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'Ballast' and slotcars

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wixwacing
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Posts: 1871
Joined: Thu Jul 10, 2008 9:22 am

'Ballast' and slotcars

Post by wixwacing »

<br><br>When to add weight and other stories (Pull up a chair)

I have revisited some old ground here in other article but have gone into a bit more depth for those who want to finely hone their models. Although it is about balancing the model, it covers all the aspects that must be right first! Adding weight to a model to improve or reduce some of the handling characteristic is an age old slot car dilemma. It’s easy to go to your local tyre store and buy a handful of stick ons but this is a science now! Not an ad hoc sequence of events to try and improve a situation. There are several things to be considered before choosing which road to go down!



1. Is this for competitive racing


Or


2. Is this for home running?


If it's just to get models to stay on at home on a plastic track you can load your model up ‘til it does what you want but if it's for competitive racing then it is a bit deeper!!

Plastic track and magnets.

Most out of the box models will run well on plastic. Low, flat models like GT's, DTM's, F1's etc. will need very little if any weight. The odd sidewinder may deslot at the start of a straight and 5 grams centrally behind the front axle may well cure this. Taller models like Rally, saloons and classic models will need some ballast to help what may be a weak or badly situated magnet.

For board track racing nearly ALL models will need systematic ballasting. I try and keep a model below a total weight of 100 grams and aim at about 85. If you are class racing and your motor is restricted or your models are restricted it is essential you don't handicap yourself by using too much weight to solve a problem resulting in the loss of acceleration or speed or both. The model needs to be weighed on a set of digital kitchen scales and if the back is too heavy i.e. more than 60% of the model weight or if the front is too heavy (40%+) then adjustments will need to be made. My target model weight is about 85 – 90 grams for an ‘S’ can Mabuchi rated at 18,000 rpm at 12 volts. If I need to address a weight distribution imbalance and have to go outside these tolerances I need to review the model before adding extra weight.

Ballasting models is the LAST step in setting up a model! Before you get to this stage you should have carried out all the body and chassis tuning checks and adjustment. The following is a list of faults which are often countered by adding weight and some alternatives as I see them, to be checked before adding weight!

1. Sloppy guide. Guide not deep enough.

This will cause the model to understeer (go straight on) at the entrance or at the apex of a corner. As the guide tilts in its hole, the angle that the guide vertical blade is in tilts and allows inertia to cause the guide to slide out of the slot.

Solution.

Either line the guide hole to reduce the side slop. (A coating of superglue, dried and careful reaming out will do). Shim the guide down so that it is as deep as it will go before lifting the front wheels clear of the track. (This may well require the top of the guide hole to be trimmed or countersunk to allow the retaining taper to work) Or replace the guide with another deeper make. Ninco and Slot.it both do guides that will bottom out on conventional tracks. The guide hole may have to be sleeved or lined, too!


2. Hard / sharp outside edge to tyres.

This will contribute to the model raising up on its outside wheels and then understeering. The edge of the tyres will dig in on hard cornering as more of the model weight is transferred to them.

Solution.

Firstly, remove the sharp edge by running the model and using medium grit wet and dry paper, sand a small radius to the outside edges of all tyres, front and rear. Then place a sheet of medium (80 / 100 grit) wet and dry on the track and after place the model in the reverse direction on the paper, carefully grind a slight chamfer to the outside edges of the rear tyres for a total of no more than a quarter of the width. This chamfer only needs to be three or four degrees from level. It’s the way tyres will wear naturally on a model and you are only speeding up the process. Doing these things will minimise the possibility of the tyre edges digging in during hard cornering and jacking the model out of the slot.

3. Eccentric (out of round) wheels and tyres.

This will contribute to the model struggling (slipping and sliding) in bends which will critically effect the corner exit and subsequent straight line speed. I have heard some racers say that other racer’s models have faster motors and must be illegal when they have been left behind after exiting a bend alongside another quicker? car. The real truth is that the ‘quicker’ car, because of superior grip, partly attributable to true wheels and tyres, actually went round the corner faster and therefore was already a lot closer to maximum revs than the other guy’s model as they left the corner!! This allowed them to make use of full motor speed for 90% of the straight instead of the other guy’s 50%.

