Orthojoe's GT4 track thread and ramblings
07-15-2016, 05:25 PM
#661
FYI, car was great last time out and I did my fastest lap ever (2:01) on bald Dunlop tires at the end of the day and felt great. I suppose toe could be out, but the car hasn't been on track since, nor driven much at all. I was struggling to get 2:03's on the RE71's. I honestly am thinking a DSC issue, it felt that bad.
07-15-2016, 05:29 PM
#662
Changing the rake has virtually zero impact on the static weight distribution.
Any weight distribution shift from a rake change comes from physically moving the center of mass forward or backwards relative to the tire contact patches. Given the incredibly small angle change of the body with front or rear ride height changes relative to the wheel base, and because the height of the center of mass is relatively low to the ground, the effective shift is next to nothing. So even tiny changes can have big impact.
For a quick and dirty over-simplified assessment, you can treat the car like a heavy rod sitting on two points at the front and rear contact patches and calculate the angle change of the vehicle due to, say, increasing the rear height of the GT4 by 1":
GT4 wheel base: 97.8"
approximate angle change: 1" / 97.8" = 0.01022 radians = 0.59 degrees
The stock GT4 has a 56% rear weight distribution, so the center of mass is 56% of the wheel base towards the rear of the car. You can calculate the change in that weight distribution due to a 0.59 degree angle change:
new distribution: 56% * cosine(0.59 degrees) = 56% * 0.999947 = 55.9971%, or a -0.0029% change
So for a 2955 pound GT4, by lifting the rear up an entire 1", you've very roughly moved 0.0029%, or 1.4 ounces of weight front the rear to the front.
But this is just a rough back-of-the envelope, and is actually a pessimistic assumption since it assumes the center of mass is on the ground directly between the tire contact patches before the rear point is raised.
If you crunch the detailed math for a more realistic car geometry by taking into account that the center of mass is roughly 16" off the ground (this was the number for the Cayman R, I'm roughly assuming it's similar for the GT3), and that the body pivot point in the rear suspension attachment is roughly 25" off the ground, then you'll actually get an answer that is almost 10 times that at -0.037%.
But even that is still just a 1.1 pound shift front rear to front. Anybody think they can feel that handling difference?
Any weight distribution shift from a rake change comes from physically moving the center of mass forward or backwards relative to the tire contact patches. Given the incredibly small angle change of the body with front or rear ride height changes relative to the wheel base, and because the height of the center of mass is relatively low to the ground, the effective shift is next to nothing. So even tiny changes can have big impact.
For a quick and dirty over-simplified assessment, you can treat the car like a heavy rod sitting on two points at the front and rear contact patches and calculate the angle change of the vehicle due to, say, increasing the rear height of the GT4 by 1":
GT4 wheel base: 97.8"
approximate angle change: 1" / 97.8" = 0.01022 radians = 0.59 degrees
The stock GT4 has a 56% rear weight distribution, so the center of mass is 56% of the wheel base towards the rear of the car. You can calculate the change in that weight distribution due to a 0.59 degree angle change:
new distribution: 56% * cosine(0.59 degrees) = 56% * 0.999947 = 55.9971%, or a -0.0029% change
So for a 2955 pound GT4, by lifting the rear up an entire 1", you've very roughly moved 0.0029%, or 1.4 ounces of weight front the rear to the front.
But this is just a rough back-of-the envelope, and is actually a pessimistic assumption since it assumes the center of mass is on the ground directly between the tire contact patches before the rear point is raised.
If you crunch the detailed math for a more realistic car geometry by taking into account that the center of mass is roughly 16" off the ground (this was the number for the Cayman R, I'm roughly assuming it's similar for the GT3), and that the body pivot point in the rear suspension attachment is roughly 25" off the ground, then you'll actually get an answer that is almost 10 times that at -0.037%.
But even that is still just a 1.1 pound shift front rear to front. Anybody think they can feel that handling difference?
Other things that happen when you change the relative ride height - you change the axis the car rolls along a bit and you change the position of the CG relative to this roll axis, which will transfer weight more quickly/less quickly F/R, which will impact under/over steer as well. Then there is also the aero impact, but I couldn't begin to quantify that.
But trust me, if you make a 10mm rake change on a GT3, and it will mean the difference between stuffing it and heaven. It seems to impact the GT4, but to a lesser degree.
07-15-2016, 05:38 PM
#663
Rennlist Member
Changing the rake has virtually zero impact on the static weight distribution.
Any weight distribution shift from a rake change comes from physically moving the center of mass forward or backwards relative to the tire contact patches. Given the incredibly small angle change of the body with front or rear ride height changes relative to the wheel base, and because the height of the center of mass is relatively low to the ground, the effective shift is next to nothing.
