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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