Solution.

Firstly remove the tyres and check for sprue flash on the wheel circumferences. This is the little piece of plastic left behind on some mouldings after they have been snapped from the sprue (tree). If this is prevalent remove it and refit the tyres. If the wheels are genuinely eccentric, check fronts, too, and consider putting the two best wheels to the back. Once the best wheels are on the rear, try running the model with the body removed and with the edge of a hobby knife carefully scrape (turn) the wheels until they are concentric. If they are too far out of round then if rules permit you may have to move on to alloy aftermarket wheels. If the tyres are out of shape, fit the best two to the rear and sand down as above. Firstly, getting both tyres flat before bevelling. Once again, if you sand too much from a tyre diameter you will be reducing the ‘rolling radius’ and therefore the distance covered per revolution by the wheel resulting in loss of straight line speed.

4. Rigid chassis / body

This will contribute to poor cornering also AND some straight line instability. Unless you are racing on a dead flat circuit (in your dreams!) you will need to allow the model to compensate for the unevenness of straights and corners especially. If a traction magnet is to work its best and if ballasting is to be more effective you need to reduce the weight which it is countering. Body weight, that is. In a level straight line and if your tyres are chosen well and doing their work, there is little work left for a traction magnet or ballast to do. In bends and corners, inertia starts to influence the behaviour of a model and it is how your model handles inertia that will determine its controllability in corners. On plastic tracks, even with the latest products, there is a degree of unevenness of the track surface. The model has to pass along this irregularity and depending on how well it can cope at speed will determine how quick you will get round it. The same applies to timber tracks and straights but to a lesser degree.

Solution

As the model rounds a bend, it will tend to ride up and down with track undulations. This is asking the magnet or (ballast) to keep the tyres 100% in contact with the track with obvious results. If the ballast or magnet has LESS work to do (ie countering less weight) it will do its job better. On decoupling the body from the equation, you have reduced the work rate by about 25 grams (depending on the model body weight.) and increased the efficiency of the ballast or magnet. The simplest solution is to back off the body mounting screws. How much varies from model to model. You need to establish at least half a millimetre up and down movement at the front and a minimum of say one millimetre at the rear. The body needs to float, uninhibited, on the screws but still stay secure. If you find the body is still ‘binding’ on the chassis and a free floating situation cannot be achieved, look firstly at the places where the body ‘pinches’ the chassis, impeding its movement. If necessary, file or scrape away the edges of the chassis and / or body until it moves freely. Next is to ensure the screw don’t stick against the sides of the holes through the chassis. Simplest solution is to either file or bore out the holes to the next size up or ‘til they are free. This will give you a body that has minimal but unimpeded travel on the chassis and it will allow the chassis to ride up and down on the undulations whilst the body has a degree of non contact, allowing the magnet / ballast to focus on the reduced weight of the chassis. A word of warning. Some models have loose or short screws and it may not be practical to slacken a screw and expect it to stay put! The solution is to either fit the next length size screw up or to put a drop of white PVA type glue into the screw holes to fix the screws in place. Taping holes or filling them with blue tack is not really a solution. Blue tack makes things messy and tape doesn’t stop the screw coming out, it just stops you losing it! Also some models seem to have more screws than they really need! These should be discarded and those little tapered screws which hold interior details in? They can go too! One other solution I am using more and more in my scratchbuilds is to make a small sleeve slightly longer than the thickness of the chassis and screw the screw through it. This allows the screw to be tightened as normal and the chassis is allowed to move on the sleeve with the head of the screw stopping its detachment.

