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Ben Lipman Goo Roo SA Soarer TT manual, plus TT track car, plus a spare shell
Posts: 3774 Reg: 04-2006
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Saturday, July 12, 2014 - 12:10 pm, by: Ben Lipman(Ben12a)
After re-reading the spring rate section of a couple of text books I decided to work out the 'motion ratio' and 'wheel rate' for the front and rear suspension. The authors focus on wheel rate, not spring rate. To find the wheel rate you need to know the motion ratio of the wheel to spring. I measured the movement of the wheels for 10mm of movement of the shock/spring. For the front (bearing in mind I have longer than stock LCAs) I came up with a motion ratio of 17mm wheel travel for 10mm shock travel, or 1.7. The rear was 12mm wheel travel for 10mm of shock travel, or 1.2.
The wheel rate is determined by the formula:
wheel rate = spring rate/(motion ratio)squared
for the front I got: WR = 18/2.89 = 6.22kg/mm
for the rear I got: WR = 9/1.44 = 6.25kg/mm
If you like your rates in lbs/in it is 350lbs/in front and 348lbs/in rear.
Either way it is pretty damn close to even front and rear. I will do a bit more reading over the next few days to determine if there is a desired or magic balance front to rear with wheel rate. My intuition is that you would probably want the wheel rate a bit higher at the front, lower at the rear. If the preference is even front to rear, it seems we have 'fluked it'. Carroll Smith simply says he uses the softest rear spring that keeps the car of the ground and balances the handling with the optimum front springs. Food for thought.
I have also measured the ride heights using the procedure specified in the workshop manual, namely measuring from the centre of the LCA pivot to the ground. (front pivot on the front suspension).
Factory ride heights are: 194mm front, and 252mm rear. Note there is a 58mm difference front to rear.
My current ride heights are: RF161, LF 162, RR 228, LR 226mm. The average front is 161.5mm and average rear is 227mm, with a 65.5mm difference front to rear. This means I am running 7.5mm more rake front to rear than the factory specs.
Now that I know the motion ration of the shock to wheel, I can accurately predict the amount of adjustment to get the ride heights even, reducing the tiresome procedure of measure, jack up, adjust, drop, measure...
The next step it to get the left to right level (with my 90kg in the seat) and then take some of the rake out of the car - maybe 7mm so I end up with the factory specs. This should help weight transfer to the rear under acceleration, and may help with some of the dive experienced previously.
After looking at the data, I think removing the sway bar did not help, so the current bar can stay on for the moment. This is also influenced by the fact the rear diffuser is riveted to the diff cradle, making it impossible to remove the roll bar without removing the front of the diffuser.
I am looking at getting some more springs to try as there is a guy in Adelaide selling off a whole bunch at $80 a pair. Might get a slightly stiffer rear spring and a slightly softer rear spring to try. Changing springs at the rear is about a 30-45 min job, meaning it is do-able between sessions on your average track day.
