Third spring

[silente]

Hi all!

I am just fitting a third spring and damper on a formula car with double wishbones and one spring-damper for each corner.

In your opinion what is the way to calculate the total wheel rate and the total damping at the wheel?
I was thinking to calculate the effect of the third spring as a spring in parallel, including its install ratio. Is it right? What about the damping force?

Thank you!

[Tony C]

Can you please explain why you are installing a third spring? What are you aimong to acheive? Was the car designed for a third spring or are you re-engineering the car? What type of car is it?

[silente]

we are installing a third spring to control heave movements under aero effects. Normally we don't use on the structure we built a real third spring but a bump stop and now also a damper, in order to reduce the high frequency movements of the car at high speeds and to have a better control on the dynamic pitch.

So i am thinking to how to calculate the overall force at the wheel under the action of springs, dampers, and all the new things (bump stops and damper on the third spring case).
I thought to calculate the effect of the bump stop or of the third spring, i we will ever use one, like a parallel spring with it's own motion ratio, is it right?

But i don't know how to exactl calculate the effect of the damper...i have to consider its own speed and its motion ratio? have you got a specific formula?

The car is a formula 3 car not initially intended to have a third spring.

thx

[phoenix]

Originally Posted by silente

Hi all!

I am just fitting a third spring and damper on a formula car with double wishbones and one spring-damper for each corner.

In your opinion what is the way to calculate the total wheel rate and the total damping at the wheel?
I was thinking to calculate the effect of the third spring as a spring in parallel, including its install ratio. Is it right? What about the damping force?

Thank you!

It is important to understand the configuration of your proposed installation as that will have a major effect on what happens to the spring rates.

If the third spring is installed so that it is mechnically in parallel with the main springs and there is no difference in the mechanical advantage that the suspension has over each spring, then the rates can be simply added together. Your total front stiffness would therefore be spring 1 + spring 2 + spring 3. A single wheel rate is half of that and can be used to calculate individual wheel frequency.

In the situation where only one wheel is displaced - i.e. by running over a kirb, then only that wheel will be trying to compress the third spring. This will increase the wheel rate to spring 1 + spring 3, which will always be greater than (spring 1 + spring 2 + spring 3) /2

The damper rebound rate should be calculated to control the third spring based on it's own installed rate and natural frequency. rebound settings on the main springs will probably remain the same as they are now.

Bump damping is another matter. This damping force will also be added to the main spring damping forces and felt at the wheel. I am thinking that bump settings will need to be reduced on all 3 dampers or the thing will turn into a kart. My thinking is that total bump force for all three dampers would need to be the same as for the two you run at present.

[silente]

we are installing the third damper + bump stop connected to the t bar we are using on the car, so that when both the wheels move the third damper move as well, but when in roll, the third damper feel only the average displacement of the two wheels.

I have to calculate the motion ratio of this kind of installation, but in this way i gues when, for example, the car has a heave movement, then you feel on an axis the effect you say and on one wheel the half of the total force.

We have already calculated the damper curve as well, what i am wondering is how to calculate, on the base of this curve, the force i will have on the wheel due to this third damper.
I guess it will dipend from the speed of the third damper which can be calculated, in my opionion, when you know the speed of the two normal dampers and their motion ratios together with the motion ratio of the third damper. I also guess i have to calculate this force simply adding it to the rest of the forces dued to the normal dampers,is it correct?

What about the total damping coefficient of the overall suspension in bump and/or rebound? How can i calculate it?

Thx

[phoenix]

Originally Posted by silente

I guess it will dipend from the speed of the third damper which can be calculated, in my opionion, when you know the speed of the two normal dampers and their motion ratios together with the motion ratio of the third damper. I also guess i have to calculate this force simply adding it to the rest of the forces dued to the normal dampers,is it correct?

What about the total damping coefficient of the overall suspension in bump and/or rebound? How can i calculate it?

Thx

The damping rates will depend on speed which I am sure you know how to calculate with the motion ratio(s) compared to the speed the wheel is moving in bump and rebound.

I don't think you can add the damping co-efficients to get the overall co-efficient, but what you can do is add the total damping forces and the total spring forces and calculate the damping co-efficient from those two results.

[flavorPacket]

do you have an accurate number for the pitch inertia of the vehicle? If not, it will take a bit of tuning to get your pitch damper correctly set up

[Inside_Info]

Why have 3rd springs suddenly become fashionable in F3 this year? Anyone know who started it?

[silente]

we had a reason to put it in and, i have to say, we have seen some differences.

I don't have data on pithc and roll inertia of the car, i have tried to understand how to measure the inertia momentum both in pitch and in roll, but without success.
Do you know how to do it?

[flavorPacket]

there are 3 ways of getting a number as I see it:

1) least accurate but easiest: get a really accurate CAD model of the car and calculate it

2) more accurate and harder: make a big swing for the car, find out the frequency of oscillation, and back out the inertias

3) most accurate and most difficult: go to an OEM who has an inertia swing.

to be honest, if you have a good data acq system on the vehicle I don't think you really need to worry about it too much. You should be able to adjust it as you test and have good results. With an F3 car the pitch mode is dominated by aero control over inertial control anyways, correct?

[silente]

i actually work on data to adjust dampers settings, also as regard the third one, but it could be usefoul to have a model and some numbers to be able to define suspensin caracteristics from a blank paper...In this sense, it coul be really useful to measure the inertia momentum and to exactly calculate damping force at ground.

As about F3 pitch mode, yes it is above all connected to aero control, but we use it also to have a control on in line weight transfer, which is very important in the brakings. But we didn't think about the third damper for this reason...