Rear Engine vs. Mid-engine

Engine location in the chassis is a direct contributor of performance capabilities as well as braking and traction.  Rear engine cars are "tail heavy" causing the rear of the car to tend to swing out in a curve or turn.  The faster the car is traveling the greater the tendency.  More weight on the rear means less on the front.  Since the primary brakes are on the front and the direction of the car is controlled from the front loss of weight there means less braking and steering control.  The shorter wheelbase of the Speedster exacerbates this tendency.  The more balanced a chassis the less swing out and the greater steering and braking on the front. Ideally, although unrealistic, a (50/50) bias would be preferred.  Rear engine VW Speedsters typically have about 38% on the front and 62% on the rear (38/62), but our mid-engine Speedster is (45/55).

Even Ferdinand Porsche knew the best handling setup was mid-engine.  The first two 356 Porsches were mid-engine, but as we all know there is no backseat for the kinders.  Family usefulness over ruled performance so the 356 became a rear engine exactly like the VW Beetle and for the same reasons.  Porsche immediately introduced a true performance car the 550 Spyder which was mid-engine and has historically been lauded for its handling acumen. The rest of the Porsche performance history is full of record setting mid-engine cars.

The point is best made by noting that virtually all world class high performance racing or road cars are mid-engine, and that includes Lamborghini, Ferrari, Jaguar, Maserti, Lotus, Porsche Boxster, Formula 1, Indy, and CART to name a few.  The few that are not are deeply rear set front engine cars like the Corvette and Viper.  The water-cooled Porsche 911 rear-engine car has alleviated the worst of the rear engine tendency by taking advantage of 40 years of design effort and making it an all wheel drive system with individual computer controlled wheel power distribution.

Front/rear weight bias affects many aspects of performance.  The most significant is the responsiveness of the car.  Front biased cars tend to under steer heavily while rear biased cars tend to over steer heavily.  Both of these types have a high polar moment of inertia, which is an engineering measure of the resistance of a rotating object to change direction..  These cars are usually altered to alleviate these tendencies, but those alterations cannot affect the responsiveness.  Cars designed and built for performance responsiveness are typically mid-engine configuration which creates a low polar moment of inertia.  Weight bias also has a large impact upon braking efficiency and traction.  Front engine cars have most of the braking resistance carried by the front brakes.  Although the greater weight is on the rear wheels of rear engine cars they still rely on the front brakes for controlled braking. The front brakes must be dominant in all cars or the car will tend to lose control on hard braking efforts.  Since rear engine cars already have this tendency in turns great care must be taken to get the braking bias forward so as not to exacerbate an already sensitive situation.   Mid-engine cars distribute the braking effort more evenly to front and rear allowing the brakes to run cooler, wear slower, and provide a balanced feel when braking.

The finished weight of a car can greatly influence the performance characteristics.  The lower weight allows the engine to deliver its horsepower and torque for greater acceleration and pulling.  However, most importantly a lower finished weight can allow the car to be negatively affected by cross winds, choppy road conditions, and noise control.  One must decide whether they are willing to sacrifice a more comfortable and quieter ride desired for 99% of their driving needs in order to be able accelerate more quickly during 1% of their driving.

A true measure of performance is the ratio of weight to horsepower.  As this number decreases the perceived performance of the car can increase due to the ability of the engine torque to overcome the inertia required for acceleration.  Obviously one can achieve this favorable ratio by decreasing the weight of the car or by increasing the horsepower.  There are advantages for both.  Heavier cars have better penetration and ride characteristics. More powerful engines cost more and consume more fuel.  Modified engines consume more fuel, generate more heat, are usually more noisy, and can be fragile and prone to failure.  One must weigh the advantages and disadvantages carefully.  Light cars are sensitive to cross winds and turbulence created by highway traffic and passing large vehicles as well as a choppier ride caused by normal road aberrations. 

Horsepower and torque ratings can be very deceiving since they are usually advertised at one rpm level which is typically the maximum rating.  If this rating is achieved at a very high rpm then you will never approach the rating at normal driving ranges (1500-4000rpm. 

Another significant factor is how the power and torque are achieved.  If an engine normally rated at 60 hp is modified to produce 120 hp the engine can become very fragile and suspect for heat and loading failure.  If the necessary alterations and additions are made to the 60 hp engine to safely achieve the 120 hp the costs will be very high and in many cases the heating and loading concerns are not entirely removed.  A true test of an engine builder's confidence in his modifications is his willingness to warranty the engine.  Most modified performance engines have no warranty.  Some of these type engines are warranted but one should read the conditions very carefully because of limitations and exemptions of use.

The most secure method of achieving reliable horsepower and torque is to design the engine from the beginning to accommodate the power provided.  Modifications and alterations create questions about reliability.  All original equipment engines (OEM) are designed, tested, and warranted to reliably deliver the power advertised.

