Tuning
Your Slot Car - Advice and Tips
by Mark Rampling - Dunton Slot Car Club
Many thanks for allowing us to use this on our site.
6.9 Rear Axle/Hubs/Tyres
It is VITAL to achieve true-running rear wheels for a car to be smooth to drive, and not to jump or judder when powering out of a corner. This is the most common failing I see in other cars running on the track. I cannot over-emphasise the importance of having the rear axle/hubs/tyre assembly run true. It gives much better rear end grip, much better braking (because of the better grip), smoothness, traction, stability and balance, and makes the car easier and more predictable to drive. A true-running rear axle assembly even allows the motor to rev faster, as there are less out-of-balance forces to overcome.
I have a long and quite elaborate process to achieve a true- running rear end, which works for me. You may choose to leave out some parts of the process.
Firstly, check that the rear axle bearings are a good fit on the axle. They should not be so tight that they are causing friction, but not be so loose that the axle can move inside the bearing. If a bearing is not a good fit, replace it.
Next remove the rear tyres, turn them inside out and remove any debris or flash from the tyres. Cut or file off any flash from the hubs.
Then run the motor at the slowest speed and look closely at the rotating rear hubs to see whether there is any trace of 'wobble'.
If you're lucky both hubs are running true, but usually one hub has a distinct 'wobble'. If a hub does 'wobble', pull it off and check the axle itself for 'wobble'. If the axle 'wobbles', it is bent and it should be replaced with a good one. If the axle is OK, then the hub is at fault. You then have to find a good replacement hub by trial and error. With some cars, the front hubs are the same size as the rear hubs, so you can try the front hubs to see whether you can find a better one. If you can't, raid your spare parts for equivalent hubs. I dislike removing a hub, as it is difficult to put it back true on the axle, or sometimes the hub can split. If the 'wobble' is minor, there are two things to try rather than remove the hub: (i) rotate the offending hub on the axle slightly, this could misalign cumulative 'wobbles' in the hub and the axle, and result in less overall 'wobble'; (ii) run the motor and try to true the hub with a file.
Now that both rear hubs are running true, the next step is to ensure that the hubs are secure on the rear axle. If it is possible to rotate one hub on the axle relatively easily, use a drop of Superglue to secure it to the axle. Again, I use an elastic band wrapped round the axle to prevent a bearing sliding down against the hub and the glue running into the bearing and jamming it up.
Next, check that the rear axle bearings cannot move relative to the chassis. In some chassis, a bearing can move slightly inside its mountings - especially SCX Formula Ones. I don't like any looseness in the rear axle assembly, as I believe it is detrimental to achieving a rear end that slides progressively. So, as a matter of course I glue the rear bearings to the chassis using Evostick. Evostick holds the bearings securely enough during normal use, but allows the rear axle to be removed by applying gentle pressure without wrecking the chassis (unlike Superglue - thanks for the tip Ken!). Ken has another tweak for SCX Formula Ones, which have bearings that are particularly loose on the rear axle. He deliberately misaligns the bearings (e.g. makes them both slope downwards in a shallow 'V') when he's gluing them into the chassis. The axle is now a much tighter fit in the bearings, so the car's handling is better, and when the bearings wear they can always be removed, rotated and glued back in again!
In the 'box-standard' classes, any Ninco model may run any Ninco tyre provided it is the same size as originally supplied on that axle. So for example hard ribbed rear tyres can be replaced with new soft slicks, which will have much more grip. We've even found that recent batches of slicks are softer than older slicks, not only from Ninco but also SCX and Fly, so there is obviously some experimenting going on by the manufacturers.
In the Modified class, any make or size of tyre may be used. My personal favourite is the ProSlot rear tyre, which is ultra-soft almost like the sponge tyres used on the more sophisticated BSCRA cars, but they don't grain on the DSCC track like the Ninco slicks do. The ProSlot tyres have a larger internal diameter, so they must be used with Fly or Ninco CLK large diameter rear hubs.
