting point.

912 guy was not a big fan of rear sway bars because he said it had a strong adverse effect on ride quality - recommended higher rate torsion bars instead.

All these things together would get my car pretty close to his 912, which was completely stock up front. That 912 did amazing things on stock sized rubber (he was running Avon tires) but it still retained it's stock look and was a comfortable driver.

Quote:

Originally Posted by Driven97 (Post 8003744)

Took a few minutes to think about this, and I'm going to take a crack at what I think is going on.

DISCLAIMER: I'm early on my quest to understand suspension, so any percentage up to and including 100% of what is about to spew from my fingers may be incorrect. There are at least 3 people on this board who actually know this stuff, so hopefully they will chime in.

I think what we are looking at is a phenomenon called jacking. A suspension of a car has a roll center. It's like a pivot point that a call rolls around under lateral load. A car also has a CG, which is a way to simplify where the mass of the car acts. What you end up with is a lever with the mass trying to lean the car roll center. I believe this is called the roll couple.

Normally, suspensions are designed with a roll center around ground level or slightly above. I think I read this gives the driver good natural feedback as the car leans.

Problem is, while the CG stays the same, the roll center is dynamic - it changes with suspension deflection. Also the rear has it's own roll center, and a line between the f&r creates your roll axis. I don't know how the rear trailing arm roll center is calculated. But with the MacStrut front, the roll center lowers as the suspension compresses, and raises as the car raises.

Now you may be thinking, cool. If you raise the roll center to CG height the car will not roll at all, and that will be awesome. Problem is, when the roll center gets too close to the CG, the car exhibits jacking where it lifts the whole car. Old swing axle Triumphs were famous for this:
http://herald-tips-tricks.wdfiles.co...ilt_herald.jpg

I think your front end is jacking. I think your aftermarket front swaybar is too stiff. I think it's not allowing your front end to compress, keeping your front roll center too high. I think the front of the car is just sort of tilting over, rather than rolling over and biting in.

http://2.bp.blogspot.com/-_InJvTZaOd.../TIRE+LIFT.jpg

I have the same suspension design and geometry as you, and bet I'm putting a fair amount more lateral load on the car. I have a lot of roll going on, but the car is hunkered down on those outside tires pretty good.

tl;dr if I'm right, there is more than one solution to try:

Easiest and cheapest is to soften that front swaybar. May seem counter intuitive when it seems like you have enough roll already, but as soon as that inside front is up it can't transfer any more load than it already is. I bet if you soften the front (move to the outer most holes) the front will compress and bite better.

Another option would be to lower the car a bit. I'm no expert, but it looks a little on the high side for performance purposes. If you have room on the front adjusters, you can give them a quick turn or two down the morning of an event. If you count your rotations, you can put it right back to where it was after the event and go back to your original alignment.

Next plan of attack would be to reduce the total roll. You'd have to go after the rear, as the front is apparently giving beyond all it can. That means more rear spring and/or adding a rear swaybar.

Thanks a lot for such a well thought answer! My front sway bar is factory, wiht aftermarket drop links. - you're the second person to recommend softening the front setting.

As stated earlier, ride heigth is Euro stock, but I think it's going to have to come down. I'll start with 30mm and probably keep dropping until it gets annoying as a daily driver.

The higher spring rate rear torsions are already in my possession. I'll be installing them sometime in the next few weeks. - probably the same time as the new shocks.

Quote:

Originally Posted by daniel911T (Post 8004060)

The higher spring rate rear torsions are already in my possession. I'll be installing them sometime in the next few weeks. - probably the same time as the new shocks.

You need to stiffen the rear, leave the front as it is.

26mm in the rear and you will have a sweet auto-x car with the right amount of oversteer available on the throttle.
Get it to what you call "euro height" and dial in as much negative camber as you can get in the front and add 0.5-1 degree more in the rear.
Slight toe-in both front and rear, maybe a bit toe-out in the front to get quick turn in, but I think I would start with toe-in first.

I have a good friend with that setup (+ stock rear swaybar) and the car, a 73S, is quick and fun.

Quote:

Originally Posted by Driven97 (Post 8003787)

According to our host the OE 21mm rear sway on your 87 is the largest factory bar. If you want to go bigger, you have to go aftermarket.

:Rant mode:

I think there is a big propensity in the enthusiast car world that the solution to every problem is to throw money at it. Buy buy buy. If you spend enough, then the problem will go away. Nobody takes the time to learn and understand what is really happening and why. These cars were blessed with a fair amount of adjustability straight from the factory. If you put in the time to learn, adjust, test, take notes, & repeat, you'd be surprised how capable you can make the car. But that's too much work, so fire up the Visa.

