Suspension design

[kishan_s]

Hello everyone. We are participating in fsae for the first time this year. We were thinking of using pushrod design by connecting it to the upper a arm. Then we saw that not many people had done something like that. Se we wanted to know if there is something wrong with the design. Thank you.

[penna]

for a given wheel rate if you make your push-rod more vertical the force that goes through it is less due to the direction of the reaction force inhibiting the wheels vertical motion ( draw a fbd ). Push rods on the bottom a arm, due to packaging, are generally more vertical which is one of the reasons for this configuration

[Claude Rouelle]

kishan_s

Let me help you to help yourself;

1. What in your opinion are the + and - of pullrod and pushrod?

2. Are pullrod and pushrod the two only solutions to connect suspended and non-suspended masses? What would be these other solutions and what would be their + and - too?

If you give a first shoot at trying to answer these 2 questions. I am sure there will be a snowball effect and I and other participants of this forum will share with you good perspectives.

[Big Bird]

I've actually seriously considered it at the rear of the car, it can circumvent potential pushrod / driveshaft issues.

Why not go one step further and turn the upper A-arm into a rocker...

[Claude Rouelle]

To Big Bird,

Been there, done that on the second Formula Ford I designed. It was heavy and I was stuck with a motion ratio I ended up not liking and could not change. Not elegant, not easily tunable, heavy. The top wishbone rocker ended up being heavy to sustain the constraints. Remember elegant (I mean esthetic) and light weight go together. Only advantage was aerodynamic (damper inside the bodywork and no pull or pushrod disturbing the air to the side pods and underwing) but is aero that critical on a Formula Ford...?

That is my experience but I welcome all arguments. At the end there is no perfect design, it is about making the car fast, knowing WHY it is fast, make it reliable, relatively cheap to manufacture and to maintain.

[Kevin Hayward]

I think Geoff (Big Bird) is talking about turning the top arm into a triangular pyramid of lightweight tubing with all members in tension/compression instead of a heavy fabricated top arm loaded in bending.

You still end up with no changes to motion ratio short of changing mount locations (maybe some different ones designed in) or a new a-arm. Wouldn't be something you would do on a open wheeler for high speed as it would disturb a lot more air. On a FSAE car no problems. (Except maybe a slightly higher COG)

You could design your upper a-arm structure to either use removable rod ends (though inboard only), cheaper bushes or bearings, and or bearings in the brackets. That would keep the cost and difficulty of manufacture low enough that it would be about equivalent to the manufacture of most rockers, excepting the simple plate designs. That way you could feasibly make a couple of different motion ratios pretty readily. Shims on the the mounts (in/out or both) coupled with normal preload would give you as much adjustment as in any typical rocker arrangement. Overall it would probably be a pretty good solution if you wanted to change the motion ratio of direct actuation.

Kev

[Claude Rouelle]

If you have a triangular pyramid then I bet its weight will be the same as the one of a top wishbone + rocker + pull or pushrod.
Big, somewhat heavy, high CG, little possible adjustment. You need a complicated jug for that too.

When you see how simple, cheap, and light it is possible to manufacture a rocker for laser cut steel today...

[Big Bird]

I was thinking as Kev had suggested, with tetrahedral A-arms. Reason was more about reducing the number of node points on the rear chassis, therefore simplifying manufacture and possibly reducing chassis weight. Depends on the overall vehicle designs you are comparing as to which would be lighter.

I'd considered the lack of motion ratio adjustment and there is no reason that you couldn't have different shock mounting points for different rising rates, or turnbuckle links in the tetrahedral arm to give different motion ratios.

Jesse Walker and I were looking at the RMIT car in 2011 and the close proximity of muffler and A-arms and rocker and dampers and pushrods and all the different chassis hardpoints. We came up with the crackpot concept of a mufflerockerarm, a muffler that that had a rotating joint that had a rocker as the end cap connecting the upper upright pivot point to the chassis. The other side of the car had a radiatorockerarm, a three dimensional structural duct that acted as the upper rear a-arm and used the structure of the radiator as a rocker on the other side. Cost - unsprung weight, some friction in the suspension motion, complexity in manufacturing the upper "Suspension" arms. Saving - quite significant reduction in complexity of the rear chassis
It was a fun conversation and some interesting sketch drawings came out of it. I'd hope that FSAE could be one of the few remaining forms of motorsport where you could entertain such ideas and have such conversations.
Suitability of such designs? Well it depends on your design problem. IF unsprung weight is the prioritized design issue for your team, then you choose the option that gives you the best returns on that criteria. If the need to finish your car and start testing it is the factor that is going to to give you the best return then it might be worth trading off some unsprung weight savings or rocker adjustment to buy some extra driver training or testing time. .
The biggest problem I see with young designers is that they lack the confidence to trade off one want against another. A good designer , to me, is one who understands their priorities and who can happily cop a "hit" somewhere to gain a bigger return elsewhere. Unfortunately at uni you will be hard pressed to find anyone who doesn't think that good design isnt just about finding the best of everything and assembling it.

[kishan_s]

The main reasons for us choosing the pushrod to upper a arm design was because we didnt want any problems with the driveshaft. so we are using this only in the rear.