997.1 GT3 HELP!!
10-09-2012, 02:54 AM
#16
Instructor
Thread Starter
All cars are sensitive to rake, but I don't think your problem is rake.
The PASM shocks in the 997.1 GT3 are not that good, but the car still drives well.
MPSC tires in the 996 GT3 (18" sizes) were race-spec MPSC, while MPSC tires in the 997.1 and 997.2 GT3 are the N-Spec tires (less grip).
I would run the car on a set of street/track tires, R888 or Trofeo, and have the limited slip diff checked as this unit wears out prematurely and cause ill handling issues (darty rear on turn exit, swinging rear under braking, a lack of progression on trail-braking.
Camber and toe are fine for combined street and track use. I like the rear toe at 4mm total (5/32"), your setup at 3mm (1/8") makes for a twitchy read-end.
The 996 GT3 is less stable and more difficult to drive, the 997.1 GT3 is excessively stable.
No need to throw parts to the car, in between alignment, LSD and tires it should get fixed. The happy rake is 20mm, and you can run the car 10mm below stock ride height.
Through PIWIS, the PASM can be deactivated, but I would rather install a set of Bilstein B16, affordable coil-over kit, PASM compatible, good for street/track use, street friendly spring rates.
A bone stock 997.1 GT3 driven by a Pro (Max Angelelli) lapped Laguna Seca in 1:39.5, so the car is pretty fast.
The PASM shocks in the 997.1 GT3 are not that good, but the car still drives well.
MPSC tires in the 996 GT3 (18" sizes) were race-spec MPSC, while MPSC tires in the 997.1 and 997.2 GT3 are the N-Spec tires (less grip).
I would run the car on a set of street/track tires, R888 or Trofeo, and have the limited slip diff checked as this unit wears out prematurely and cause ill handling issues (darty rear on turn exit, swinging rear under braking, a lack of progression on trail-braking.
Camber and toe are fine for combined street and track use. I like the rear toe at 4mm total (5/32"), your setup at 3mm (1/8") makes for a twitchy read-end.
The 996 GT3 is less stable and more difficult to drive, the 997.1 GT3 is excessively stable.
No need to throw parts to the car, in between alignment, LSD and tires it should get fixed. The happy rake is 20mm, and you can run the car 10mm below stock ride height.
Through PIWIS, the PASM can be deactivated, but I would rather install a set of Bilstein B16, affordable coil-over kit, PASM compatible, good for street/track use, street friendly spring rates.
A bone stock 997.1 GT3 driven by a Pro (Max Angelelli) lapped Laguna Seca in 1:39.5, so the car is pretty fast.
First of all, OMG, I can't believe I didn't figure this out. The MPSC's are indeed different! I just got done looking at pictures online, and I can't believe I didn't notice this before. I thought they were just different sizes and a newer version of the same tire. Now I even wonder if I wound up with a mismatched set, with stickier on the front and N-spec on the rear. Sigh. Either way, this certainly could be one contributing factor to my frustration.
I think the diff is fine so far, and haven't noticed what you describe.
I did want to get the rear toe-in to 4 mm based on 997gt3north's prior suggestion, but since they measure it in degrees on most machines and the conversion isn't all that clear, it was a little harder to achieve. I didn't think it would make as big a difference as you say - that surprises me. I wonder if that again is a contributing factor.
Forgive me for being ignorant, but when you say rake of 20 mm, does that mean 20mm lower in front that in rear? And then 10mm lower than stock, does that then mean 10mm lower in the rear and a combined 30mm lower in front (10mm from stock plus 20mm rake)? How big a difference will this make to the handling of the car and in what way?
Thanks again,
-Christian
10-09-2012, 04:00 AM
#17
Addict
Rennlist Member
Rennlist Member
The 997.1 GT3 comes with a standard ride height of 128mm front and 150mm rear, so there is already a 22mm rake. You can run it at 120mm front and 140mm rear (20mm rake) with no need to change springs.
