We can define transmission the part responsible, thanks to a series of gears and some tools, to move the kart in a straight direction, even if the engine movement is rotatory.

First of all, we have to notice the engine push is done only by the right side and the kart is lacking in differential gear. Anyhow, none of these two factors has an impact: in fact the kart doesn’t run obliquely or better it runs so slightly obliquely that it is not noticed, also because straight stretches are short and you have to correct continuously the direction, so you cannot realize the kart is going more towards one side… The differential gear lacking is not a problem because this means doesn’t need to follow a straight and “tidy” trajectory. Continuous adjustments and countersteerings let you realize how a differential gear would be completely useless.

We can now start talking about more interesting things; transmission is made by

Chain: it connects pinion and sprocket and let them turn together

Pinion: small gear

Sprocket: wheel with teeth

Now we can study details of each component.

CHAIN

Here you can see my kart’s chain. The price can vary between 20 and 35 euro (maybe you can find something around 10 euro, but I’m not sure), based on quality.

In particular, this one costs around 30 euro and they told me it is better than others, and professional pilots use this type of chain; in fact it has the peculiarity of being pre-lubricated. Its brand is D.I.D.

Kart chain is generally different than chains of other means of transport (except for regular bicycle ones): even though it is submitted to big tractions, it is quite small and it has a “lean” structure, and that is the reason why it’s very often broken. This fragility is mainly due to 2 reasons: first one, since it’s small, it provokes quite less friction and so the acceleration will be bigger; second one, in case of wrong setting of the corona - pinion alignment, it doesn’t damage these 2 parts (as worst thing, you could have a corona wear), or even worse, the shaft. It’s better a chain breaking than worse damages. Anyhow the chain doesn’t need particular maintenance, only 2 attention points have to be followed.

The first one is the lubrication done by a special kart greasy-spray (the cost is lower than 10 euro per package and it lasts a lot); without this lubrication the chain is going to wear sprocket teeth, to fall down and to break. In this web site you can buy lubricants for FI.MO chains directly on line!

The second attention point to be followed is that sproket and pinion have to be always in line: on the other hand the chain will wear sprocket’s teeth on its sides (and not too much it will wear pinion’s ones, because it’s smaller, compact, and harder) and in this case it’s going to fall down (it’s not likely it is going to break). In order to check the alignment, in addition to your sight, you can simply use a ruler which has to be put between sprocket and pinion: if everything is in line with the chain (parallel), it means it’s ok, otherwise you have to move the sprocket on the shaft (that’s because you cannot move the pinion, already put on the engine). To do that action, look at the next section, concerning the sprocket. Chain length is not fixed, everyone can choose as wanted, based on chassis model; theoretically shorter the chain is and better is….theoretically… actually you can choose what you prefer.

The last attention point is about when the chain has to be tightened: it’s very difficult to say just by words, but also by facts, because each driver has probably a different opinion about it. There are people who have the chain very slack (and they don’t have particular problems… but I really don’t know how it can work...), and people who have it very hard and tightened (I’m one out of these people). Now, in the first case the chain will going to “jump”, with the risk of falling down, or even worse it could damage the pinion and the sprocket (and maybe it could damage the carburetor, if you have it a 100-valve with a carburetor close to the pinion). If you keep your chain very hard and tightened, the worst problem will be the excessive sprocket consumption and the risk of bowing the engine shaft. Based on how much the chain is tightened, the kart should have a different behaviour; if the chain is very tightened, when you start accelerating the sprint should be fast and immediate, but it should also be bigger the friction between corona and chain-pinion; if the chain is very slack the sprint should be slightly less fast and sudden, but the acceleration shouldn’t be penalized by the friction. So it’s completely up to you finding the right setting: in my opinion you should tighten the chain as much as you can and then, using a screwdriver (with the engine not yet fixed) you have to push the chain down, ‘til you gain some clearance, but not too much. The clearance should be adjusted when the engine really starts working, but it’s better setting everything from the early beginning.

The chain doesn’t have a defined wearing time. If you want to be sure your chain will last until the end (during training you can use an old chain; it doesn’t matter if it breaks down- even if it shouldn’t break down when you are strongly accelerating or your engine will be too much revved up – but during races you have to be sure your chain will last until the end) check all small cylinder which keep links together and see if they are in good conditions and they are not deeply scratched or ruined. You don’t need to check each of them, it’s enough checking some of them and you will have a kind of picture concerning the chain condition. It is said the wearing time is about 4-5 uses, unless anything unexpected happens.