Brakes are an obvious important consideration in a performance car.  Their effectiveness is greatly influenced by what engineers call normal force.  Normal force is the force the tire is exerting directly against the ground and is created primarily by the weight of the car. Dynamic and aerodynamic influences also affect this force, but for a normal road car the weight is dominant. Formula 1 racing cars create aerodynamic forces that increase the normal force at speed as much as 3 times.  Many characteristics of handling, braking, acceleration, and even tire wear are directly linked to this force. Aside from the obvious requirement of the braking system having adequacy for the platform, such as swept contact area vs. power and weight of the car, the normal forces prevail.

The ability of a tire (not the brakes) to stop the car in a controlled manner is more often than not overlooked.  The cheapest and most expensive brakes in the world are both limited by the traction created by the tires.  If the tires lose traction then braking and steering control is lost.  This is a phenomenon that is frequently experienced by drivers who find themselves on icy road conditions.  The brakes work but the tires have no traction with a subsequent loss of control.  This extreme example can be used to describe the same results even under dry conditions.  Wet, sandy, or oily roads only exacerbate the traction loss  phenomenon. Modern automobile design incorporates Anti-lock Brakes (ABS)  which controls the sliding tendency of the tires thereby increasing braking effectiveness.

Tire traction is complicated and is influenced by the compound used to make the tire, tread design, sidewall flex, temperature, and road surface conditions.   Coefficient of friction is an engineering term that describes quantitatively the resistance two surfaces exert when you try to move one over the other.  There is a static and dynamic value of this coefficient.  The static value describes the effort to break the two surfaces free from each other and the dynamic value describes the effort to keep them sliding.  The coefficient is a constant for a given set of parameters.  Traction is a product of Normal Force and coefficient of friction  so as you can see the normal force becomes a variable of extreme consequence.

Front brakes and traction must dominate the stopping of the car.  Braking bias is adjusted so that the front brakes dominate the requirement.  If the rear brakes are allowed to dominate directional control is easily lost.  All effort should be given to maximizing the traction on the front tires in order to maximize the effectiveness of the front brakes. If all things are kept constant  it becomes obvious that the more weight you have on the front tires the greater will be the Normal Force and that force multiplied by the coefficient of friction will define the traction force.

From this presentation comes the obvious reason why a rear engine VW Speedster with a 38/62 weight bias has less front tire traction force (normal force) than our mid-engine Speedster with a 45/55 weight bias.  For an 1800# car this means 810# on the front tires vs. 684#.  Our mid-engine Speedster has 19 percent more weight on the front wheels.  In order to keep from losing traction on the front brakes of a VW Speedster but at the same time not let the rear brakes dominate the entire system has to be diminished to the ability of the front traction.

It should be noted that in addition to improved braking the normal force and subsequent traction force greatly influences directional control and high speed stability.

Fuel delivery to a normally aspirated (not super or turbo charged) internal combustion engine has probably had more impact on engine performance than any other single engine system.  It is very complicated and must be integrated with the other engine systems to achieve a balance for proper use.  Two methods of delivery have been pioneered since the invention of the internal combustion engine - carburetion and fuel injection.

Carburetion is totally dependent on ambient conditions, attitude (hills, curves, etc) and paths of delivery to the cylinders.  Carburetion is not dynamic but rather statically set up to provide acceptable delivery over the normal range of use of the car.  It cannot accommodate changes in altitude, barometric pressure, humidity, or temperature.  It just delivers a generally correct fuel/air mixture for general driving conditions.  To exemplify this limitation:  professional drag racers have as normal equipment devices to measure temperature, barometric pressure, and humidity so they can immediately change jet setting in their carburetors when conditions change at the race track.  This allows them to achieve the maximum power out put for that engine, at that track, under those immediate conditions.  This is not something you can do while driving to visit grandma in the country.

Fuel injection has been around for a long time and has been primarily mechanically controlled.  These mechanical systems are used on diesel engines as well as famous cars of the past such as the Mercedes, 57 Corvette, etc.  These systems were a great leap forward in that they were able to deliver precise amounts of fuel over a broad range of conditions thereby maximizing power out put.  However they did have limitations.

Welcome to the 20th/21st century.  Electronically controlled fuel injection found on just about every car on the road today started appearing in great numbers in the early eighties.  With the miraculous advances in computer chips and miniaturization fuel delivery today is truly state-of-the-art.  The Engine Management system monitors every conceivable condition which could affect proper fuel delivery ratios and instantly makes adjustments to every cylinder while driving.  In other words your engine is producing the maximum power for the conditions monitored at just about every instant of use.  This is why fuel injected cars can produce more horsepower while at the same time deliver greater fuel mileage. Try driving  from the coast to Pikes Peak with a carbureted car versus an electronically controlled fuel injected car and you will immediately see the reasons why carburetion is no longer a viable method.

Our 356A replicas all have electronically controlled fuel injection.

Engine displacement is a surefire way to gain horsepower, but it is done at sacrifice of other desirable features.  Large displacement requires large engine blocks which add weight, higher fuel consumption, and often aerodynamic losses due to the large shape of the body required to house them.  There are circumstances where displacement is advantageous, but for a high performance sports car one must consider more than brute power.  Fuel economy, space, handling, braking, noise levels must be balanced with power to achieve the "perfect" compromise.  This argument has never been nor will ever be settled amongst car enthusiasts.

