The Mark Ortiz Automotive

CHASSIS NEWSLETTER

Presented free of charge as a service

to the Motorsports Community

December 2006

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WELCOME

 

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions.  Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by    e-mail to: mortiz49@earthlink.net.  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.

 

 

COIL-BINDING SETUPS IN STOCK CARS FRIEND OR FOE?

 

In reading the November newsletter, a couple of thoughts came to mind.  My application is pavement late model racing (ASA, CRA, etc.).

1) To remedy coil binding, what about using torsion bars instead of coil springs?  What I am thinking of is mounting the bars parallel to the front-to-rear vehicle centerline, connecting to the lower control arms (Chrysler-style).  This would get some weight lower in the vehicle as well.

 

Rules permitting, certainly you can use longitudinal torsion bars.  However, the coil binding currently found in upper-division NASCAR racing is not an unintended problem; it's a deliberate strategy, aimed at working the ride height and spring rules, and better controlling the car's aerodynamics.  The idea is to have the car pass ride height inspection, yet come down as readily as possible to a ride height where the valance just clears the track surface, and then go solid, so the valance stays at that height and doesn't rub or lift.  This is awful for riding bumps, but if the track is smooth you can accept that in return for good control of the aerodynamics.

 

Perhaps a better compromise is to have the suspension go very stiff, rather than solid.  This used to be achieved by using bump rubbers instead of having the springs hit coil bind.  But NASCAR outlawed the bump rubbers.  At about the same time, they instituted a minimum spring rate of 400 lb/in.

 

To make the spring coil bind, you either shorten its length or increase the wire diameter and number of coils, leaving the length the same.  To make the spring operate in coil bind as much of the time as possible, you minimize the spring-to-wheel motion ratio, and create as much downward jacking with the suspension geometry as you can.

 

One insider I talked to recently tells me that crew chiefs are now using so much pro-roll on the right front that the cars are now sitting considerably higher at static than necessary to pass ride height inspection.  Apparently, some people believe that if you have more "drop" to coil bind, that somehow adds load to the tires.

 

 

It doesn't.

 

It occurred to me that maybe having the nose of the car ride higher down the straights might reduce drag, perhaps due to the roof masking the rear spoiler more.  I asked Gary Eaker at AeroDyn about that possibility.  He says no, having the nose higher is bad in terms of both drag and lift.  The increased air under the car adds more drag than you save by masking the rear spoiler.

 

 

FALLING-FORCE SHOCKS

 

[Continued from previous question]

2) For shock damping on bumpy tracks, it seems that if damping forces at high shaft speeds could be significantly reduced, grip could be enhanced.  I don't mean simply digressive valving, but significantly dropping damping force, say above shaft speeds of maybe 5in/sec.  I don't know if this is even possible?

 

What you are describing is a shock with a negative damping coefficient over part of the velocity range.

 

I'm not sure it's impossible, but it's tricky to create such an effect in a package that would look like the shocks we use today.  It is possible to create a falling-rate leaf spring or Belleville washer (and a shim in a damper is essentially a round leaf spring, or diaphragm spring), but usually we can't actually obtain a negative rate, or a force that decreases with increasing displacement.  There is one exception to this, which I will discuss below.

 

Although it is normally not possible to create a negative-rate or falling-force spring, it is possible to create a springing device where the force diminishes or even reverses as displacement increases, by combining a spring with other mechanical elements.  One example would be the over-center spring found in some clutch linkages.  These are coil springs arranged so that after about half pedal travel, the spring tries to pull the pedal to the floor, rather than pull it up.  These forces act in parallel with the springs in the clutch, which resist pedal force at a steadily increasing rate with respect to pedal displacement.  The net result is a falling-rate resistance at the pedal, but not necessarily a falling-force resistance.  However, if the over-center spring is strong enough, it is possible to create a falling-force characteristic.  On some cars, the over-center spring tension can be adjusted to create the desired pedal feel.

 

To produce the over-center action, the coil spring needs to have a lever or rocker to act on.  I can't think of a way to incorporate that into a shock.

