HOW TO CHOOSE AND OPTIMISE THE GEAR WHEEL/SPROCKET RATIO OF THE KART ON A TRACK YOU DON'T KNOW

A guide that accompanies beginners and non-beginners in choosing the optimal ratio to use on the track

Imagine you find yourself driving on a track that is new to you: what gear ratio should you use on your kart? Whether it's a Shifter, a Single Speed or a MINI, you'll probably start with a gear suggested by someone, or with the one you already use on a track with a similar layout. But finding the truly optimal ratio is a whole other story. The choice of the right gear wheel/sprocket combination ,a crucial element to make the most of the engine and obtain the best performance on the track, it is an art that can be learned and mastered independently, by analysing different parametres. To understand what these parametres are, we rely on Michele Zampieri, a highly experienced mechanic and co-owner of CKR, an Italian manufacturer of racing chassis. Here are, in his opinion, the fundamental elements for choosing the ratio:

type of engine used
circuit layout
track conditions (dry or wet)
grip level available
compound and tyre size
physical characteristics of the driver
driving style

The first two factors are used to identify an indicative starting ratio, useful for starting to race on an unknown track. The others, and many more, which we will explore in future articles, are essential to refine the choice and really optimise the ratio according to the specific conditions of the day on the track.

1 Why do we need to choose the gear ratio of the kart? What is it for?

The choice of gear ratio has as its main objective to make the most of the engine's performance, adapting it to the characteristics of the track and the specific conditions of use (such as, for example, the compound of the tyres available, the grip offered by the asphalt, etc.). In concrete terms, the optimal ratio is the one that allows the engine to be kept within the rpm range for as long as possible when torque and power reach their maximum values. Analysing a torque/power graph of a two-stroke endothermic engine, in fact, an ideal rpm range is identified in which the engine expresses its maximum capabilities both in terms of torque and power. The gear ratio must allow this range to be reached quickly and to be used for as long as possible, thus optimising both acceleration and top speeds.

2 What are the main differences in the way the ratio is defined when using a single-speed, MINI 60 cc, or Shifter engine?

In the case of single-speed and MINI 60 cc engines, the management of the gear ratio follows the same principle: a single fixed ratio must adapt to the entire track. The choice therefore represents a compromise between top speed and readiness out of bends. However, with a MINI engine, with lower power and torque than a single-speed with a higher displacement (whether it is a 100 cc or a 125 cc), this adjustment is more delicate. The MINI struggles more to sustain long gears, which is why it is essential to intervene precisely to keep it as much as possible in its optimal performance range. With a Shifter engine, on the other hand, the situation changes radically: you don't work on a single gear, but on the use of gear changes throughout the lap. The great advantage is the possibility of keeping the engine constantly in the ideal torque and power range, without having to cover a wide range as is the case with fixed-ratio engines. In this case, the goal is to make the most of a narrow area of the graph, the one in which the performance is maximum and most stable. For this to happen, it is essential that each gear is brought up to the correct speed before shifting.

3 How can you tell if the engine is working within its optimal range of use?

To understand if the engine is working in the optimal range of use, it is necessary to carefully observe the response of the kart and, when possible, analyse the data provided by the acquisition system. If the kart struggles to regain speed out of bends and the engine takes too long to rev up, it means that the ratio is too long: the engine works under torque and the thrust arrives late, often suddenly. This effect is also recognisable by the sound of the engine, which appears "empty" before returning to "full". In these cases, shortening the ratio helps you get into the useful range more quickly. Conversely, if the engine reaches torque speed quickly, but remains over-revving for a long time on the straights, it is likely that the ratio is too short. In this scenario, the engine stops pushing before the end of the straight, limiting top speed and progression. For more precise regulation, data collection is a key tool. By analysing the rpm graph and comparing it with the torque/power curves of a two-stroke engine, a check can be carried out as to whether the engine enters the peak thrust at the right point (typically out of bends) and if it remains within the ideal range as much as possible throughout the lap. In any case, the goal is to keep the engine "pulling", preventing it from working under torque in restarts or overrevving on the straights for too long.