25mm rake starts to make the rear a little loose, but you could compensate with a bigger wing, so the car would rotate better on low speed turns, while still being hooked on high speed turns. Not your case, as the 997.1 GT3 doesn't have good aero. 25mm rake works great for the 3.8RS and 4.0RS.
To convert from 4mm or 5/32" to degrees, it is 0.25 degrees toe-in per rear corner, or 0.50 degrees total toe-in. The extra 1mm makes a significant difference. I have the settings from multiple race teams that shared the info with me in the past, some club racers, DE drivers, and 4mm (0.50 degrees total toe-in) is what works for most, it is also the factory recommendation.
Rear toe is the most sensitive alignment setting on these cars, as the rear suspension by design tends to toe-out under compression and under braking. So, the less the toe-in at the back the easier to get bump steer conditions, especially with such soft springs (550#-600#) supporting around 2,200 lbs of static weight.
25mm rake starts to make the rear a little loose, but you could compensate with a bigger wing, so the car would rotate better on low speed turns, while still being hooked on high speed turns. Not your case, as the 997.1 GT3 doesn't have good aero. 25mm rake works great for the 3.8RS and 4.0RS.
To convert from 4mm or 5/32" to degrees, it is 0.25 degrees toe-in per rear corner, or 0.50 degrees total toe-in. The extra 1mm makes a significant difference. I have the settings from multiple race teams that shared the info with me in the past, some club racers, DE drivers, and 4mm (0.50 degrees total toe-in) is what works for most, it is also the factory recommendation.
Rear toe is the most sensitive alignment setting on these cars, as the rear suspension by design tends to toe-out under compression and under braking. So, the less the toe-in at the back the easier to get bump steer conditions, especially with such soft springs (550#-600#) supporting around 2,200 lbs of static weight.
10-09-2012, 04:24 AM
#18
The 997.1 GT3 comes with a standard ride height of 128mm front and 150mm rear, so there is already a 22mm rake. You can run it at 120mm front and 140mm rear (20mm rake) with no need to change springs.
25mm rake starts to make the rear a little loose, but you could compensate with a bigger wing, so the car would rotate better on low speed turns, while still being hooked on high speed turns. Not your case, as the 997.1 GT3 doesn't have good aero. 25mm rake works great for the 3.8RS and 4.0RS.
To convert from 4mm or 5/32" to degrees, it is 0.25 degrees toe-in per rear corner, or 0.50 degrees total toe-in. The extra 1mm makes a significant difference. I have the settings from multiple race teams that shared the info with me in the past, some club racers, DE drivers, and 4mm (0.50 degrees total toe-in) is what works for most, it is also the factory recommendation.
Rear toe is the most sensitive alignment setting on these cars, as the rear suspension by design tends to toe-out under compression and under braking. So, the less the toe-in at the back the easier to get bump steer conditions, especially with such soft springs (550#-600#) supporting around 2,200 lbs of static weight.
25mm rake starts to make the rear a little loose, but you could compensate with a bigger wing, so the car would rotate better on low speed turns, while still being hooked on high speed turns. Not your case, as the 997.1 GT3 doesn't have good aero. 25mm rake works great for the 3.8RS and 4.0RS.
To convert from 4mm or 5/32" to degrees, it is 0.25 degrees toe-in per rear corner, or 0.50 degrees total toe-in. The extra 1mm makes a significant difference. I have the settings from multiple race teams that shared the info with me in the past, some club racers, DE drivers, and 4mm (0.50 degrees total toe-in) is what works for most, it is also the factory recommendation.
Rear toe is the most sensitive alignment setting on these cars, as the rear suspension by design tends to toe-out under compression and under braking. So, the less the toe-in at the back the easier to get bump steer conditions, especially with such soft springs (550#-600#) supporting around 2,200 lbs of static weight.
Dropping the front without the rear does make the front a lot better but be careful how far you go before you overwhelm the shocks/suspension, and expect mid-corner oversteer. I'm not sure I agree that a 20mm rake is the way to go, I think its very sensitive to driving style, and, believe it or not, fuel load.
