hijacked from another forum. some may have seen before, others maybe not..
Firstly, it’'s important to understand how slot car tyres work (the science bit) so I can explain the clever stuff that I’'ve discovered.
Most people (including me) begin by thinking that more rubber on the track equals more grip. It doesn’'t. You can see this with a simple experiment. Get yourself a school eraser. Place the eraser flat on the track, apply a small amount of pressure, and try to slide it. You’ll find it moves quite easily. Now put the eraser on its edge. Apply the same pressure and you’'ll find it's more difficult to move. This proves that less rubber on the track equals more grip.
If you’'re driving with a magnet, then this isn’'t much of a problem. But, if you'’re into serious competition slot car racing, it’'s very important. The reason why the edge of the eraser had more grip is to do with down pressure. Obviously, the more down pressure you apply, the more the eraser will grip the track. But, if you'’re driving without a magnet, the only down pressure comes from the weight of the car; and, as you know, lighter cars run faster. When I first started out, I used to put lead weight over the rear axle to get some down pressure on my tyres. It worked - sort of - but made the car slow and that weight down the back end caused the car's tail to kick out. The real secret is to get more grip from your tyres without adding any weight to the car. These days, I seldom add any weight to my cars and, providing I've correctly prepared my tyres, they grip the track beautifully.
The next thing we need to understand is how tyre size affects grip and performance. We’'ve already established that a narrow tyre will give more grip than a wide tyre. A narrow tyre will also result in less drag along the straights; I.e. Your car will go faster. That said, a very narrow tyre won’'t corner as well as a wider tyre. That'’s because the car ‘leans’ onto its outside tyre as it sweeps around the bends. This means that the car'’s weight is transferred onto the outside tyres. Remember that more weight (down pressure) equals better grip, so we can get better performance if we use wider tyres (operating under increased down pressure) for going around the bends.
When the slot car is properly set up - loosening body and motor pod screws, fitting a suspension, and all that stuff - you get considerable downforce on the two outside tyres, while the inside tyres, especially the inside rear one, will lift and get less grip. (Note: fitting a spring suspension will help reduce the lifting of the inside rear.) This means that ALL of the motor’'s power is now going through just one tyre. That’'s another reason why you want a wider tyre for cornering; otherwise the tyre has a tendency to skip-and-judder as it can’'t maintain sufficient grip to transfer all of your motor’'s power to the track; spread across two tyres and everything was fine, but with that inside thre lifting, you're going to need to give more grip to that outside tyre. (Note: this same symptom can be attributed to so-called "Ninco hop", where the motor is twisting in its mount. You can usually spot the difference, as what I'm talking about will only occur when cornering.)
If you’'re still with me, you now understand that you want a thin tyre for great grip and low drag when running flat out along the straights, and you want a wide tyre for maximum grip - stopping the rear end from sliding and giving good transfer of accelerating power - when cornering. It sounds like you’'re going to have to go for a compromise tyre width for optimal performance. But, if you’'re into serious racing - like the Slot.It Challenge - there are rules that specify what tyres you have to use. So, we need a way to get our tyres to change their characteristics depending upon whether we’'re accelerating down the straight, breaking hard before entering a corner, minimizing tail slide on entering the corner, and maximizing power put down as we accelerate out of the corner. This is the secret of tyre preparation and conditioning. Now we’'ve discussed the science bit, we'’re almost ready to look at the ‘'how to'’.
One last thing before we get down to business. Tyre diameter also affects gearing. The larger the diameter, the higher the gearing effect. The smaller the tyre diameter, the lower the gearing effect. So you want tall tyres for maximum speed along the straights, but you need a smaller diameter for low gear grunt when accelerating up to maximum speed. Again, we have a conflict: and, again, correct tyre preparation (“conditioning”) will resolve the problem.
MOST SERIOUS RACERS in Slot.It Challenge type races use one size of wheel and tyre. As it’'s a ‘Slot.It’ race, they use that manufacturer’'s 15x10 hubs with 19x10 rubber tyres. This choice is supposed to give an ideal diameter for correct gearing (don't ask me: that's what the experts told me). Ventilated ‘Air Hubs’ are the best choice (as we’'ll see later). Tyre compound depends upon the type of track you'’re running on. Also, to answer the perennial question of “"which tyres are the correct size for my car/hubs?";” (something that drove me crazy when I first started racing); with the exception of F1 tyres, ALL Slot.It tyres will fit all Slot.It hubs. What you need to look at is the overall diameter (the first number) and the tyre width (the second number). So, 19x10 tyres fitted to 15x10 hubs will give an overall diameter of 19 mm and a width of 10 mm. You can mostly ignore the 15 in the hub size. Someone else can explain that bit...
GLUING & TRUING isn’'t just about ensuring the roundness of the tyre and that it doesn’'t come off the hub. You glue just the edges of the tyre. It’'s crucial that you never get glue on the central area of the tyre, just the edges. (We'’ll see why later.) As for truing. You can use a bit of abrasive paper under the rear tyres, but you'’ll get much better results if you use a professional tyre truer. This is because the abrasive action of truing the tyres doesn’'t just ensure that they’'re round with a flat contact surface. Of more importance is how the abrasive ‘roughs up’ the outer surface of the tyre tread; what I call 'scuffing' or breaking up of the tread surface.
