Coil Over spring rate calculation 2

[tim080772]

Ok, I've read the forum and a load of other sources and this is my conclusion;

Vehicle Weight: 740kg
Split 60/40 (r/f)
Shock absorber available travel 88mm (3.46 inch) + bumpstop 4 inch total
Front shock/springs mounted at 30 degrees
Front lower arm MR = 0.62 (Arm pivot to tyre = 430m, to spring mount = 270mm)

If I convert this all to allow for up to 2,5g I come out at a very high spring rate. I am currently using 275lb and there is not noticable suspension movement. Before I guess and go for a 180/200/225 spring rate, I want to get the calculations right. Can anyone help further?

Vehicle weight: 740kg
Front Weight : 740kg*40%=296kg (653lb/2=327lb per side)

327lb * 2.5g / spring travel = 204lb/inch required at the wheel

Take in to account the shock/spring is mounted at 30degrees this means it is only 75% efficient, also the ratio in the lower wishbone is 1.59 (or should that be 0.62?).

(204lb/3)*4 = 272lb (To allow for mounting angle)

272*1.59 = 432lb (Arm ratio)
272*0.62 = 169lb (Alternative arm ratio)

Thanks in advance!!

Tim

http://devil.russellweb.eu

 

[BrianPetersen]

I think it will have to be even higher than you think. There is a squared relationship for the mechanical advantage between the "wheel rate" and "spring rate".

to use easy numbers, suppose the mechanical advantage is 2. then 1" wheel rate means only 0.5" spring movement but the forces are also doubled, so the spring rate is the wheel rate multiplied by the mechanical advantage squared.

For your "shock absorber available travel" are you talking travel at the shock, or travel at the wheel?

Keep in mind also that when the suspension is fully extended, most likely there is still going to be some preload of the spring.

What you should do is figure out how much force will be on the spring (and the length of the spring) when you have the car at nominal ride height, and repeat it when fully compressed with the amount of spring-force you want to have under those conditions.

Where did the "2.5 g" come from, does the body have a lot of down force, or are you allowing for that much to allow for going over dips in the road (like the "gravity cavity" in the old configuration of Road Atlanta)? If you are looking at cornering loads, the antiroll bars will take up some of it.

But, nevertheless, I'm not too surprised at your findings so far.

Expect to have to fine-tune spring rates when you actually drive the car.

 

[tim080772]

Hi Brian, thanks for the response. I'm not quite clear however what you mean with squaring the force.

I have a shock absorber with 4 inches of travel.
The car when resting on it's wheels has 4 inches of ground clearance. The ration of wheel to shock absorber movement is 0.62. This means if I don't want the car to hit the road, I want to allow 4 inch maximum of wheel travel, which equates to 2.5 inch of shock/spring travel. If I remove the unsprung weight per wheel (+/- 25kg) then I have 125kg per side.

On the basis of the current springs rated at 275lb, I can calculate that when the car is lowered the springs will compress by (1 inch * 0.62) = 0.62 inches. Or do I need to square the effort?


Tim

http://devil.russellweb.eu

 

[NormPeterson]

Either forces (by themselves) or displacements (ditto) use the ratio directly. Whether you'd use 1.59 or 0.62 depends on which way you're going (spring/shock ? wheel or vice-versa) and which parameter you're looking at.

Rates involve both of the above parameters, so if the spring load is 1.59 times the wheel load and the spring displacement is 0.62 times the wheel displacement . . . BTW, rate computations due to spring/shock angle would be treated in analogous fashion.

Consider that at 1g the suspension is at rest, presumably somewhere near the middle of its travel. IOW, the capability to withstand 2.5g may not be uniformly distributed over the total wheel travel.

Unless this is a firmly damped dedicated track/autocross car that sees little or no street duty, you may also want to investigate ride frequencies and pitch behavior.


Norm

 

[tim080772]

Thanks for the input Norm. The photo is from about a year ago, I've been driving the car on the road about three months now, and covered around 2000 miles. In corners when I hit a bump the car jumps and looses front end traction. For this reason I wish to change the front springs.

I understand your uniform calculation, and that, I'm afraid is going to be trial and error, or progressive springs.

I think I'l just try some 200lb springs and see how that changes thinkgs. I can always adjust the preload and the shock damping to compensate a little.


Tim

http://devil.russellweb.eu

 

[NormPeterson]

I'll have to wait a bit to see the picture, as all I see is a red "X" or a "blockpage" due to corporate internet filtering at my end. Lotus/Caterham/Locost or other clone?

From that 30° inclination note I'm thinking that the overall motion ratio will not remain constant, and that the effective wheel rate could soften with increasing suspension bump travel.


Norm

 

[GregLocock]

Find 50 kg. Place it above the front axle. tell us what the change in ride height is.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BrianPetersen]

"In corners when I hit a bump the car jumps and looses front end traction"

My immediate thought is too much high-speed compression damping, or maybe too much unsprung weight relative to the vehicle.

 

[BobM3]

You're sure that the whole car body jumps? Or does the body not move and instead the tire/wheel hops off the pavement?

 

[tim080772]

If you watch this video you'll see the amount of movement in the front right suspension is almost non existant.

http://www.youtube.com/watch?v=5x0NUFQvUrE

As Bob says when the car hits a bump I am ouncing like a skippy ball because the springs are too stiff and only the tyres are suppling any suspension.


Tim

http://devil.russellweb.eu

 

[GregLocock]

Without delving into the motion ratios, your estimated required wheel rate targets are extremely stiff, compared with the static weight. It looks as though you have no sta bar, but I think you could safely drop the target rate to 120 lb/in, and if you have a sta bar then you could expect to target 80 lb/in.

I suspect you may have to compromise because your coilovers lock you into may restrictions.

Also don't be afraid of jounce bumpers.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tim080772]

Thanks Greg.