# Thread: how to design a bell crank for push rod suspension

1. ## how to design a bell crank for push rod suspension

hi sir i am nikhil mehrotra age 20 . i am currently working with my team to design an electric car for formula14.
As i am new to the project, i am currently facing problems in designing my bell crank.
Both the front and the rear geometries are completed and now we are working on designing our bell crank.
we are using pushrod suspension for both front and rear.
plzz help me so that i get some idea on how to design my bellcrank.

2. What problems are you facing? What design work have you done so far?

3. First you must connect your shock to the bellcrank.
Then you must connect your pullrod to the bellcrank.

Match your motion ratio in your geometry, that's the point it must rotate around attached to the chassis.

Connect the dots and make it a solid part. Done!
Bellcrank is designed. Great victory today!

4. sir i have no idea on how to calculate the motion ratio. i have completed my 2-D design for front and rear geometries . How should i mount my bell crank and what should be the first criteria in designing bell crank for my car. (only the 16 points of a-arms have been decided so far , next step is to desig a suitable bell crank , how shoul i do it in solid works??)

5. Hi sir i have completed my 16 points for the mounting of the A-ARMS . now what should be my next step. how should i design my bell crank... what steps should i follow to make it.
I have no idea on how to calculate the motion ratio??? how should i begin??

6. Have you ever followed a course or read a book or watch a video on critical thinking? That is what you need the most I can tell you.

7. Two quick tips and questions

One

- Your lower wishbone outside rod end / spherical joint center (or top wishbone if you have a pullrod)
- The lower pushrod rod end center
- The upper pushrod rod end center (which is also one of the rocker point) (rocker = bellcrank)
- The center of the axis of your rocker
- The damper pick up point on the rocker
- The damper pickup point on the chassis
- The damper axis

Should all be in the same plane. DO YOU KNOW WHY?

Two

Your front 2 pullrods or 2 pushrods should in a vertical plane (or close to a vertical plane) to the ground. Same on the rear DO YOU KNOW WHY?

Questions:
- How do you come with 16 points? I think you know but I would like to read it from you.
- What Ackermann and what bumpsteer (if any) do you have?
- Why 2D and not 3D?

8. hi sir, sorry for replying late!!

The answer to your first question should be that all these things need to be in the same plane because the force transfer from the wheel to the bell crank can easily be resolved in a single plane. The force from the pushrod will try to break the bellcrank, the force from bell crank is transferred to the dampers. The advantage of having everything in one plane is that all the forces can easily be resolved.The force transfer is uniformally distributed if it is in a single plane.

got the
Sir my 16 points were decided on a specified geometry which was 3D only( sorry for mentioning 2D in the previous posts). After doing the solid works , we got the main A arm points, those points were then put up to optimum k. Our geometry was stable in optimum k.

We are using 50 percent Ackermann and have a bumpsteer of .05 degrees at 1.5 inches of heave.

9. Nikhil,

OK so in that case, unless huge other compliance, you probably won't have any REIB (Rod End In Bending), good!

So lets's go for the next steps

What are your criteria to chose (assuming you create a double wishbone suspension, not necessarily THE option but let's say)
- your front and rear tracks
- your front and rear, top and bottom wishbone upright pick up points on the king pin axis
- your lateral VSAL and therefore you camber variation in roll and heave
- your front and rear roll centers height-
- your ratio between top and bottom wishbone length
- your longitudinal VSAL and your X and Z position of your left and right pitch centers
- your front and rear springs position (that means withinin other things pullrods or pushrods choice) and front and rear ARBs position on the chassis
- your steering wheel and steering column (or steering column parts) angle and position
- your front and rear spring motion ratio
- your front and rear ARB motion ratio

The design of the rocker (= bellcrank) is one part of the puzzle and the answer to the questions above need to be part of thst design. There are no perfect answers but I think the list of criteria question is presented in a logical order.

What do you call a "stable" geometry in OptimumKinematics? Do you have a legal license for this software?

10. Originally Posted by Claude Rouelle
Two quick tips and questions

One

- Your lower wishbone outside rod end / spherical joint center (or top wishbone if you have a pullrod)
- The lower pushrod rod end center
- The upper pushrod rod end center (which is also one of the rocker point) (rocker = bellcrank)
- The center of the axis of your rocker
- The damper pick up point on the rocker
- The damper pickup point on the chassis
- The damper axis

Should all be in the same plane. DO YOU KNOW WHY?
Not all of the mentioned above need to be on the same plane, easier in terms of force calculation yes but geometry can easily change the locations of the following without creating RIEB:

- The center of the axis of your rocker
- The damper pick up point on the rocker
- The damper pickup point on the chassis

Just do realize that most of these are two force members and and not to put bending loads on them and then organize your geometry. a better description would be that any consecutive two points (in reality not the list) mentioned will need to be on the same plane to avoid unwanted/needed loads and have a decent load path.