Refit the rear tyres, making sure that they sit properly on the hubs with no odd bulges. In the Modified class, if you are using soft tyres and a fast motor, it is possible that the tyres will grow at high revs, shift on the hub and end up out of true. I recommend gluing the tyres onto the hubs with Evostick to prevent this happening, they can still be peeled off again when the tyres are worn.
Now comes the truing process. I have spent many hours getting this right by trial and error. The results vary depending on tyre make (e.g. Ninco, SCX, Fly, Scalextric, ProSlot), tyre compound (e.g. standard, soft), and even tyre batch (nominally identical tyres bought at different times can be different!).
I use:
- My set-up block (described in Section 5)
- Coarse sandpaper
- Variable-voltage power supply (running 12V with Ninco NC1 & SCX motors, 9V with faster motors)
I prefer to have the body removed from the car, so that I can clearly see the rear tyres.
Holding the sandpaper down on the set-up block (otherwise the sandpaper will flick backwards), and blocking the front of car (otherwise it will fly forwards!), I lift the rear end of the car up.
With the power on and the motor running, I very gently lower the rear end until the tyres just start to touch the sandpaper. I'm supporting most of the rear end weight, and the motor speed should only drop very slightly. The objective is to very lightly skim off the peaks on the tyres. Ideally I should see small slivers of rubber appearing on the sandpaper; I know then that the combination of sandpaper coarseness, motor speed and downward pressure is cutting the rubber cleanly. As the tyres get more and more true, gradually increase the downward pressure. If the rear end starts to jump around, then too much downward pressure is being applied too soon. Be careful to apply the downward pressure evenly to both tyres, otherwise one tyre could end up smaller than the other! Check that roughly equal amounts of rubber slivers accumulate on the sandpaper behind each tyre. Swing the rear end slowly from side-to-side, to prevent localised peaks in the sandpaper cutting ridges in the tyre surface.
If the surface of the tyre develops tear marks, the sandpaper is too coarse and needs to be changed to a finer grade. If shiny black stripes develop around the tyre, the rubber has overheated and smeared over the tyre surface. Stop, clean the tyre surface with lighter fuel to remove the shiny stripe, select a lower motor speed, and start again. To prevent the rubber from overheating again, lower the tyres onto the sandpaper only for a few seconds at a time, lifting in between to allow the rubber to cool off.
You can tell out-of-roundness by: (i) the rear end vibrating when it is running on the sandpaper (though don't be fooled by slivers of rubber rolling between the sandpaper and the tyre surface); (ii) the motor speed oscillating when truing the tyres; (iii) the profile of the tyre tread being blurred when the motor is running. When looking at the stationary tyres, only some of the surface will have been skimmed away. Keep going until the tyres feel true (there is no vibration when cutting), sound true (the motor runs evenly when cutting) and look true (the tyre tread doesn't 'wobble'). Check that there is no 'wobble' at the slowest motor speed. The tread of the rear tyres should end up flat on the track across the entire width of the tyre (though this will change with use, see section 6.8). The tyre surface should have no patches and should be a uniform appearance all round.
A word of warning: cars with the relatively slow Ninco NC1 motor are sensitive to the diameter of the rear tyres. If you take too much off the tyres during truing, the overall gearing is affected and the car will be noticeably slower down the longer straights. Take the minimum off, even if it means that the tyres are still slightly out of true, as this is the better compromise for the DSCC track.
If you have a set of callipers or a micrometer, you can check that the rear tyres are the same diameter after truing. Different tyre sizes can cause the rear end to skew sideways under braking.
If the edge between the tread and the sidewall is sharp, this could cause the tyre to dig in and tip the car out of the slot. With the motor running, use a piece of sandpaper or a file to gently cut a small radius on the outside and inside edges of each tyre. This should allow the rear end to slide much more progressively.
When you're totally happy, use a brush to clean out all the rubber debris from the chassis, and lubricate the rear axle bearings with a drop of oil. Fit the body and do one last skim of the rear tyres using the full weight of the car complete with the body. Then with the power off, re-check on the set-up block that all wheels touch the track, and that each front wheel lifts by the same small amount.
