Question : What are the important things to watch for when building a chassis ?
At the fear of boring to tears the non builders and the builders, the hobby is a multifaceted experience that can be delved as deeply as you get an urge to, or just be kept to the lets buy plastic track and set it up when and where we can and play till we drop. None of which is wrong or right. Many start the plastic track way and then let it go from there or not. The old timers like myself had to start building our own if we wanted something that appealed to us. I was going to Community college in 1963 and found a small store in a small shopping center that was starting to carry slot car parts and was building small routed track layout in a small alcove shop right across the hall. This guy had nothing but parts for cars, some wheels, tires, axles, bearings and some bodies in clear vacuum form hanging on the wall. He had some music wire, and brass tubing and some pictures of cars from the LA area he came from. (We were basically out in the sticks) I used Ho gauge train motors I swiped out of locos I had in storage, mostly Pittmans or Lindsay's. That first Porsche 904 body in 1/24th scale did me in, I had to have it. A couple of the locals were attempting to get some cars built and they were pretty crude as most knew nothing about soldering. So I said I had done some were I had worked and would help out. In a couple weeks we had 6 guys with cars that ran pretty good and the racing started from there in the area. I learned what I needed for a car and helped others learn how to cut the metal, clean it and how to solder it together. From there it took off and I have been doing it that way for 39 years. Hard to stop now :) But my background was as a tool and die maker apprentice before the Army got me. Depends on your skills and ability to use them or learn to use them. It can be just learning to cut plastic in a straight line and how to glue it all together or soldering hunks of metal together. Many skills are needed you just have to decide how far you want to jump in. Some build great chassis and can't paint or detail a body worth a dang. Others paint and detail bodies that you would not want to put on the track, takes all kinds and most are happy with what they like to do. After a while, the competition part if it began to creep in, as more and more drivers became competitive. Winning became too important for a while till I decided that that was not fun anymore. (15 years later) And many now, like us old Pharts just like to tool around and threaten to beat each others creations to smithereens in a match race. Ain't gonna happen. Still having fun and making new friends almost every day online or in person, like at the convention in Las Vegas in April. Join in, the waters warm and fun. It's only as deep as you want it to be.
For the purpose of this article certain details of chassis construction are assumed to be obvious.
1.) The chassis must be flat and true . . . this cannot be emphasized enough ! 2.) The rear wheels and axles must be aligned to track straight and true. Any untrue tracking or twisting, will cause problems with the car swinging from side to side under acceleration or behaving erratically in corners. 3.) Front wheels should be just slightly, evenly spaced off the track surface so as to not cause drag on the straights. 4.) The front wheels should be free to spin if body weight transference causes them to touch in the corners.
Most of the art of slot car handling and tuning is trial and error. This article will attempt to explain some of the theory and reasoning covering the basics of tuning and setting up the cars. These assume we are beginning with an excessively rigid chassis/body package. Such a condition will limit a car's ability to flex during cornering. A full-size racing automobile relies upon an infinitely adjustable chassis that can be changed for differing or changing track conditions, types and surfaces. This is accomplished with changes to suspension, shocks, springs, torsion bars and tire pressures, etc. These same changes are difficult and, depending on the starting point, may be impossible to duplicate on a smaller scale such as applies to slot cars.
NOTE : Actually, if you are serious, you can do a lot of subtle tuning from track to track based on the stiffness of the main rail section, literally changing the diameter of the rails on a wire frame, along with the hardness of the tires AND the dimensions of the rear tread. Gary Gerding, the year he was national champ, thought it was particularly important to dial in the track in such a way as to use a different width/height/shortness tire ON EACH LANE. [rfr]
However, these same basic adjustments which apply to full-size cars are also applicable to slot cars - there are just different methods of achieving these changes and settings. Cornering forces on 1/32nd scale tracks are greater in comparison to the big cars and the more movement you can allow a slot car body/chassis to have, the more it can be adapted to varying conditions. A laser-cut or brass/wire chassis has built-in provisions in the design for adjusting most types of chassis movement to allow for the various forces at work on a slot car in the corners. One major theory relating to slot car handling is that the cars are really tripods based upon the placement of the rear drive axle and the guide shoe pin. This theory further states that the front wheels assist only in an outrigger fashion during hard cornering. The longer the effective distance between the centerline of the rear axle to the pivot point of the guide pin can be, the smoother the car will handle.