5. Slack bearings

Rear axle bearings are another source of destabilisation. The natural torque reaction is to push an axle against one particular side of the bearing under drive load and then on the opposite side on over-run. In the long term this will cause the bearing to wear in an oval manner. Other models come from manufacture with slack bearings. Either the axle is loose in the bearing or the bearing is loose in the chassis. In a traction magnet situation this isn’t a great problem as the downforce generated will remove the running slack from most drive train components. In a non magnet / board track scenario the slackness can manifest itself as a hop or a judder under acceleration and some times on the over run. This will cause the model to loose valuable grip on corner exits, straight line acceleration and in the worst case the model will even hop from the slot. Front axles are not so critical and tyre truing should suffice.

Solution

The easiest solution is to fit aftermarket bearings that are snug on the axle and in the chassis. Failing this, the inner bearing surface needs to be built up. Brass bearings can have solder floated into them and the holes redrilled with the respective diameter drill. Another fix for brass and nylon bearings is to coat the inside of the bearings with cyno acrylate (super glue), applied with the stock end of a small diameter drill, allow it to cure overnight then once again run the relevant drill through the bearing. Super glue is surprisingly hard wearing and you may well see the axle wear before the glue! Both of these fixes require wheels to be removed and I for one would be reluctant to do this on some modern models. The manufacturing trend is towards fitting wheels and contrates onto heavily knurled sections of axle. These areas are greater in diameter than the plain axle and removal of a wheel may tear a degree of material from inside the hub and regluing may see a wheel refitted with bad run out (wobble). If a wheel has to come off, I recommend soaking it in a cup of very hot water for a minute, then holding with a cloth; pull the wheel from the axle with a slightly oscillating motion. This should soften the plastic a little and see the wheel sustain minimal damage. Always trial fit the wheel before regluing. There’s nothing worse than adding glue to an axle and pushing the wheel on only to have it stick half way!! Finally, some people advocate running superglue into the fitted axle bearing and letting it set, then freeing the grip later and lubricating??? I would need to do this before recommending it!! I have accidentally got glue in axle bushes after regluing errant wheels and believe me, even with oil in the bush, it was no fun!

6. Bent axles

Exactly what it says. Symptoms are juddering at all the vital moments with reduced corner speed and even deslotting the result.

Solution.

Simple really. It has to be replaced. There are no alternatives. Either the manufacturers recommended axle or an aftermarket product, or even a length of piano (music) wire. Trying to straighten an axle is not worth the time it takes for a half botched job. Ninco axles are prone to being bent from the factory!!

7. Loose motor

A loose motor is the prime cause of hop or judder in a model. On magnet tracks it is not a big issue but models can still lurch on hard acceleration putting the model slightly off line. A loose motor is also a potential ‘jump out’. One hard acceleration too many or a heavy deslot will see the motor leave its mounts with obvious results. On board track, some models will hop and judder on every acceleration opportunity. Once again allowing tyres to loose contact with the race surface with a consequent loss of corner stability and / or straight line speed. Motors must not move or rock in their mounted AT ALL! I cannot overstress this. I have even had motors that were hot glued in STILL JUDDER!

Solution

The motor has to be fixed rigidly in the chassis. Screw or glue, it’s not important. There aren’t many opportunities to screw motors into RTR models. So gluing is the next option. What glue? Mmmmmmmm? Hot glue has been a popular way but by no means infallible! If you are going to hot glue a motor in there are a couple of things to consider. Firstly the contact surfaces must be dust and grease free. Secondly, the glue gun has to be hot, as hot as you can get it! If your glue gun leaves a stringy residue and a convex bead of glue after gluing, it isn’t hot enough and the bond to the surface will be poor and probably let you down at a critical moment! The glue has to come from a gun in liquid form, about the consistency of refined honey. This will ensure it flows to all the contact area and do its job properly. This will be confirmed by a neat concave bead of glue along the sides of the motor. Secondly, it will only need a minimal amount to fix in place. In this age of detailed interiors, you may well not be able to refit a body if you have billowing mounds of glue holding everything to the chassis. Also, hot glue has a specific gravity and in applying too much glue you may well be causing a centre of gravity shift to your model, minimal though it may be. One other point to watch for is that some motors will have foil/paper wrappers around them. It has been known for the foil to separate in time. If you have any qualms, slice off the foil at the contact area and glue directly to the can. But hot glue isn’t the perfect solution!!! If you race long distances or repeated rounds a motor heats up. Some motors get VERY hot. Hot glue isn’t a curing type of glue and its state doesn’t change as it hardens, unlike a chemical solvent glue. Hot glue will soften, the hotter it gets!! And with motors that run very hot you will see the old juddering and hopping traits return as the motor moves in its soft hot glue cradle!! So be warned. Also with hot glue it can be a problem to remove a glued motor but fortunately I have found that in applying ‘Shellite’ (lighter fuel, pure hydrocarbon) the hot glue will release its grip of metal and plastic surfaces.

