Track suspension set up ?
06-18-2012, 10:11 PM
#91
Bob, thanks for your comments. I think I have my head around what's going on but haven't had a chance to experiment develop a seat of the pants feel for how less vs. more behaves yet. Have you swept the range of adjustment to see how the car responds?
Here are a few more plots that show what's going on in the rear of a 993.
I think most have seen this one (996 looks remarkably similar). This is the toe / camber curve vs. vertical wheel movement:
Here's part 1 of the kinematic toe effect, i.e. toe-in due to lateral accel:
Here's part 2 of the k-toe effect, toe-in due to braking (longitudinal accel):
Note the units, m/s^2 so using a modern tire results in operation a fair bit off the right side of these graphs.. fortunately the effects look to be linear.
Couple of things come to mind:
1) base 'static' toe per side = 15' seems high. this was reduced to 5' in the 996
2) braking @ a moderate 0.5g results in approx 8' additional toe for a total of 46' (0.77 deg). This increases to >1deg during max decel. No wonder rear tire life is notoriously short on these things!!
3) The inclination of the instant axis (caster) in the rear is 4.5deg @ stock settings. This results in additional camber gain due to kinematic toe-in that isn't captured in these plots. More camber results in more camber thrust - I suspect this is what causes the spooky rear behavior when both sides don't match.
4) According to the GT2 set-up sheets posted before, rear caster is reduced to 3.5deg in the race cars(https://rennlist.com/forums/7159535-post1.html)
This might be the result of tilting the rear subframe down in front, but that can't be the *only* reason for the tilt kit as the same effect can be achieved by just adjusting / lengthening the KT-arm. (ahem, reduced antisquat). I suspect Porsche's goal was to reduce movement back there & make the car more predictable but need to ponder it a bit more as there might be more going on.
Bill, not sure what you mean. The upper arm arrangement is basically the same 993 <> 996. The lower arms are similar too, except that the lower A-arm from the 993 is replaced w/ a 2pc lateral / trailing link. The inner lateral bushing (below the 'LH' marking in your photo) is *much* stiffer than its 993 counterpart, essentially eliminating the k-toe effects in the later cars.
Cheers,
Tom
Here are a few more plots that show what's going on in the rear of a 993.
I think most have seen this one (996 looks remarkably similar). This is the toe / camber curve vs. vertical wheel movement:
Here's part 1 of the kinematic toe effect, i.e. toe-in due to lateral accel:
Here's part 2 of the k-toe effect, toe-in due to braking (longitudinal accel):
Note the units, m/s^2 so using a modern tire results in operation a fair bit off the right side of these graphs.. fortunately the effects look to be linear.
Couple of things come to mind:
1) base 'static' toe per side = 15' seems high. this was reduced to 5' in the 996
2) braking @ a moderate 0.5g results in approx 8' additional toe for a total of 46' (0.77 deg). This increases to >1deg during max decel. No wonder rear tire life is notoriously short on these things!!
3) The inclination of the instant axis (caster) in the rear is 4.5deg @ stock settings. This results in additional camber gain due to kinematic toe-in that isn't captured in these plots. More camber results in more camber thrust - I suspect this is what causes the spooky rear behavior when both sides don't match.
4) According to the GT2 set-up sheets posted before, rear caster is reduced to 3.5deg in the race cars(https://rennlist.com/forums/7159535-post1.html)
This might be the result of tilting the rear subframe down in front, but that can't be the *only* reason for the tilt kit as the same effect can be achieved by just adjusting / lengthening the KT-arm. (ahem, reduced antisquat). I suspect Porsche's goal was to reduce movement back there & make the car more predictable but need to ponder it a bit more as there might be more going on.
Cheers,
Tom
06-19-2012, 10:26 AM
#93
993 GT2 Evo, like 993 RSR uses all uni-ball joints, So yes they set the rears up the same, they want min caster equal on both sides(certainly a car set up for all rights will be set up differently)
here, is the GT2 rear parts list, it's the same as the RSR
here, is the GT2 rear parts list, it's the same as the RSR
06-19-2012, 10:29 AM
#94
06-19-2012, 02:10 PM
#95
Rennlist Member
The comment was more of a place holder to keep me updated on the thread.
However, you are so right on "chasing" the adjustment to get it right. This is part of the reason shops charge so much for an alignment on these cars.
I can remember the first time I did the rear alignment, it was very frustrating, I would get everything close then go back to check and the adjustment had changed
However, you are so right on "chasing" the adjustment to get it right. This is part of the reason shops charge so much for an alignment on these cars.
I can remember the first time I did the rear alignment, it was very frustrating, I would get everything close then go back to check and the adjustment had changed
Steve - I have a spare set of new ERP kinematics sitting in my garage when you're ready
06-19-2012, 05:09 PM
#96
Race Car
Thanks for the offer, I have a full set in the car now, the only non monoball mounts in the car are the front a-arms (Walrod), and I am sure that will change once the engine rebuild is out of the way
.....
06-20-2012, 06:03 PM
#98
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Lost again but sure it will all come in very handy sooner or later :-)
So if the kw and standard spring ID is the same, 60mm, problem of the bilsteins is purely the perch (spring top hat) ?
So using rs top hats would solve the problem or is this a too easy solution ?
Are bilsteins pss10's then a smaller inside diameter ? Don't see how else jackal would have had this problem ? Aside from hole on the top hat that is too big to work with the rs top mounts. Or am I wrong ?
So if the kw and standard spring ID is the same, 60mm, problem of the bilsteins is purely the perch (spring top hat) ?
So using rs top hats would solve the problem or is this a too easy solution ?
Are bilsteins pss10's then a smaller inside diameter ? Don't see how else jackal would have had this problem ? Aside from hole on the top hat that is too big to work with the rs top mounts. Or am I wrong ?
Last edited by pvdw; 06-20-2012 at 06:05 PM. Reason: Correction
06-20-2012, 06:38 PM
#99
No, you're right. KW & pss10 are both 60mm. Oem RS is larger.
Rear needs new top hats. Front is a bit more tricky but one possible work-around is shown on jackal's site. I'm working on something different & will post when done.
Rear needs new top hats. Front is a bit more tricky but one possible work-around is shown on jackal's site. I'm working on something different & will post when done.