I agree 100% with Rad about the rear Toe. 2mm each side is excellent, I recently decided to go "aggressive" and went 1.8mm each side, in an attempt to loosen the rear, and the difference was noticeable. Will probably go back to 2mm on next alignment. So if you're at 3mm total (or 1.5mm each side) that sounds like the rear will feel very unsettled.
10-09-2012, 04:49 AM
#19
Instructor
Thread Starter
In reviewing my notes, I see that they set my car to have 0.21 degrees toe-in each side in the rear. I tried to figure out the conversion ratio for 19" wheels on the web, and found (probably incorrectly) that 2mm would be more like .227 degrees, and that my 0.21 degrees corresponded to 1.85mm.
If I try to do the math myself, I get the following answer. Assuming the arc over a small angle is approximately like a straight line, and assuming D=19" Pi=3.1415: 1 degree of rotation of the wheel makes each edge of the rim travel = 19in x 25.4mm/in x 3.1415/360deg. = 4.21mm/deg. So total difference between the opposite edges is 8.4mm/deg. I.e 2mm = 2mm / 8.42mm/deg = 0.2375 deg. So my 0.21 degrees should be equal to 1.77mm. According to what you two are saying, that would be uncomfortably unstable??? So this could be a big part of the problem in my case then???
-Christian
If I try to do the math myself, I get the following answer. Assuming the arc over a small angle is approximately like a straight line, and assuming D=19" Pi=3.1415: 1 degree of rotation of the wheel makes each edge of the rim travel = 19in x 25.4mm/in x 3.1415/360deg. = 4.21mm/deg. So total difference between the opposite edges is 8.4mm/deg. I.e 2mm = 2mm / 8.42mm/deg = 0.2375 deg. So my 0.21 degrees should be equal to 1.77mm. According to what you two are saying, that would be uncomfortably unstable??? So this could be a big part of the problem in my case then???
-Christian
Last edited by csmarx; 10-09-2012 at 05:05 AM. Reason: brain fart
10-09-2012, 05:18 AM
#20
Instructor
Thread Starter
On a tangential note, perhaps vaguely related, I noticed that different brands of tires despite having the same designation can wind up with pretty different outer diameter. For example, the stock tires for this car have diameters of 25.5" front and 26.3" rear. I found a different set of R compounds, where the rears were more like 25.5" i.e. matching the fronts. That's an almost 4% difference in diameter for the rears. How bad would that be? Aside from messing with the actual speed indicated, is it enough to mess with the ABS, the TC, the PASM, etc, etc???
-Christian
-Christian
Last edited by csmarx; 10-09-2012 at 05:19 AM. Reason: brain fart
10-09-2012, 06:09 AM
#21
In reviewing my notes, I see that they set my car to have 0.21 degrees toe-in each side in the rear. I tried to figure out the conversion ratio for 19" wheels on the web, and found (probably incorrectly) that 2mm would be more like .227 degrees, and that my 0.21 degrees corresponded to 1.85mm.
If I try to do the math myself, I get the following answer. Assuming the arc over a small angle is approximately like a straight line, and assuming D=19" Pi=3.1415: 1 degree of rotation of the wheel makes each edge of the rim travel = 19in x 25.4mm/in x 3.1415/360deg. = 4.21mm/deg. So total difference between the opposite edges is 8.4mm/deg. I.e 2mm = 2mm / 8.42mm/deg = 0.2375 deg. So my 0.21 degrees should be equal to 1.77mm. According to what you two are saying, that would be uncomfortably unstable??? So this could be a big part of the problem in my case then???
-Christian
If I try to do the math myself, I get the following answer. Assuming the arc over a small angle is approximately like a straight line, and assuming D=19" Pi=3.1415: 1 degree of rotation of the wheel makes each edge of the rim travel = 19in x 25.4mm/in x 3.1415/360deg. = 4.21mm/deg. So total difference between the opposite edges is 8.4mm/deg. I.e 2mm = 2mm / 8.42mm/deg = 0.2375 deg. So my 0.21 degrees should be equal to 1.77mm. According to what you two are saying, that would be uncomfortably unstable??? So this could be a big part of the problem in my case then???
