Spring rate...

[Cement Legs]

I'm just curious what you guys with experience think about setting up spring rates. Using moutain bike shocks what is a good starting point (after leveraging) for an est. 425lb car with average rider about 170 lbs. I know there is an infinite number of possible iterations, I guess I'm looking for an average or something. ie, is 250lbs/inch a good place to start.

[Greg H]

70 lb driver? Are we allowing elementary schools to compete now?

[Angry Joe]

If you want to do it by the book, you need to start with the wheel frequency and roll stiffness you want, and work back from there. Miliken & Miliken goes through it pretty thoroughly.

[Angry Joe]

By the way, your spring motion ratio will have a huge effect on your spring rate. You can't even ball park it without knowing that...

[Cement Legs]

k... but you dont have to be so angry about it...http://fsae.com/groupee_common/emoticons/icon_smile.gif

Oh and thanks... I've got the book at home so I will back up a couple of steps.

[Cement Legs]

We have 450 and 500 # spings and for our motion ratio I was kinda playing with 2-1 to get our rates down to 225 - 250 at the wheel. I checked for wheel frequency but could not find anything... can you quote a chapter or page. Tks

[fade]

i think its titled wheel loads

[Denny Trimble]

Definitions:
Motion Ratio (MR) = Wheel Travel / Spring Travel (Carroll Smith)

Installation Ratio (IR) = Spring Travel / Wheel Travel (Milliken)

Ks = spring rate (coil itself)
Kw = effective rate at wheel
Kt = tire spring rate (vertical)
Kr = ride rate (Kw + Kt in series)

Kw = IR^2 * Ks = Ks / (MR^2)

Don't forget the squared term...

1/Kr = 1/Kt + 1/Kw

Natural Frequencies should be around 3Hz (+/- 3 http://fsae.com/groupee_common/emoticons/icon_smile.gif )

Natural Frequency (Hz)= (1/2pi) sqrt((Kw or Kr)/M)

M = quarter car sprung mass

Units need to be consistent; K in newtons per meter or pounds per foot; M in units of kg or slugs.

I hope that helps... If I screwed something up, let me know.

[Carlos Torres]

Wheel rate should be around 100 lb/in
Check out the ride rates too. Chapter 16 Milliken
Should be around 2 Hz

What do you think about a 4.5 Hz Sprung natural Frequency? Is too high???

[Paul V.]

Alright,
I am soooooo glad you brought this up, I came up with frequencies around 230 cyc per min. this corresponds to similar wheel rate as above with damping ratio of around 1.06.
In testing we cannot get the rear to hook up!! do you guys like to fix this by merely softening the rear springs, or do you change geometry? I am weary of soft spring for no good reason other than I am scared of bump stop contact on a big bump.

Paul Vaughan

[Cement Legs]

Off topic... I love this forum... you can actually get really good feedback if you don't mind listening to people who have different but really good ideas!

There should be FSAEU or FSAE University. Because that's where I would go and work on cars ALL of the time... ok throw in a few hours for the wife...http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Denny Trimble]

You shouldn't be running on the bump stops or skid plates, but hitting them once in a while in testing is what they're there for - to let you know how far you can adjust ride height and spring rates.