Alternatives can be as follows. A bead of superglue applied at the front and rear motor mounts has worked quite well for me. The glue is applied to the motor in situ and is put on with the tip of a modelling knife and allowed to cure. The method I am experimenting with at the moment is PVA glue. This comes in many brand names but is basically the same in all. It is a milky white glue commonly used for wood working. I remove and clean the motor and mounts and lightly coat both mounts with it. Pressing the motor back into place, I allow an overnight curing period. So far, so good. The plus with this is that if you want to remove the motor, gently bending the chassis whilst levering the motor will see it pop out, leaving you to peel off the old residue ready for another application.

8. Tyre choice

Tyres are almost the vital connection between your model and success. If you intend to race competitively you will need to choose the absolute best tyre for the model and track. Don’t waste time trying to get the OE tyres to work if they don’t want to! Racing a model on half baked tyres is a half baked model (unless rules stipulate, of course). There will be a thousand recommendations from friends and foe alike, they may be well intended but will lack that little input that your driving style will need. What to choose? Silicone, natural rubber. Compounds etc..... Track time will be the arbiter! Silicones are not god’s gift to all racers, Natural rubber has its place and compounds, too, will work some places and not others. But whatever you choose it will be the major factor in what your model is going to behave like. It is my view that silicone tyres are for the lazy. They work well, yes but they are hostile to some tracks and some tracks are hostile to them, Lazy tyre? What I mean is that they are the simplest short term solution as they may well run well out of the box but their peak of performance is not as high as some tyres that need pre race attention. Natural rubber tyres which we hardly see these days are great on shiny surfaces and with a bit of pre race fettling will stand up well against other types. Compound tyres are my first choice. These are ones that come with a variety of grip factors i.e. soft, medium, hard etc. They don’t like dusty tracks or cold weather but once both these things are countered, to me, they offer the best solution. Pick your tyres carefully for the track surface you are racing. Each medium will have its best tyre solution. Personally, once set, I will clean the tyres with hydrocarbon and then dress the tread area with a film of light oil. I use singer sewing machine oil and I treat my tyres as I come OFF the track from a race! This is allowed to soak in to the compound or if there is not enough time, I will rub off the surplus with a dry cloth. Others will spray a film of non silicone lubricant like WD40 or CRC on the tyres and rub it in, again drying the tyre afterwards. None of this is messy and tracks do not end up ‘Sticky’ as I have heard from people that obviously haven’t tried it! Once again tyre treatment may vary from track to track. I have found that smooth tracks like Scalextric sport and Carrera like compound tyres that have had the full treatment whereas the coarser tracks like Scaley classic, SCX and Ninco just need the hydrocarbon treatment and you can forego the oil.

