Pushrod in upper wishbone

[JuanJo]

Hi all, I was hoping someone can help me about this issue: I'm a MSc student and for my thesis I'm designing a suspension for a formula student car, the issue is that we want to have 4WD inwheel engines and to achieve that I want to mount the pushords in the upper wishbone in order to clear the space between the wishbones and mount the engine there.
Is there anyone who can help me about this topic? Mostly about the consequences of having this configuration and also the difference between having a pushrod in a lower wishbone and having it in the upper one.
Thanks in advance for your help.

[Tim.Wright]

Why are you so determimed to use pushrods? If you are stuck with attaching to the upper wishbone why not use a pull rod or even better, use a direct acting setup to reduce unsprung mass, overall mass, number of parts, number of interfaces vertical frictio and improve your packaging?

[Per]

He's not stuck to attaching to the upper wishbones, he just needs the space in between the wishbones to mount the motor. So a pullrod is out of the question.

This setup (pushrod from upper wishbone) has already been done many times by other teams in Formula Student Electric. There is no reason why it won't work, all you need for an efficient mechanical design (with low compliance) is that the angle between pushrod and wishbones needs to be as large as possible, meaning the inboard location of the push rod needs to be as high up as possible (e.g. on top of chassis).

From an MSc student it could be expected that you are able to draw a free body diagram and figure this out for yourself.

[Scootin159]

Some of the very first "inboard suspension" designs of various early 70's race cars had setups like this. They should be pretty low drag (just a lower control arm, and an upper control arm - the larger upper control arm could shroud the steering link) - but I imagine they probably went out of favor due to weight and/or rigidity concerns. Depending on the extremes you need to go to in order to get a pushrod/pullrod setup to work, this might be something to consider.

[Per]

That's just very poor design. The upper control arm has to carry an enormous bending moment, the weight penalty will be ridiculous. Drag has just about zero importance in Formula Student, compared to weight.

[Tim.Wright]

If weight's so important why recommend adding push rods & rockers? They add weight, friction and unsprung inertia to the system. I personally see the blind application of pushrod suspensions as poor design...

[autogyro]

Torsion bar and friction damping should do it.

[Per]

You are right, you can use a direct acting system as well (this has also been done successfully in Formula Student) but it still has to fit in somewhere so that it does not clash with the in-wheel motors.

These guys had direct acting at the rear and pushrod/rocker at the front due to different packaging and load introduction considerations. The direct set-up coming from the lower control arm seen at the rear is only possible because the motor is not in line with the wheel axle. If your gearbox design is different and the motor sits in the middle of the wheel, there is no way you're going to get a pushrod or direct setup coming off the lower control arm.

http://media.formulastudent.de/FSG13/Ho ... -RhfqRWs/A

[Roni]

I would say pushrod from upper wishbone is a pretty decent and common design. Instead of loading your lower wishbone you are loading the upper arm. But in order to do that your chassis side rocker mount must be higher (than what it would have been if it were from the lower arm) or else you will end up with an extremly bad pushrod angle which will reduce your motion ratio greatly.