should i drop the rear

[Alan L]

Originally Posted by JFairman

The spring plate bolts can be more difficult than they look. If adjusting them while on the car you'll need a really thin special open end wrench to fit between the longitudinal and the spring plate to get at the big bolt/nut and they are on there really tight. I think they are eccentric bolts that work kind of like a cam lobe while you turn them. I've never done that though so I don't know..

Correct.
I made a special tool out of something like 1/8" flat.
Ideally the spring plates want to be near center when you do the splines - that gives you leeway either way if you want to tweek it later. Or for corner balancing later - which you will prob want to do if you keep doing track work. Doing the splines is a bit of a PITA, so you don't want to be fiddling with them too much.
As soon as I am home again (sometime in next few days ) I will post the spline calcs. You can get very fine adjustments with them. Meantime you may want to make up a spring plate adjuster tool.
Meantime you can remove a rear wheel and see where the spring plate rests. Scribe a line on the body along the spring plate top edge. This is your reference mark before doing the splines. If you want 1/2" lower then you should be RAISED approx 1/4"at the end of the spring plate relative to your reference line when done with the splines/spring plate adjustment. The spring plate is about 1/2 the length of the suspension arm from memory -so the end of the SP changes by 1/2 what you want at the axle.
Regards
Alan

[Alan L]

The calcs are approx like this. 40 splines = 9.0 deg rotation. 44 splines = 8.1 deg rotation. So one major spline rotation and one minor spline counter-rotation = 0.9 deg rotation. Knowing the length of the suspension arm to the axle, you can do the trig and calculate what 0.9 deg translates to in height adjustment. From memory it is around 3-5mm.
So depending where your spring plate is adjusted you can work out how much height adjustment you need from the splines and do the numbers of rotate/counter-rotate required to get 1/2" or 1", or whatever. One 40 spline rotation and no counter- rotation would give you somewhere around 30-50mm. I will confirm that number in a few days.
If you were to do that process, I would scribe your reference mark, then center your spring plate, and then do the spline calcs and work from there. You will get to where you need to be then and have the SP centered at the same time.
Regards
Alan

[T77911S]

i was told there was no math involved.
just kidding.

thanks for all the info.

I will check the ride height first.
anyone have the factory ride height specs.

[JFairman]

Factory USA ride hight specs will have the car looking like an off road vehicle because USA DOT specs wanted all cars to have matching hight 5 mph impact bumpers back then.
Euro ride hight specs are lower and look better. I would suggest you set ride hight where you like it and then have the car professionally aligned and corner balanced.

[JFairman]

When I've done what Alan talks about in his excellent post above I removed the bottom bolt and it's short steel sleeve in the spring plate cover so the spring plate would go all the way down to where the torsion bar is relaxed. Then I put a digital angle finder gauge on the top edge of the spring plate and wrote down what it said.

Scribing and measuring should work well too. The digital angle gauge is just quicker and more accurate. All the angle gauges I've seen, digital or mechanical will work and they usually have a magnetic edge on the bottom so you can stick it to the top edge of the spring plate and leave it there while working on it.

The digital ones have magnets on 4 sides and with one of those you can stick the gauge to a brake rotor while the car is up on jack stands while doing camber adjustments and keep track of what you're doing.

When the car is up on jack stands and some of the spring plate bolts are removed the trailing arms will drop down farther and the rotors will be at positive camber so you'll have to figure out how all your measurements relate to when everything is bolted back together and the wheels are on the ground and rear wheel camber is between 0 and -1 degrees or wherever you want it.
On a street car 0 degrees or just a tiny bit of negative camber will let the rear tires wear more evenly.
For a track car you may want more negative camber in the rear wheels to reduce oversteer. That will also wear down the inner edge of the rear tires faster than the outer edge depending on how you drive.

It is a lot of work and it takes practice to get it right and you will learn a lot while doing it.
Or pay a good Porsche shop to do it.

[Bill Verburg]

Originally Posted by JFairman

Factory USA ride hight specs will have the car looking like an off road vehicle because USA DOT specs wanted all cars to have matching hight 5 mph impact bumpers back then.
Euro ride hight specs are lower and look better. I would suggest you set ride hight where you like it and then have the car professionally aligned and corner balanced.

That's only true though '83, from '84 up there is just one spec worldwide

from '84 w/w spec for Carrrera & t look is 108+/-5mm front, 12+/-5mm rear
for '84 up 930 it is 94+/-5mm front, 16+/-5mm rear

note that these #s mean the front of a stock Carrera or t-look is lower than 930 & the rear is higher

[mark houghton]

I can contribute absolutely nothing to this discussion, with all things alignment etc. way over my head. One thing that keeps popping up....tire pressure....has always stumped me vs. factory specs. Why is it that many folks recommend considerably less psi than the owners manual 29 front/44 rear? What was the good Dr. Porsche trying to accomplish with that extreme F/R bias?

