A glance at a modern scratch-built 1/32nd scale sports car chassis shows that the motor is mounted across the chassis, at a slight angle to the rear axle. The reason for this crossways fitting is mainly to derive benefit from the gyroscopic action of the rotating armature to aid chassis handling. The gyroscopic force concerned is called precession, which is the reaction of a rotating mass at 90 degrees to the force applied.
With a traditional in-line design chassis, precession causes a weight transfer from one end of the car to the other, producing a tendency to de-slot when turning one way and excessive tail-slide turning the other.
With the motor sideways across the car, the weight transfer is also across the car and with the armature turning in the opposite direction to the wheels (due to the gears), the weight transfer is always toward the inside of the corner, which helps to keep the car stable and reduces tendency to tip.
In order to use smaller a diameter gear and rear tyres than a full sidewinder design will allow, the motor is set at a slight angle. The 'anglewinder,' as it is known, represents the best compromise between the conflicting requirements of theory and practicality.
Forward of the motor the chassis may be constructed along several different approaches. In general, these use piano-wire rails, several wire and tube hinges and wide brass side-pans. All this metal is as low as possible, and the ground clearance will also be set as low as possible. Some clubs restrict ground clearance to 1/16 inch minimum, but at others the front of the chassis in particular is dropped to be only a few thousandths clear of the track, in the constant search for greater stability and hence higher cornering speeds.
Detailed descriptions of different types of chassis can be confusing, especially as there is no proven best design and, indeed, many of the most successful chassis builders themselves find it difficult to analyse why a particular car handles better than another.
The main approach used today is a flexi-iso or flexi-board chassis design. The "flexi-board" design, pioneered by Ian Fisher in the late '70s or early '80s, is a good all-round design which works well on most tracks.
The "flexi-board" allows the front and back ends of the chassis to flex or twist independently by means of the central longitudinal pivot, called the "flexi hinge". This effectively absorbs some of the cornering forces, while the side rails dampen longitudinal twisting, helping to keep the guide blade vertical without being affected by what's happening at the rear, since the front end of the chassis remains relatively fixed.
The side rails contribute to the torsional stiffness of the chassis, controlling the degree to which the rear end can twist. Too much traction or 'grip' causes the car to want to tip over when cornering and adjusting the stiffness or degree of flex can be achieved by experimenting with different thicknesses of side rails. I've found 18 gauge piano wire works well.
The side rails, or "torsion bars" as they are sometimes referred to, do not contribute much to the beam stiffness of the chassis -- in fact if you press down the middle of the central spine, it doesnt take much force to make it touch the track. To overcome this to some degree, I extend the central "spine" all the way to the front, as pictured below:-
The "plumber" effect is best illustrated by the way that dragsters lift their front wheels off the ground when accelerating; the opposite torque effect - when decelerating - is not so obvious but it is there in a big way. The "flexi-iso" design is particularly good at transferring these forces.
Chassis can be built so that the side-pans have a small amount of additional sliding motion, allowing the body to move slightly forwards and backwards in relation to the centre section, guide and rear wheels. This also has the effect, under braking, of transferring weight to the front and, under acceleration, towards the back, aiding traction.
Side-pans or "bat-pans are hinged to aid the weight transfer of the body from one side of the chassis to the other, caused by cornering forces. If weight is transferred to the outside rear tyre while cornering, it will provide more traction and thereby reduce excessive sliding.
Modern Eurosport chassis are laser or EDM cut from spring steel, which does away with the need for piano wire and hinges to achieve dampening or flex. Current Eurosport chassis are also designed so that the motor-box/rear axle unit is free to move slightly from side to side. With this arrangement, often called "rear-end steering", the rear of the chassis and body is allowed to move out slightly, being pulled back into line as the car straightens up when accelerating out of the corner.
With brass and wire chassis, the gauge or thickness of the piano-wire I use for the main rails depends on how flexible or how stiff I want the chassis to be, which in turn depends on a combination of factors. It's really all about getting the power down so that the car can be driven close to the limit in a manageable way; i.e. having the "right" amount of grip or traction.
The combination of motor and tyres (often predetermined by the rules), the length of the chassis -- the critical measurement being from the centreline of the rear axle to the centre of the guide swivel -- and the intended weight of the car, plus of course the track design, are all factors to consider.
