Suspension Tuning

[topless94style2]

Thought I would share what I have come up with so far. I tend to take things farther than necessary and in some cases attempt to recreate the wheel. In this case I purchased my suspension design based off research done here on the board. I am satisfied with what I purchased, once the car is on the road I'll be able to really tell how it handles. In any case, the second design of the suspension has started based on the numbers I have pulled below and to be tweaked once I understand what the current spring rates do to ride quality and handling. These apply to the convertible only, and will be slightly different for other models.

 

These weights are estimated based on stickers on the car to find curb and weight dist, without driver.

 

Numbers:

 

GVWR 4547

GAWR FRT 2620

GAWR RR 1927

Payload 758

F/R Distribution 58/42

Curb 3789

Front Corner Weight 1099

Rear Corner Weight 796

 

Scaled Weights:

Stock:

Subframe with stock Links 20lbs

Stock Struts 15lbs ea

Stock Sway bar with links 5lbs

Leaf Spring 5lbs

Knuckle 14lbs ea

Auxillary spring 2lbs ea

Total: 92lbs

 

Modified:

Subframe with Gen 2 Links 21lbs

Coilvers 18lbs

Addco bar with links 12lbs

Knuckle 14lbs ea

Total: 97lbs

 

Estimated unsprung weights (wheel/tire and brake components estimated at 38lbs):

Stock Rear unsprung weight 62lbs

Modified Rear unsprung weight 65lbs

 

Now that we got the weights out of the way, lets get into some measurements.

 

Sway Bar Measurements (Circumference):

Stock:

Front 2.75"

Rear 1.57"

 

Modified:

Front 2.75"

Rear 2.75"

 

Front Motion Ratio: .833

Rear Motion Ratio: .866

Wheel Base: 107.5in

Width: 71in

 

Stock front spring rate(still calculating): 300-340lb/in

Modified front spring rate: 400lb/in

 

Stock rear spring rate: 232lb/in

Modified rear spring rate: 350lb/in

 

I used the lower est stock front spring rate in this calculation, I find it hard to believe OEM would set up the stock frequencies as such. Frequency tells us two things, first how well the suspension is tuned to the vehicle, this is a good way to compare suspensions across different vehicles. Second, it will tell us what the optim ride speed will be. Pitch optimisation is a measure of how much the car will pitch, or shift weight between front and back while in motion. In this case, stock pitch optimized speed is 0mph, and modified is 44mph. Goal frequency(for me) is 60mph, you want the optimized speed to be the speed at which the car is going to see the most frequent.

 

Stock Frequencies F/R: .7 / .7

Modified Frequencies F/R: .8 / .9

Goal Frequency F/R: 1.5 / 1.6

 

Setting Ride height:

Since KYB are direct replacement equipment and have a fully extended rod height of 8in, and goal is to have 40-50% compressed strut at normal ride height, I used stock shock ride height at 4". This could alter the above stock calculations slightly if it proves different. I need to get the wheels back on the car before I dismantle the front to get some more accurate spring rate and shock ride height calculations than what I have been able to find so far. The front should also be set slightly lower than the rear.

 

I will be using a bump stop in my setup, I agree with the notion that if you are not using a bump stop you are using the wrong spring rates or living dangerously. Stock bump stop is 4 inches in length, so to have a shock ride height of 4 inches isn't feasible. Again I don't know what the stock set up looks like, I ripped into it before I measured so I'll be tweaking the coil overs today to find out what happens when weight is on them.

 

In any case, with 350lb springs in the rear that fit snug between top mount and lower spring perch I expect the stock shock ride height to be 5.5", which is a little high. I will lower the spring 1 inch and check bump stop clearance, if it is an inch or less I may be modifying the bump stop. Pictures and results to follow.....

 

Goal Suspension:

As shown above, the frequency is 1.5 / 1.6 for 60mph. To acheive this, the next suspension set up could look something like this:

Front spring rate: 970lb

Rear spring rate: 650lb

 

 

 

 

[topless94style2]

 

I removed the stop bump stop and instead modified the KYB one to fit and allow enough travel. 14 inch springs are at their lowest and I got about 1.5-2inch drop.

[RobertISaar]

Goal Suspension:

As shown above, the frequency is 1.5 / 1.6 for 60mph. To acheive this, the next suspension set up could look something like this:

Front spring rate: 970lb

Rear spring rate: 650lb

not that i know anything about suspensions, but won't this be rougher than a corn cob up the ass?

[skitchin]

Pics of top mounts/bump stop? Curious how you've modified things. I'd have to suspect the suspension is going to ride like concrete, but you're way deeper into researching than I am. Still, I wonder if the added jolt from stiffer springs can be harmful to the sub-frame or any other components? I know I've heard of honda's etc running real stiff spring rates like this, but would be the first I've heard in a w. Someone on here used to have a 2nd gen GP w/ air bags though.

[GaPrix]

Goal Suspension:

As shown above, the frequency is 1.5 / 1.6 for 60mph. To acheive this, the next suspension set up could look something like this:

Front spring rate: 970lb

Rear spring rate: 650lb

 

This has me confused

[topless94style2]

Yes the spring rates i got out of the formulas seem crazy high thats why ill run with what i have and if this is stiff i wont be going any higher. Best i can make of it is from what i have read autocross cars are set up with frequencies in the 2.5-3 range and an Evo for comparison sake is the 1.6. Which is where i got my goal however working back through the formulas i get these sick high rates, which maybe i am missing something.

[skitchin]

I don't think you made any errors in your calculations, these are just numbers for an all out race setup.

[topless94style2]

i based it off reading someplace that an evo having a spring frequncy of 1.6 and since they handle pretty well i figured it was a decent starting place.

 

Formula for pitch optimization is 3.6(wheel base / ((1/ff frq) - (1/rr frq))) which gives a result in km/hr. Wheel base in meters also.

 

Working these frequencies back through first the frequency formula to find wheel rate required. Use this wheel rate back through wheel rate formula to find spring rate. I know there was some rounding involved to the nearest available spring rate on summit so with these rates of 970 and 650 we can find that front wheel rate 668 and rear 481. Frequencies then are 1.3 and 1.5.

 

I dont haveany better pics of how the set up looks but i used the same setup as you skitchin except with a shorter bump stop. Maybe two inches long instead of 4

[skitchin]

Yeah, trimming that stop is on my list of things to do.