Emission compliance has become an issue of concern for kit cars, replicas, specially constructed cars, and others falling under this general description.  The past practices of buying an old title for an abandoned car or using the title of the donor car which in no way looks like the new replica are successfully being attacked by the state DMV offices.  They are viewed as fraudulent registrations that often times are used to avoid the proper taxes due as well as to disguise the driveline in the car thereby avoiding proper emission evaluation.

EPA has issued a requirement for emissions compliance for these type cars.

http://www.epa.gov/otaq/imports/kitcar.htm

 and I have covered this issue in detail at  http://www.specialtyauto.com/registration_requirements.htm

One should be concerned about building an expensive car that does not address these requirements since it can subject the builder/owner to EPA fines, cause the car to fail registration requirements, and/or prevent the car from being transferred in the future to second owner for the same reasons.

All Specialty Auto-sports, Inc. 356A and XK120 replicas are delivered with conforming drivelines that meet the EPA requirements "Replica definition". 

There are two basic types of steering control found on most vehicles.  A link type or rack and pinion.  Link type steering was found on almost all early vehicles up to the 1960's.  Since that time the industry as almost completely evolved into rack and pinion.  The reasons for this evolution center around the advent of front wheel drive cars, weight, cost, space, and the very positive steering feel provided by rack and pinion.

Link type steering is plagued by having too many connection points each having a required degree of looseness, and each being further loosened by wear.  The end result is a sloppy feeling at best and a dangerous reaction at worst.  The link steering has a steering gear box with a complicated recirculating ball bearing, and at least 5 tie rod connections.  This is further complicated by the extraordinary angles the links are required to achieve during normal driving. The rack and pinion has a gear driving a flattened cog gear and 2 inner ball connections and 2 tie rodends.  This method has proven to be the only used on racing cars and is where the technology evolved.  Most all cars today have rack and pinion steering.  The VW Beetle uses a link type steering.  The last Beetles imported into this country called Super Beetles had rack and pinion steering, so even VW eventually recognized the importance of rack and pinion steering.  Super Beetles are not used to build VW Speedster replicas.  All Specialty Auto-sports, Inc. cars have rack and pinion steering control.

There are generally three types of frames used with replica automobiles.  Ladder rail, pan, and structural tube.

Ladder Rail Frame - this is a frame that is found on most cars and trucks prior to the mid-60's.  It is composed of two parallel main members and cross braced with diagonals or cross beams. Attached to these members are the connections for the suspension members, drive line, and body components.  The body contributes little to the structural integrity of the car, and the loads and forces are borne by the frame structure.  Remove the body and you have an non-drivable vehicle. These frames have almost been completely replaced by uni-body construction where the engineered body structure contributes the major structural integrity of the vehicle.  A fiberglass replica body cannot serve as a uni-body type structure because fiberglass does not have the structural properties required for enduring the loads and fatigue causing inputs from automobile driving.

 

Pan Type Frame - this is a pseudo-frame in that all of the suspension and driveline components are bolted to the pan, but the pan has insufficient integrity for its intended use without the VW body attached.  The pan requires the additional structure provided by a welded steel body.  Although all of the driving components are on the pan that allow it to be driven it does not have the strength or rigidity to serve as a stand alone chassis.  All VW Beetles were built with pan/body construction.  When the Beetle body is removed in order to use the pan for a replica Speedster foundation all of the required structural integrity afforded by the body is lost.  Most Speedster replica companies attach a perimeter sub-frame to the fiberglass body which serves as a stiffener and an interface between the fiberglass body and the pan.  This is not a structural tube frame.  It is a perimeter reinforced pan.  It is obvious that this sub frame does not provide any impact protection.

 

This type VW sub-frame is found under virtually every VW speedster replica.

 

Structural Tube Frame -  This is the most abused term used in the replica industry.  People with no engineering education or background seem to have the belief that if you cut up some tubes and weld them together you have a structural tube chassis.  Well as you can see from above the ladder and sub-frame have welded tubes but they are not stand alone chassis.  The true test of whether a chassis is structural tube or not is simple.  If you could wave a magic wand over the car and remove all of the fiberglass body components and then walk up to what remains, open the door, sit down, start the car and drive away knowing the chassis could be driven anywhere the finished car could be driven.... you have a structural tube chassis. (Be sure to take your raincoat)   The real challenge is the engineering required to ensure that it is of sufficient strength and has fatigue input factored into its design.  This is not body shop type work.

 

       

compare these SAS frames with the VW sub frame above.  The protection should be obvious

 

steel reinforced doors for strength and safety - you won't find this on VW speedsters

 

All Specialty Auto-sports, Inc. Speedsters, Cabriolets, and Coupes are built on a true Structural tube frame not a sub-frame that is misrepresented as a structural frame.