My car "looks" pretty well balanced (so very deceiving are looks) and handles OK I guess, having no properly suspension tuned 930 to compare against. If I were to track it I would probably mess my pants.

[Alan L]

When I first put mine on the track I messed my pants. It felt like a jelly fish on P. But 30+ yr old rubber suspension bushes etc.
Wrt to the tyre pressures - consider also, for hwy driving, you are driving on 'wets' - treaded tyres. What you have regarding pressures and tyres is a compromise which is a long way from track settings. You are not expected to heat cycle your tyres on the hwy anything like the track. Therefore you run higher than track pressures - to keep some rigidity and performance from a relatively cold tyre.
Start hammering it on a track with those settings and quickly the tyre will overheat for the compound used for a 'wet' cold tyre. And it turns to a blob of jelly. The profile is wrong, and the rubber will be overheated. etc.
Alan

[puddy]

Originally Posted by mark houghton

I can contribute absolutely nothing to this discussion, with all things alignment etc. way over my head. One thing that keeps popping up....tire pressure....has always stumped me vs. factory specs. Why is it that many folks recommend considerably less psi than the owners manual 29 front/44 rear? What was the good Dr. Porsche trying to accomplish with that extreme F/R bias?

My car "looks" pretty well balanced (so very deceiving are looks) and handles OK I guess, having no properly suspension tuned 930 to compare against. If I were to track it I would probably mess my pants.

Hi Mark, FYI I run R888's on street and track. Car is approx 2500 lbs. I run 255 x 18" front and 315 x 18" rear. Set cold pressure at 24 front and 26 rear. This seems to be a consistent pressure I've used and liked for keeping the car balanced under hard braking on track.

[mark houghton]

Originally Posted by puddy

Hi Mark, FYI I run R888's on street and track. Car is approx 2500 lbs. I run 255 x 18" front and 315 x 18" rear. Set cold pressure at 24 front and 26 rear. This seems to be a consistent pressure I've used and liked for keeping the car balanced under hard braking on track.

I've been running with a 4 psi differential between front and back on stock tires and wheels, something like 32/36 respectively, for no well thought out or tested reason. Guess I'll go back to the drawing board and push the other end (lower psi yet) of the spectrum which for the highway might be too low. Following Alex's description, I would presume that even with your low(er) pressures you're probably much higher once tracked and heated up. What's your hot pressure after a good track run?

[T77911S]

ok, I got a better offer this weekend so all I did on the car was check ride height, I did not even do the brakes,

here is what I have.
all measurements are C to C
RR- 20mm center of wheel to Tbar
RR- 11 7/16 in Tbar to ground

LR -15mm wheel to Tbar
LR- 11 3/4in Tbar to ground

left rear is higher no one in the car


the angle of the Tbars was sloping down plus I could only see the FRONT end of the Tbar and the wheel center. I used a lazar level to shoot a line to the center of the Tbar and the wheel. then I measured from the center of the wheel down to the lazar line.

RF- 6 in (152mm) wheel to Tbar
RF- 6 1/4 in to ground

LF- 6 in (152mm) wheel to Tbar
LF- 6 1/4 in to ground


weather is suppose to be really nice this week so I wont tackle the rear just yet.
I may drive it down to Charleston today and buy the wife a ruger mark iv .22 for Christmas.

[puddy]

Originally Posted by mark houghton

I've been running with a 4 psi differential between front and back on stock tires and wheels, something like 32/36 respectively, for no well thought out or tested reason. Guess I'll go back to the drawing board and push the other end (lower psi yet) of the spectrum which for the highway might be too low. Following Alex's description, I would presume that even with your low(er) pressures you're probably much higher once tracked and heated up. What's your hot pressure after a good track run?

OP sorry for hyjacking Mark, Track pressures grow 6-8 lbs when hot so I'm roughly 32/34 when hot.

[T77911S]

here is what I have found. hope it is correct:

40 on inside. 1 tooth=9*
44 out. 1 tooth=8* 10min.

1*=8mm
50min=6.5mm

inside one direction. outside opposite= 50min=6.5mm

ride height(?)
front=108
rear=12. Tbar ABOVE center line of wheel

here is the kicker. for my rear the Tbar is BELOW the wheel.
that would make my car about 40mm low in front and 30mm low in the back

looking at it it looks like there is only ONE combination for the ride height I am at.
I could not image how hard it would be to put in NEW Tbars. still working on this.

[Alan L]

Have access to my calcs now. These are based on my SC. A critical measurement is the distance between the center of the torsion bar and the rear axle. On the SC it is approx 460mm. If the 930 is different, give me the number and I can recalculate.
On the measurements for the SC, 5.0 deg = 40mm adjustment. So 0.5 deg = 4mm etc.
Regards
Alan

[T77911S]

that is what I read in bruce andersons book.

1*=8mm

460mm? that's 18in

[Alan L]

1 deg = 8mm, 5 deg = 40mm. We agree.
Yes, 300mm is close to 1 ft.
Alan