9. Front end drag

This is again exactly what it says. The symptoms can be a model dramatically slowing in a corner or a reduction in straight line speed. The problem is caused firstly by a model’s front tyres. A lot of RTR model now come with a fixed front axle as compared to a floating type or separate stubs. This is because there are a lot of narrow models finding their way into the world. These are true scale replicas of highly desirable cars which the manufacturers can cash in on. Unfortunately their rolling chassis dimensions are not competition friendly. They are great for a casual saunter around your home circuit but in the heat of competition their lack of critical dimensions will let them down. On of the most critical is the axle width (sometimes incorrectly referred to as track). On a low flat model with an axle width around the 50 to 55 m.m. mark like a GT car, the axle width to car height ratio will be low, giving the model inherent stability in corners because it has a low C of G. But on some of the classic and Saloon / sedan models the axle width will be markedly smaller and the body height greater giving the model a high C of G. This model will be prone to roll over deslots. One way to reduce this is to give the model more stability by making it run on its front wheels and therefore creating an outrigger effect, unlike tripod models which run on the guide with wheels akimbo. Fine for low flat models but hands up all those who have driven Scalextric Saloons from the seventies and eighties!! Let Sierra Cosworths be a lesson to us all!! Any way, I digress. Models that sit on there front wheels are prone to tyre drag in medium to tight corners simply because they don’t have a way of differentiating wheel speeds in a corner. The outside wheel in a bend always wants to go faster than the inside and with a fixed front axle there has to be actions and results. In this case neither wheel gets to travel at track speed and one wheel tries to over and under drive the other. The result is a model that slows noticeably in corners. The best test of this is to drive a tight bend on a low throttle then either remove the front tyres or the axle completely and re drive the bend. Nearly all fixed axle models will show a marked improvement but on narrower models their corner stability will be reduced. Some models have ’stub’ axles at the front to eliminate this but they are also ‘tripod’ type models in that the tyres only lightly touch the track and the model weight is taken on the guide and rear wheels.

Another source of front end drag believe it or not is the guide!!! Place a model which runs on its guide on a piece of spare track and place a similar model which runs on its front wheels beside it. Lift one end of the track up ‘til one of the models rolls. It will invariably be the model which utilises its front wheels which moves first. This demonstrates that the model which runs on its guide has to overcome a greater drag force than the other. This compounds with speed. Lastly, a model on all fours is subject to overcoming tyre side slip in bends. Because very few models have steering, when they corner, the tyres are encouraged to slip sideways to a degree varying with the tightness of the corner. So if we are going to run models on ‘all fours’, what can we do about drag in corners.


Solutions

The problem is simpler that it first appears. If you want to retain the stability that running on all fours can give then you need to reduce the grip of the front tyres. Some manufacturers are now producing ‘low grip’ tyres especially for the fronts. Simply a case of popping them on. Some models have a built in solution which takes a little while to manifest itself. These are Fly and Cartrix who’s tyres appear to go rock hard and sweat oil for no reason. A phenomenon first experienced by me with MRRC models from the seventies. But the up side is that these tyres have minimal drag even if they are noisy and do clatter a bit round the track. Another solution is to grind the front tyres on a dummy axle in a drill and profile them convex so they run on a very narrow strip down the centre of the tyre. Thirdly, a practice which has been around for a long time is that of coating the tyre, usually with clear nail polish, to achieve the same low friction results. What ever you do, don’t forsake the stability and reduced friction that a model which runs on all fours has, you may regret it!

