The crossmember is thick tube, to prevent deflection. The upper bars will pull on it, while the lower bars are pushing, but either way there is stress on the crossmember. There is more force in the lower bars however. In a car with higher horsepower I support the crossmember to another point on the frame to stiffen it up. I didn't feel it necessary on a 45 HP vehicle.
I posted a picture below of a truck pushing more horsepower, with more triangulation and support for the upper bars.
Thats a good looking setup. Most of the time you see the upper bars opposite yours. Thanks for the reply.
Yes, generally you do. "Technically" (and I use the term loosely) that is not the correct way to set up a 4-link. In theory the bars should be wide on the axle, for more support, and to eliminate or reduce torque steer. Especially on higher horsepower cars. They also are supposed to be wide on the axle, and narrow on the frame so that the roll center of the vehicle is located on the frame. When it is located on the frame, the roll center stays in the same place in relation to the vehicle. So the vehicle will perform the same in every corner, at every speed. With the bars narrow on the axle, the roll center is located at the axle, and it moves with the suspension. This means it will perform differently depending on how the car is loaded, how the body is rolling, how the suspension is compressing, etc. Its not a big concern to some people, but I really like to drive my cars hard. With the bars wider on the axle, it also produces better roll steer characteristics, which also keeps the suspension very predictable in turns.
You can also triangulate the lower bars, which is known as a Satchell Link. This creates a much lower roll center. I like this setup, because most vehicles have a low roll center in the front, and lowering it in the back makes the car feel more balanced. Here is an example.
I plan on using this setup on my 30 Chevy. If you have a transverse front spring, your front roll center is very low, sometimes even at the ground. I want my car to feel as balanced as possible, and handle as well as ti can on straight axles.
Another thing you need to consider, is your instant center. This is the point where you link bars would intersect if they where extended out into space past their pivots. The "technical" way once again, is to find the vehicles center of gravity and put the IC there. This is kind of aggressive for a street driven car, and provides a lot of anti-squat, to the point that the rear of the vehicle will lift up (pushing tires down for more traction). I set my roll center on most vehicles around the bumper or grill area if its under 120" wheelbase, because it provides a pretty neutral easy to drive setup that is daily driver friendly. Here is a view from the side.
You don't want your IC behind the axle. Your car will squat hard under acceleration. I like a vehicle that launches hard without lifting or lowering the rear end much at all.
Hope that helps a little bit. I'm glad to answer any other questions, and take criticism as well. I try to spread the word about 4-link geometry, because it seems to be something often overlooked.
-Edit- Pay no attention to the fruity colors. Not my pick, lol.
