The Mark Ortiz Automotive

CHASSIS NEWSLETTER

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October 2011

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

 

 

THE DELTAWING CAR

 

What are your comments on the Ben Bowlby DeltaWing concept as it now appears that a version of this configuration will be built for LeMans in 2012.  I have read the article in the August 2011 "Racecar Engineering" and really have a difficult time accepting Mr. Bowlby's concepts as something that will be a workable race car.  It appears that the designís method of making the car turn in is an electrically driven differential drive that will provide additional torque at some programmed amount to the outside drive wheel, adjustable stagger if you will.  Is this a workable concept??  It certainly appears that the front wheels will have very little effect of turning the car at race speed.

 

Racecar Engineering has actually carried at least two articles about this concept.  The first appeared in the April 2010 issue, and related to the project in its original form, as a proposed spec chassis for Indy car competition.

 

Do I think the car can win at Le Mans?  Probably not, overall.  Can it win its class?  Undoubtedly, depending on what you call a win.  Itís in a demonstration class, all by itself.  It has no competition. It canít lose.  Whether thatís winning, I will leave to the reader.

 

Do I think it can be made to turn in acceptably?  Probably it can.  In fact, I think the designers are correct that it wonít even need the trick diff for that.  I think they are correct that the front tires have a long enough moment arm about the c.g. so that they will make enough lateral force to produce acceptable yaw acceleration.  The car will be a bit deficient with respect to yaw moment created by toe-out at very small handwheel inputs, compared to one with a wide track, but that wonít necessarily slow it down.

 

The hard thing will be to get it to put power down with the inside rear wheel, while exiting turns.  I am sure the designers are correct that traction will be good in a straight line.  However, even with a rear track thatís a bit on the wide side, the car has a mean track considerably narrower than a

 

 

conventional car.  Overall lateral load transfer will accordingly be greater than in a conventional car, and the front wheels will not be able to contribute a share of the roll resistance moment that is larger than their static weight percentage, as they would in a conventional rear-engined car.  This means the inside rear wheel will unload considerably more.

 

Merely using a limited-slip diff or locker to deal with this would tend to cause a push.  So where the trick diff comes in is overcoming the problem without this drawback.

 

Applying the inside rear brake would also work, but the designers are trying to create an unusually energy-efficient car, so they are looking for a different way.  I must confess I donít fully understand from the published descriptions how their diff is supposed to work, so I canít comment on its likely efficacy.  However, if it involves power augmentation to the outside wheel, that should have the desired effect on yaw moment, provided it can be managed appropriately and be reliable.  If it can, it could have potential for conventional cars too.  It would probably require a rule change, because it would be a power augmentation.

 

It might be possible to link the two output shafts with a motor whose armature spun and was connected to one output shaft, while the field was connected to the opposite shaft.  Maybe thatís what the trick diff does.  I canít tell from published information.

 

Of course, we will have to see if all this really does work once there is a running car, but theory suggests that the car can be made driveable.  Outright tricycles can be made to corner acceptably, provided the wheels are spread out and the center of mass is low and toward the wide end.

 

But, does that mean the concept makes sense or is an improvement over existing cars?  Thatís a somewhat different question.

 

To address this broader question, we need to examine the broader nature of this effort, because it is highly unusual in this regard.  The whole thing has been a spec car design exercise from the outset, albeit with some unspecified variations permitted, and a sort of ďopen sourceĒ rule to allow multiple suppliers for any given part.  It was never conceived as something to run under existing rules, and beat other cars designed to those rules.

 

Ordinarily, when one sets out to build a race car, the first step is to tentatively pick an existing class, and read the rule book.  The second step is to read it again.  One studies existing cars in the chosen class, and tries to identify their strengths and weaknesses.  One studies the tracks.  One tries to figure out ways to work the existing rules.  When one gets a bright idea, one tries to predict whether it would immediately be outlawed.  One tries to predict what changes to the rules others might make.  Other than this, one does not contemplate changes to the rules.  One accepts the existing rules, and tries to build a car that can win races under those rules.

 

The DeltaWing project started out as a response to the need for a new spec chassis for the Indianapolis 500.  The event was a given, and so was the track, but the car didnít have to beat any

 

existing cars, or comply with any existing car specifications.  The project was to design a vehicle that could put on a good show in the Indy 500, be as safe as possible, be reasonably cost-effective,

and address to some degree the environmental issues that racing raises.  The vehicle didnít have to beat any conventional vehicles, built by others to a common set of rules.  It was to be the only design allowed.  Modifications were to be permitted, but all related parts were to be ďopen sourceĒ Ė documented by drawings published on a class website for all competitors to see and reproduce if desired.

 

The designers thus faced the kind of challenge hypothetically addressed in some of Paul Van Valkenburghís later columns in Racecar Engineering: if you wanted to design a car to solve the existing problems in racing, and you could write the rulebook too, not just design the car, what would you do?

 

Three problems that might be addressed would be:

1.      The cars use a lot of fossil fuel.

2.      The engines are exotic and expensive.

3.      Itís hard to pass, which makes the races dull.

 

The DeltaWing car attempts to address the first two concerns by having low drag, so that sufficient speed can be attained to make for interesting racing on a very large, high-speed track, with considerably lower engine power than in current cars.  It seeks to address the passing issue by not relying on its front end for downforce, having minimal upwash at the rear, and having small plan view area at the front so that the front will not produce much positive lift either, when it does encounter upwash.

 

I do think the design looks like it should accomplish those things.  However, I am not convinced that the narrow front track is really necessary for this.  Rather, I think the objectives could easily be achieved with more conventional wheel positioning Ė and probably with greater safety, at least with respect to discouraging rollover.

 

Existing Indycars have little plan view area at the front, and little upwash at the rear, if you just take the wings off.

 

The narrow front track does look different.  This is good for getting media attention, as has already been demonstrated.  But if such a layout were to be adopted as a spec configuration, media attention to it would evaporate in short order.  A field of look-alike ďDeltaWingĒ cars is no more interesting than a look-alike field of some other sort of cars.

 

I donít know if the requirement to lap Indianapolis at about 200mph was part of an RFP to which the design was originally a response, but really it isnít necessary for cars to go that fast to put on a good show at Indy.  What is necessary is that the cars must need to slow down for the turns and take them at the limit of adhesion, and be able to pass each other and run in close proximity.  This is true for

 

 

other tracks as well.  It also helps if there is a diversity of car designs, yet sufficiently close competition to make the outcome unpredictable.

 

The first year I paid serious attention to the Indy 500 was 1960.  As I recall, a lap just over 140mph was good enough for pole.  A 150mph lap was considered a distant dream.  The race was still an interesting show, and drew a good crowd.  Most of the cars, including the winner, were similar in design, but there were a few oddballs to spice things up.  All but the Novis had less than 500 horsepower, and the Novis seldom finished.

 

Wide tires and rear engines raised lap speeds into the 160ís.  Then came wings and turbos, then ground effects.  That was what it took to raise speeds above 200mph.  But was the show more entertaining?  The wings needed clean air to work well, yet the cars created large wakes that denied a following car clean air.  This meant that rather than speeding up in another carís draft, a car slowed down, at least in the turns.  That discouraged passing.

 

The hope is that the DeltaWing can be made to work without wings, yet still corner quite fast, by relying almost entirely on tunnels or undercar venturis.  Past attempts at this, with more conventional wheel positioning, have not been very successful.  The problem is not that tunnels donít work in the wake of front suspension components.  On the contrary, they make lots of downforce, and do it very efficiently.  The problem is that it has proved difficult to control the location of the center of lift (downforce) with sufficient precision and adjustability, without adding front and rear wings.  Also, a rear wing can help drive tunnels or a diffuser, so there is a synergistic gain thatís hard to pass up.  And once thereís a rear wing, a front wing to balance it becomes highly desirable.  Together, the wings can serve to adjust both the front/rear downforce distribution and the overall amount of downforce and drag.  Different wing packages can be used for faster and slower tracks.

 

So wings are highly attractive.  They just present problems when two or more cars are running in close proximity.  When cars are nose to tail, the front downforce generating element of one is in the wake of the rear downforce generating element of another.  When wing packages are regulated by rules that assume that front and rear wings are inevitable, the stage is set for races with reduced passing.

 

The DeltaWing team are therefore onto something by keeping the downforce generating elements toward the middle of the car.  This achieves roughly a car length separation between downforce devices, instead of a few feet or inches.  How much difference this can make would best be established by comprehensive testing and/or CFD, but we can get some indication from the example of winged sprint cars.

 

These cars are allowed large wings, but they have to be on top of the roll cage.  They also have little front ones, but theyíre on top of the nose, and backed by the bluff portion of the nose that covers the induction system, so most of the downforce comes from the top wing.  Top wings are adjustable both for attack angle and for front/rear position.  The tails are required to look like old-fashioned race car tails, from the days before people caught on to the importance of downforce.  They do not

 

generate upwash.  The wings do disturb the air going to a following car, but at least some distance between wing and wing is maintained.

 

The cars are able to run in close proximity.  Thereís lots of passing.  The show is generally entertaining.

 

So, if we want downforce devices amidships, should they be wings?  Tunnels?  Both?

 

Tunnels have the advantage of being more efficient in terms of lift/drag ratio.  Arguably, they have an aesthetic advantage.  However, wings are more adjustable and adaptable.

 

Both wings and tunnels depend on having the car running more or less straight: they are yaw-sensitive.  That is not good.  Probably tunnels are worse than wings in this regard, but in both cases, if you get the car sideways, the downforce largely goes away, and if you spin the car and itís going backwards, you can get lift.  The downforce devices are your friend until something goes a bit wrong or you make a marginal mistake, and then they abandon you or even turn on you and bite you.  And with the higher speeds enabled by the downforce devices, the ensuing crash is a harder hit.  So why would you race with wings or tunnels?  Because if they are allowed, you canít win without them.

 

Therefore, there is a strong case for prohibiting wings, tunnels, and other downforce devices relying on free air, altogether.  Can this actually be done?  It is done, pretty much, in Formula Ford, in midgets, and in unwinged sprint cars.  It really is possible to regulate bodywork tightly enough to largely eliminate downforce.  The quality of racing in these classes speaks for itself.

 

Is there any advantage to the narrow front track at all?  In terms of passing, itís actually a minus.  For comparable roll stability, a triangular-plan car has to be wider at its widest point than a rectangular-plan car.  The carís ability to fit through a hole is governed by its width at the wide end.  Faired wheels do reduce drag, but rectangular-plan cars can have faired wheels too.  The structures required can be outrigger or pannier fairings or fenders, and can add side impact protection.

 

Triangular-plan cars, with three wheels, do have some interesting possibilities for road use, but only because they are considered motorcycles, and do not have to meet automobile crash standards.  This plus the reduced need for torsional rigidity allow a street-legal trike to be considerably lighter than a street-legal 4-wheeler.  However, it is likely that the licensing and regulatory status of trikes would change if they became more popular, while their disadvantages in terms of space utilization and roll stability would remain.