Macpherson Suspension system

[Som]

Greetings,

My team is participating in Formula Student India-2015. I am working in the design and suspension department of our team. Being a first year team in a Formula Student event we are using the Macpherson suspension system in our car.

At the rear part of the car we are coming across the problem of fitment of the shocker. I want to know that if we mount the one side of the shocker to the upper A arms (instead of lower A arms) and other to the node shown in the pic in a brown circle, then will there be any problem. If yes, then let me know what can be done because drive shaft will interfere with shocker at rear if we mount at lower A arms.

Som
DIT MOTORACING

[Kevin Hayward]

It looks like a double a-arm instead of Macpherson. You can have a damper mounted to the upper a-arm, or just sort out the clearance issue to the driveshaft. This is a fairly common packaging issue and has been solved a number of times. Have a bit of a look around.

Remember these are 3d spatial problems. Trying to do effective car packaging in 2d (or with a 2d mindset) is going to end up with a very ordinary (i.e. bad) car.

As a first year team maybe you should have a look at some of the cars with simpler suspensions being developed by a few teams (maybe the old buffalo cars). A beam is much easier to package and doesn't need much in the way of rear frame structure.

Kev

[The_Man]

Good to see some Formula Student India discussion here.

As Kevin pointed out that this is a fairly common packaging issue that teams have been encountering for years. A lot of the teams have come up with a lot of solutions to this problem. The design that you have proposed will work, but it will not be a very elegant solution to the packaging problem you have. You will have a higher CG for your rear suspension sub-system and if the spring - damper is inclined too much, you will have variable motion ratio. It is very difficult to get a damper with a spring mounted directly on the lower shock packaged with the drive shaft around. It is one of the many reasons people have used push / pull rods in the past. Google a little look at some of the Formula Student cars around, many have solved this problem in very interesting ways.

[Som]

Thanks a lot for your replies. I wrote it by mistake Macpherson instead of double wish bone. Well I have tried to Google it a lot but most of the cars have been using push/pull rod mechnism. We somehow managed to adjust our things properly. We are not able to find an alternate solution other than fixing our damper on the upper a arm. The drive shaft is interfering with the damper in other cases.

Now we are coming across a new problem. When we are fixing our rear a arms the tire's motion in vertical direction has been restricted, since the spherical bearing has been full rotated. Our rear upper a arm is at an angle of 22 deg from horizontal in anti clockwise direction (left side-front view). What should I do to solve these things out.

Somnath

[MCoach]

Sounds like it's time to get an iterative process going. Is this the first concept that you've considered?

Are you saying that your control arms rotate 22 degrees or starts 22 degrees from static?

[Mbirt]

At the rear... we are coming across the problem of fitment of the shocker... the node shown in the pic in a brown circle... drive shaft will interfere with shocker at rear if we mount
My good friends from the band Steel Panther can help explain the fitment of the shocker: http://www.youtube.com/watch?v=PNrS9Ot0wu4

Also, maybe the gentlemen of Wichita State University Shocker Racing can elaborate further on the fitment of the shocker:
http://blogs.riverfronttimes.com/dailyrft/MVC024mvc_tourney_2010.jpg

[Kevin Parks]

Som,

For your problem of spherical bearings, maybe you can put them verticaly instead of horizontaly. It will allow you to have much larger possible angles.
Otherwise, try to have longer wishbones.

Kevin

[mech5496]

Or angled brackets. Or "angled" uprights. Many ways to skin a cat.

[Claude Rouelle]

To elaborate on Harry comments.... or five a different perspective....: The screw going through your ball joint should, ideally, be perpendicular to the wishbone plane but it doesn't need to be perpendicular to the ground!

[Som]

Thank you all of you for your replies.

Well 22 deg is the static inclination of rear A arms. Our upright has already been made and somehow we managed to get some vertical motion at rear, but not that much. I have attached pic of our knuckle. Please have a look and suggest me the ways that I can go for to get a solution to this problem. Also, if we will increase the length of A arms, simultaneously the rear rear track width will also increase, which we do not want. We have kept the 22 deg angle so that we can get the required roll center point of rear at the appropriate height from ground, at rear.

Som

[Som]

One more thing i want to ask. If the bracket of the A arms is mounted at one of the suspension points of chassis but the hole in the bracket is somewhat above from the node, so that the rod end on chassis side is above that node.Will there be any problems we can come across if this is the case.

Som

[Pat Clarke]

New wishbones Som!

