1) Prevent the wheels from moving left/right or forward/backward relative to the body.
2) Allow the wheels to move up and down together (bump)
3) Allow the wheels to move up down independently of each other (roll)
4) Minimize weight
Since my plan was to build this car from the mustang donor, I committed to a solid axle rear end as opposed to independent rear suspension. I think many people poopoo solid rear suspensions. It isn't really that bad:
1) If the road is smooth, they don't really work worse than IRS.
2) They are cheap.
3) You don't ahve to worry about camber changing, toe changes, and scrub.
As I alluded to previously there is a balance to be had between slapping something together, and making something that works well. Here are a few ways that attention to the design can help:
Roll steer
When the car body rolls in a turn one wheel may move back more than the other or one moves back and one moves forward. This changes the direction that the rear of the car is going.
Top: No roll steer. Axle pointed in same direction as the body. The line from each wheel intersects the line from the axle indication instant center. This is the point that all four wheels are turning about.
Middle: Roll understeer. The outside wheel moves further forward than the inside wheel when the car body rolls. Although this appears to move the rear of the car into a tighter turn, it actuall moves the instant center out, and makes the car turn on a larger circle. This is stable. Also, note how this took our perfect steering ackerman and turned it into pro ackerman.
Bottom: Roll oversteer. The inside wheel is moves further forward than the outside wheel. This tightened up out turning radius. This is unstable. The car roll made the car turned tighter, which will make the car body roll more, which will make the car turn tighter still, which mill make the body roll more... until you spin. Note that this also creates anti ackerman.
It is not the goal to make a design that has no roll steer for the entire range of suspension travel and roll. I don't even know if it is possible, but if you could you would almost certainly have terrible handling as you abandoned all other design goals. The goal should be to keep roll steer to a minimum, and to sacrifice whatever is needed to be sure we have roll understeer.
Antidive and antilift
If side view instant center for the rear suspension is in line center of gravity then the thrusting force from the wheels will be applied directly to the CG and the car will not squat when accelerating. The flatter the swing arm angle relative to the ground, the larger the moment created by the thrusting wheel force compared to the CG, and the harder the car will squat. Fine for a drag race. You generally want a little squat, and a little dive on braking so that the driver can feel what is going on.
Top: 100% anti squat
Bottom: 0% anti squat. Amount of squat depends on spring rate.
Roll center
If we imagine being aft looking forward. We can imagine that there is a point that the car will roll about.
Center: Roll center at axle height
Right. Roll center near CG.
It should be noted that the body will not roll on the paths shown. As soon as the body rolls a small increment, the suspension geometry changes and the location of the roll center changes as well.
The centrifugal force is acting at the car's CG. So the lower the roll center is greater the body roll will be in a turn . Cars counteract the body roll with sway bars, which essentially increase the roll stiffness without raising the bump stiffness.
A higher roll center will cause an increased jacking affect. In other words, the more the body leans, the higher it will get. High roll centers also cause camber to change significantly in roll (although we don't care about this with a solid rear axle!)
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