-Christian
10-09-2012, 08:13 AM
#22
you are most likely feeling the tire difference as the 19" mpsc have no where near the grip of the real 18" mpsc you are comparing them to
if you want to compare the cars you are gong to need comparable tires
it was the first huge eureka moment I had with the car when 4 years ago I got a ride in a 997gt3.mk1 with Hoosier R6s having got out of mine with the OE 19" tires - it blew me away - i was getting frustrated with the car as it sounds like you are - put on a new set of sticky tires and then see what you think
if you want to compare the cars you are gong to need comparable tires
it was the first huge eureka moment I had with the car when 4 years ago I got a ride in a 997gt3.mk1 with Hoosier R6s having got out of mine with the OE 19" tires - it blew me away - i was getting frustrated with the car as it sounds like you are - put on a new set of sticky tires and then see what you think
Last edited by 997gt3north; 10-09-2012 at 01:19 PM.
10-09-2012, 10:43 AM
#23
Three Wheelin'
CSMARX
I wasn't bored one bit reading your experiences with driving and with different cars. A lot better posts than the usual IMO. Keep it up. Good luck with your mk1 GT3. Me I have only been on the track a few times with mine but am looking forwards to a full season next year. Our usual Pacific NW rain beginning later this week. Should last till July.
I wasn't bored one bit reading your experiences with driving and with different cars. A lot better posts than the usual IMO. Keep it up. Good luck with your mk1 GT3. Me I have only been on the track a few times with mine but am looking forwards to a full season next year. Our usual Pacific NW rain beginning later this week. Should last till July.
Last edited by seapar; 10-09-2012 at 02:29 PM. Reason: editing
10-09-2012, 12:10 PM
#25
Instructor
Thread Starter
I have looked at the Hoosier R6's, but they really aren't drivable on the street. I notice that the Pirelli Trofeo's and the Toyo R888's both come in appropriate sizes for the stock 19" rims. Which would be the better choice and what is the trade-off in terms of grip and wear? Remember that I drive the car for 3 - 6 hours on highway to get to the track, and that I also use the car a significant amount on roads.
Thanks!
-Christian
Thanks!
-Christian
10-09-2012, 12:24 PM
#26
Instructor
Thread Starter
I would still love to hear more thoughts and insights from you all.
Thanks so much,
-Christian
10-09-2012, 12:51 PM
#27
Drifting
I have commented on this a few times but what you are feeling (or not feeling) is the difference in the chassis stiffness in the cars. The 996 gt3s would bend in the corners and thus you feel more - stiffer more modern chassis bend less and the suspension thus can work better.
Great point Rad - and I think this is another contributor to the different "feel".
10-09-2012, 01:45 PM
#28
Instructor
Thread Starter
One more question: some say that going with 18" rims will give more grip and better feel. The Toyo R888 tires come in a 305/35-18 that has the same diameter of 26.3" as the OE 19's. For the fronts they also have a 235/40-18 that have the same 25.5" diameter as the OE 19's. How would that impact what I'm struggling with, vs. going with 19" R888's for example? How important is it to keep the same outer diameter when minus sizing? For example for 18's you can get the original stickier Cups in a 295/30-18, but the outer diameter is only 25.2". What would you guys do here?
-Christian
-Christian
10-09-2012, 01:51 PM
#29
Instructor
Thread Starter
CSMARX
I wasn't bored one bit reading your experiences with driving and with different cars. A lot better posts than the usual IMO. Keep it up. Good luck with your mk1 GT3. Me I have only been on the track a few times with mine but am looking forwards to a full season next year. Our usual Pacific NW rain beginning later this week. Should last till July.
I hope you'll join Rennlist and be a member.
I wasn't bored one bit reading your experiences with driving and with different cars. A lot better posts than the usual IMO. Keep it up. Good luck with your mk1 GT3. Me I have only been on the track a few times with mine but am looking forwards to a full season next year. Our usual Pacific NW rain beginning later this week. Should last till July.
I hope you'll join Rennlist and be a member.
-Christian