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March 2007

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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:  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.





Attached are illustrations of a proposed car for F-1 racing, suggested during the early 1990s.  [For newsletter recipients, one of these illustrations is included as a separate attachment.] The car has a single steered but unpowered front wheel.  A pair of wheels is located further aft located on an axle passing close to the COG or possibly ahead of it.  These wheels are non-steered but are powered.  Finally there is a powered and steered single wheel right aft.  The car is a diamond in plan view.  I understand the idea was to provide a chassis with better aerodynamic downforce and less drag than a conventional layout.  In addition the car featured better traction by being able to drive three wheel drive (the rear three).  Only the front wheel was not powered.  This nicely got around the rules banning four-wheel drive in F-1!  


How would such a car handle?  Assuming the front wheel and the rear wheel accomplish the steering task (that is the non-paired wheels steer whereas the “axle” pair remain fixed with respect to steering) what would happen as the car reached maximum slip angle and the driver decided to increase the radius of cornering path (that is, reduce lateral acceleration)?  In this case he’d be adding to the rear wheel slip angle while simultaneously reducing the front wheel slip angle.  What would occur if the rear wheel lost static traction because of this?  Would the effect of the single rear wheel losing grip dominate?  Would the increase in slip angle at the rear tend to over steer the car putting it into a slide or a spin?  Or would the combination of the three other wheels prevent oversteer and slides?  


An alternative arrangement would be to keep the single rear wheel fixed while steering the other three.  There would need to be some sort of linkage to do this- a sort of modified Ackerman geometry needing to be implemented.  Would this be a better arrangement?  The designer must have thought about this possibility and thought not.  It would be interesting to understand why.


An interesting feature is that it would seem that only a single differential unit would need to be used since the distance of the path traversed by the rear wheel is exactly equal to the mean of the wheel



paths of the middle wheels.  The rear wheel could be driven from a shaft taken from the crown wheel.




The first attempt at a diamond layout that I ever saw was a model that somebody submitted to the Fisher Body Craftsman's Guild styling competition back in the 1960's.  To my knowledge, nobody has ever tried to actually build a race car this way.


I love this kind of outside-the-box thinking.  To give a brief answer first, I think the diamond layout holds more promise for straight-line cars such as dragsters or speed record cars than for road racing cars.  There is, at least potentially, an aerodynamic benefit.  There is also, unfortunately, a handling penalty.


There would be an advantage in forward traction with three wheels driven.  Whether this would actually be allowed would depend on the exact wording of the rules, and their interpretation by the officials.  Even if three-wheel drive were ruled legal, the rules could be changed to prohibit it if the FIA saw fit.  Any dramatic innovation that obsoletes existing cars in any class faces the same problem.  Even if it is clearly legal when introduced, it can still be prohibited by a subsequent rule change if those in charge deem this desirable.


Whether the diamond layout stands to win road races or not, it is fascinating to consider what its pros and cons are, and what would be involved in optimizing it.


Perhaps the most obvious problem with the diamond layout is that roll is only resisted by a single wheel pair, at least up to the point where one wheel lifts.  In low-speed turns, with minimal aerodynamic downforce, and with racing tires, the inside middle wheel will lift before the tires will slide, just as we know one wheel would lift in a conventional layout, with all the roll resistance at one end.  Beyond that point, the vehicle is a tricycle, or maybe a sidehack, and any further overturning moment  is resisted by the front and rear wheels and the outside middle wheel, acting on half the track width.  The vehicle is unlikely to flip, as long as it doesn't snag an edge while sliding, but it is not making very good use of its tires.  If we are driving the middle wheels, the diff had better be able to lock with one wheel in the air.


In a road car, we would have much less grip, although we'd also have a higher c.g.  Maybe a road car could be made to keep the inside middle wheel on the ground at the limit of adhesion.  Even then, however, the middle suspension would need to be very stiff in roll to keep the roll angle within reason.


In a conventional layout, we can control the car's oversteer/understeer balance by juggling the relative roll resistance for the front and rear wheel pairs.  With the diamond layout, we can no longer do that.



Another problem is that we cannot achieve camber recovery in roll on the front and rear wheels.  Short of the point of inside middle wheel lift, we have no suspension displacement to work with on the front and rear wheels.  After the point of inside middle wheel lift, the front and rear wheel suspensions extend whether the car is rolling to the right or the left.  So we really have no choice but

to make the front and rear wheels move without camber change in ride and lean with the sprung mass in roll – and, as we have noted, the sprung mass is apt to lean considerably.


As with three-wheeled vehicles, the worst case for overturning is a combination of longitudinal and lateral acceleration.  If we compare conventional and diamond layouts, with identical wheelbase and track dimensions, with the diamond layout the center of mass is much closer in plan view to the nearest line connecting two contact patches.  That means that the diamond layout will bicycle more easily in some combination of lateral and longitudinal acceleration than the conventional layout will in its worst case, which normally is pure lateral acceleration.


Short of the point of bicycling, a layout with poorer overturning resistance will experience greater load transfer.  It will load its tires less equally, and will consequently use them less effectively.