NOTE : This concept is a ‘sorta true'. There are two problems to consider here - 1 .) The longer the car, the SLOWER it is in the corners. It is slower because the rear swings out and covers a longer ARC through the corner than the guide does in the Slot. The longer the car, the longer the arc, the more time in the corner, the slower the lap time. This defines why NASCARS are usually slower than GTPs on a given track. Ironically, if built to correct scale, most modern F-1s and Sports cars would have 4.5" wheelbases in 1/24th scale, but everyone runs them at 4" for this reason. 2.) The second problem is FEEL. When I build for someone, I like to watch them drive. John Cukras doesn't count - he can drive anything ! One old pro I have built a lot of cars for likes hard nervous cars, because they FEEL fast. However, he cannot drive that kind of car. I have to be a tyrant with him and insist on the softer car. This is the car he can dial in and run perfect heats with. On the other hand, another pro I build for NEEDS a short nervous car. He cannot seem to stay focussed if the car is easy. And loses all the heats. If I give him the short nervous car, he concentrates beautifully and for 20 minutes will run with anyone. But he usually loses at the end. [rfr]
Due to its momentum, a slot car's tendency is to go straight upon entering a corner. If the chassis is too stiff, it will react stiffly and resist the change in direction. If you can soften the initial pressure of the wall of the slot against to the guide shoe blade during cornering and allow the car to react in a ‘softer' manner to the change in direction, it can be kept in place in the slot more efficiently. That is why certain chassis have what some drivers call rear steering. The theory of three point chassis construction states that the guide pin is attached to the rear wheels by a center section that should have some flex in relation to the rest of the chassis and allow a slight springing away from the wall of the slot by the guide blade. The rest of the chassis and body are then allowed to swing out slightly and are pulled back into line in a shock absorber type movement. Within that tripod center section there can also be a built- in torsion bar effect. Such a device, when successful, resists the twisting of the chassis in the corner, but also allows absorption of some of the cornering forces. Up until now this article has dealt with the placement of the guide shoe, the rear end, the center section and the rest of the chassis. The body, whether injected or vacuum-formed, must be dealt with separately. If rigidly mounted to the chassis, any body will restrict the movements of the chassis and hinder them. Most builders want to float the body to some small extent, either loosening the body mount screws a small amount, or building body mounts that float separately from the chassis movements and help isolate the body from the frame as much as possible and still keep it on the car. Bat pan floppies are hinged to allow some weight transference (lifting) of the body from one side of the chassis to the other in a corner. If weight is transferred to the outside wheel, it will give more traction on the wheel and the car is able to resist excessive sliding to the outside of the corner. At the front, this same weight transference puts some weight on the outside front wheel to cause it to act as an outrigger and so prevent the tipping out of the slot of the guide shoe. Weight transfer is as important to the handling of a slot car as it is to a full-size car for three reasons : 1.) at rest 2.) while in motion 3.) in a change of direction either fore and aft or side to side. Most slot cars have too much weight on their rear wheels because of construction limitations (inline or sidewinder drives). The best place for weight or the transfer of weight under braking is to the front onto the guide shoe, pickup brushes and (minimally) to the outside drive wheels. The more weight on the shoe in cornering, the harder it is for the guide to come out of the slot.
A ‘plumber' type chassis allows the transfer of weight to the front under braking and, with appropriate hinging, also allows the rear of the chassis/body to lift slightly and thereby cause even more weight to be transferred to the front. This keeps the guide shoe planted in the slot and provides a better power pick-up for braking. Some chassis are designed with built- in sliding motion of the body and chassis, to move for and aft in relation to the center section, guide and rear wheels. This has the effect, under braking, of transferring weight to the front and then, under acceleration, the weight can move back and help with traction to get up the car up to speed quicker. This movement should be minimal and only experimentation will determine what is correct for any given car/chassis on a given track. Most of the preceding particulars apply to scratch or custom-built cars. But the principles apply to the home set car as well. Bent or twisted injected frames are difficult to true and make run correctly. A miscast frame without true axle alignment can hurt as well. Freeing up the body as much as possible will help and maybe some small rubber washers between the body and the chassis mounts can isolate the vibrations that upset the cars handling. Use of traction magnets masks many of these problems, but does not eliminate them. A magnet car will run much better when the basic principles are applied to them as well.
[Larry Shepard - Notes by Rockland F. Russo]
Question : What are the basic principles that we must acknowledge when building a chassis ?
1) The 3 Point Principle 2) The Weight of a Frame 3) The Frequency and Cross-flex of a Frame.
The 3 Point (Triangle) Principle - Frames work on a three-point principal. The center of the guide pivot and the rear tires make up the ‘triangle'. The smaller the angle created at the guide pivot the deeper your car will go into a turn; the angle at the guide pivot is inversely related to the distance between the guide pivot and the rear axle -- the angle gets smaller as the distance increases. The ‘increased' distance enables your car, aside from going deeper into a turn, to be less responsive to your trigger, which is a welcomed trait on a small tight track. The ‘triangle' principle works best when the front wheels are able to ‘float' on the track surface. The ‘float' effect can be attained by incorporating an iso-fulcrum (hinged front axle tube), or an oversized axle hole in the front wheels, or an oversized axle tube.