10. Adding weight!!!!

Lastly we come to the topic subject matter. I hope I haven’t bored you too much but believe me, if I didn’t think all this was important, you wouldn’t be reading it now. Adding weight is the final stage of body / chassis tuning. It can cure a multitude of sins and in itself create them. Once you have the model running as sweetly as you can by carrying out all the above measures, you may find there is no need to add weight!! But if you still find the need then tread cautiously. Ideally it should be about 5 to 15 grams at the most ( unless you are tuning a very light model, the Cartrix GP Legends weigh in at 45 grams!!) This then has to be placed according to model behaviour as it is this we are trying to cure. Under NO circumstances add weight behind the back axle. This may well improve a model somewhat but the advantages are outweighed by the increase in the ‘pendulum effect’ you have created and further testing will show half the weight added elsewhere may have double the advantages. Models which deslot on acceleration or understeer into a bend consistently can take a piece up front, normally behind the front axle, and five grams or so placed as a single piece on the chassis centreline should rectify this. Models which slew round or oversteer can have weight added anywhere from behind the front wheels to in front of the rears or both. Added equally to both sides. You will need to experiment to find the best spots. Dividing a weight up will reduce the overall weight required as two smaller bits can be placed over the trouble points whereas one large piece has to do both jobs and sit between the problem zones. If a model has the ideal weight split then adding to both sides at the centre of the model is a good start. Either side of a motor may well increase rear end weight and the pendulum effect. Mount the weights as far out to the sides as possible and don’t ‘huddle’ them along side the motor. This contributes to stability. Much like a tightrope walker carries a long pole for stability or a skater holds their arms out in a spin to slow up, outrigger weights reduce the ‘roll’ characteristics of a model and it takes more inertia to overcome the tipping moment. Having said that, once a model starts to tip and has overcome the effect of these weights it will then add to the tipping moment! But with all these mods we should no longer be driving ‘on the edge’. With a better handling model we can relax more and get on with the job of racing!! If a model has a high C of G then it’s time to rethink convention. Almost exclusively I see people putting weights inside a model ??!!??. If you have a C of G problem (i.e. Mk1 Cortina, Simca 1000, Trabant!!! etc.... then you want the weight as close to the track as possible!! This ISN”T inside the model!! Thinking outside the square (and outside the model) it is time to break with convention and add weight UNDER the chassis, you heard it right!! Weight placed under the model and in the right places can halve the weight required and minimise the effects of over weighted models. Plus there is usually a lot more usable space under a model. Almost exclusively all my recent scratchbuilds carry their ballast underneath. My RTR’s, too, get it there as a first option if I’m trying to counter a high C of G.

I realise I have created a dilemma here and a lot of people will be thinking Uh! Uh! Well, if you are then you need to think outside some more squares!! The most common and most unsuitable method of ballasting a slotcar is with stick on wheel balance weights!! Correct? Sorry to dishearten you but this is the lazy way of doing things and it limits your options. Stick on weights are too thick for under most models AND, because they rely on a sticky tape, are a liability!! In a heavy deslot they DO come loose! I continually see people who’s models start rattling half way through a race or slow noticeably emitting odd sounds and even stop!. The cause usually being a wheel balance weight or a similarly tape stuck ballast which has come adrift and is working itself around the inside of the model, shorting on its way and eventually jamming up some vital function. So please, use stick – ons to determine the area to be ballasted but move on to gluing the weight in when you are happy with the positions. Another popular way is the fishing sinker or ledger method, these are hammered flat on a suitable piece of concrete or metal and are then glued in. Infinitely better than the stick on if they are stuck in but still crude. Their shape dictates where you can put them rather than the model’s behaviour!!??

The best way, hard though it may be, is to track down some thin sheet lead like plumbers flashing. This is about 3 m.m. thick and will fit most places once neatly cut to shape. If hammered lightly it will also go thinner and make under model ballasting easier. I usually get the weight where I want it and trim it to fit the spot. I then cross hatch the contact side of the weight and the chassis. A reasonable amount of superglue is then applied to the weight and it is placed in position using tweezers. You have a few seconds to position the weight after which the weight is pressed and held in place for twenty seconds or so. When totally happy, the weight can be painted with something that matches the underside of the model. All going well and with attention being paid to detail, you should now have a model which is going to do the best it can.

Other competition tuning such as motors and magnets is another story and I have not touched on them here as it moves into a minority aspect of the hobby and is best left to those who get satisfaction from it. A lot of rules are still applicable but the cost can outweigh the end result

11. Still not winning!

The idea of all this is not necessarily to make someone a winner but to make a model more predictable and easier to race, which will contribute to winning, or driving better at least! With all these things in place it still doesn’t guarantee a person will dominate the results but hopefully it will see them move up the results list. The final buck stops with the racer. After fine tuning the model it may be necessary to fine tune your driving skills, braking points corner techniques, driving tactics. All these will need to be overhauled too, but I hope this lengthy piece will help a few more understand the mechanics of setting up a potential race winner and encourage them to look afresh at their hobby.<center>
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When I'm not racing slotcars,
I'm out in the back yard, burning food!!

When I win, it's because of my talent, not my car or my controller!
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