One might argue that a fairer basis of comparison would be a case where the diamond layout has similar worst-case overturning resistance to the conventional layout.  That would imply a longer wheelbase and wider track for the diamond layout.  Ordinarily, our vehicles are constrained by the width of the lanes on our roads, and the length and width of our parking spaces.  The envelopes thus defined are rectangular rather than diamond-shaped, and therefore a conventional rectangular vehicle fills them more effectively, with its wheels spread further from its center of mass, than a diamond-shaped vehicle.  So a diamond-layout car needs to take up more room on the road, if it is to have comparable overturning resistance and comparable load transfer.


In racing, we usually have an overall width limit, an overall length limit, and sometimes a minimum and/or maximum for wheelbase and/or track.  We also have some advantage from a narrow car, in the line we can use, especially through doglegs and slaloms or esses.  All these factors favor the conventional layout over the diamond.


For the diamond layout, many of the steering, differential, and suspension design considerations are common to more conventional three-axle vehicles, and in some instances even two-axle vehicles.


Even when the two rear axles are quite close to each other, as in heavy trucks, in tight turns at low speeds there is a significant difference in the speed of the rearmost axle and the middle one.  Heavy trucks consequently have three differentials, two of them housed in the forward drive axle.  Many trucks have a driver-controlled lock for the center diff.  There is a noticeable increase in tire scuffing in low-speed maneuvers with the center diff locked.  I am not sure a second diff would be necessary in a diamond-layout race car, but the mean speed of the middle wheels would not be identical to the speed of the rear wheel in all situations.




As for steering, in multi-axle vehicles it is most common to steer the front axles and not the rear ones.  This gives the most predictable high-speed behavior, at the expense of turning circle.  A neighbor down the road from me operates a fleet of concrete pumping trucks.  The largest of these are non-articulated trucks with seven axles.  The front three steer.  The linkage is designed so the front axle steers the most, the second axle less, and the third axle still less.  The rearmost axle is a non-driven tag axle that is lifted off the ground in low-speed maneuvering.


Looking at the drawings by Mr. Scalabroni, it doesn't look like the front wheel could steer very much without hitting the leading arm that locates it.  A different design could be used, of course, but as drawn, the car would need to have the rear wheel steer just to get around the tighter turns.  If the rear wheel is driven by a longitudinal shaft, there will be problems making it steer much either.


Making a single front wheel steer a reasonable number of degrees presents some packaging challenges.  There has to be some fairly bulky structure either alongside the wheel or above it.  We can't have the steering axis behind the wheel; we would have negative trail.


Older readers will recall the four-wheel-steer cars sold by Honda and Mitsubishi during the 1980's.  These were originally designed to cope with cramped roads and parking areas in Japan.  The rear wheels steered out of the turn, adding yaw moment, in some situations, and steered into the turn, reducing yaw moment, in other situations.  The Honda system was purely mechanical.  It was ingeniously arranged to make the rear wheels steer into the turn at small handwheel (steering wheel) displacements, and steer out of the turn at large handwheel displacements.  The Mitsubishi system, called HICAS, was electronic and computer-controlled.  It steered the rear wheels out of the turn at low vehicle speeds and into the turn at high vehicle speeds.  That is really what we want.  The Honda system was intended to approximate this with a simpler, passive system, the thinking being that large handwheel inputs mainly occur at low speeds.  In both of these systems, the rear wheels steered much less than the fronts.


There was a four-wheeled truck made in the 1930's that steered the front and rear wheels equally and oppositely.  The idea there was to allow full-time four wheel drive with no center differential, and have a good turning circle without excessive U-joint angle at the outboard end of the axle.  Apparently, high-speed operation was not contemplated.


In the Scalabroni, would there be a possibility of the rear tire going to a slip angle past peak lateral force when catching an oversteer slide?  I think so.  In fact, this is fairly common with unsteered rear wheels.  It is also common for a driver to push the front wheels past optimum slip in an understeering car, and actually have the front end unexpectedly grab, or bite better, as the steering is unwound on exit.  Stock car drivers sometimes call this a "push loose" condition.  So, yes, it could happen, but the problem is not unique to the Scalabroni design.


The reader will note that we have so far assumed purely passive suspension.  Current F1 rules do require this.  However, active suspension could very significantly enhance the properties of the



diamond layout.  I doubt if it would equal a conventional layout even then, but again the possibilities are interesting to contemplate.


One possibility with active suspension would be to make the car lean into the turns, essentially making it a large motorcycle with training wheels.  Very good computer control would be needed, along with a lot of travel for the middle wheel suspensions.  Why not just have a two-wheeler instead?  Well, the diamond-layout car could be larger, and would be less prone to falling over to the inside when traction is suddenly lost, and less prone to high-siding when traction is suddenly regained.  A disadvantage versus a two-wheeler would be that the operator would not have the advantage of being able to straighten out the turns as much as with a single-track vehicle; the line could not be as good, due to the width of the vehicle.


It appears that the designer of the Scalabroni was thinking more of aerodynamics than mechanical dynamics.  I think that is where the concept's advantages lie, and therefore I think there are some really interesting possibilities for diamond-layout speed record cars.  By putting the middle wheels about a third of the way back, it would be possible to have a nice teardrop shape in plan view.  Large amounts of steering lock would not be needed.  Probably steering just the front wheel would be fine.  All four wheels could be driven.  If the rules required, the front and rear wheels could be slightly offset, rather than directly in line.


In any case, it is heartening to see this kind of original thinking.