suspension - statics

[GrannySmithApple]

Hi all, I'm fairly new to SAE and I want to understand more on designing and developing calculations for suspension systems. So I hope you can help me.

I want to develop a basic 2D static analysis excel spread sheet that can determine the loads on the upper and lower A-arms (whether they are in tension or compression). The diagram below is a front view of a double wishbone with a pull rod suspension. Just want to check whether I am on the right path. How would I calculate the force loading on the pull rod?

If you could provide some guidance or any help, much appreciated!


http://img9.imageshack.us/img9/9767/staticanalysis.jpg

[GrannySmithApple]

Hi all, I'm fairly new to SAE and I want to understand more on designing and developing calculations for suspension systems. So I hope you can help me.

I want to develop a basic 2D static analysis excel spread sheet that can determine the loads on the upper and lower A-arms (whether they are in tension or compression). The diagram below is a front view of a double wishbone with a pull rod suspension. Just want to check whether I am on the right path. How would I calculate the force loading on the pull rod?

If you could provide some guidance or any help, much appreciated!


http://img9.imageshack.us/img9/9767/staticanalysis.jpg

[Nick Renold]

For 3D analysis:

http://fsae.com/eve/forums/a/t...20226151#21020226151 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/98420416151?r=21020226151#21020226151)

For 2D analysis:
Write force balance and moment balance equations for each point. Solve.

[GrannySmithApple]

Thanks for your quick reply,

I used "method of joints" by taking the moment at B and then found the remaining forces in the members by doing a force balance in each point as you said. Top arm and lower arm is in compression, with the pull rod in tension. But you said to write moment balance for each point? Could you explain why this is necessary?

thanks!

[Nick Renold]

For your simplified front view, it sounds like you did all the necessary steps.

If you wish to move to 3 dimensions it helps to start with the equilibrium equations (force and moment balance). That is all.

[edd_16]

In the above figure Sdyn and Gdyn represent cornering and ground reactions right?
i am also new to FSAE suspension so its all very confusing to me as well
To solve the equation we would need 2 force equations and a moment equation right?
The force equation could be from the ground and cornering but what about moment equation? do we need to take the roll centre distance as well and make a moment equation such that there is no roll over???
Is this making any sense:confused:

[Ray19]

http://www.fsae.com/forums/showthread.php?11179-analysis-of-wishbones&highlight=analysis+wishbones
This thread will give you a great idea on how to proceed in calculating wishbone forces. (I am getting a 404 error on the page Nick posted above so sorry if it is a repost)

[edd_16]

Thank you for the fast reply ray
But as explained in the link u posted the forces are for static condition right?
What about the dynamics like cornering accelerating etc.???

[Ray19]

Your previous question about moments can be answered by the thread I linked to although it is in 3d. What do you mean by dynamic conditions? In the above method you can input cornering and acceleration forces at the tire contact patch and see the output as forces on the links. Do you mean to say that you want to analyze on moving links? Also a suggestion, the above FBD approach is the best place to start(and in many cases more than enough) and I would like to see you solve that and get some values first before moving on.

[edd_16]

Thank you Ray
Now i have a clear understanding on many concepts of the foorce and moment calculation
I am learning to prepare the matrix and at the same time working out the FBD approach as well.
Just one little question suppose my tire left front contact patch has co ordinates of x=0,y=0.66m,z=0 and origin is the centerline of my vehicle .
Now if i wanted to calculate my moments on the rear left tyre that has corodinates of x=1.62m(wheelbase) ,y= 0.60m(trackwidth) and z=0(vertical height) with Fx=Fy=Fz=1000N (assuming) the calculations will be
Mx = Fz.Ry - Fy.Rz = 1000x0.60 - 1000x0 = 600 Nm
My = Fx.Rz - Fz.Rx = 1000x0 - 1000x1.620 = -1620 Nm,
Mz = Fy.Rx - Fx.Ry = 1000x(1.620) - 1000x0.60 = 1020 Nm
Is the approach correct???
If yes then you have helped me solve a major problem in my understanding of suspension dynamics

[Ray19]

Yes those numbers are correct. But I hope you understand what it actually means. The moments that you get will be about the axes that you have defined through your origin. In the above case the My will be about an axis passing through the contact patches of the two front tires while the value of Mz is about a vertical axis at the midpoint of the front track. Now if you change the origin to somewhere else the moment values will obviously change to show the moment about those newly defined axes but bear in mind that the force values on the links should not change due to this change in origin. I am happy to help out. Also can you explain what suspension dynamics problem of yours is getting solved by this? I would love to know and I am sure it might help someone else out there in the future.

