Double Wishbone Suspension

[Raghav_Jagan]

Hey, guys. I'm new to this forum.. and i don't know a better place to put forth my doubts.
"m currently doing a suspension design for my vehicle (Double wishbone on both front and rear) .The total weight of my vehicle is around 300Kg, with a 60-40 split( F-R).. However, i am stuck at one point.. In the front, IT is understandable that the upper wishbone has to be inclined upwards, so as to get a lower rool center.. would there be any significance, if i also keep the lower wishbone inclined ( i.e. parallel to the upper wishbone)??

[Black Lotus]

The instant center would move to infinity. Then you would would have to determine your roll center by simply offsetting your sloped line from the lower control arm to the center line of the tire. Same as you would with a beam axle. Of course, this would only apply to that particular ride height where the arms are parallel. If you are silly enough to make your upper and lower arms the same length, well, have fun.
Anyway,the roll center would probably move lower.
If the slope of the control arms is steep enough, the RC could go underground.
Which is neither here nor there, except that you might have to contend with the car down-jacking in the corners.
If your car has the engine in the front, you might want a higher roll center to account for the usually local higher CG associated with lumps of aluminum and iron at that end of the car. (and keep in mind the CG height of the car along its length vs the roll axis of the front and rear suspension)
In the pic, I had to roll the car 2* to get the extension lines to show. Notice the roll center only moved sideways a half inch as well. At least that is good.

[Raghav_Jagan]

Thanks. I understand the concept of Instant center now..
Anyway, i have another doubt..
Lets say , i have a positive camber on the Front. This would mean, that the Upper wishbone would be inclined downwards. In that case(Non-parallel wishbone), the Instant center would be outside the vehicle. I don't understand,why they don't prefer instant center being outside the vehicle, even though, they prefer a positive camber..

[Black Lotus]

Had to break out the books and look it up, because this setup isn't given much coverage, but it seems to be used a lot in older cars, trucks and RV toys.
Its called Negative Swing Arm Geometry, or Reversed Swing Arm.
"Used as a solution to the wheelfight problem"
Wheelfight being similar to "shimmy". The steering wheel being the "wheel".
The tire and wheel assembly gets into a mode where it wobbles back and forth after being deflected. (thinking of an off-road type vehicle here)
Apparently one of the ways to damp these oscillations out is to add some positive camber to the tire and wheels assembly as it rises. If some toe out gets added, that'll help damp it out as well.
Generally frowned upon for high performance cars as it doesn't add any negative camber as the suspension deflects upward.

[Raghav_Jagan]

Actually, I'm doing for a F1 car. I understand, the bump would be more in case you mentioned.But, what i learned through my source is, The F1 cars, usually employ multiple lniks in the wishbone. ALthough, i don't understand how it gives a negative camber gain.
But, we can't really afford to use a 5 link system. We planned out a basic Double wishbone suspension.. and i am stuck at this point. Is it possible to have the upper arm inclined upwards, with a positive camber in the front?

[Black Lotus]

I don't know what's going on with F1. But they run negative camber all around.
Since they are simply a design solution to run on very smooth purpose built courses, their design doesn't seem to be relevant to street driven cars, so I don't pay any attention to them.
My GUESS is that they run a high roll center in the front, which means that the front would take much of the lateral load transfer. This would mean that they would tend to understeer, but they have bags of front negative camber and fat front tires, compared to the rear. The raised a-arms also match up well to the raised forward end of the body shell, which allows for smooth airflow about the front wing. And the raised nose is a convenient place to put the raised up legs of the driver, to keep him stuck in the seat under braking.
The cars are basically go-carts with very little suspension movement.
Since the front end doesn't move much, it doesn't really matter what the camber curve is. It matters more to keep the body level and to keep the wings at the design angle of attack.
I really don't know what the design philosophies are in F1. They are just too weird a car for me, and like I said, nothing about them is relevant to road cars.
Maybe somebody else can chip in and comment.