Pat

[Adam Farabaugh]

One more thing i want to ask. If the bracket of the A arms is mounted at one of the suspension points of chassis but the hole in the bracket is somewhat above from the node, so that the rod end on chassis side is above that node.Will there be any problems we can come across if this is the case.

Som

Sure. If you know forces in each suspension member (there have been some good threads here which helped me personally a lot with this problem), then you can find a moment arm about the tube and by hand calculate a torsional moment. Then figure out if that is significant enough to be a problem, either for yielding the member or excessive compliance. There's going to be a lot of judgement calls required... i.e. is 10 thou compliance too much.

P.S. this process is called "engineering"... you have the inputs, you've solved similar problems for your classes, now apply it to a real problem and make a decision.

[Claude Rouelle]

Som.

Why would you want to do that?

Why wouldn't you want to have the wishbone member axis to go through one of the chassis node center?

Is there any difficulty to do so?

[Som]

Som.

Why would you want to do that?

Why wouldn't you want to have the wishbone member axis to go through one of the chassis node center?

Is there any difficulty to do so?

The problem I am coming across while trying to fix the axis of my wishbones at the center of the node on chassis is related to spherical bearing. The 22 deg angle inclination in static conditions have almost finished the motion of spherical bearing at upright side. Due to this the vertical motion of tyre which has to be there in case of jounce and bounce has been restricted. Now if I will relocated the point to somewhere above the node center my problem perhaps will solve, simultaneously my rear roll center will also change.
I am looking for an optimum solution by which my roll center does not change.

[Som]

New wishbones Som!

Pat

Thank you for your reply.

If I will go for a new wishbone, ultimately my rear track width will also change, which is almost impossible for us to do. The drive shaft is opened to its maximum, therefore its length cannot be increased further. Once we think for new wishbones, in terms of length, at the same time we also need to think for changing our drive shaft. The dive shaft, we are unable to replace with any other because of lack of availability at our place.

[Som]

Thank you Adam

I will definitely try to do that and will try to get a solution out of it.

Som

[Claude Rouelle]

Som

Unless I misunderstand your problem, it seems you put yourself in a box. May I repeat what I said earlier: if the screw which goes though your rod end is perpendicular to your wishbone plane the rod end angle is 0 at rest. That screw does not need to be perpendicular to the ground. It is all about how you incline the upright and chassis bracket. Do you understand what I mean?

[Adam Farabaugh]

It seems like (1) your chassis is built and done and (2) your knuckles are built and done and (3) you have interference between a-arms and upright BJ location at the extremes of your travel.

The ideal is obviously to fix those clearance problems with the chassis and the upright, not by modifying geometry... is that not a possibility?

[Som]

Som

Unless I misunderstand your problem, it seems you put yourself in a box. May I repeat what I said earlier: if the screw which goes though your rod end is perpendicular to your wishbone plane the rod end angle is 0 at rest. That screw does not need to be perpendicular to the ground. It is all about how you incline the upright and chassis bracket. Do you understand what I mean?

Our motive is to fix the plane of wishbone perpendicular to the screw which goes through our rod end, and we have achieved it so far in respect to the chassis side. But when I talk about upright side then the screw which passes through the spherical bearing is not perpendicular to the plane of axis of the wishbone. The holes made in the upright are making the screw perpendicular to the ground. Also our upright and chassis have been manufactured and fabricated already.

[Som]

Adam

The clearance as you might have seen in my pic of upright is enough. The problem is that the spherical bearing is rotated so much that the housing of it is interfering with the screw passing through the same bearing. This is in respect to static conditions.

[apalrd]

You could rotate the spherical bearing 90 degrees so the screw does not hit the A arm.


Or you could design your upright such that this isn't a problem, or your suspension geometry such that this isn't a problem.

Any way you do it, you have to do some sort of engineering.

[Claude Rouelle]

Som,

".......The holes made in the upright are making the screw perpendicular to the ground. Also our upright and chassis have been manufactured and fabricated already"

You did fall in one of the most important traps of FSAE/FS: you started to manufacture parts while ALL you drawings were not finished.

You learn from your mistake. Back to the drawings and but to the machine shop. Not yet

[Som]

Yes i do agree to the fact that we started manufacturing without completing our drawings completely. Being a first year team and inexperienced we somehow struggle to manage engineering and management both at the same time. Well I will go back to my designs again for corrections.
Thank you for you support.