Importance of Rake in setting suspension heights
07-20-2018, 04:32 AM
#11
Dumb question....from where are we all measuring to get rake measurements, and how to achieve zero, is this relative to stock ride heights? Alignment bolts, pinch weld, hub to fender?
07-20-2018, 02:03 PM
#12
Experiment. Too many variables to give a one size fits all answer. Generally a negative rake (rear lower) is going to have a bit more stability under high speed hard braking, but brake bias plays a part too ie pad selection and tire stagger. But for the sake of example, if the car feels a bit unsettle like the rear is squirming around on you a bit under hard braking, try lowering the rear. If the car just doesn't turn in as crisp and responsive as you would like without losing traction, lower the front to further weight it while lightening the rear. Generally you can influence under/over steer in other ways through driving style with throttle inputs and speed of the corner/down force available, sway bar weight etc, so generally a level S2k is a good place to start.
Rake effects two factors: suspension geometry and aerodynamics. There is nothing magic. Unless significantly raising or lowering the CG it shouldn't affect weight transfer. If lowering causes the car to bottom, that is a different issue. The anti-roll bars may need to be reset to remove preload, bumpsteer may be affected, and alignment in general. All things that should be set intentionally and not coincidentally and need to be reset to a baseline condition. Part of the geometry changed can be to put the car in a different part of its camber change curve.
Raising the center of gravity does effect weight transfer though. How many MM to perceive a handling change is debatable. You can see the rake adjustment reflected on a corner balance scale. We all know ride height effects wheel alignment/toe/camber change, so its assumed those are re calibrated with the rake/height change as a control measure for testing. I have gone back and forth over the years running an added 80lb up front with a supercharger or just NA, and I would generally go back and forth between a negative rake to help compensate for the added weight up front or normal/even ride height when running NA.
07-20-2018, 02:09 PM
#13
Ride height is typically the distance between the center of the hub to the edge of the fender. That way the size of the wheels and tires has no effect. In practice, I measure from the outer edge of the wheel lip to the fender as it's easier to be precise and add to it half the wheel diameter measured lip to lip.
07-20-2018, 02:16 PM
#14
Also, since you mentioned numbers, I am 324mm / 318mm.
Penske DAs with 850lb / 600lb springs
Karcepts STR bars, one step off full hard up front, change the rear depending but I usually use the softest 3 pairs holes and this is all I really mess with at the track if I want the car to rotate more or less easily.
17x9 +45 w/ 255/40 square
DTC-60 stock rotors / HP+ Urge rotors rear
No aero
I have no idea where you live, but you're welcome to drive the car if we end up at the same event. Always good to compare notes.
Last edited by Chibo; 07-20-2018 at 03:01 PM.
07-20-2018, 02:52 PM
#15
^^^^^^^^^^^^^^^
I'm tempted to try a negative rake, but if I go any lower I'll need to roll the rear fenders. Heights are currently F 340 mm, R 341.5 mm.
so..
Could raise the front 1.5mm then, would be less intrusive on geometery,toe out a little more and that would not be much with 1.5mm change, would not notice camber/ caster&Roll center would get closer to oem also & that's a good thang
I'm tempted to try a negative rake, but if I go any lower I'll need to roll the rear fenders. Heights are currently F 340 mm, R 341.5 mm.
so..
Could raise the front 1.5mm then, would be less intrusive on geometery,toe out a little more and that would not be much with 1.5mm change, would not notice camber/ caster&Roll center would get closer to oem also & that's a good thang
Last edited by noodels; 07-20-2018 at 03:00 PM.
07-20-2018, 03:48 PM
#16
Ride height is typically the distance between the center of the hub to the edge of the fender. That way the size of the wheels and tires has no effect. In practice, I measure from the outer edge of the wheel lip to the fender as it's easier to be precise and add to it half the wheel diameter measured lip to lip.
Last edited by s2000Junky; 07-20-2018 at 04:01 PM.
07-20-2018, 06:22 PM
#17
I’ve been meaning to try less rake. I may do so for my next autocross, but won’t have time for another alignment. What happens to alignment in the front if I raise it? Increase positive camber, but what else?
07-20-2018, 06:36 PM
#18
Camber actually wont change that much in the front, not as much as the rear given the same ride height change. But you will get toe 'in' (front of tires closer together)
07-21-2018, 01:16 AM
#19
^^ All depends on the angle of the rod and rack ie: pointing down <rack higher than rod ends>= toe in & pointing up = toe out with raising ride height.
07-21-2018, 10:07 AM
#20
Correct. I'm assuming the rack tie rods are at typical parallel at his current lowered ride height at rest, which when raising the rack/ride height above the tie ends, will cause toe in. If at his current ride height if the tie ends are higher then the rack, the front will toe out if raises ride height and putting them to parallel. OP will just have to look and see where he is currently at.
Last edited by s2000Junky; 07-21-2018 at 10:11 AM.