The Weight of a Frame - The next issue in building a frame is its weight. Here we must look at the variables that determine the weight of a car. The variables are weight of body, weight and strength of motor, track size and tightness of turns, track surface texture and finally the material being used to make the frame. Body weight : The heavier the body the heavier the frame. The frame must counter the tilt factor of the body; as you increase the body weight you will need to also increase the frame weight.
Motor weight and strength : The heavier the motor the heavier the frame. Heavier motors will need more weight up front in order to maintain the proper front to back balance. Keeping the weight of the motor the same (same setup) but installing a more powerful armature will necessitate extra weight to be added to the frame. Track size and turn tightness : The larger the track the lighter the car. The increased running surface of a large track allows the body to generate and transfer more downforce to the frame; therefore your frame can be lighter. With the limited downforce attainable on a short tight track a heavier frame is needed. Smooth track surfaces offer more ‘bite' therefore a lighter frame can be used; bumpy track surfaces require a heavier frame to obtain better traction. Materials : The stronger the frame material the lighter the frame. Brass frames are heavier than steel frames since brass is a softer yet heavier material than steel. A thicker brass plate is needed in order to attain the strength of steel. The ‘new' frame material, 6160 aluminum, used in wing car frames, is lighter and as strong as steel. It has become the modern lightweight frame material.
The Frequency and Cross-flex of a Frame - A slot car frame is like a rolling tuning fork. The vibrations induced by the motor (starting, topping out and braking), out of round tires, bad bearings, and bumpy track surfaces, etc. can send the frame into chaos causing you to de-slot. To control these unwanted vibrations a frame must be built very stiff in the front to back direction; adding a reinforcing rail atop the center rail or pan generally does the trick. A frame should not flex when downward pressure is applied between its guide flag and rear end ! The stiffness raises the frequency point of a frame; the terminal velocity of a frame increases as the stiffness increases. Note: this does not apply to one-piece wing frames.
Cross-flex is the side-to-side flex of the rear end while keeping the front end stationary. The tightness of the turns on a track and the power of the motor determine the amount of cross-flex. Too much cross-flex will cause your car not to straighten out fast enough exiting a turn in order to accelerate up the straightaway. Too little cross-flex will cause your car to chatter and de-slot through the turns. It is best to start out with too much cross-flex and gradually stiffen the main rails by soldering piano wire on top of them. Start from the front of the rail and work your way back with longer pieces until the desired amount of flex is attained. This method is preferred to adding lead. [Pete Sardella]
Question : What is this WOMP I see referred to ?
In the late 1960's, an American slot car company by the name of "Riggen" (from its founder Al Riggen) devised and built a 1/32 scale car with a one-piece in-line stamped brass chassis. After they sold to Gayla Industries in 1972, the tooling for this chassis was used to produce aluminum cheaper versions (brass is a more expensive metal) using the seemingly inexhaustible stock of leftover Mabuchi FT16 end bell-drive motors from the 1960's "heydays". The Gayla bailed out of the slot stuff and by 1977, REH inherited most of it, while Parma got the chassis tooling. From this, they expanded the tool and created a new, low-cost RTR series called poetically "Womp-Womp" (don't even ask why). The Womps as they are familiarly known, are as basic as it gets, but as long as the tires are fresh, they do work decently. Bodies ranged from Porsche 911 in flared-fender varieties to the wildest and ugliest beast that the moldmakers-on-drugs of the late 1970's to the mid 1990's could devise. Parma sold thousands of them worldwide, and they are still in their catalogues today. The Parma Womps can without doubt be called the "anti-scale" slot car. Now I know what a WOMP is, but wouldn't know it if it bit me ! [Philippe]
Question : What is the "tripod" concept and why is this important in slot car chassis dynamics ?
