The Mark Ortiz Automotive

CHASSIS NEWSLETTER

Presented free of charge as a service

to the Motorsports Community

April 2009

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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.

 

 

"MINDING YOUR ANTI" VIDEO NOW ON DVD

 

"Minding Your Anti: Understanding Factors in Load Transfer (Weight Transfer)" is a video of a one-hour lecture I delivered at UNC Charlotte in 2003.  Until now, it has only been available on US-standard VHS video cassettes.  I now have it on DVD.  I also have a few VHS versions left in stock.  Price for either format is US$50.00, which includes shipping and handling worldwide.  To order, send check or money order to me at 155 Wankel Dr., Kannapolis, NC 28083-8200, USA.

 

 

RAISED LOWER CONTROL ARMS ON 2009 F1 CARS

 

I'm sure you've noted that recently there have been radical changes in the suspension geometry at the front of F-1 cars.  The changes  permit the front of the cars to be raised in order to increase the volume of air supplied to the aerodynamic features under the cars which produce down force. 

 

Could you comment on the new geometry and how it affects the camber change curve, virtual swing arm length, instant center location, roll center height and any other significant ramifications.

 

First, a bit of explanation for readers who haven't been following Formula 1 lately: this year's cars have the lower control arms dramatically higher than previous cars.  They are higher at both the inboard and outboard ends.  Purpose of this is to get the arms up out of the airstream.  This allows aerodynamic elements under the driver's legs to be more effective.

 

It's all related to efforts to keep the cars from losing so much front downforce when running in another car's wake, and thereby enable more passing.

 

The noses aren't really higher than before.  Drivers' feet were already up to where they could barely see over their toes.  What's different now is that the region below the footwell has been aerodynamically cleaned up.

 

 

Previously, there used to be either a single projecting pylon under the nose, or two of them, to pick up the inboard ends of the lower control arms.

 

If designers were just raising the inboard pivots of the lower arm, that would increase camber recovery in roll, increase camber change in ride, and raise the roll center.  But that's not what they're doing.  They're raising the outboard end along with the inboard points.  This keeps the instant center position about the same as before, so the camber properties and geometric anti-roll are similar also.  Looking at pictures of the cars, they all seem to have very little camber recovery, and roll center heights from 1 to 3 inches, which is about what they had in previous years.

 

In theory, it is possible to have any instant center with any control arm heights, packaging constraints permitting.  The inescapable penalty if we raise the lower arms, or lower the upper ones, is that the loads on the arms in cornering and braking increase (assuming outboard brakes).  So do the ball joint loads.  That's why designers didn't raise the lower arms years ago.  Now most of them have decided that aero considerations trump structural ones, under present rules.

 

Under present rules or previous ones, it is aerodynamically desirable to get the nose as high and as slender as possible.  That means the master cylinders have to go in front of the pedals, rather than above or below, and the pedals have to pivot at the bottom.  All of that means the steering rack has to lie above the master cylinders.  Finally, for reasons of steering geometry and aerodynamics, the upper ball joints need to lie inside the wheel rim, whose diameter is dictated by the rules.  These constraints require the tie rod and upper control arm to angle downward somewhat from the tub to the wheel.

 

With the upper arm angle constrained that way, the only way to get any serious camber recovery in roll would be to have a high roll center, which is undesirable because it causes jacking.  So the designers accept having poor camber recovery in roll, and deal with this by just making the suspension so stiff that very little roll occurs, and running static negative camber.

 

 

PULLROD SUSPENSIONS IN F1

 

I am hearing all sorts of outrageous claims about the pullrod suspension used on the rear of the highly successful Red Bull F1 car.  Does pullrod suspension actually offer any clear advantage over pushrod?

 

In terms of suspension dynamics, no.  Both layouts can be made to have the same wheel rate, the same motion ratio/displacement curve, and so on.  Both affect dynamic load transfer in the same manner.

