Quote:
Originally Posted by Shankenstein
New discussion point: Sway bars!
Roll center (according to SAE) - The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll.
Layman's definition - This is a neutral point for your suspension. Applying lateral force at this height will generate no roll (vertical movement at either corner).
Let's consider a 1.0 g turn in a 2645 lbs car. That means ~2645 lbs force will be applied to the center of mass/gravity. Here's an illustration:
4 possible options:
1) Roll center height = center of gravity
There is no roll. If there is sufficient grip in the tires, your car will turn like a go-kart or a door-hinge (flat). This does sent alot of force through the control arms, and the spring/damper are not used at all.
2) 0 < roll center height < center of gravity
There will be a moment (torque) generated, since the lateral force is applied at a different height than the reaction force.
Torque = Force * distance
Roll Torque = 2645 * abs(center of gravity height - roll center height)
Since there is a torque, there will be reaction forces. Typically this duty falls on the springs and sway bars. Most race cars try to keep the roll center height at 15-30% of the center of gravity height.
3) roll center height = ground height
The control arms won't be loaded, and all forces will be sent through the spring/damper. Not horrible, just sub-optimal.
4) roll center height < ground height
The spring/damper will see an amplified force, and can cause the control arms to see the wrong type of force (compression vs tension). This isn't necessarily bad, but I can't recommend ever having an underground roll center, unless you overbuild the spring/damper to compensate for it.
TL;DR - Try to keep the roll center between the center of gravity and the ground. Lower is better, but don't go underground.
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Didn't see this got stickied. I applaud your efforts. I want to develop a full system model, including a driver, using state space equations so you can change the initial conditions easily for simulation purposes. The problem is I hate matrices lol.
Be careful with roll centers. Over the last few years, it has become one of the most misunderstood terms on the internet. You are using the proper, SAE definition, but you are giving 2D, kinematic scenarios of roll center height. What we need to determine are the force application points (FAP) and use the force-based roll center, not the kinematic roll center.
The KRC tells you an arbitrary point in space. It only works in a symmetrical 2D case. As soon as you turn the wheel and the suspension begins to compress/decompress as the car begins to roll, the model is no longer valid. So the KRC only works when the car is static and what good does that do?
Worrying about roll center migration is also a load of crap. If the roll center can migrate up-and-down and side-to-side, what happens when the roll center is outside the wheelbase? All four corners would simultaneously be in tension or compression, a physical impossibility.
Mitchell himself discusses the KRC vs FBRC here:
http://www.neohio-scca.org/comp_clin...namics2007.pdf
There is nothing wrong with having a KRC below ground per se. Sometimes, such as in the case of F1, there's a below ground roll center, but that's because suspension as a whole is compromised around the aero. But a below ground roll center means the suspension has anti-jacking built into it, creating a more stable aero platform in return. If you are going to discuss roll center height, you must include jacking/anti-jacking forces as a result.