For a quick and dirty over-simplified assessment, you can treat the car like a heavy rod sitting on two points at the front and rear contact patches and calculate the angle change of the vehicle due to, say, increasing the rear height of the GT4 by 1":
GT4 wheel base: 97.8"
approximate angle change: 1" / 97.8" = 0.01022 radians = 0.59 degrees
The stock GT4 has a 56% rear weight distribution, so the center of mass is 56% of the wheel base towards the rear of the car. You can calculate the change in that weight distribution due to a 0.59 degree angle change:
new distribution: 56% * cosine(0.59 degrees) = 56% * 0.999947 = 55.9971%, or a -0.0029% change
So for a 2955 pound GT4, by lifting the rear up an entire 1", you've very roughly moved 0.0029%, or 1.4 ounces of weight front the rear to the front.
But this is just a rough back-of-the envelope, and is actually a pessimistic assumption since it assumes the center of mass is on the ground directly between the tire contact patches before the rear point is raised.
If you crunch the detailed math for a more realistic car geometry by taking into account that the center of mass is roughly 16" off the ground (this was the number for the Cayman R, I'm roughly assuming it's similar for the GT3), and that the body pivot point in the rear suspension attachment is roughly 25" off the ground, then you'll actually get an answer that is almost 10 times that at -0.037%.
But even that is still just a 1.1 pound shift front rear to front. Anybody think they can feel that handling difference?
Any weight distribution shift from a rake change comes from physically moving the center of mass forward or backwards relative to the tire contact patches. Given the incredibly small angle change of the body with front or rear ride height changes relative to the wheel base, and because the height of the center of mass is relatively low to the ground, the effective shift is next to nothing.
For a quick and dirty over-simplified assessment, you can treat the car like a heavy rod sitting on two points at the front and rear contact patches and calculate the angle change of the vehicle due to, say, increasing the rear height of the GT4 by 1":
GT4 wheel base: 97.8"
approximate angle change: 1" / 97.8" = 0.01022 radians = 0.59 degrees
The stock GT4 has a 56% rear weight distribution, so the center of mass is 56% of the wheel base towards the rear of the car. You can calculate the change in that weight distribution due to a 0.59 degree angle change:
new distribution: 56% * cosine(0.59 degrees) = 56% * 0.999947 = 55.9971%, or a -0.0029% change
So for a 2955 pound GT4, by lifting the rear up an entire 1", you've very roughly moved 0.0029%, or 1.4 ounces of weight front the rear to the front.
But this is just a rough back-of-the envelope, and is actually a pessimistic assumption since it assumes the center of mass is on the ground directly between the tire contact patches before the rear point is raised.
If you crunch the detailed math for a more realistic car geometry by taking into account that the center of mass is roughly 16" off the ground (this was the number for the Cayman R, I'm roughly assuming it's similar for the GT3), and that the body pivot point in the rear suspension attachment is roughly 25" off the ground, then you'll actually get an answer that is almost 10 times that at -0.037%.
But even that is still just a 1.1 pound shift front rear to front. Anybody think they can feel that handling difference?
07-15-2016, 05:51 PM
#664
07-15-2016, 05:54 PM
#665
Since the rear suspension pivot relative to the front suspension pivot is harder to estimate while bench racing and does impact the result, a more aggressive weight transfer estimate would be to assume the vehicle angle change simply pivots a nominally horizontal line between the front and rear wheel contact patches about a point at the front wheel contact patch, while the center of mass still sits elevated ~16" off the ground at a point 56% rearward of the front wheels.
That then predicts a -0.30% weight transfer shift, which is about 8.8 pounds. Maybe Randy Pobst could feel that one. To get more accurate, I think you'd need a detailed suspension model...
That then predicts a -0.30% weight transfer shift, which is about 8.8 pounds. Maybe Randy Pobst could feel that one.
To get more accurate, I think you'd need a detailed suspension model...
07-15-2016, 06:16 PM
#666
Rennlist Member
Eric. I believe this is a DSC issue. Refer to prior software version then go out and drive the track again before you make any changes to your current set up or else you will be chasing ghosts. I've run the DSC on the GT3 a fair bit with different software and it can really affect the cars rear stability...
07-15-2016, 06:40 PM
#667
Instructor
Eric. I believe this is a DSC issue. Refer to prior software version then go out and drive the track again before you make any changes to your current set up or else you will be chasing ghosts. I've run the DSC on the GT3 a fair bit with different software and it can really affect the cars rear stability...
07-15-2016, 06:59 PM
#668
Thanks Macca and Leong, I will ask DSC for the previous file and load it on to try again. I used the most recent file from June and have stock links.
07-15-2016, 10:21 PM
#669
GT3 player par excellence
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Joe (or anyone who would like to answer), I tried the RE71R's on track yesterday and am now getting some pretty extreme oversteer. On the Dunlop's, the rear end had far more grip and wondering if 10 mm (285 vs 295) could really make that much difference. Visually, they do look significantly slimmer. I am running 2.25 camber front and 1.25 rear with full soft front and full hard rear bars. Perhaps I need to adjust the rear bar or maybe 1.25 is not enough camber in the rear for these tires? I tried 30-35 hot pressures with 30 being really loose and 35 better. Any ideas? I am now 1.5 seconds off pace and lacking confidence in the car as it feels very unstable. Thanks in advance.
your camber is 1deg split, that's WAY too much. ..5 is common. i am surprised that you were ok on dunlop!