 

The differences come down to packaging, with small effects on overall c.g. height, aerodynamics, and component accessibility.

 

 

Pullrod suspension generally places the shocks, springs, and rockers low in the car.  Depending on whether something else has to be moved to make room, this generally lowers the overall c.g. of the car, which is good.  Since these components are a fairly small part of the total mass, the benefit is correspondingly small, but it is real.

 

Accessibility of the shocks, springs, and rockers generally suffers.  It is easier to get at these parts when they are on top of the transaxle or footbox than when they are down under other components, a few inches off the ground.  However, when millions of dollars are on the line, it can make sense to put up with some extra hassle to gain a small performance advantage.

 

The shocks, springs, and rockers take up space, and create a wider package near the ground.  If they get in the way of under-car airflow, it is often a better choice to use pushrods.

 

This last consideration was crucial in making pullrods a rational choice for the rear of the car under th new 2009 rules.  As part of the effort to reduce and narrow the upwash behind the car, diffusers were required to start further back than before, and the flat portion of the underside had to extend further aft.  That creates the opportunity to put components low down alongside the transaxle, without aerodynamic penalty.  The Red Bull team simply saw this opportunity, and used it.

 

Some have suggested that there is a reduction of overall weight with pullrods.  I am skeptical.  It is true that a pullrod can be made more slender than a pushrod, and perhaps lighter, because it does not have to withstand large compression loads, and therefore does not need as much buckling resistance.  However, compressive loadings on the upper control arms increase, and the weight can easily come back there.  Any net weight reduction probably depends on the particulars of the individual design.

 

The Red Bull car's overall success should not be attributed to this one feature.  Rather, it stems from a larger willingness to consider new possibilities, combined with the engineering understanding to properly evaluate, select, and apply such possibilities, and to integrate and optimize the total package.

 

 

RATE AND LENGTH NUMBERS ON TORSION BARS

 

What do the numbers stamped on the end of torsion bars mean (race car ones, like for sprint cars and midgets)?  I understand one is the length and the other is the rate.  The one for the length makes some sense: it appears to just be the overall length in inches.  But I can't see what the one for rate relates to.  Can you explain?

 

The number people sometimes call "rate" is actually the effective diameter for the active part of the bar, in thousandths of an inch.  The active portion is the turned-down portion in the middle, where most of the twisting occurs.

 

 

 

The effective diameter is the actual diameter, if the bar is solid.  If it's hollow, it's the diameter of an equivalent-rate solid bar.

 

From these numbers, you can calculate a rate at the end of a known-length lever arm that equates to the rate of a coil spring acting at a similar point.  In most cases, you don't need to do that, because torsion bar manufacturers offer charts in their catalogs and on their websites.  But here's the equation:

 

                                       S = (πGd4) / (32R2L) = (.098Gd4) / (R2L)                                 (1)

        

where:

                  S = rate at lever arm end, lb/in

                  G = shear modulus modulus of material

11,500,000 lb/in2 for most steels, 11,800,000 lb/in2 for spring steel

                  d = diameter of the active portion of the bar, inches

                  R = effective length (moment arm length) of lever arm, inches

                  L = length of active portion of the bar, inches

 

For d, you use the "rate" number stamped on the bar, with a decimal point three places from the right.  For L, you use either your own measured length for the active portion, or, as a rule of thumb, the stamped or cataloged length minus four inches.  R is the distance from bar axis to lever arm end, not along the arm if it's curved or angled, but perpendicular to the bar axis.

 

If you have a bar with no stamped numbers, or with the numbers worn off, and it's hollow, you can calculate the rate using measured inside and outside diameters of the bar, with the following formula:

 

                      S = (πG(do4 di4)) / (32R2L) = (.098G(do4 di4)) / (R2L)                         (2)

 

      where:

                  do = outside diameter, inches

                  di = inside diameter, inches

                  All other variables are as in Equation (1).