I generally use two single pieces of 16 gauge piano wire (0.047") main rails for chassis in classes that require "hard" bodies, such as injection-moulded plastic or resin bodies, and where no hinges are permitted. I've found that a slightly stiff chassis work best when there is a high mass, such as with a "hard" body. Of course, how far apart the rails are spaced also affects the amount of flex. For lightweight vacuum-formed bodies, I use 18 gauge wire (0.040") for the outer rails. In both cases, I use 16 gauge wire for the central pivot.
My preferred wire and brass design -- the design that I've found works well on most tracks, i.e. not necessarily the best design for any specific the track, but a good all rounder -- is the basic 25 year old "flexi-iso" chassis design, which dates back to the late 70s or early 80s and was pioneered by North London's Ian Fisher, such as the one pictured above.
Assuming that everything else is equal (armature, timing, magnets, etc.), spending some time fiddling with spring tension is the one area where tremendous gains can be achieved in motor performance.
The first step is to ensure that the brush hoods are properly aligned. The face of the brush should sit perfectly on the centre of the commutator (aligned with the armature shaft) or you will get uneven brush wear. Any variation in the alignment will also affect the timing of the armature. Use a brush alignment tool to do this, such as Slick-7's brush and bushing alignment tool (ref. S7163).
For 1/32nd scale low-power racing, I use Camen light tension double overhead springs (ref. #1620.20). Insulate both ends of the spring using Pro Slot Teflon Spring Insulation (ref. PS-611). This will ensure that the heat from the brush hoods and endbell plates is not transferred to the springs. The cooler you can keep the springs, the more consistent the tension.
I always use Pro Slot PS-900 Goldust Pro Motor Brushes and consider them the best available. Lightly polish all four sides of the brush by rubbing it against a Crocus cloth or just use a piece of paper, placed on a flat surface, and lightly rub the brush surfaces up and down it. This will ensure that the brushes slide smoothly inside the brush holders. Make sure that the brushes don't wobble in the brush holders. If you use a break-in tool on the face of the brush, make sure that it isn't too rough. Pro Slot makes a very nice diamond-coated brush radius tool (ref PS-654) which has an anodized knurled knob for easy use.
The next thing to do is to widen the slot in the brush where the short leg of the spring sits, so that you can install shunt wires. Shunt wires help to transfer the electrical current to the brushes directly. If you dont use the proper tool for this it can be tricky and you may end up with lots of broken brushes. The width of the slot should be able to accommodate the shunt wire and the insulted spring in such a way that the spring seats properly. Use a small round jeweller's diamond file or better yet, a special brush slotting tool like the one made by Magnehone. I use Pro Slot PS-613 Silver Shunt Wire. The silver content assures good conductivity.
Before installing the brushes, clean them off with contact cleaner. Any electrical contact cleaner will do. Slip the first brush into its hood, attach its spring and check that there is no binding and that the insulator on the short leg clears the brush hood. Now, attach the shunt wire. I bend a 'U' in the end of a length of shunt wire. Place the short end of this 'U' into the brush spring slot and latch the spring down onto it, making sure to get all the strand ends tucked neatly under the spring insulator.
The shunt, like the insulator, should cover the entire length of the brush slot for maximum contact but not interfere with the brush hood. Now route the shunt to the buss bar; using as little wire as possible but leaving enough slack for the brush to move during wear, also allowing clearance for spring removal and making all this look as neat as possible. Solder the buss bar end of the shunt in place. Do not use too much solder as it will flow into the shunt and restrict movement. Repeat this process for the other brush.
Bend the springs to 90 degrees, by placing a tight fitting tube through the coils and with your thumb against the short leg of the spring, lightly squeezing the long leg with your forefinger. Never adjust the angle of the short leg as this is set to correctly seat in the slot of the brush.
Now you need to bed in the brushes. Install the springs and fit lead-wires, lightly oil the bearings and place the motor in a glass of water. Slowly increase the power of the power supply to about 5-volts with the motor running under water. Let the motor run for about minute. The water will discolour from the carbon of the brushes. Remove and blow dry the motor. Don't forget to lightly re-oil the bearings.
Now adjust the spring tension. You really can't do this without a decent power supply, one which has a volt and ammeter. I use a Slick-7 Motor Analyser, but Camen, Koford, BSP and Wrightway all make good power supplies. They are a good investment and you need one anyway to power your tyre-truer. Run the motor at 3 to 5 volts for about twenty minutes. When you see uniform arcing under the trailing edge of the brushes it will indicate they are fully seated. You will notice that the amp draw will initially be high, then gradually come down as the brushes become fully seated and stabilize in the range of 1.5 to 2.5 amps.
