Double Wishbone Shape

[tomgwill98]

Whilst designing the chassis, I'm trying to decide at the rear, whether to have a standard shape double wishbone (A shape) or have the wishbone arms swept and facing forwards, in order to shorten and save some weight on the chassis.

Are there any advantages or disadvantages to these two shapes of wishbones?

[swong46]

Don't you mean swept backwards? Thus shortening the chassis whilst keeping the 60" minimum WB?

When will the 55" revision be implemented?? http://fsae.com/groupee_common/emoticons/icon_wink.gif

[sbrenaman]

Deformation = P*L / A*E if I recall correctly.

What is the P on each of the control arm tubes? Draw a FBD.
What is the deformation of each of the control arm tubes?
How does this effect your rear toe?
Is that deflection an acceptable amount of toe deflection?
How much toe deflection is too much?

It's kinda one of those things you have to make a judgement call on.

[windsoreng]

In addition to all of the great considerations Scott pointed out, you must consider a few things:

The chassis geometry and location of nodes will be dramatically affected by the geometry of the wishbones, engine location, and a host of other factors.

The shape / geometry of the wishbones should accommodate the suspension points that provide you with what you or your suspension team have decided to be the optimal kinematic performance of the system.

Load paths from the wheel to the chassis and the impact of control arm geometry on them (ie. acute angles may amplify loads input to the frame).

[PatClarke]

Tomgwill98,

We see these swept back wishbones on a few cars each year. The judges call them 'Supersonic A arms'.

I have never seen a car like this that had a satisfactory lack of toe compliance and on track the cars are unstable and very difficult to drive.

That is not to say it cannot be done, but if you do you can expect to sit blindfolded in a dark soundproofed room with the Judges, with a bright light shining in your eyes and a cattle prod in the judges hands! =]

In other words, be prepared for an inquisition!

Try to keep your load paths direct!

Pat

[sbrenaman]

Pat - I'm not sure I agree with the fact that the cars are unstable and difficult to drive.

If packaging necessitates a supersonic A-Arm, then you just have to make A and E super stiff! This comes at the expense of added unsprung weight.

For instance, if you needed to get the rear leg of the RL wishbones out of the way of your double diffuser... http://fsae.com/groupee_common/emoticons/icon_wink.gif
http://i837.photobucket.com/albums/zz299/n_anirudh/rb6nakedaylgturk.jpg

But silliness aside, the interesting part is there's no minimum weight in FSAE. And having 'isosceles' or 'right' triangle RCA's (in top view) will most definitely necessitate the addition of structure rearward of the rear axles.

I'm not sure what the correct solution is, though hopefully someone designing the rear end of an FSAE car will read this and bring more info to their next design judging event.

[PatClarke]

Scott,

Your Red Bull example does not illustrate what I am talking about. Rearward trailing arms as the forward link in rear wishbones has been successfully used for eons.

What I was describing is a design where both front and rear arms in the wishbone/A arm sweep backwards.

In the Red Bull, the rear links are approximately at right angles and so react the cornering forces directly.

Your picture is taken at an oblique angle and so is somewhat distorted. Look at this view from above and you will see the rear legs are about at right angles to the vehicle centerline.

http://www.f1-site.com/wallpap...r-f1-car-2009-10.jpg (http://www.f1-site.com/wallpapers/2009/presentation_redbull/redbull-rb5-wallpaper-f1-car-2009-10.jpg)

Pat

[shark.ashwa]

Originally posted by PatClarke:
Tomgwill98,

We see these swept back wishbones on a few cars each year. The judges call them 'Supersonic A arms'.

I have never seen a car like this that had a satisfactory lack of toe compliance and on track the cars are unstable and very difficult to drive.

That is not to say it cannot be done, but if you do you can expect to sit blindfolded in a dark soundproofed room with the Judges, with a bright light shining in your eyes and a cattle prod in the judges hands! =]

In other words, be prepared for an inquisition!

Try to keep your load paths direct!

Pat
Thank you for the insight Pat. We did have these "Supersonic A-Arms" on our '09 car but I did not get any news from my seniors that they were keenly looked at, but there were other issues then. I agree with your lack of toe compliance opinion and this part of the design should be looked into as well as keeping the load paths direct.
Thanks again Pat!

[buggaero]

Originally posted by PatClarke:
Tomgwill98,

We see these swept back wishbones on a few cars each year. The judges call them 'Supersonic A arms'.

I have never seen a car like this that had a satisfactory lack of toe compliance and on track the cars are unstable and very difficult to drive.

Try to keep your load paths direct!

Pat

http://imageshack.us/photo/my-images/64/topview.jpg/

is that exactly what a supersonic-Aarm is (at rear upper wisbones)

[Tickers]

We used 'supersonic' wishbones for the past few years, and had no problems with rear stability.

In fact Pat, you put your 'seal of approval' on our sticker at FSUK this year. One of the design judges at FSG also mentioned that our suspension system was one of the best present (though having our entire unsprung department missing at FSG didn't help)

[PatClarke]

Buggaero,

I would call those upper rear arms 'High transonic A arms' =] =] =] That is a pretty mild example. We see cars with far more exaggerated sweep back each year and they are usually very compliant when cornering loads are applied.

Tickers,
See above, and when I sign a teams car, that doesn't mean 'This is the best thing I have ever seen in my whole life'! Rather, it means I really appreciate the effort the team have put in and approve of what they have achieved, warts and all!

Dont let this discussion degenerate into an argument about what has been done and what is acceptable. Just remember that all examples of poor load paths, including swept back wishbones, are usually poor design and will be difficult to defend in the Design facet of the competition.

Pat

[Ben Beacock]

Originally posted by buggaero:

http://imageshack.us/photo/my-images/64/topview.jpg/

is that exactly what a supersonic-Aarm is (at rear upper wisbones)

When I saw that layout and figured out what was being discussed, something in my gut tied a knot and my brain instantly buckled the tubes. I don't have any equations to explain this analysis though. Cheers http://fsae.com/groupee_common/emoticons/icon_wink.gif