JeffDC

Wow, thanks for all of the input. It sounds like there is a real consensus around the front bushings and toe. As I said, car was aligned to roughly RS specs (more rear camber) about 1 year ago. Only about 4,000 miles since then but things could have changed, I guess.

Bill, my alignment report is attached. My toe numbers are in degrees, your chart is in mm. What am I missing there?



I don't know spring rates but will get the car up and look for markings. I saw the standard H&R Super Cup rates somewhere on-line and IIRC they were 625/450. I have no reason to believe that the shock valving was ever changed, but can't be sure (H&R setup was by PO).

I would say that I do not feel the straight line twitch at the seat. It is more a sense of hyper-sensitivity to and lightness of steering input.

On this bump steer issue -- I suspect that I have it (given what you guys have to say about the original uprights and RS ride height) but how does it manifest itself?

The car does not feel loose under heavy braking.

Bill Verburg

I never edited the left field, the units are as specified in each field
your front toe is 6' on the left and 18' on the right compared to mine 0' on each side, that will case the car to pull to the left, you also have caster#'s that will cause a pull to one side.

All in all you alignment is poor and most likely is the problem, I'd go to a shop that knows what they are doing and have it redone.

The shocks you have are a great starting point but the stock valving sucks, I'd send them to Bilstein San Diego for a digressive revalve

JeffDC

Thanks Bill. I will read up on revalving and talk w/ Bilstein. It sounds like my winter project list is shaping up.

Sorry if I am being dense, but how do you arrive at the 6'/18' for front toe? Is there more to the story than the 0.05 deg/0.05 deg (3'/3') shown on the report?

Bill Verburg

My mistake I was looking at the before #s,

the after #s look good

was the car corner balanced? if so then bushes or tires or tire pressure seem to be the issue

NC TRACKRAT

Concur with JSCOTT82 and Bill Verburg on the rake issue. Did not see a reply on your rake angle. Rear engined car=wt. biased to the rear with a light front end. Get air under the front and you lose grip=sphincter pucker!

JeffDC

I finally got out to the garage today -- I used Bill's older post with instructions for measuring ride height:

Ride height (F/R): 126/119

Rake: 1 degree rear up (at sill)

So, the front looks to be at RS height but the rear is about 10mm high, likely because that's as low as it would go with the sway bar above the toe links.

Conclusions?

- Rake is ok? (In fact, more rear-up than RS spec)

- In addition to front bushings, should I consider rear RS drop links so that I can get the rear down? Would that help with planting the car in fast sweepers?

Many thanks for the continued help...

NC TRACKRAT

Your rake measurements would appear to be o.k. Question: Do you equate the lightness in the front end to fuel level? If so, try to maintain 1/2 tank or more. Do you run with the spare in place? If not, try it for possible improvement. (Safety measure,too.)

Bill Verburg

Rake is fine, at that height you will want the RS wheel carriers, if tracking the car mono-ball tie rods

if you are feeling the twitch in front replace the front bushes, if you fell it in the seat do the back, when i did mine I did both ends at once.

taking a while to take a set can be a result of using tenders springs, that's why I don't use them.

you don't mention spring rates, it sounds to me that the car like most 993s is understeering a lot, wider front wheels and or tires help, you want the rear to be ~10 - 20% stiffer than the front, 10% is less understeeer 20% is a little more that can still be dialed out relatively easily, and w/ as little rear roll steer as posible

JeffDC

I typically run with about 1/2 tank and have not noticed any difference as fuel level changes, though I will now pay attention to that.

I don't even know if my spare will fit over my brakes -- it's been up on a shelf in the garage since the day I got the car -- I guess I should check that!

I've now involved my trusted independent Porsche mechanic. The guy's depth of knowledge of these cars never ceases to amaze me. He is on-board with your thinking Bill (and many others that have kindly weighed in) -- front bushes and tie-rod monoballs for sure and a hard look at spring rates and either shocks or re-valving, though Taylor is concerned about what I might spend re-valving and the risk of the re-valved shocks really not working well with the springs -- I will speak with Bilstein about this, as you've suggested.

Taylor did mention something about "tie rod end height adapters" as a less expensive way to deal with bump steer in some situations. I will gather more information on that. Do you know anything about them?

I am still looking to see if I can find my spring rates...

Continued thanks

Bill Verburg

Yes, Geoffrey uses the ball joint extensions in addition to RS uprights on his 964

From his racecar build thread
top offset tie rod outer, bottom RS tierod outer


offset tie rod w/ RS wheel carrier


AFAIK the mono-ball inners are not available as a separate part, you have to buy the outer/inner assembly which only comes in RS form.

valving is matched to the spring rates and car weight, it cost ~ $150/shock and is worth every penny

JeffDC

I have a deep-in-the-weeds question about bump steer for you guys that have weighed in:

I've been reading about bump steer and now have a basic understanding of the geometry and kinematics involved but I can't feel it when driving the car. I confirmed that I still have stock wheel carriers and tie rods and that tie rods are indeed at an angle from the horizontal (as you all knew).

I took the car out twice over the weekend with the objective of feeling its effects. Driving with little more than finger tip pressure on the wheel, I expected to feel the car want to steer off-line when going over bumps. I did this on our bumpy DC area roads at both slow and highway speeds. Nothing. She continues to track straight when either or both wheels go down into dips or up over bumps.

Is the bump steer effect more subtle than that? Is it possible that what I am calling the "delay in taking a set" in faster sweepers (i.e., Hog Pen) is caused by bump steer -- by a change in front toe caused by the outside suspension being compressed more than the inside?

certz

I will also be interested to read what the "experts" have to say on this, but it was always my understanding and impression that bump steer is felt (and is definitely more dramatic) when cornering over rough and bumpy surfaces. The poor geometry tries to straighten the wheels in this situation. I am not saying that there is not an affect when travelling straight, just not near as dramatic. Nonetheless, let the guru's weigh in...

Bill Verburg

There are 3 things that mask bump steer in a stock 993
1) the very sloppy front A -arm bushes
2) the power steering mechanism
3) the scrub radius of the front end


In a pre 964 car the steering feels alive, like a snake in your hands, it is always making small L/R movements, when you have bump steer in these cars it is very noticeable, the more the arm movement differences L/R the more it is felt, in 964/993 you need much larger A-arm movement differences to feel the same thing, but it is still there.

If you get a chance to jump into a car w/ the stiffer bushes, RS wheel carriers/monoball tierods and wider front wheels you will easily discern the difference

dgmattingley

I'm also interested in hearing from the experts on this. I "think" I notice it as an unsettled feeling at the front end over bumps and crests that gives the car a general unpredictable nervousness, if only briefly in most cases.