Ronin of the Spirit

Because reality is beautiful.

Suspension Primer

So, I don’t write many car posts.  I am a freak about cars, so I know a lot about cars.  I don’t just know about the cars themselves, but the companies, the people that designed them, the engineering, etc.  I take cars very seriously, and having the position of knowledge that I’ve worked for over the years, I say things that people without my background don’t understand, and they think that I and not they, am the moron.   (There’s two approaches to this. One, I can explain everything from start to finish.  By the time I’ve given them the background to actually know what I am talking about, my point is lost.  Alternately, I can make my point, have them tell me how stupid I am, and then spend an extra hour on the lecture defending every single point, since they already know the final point and don’t want to agree with it.)  I am going to spin this into a critical thinking post in the next installment, so I am doing it anyway.

OK, some basic physics.  Cars are heavy.  In physics terms, this means cars have a large mass.  Mass resists being moved, and once moved, resists being stopped.  Roads are not flat.  They go up and down, so anything going accross them goes up and down.   When a car goes up, the wheels aren’t sticking to the road.  When it goes down it the wheels are diving into the road really hard.  Also this isn’t good for the car.  A car is made of many pieces, if they are all made flexible, they bend and rub each other till they break.  Make the car rigid, and the constant force without flex to absorb it will also break it.  So we we make suspension.

Springs in the form of wood have been used on chariots since Egypt.  By the 19th century, they were steel.  Cars inherited these steel springs.  The problem was that cars went faster than horses, so something had to link the axle to the car besides the springs, to keep the springs from just bending out of the way.  This is called linkage.  As cars got faster still, on new problem was found.  The car would bounce on the springs so fast that it would vibrate the car to pieces, so the shock absorber was invented.  The shock absorber lets the spring bounce but slows it down, like the difference between swinging your arm in water instead of air.

So, their are 4 parts to suspension.  Axle (what the wheel spins with or around), springs (which connect the axle to the car), linkages (which keep the spring from flexing right out of from between the car and axle) and shocks (which keep the spring from bouncing excessively.) Whether we are talking about a Model T,  or a Formula 1 racer, that’s it: 4 basic parts consisting of axle, springs, linkages, and shocks.

The truly astute will notice that we have not solved the first problem, just make it smaller.  Remember that moving the car horizontally causes vertical motion.  Mass resists moving, and once moving resists stopping.  Now, it is the much lower mass of the suspension doing the vertical movement while the car pretty much floats over.   This car is mounted on springs, thus is “sprung mass”.  The wheel and axle are not, thus “unsprung mass”  (Some parts are both, the part of the spring which is fixed against the car is sprung and the part attached to the bouncing axle is unsprung.  So we figure 1/2 the mass of the spring is unsprung mass.)

But cars don’t just go straight.  They turn.  Remember that an object in motion wants to stay in motion and that the car is heavier than the suspension?  When you make a hard right, the lighter suspension pretty happily changes direction.  The rest of the car wants to follow the old path of motion, now to the left.  It tries do slide to the left, and pushes against the suspension, so it “rolls” left, squishing the left springs and stretching out right springs.  If the body rolls enough it will pick up the right-side wheels off the road.  Sometimes this is no big deal.  Sometimes you die in horrible agony.  Depends on the road, and the car.  Anyway…

So, to recap so far: The suspension has one job, to keep the tires on the road.  It must keep the tires on the road when the road tells the wheel to pull away, or when the cars body roll pulls the wheel away. It does this with 4 parts: axles, springs, linkages, and shocks.

Totally unrelated to the problem of keeping the rubber in the road is the task of passenger comfort.   And totally unrelated to that is the production engineering.  Bearing that in mind, here is the normal time line of front suspension development. (For reasons I’m not going to explain, new technology goes into the front end first, then is translated into the rear suspension.)

Ok, so first is the beam axle.  Its a big pole with wheel on each side.  Then comes the swing axle which is the same thing with a pivot in the middle.  Then comes the McPherson Strut, then the equal length wish bone, then unequal length wishbone.

The problem is?  That’s crap.  Despite the fact that absolutely everyone says that the time line, it’s not.  And it doesn’t go from bad handling to good handling in good order, which is it’s usually presented: a timeline with improving ability with each development.

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January 1, 2009 - Posted by | Engines, Microcar, skepticism, Small Car, Transportation, Uncategorized | , , , , ,

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