If you look at a tyre that has been prepared on a professional tyre truer, you’'ll see that the outer surface of the rubber has a ‘'scuffed'’ appearance, where the relatively soft compound is difficult to abrade, it breaks up rather than coming out smoothly sanded. This is precisely what you want. (Some people have suggested ways to get a smooth finish on the tyres after truing. Ignore them. That slightly scuffed surface is of huge advantage when we use oil to ‘condition’ the tyres.) I won’'t go into truing in detail - there are already many excellent threads on this subject elsewhere on Slotforum. Just be aware that tyre truing must be done slowly, else heat from the abrading action will ‘'cook'’ the outer surface of the rubber, making it hard, and impossible to complete the essential oil ‘conditioning’ process.
OIL CONDITIONING is the real magic! You might think this is simply a way to soften the rubber and give your tyres more grip on the track. In fact, we are seeking to soften the outer surface of our tyres, but that’'s only the beginning of the story. Remember that lengthy preamble about tyre widths and tyre diameters? This is why it'’s important you understood the science.
When you apply oil to the tread of your tyres it'’ll damage, or ‘'degrade'’, the rubber. The rubber will both soften and expand. (Try dropping a new tyre into oil and leaving it a couple of days. It’'ll come out softer. It’'ll also come out about 5% bigger!) That'’s why it’s important we only apply oil to the outer surface of the tread. What we want is to cause just the outer surface of the tread to soften and expand. This is where the ‘scuffing’ of the rubber, created by professional tyre truers, helps. The roughened tread has more surface area (the gaps the scuffing created in the slightly broken up tyre'’s tread) which will absorb that rubber-degrading oil much quicker, as well as causing expansion upwards/outwards, the cracks in the scuffed tread will expand sideways. Thereby we get a 'mushrooming' effect on the outside tread of our tyres. It also reduces the risk of oil permeating too deep into the tyre. You only want so soften and mushroom that outer surface of the tread.
Apply the oil in very small amounts. Just a smear. Do NOT attempt to speed things up by applying too much oil. It will soak too deep into the tyre with disastrous results. The cracks in that scuffed roughened tread will hold enough oil for our needs. So wipe away any excess. I usually apply one smearing of oil every day, and no more than every 12 hours. The degrading action of the oil takes at least one week before you’'ll see the desired result. (In fact, Slot.It Challenge enthusiasts usually have several sets of glued and trued wheels and tyres on the go. As we always use the same size hubs and tyres, this means we can put our best conditioned tyres on the car whenever we race.)
You’'ll know when your tyres are ready because you'’ll see a slight bulging or mushrooming (termed ‘blistering’) of the rubber around the centre of the tread. This blistering, caused by the rubber expanding across the tread, is that little bit of magic that we’'re after! If you’'re using normal hubs, as versus those with holes drilled through them, you can help the formation of the blistering by injecting air under the centre of the tyre using a hypodermic syringe. In part, this is why we didn’'t permit any glue to get onto the central ridge of the hubs; we need that space for injecting air. We also need the rim of the tyres to be airtight enough to hold the injected air while the oil is doing its stuff. Don’'t inject too much air, we’re only after a swelling of about 0.5mm, and go right around the tyre to ensure even inflation. (Note: over time, the air will leak out of he tyre, but doesn’'t need replacing, as the action of the oil will already have done its job in permanently reshaping the tread.)
Ventilated hubs and Air Hubs work slightly differently. Those ventilation holes, or the air inside the Air Hub, facilitate easier expansion of the tyre (its overall diameter) due to centrifugal force as the wheels spin faster. Also, the Air Hubs, where the tyre sits on a pair of thin disc supports without a solid centre, have the additional advantage that the softened ‘bulged’ part of the tyre has nothing underneath the bulge. At speed, the tyre takes on a narrower more bulged shape entirely supported by centrifugal force. But, as the car decelerates, the centrifugal bulging reduces AND there'’s a hollow space into which the tread can drop; so that the tread becomes completely flat, giving that wider flat tread we want for breaking, cornering, and transfer of power under acceleration. (Note: if you examine a correctly conditioned tyre that's fitted to an Air Hub, you won't see as pronounced a mushroomed bulging as you'd see on a standard hub, where the tyre is supported from underneath. Instead, the test of when the tyre is ready is to look for a very slight bulge combined with the tyre being very soft when you press it down into the gap of the Air Hub.)
If you’'re using standard or hubs with just ventilation holes, there is no air space for the bulged tyre centre to drop into, so although the conditioned tyres will change their shape according to how fast the wheels are spinning, your tyre's treads will never become completely flat when the car slows and leans all of its weight onto the outside wheels during cornering. This is why the Ventilated Air Hub is your best choice.
Finally, it is possible to over-condition your tyres. With standard hubs you only want a maximum mushrooming of about 0.5 mm. With Air Hubs, it's the softness that matters: this is easiest tested by having an untreated tyre to compare with the one you're conditioning. You'll know when you've got it right, because your lap times will drop incredibly and the car's handling will be superb.
So, there you have it. Tyres that become wider or thinner, and taller or shorter, always giving maximum grip and minimal drag, with a self-adjusting 'virtual gearing' effect, according to your car’'s needs. It took me a long time and a lot of frustrating mistakes before I discovered all this, so I hope it will be useful in helping others to understand what they need to get to grips (sorry, couldn’'t resist) with the dark art of slot car tyre technology. Clever stuff: I wish I'’d invented it!