Spyerx

Bill Verburg

There were several changes made on the 993 compared to the 964 to improve the steering
caster was increased, ( this slows the steering response)
steering o/s(aka scrub radius was decreased from 0 to -11.5 mm, (this increases feedback at the wheel)
the steering ratio was changed to 1:16.5 w/ a modified assist curve, (but they don't go into more detail about that)
Reduction of 'retarder lever arm', results in reduction of negative influences on steering( I haven't a clue about this one)

Spyerx

SOOOO Helpful as usual !!! :-)

A few questions:

caster was increased, ( this slows the steering response)
**OK this is adjustable at the upright, bottom of the A-Arm right? So, with 993 uprights, 993 tie rods, etc you're effectively adjusting this anyway? IIRC, my shop will max caster as much as they can within limits of tire rub/alignment, etc.

steering o/s(aka scrub radius was decreased from 0 to -11.5 mm, (this increases feedback at the wheel)
**What impacts this? In the rack or the alignment of the upright to the tie rods/etc??

the steering ratio was changed to 1:16.5 w/ a modified assist curve, (but they don't go into more detail about that)
**I suspect this is the rack. I guess it's possible the ratio off center is different than the 964.

Reduction of 'retarder lever arm', results in reduction of negative influences on steering( I haven't a clue about this one)
**What is a "regarder lever arm"!?!?!

Bill Verburg

yes, most of us want this maxed anyway and it is difficult even to get to spec, most track guys end up w/ ~4.7° - 5°, spec on 964 was 4°25'+15'-30' and on 993 Cups/RSR 4°, 993GT2 Evo 4° 30'
The less caster the faster the end of the car reacts to any input, also the less self centering of the wheels

[QUOTEsteering o/s(aka scrub radius was decreased from 0 to -11.5 mm, (this increases feedback at the wheel)
**What impacts this? In the rack or the alignment of the upright to the tie rods/etc??][/QUOTE]
The closer to zero + or - the less feedback and self centering, ABS equipped cars need neg for the feedback loop to work in a positive manner to control the car w/ uneven grip side to side. This is a function of the SAI(steering axis inclination) aka kingpin angle(KPI) and wheel ET

Peteinjp

Spyerx

Right, so on the 993, how did they impact this? I know the front end was pushed out 10mm / side if I recall correctly via the A-Arms, and maybe the wheel offsets different OEM?

I guess reason I'm asking is real feel difference in the 993 rack. I def. don't feel like I need better off center feel or a quicker, more aggressive rack.

Bill Verburg

The 993 longitudinals push the control arms out board 12.5mm more than on a 964. Since the change in scrub radius was 11.5 that accounts for most of the Δ.

The rest is either in wheel o/s or repositioning the top of the strut.

any time the ET of the wheel goes numerically down the scrub goes up by the same amount.

ThomasC2

I've had the 964 manual rack for many years. A couple of years ago I did a make over of my front end and put on 993RS uprights, 993 longitudinals and 993 tie rods etc. I felt that the steering became better after these mods. More sharp and with a more direct feel.

tjb616

Thomas, could you offer any more detailed insight into your process? I have all the parts and will be talking the same project this winter. Any alignment issues or hiccups installing the longitudinals? Did you use the same 964 shocks? EDIT: Found your Report thread, thanks!

One issue I know I will face is with the 993 master cylinder and brake booster lines mating to the 964. Did you happen to do that upgrade as well?

Peteinjp

ThomasC2

Yes I have the 993 cylinder/booster/lines too. The hard lines are from BMW but the soft ones are the same so re used them.

tjb616

That is precisely what I am trying to find out. Ive only read anecdotally of the issue.

Thomas, do you have a part number or source for the BMW line? Is it a bolt-in solution to mate the two pieces? Require any modification?

ThomasC2

I may have pictures of the stickers with the part numbers. I'll see If I can find them. But...there are other threads here with that info.
The good thing with these two pipes are that the length is right and the ends have the correct nut size, 12 mm in one end and 11 mm in the other which is what you need for the 993 parts. (or if it is 11 to 10, I don't remember,)
So the only thing you have to do with these pipes is to bend them the right way to make them fit.You could also build your on pipes with standard brake line parts. The key issue is that the fittings on the 993 parts are 1 mm bigger.

Bill Verburg

These are the BMW 10x12 brake lines, part #34-32-6-752-405

Spyerx

You can see the parts in my build thread to go to the 993 M/C. You don't need the adapters if you use the BMW lines. They are straight bolt on, but, you do need to bend the tubing from the m/c to the chassis connection point. It's not a hard job. The lines are the right length and sizing on both ends/flared correctly.

Regarding this conversation, I have access to a set of 993 longs / a arms but I just calculated given my current wheel offsets and my front end will push out too far. I'm just going to leave well enough alone :-)