SPROCKET

In the picture you can see a 85-tooth sprocket, which together with a 10-pinion gives a ratio of medium lenght. Its price can vary between 13 and 20 euro. In this picture, the corona has to be thrown away, in fact you can notice all teeth are almost gone through and bended in the rotation direction (clockwise). The breaking cause is the lack of lubricant spray. A direct consequence of this wearing, is the falling down and the breaking of the chain. Never try to use again sprockets in these conditions. Sprocket is one of the factors (together with the pinion) affecting speed and acceleration (on the same engine conditions), since it let the ratio vary; the bigger the corona is, the shorter the ratio is (or bigger, if we talk about pinion/corona ratio). So between ratio and sprocket dimension there’s an indirect proportion. Now, based on track features, you should choose the right sprocket; it means a sprocket which can bring you to the straight stretch limit (slightly before if you have a great and powerful engine). The 100 – sprockets can generally have from 77 teeth to 94 teeth. This point is not so important, but you should notice something: if you have a rotating valve engine, maybe old (as I have), notice the chain is very close to the carburetor (since the pinion is close to the carburetor) and it could happen (as it happened to me) it goes bumping against the carburetor and it could be ruined. In order to avoid this problem, you could use smaller pinion and sprocket, and so keeping the same ratio between them. This is simply an advice.

Now let’s compose a formula, to give us some knowledge about kart speed (this formula is useful for 100-kart; I don’t know how to calculate it if you have a 125, since they have gears):

S = n / N x C x r x 0,06

where S is speed; n is pinion’s teeth, N sprocket’s teeth, C is the back wheel circumference, r is the number of engine revolutions… is that ok? If there’s something weird about the 0,06… don’t worry: it’s important to calculate the speed in km/h, on the other hand you will get it in cm/min.

Coming back to more practical things, in order to align sprocket and pinion, you have to slacken the screw (generally it’s an allen one) put in the place where the corona is linked; once slackened, you have to move it to the left or to the right side (it doesn’t matter the direction…..; you can use a hammer, but not a hard iron hammer; it’s better if you use a copper hammer or a wood chisel) and then screw it again. Be careful that the small key has to block the support on the shaft.

Attention: if the sprocket is ruined only in some parts (even if only few teeth), it could cause an ovalization of the sprocket; so the chain is sometimes more tightened than others. You have to replace the corona, on the other hand you could break the chain too (and if it’s tightened to face a corona’s defect, you risk to twist the engine shaft).

To replace the sprocket you have to remove the wheel and then the screws which keep the corona connected to the support; at that point you have to do the same operation once again, but starting from the end. It’s not necessary screwing too much the screws, or better in order not to twist it, screw and tighten in a symmetrical way.

Also the sprocket doesn’t have a precise lasting time; it depends on how you maintain it. It has to be replaced when teeth are too flat, or they are too tight or when some teeth break down. When a sprocket is old, it has to be replaced, because on the other way it could cause damages to the chain.

PINION

In the picture you can see a simple pinion with 10 teeth. Remember you have to strongly (very strongly) tighten the nut.

Concerning the pinion we don’t have too many things to say; the price is a standard price, about 10 euro in all Kc, it’s lower if you buy it for example in a factory. For 100 – engine, the pinion can vary from a 9-tooth dimension to a 11-tooth one. The most popular one is the 10 one, the 9 one is not so common.

The pinion is very difficult to be broken down, but it’s possible when submitted to big bumps or wrenches, which are reflected by the chain. When it is damaged (and it’s very visible), you need to replace it, but this operation is not so easy to perform. If you own a puller, everything is easier; first of all you have to unscrew the conical nut, keeping still the engine (on the other hand it would rotate), from the left side by a Slip-joint pliers (for example) and then you have to extract it, using a puller. If you have a suitable tool you can keep the piston still and then unscrew the conical nut. After removing the pinion, it’s fundamental checking the small key: if it’s in bad conditions it has to be replaced (the cost is about 1 euro). I advice you NOT to underestimate this cheap piece of iron: since it’s so cheap it’s better replacing it every time you disassemble the pinion (rarely). And please don’t try to do that by yourself, you cannot imagine what kind of damage you could cause…(it could lead to scrap the final shaft part, and so the total shaft and sometimes even the engine…).

About the ratio, it works exactly the opposite than the sprocket ratio.

Since it’s very difficult having a pinion weared out, you have to replace it when it’s broken. It’s very rare using the pinion for so much time that it is weared and consumed; during this time it’s more likely some teeth will be broken (due to chain wrench).