Bear in mind that the brushes generate friction against the commutator. Logically, less friction should mean greater performance. This is true to a point but just the opposite can happen. Too little spring tension can increase arcing, which generates heat and heat reduces spring tension. A certain amount of spring tension must therefore be maintained to provide the optimum conductivity between the brush and the commutator. Since heat will reduce spring tension, you need to actually load the spring with a little more tension than actually needed, since we're setting the tension while the motor is cold.
One of the best tools you can use for measuring spring tension is a Fiddlestick, made by Sonic. It will allow you to take spring tension readings. Without these readings you will only be guessing. Experimentation is the only way to find the right tension. Reduce the tension until the motor starts getting hot, then raise it back up. Camen light springs usually have a little too much tension. I try to get my motors to run at around an amp to an amp and a half at 7 volts for low power tracks. But as mentioned, its a matter of experimentation.
Now install the motor in the chassis with gears and wheels. Check for drag or binds. The amp draw with the car ready to run should not be more than .5 amps higher than with no load. More than that and you need to check for binding by checking the axle and bearing alignment and getting the gear mesh perfect.
The process of reducing the out-of-balance forces that cause vibration in rotating machinery is called balancing. The reason for balancing an armature is to reduce excessive vibration, because vibration causes inefficiency. The unbalance is caused by the displacement of the armature's mass centre-line from its true axis, usually due to some mass eccentricity in the windings.
The process of balancing is the removal (or addition, rarely done these days) of weight to the armature, so that the effective mass centre-line approaches the true axis, i.e. the armature shaft.
The simplest form of balancing is static balancing, i.e. placing the armature on low friction bearings (such as done with razor blades back in the '60s) and allowing it to rotate and settle with the heaviest point falling to the bottom. Material is then removed from this point (or added at the top point) and the armature is gently rotated again until, when stopping, the new heavy point again falls to the bottom. This process is then repeated until no obvious heavy point exists.
Nowadays balancing is done using purpose-built dynamic balancing machines. The bearings that the armature shaft runs in are connected to sensors that detect the heavy point while the armature is being rotated.
Dynamic balancing is relatively cheap. I get my armatures balanced by Missile Products in the UK. For £4.75 you get the comm ultrasonically cleaned and diamond trued and the armature balanced.
Most, if not all, magnets used in motors suitable for slot cars are of the sintered ferrite, or ceramic - whichever you prefer - type. In the 1960's, cast Alnico magnets were mostly used. Cast magnets are manufactured by pouring a molten metal alloy into a mold and then further processing it through various heat-treat cycles. The resulting magnet has a dark gray exterior appearance. Sintered magnets are manufactured by compacting fine Alnico powder in a press, and then sintering the compacted powder into a solid magnet. Ferrite magnets are sintered permanent magnets, composed of Barium or Strontium Ferrite. This class of magnets, aside from good resistance to demagnetisation, has the advantage of low cost.
The very powerful "rare earth" or Cobalt types of magnets, as used in Eurosport strap motors, are a hybrid of casting and sintering but because of the many special steps involved in manufacturing they tend to be somewhat expensive. As with everything, you pay for performance!
Magnets are easily demagnetised if not handled with care. Special care should be taken to ensure that the magnets are not subjected to adverse repelling fields, since this could partially demagnetise the magnets. Magnetised magnets should be stored in such a way as to reduce the possibility of partial demagnetisation.
Also, bear in mind that magnetic flux density (field strength) is affected by many external forces. Heat is not as serious as one might expect. Typically, a ferrite magnet will lose 3-5% of its strength by being elevated from room temperature to 100 decrees Celsius, the boiling point of water. When it cools, the strength returns to very near normal, so this is a reversible effect. There is a point where a magnet will lose all of its strength, never to return until re-magnetised. This is called the "Curie temperature." For a ferrite magnet, this is somewhere around 450 degrees C, about 850 degrees Fahrenheit. Cobalt curies at about 800 degrees C.
