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

1. Nikhil,

Why do you want bellcranks?

What is the point?

More specifically, how many more competition points do you think you will get with bellcranks, compared with not having them? Keep in mind that bellcranks cost you more time, money, resources, etc., than not having them.

Please provide at least one good reason for how/why bellcranks will make your car "better".

You might ask Claude why he always wants to complicate things first, without ever making them simple, but I doubt he will "defend his decisions" (DJ privilege, I guess?).

Z

2. In principle it is very simple to design a rocker. Start simple and work your way through.

a) make sure that the rotational axis of the rocker is perpendicular to the push-rod angle in the front/rear view and is aligned to the sideview angle of the push-rod
b) on the rocker make sure that the line from push-rod attachment point to rocker axis to spring/damper attachment point is rectangular (90°) and is all in one plane This will give you a linear ratio. By making the angle less than 90° one can create a rising rate motion ratio.
c) the base ratio of the rocker can be calculated by dividing the length of attachment point spring/damper to rocker axis by the length of push-rod attachment to rocker axis. This value is usually greater than 1 and needs to be multiplied with the ratio of the push-rod to wheel. This ratio can be calculated with all kinds of kinematic programs.

happy racings !

dynatune, www.dynatune-xl.com

3. I guess on being asked about the genuinity of Software, Nikhil has left.
I want to be sure of my answer to the need of bell crank question. -

*The bell crank multiply the motion of wheel before transmitting the motion to dampers. (this is motion ratio I guess)
*This means the dampers compression/tension is more than the displacement of wheels. This makes the suspension "Stiffer" as compared to the situation when pushrod/pullrod is directly attached to damper.

So we use Bellcrank so that we can make suspension more stiff ?

Just an on spot question which came to my mind while writing this post, if we use pushrod/pull rod directly attached to damper, how Anti Roll Bar is attached in the suspension ?

4. Originally Posted by Ashir
I guess on being asked about the genuinity of Software, Nikhil has left.
So we use Bellcrank so that we can make suspension more stiff ?
If that is the only purpose of a bellcrank, why not simply use a stiffer spring to make the suspension more stiff?

Originally Posted by Ashir
Just an on spot question which came to my mind while writing this post, if we use pushrod/pull rod directly attached to damper, how Anti Roll Bar is attached in the suspension ?
The antiroll bar can be attached to any part of the unsprung mass, it's up to you to decide which solution is best. I'd recommend not attaching it to any of the spinning parts for starters. Of course, this also brings up another question- why do you need an antiroll bar?

5. Ashir,

To add to what JT A. said, you may be a bit confused about what a push/pull rod is. If you are not running a rocker or bellcrank then typically you don't have a push/pull rod, because you are attaching the damper directly to the unsprung mass at one end and the chassis at the other. There are many road going cars that have direct actuation (generally the term to describe a suspension without push/pull rods + rockers) combined with anti-roll bars (which are typically connected to the A-arms), but as alluded to above, I think you might need to have a think about what your suspension is required to do. There are a great many posts, particularly by Z, regarding this.

6. OK,I went in more depth of search results and visited this topic
http://www.fsae.com/forums/archive/i...hp/t-408.html?

Same discussion is already carried out and the conclusion seems good for me.

I myself had thought that by mounting damper directly, we would increase the force acting on it. But then I thought, the force is absorbed by spring not damper.
Then I looked for more information. And turned up with the effect of motion ratio on "effective damper travel" which I interpreted as being stiffer. But again spring's stiffness can be increased in direct mounting of damper as told above.
After this I started thinking about "effective" damper travel. Now I am interpreting it as for the same wheel travel, the damper will push more fluid thus making more effective damping.
Still not completely satisfied, I went for in depth search and visited above mentioned topic. And yes now I would say, my objective is to design the suspension based on the damper we already have. This will save us lot of money (more than what it will be saved on using direct damper mounting by removing bell crank and pushrod costs).
So now I have to make my suspension stiff without changing damper, I will use bell crank to chance motion ratio.
I hope I am on right track. Please correct me if I am wrong or incomplete anywhere!

Edit: I missed out arb question.

ARB: Anti Roll Bar, is used to "fine tune" the suspension. When the chassis roll, the arb blade twists the spring steel rod. The resistance to twist in this rod, creates resistance to roll in chassis. But this resistance is small and thus if required, it can be ommited. When fine tuning is required accounting to track conditions, ARB can play good role.

7. Originally Posted by Jay Lawrence
Ashir,

To add to what JT A. said, you may be a bit confused about what a push/pull rod is. If you are not running a rocker or bellcrank then typically you don't have a push/pull rod, because you are attaching the damper directly to the unsprung mass at one end and the chassis at the other. There are many road going cars that have direct actuation (generally the term to describe a suspension without push/pull rods + rockers) combined with anti-roll bars (which are typically connected to the A-arms), but as alluded to above, I think you might need to have a think about what your suspension is required to do. There are a great many posts, particularly by Z, regarding this.
Sorry I didn't include your reply in above post as I went for a bath while writing above and didn't see your post.
The link joining bell crank to lower wishbone can be push rod if it is in compression, and pull rod if it is in tension. To add to the benefit of using bellcrank, they allows to adjust ride height without changing spring stiffness.

I am sorry I am not a suspension guy but I have read basics. So please excuse me for my lack of research. I will do some more research and come up with better answers

Till then any help and/or comment is apritiated.

8. Where did you get "effective" damper travel from? There is wheel motion, and this is translated to the damper. There isn't really anything "effective" here. Don't confuse yourself with motion ratio. It doesn't necessarily make whatever suspension mode stiffer; it can make it stiffer or softer or leave it the same, or it can be rising/failing rate.

9. Ashir,

Just because you have an existing damper does not mean you cannot mount it in a direct acting fashion. We (University of Cincinnati) and Monash both ran TTX25's (I believe Monash ran TTX25s, if not, someone please correct me) in a direct acting manner in the past year, and those are about as small of a damper as you're going to find. There is no reason you cannot use your current damper in a direct acting installation, you may just have to be a bit more creative in your packaging/frame design.

-Matt

Ashir,

I recommend you take a look at a Honda Civic (just the first that came to my mind) front suspension arrangement (any Civic from 88 to 2001, at least). Many dirtbikes (and bicycles) do not present rockers and they still get the desired motion ratio (and motion ratio curve, which in long travel applications may be way more important than in our toy FSAE cars). Other dirtbikes (and bicycles) present rockers with their designers claiming that this way they could control better motion ratios curves. In many cases, packaging drives a lot of this, in other cases, even marketing can drive design tendencies (bicycles, many times). Note that in any of these applications (with or without rockers) you can adjust ride height without changing spring stiffness.

In FSAE (as in dirtbikes, and bicycles), there are fast cars with all kinds of configurations. So, you don't need to know anything from vehicle dynamics and chassis tuning to understand that there isn't a holy winning formula.
The answer to "why am I using rockers" and conceptual questions like that (these type of question I find EXTREMELY important) may well be particular for your team's needs, realities, targets, etc.

JP