The Mark Ortiz Automotive

CHASSIS NEWSLETTER

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August 2008

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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.

 

 

DIRTY TRICKS

 

I have a request...we are trying to get an asphalt Late Model working with some ideas normally reserved for dirt suspension.  I see some of these techniques used more and more.  I am thinking about:

         LR spring in front of rear axle

         Double springs within single coilover

         Double link on top link for rearend.

We play with these but don't necessarily understand all the concepts.

 

All of the features mentioned are used on dirt cars for one purpose: to add wedge or diagonal percentage (total of left rear and right front tire loads as a percentage of the total for all four wheels) when power is applied.  Dual springs can be used for other purposes as well.

 

The reason we want to add diagonal percentage under power is to tighten the car (add understeer), so that the rear tires' grip is not entirely used up for cornering, and we have more available for propulsion.  That lets us apply more power and get a faster exit.  We want this understeering influence to come in as we add power, so the car won't understeer excessively the rest of the time.

 

We can have too much of this effect.  More isn't necessarily better.  If we overdo it, the car can have a power push.  The idea is to tighten the car enough for good exit, but not tighten it excessively.

 

Having the left spring ahead of the axle and/or the right spring behind adds wedge under power if and only if the springs mount to the axle, not the birdcages, and if the torque reacting mechanism (pull bar/top link or lift bar/torque arm) is compliant and lets the axle rotate backward under power.  The axle tries to do this because the pinion gear wants to climb the ring gear and lift the nose of the third member.  Or to state it another way, in a live axle suspension, drive torque reacts through the suspension linkage.

 

 

 

With the left rear spring ahead of the axle and the right rear spring behind, the left rear corner of the car is lifted by axle rotation and the right rear corner is let down.  That rolls the car to the right and adds diagonal percentage.  Again, this only happens if the linkage controlling the axle lets it rotate.

 

Indeed, all strategies for adding wedge under power that involve the rear suspension work by rolling the car to the right.  All strategies for adding wedge under power that involve the front suspension work by resisting roll to the right, or trying to roll the car to the left.

 

In a rear-drive car, we don't have any torque or any thrust at the front wheels under power, so we have less to work with.  However, we do have the tendency of the front end to lift as load transfers to the rear.  If the left front suspension resists lifting more strongly, or the right front resists lifting less strongly, that adds wedge under power.  If the right front spring is softer, or the left front is stiffer, that encourages the front end to rise more on the right side under power than on the left side, and adds wedge under power.  If that sounds backwards, remember that spring rate is the rate at which force changes with displacement, not the magnitude of the force.  A stiff spring is more reluctant to extend, or creates more load change per inch of extension, than a soft one.

 

Dual springs are used on the left front of dirt Late Models to make the springing on that corner of the car get stiffer when the front end rises.  Contrary to what you might suppose, the dual springs are used to give a falling rate rather than a rising rate.  The rate increases when the suspension extends to a certain point, rather than increasing when the suspension compresses to a certain point.

 

This is done by stacking two springs on top of each other on the coilover, with a slider between them that can slide up and down on the coilover.  The slider can slide upward as far as it wants, but there is an adjustable collar that stops it at some point when it slides down.  When the slider can slide freely, the springs are both working, in series.  Once the slider tops out against the collar, only the lower spring can extend further.

 

Two springs acting in series are softer than either of them acting alone.  More precisely, the rate of the combination is equal to the product of the individual rates divided by their sum.  For example, a 100 lb/in spring stacked on top of a 200 lb/in spring gives a combination rate of 20,000/300 = 67 lb/in.  If either of the two springs is immobilized, we are left with the rate of the remaining one.

 

Applied to the left front suspension, the idea is to have the upper spring working until the driver gets on the power, and then top out.

 

The driver may or may not be able to actually feel the top-out point.  If the driver isn't sure where the spring is topping out, or even whether it's topping out, there are ways you can still find out.

 

Using electronic data acquisition, you simply find the suspension displacement where the upper spring tops out, and then look at where the car is on the track when the suspension reaches that point, and what the driver is doing with the controls and what the driver says about the car's behavior.

