Orthojoe's GT4 track thread and ramblings
07-14-2016, 07:37 PM
#646
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I'm getting some custom street wheels and want the 265 fronts for sure.
07-14-2016, 07:51 PM
#647
It's pretty flush from top down. Maybe a few mm out.
Hard to get a perfect orthogonal pic, but you get the idea here:
07-15-2016, 01:25 AM
#648
Rennlist Member
Leong. They won't even stock the RE71 in NZ in our sizes, they will be special order only. However Japanese rubber in NZ is always at least as competitive as US retail I've found over the years with my 993 (re11, SO2 etc). We just need to approach Bridgestone HO and order direct...
07-15-2016, 03:24 AM
#649
GT3 player par excellence
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front et 50 is optimal
you need camber to fit 265
rear 10.5 at et 43-45 is fine
you need camber to fit 265
rear 10.5 at et 43-45 is fine
07-15-2016, 12:02 PM
#650
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.
07-15-2016, 12:13 PM
#651
Burning Brakes
Tire feedback from the RE71R instantly gave me MORE confidence along with what I perceived as slightly more grip (even though the rear tire and rim are slightly smaller) vs the Dunlops. They also lasted a full 30 min track session with consistent grip vs the Dunlops rear tires always going "off" (too much heat or pressure) after about 20 minutes.
I know this doesn't help you determine your issue, but I feel like you've got some other issue going on. Perhaps one of your sway bars came loose, or your toe setting fell out of spec?
07-15-2016, 12:17 PM
#652
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.
07-15-2016, 12:46 PM
#653
+1, what are the front/rear size you're running. You can change the handling a lot with just a change in front->rear diameter ratio (changing rake). If the rear tire is shorter relative to the front, you've moved static weight to the rear, which will result in more understeer at certain parts of the corner.
07-15-2016, 01:07 PM
#655
I had 6 or 7 major moments where the rear end snapped out mid-corner and I barely caught them. Prior to that I only had one or two on the Dunlops and those were my fault for sure. Tire wear looks good though.
Thanks for the responses.
07-15-2016, 01:13 PM
#656
Three Wheelin'
what tire sizes RE71R on 19"?
I am running OZ ultra legerra HLT 19x 8.5 and 10
i want to run the bridgestone RE71R
I spoke to tire rack yesterday and they say that 245 front and 285 rear works
there seems to be a 255 and 265 tire but they seem to drop the front diameter too much which might throw of the electronics...
I wonder if anyone has investigated further or has even tried it before I just take the easy out and order the 245/285 combo
i want to run the bridgestone RE71R
I spoke to tire rack yesterday and they say that 245 front and 285 rear works
there seems to be a 255 and 265 tire but they seem to drop the front diameter too much which might throw of the electronics...
I wonder if anyone has investigated further or has even tried it before I just take the easy out and order the 245/285 combo
07-15-2016, 04:46 PM
#657
Originally Posted by Eric5280
Correct, and the front height is like 45 and rear is 40 I believe which they tell me keeps the same overall height size as the 20" stock tires. The only other item I changed was updating the DSC Sport to the most recent software released in June. Maybe I made a mistake loading that or maybe toe is indeed out of spec.
I had 6 or 7 major moments where the rear end snapped out mid-corner and I barely caught them. Prior to that I only had one or two on the Dunlops and those were my fault for sure. Tire wear looks good though.
Thanks for the responses.
I had 6 or 7 major moments where the rear end snapped out mid-corner and I barely caught them. Prior to that I only had one or two on the Dunlops and those were my fault for sure. Tire wear looks good though.
Thanks for the responses.
07-15-2016, 04:51 PM
#658
+1, what are the front/rear size you're running. You can change the handling a lot with just a change in front->rear diameter ratio (changing rake). If the rear tire is shorter relative to the front, you've moved static weight to the rear, which will result in more understeer at certain parts of the corner.
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, 04:55 PM
#659
Three Wheelin'
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?
Thanks.
07-15-2016, 05:07 PM
#660
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?
