Starting Point For Springs & Bars

[VT-R]

Ok I’ve been trying to calculate some starting point spring and anti roll bars rates, just looking for a few pointers and a bit of feedback really. I’m obviously prepared to have a play around with the rates, but I’d like to at least start with something in the right ball park.

Car is for UK tracks runing on slicks

Small hatchback mid-engine rear drive

~40/60 F/R weight distribution

~850kg with ½ tank fuel & driver

Wheel Base: 2380mm

Front track: 1545mm

Rear track: 1470mm

Front suspension MR: 1.15

Rear Suspension MR: 1.1

Front/ Rear Roll Centers both either on or very near ground level.

Front splitter (50mm ground clearance), flat underfloor with rear diffuser and single plane rear wing (No idea on the magnitude of forces developed)

Now I’ve used Optimum G spreadsheet and a few other calculations and for spring rates I was considering the following:

Front Springs: 350lb/in (~61N/mm) giving spring frequency of ~3.05Hz
Rear Springs: 600lb/in (105N/mm) giving spring frequency of ~3.21Hz

Then when I add 50N/mm bars front and rear (MR=1) this gives me 1 degree of total roll per g of cornering force and biases the total lateral load transfer (‘Magic Number’) 5% towards the front.

My starting point to calculate the springs was to calculate the weight transfer under braking and the resultant dive and then spec a front spring that would keep the splitter off the ground and still allow some additional clearance for bumps etc.

Do the bars sound a bit soft, and spring rates possibly a bit high?
Any feedback welcome as I don't really have any practical knowlege, everything I know is based on what I've read or picked up in the paddock.

[Al Weyman]

You need to know the mechanical ratio of the suspension before you can make any assumptions or calculations. For example if it was a double wishbone front suspension with the spring half way along the lower wishbone then you would have to square it (divide by 4) to find the actual wheel rate whereas if it was a McPherson strut it would virtually be 100% of the spring rate, same goes for the rear.

[tristancliffe]

3Hz on a hatchback? They sound a bit hard to me, but it does depend on your specific situation. Were these calculated using the wheel rate or the spring rate (as Al mentions)?

Personally, with the little information I have, would be softer springs and the same bars unless camber control is a problem. Alternatively, ask people who run similar cars...

[VT-R]

Sorry I didn't make it clear...

Front suspension ratio: 1.15

Rear suspension ratio : 1.1

i.e. wheel rate of 100 would be spring rate of 110

And yes everything was calculated using wheel ratio.

Thing is I don't have anything really to compare with, my car is pretty much a one off, its that far removed from production spec.

Camber compensation is a concern and I would like to aim for 1deg per g, so if I go softer on the springs I'd need to up the bar rates.

What sort of frequencies should I be looking at?

[phoenix]

Without knowing your unsprung weights front and rear I can't give you an answer with numbers for spring rates.

However, I do think for the total weight of yur mid-engined car, the weight distribution and the front/rear track that you have, the front should be about 2.76 Hz and the rear a litttle lower - in the range 2.45 to 2.73 Hz. Then with only a front anti roll bar you should be able to find a good balance.

You didn't mention the motion ratio of the anti roll bars, which makes it impossible to tell how stiff your bars really are when operating, and so I can't comment as to whether what you have is too stiff or too soft. I would suggest that you will find the car easier to drive if the wheel rate of the front anti roll bar is equal to or less than the wheel rate of the front springs. This in my humble opinion is probbaly more important that roll angle, unless you run out of camber compensation on the loaded front tyre.

[ubrben]

Originally Posted by VT-R

Sorry I didn't make it clear...

Front suspension ratio: 1.15

Rear suspension ratio : 1.1

i.e. wheel rate of 100 would be spring rate of 110

Not sure you're right here.

E.g. on the rear. If your ratio of 1.1 is wheel / spring (motion ratio) then your wheel rate is spring rate / motion ratio ^2 so in this case the wheel rate with a 100 spring is 100/(1.1 * 1.1) = 82.64
Alternatively if you're ratio is spring / wheel (installation ratio in RCVD) then your wheel rate is spring rate * installation ratio ^2 so in this case we have wheel rate = 100 * (1.1*1.1) = 121

The displacement of the spring is factored by the ratio, but so is the force, this is why the ratio is squared.

Ben