 

 

If you are testing without data acquisition, you can cut a hole in the top of the fender and attach a rod or dowel to the spindle or some other part of the suspension, extending upward through the fender, as a travel indicator.  Mark the travel indicator with bright-colored tape or paint below the point

where the upper spring tops out.  When the color disappears, the upper spring is topped out.  The driver can watch this indicator, and so can trackside observers.  You can also use video to record what it does.

 

The usual way of using compliant upper links on the rear axle is to have both of them near the middle of the car, and have one act in compression, for braking, and the other in tension, for power.  If you use two rigid links, or even one rigid link and one compliant one, they fight each other and you have a bind.

 

The compliance should be damped somehow.  One common strategy is to use a shock absorber (damper), with a bump rubber on its shaft, as the compression link.  The damper acts as a compression link when it bottoms out on the rubber, and it damps axle rotation in both directions.  On dirt cars, the shock is normally angled up at the front, and this creates some pro-lift when the shock sees compression.  The pro-lift helps prevent wheel hop, which can otherwise be a problem as dirt cars often use a lot of rear brake.

 

Engine braking torque always acts through the shock.  Torque from the brakes themselves may or may not.  If the calipers are mounted to the axle tubes, brake torque acts through the shock.  If the calipers are mounted to the birdcages, the torque from the brakes acts through the birdcage linkages instead.

 

You may not want pro-lift on a pavement car.  It depends partly on how much rear brake your driver likes.

 

I like mounting at least the tension link offset to the left, behind the driver.  Rules permitting, I also like the idea of using a two tension links, one to the left of the other, and choosing spring rates and preloads so that light drive torque acts mainly through the left link, and the right or center link takes up and absorbs more of the load as torque increases and the axle rotates more.  This allows the rear suspension to add wedge most when the car needs it most on a slick surface while not adding excessive amounts when grip is good.

 

Pavement Late Model rules in the US are a real hodge-podge.  NASCAR has three different Late Model classes, the differences being mainly in the engine rules.  They prohibit most of the suspension tricks found in dirt cars.  There are several regional sanctioning bodies, and I think all of them are more permissive than NASCAR.  Then there are tracks that have their own unique rules.  Consequently, the legality of the ideas described above will vary drastically depending on where you're running, and what class.

 

 

 

 

PORSCHE 993 REAR SUSPENSION

 

In your article on the Corvair rear suspension, you stated you have a V8 Crown Corvair.  I'm building a V8 Corvair using a Porsche 915 5 speed and a complete suspension from a C2 993 911.  They're complete sub-assemblies that unbolt from the body like the Corvair front end.  I'm  wondering what your opinion is of the Porsche 993 suspension and what sort of things I should be on the lookout for when I get to installing these sub-assemblies.

 

For an illustration of this suspension, see http://www.autozine.org/technical_school/suspension/tech_pic_sus_dw.jpg.  This is a rear three-quarter view of a right rear assembly, removed from the car.  Also see http://www.autometricsmotorsports.com/images/cars/rsr/933_strut_rear.jpg.  This is a rear three-quarter view of a left rear assembly, in the car.  The September 2008 Road & Track has a picture on p.77.

 

Another correspondent suggested to me that maybe Porsche vindicated GM by imitating its Corvair design for the 993.  However, it will be seen from the pictures that the Porsche design bears more resemblance to current Corvette rear suspension.  It is a true 5-link system, with the driveshaft not serving as a suspension link.  It does have a bit in common with the Corvair and C2/C3 suspension, in that all of these are short-and-long-arm suspensions, and it does resemble the Corvair layout more than the 911 semi-trailing arm system did.

 

But if the 993 is like another car, that would be the front-engined Porsche 928.  That was the first Porsche to have 5-link rear suspension.  At the time, Porsche called this design the Weissach Axle.  As used in the 928, it had compliance characteristics intended to toe the rear wheels in on trailing throttle, minimizing drop-throttle oversteer.

 

I have not worked with 993's, and I can't tell what the system's side-view geometry is like.  It looks like Porsche's thinking on front-view geometry pretty much agrees with mine: camber recovery around 50%, roll center height somewhere in the 3 to 4 inch range.  Road tests I've read don't contain comments that would point to any severe geometry problems.

 

So, absent better information, I would advise sticking close to the geometry Porsche uses, and mounting the structure to try to preserve that, while also trying to maintain solid structure and decent load paths.