spring constant calculations

[A]

hello guys,
i am working on calculation of spring constant for our car and here are d calculations,plz correct me if im wrong
i assumed 0.8deg/g of roll gradient,
and calculated roll rates using formula in rcvd (roll gradient=wt of car*cg to roll axis distance/(roll rate front+roll rate rear)).our both roll rates are equal(wt distribution is45:55).and mass of car is 280 kg.so i got roll rates as515Nm/deg.
then i calculated the wt transfer in front and rear by formula in rcvd .and got values as 496 N and 530 N resp.here i have assumed no banking and lateral accn of 1.5g about which i am not sure.(can u plz tell me how to assume this value??we dont know how much will be the speed in turn and radius of the turn,do these race tracks have the bankings???)
then i calculated ride rates by assuming total travel to be 3 in and got value as 6509.2N/m in front and from this value i got frequency in front to be 1.6 hz(is this freq of chassi or wheel or both??,formula in rcvd) and then i calculated k value in front by formula in opt g(motion taio 1:1) to be20.2N/m...i think my freq and k values are small,guys also let me know whether it is good idea for the first year team to opt for ARB or not
thanx in advance

[NickFavazzo]

The frequency (1.6) would be that of the chassis in ride, to find the unsprung natural frequency you will need the tyre stiffness also typically the unsprung natural frequency will be much higher than 1.6 Hz.
you can quickly check the stiffness values by checking loads and seeing if the resulting displacements are what you expected/designed.

As for using an ARB, research why they are used and the effects of roll stiffness (from corner springs) have on stiffnesses in other suspension modes. ARB's by design work in roll and increase the roll stiffness without adding to the stiffnesses of other modes.

As for assuming lat G's 1.5g probably isn't too bad for a starting point for a first year car. Corner radius can be found by sketching slaloms/corners from information in the rule book, then draw a "race line" based on your CG position from the cone/corner marker, this will give you something to work with.

[A]

hey thanks Nick..but im still confused about freqency factor..i mean there are two frequencies
1 chassi unadamped freq
2 wheel undamped freq
which one is of importance??
i have read on forums,teams keeping their ride freqency between 2-3 Hz ..which freqency is this???and according to formula in rcvd (for frequency calculation) wt used for calculation is total wt at one corner ,so isn't it the freq for sprung + unsprung mass system???

[NickFavazzo]

The chassis natural frequency is damped, whilst the unsprung natural frequency is not damped (well not by the damper) they are both important but in different ways, the ride frequency you are referring to as between 2-3 Hz is the chassis (or sprung mass) natural frequency. The natural frequency is an indication of how soft or hard the car is

I take it you are using chapter 16 of RCVD? The equations used in that chapter are simplified, for more accurate calculations follow chapter 18 which goes into much greater depth about wheel loads which can be transferred into spring loads.
frequency calcs are a function of the sprung mass and the spring. in the case of the sprung portion of the car it is the sprung mass, and the corner spring (assuming a simple 4 corner spring car) whilst the unsprung frequency will use the unsprung mass the the two springs that support it, the tyre and the corner spring. Using total corner mass will just give you an approximation.

[A]

do they have bankings there???

[NickFavazzo]

banking where?
I would ignore banking, there isn't enough to warrant changes to your design, if there is banking and you plan on going to the same competition again the following year you have some data to make decisions about including it into the calculations or not. Remember that RCVD is not a FSAE design book, it covers vehicles that run in many different style races. it is not a set of instructions about how to build a FSAE car.

[A]

is my assumption of 1:1 distribution of roll rate correct?? i have assumed it so because of 45:55 wt dist (45+5=50..magic no concept)....in this case i have understood every term and i know how to cal the things ...but i dont know what value of roll rate ,wheel rate or ride rate i want

[MCoach]

Originally posted by A:
is my assumption of 1:1 distribution of roll rate correct?? i have assumed it so because of 45:55 wt dist (45+5=50..magic no concept)....in this case i have understood every term and i know how to cal the things ...but i dont know what value of roll rate ,wheel rate or ride rate i want


From reading through your many posts of the forums as of recent, it's nice to see that you are reading. However, this is more than just a "perfect numbers" game. Is your assumption of 1:1 correct? I don't know. Wheel rate, ride rate, etc. all value to a large degree based on what you decide as a design engineer. Since you have RCVD, you can reference that for to folow my points. Table 16.1 in RCVD lists typical roll gradients.

soft--------------7.5deg/g
Firm--------------5.0deg/g
very Firm---------4.2deg/g
HARD-----------1.5deg/g

This gives you a nice place to start.
If you notice the sentence right below the chart, "The problem the designer faces is to establish ride rate requirements for the particular car in question."
So, determine your wheel loading and designed wheel travel. Start with a estimate, iterate as better data becomes available. Since, I will also guess that you don't have any data on your own car for natural frequency, I'd recommend starting with the value of 2Hz (Don't assume I started anywhere far or close to where you should end up) front and rear. 2 is just prime and easy to work with, I like that. Iterate from there.

Jumping to "Transferring Ride/Roll Rates to Wheel Center Rates". The analysis in this portion will help you get an idea of how to select your ride and roll rates (assuming you've done the leg work in the pages between here and Table 16.1 with estimated data). Find your roll rates/gradients.

Jumping to Table 16.3, this is what it should look like when you've calculated the necessary rates and load transfer functions.
Perform the ride and roll analysis. Then reiterate the whole entire process until you get numbers you are happy with.

Also, keep in mind this:
Your car may be "optimally" designed by the numbers, but if the driver's cant keep it underneath them, then it's still useless. Always leave room for tuning, listen to the driver when he says something is wrong. Gaining confidence in the driver is worth more time than having him carefully tip toe the car to its ultimate limit.

[bharath supra]

Originally posted by A:
hello guys,
then i calculated ride rates by assuming total travel to be 3 in and got value as 6509.2N/m in front and from this value i got frequency in front to be 1.6 hz(is this freq of chassi or wheel or both??,formula in rcvd) and then i calculated k value in front by formula in opt g(motion taio 1:1) to be20.2N/m...i think my freq and k values are small,guys also let me know whether it is good idea for the first year team to opt for ARB or not
thanx in advance

you keep on saying that you read each and every concept in the vehicle dynamics books.
if you got the ride rate as 6509.2N/m and if your motion ratio is 1:1 how did you get 20.2N/m
spring rate??
first be clear with your basics and then star reading the difficult books. you won't have this many confusions.
if you think your values are small, compare with real world values..
if you don't know what are your desired roll and ride rates are how did you assume 0.8deg/g roll rate.
first thing depending upon your C.G height and weight and roll axis your car is being exerted a moment of 515nm per g of cornering force not per deg. i think you have to mention 515Nm/g not 515 Nm/deg.