[edd_16]

Thanks again for the response ray
Actually i wanted to know all the forces on my wishbones + rocker
If i knw the forces then it will greatly help the design team to optimize the dimensions of my wishbones which in turn might help us save weight on our car.
Additionally until and unless i dnt know the forces on my pushrod how will i find the reaction on the fulcrum pic of the rocker and spring which is of coursee the basic essence of designing rockers.
Also learning about dynamics is never a bad thing being a automobile enthusiast i think it will benefit me in the long run as well.
Now i wanted to ask you this i was looking for an alternative material for pushrod( aluminium actually) instead of mild steel so i have some basic questions :-
1) Do i need to apply Euler's buckling theorem i.e. P= [(pi*pi)*E*moment of inertia]/(Length of pushrod*length of pushrod)??? or anyother formula to determine the dimensions since the pushrods buckle raather than yield???????
2) Is aluminium the right choice cuz its significantly weaker than m.s (E= 70 Gpa while for steel it is E=205 Gpa)???
3) What abut Yaw and pitch Geometry? Do i really need to do calculations on them since i have done roll calculations??
4) Do you think i am an idiot for nt knowing sch things:confused:

[Ray19]

Can you please tell us which team you are from? Is this your first car? If so then I would suggest that you do not go for alternative materials for critical components like pushrod. What you need to focus on is a working suspension geometry that can be put in an fsae car that will complete all dynamic events. As for your 3rd question, what do you think? Are they important in defining your car behavior? How does it become less important "since" you have done roll calculations?

[edd_16]

yes this is my first car hence going for the alternative cud be a threat
RCVD roll calculations dnt necessarily involve much pitch geometry except for slight anti dive and anti squat
As far as yaw is concerned i dnt really think it would be of much use. i cud be wrong though
And one more thing i also tried to develop a matlab program as suggested in the link given by you but my values are coming in the range of 1.3*10^5 which is very very high i must be doing something really really wrong right???
The matrix is a 6*6 matrix with the unknows being
1. Lower front wishbone
2. Lower rear wishbone
3. Upper front wishbone
4. upper rear wishbone
5. pushrod
6. trackrod isnt it???
Also u did not answer my first question in the previous post -Do i need to apply the euler theory of buckling for pushrods??

[Ray19]

Have a set of goals ready for the car and the suspension. I would say right now top priority should be reliability. To achieve that the biggest factor is testing. So look at your timeline, can you accommodate the extra time to study up and properly design the car for pitch and yaw or should you just do the basic minimum in these to make it into a working geometry. Take the decision yourself. Don't get me wrong, FSAE is all about learning but at the start you may need to take these steps. Once you have the reliability you can move on to better design.

For the force calculation go back to the thread I posted before and go through all the posts, there are a set of input and corresponding output values with which you can verify your program/sheet.

About the pushrod it also is up to you. Think of the forces acting on it. Check if there are any tensile loads and/or possibility of bending loads. If your answer is no and the only significant load is compressive then look at buckling. Again check to see whether Euler's theory applies to your case or is there any other way to do it. Decide accordingly taking all factors into consideration and you will have lesser chance of it failing.

[edd_16]

Ray you were right i worked hard and have finally worked out the matlab program and the loads seems to pretty accurate so thank you!!!!
And yes as for pushrods i have been looking at alternate loading conditions and now with the matlab program the loads have been calculated and now i can start to work on the dimensions and yes i have also taken your earlier advice and have decided to use mild steel only for push rods:cool: