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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: firstname.lastname@example.org. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
MORE EXOTIC VEHICLE LAYOUT IDEAS
Thanks for an interesting commentary on the diamond car recently. That certainly has had me thinking. Below are some topics I've been pondering.
Another unusual race car
Following up on the diamond shaped car, I came across another unusual car. This one is a special based on a Mazda Familia hatchback. The original car had four-wheel drive and was powered by a transverse mounted engine and gearbox assembly. The engine was a turbo four (of 1.8 litres I think) with a five speed manual box. The box drove all four wheels via a “central differential”. The car was modified by moving the original engine/gearbox and front wheels rearwards. Another engine/gearbox (this time an automatic gearbox) assembly was mounted ahead of it. This powered an extra pair of road wheels up front. The car became a six wheeler with all wheels driven! The front four were steered. The car is reported to need more development work but has been successful on dirt and hill climbs, outperforming V-8 competitors on occasion! It is a good handler and fast with plenty of traction.
I’d be most interested in reading your analysis of this type of vehicle.
A second type of six-wheel car I’ve thought about is one with only the front four wheels driven. In this case the plan would be to use a pair of front wheel drive engine/gearbox/suspension/road wheel assemblies mounted at the front of the car. The two power trains would operate independently of each other. All four front wheels would be steered. It may be necessary to use a balance beam linkage in the steering to ensure that the front wheels on each side of the car developed similar lateral forces whilst cornering. The final pair of wheels at the rear of the car would be un-powered and non-steering, as is normal for front wheel drive vehicles.
I am aware of various attempts to use independent power-trains in four wheel vehicles. Famous examples include the Twinnie Minnie, which had an engine at each end. VW tried this for the Pikes
Peak hill climb, as did Monster Tajima in his Suzuki at the same venue. There are some interesting handling issues to be tamed with this type of system (where there are no links between the two power-trains). VW used traction control and an early form of throttle by wire. In the end Monster added a shaft linking his two engines via a computer controlled electro-magnetic clutch. The Twinnie and various replicas of it have encountered some handling problems, which appear to be solvable, or at least made controllable by careful engine tuning and modification to engine response and power delivery characteristics. Still there are questions about the ultimate handling behaviour on the limit with this type of set up.
In the case of the six wheeler with only the front four wheels driven it may be that these problems are not present since the driving wheels are close to each other and not at opposite ends of the car.
What is your analysis of these set-ups?
In most classes of racing nowadays, we don't have the option of more than four wheels, or more than one engine. For the most part, the rules prohibit such things. Nonetheless, there are scattered venues where one can at least run such vehicles for fun, and it is fascinating to consider the possibilities.
One fundamental argument against using a larger number of anything is that, as a rule, two little ones weigh more, cost more, and take up more room than one big one. This applies to engines, cylinders in the engine(s), and wheels. That doesn't mean that adding a component is always a bad idea, but there has to be a compelling functional advantage in adding anything to the car, to override this disadvantage.
Multiple engines are usually used for one of two reasons: either the engine in question is small, and using two of them is an economically appealing way to build an all-wheel-drive special, with much-needed additional power, or the vehicle is really large, as with land speed record cars, and high-output engines of sufficient size are either unavailable or unaffordable.
Until recently, controlling two unconnected engines driving the front and rear wheels was a real problem. Perhaps with the advent of by-wire controls, this can be overcome, and potentially even turned into an advantage. The question is, who is going to invest the money to do this, when the car is ineligible for any big-money racing class? Not only do we need to appropriately control the power and rpm of the two engines, we also have to make the two clutches take up in sync with a single pedal or other control, and make both transmissions shift in sync from a single lever or other control.
If we are talking about two identical engines, with unconnected two-wheel-drive transaxles, driving the front and rear wheel pairs, we have a 50/50 torque split front to rear, and no connection between the front and rear wheels. This means that unless the car is severely nose-heavy, the front wheels will tend to spin prematurely in hard forward acceleration. Either the driver will have to lift to control front wheelspin, or, if we have traction control, the power of the front engine will have to be
suppressed, meaning we are not making full use of that engine. If we do make the car sufficiently nose-heavy so both engines can deliver full power, that will compromise cornering and braking. Linking the two transaxles with a driveshaft and some form of clutch allows some transfer of power from the front to the rear, but requires the front and rear wheels to be constrained to the same rpm at the traction limit, which compromises handling – unless we use different final drive ratios at the two ends, which would bring a whole set of new problems, chiefly in transmission control and rev limits. Adding such a mechanism to two existing two-wheel-drive transaxles also involves a lot of expense and special parts, which erodes the original economic appeal of using two cheaply available economy car powertrains.
All things considered, if we are designing from a blank sheet of paper (or screen), it is probably more appealing to have one large engine, with one transmission, and, if we want to drive all the wheels, to control torque distribution with the differentials and clutches or other locking devices in the drivetrain. Fundamentally, we have fewer parts, no synchronization issues, less metal, and an easier time getting the torque distribution we want.
It is desirable not only to have the power, or at least a preponderance of it, go to one end of the car, it is desirable that this be the rear end. One advantage of rear wheel drive in terms of car control is that it affords us some measure of independent steering control at both ends of the car. This allows us to create yaw accelerations, and control the car's attitude, with the rear wheels, even when the front wheels are at the limit of adhesion. With front wheel drive, we have two ways of controlling the front end, but nothing except perhaps the brakes to control the rear. If we wish to drive all the wheels, it is desirable, at least in a high-speed road or road racing car, to make the car throttle-steer somewhat like a rear-drive car.
If we are designing a front-engine car with all wheels driven, and we want to have a back seat and a reasonable overall length, we will have somewhere between 50 and 56 percent of the weight on the front wheels. To make the car throttle-steer controllably, we want at least 60% of the torque to go to the rear wheels. If we are designing a pure performance car, it is better to choose a rear-mid-engine layout, which will typically give us around 60% static rear weight. If nothing else, this layout gives us better braking and less yaw inertia than a typical front-engine layout. We then want somewhere in the range of 75 to 90 percent of the propulsion to come from the rear wheels.
There is a definite advantage in driving the front wheels, even if they receive a small percentage of the power. It may not be obvious, but we always have to drive the front wheels, particularly while cornering. When the front tires are running at an appreciable slip angle, they are generating significant drag, which must be overcome. This uses power, which must be supplied somehow. In a pure rear-drive car, the power to drive the front wheels is delivered from the rear wheels via the road surface. That means that this portion of the car's power passes through the contact patches, and uses up a portion of the tires' friction circle or performance envelope. If we power the front wheels, even moderately, by some other means, we have more grip available for cornering. This explains why cars with all wheel drive perform as well as they do, even in pure cornering, despite their inherent weight penalty.
The basic front/rear torque split is generally established by a planetary center differential. The planet carrier is driven in some manner from the transmission output shaft. The ring gear or annulus drives the rear diff. The sun gear drives the front diff. The torque ratio is the same as the pitch diameter ratio of the sun and annulus. If the planet gears are the same diameter as the sun, the diameter ratio is 3:1. The torque split is then 25/75. To get a 20/80 split, the planets need to be 1.5 times the diameter of the sun. Planets twice the size of the sun yield a 5:1 annulus to sun ratio, or a 16.7/83.3 split. To get a 10/90 split, the planets need to be 4.5 times the diameter of the sun. To get a 40/60 split, the planets need to be ¼ the size of the sun. It will be apparent that there will be practical limitations precluding extremely unequal torque splits, or ones between 40/60 and 50/50. (Actually, splits between 40/60 and 50/50 are possible, if we are willing to machine a conventional-style bevel gear differential with unequal-size side gears, cut with different bevel angles, and the pinion gears cocked at an angle. Opposing pinion gears have to be on independent shafts, rather than a common shaft running through the carrier.)
Usual practice is to design a drivetrain so that the front and rear pinion gears and propshafts run at equal speeds when there is no wheelspin and the car is running in a straight line. This minimizes wear on the center diff. However, it is possible to design the center diff to withstand having its gears in relative motion most of the time, and have the propshafts run at different speeds.
Running the propshafts at differing speeds has two effects: first, the split in propulsion forces changes, because we are using different ring and pinion ratios at the front and rear, and the torque multiplication at these gear sets is correspondingly unequal. The end with the faster-spinning prop shaft gets more power, for any given torque split at the center diff. Second, the car's response to center diff locking devices changes. It is possible to have the rear wheels slipping or overrunning the front wheels by a predetermined percentage when the center diff locks, rather than running at the same speed. This allows us to tune the car's throttle-steering characteristics. Ordinarily, we would arrange for the front propshaft to overrun the rear most of the time, and lock the center diff when the propshaft speeds equalize, or when the rear one overruns the front by some amount.
With electronic control of the lockup, we really don't need to juggle the front and rear final drive ratios. We can pretty much get whatever torque distribution and rear overrun we want, just with the center diff and the lockup strategy. However, there is some effect on car behavior when the center diff is completely or largely locked, even if the lockup is electronically controlled. With simpler, passive, mechanical lockup devices, juggling the front and rear final drive ratios offers some interesting possibilities.
For example, it would be possible to set up the ratios so that the front propshaft overruns the rear by 30%, and add a roller clutch that locks the front propshaft to either the planet carrier or the annulus whenever either of these tries to overrun the sun gear, but freewheels the rest of the time. That would allow the driver to spin the rear wheels enough to throttle-steer, but prevent runaway rear wheelspin. Front to rear propulsion force ratio short of lockup would be sun/annulus ratio, times the ratio of front final drive ratio to rear final drive ratio, times the ratio of rear tire effective radius to front tire effective radius.
Another possibility is to drive the front wheels hydrostatically or electrically, rather than mechanically. This could be a way of powering the front wheels very modestly, with far less hardware and more attractive packaging. One way would be to use a small gear pump, perhaps one stage from a dry sump pump, as a motor on each front wheel. These might be driven by a similar pump at the transmission, and the circuit might include a cooler for the fluid. It might also include a relief valve that would limit pressure to the motors.
It would only be possible to transmit small amounts of power this way, but we could eliminate or reduce the power otherwise transmitted through the contact patches to overcome the drag of the front tires when operating at a slip angle. If we can do that, we get the main cornering advantage of mechanical all-wheel drive, with a smaller weight penalty.
Hydrostatic motors and pumps are available in a wide range of sizes. However, power losses in hydrostatic drive systems are high, so transmitting large amounts of power hydrostatically is not an attractive proposition for a high-speed vehicle.
Electric drive, as used in locomotives, offers some of the same possibilities as hydrostatic drive, although it cannot be made a part of a transmission cooling circuit. Also, electric motors are more delicate than a gear motor, so mounting them outboard is probably not a good idea. Electric drive does lend itself nicely to computer control of torque distribution.
Mounting both motors and brakes inboard offers some interesting advantages. Inboard brakes do add weight and cost, and they occupy space. So does the ducting required to cool them. However, inboard brakes do offer advantages not commonly recognized. Everybody knows they save unsprung weight. But additionally, they make it easier to get balanced airflow to both faces of the rotor, reducing the rotor's tendency to dish as it heats up. Potentially, they allow a larger rotor diameter, at least in some cases. This is particularly true in low-built race cars, where the floor hangs below the wheel rims. Finally, they allow relatively unobstructed airflow through the wheel. In a full-bodied car, this greatly increases the effectiveness of the wheel openings as ducts for extracting under-car air.
Inboard front brakes do require beefy halfshafts. The torque transmitted can easily be as great as that transmitted by the rear wheels under power, even in a tail-heavy car, and the consequences of shaft or joint breakage are really nasty.
Either electric or hydrostatic drive to the front wheels offers the possibility of integrating the front drive system with an energy recovery system.
So much for the pros, cons, and nuances of driving more than two wheels. What about having more than four wheels on the vehicle?
In the case of tires, more rubber generally gives us more grip, at a penalty in speed on long straightaways, for reasons I have discussed in previous writings. We are limited, however, by our ability to keep a wide tire upright enough to use the full width of the tread. As the tire becomes really huge, suspension packaging can become problematic. So can steering geometry, in the case of a front wheel. Frontal area of the tire increases in direct proportion to width, or height, with a corresponding aerodynamic drag penalty. If there is standing or streaming water on the road surface, a wide tire is more prone to aquaplaning. If we are going to have to deal with snow, a wide tire is a disadvantage because of the greater force required to move it through the snow.
So there are reasons to consider having two small tires in line with each other, instead of one big one, or perhaps two big ones in line with each other to provide twice as much tire, or some compromise in between.
Two strategies immediately present themselves: double the number of wheels, and use eight, or make the car heavy at one end, and use four at the heavy end and two at the light end.
Those who have studied Formula 1 history will recall the Tyrrell P34 6-wheeler of 1976, which had the extra wheels at the light end. The idea there was to use much smaller front tires than usual, and reduce aerodynamic drag. This required special tires. Conventional, large tires were used at the rear. The reasoning was that the airflow was so disrupted by the time it reached the rear wheels that small tires there would not return as much benefit. Also, it is easier to add more non-driven wheels than to create the hardware to have more driven ones. The Tyrrell actually won The Swedish Grand Prix that year. In fact, the team got a 1-2 finish.
Because rear grip was essentially the same as a conventional car, there was no benefit in exploiting the potentially greater front grip from the four front tires. F1 cars of the time already cornered with the inside front very lightly loaded, so there was little room for adding more front roll resistance to use enhanced front grip to help the rear end stick. Consequently, the advantage of the design was mainly aerodynamic, and this came at a penalty in weight.
Shortly following the Tyrrell effort, March built a 6-wheeler the more obvious way: four rears, all driven. The transaxle was an adapted Hewland, with no center differential. The car was tested but never raced. March was in financial straits at the time, and development was lacking. In 1979, a retired March F1 car with the 6-wheel setup achieved considerable success in British hillclimbing.
There was also a similar car built by Williams in 1982, which likewise was never raced. After this, the FIA banned all four-wheel-drive systems from F1.
It would be possible to have four rear wheels and only drive two. The car wouldn't throttle-steer as one might hope, and propulsive traction would be poor, but it might be possible to get good lateral acceleration numbers that way, and perhaps better braking than with four wheels.
The idea of using four front wheels, and driving the frontmost pair from a separate engine, with a automatic transmission, is interesting. One drawback would be that the torque distribution would vary in a largely uncontrolled manner, because the torque multiplication at the automatic would not have a constant relationship to the torque multiplication in the manual transmission. The two transmissions would have differing ratios and wouldn't shift at the same time.
The questioner asks about steering four front wheels, and wonders about using some form of balance bar. I don't think a balance bar is appropriate for the steering. All four wheels need to have a fixed relationship to handwheel position. I think Tyrrell got it about right. As I recall, they steered the frontmost pair of wheels from a rack and pinion in the usual manner, and ran a drag link back from each of these to the wheel behind it. It would be normal practice to make the second wheel pair steer a bit less than the first. This could be considered a form of Ackermann effect. For conventional, or positive, Ackermann effect with four front wheels (i.e. to optimize for a turn center behind the front wheels), the second pair of wheels should steer less than the first because they track inside and behind the first pair. This is certainly the way it's done in heavy trucks with multiple front axles.
We have noted in previous newsletters that we want different dynamic toe-out for high-speed turns than for low-speed turns, and possibly even toe-in for the high-speed ones. I don't think we have this kind of reversal regarding how much the second wheel pair steer compared to the front pair. The second pair should steer less than the first pair whether the turn center is behind the front wheels or ahead. (At least, this is true as long as the front wheels are steered into the turn, as opposed to counter-steered to correct a rear wheel slide.) We might say that this is so because the second wheels always trail the first ones, whereas the outside wheel may either lead or trail the inside one.
Tyrrell did use balance bars on either side for springing and damping, and it seems to have worked okay. The first and second wheels on each side were linked by a balance bar, and a single pair of coilovers and a single anti-roll bar acted on these balance bars. This meant that, considered as a four-wheel group, the front wheels had a wheel rate of zero in pitch and warp, and were also undamped in these modes.
I'm not sure a pure balance beam arrangement like Tyrrell's is fully optimal, but the fact that it worked acceptably points up a potential advantage of having two close-coupled pairs of wheels rather than a single pair: we can make the system very soft in absorbing small bumps, while still having good control of sprung mass pitch, roll, and heave.
Does it make sense to have eight wheels, then? It might, if the application makes traction a priority over power-to-weight ratio and compactness. The optimum might be a rear-mid-engine configuration, with the rear four wheels driven mechanically and the front four driven hydrostatically or electrically, and inboard brakes on all eight. The most advantageous course for such a vehicle would have low grip and plenty of chatter bumps, and be uphill – but not be confining enough to make compactness too crucial. And of course, the rules would have to be set up to encourage innovation, rather than create a drivers' class with standardized or closely matched cars.