Force analysis

[Cement Legs]

Hey everyone. I'm wondering what most teams are doing to get their moment of inertia for initial force analysis before the design is complete. Is there a different method that people are using to find the forces traveling from the surface throught the tires and wheels and other components right up into the frame? Since we are working on our first car we dont really have any prior data to make a 'good' engineering estimate so I'm kinda stuck. I could pull some numbers out of my butt to use for FEAs, but the problem is that I would be pulling numbers out of my butt. Any suggestions would be very koo.

[Cement Legs]

Hey everyone. I'm wondering what most teams are doing to get their moment of inertia for initial force analysis before the design is complete. Is there a different method that people are using to find the forces traveling from the surface throught the tires and wheels and other components right up into the frame? Since we are working on our first car we dont really have any prior data to make a 'good' engineering estimate so I'm kinda stuck. I could pull some numbers out of my butt to use for FEAs, but the problem is that I would be pulling numbers out of my butt. Any suggestions would be very koo.

[D J Yates]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Cement Legs:
Is there a different method that people are using to find the forces traveling from the surface throught the tires and wheels and other components right up into the frame? <HR></BLOCKQUOTE>

I had this problem when attempting my first suspension design. Initially, i avoided any 3D analysis of the forces - thought it'd be too hard - but eventually gave it a try and haven't looked back since.

I was looking to find the forces at the ball joints. I set up a FBD of the wheel/upright and then solved it to get six simultanious equations. Using vectors and vector products for the moments, the equations can be aranging in the form AX=B,(A is a 6x6 matrix, X a 6x1 and B a 6x1) and solved using the Matlab X=A\B function (or by hand if your a sadist), where X is a matrix of the vector components i was trying to find. The best thing about this is that once i'd set the whole thing up in matlab - which did take a while - i could put in any load case (vector force at the tyre contact) or any geometry to get the reactions at the ball joints by hitting enter.

Using the same method i extended the problem to the wishbones to find the forces in them and therefore, the reactions on the chassis.

I got the load cases by setting up a spread sheet to find the normal tyre loads and then used some assumptions and data for the lateral and longitudinal. Although, for my nexy effort i'll make this a function of the Matlab script that solves the forces. Actually, i intend to use the same script to to anayse the geometry (force based roll centers!) and calculate all the stresses and max loads based on specifed materials and parts; then automatically export the results to excel which will then import into Inventor to dimension the parts. Sounds like a lot of work but is actually so much easier than doing each step manually - really just using the tools at your disposal.

I got this idea from Road Vehicle Suspensions by Wolfgang Matschinsky (ISBN 1 86058 202 8) and plan to take it further for the next car. Although i now have access to some dedicated simulation software, it takes time to learn to drive it and requires a much more complex model than i can specify at the early design stages. I can only really use it to optimise the design at a later stage.

I will admit that it took several days to set up the script. Although once it's done it means you can go through many iterations in an afternoon to find the best design without having to all the basic remedial work that is nessecary when doing such calculations by any other method.

If you want to know more i can send you a copy of the script to save you some time figuring it out from scratch. However, the instructions and explanation of the parameters is even longer than this post. http://fsae.com/groupee_common/emoticons/icon_wink.gif

Hope this is some help to you.

[Cement Legs]

Yeah that would be very cool, my email is cement.legs@unb.ca I'll have a look through it and see if it helps to clarify the process a little as well. Thanks http://fsae.com/groupee_common/emoticons/icon_biggrin.gif