The silent killer of magnets is called "contact demagnetisation." Every time you allow a magnet to stick to something, the magnet loses energy! The effect is not as serious on the poles as it is on the sides or ends. The worst possible way to package magnets is in a poly bag, stapled to a card where they are allowed to contact everything and stick together. Even sliding the magnets in the can will lose some flux density. By removing the armature, the flux path will be partially opened and the magnets will be weakened to a degree. Impact has an effect on the flux strength, but it really isn't significant.
If magnets are partially demagnetised, they may be easily re-magnetised, or zapped.
Can the magnetic strength be increased through zapping? With, for example, Mabuchi motor magnets, it is more an issue of "saturation". I have found that the magnets are mostly pretty much saturated, anyway. I've tried "zapping" them but there was no marked difference. When a magnet is saturated, it contains all of the magnetic field it can hold and will be as strong as it can and ever will be.
Pro Slot Euro 37000 rpm: 100 turns of 32 ga (.21mm wire) 0* timing .370" stack
Pro Slot Euro 47000 rpm: 80 turns of 31 ga (.22mm wire) 25* timing .370" stack
Pro Slot D3 American Made: 80 turns of 31 ga (.22mm wire) 25* timing .370" stack
Group 27: 38 turns of 27 ga .440" stack
Group 20: 38 turns of 27 ga .440" stack
S-16-C: 55 turns of 28 ga .490" stack
S-16-D: 60 turns of 28 ga .490" stack .523" dia
16-D: 70 turns of 30 ga .600" stack .513" dia
X-12: 50 turns of 29 ga .350" stack
Contender: 55 turns of 30 ga .440" stack
Hornet: 60 turns of 30 ga .350" stack
JK Falcon V: 75 turns of 31 ga 5* timing
Plafit Cheetah/TSRF: 95 turns of 31 ga
Plafit Fox: 130 turns of 33 ga
Rabbit, Carrera: 150 turns of 35 ga
Fly, Scalextric: 180 turns of 38 ga
Vacuum-formed bodies:
Personally, I prefer Lexan (polycarbonate) vacuum-formed bodies for racing, as they are extremely light yet withstand heavy crashes. I like 10 thou thick bodies, which are very flexible, but they do split at sharply curved areas if not reinforced. 20 thou thick Lexan bodies can probably withstand even higher impact than injection-moulded plastic bodies but are obviously more rigid than 10 thou thick bodies.
The problem with Lexan is that it is a difficult material to vac-form and detail is often lost. Some vacuum-formers are better than others though, the nicest and most detailed Lexan bodies that I've ever seen are those produced by Victor Ferguson of Truescale Products.
PETG (polyethylene terephtalate glycol) is easier to mould and produces bodies with good definition but is not nearly as impact-proof as Lexan. Polystyrene is also light but needs to be painted on the outside, as you would paint a plastic body.
In terms of the paint adhering to the body, as with any painting it is down to the proper preparation. I always wash the body in lukewarm water with dishwashing liquid soap (whether a vac or injection moulded body), rinse it clean and thoroughly dry it with a lint free cloth prior to painting.
I always paint clear bodies from the inside, for Lexan I use special paints made for polycarbonate such as Pactra's RC paint or Tamiya's polycarbonate paints. I use water-based acrylics on PETG bodies. It's best to use an airbrush and do a number of light passes. I've never had paint flaking. Using enamel paints on Lexan or PETG is a recipe for disaster as it peels off very easily, even with the best preparation. Details can be drawn onto the Lexan using a Rotring Isograph 'P' or similar pen with etching ink, from either the inside or the outside.
Polystyrene bodies can be treated in the same way as you would paint and clear-coat a plastic body.
Resin bodies:
Painting resin bodies for the first time can be daunting. The good thing is that if it all goes wrong you can easily strip the paint and start all over again.
With my first resin bodies, I airbrushed thinned enamel or lacquer paints, with mixed success. I've always fancied the convenience and vast colour range of the automotive aerosols available from most hardware stores but the early cellulose paints often reacted with plastic. A few years ago I was advised to try the aerosols and I have never seen any reason to change. If I cannot get the colour I want off the shelf, most suppliers will mix up a matching colour.
Preparation
The secret to painting any body is in the preparation. Most resin bodies have some imperfections to remove and there will often be air holes in some of the parts. On the body these usually occur along the base of the sills and front and rear aprons. There is also often some excess material to remove.
Firstly I open up all the blowholes to make them easy to see and fill. Use a soft automotive filler paste. It is important that the excess filler sands off before the resin. Sand down the filler using first a sanding block and coarse emery paper. Once the bulk shaping has been achieved, finish with a piece of wet 1200 grit emery paper.