This evening I was bolting together one of the cars I plan to take to Las Vegas. It is a Slot Classics Ferrari. I was testing a few modifications because as originally specified, with the Slot Classics wheels and tires and a Ninco NC1, it was embarrassingly slow. So I changed a few things : motor, gears, axle, wheels and tires and guide shoe - (to what, you ask - Does Macy's tell Gimbels ?) I put it on the track sans front axle - for a quick check - it was nearly undriveable - despite respectable speed, I had trouble equaling Ninco 120 time. I could see it rolling into the corners, tilting to the outside at the front, lifting the inside rear and then either tripping on the outside rear and flipping or lift the guide and sliding off. Of course, despite some significant grinding of the superstructure to remove excess resin, this is still a top-heavy car. I added some weight - low just in front of the motor - small improvement - not much faster but a little more predictable. Then I fitted the front axle and wheels - and now it is a very respectable little car - quick enough, I hope, to run with others of similar vintage. This may well be a special case. I am sure the C. O. G. is not far below the axle center line - but there may be others like it. FWIW [Alan Swartz]
Now you've gone and done it! We are now going to hear ALL the pros and cons of touching wheels vs. the "TRIPOD" theory... I'm with Al... the fastest way around my track is with the wheels on the track... All the wheels... that and some weight and you are on your way.. [J Briggs]
[Chris] ....... Well, shall I join in this one or just put some marshmallows on a stick and wait 'til it gets warm enough to toast them ....... O.K. why not - Assumption : A slot car is a 3 dimensional object operating in a 3 dimensional space. behavior in all 3 dimensions must be considered. To simplify matters, let us consider motion around 3 axes :
1.) The vertical axis : This is the axis that describes the angle of the car with respect to its direction of travel and is also the axis around which the behavior of a slot car differs most from the real thing. Only when the vertical axis of rotation passes through the guide pivot or pin is the car functional (If the car rotates around any other axis, the guide/pin is out of the slot and nothing else matters)
2.) The longitudinal axis : This axis passes through the car lengthwise and can be described as similar to the roll axis of a real car. To the extent that a slot car may have some flex that is similar to a suspension, the longitudinal axis may be somewhere else than passing through the point of contact of the front and rear tires on the outside of a turn.
3.) The lateral axis passes cross-wise through the car. Absent any real suspension, it probably can be simplified to 2 states : at the point of contact of the rear tires under acceleration and at a similar point at the front tires when decelerating.
In addition to these dynamic considerations, slot cars also require some level of pressure on the braids to assure adequate power flow. I would offer the following observations and propose an explanation of these observations based on the above : (Caveat : these observations and explanations apply to scale cars with relatively rigid chassis, normal width tires and length/width/height rations close to the real thing. I have no experience with thingies or wingies). Slot cars will run as triangles. The addition of front wheels can increase of decrease the performance of a given chassis depending on the implementation. Implementation options include: solid front axle vs. independently rotating wheels; degrees of unloading of the guide system; vertical movement of the front suspension; rotation of the front suspension about a vertical axis and/or steering; tire composition and profile. Considering the 4 factors:
1.) Rotation about the vertical axis - Repeat after me : "A slot car can only be permitted to oversteer, understeer is immediately and irreparably fatal". Oversteer and the control thereof is primarily a question of rear end adhesion via tire choice, weight and some effect of roll and weight transfer (getting tricky - invoking more than one parameter at a time) with some potential front end effect : to wit : a solid front axle with good contact and traction will tend to reduce oversteer by resisting rotation about the guide pivot or pin assuming that this pivot point is not directly under the axle. On the other hand, a solid axle will create a pivoting moment, proportional to the displacement of the axle from the guide pivot, which will tend to push the guide to one side of, and potentially out of, the slot. Independently rotating front wheels will minimize both effects. There is no right answer. The best balance of these effects will differ for each chassis.
2.) Rotation about the longitudinal axis : Upside down is slow ! - But a little "roll" can result in weight transfer to the outside rear wheel which can improve traction and limit sliding. The pure triangle case depends on rear track, friction and roll center - and the vertical distance from the roll center (effectively at the track surface) and the center of gravity to determine and resist roll. A fixed front suspension ads another element of roll resistances significantly reducing the tendency of the guide to tilt in the slot. A front axle with limited vertical movement (slots instead of holes) or one pivoted at the center to provide limited rotation about the longitudinal axis will provide additional stability once some initial roll has occurred. The choice between some weight transfer and maintaining the guide vertical is again car dependent. In my experience, cars with a rear track that is narrow compared to their length, ( The Vanwall and most 30's GP cars come to mind) benefit from some limited roll and I prefer the pivot to the slot. I would guess (emphasis on the 1st word) that wider, lower cars are less affected by these issues.
3.) Rotation about the lateral axis - Wheelies, anyone ? Under acceleration, it is likely that the only contribution of a front suspension is to the weight at the front end - wheels, lead, etc. - all the same. On deceleration, forward weight transfer in the absence of an effective front suspension will be carried entirely by the guide, tending to force it into the slot - no bad thing entering a corner - but - too much pressure on the guide will increase friction and perhaps promote rear end over rotation (spin out) - but I am comfortable suggesting that this is probably the last of the actions to be of concern. And finally the issue of power pick up - this is simple but difficult to measure. There is certainly a curve of decreasing electrical resistance with increasing braid pressure - and it is almost certainly dynamic varying both with current draw and speed of the siding contact. I can't even begin to suggest a way to measure it. There is also a curve of frictional resistance as a function of braid pressure - and somewhere there is an optimal point where the combination of resistance and friction yield the best speed - for a given car on a given track. The formatting of these observations into expressions and equations amenable to explicit solution is left as an exercise for the student. Marshmallows, anyone ? [EM]
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