also full sof front and full stiff rear only add the the oversteer.
i would try change bar to see if it does anything positive for you
07-16-2016, 12:02 AM
#670
Advanced
Joe (or anyone who would like to answer), I tried the RE71R's on track yesterday and am now getting some pretty extreme oversteer. On the Dunlop's, the rear end had far more grip and wondering if 10 mm (285 vs 295) could really make that much difference. Visually, they do look significantly slimmer. I am running 2.25 camber front and 1.25 rear with full soft front and full hard rear bars. Perhaps I need to adjust the rear bar or maybe 1.25 is not enough camber in the rear for these tires? I tried 30-35 hot pressures with 30 being really loose and 35 better. Any ideas? I am now 1.5 seconds off pace and lacking confidence in the car as it feels very unstable. Thanks in advance.
The biggest change from the OEM spec. 20" Dunlops and the 19" RE71r's appears to be the Max Load/stiffness of the tires.
Oem Dunlop
245/35ZR20 1356 lbs
295/30ZR20 1819 lbs
RE71R
245/40R19 1653 lbs
285/35R19 1709 lbs
How this can influence oversteer someone else maybe can explain. Something to do with roll rate.
07-16-2016, 12:05 AM
#671
Nordschleife Master
Thread Starter
Joe (or anyone who would like to answer), I tried the RE71R's on track yesterday and am now getting some pretty extreme oversteer. On the Dunlop's, the rear end had far more grip and wondering if 10 mm (285 vs 295) could really make that much difference. Visually, they do look significantly slimmer. I am running 2.25 camber front and 1.25 rear with full soft front and full hard rear bars. Perhaps I need to adjust the rear bar or maybe 1.25 is not enough camber in the rear for these tires? I tried 30-35 hot pressures with 30 being really loose and 35 better. Any ideas? I am now 1.5 seconds off pace and lacking confidence in the car as it feels very unstable. Thanks in advance.
Sorry for the late response. Anytime it takes me longer than usual to respond, it usually means...... I'm at the track.
Looks like we have a lot of good folks chiming in with good ideas.
Did you change anything else other than the tires and the DSC reprogram? If not, DSC seems to make the most sense. Perhaps it's the wrong profile that was uploaded.
I would also agree with mooty that your rear camber seems way too low and is likely contributing to oversteer, although it is curious that it wasn't an issue with the dunlops with this alignment.
My car still understeers with front sway bar middle, rear bar stiff, camber -2.8 front and camber -2.4 rear, 245 RE71R front and 285 RE71R rear
07-16-2016, 01:34 AM
#672
I am about the same setup as yours and it was only once I added the additional rubber up front 265 (R7's) that the understeer became negligible. I am experiencing a bit more corner entry oversteer now though and have to very careful with that last bit of brake release.
07-16-2016, 07:11 AM
#673
Instructor
07-16-2016, 10:49 AM
#674
Thanks, Mooty. I asked the shop to do max camber on stock parts for GT4 and my E36 NASA TT car runs a 1 degree differential so I thought that was normal. First Porsche for me, will move rear bar from full stiff to middle.
07-16-2016, 11:07 AM
#675
Hey Eric,
Sorry for the late response. Anytime it takes me longer than usual to respond, it usually means...... I'm at the track.
Looks like we have a lot of good folks chiming in with good ideas.
Did you change anything else other than the tires and the DSC reprogram? If not, DSC seems to make the most sense. Perhaps it's the wrong profile that was uploaded.
I would also agree with mooty that your rear camber seems way too low and is likely contributing to oversteer, although it is curious that it wasn't an issue with the dunlops with this alignment.
My car still understeers with front sway bar middle, rear bar stiff, camber -2.8 front and camber -2.4 rear, 245 RE71R front and 285 RE71R rear
Sorry for the late response. Anytime it takes me longer than usual to respond, it usually means...... I'm at the track.
Looks like we have a lot of good folks chiming in with good ideas.
Did you change anything else other than the tires and the DSC reprogram? If not, DSC seems to make the most sense. Perhaps it's the wrong profile that was uploaded.
I would also agree with mooty that your rear camber seems way too low and is likely contributing to oversteer, although it is curious that it wasn't an issue with the dunlops with this alignment.
My car still understeers with front sway bar middle, rear bar stiff, camber -2.8 front and camber -2.4 rear, 245 RE71R front and 285 RE71R rear
Yeah, great community on here. Nothing other than it has been a while since I have driven the GT4 due to a lot of E36 driving in TT lately. Admittedly, the driver software (me) is probably out of date as the two drive night and day. I have an email in to DSC, maybe I messed it up.
Last time out, Dunlops were completely toast on the front and some life left in rear. Maybe this played a part in the feel, but even on bad fronts, the car felt great by comparison. I kind of want to give the Tarett GT4 spring upgrade package a try next to reduce body roll, but maybe I need links first to correct camber.
Any reason for not going full soft on front to help understeer? I am thinking I will adjust my rear bar to middle first and then load on the previous DSC file at the track if the bar is not enough. I'm open to any bar config suggestions, though.
Thanks again to all for the help.