 

Ordinarily, the over-center spring is used to make a Borg & Beck clutch feel similar to a diaphragm clutch.  The spring in a diaphragm clutch is an interesting case.  It can actually have a falling-force characteristic not over its entire deflection range, but over a portion of it.  I have not actually tested

 

clutch diaphragms, but I have an old textbook that shows a force-vs.-displacement curve for a typical clutch diaphragm (Herbert Ellinger, Automechanics, Prentice-Hall, 1972, p. 329).  The curve is S-shaped.  It starts out fairly linear, then becomes concave-down, and continues to bend downward until the slope becomes negative.  Then it becomes concave-up, and continues to bend upward so that the slope becomes positive again.  The force never becomes negative, but the rate does, over an interval.

 

A clutch diaphragm is somewhat like an ordinary Belleville washer, but with some differences.  It has substantially more dish, and it has radial slits running from its inner diameter out more than half way to its rim.  In other words, rather than being a dished continuous disc, it is a ring with fingers extending inward.  Additionally, a clutch diaphragm has a perimeter outboard of the ring it bears against, that moves oppositely to the motion at the inner ends of the fingers.  This feature takes the place of the release levers in other clutch designs, and retracts the pressure plate.

 

It would be possible to make a diaphragm spring like that, minus the reverse-motion portion of the rim, and use it as part of the stack in a deflective-disc shock.  It would then be possible to have at least a portion of the valve stack have a falling-force characteristic.  That would still not necessarily create a damping force that falls as absolute velocity rises.  The stack force does not directly determine the piston force.  Rather, it determines the flow characteristics of the orifices in the piston which the shims mask.  The piston force then depends on the resistance to oil flow through these partially masked orifices.

 

A falling-force stack could, however, produce a more highly digressive shock than a stack with a modest rate and high preload, which is the usual approach.  It might even be possible to have a true falling-force or negative-damping-coefficient shock, but a falling-force stack would not necessarily produce this.

 

It would be possible to mount the shock so that its force would diminish with increasing displacement, but that is not the same thing as having the force diminish as velocity increases.

 

It would be possible to deliberately make the shock acceleration-sensitive.  (Velocity is the rate and direction of displacement change; acceleration is the rate and direction of velocity change.)  Edelbrock has been selling acceleration-sensitive shocks for some years now.  Acceleration-sensitive shocks generally use a weight of some kind, working against a spring.  The weight can either open an orifice or move a needle to change the area of an orifice, or it can vary the effective preload on a stack.

 

Finally, there is one type of shock that produces a negative damping coefficient over a small portion of its velocity range, and a damping coefficient of zero over most of its velocity range.  Furthermore, it is easy to make the damping force adjustable, while retaining the zero coefficient for all adjustment settings.  What kind of advanced design might that be?  Surprise: a friction shock!  A friction shock makes the same force regardless of velocity, except when it's just starting to move.  Then it has a "stiction" zone, where the friction force is greater than the force we get once it's

 

moving.  As the stiction goes away, the velocity is increasing; the damping force is decreasing; the damping coefficient is negative.

 

Nobody uses friction shocks anymore, but all suspensions have some sliding contacts in them.  These generate what we call Coulomb damping: friction that is independent of velocity, except for the stiction effect.

 

Some cars still use leaf springs.  Leaf springs have a lot more Coulomb friction than coils do, because of inter-leaf friction.  In the days when hydraulic shocks were primitive, this was actually an advantage, and potentially it still could be today, on dirt.  Much of this depends on rules, but in some dirt classes, leaf springs are actually faster than coils.  Unfortunately, they are also a pain to run, because to get the soft rates needed for dirt, the leaf springs have relatively few leaves, and stresses are high.  Consequently, the springs fatigue and lose ride height rapidly, and you have to replace them all the time.

 

The normal assumption is that friction is bad, so it's smart to minimize the number of leaves.  But if Coulomb friction is actually okay, because it gives us damping that's stiff at low velocities yet no stiffer at high velocities, maybe there's a future in leaf springs for dirt that use a larger number of thinner leaves than those we see today.  That would increase inter-leaf friction, and also increase weight, but result in lower stresses, longer life, and better ride height retention.