With this elaborate process I still find that it takes 15-20 laps on the track to finally bed the tyres in and allow them to perform at their best. I've even heard that some racers have a special circular track with a Sandtex surface, and they run the car round and round to put a final 'cut' on the tyres!
A useful tweak: the tyres pick up dust and debris from the track surface, or they can grain and form little 'baubles' that stick to the tyre surface. The effect is to reduce rear end grip, increase braking distance, and even cause the car to judder. To clean the tyres between races, I now roll the rear tyres on the sticky side of some masking tape, and then double-check to make sure the 'baubles' have been removed (sometimes they need picking off). Try it, you'll be surprised at the improvement this makes!
Some clubs allow oil or other liquids to be applied to the surface of the rear tyres, to soften the rubber and give more grip. Another tweak is to apply varnish to the surface of the front tyres, to make a hard layer and reduce rolling resistance. Neither of these tweaks is allowed at DSCC.
6.10 Body
With fully tightened fixing screws, check to make sure that the body is sitting properly on the chassis, and that it doesn't catch on anything, trap the lead wires or tension the chassis.
A performance tip is to loosen the front and rear fixing screws by about ¾-1 turn, allowing the body to run loose and float independently of the chassis. This seems to make the car more stable and allows it to be driven harder into the corners. If a screw feels very loose and about to fall out, cover it with a piece of tape to prevent any trouble on the track, or fit a longer fixing screw (supplied by Ninco). If the body is very loose, it can vibrate against the chassis and make a very disconcerting noise, particularly under braking. I haven't found this to be detrimental to performance, though it is a little off-putting! The ideal seems to be to still have the body movement, but to have it damped a little by friction against the chassis - this gives the handling benefits without the noise!
It is possible that the chassis will rub or catch somewhere against the body and restrict its movement, even with loose fixing screws. For example the Ninco Ferrari F50 and Mercedes CLK have the exhaust detail moulded on the chassis but poking through tight-fitting holes in the body, thus preventing the rear of the body from running loose. For the 'box-standard' classes at DSCC, we allow specific chassis detail to be cut away on specific cars, please check the latest rules. In the Modified class, the chassis can be cut away on any car to ensure unrestricted movement.
Some of the Scalextric/Hornby models are fitted with working lights. Again, at DSCC it is permitted to remove them provided that the external lamp detail is retained.
Some people like to repaint the body in their own personal colour scheme. If you're going to do this, (i) don't choose a colour which is the same as a lane colour, because the lane sticker won't be so easily visible to the marshals, and (ii) keep the paint as thin as possible, as you will be adding weight high up and raising the centre of gravity. It does make a difference!
Another tweak for the Modified class is to replace the standard interior with a lightweight vacuum-formed alternative from Pendle Slot Racing. This has the effect of lowering the car's centre of gravity, and it makes a surprising improvement!
6.11 Weight
A car's handling can be fine-tuned by adding weight and subtly altering the weight distribution. The added weight improves cornering, reduces sensitivity to bumps and makes the car more robust in clashes on the track, but it worsens acceleration and braking.
As a rule-of-thumb if the front of the car shimmies down the straights or if the car tips too easily, add weight right behind the front wheels as far outboard as possible. If the rear end slides too much, add weight at the rear as far outboard as possible. Beware though of the pendulum effect (once all that weight at the back does break into a slide, it will be very difficult to stop!).
Some people use plasticene when experimenting with the position of additional weight, as it is easy to move or reshape. Once you have found the ideal position, it is better to use small pieces of lead sheet to keep the centre of gravity as low as possible. I usually use Evostick to glue the lead to the chassis, so that I can remove the lead again if necessary.
6.12 Controller
I do little tuning of the controller, other than to ensure that all electrical connections are clean and tight for minimum resistance. Adjust the pressure of the wiper button on the resistor to give a nice smooth action rather than a coarse one. With the Parma Plus controllers, also check that the brake stop and the full power stop are actually the end stops for the wiper movement. On one of my controllers a part of the casing prevented the wiper from getting to full power, until I removed the offending piece of plastic!