rear suspension overview

So I have been reading about rear suspension design in a few books.  To begin, the job of any rear suspension is to:
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.

Left:  Roll center below the ground.  Notice body moves about roll center
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!)

cobwebs and crickets

A little over 2 months since the last post.  Embarrassing.  The main reason for this is that we got some attached bicycles to ride around with the kids.  They are too big and heavy to disconnect from our bikes and take to the basement after each use, so guess what: they all now live in the last open space in the garage.  They make it completely impossible to open any drawers on 4 of 6 tool boxes, impossible to use the lathe, press, or workbench, and they use up the last spot of floor space where I could work.  Epic.

As if that wasn't bad enough spacewise, the mustang has finished shedding parts into my garage, and then I got this beauty off CL which is taking up some space I didn't even have:

At this point only the fact that I have a clear goal is separating my garage from that of a hoarder's.

Also school started 2 weeks back and I am teaching a new course in the afternoons.  However, my luck is turning.  The course I am teaching is a subject I know really well so it hasn't destroyed my free time. 

I also finally bought this:

 

I wish I had gotten it much sooner.  I am starting into the rear suspension design which would have been impossible without this book.  I say this already having 4 books on the subject.  One of the suspension chapters begins with an awesome quote:

 

"Figuring the suspension of a car is almost entirely a matter of making useful approximations.  It is not an exact science.  But neither is it a blind application of rule of thumb principles."

 - Maurice Olley 1961

I think we all tend to think that someone who analyzes less than us is a maniac, and someone who analyzes more than we would is too academic for the real world.  It is hard to know how much is the right amount.  I think this quote sort of helps ground that effort.  It makes me think I am doing things about right actually.  Although perhaps no matter what one's level of care were, it may make him think that!

I hope to be posting again soon with some rear supsension discussion!