People buy adjustable shocks when you have no idea how to adjust them. Pick spring rates at random, choose your alignment specs from some random internet poster, never bother to see how something as simple as tire pressure changes affects the dynamics of the car. They listen to someone who says you need to buy this thousand dollar super adjustable widget because so & so has it on some race car.

I say, take advantage of all the options you have available to you for free first. Learn what everything you already have does, how it works, and why it was made adjustable in the first place. Yeah, it's a lot of effort. Hard work, even. But it's really fun work. When you exhaust all of your options, you'll have a deep understanding of what real problem you have. Then you'll know what part you actually do need to buy to progress instead of just throwing money and hoping it sticks.

:Rant off:

I hear and agree with your rant.

Please allow me to briefly explain myself. My car is 45 years old - and was not in my control for about 35 of those years. There are some components, like shock absorbers, that are of unknown age, but very well may be original to the car. Judging by the extreme deterioration of the rear shocks I removed I think there is a very good chance that they were equipped at the factory. Things like shocks wear - and in my case are likely worn. When I bought new ones, I didn't go for some fancy racer's special, I got the OEM stuff - which was originally adjustable when out of the car.

I'm also starting at a significant disadvantage: my car was factory equipped with the "comfort group" which does make the car ride well, but handle awfully. I will admit to having furthered the comfort philosophy by insisting that all suspension settings be stock.

At this autocross there was an expert driving a '68 912 with very heavy patina - it was brilliant on the track. I'm following his suggestions to get my car to the same point using secondhand stock Porsche parts and OEM replacements, and a lot of elbow grease.

As long as it's a daily driver, it probably won't be a savage autocross beast, but I've seen how brilliant these old chassis can be when properly tuned, and how much drifty fun can be had. I'm looking forward to getting it set up a bit more competitivley. :)

-Dan

A couple of observations. Obviously suspension design and modifications are a complex issue.

1. Forget 912s. Different animal. There are two here in Nor Cal that will tear up an autocross. (very annoying)

2. Sway bars. They serve two functions. To limit roll of the body AND to balance the roll stiffness of each end of the car. Meaning, if you just add a rear roll bar to your car (I am assuming that it never had one in the first place and it was not just removed by a PO) you will add roll stiffness to the rear. Problem is now you may have a car that wants to have the rear end slide out. Solution, increase the size of the front bar when you add a rear bar. (you should research stock sway bar sizes, there were several options) Keep in mind that going to larger rear torsions will add roll stiffness as well.

3. Shocks. Since you have Konis on order, set them soft and install them. If the car feels mushy, you can always stiffen them up. There are some advantages to Bilstiens, but, for them to work best they need to be custom valved to the weight and suspension of the car.

4. Ride quality. Any time you make an "upgrade" to your suspension, you probably will make the ride less smooth. However, if you do your homework and don't go overboard, you can firm up the ride without being overly harsh. (a friend of mine has externally adjustable shocks and stiffens them up at the track). If you stay with rubber suspension bushings and don't go too large on sways, you should be OK with ride quality.

Questions. What size front sway do you have now? Is there evidence of a rear sway ever on the car?

Quote:

Originally Posted by Trackrash (Post 8004102)

A couple of observations. Obviously suspension design and modifications are a complex issue.

Questions. What size front sway do you have now? Is there evidence of a rear sway ever on the car?

Can't guarntee, but I'm pretty sure it's the factory front sway bar with adjustable drop links added. I'll measure it just to be sure.

There are mounts for a rear sway bar, but no indication that one was ever installed. This seems consistent with a factory '69T speced with the comfort group.

Intresting aside- I've read that the self leveling suspension was standard issue on all comfort group cars, but this one really looks like the Konis were on it from the beginning. I'm guessing you could custom order and delete the fancy suspension if you wanted to.

-Dan

Quote:

Originally Posted by daniel911T (Post 8004124)

A friend of mine has a '71E that had the hydro pneumatic suspension originally, and was replaced with standard shocks. In his case the front struts were modified to receive Konis. If your car had that self leveling suspension removed are your front struts the originals?

Quote:

Originally Posted by Trackrash (Post 8004143)

The struts on the car are the complete '69 Koni factory setup. It's nice because all the normal suspension parts, like ball joints, pinch bolts, inserts, etc... fit without any effort. They're even still red. :)

(Although Koni's idea of red will always look orange to me)