The body will now be covered in sludge of dust and grease from fingers. Scrub the body with a mix of washing up liquid and cream cleaner and an old toothbrush. The soap is then washed off with the toothbrush under streaming clean tap water and the model carefully set aside to dry.
Priming
Try to use a primer made for plastic, these primers are intended for use on the plastic bumpers of modern cars. They seem to have an etch effect on resin but perhaps more importantly they are flexible and thus more able to resist any bending or crashing. You can get the primer in white, grey and red.
Mount the body onto something. I usually use a bent wire coat hanger and attach the body with a large blob of tacky putty - the type your daughter uses to stick posters all over the bedroom walls!
First spray a mist coat. Inspect to see if any awkward areas have been missed. The mist coat is so thin that it dries matt on contact and the colour of the resin can still be seen but it is enough to see if there are any flaws in preparation.
After about one minute a second coat is sprayed. This coat should be thick enough to almost obscure the panel lines and will have an initial gloss finish.
When the primer dries, it will leave a good even smooth coat with clear sharp panel lines. This should need no preparation before the topcoat is applied.
Painting
Once the primer has dried for 24 hours in a warm place, we are ready to apply the paint. Warm up the can and the body. Warm paint on warm resin is a good starting point towards a good finish. Moisture and dust are perhaps the greatest enemies to a good finish.
Spray in the same way as the primer, one mist coat then after one minute a couple of heavier coats. The fewer coats the better and also the fewer coats the less chance of contamination. Then put away in a warm dust free area to cure.
If it all goes wrong it is easy to strip the paint and start again. Automotive paint thinner will remove paint easily without damage to the resin. The removal should be done quickly with an old toothbrush. If the resin is left to soak some may be damaged by thinners but if done quickly there is very little chance of damage.
Finishing
I like to leave paint for about a week before I finish the body. I do not like clear coats. Wash the body again and after drying, polish it with Finesse-it, a 3M polish.
A gentle polish with Finesse-it will remove greasy finger marks before applying decals. When the decals are firmly set a further gentle polish with Finesse-it will remove watermarks. I then polish with a good liquid furniture polish; most seem to contain an anti-static agent which helps keep the dust away.
PLANS
Can Am Cars
In 1966 the Sports Car Club of America (SCCA) along with the CASC (Canadian Automobile Sports Club), founded the Can-Am series for Group 7 sports cars. With sponsorship from Johnson Wax, the Can-Am was the best paying racing series in the world.
The series only lasted nine seasons (1966-1974) but while they raced the cars were the pinnacle of motorsport technology. The Can-Am pushed the limits of engineering. Advances were made in aerodynamics, turbocharging, and the use of materials such as fiberglass, aluminium and titanium. That is until the fuel crisis, rule-makers and the increasing cost of technology ended it.
The Can-Am cars were some of the biggest and most powerful racing cars ever built.
In terms of wheels and tyres, from 1969 to 1974, most of the Can-Am cars used 15 wheels, anything from 9 (.281 7.1mm in 1/32nd scale) to 12 wide (.375 9.5mm) in the front; and from 12 to 17 wide (.531 - 13.48mm) at the rear. Tyres were generally 24 (.750 19mm) in diameter at the front; and 26 (.812 20.62mm) up to 26.8 at the rear. (Dimensions in brackets represent 1/32nd scale).
From the smallest to the biggest.
The 1970 AVS Shadow (or Shadow Mk. I) was a full 14 lower than the McLaren M8D. Front tyres were 17 (.531 13.48mm) in diameter and 11 wide; rear tyres were 19 (.593 15mm) in diameter and 17 wide, with a stubby 86 wheelbase.
Of the bigger cars, the Ferrari 612P was 88.9 wide, while the March 707 was the widest of them all, measuring 93 wide (2.9 73.66mm).
Can-Am Racing Cars 1966-1974, compiled by R.M. Clarke
Can-Am Cars 1966-1974, David McKinney
Can-Am, Pete Lyons
Can-Am Photo History, Pete Lyons
Model Cars magazine, various
Russell's Slot Cars About Me A Brief History of Slot Racing Cars I Have Built Chassis Design and Motors
My Slot Car Collection Various Scale Plans South African F1 Championship My Favourite Slot Car Links
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