Adjusting Rake for Tire Diameter
01-07-2019, 08:31 PM
#1
Advanced
Thread Starter
Adjusting Rake for Tire Diameter
I am setting up a GT3 for Track use only.
For a 997.2 GT3, Porsche recommends ride heights of 102mm Front and 137mm Rear (35mm Rake). This is using Tires with 25.8" F and 26.5" R diameters.
Now I have heard many say that they prefer less rake, as in 20mm. Assuming you want to maintain the factory camber curves and geometry on the front, this would mean dropping the rear to 117mm. Or if you dropped the front 5mm, you would have to drop the rear to 112mm.
However, what is not often discussed is how tire diameter changes come into play.
Lets say we are putting on a 255-18 Hoosier on the Front that is only 24.8" tall (1" shorter than OEM) and keeping the rear tire the same diameter.
The front then drops to 89mm. From what I have read there are no negatives. You are lowering the center of gravity, lowering the roll center, and there is no change to the suspension geometry with a reduction of tire diameter as the control arm is sitting at the same angle. But if you kept your rear tire size and height the same, you now end up with a rake difference of 48mm. You could lower the rear 12mm so you get back to OEM rake, but if 20mm rake is felt to be ideal you would have to lower the rear 28mm to get a 20mm rake. This would may significantly change your geometry and who knows what this is doing to your roll center and camber curve? Maybe it's too much of a drop?
So the question is what is the best way to set up ride heights and rake when you make a significant change with tire diameter at one end vs the other? Do you adjust the other sides height to target a particular rake value, or do you leave it alone to maintain consistent suspension geometry at both ends?
In the past what I have done is set up the car with my desired ride heights and rake with OEM sized tires, and just left it there even if I run shorter or taller tires. My geometry stays the same but I end up with different ride heights and rake (and different relative roll centers) so I am not sure if this is the way to go.
Thanks.
For a 997.2 GT3, Porsche recommends ride heights of 102mm Front and 137mm Rear (35mm Rake). This is using Tires with 25.8" F and 26.5" R diameters.
Now I have heard many say that they prefer less rake, as in 20mm. Assuming you want to maintain the factory camber curves and geometry on the front, this would mean dropping the rear to 117mm. Or if you dropped the front 5mm, you would have to drop the rear to 112mm.
However, what is not often discussed is how tire diameter changes come into play.
Lets say we are putting on a 255-18 Hoosier on the Front that is only 24.8" tall (1" shorter than OEM) and keeping the rear tire the same diameter.
The front then drops to 89mm. From what I have read there are no negatives. You are lowering the center of gravity, lowering the roll center, and there is no change to the suspension geometry with a reduction of tire diameter as the control arm is sitting at the same angle. But if you kept your rear tire size and height the same, you now end up with a rake difference of 48mm. You could lower the rear 12mm so you get back to OEM rake, but if 20mm rake is felt to be ideal you would have to lower the rear 28mm to get a 20mm rake. This would may significantly change your geometry and who knows what this is doing to your roll center and camber curve? Maybe it's too much of a drop?
So the question is what is the best way to set up ride heights and rake when you make a significant change with tire diameter at one end vs the other? Do you adjust the other sides height to target a particular rake value, or do you leave it alone to maintain consistent suspension geometry at both ends?
In the past what I have done is set up the car with my desired ride heights and rake with OEM sized tires, and just left it there even if I run shorter or taller tires. My geometry stays the same but I end up with different ride heights and rake (and different relative roll centers) so I am not sure if this is the way to go.
Thanks.
01-07-2019, 10:38 PM
#2
Race Car
Change rolling diameter and you will get ice mode from the ABS unless you plan on putting a Motorsport ABS unit in the car
...so don’t change rolling diameter is what I’m saying, then you don’t have to worry about the tire’s effect on rake.
...so don’t change rolling diameter is what I’m saying, then you don’t have to worry about the tire’s effect on rake.
01-08-2019, 12:53 AM
#3
Advanced
Thread Starter
I have actually ran significantly different rolling diameters in RE71Rs and never have ice mood. Hoosier doesnt make an appropriate sized front and rear tire to not change rolling diameter at least somewhat.
01-08-2019, 12:19 PM
#4
I think ABS and such needs to maintain close to the OEM ratio between front and rear rolling diameters, which has nothing to do with the height of the tire. To properly setup the suspension you usually have to select a tire you want to run, mount it and setup the car with those tires in mind. If you are like most and run multiple tires (e.g. Hoosier, BFG, Cup2, etc.) then you need to find a "general" height setting that works for you based on your springs, suspension, tracks you drive, etc.
It all sounds complicated, but generally speaking for most of us, you just need to set it and go and drive to see how it feels. I have used NT01, RE71R, Cup2, R1S, Hoosiers all without modifying my ride height and they all yielded fast results.
It all sounds complicated, but generally speaking for most of us, you just need to set it and go and drive to see how it feels. I have used NT01, RE71R, Cup2, R1S, Hoosiers all without modifying my ride height and they all yielded fast results.
01-08-2019, 03:20 PM
#5
Advanced
Thread Starter
Yes, I am not worried about ABS at all. I have ran many different tire diameters and it's never been a problem. My worry is that if I run a really short tire and i set the rear rake to match this tire, I may be way outside the optimal geometry (camber curves, roll center height) so it may just be better to keep the rear as it usually is and not lower it to compensate for the short front tire.