It is the layout configuration that provides all the information you need to orient yourself in the choice of the ratio

4 What should a novice kart driver do who arrives on an unknown circuit without being able to ask anyone for advice and must decide the ratio to use?

When you find yourself facing an unknown kart track, without references or data available, the first thing to do to get the most out of your kart, both in terms of acceleration out of bends and top speed on the straights, is to carefully analyse the layout of the track. In fact, it is the layout configuration that provides all the information necessary to orient oneself in choosing the most suitable gear ratio. For me, I always focus on three fundamental elements:

The type of bend that leads into the main straight, because it directly affects the exit speed and therefore the progression on the straight;
the length of the main straight, the total number of straights and their extension, to understand how much the engine will be able to "stretch";
the number of restart points after "tight" bends, i.e. those sections where the engine drops to the lowest revs and must quickly return to the optimal range to relaunch the kart.

5 What is the importance of the curve that precedes the straight in the choice of the ratio and how should it be analysed?

The last bend before the straight is a key element in the choice of the gear ratio, since it determines the speed at which the kart enters the straight. A high exit speed will lead to maximum engine speed being reached quickly, even before the end of the straight, while a reduced exit speed limits acceleration potential and requires gear adjustment to keep the engine in optimal operating speed. In detail, if the last bend is tight (such as a 180° hairpin), the exit speed will be low. In this case, it can be assumed that the track prefers shorter ratios, to facilitate a faster restart and prevent the engine from working under torque. If, on the other hand, the last bend is fast and flowing (such as a 90° curve travelled at high speed), it can be assumed that the track favours longer ratios, taking advantage of the accumulated inertia to maximize the speed reached along the straight.

The length of the straight determines the time and space available to increase the engine revs and reach top speed

6 How does the analysis of the main straight, along with the number and length of straights on a track, affect the choice of gear ratio?

The length of the straight determines the time and space available to increase engine revs and reach top speed. If the straight is particularly long, for example 180 metres or more, you may think that it is a track where it is better to use longer ratios, taking advantage of the greater distance to develop high speeds without the engine reaching the limiter too soon. On the contrary, if the straight is short, for example 120 metres or less, it can be thought that the track prefers shorter ratios, so as to reach the maximum speed available more quickly, without wasting time with slow and progressive acceleration, to be carried out on a stretch of straight with a limited length. As far as the number and overall length of straights on the track is concerned:

If the circuit has several long straights, it will be natural to think that it is a track where it is better to adopt longer gears, so as to maintain a higher average speed throughout the lap;
if, on the other hand, the straights are few or particularly short, it can be thought that the track requires shorter ratios, to favour the readiness of the engine in accelerations and restarts.

7 What are restart points, why are they so important?

The restart points are those sections of the circuit where, after a very slow bend, the engine drops to low revs and must be able to quickly relaunch the kart. They are particularly important because, at the starting points, the ability of the engine to effectively return to its optimal torque range is measured: if the engine remains too "under torque" due to an unsuitable ratio, recovery will be slow and precious tenths will be lost. In practical terms, the number and distribution of restart points provide very useful information on the choice of the gear ratio:

If the route has many restart points (for example, more than three), it will be preferable to move towards shorter gears. A short ratio helps the engine to return to the useful torque zone more quickly, improving acceleration and ensuring more effective restarts.
if, on the other hand, there are few restart points, the engine will spend more time at medium-high revs and it will be possible to think about longer ratios, favouring top speed and fluidity in the flowing sections of the circuit.

8 Once all the main aspects have been analysed, how do you combine the information gathered to determine the gear ratio with which to start on the track?

To answer this question in a concrete and effective way, the best way, in my opinion, is to analyse three different circuits, each with specific layout characteristics. In particular, I will consider the Pista Azzurra in Jesolo, Italy, which is located about 50 km from Venice, which is an example of a track where a short gear ratio is used. Then the South Garda Karting in Lonato, about 130 km from Milan, which is a circuit where an intermediate transmission ratio is adopted. Finally, the Atlanta Motorsport Park in the United States, in the state of Georgia, which allows us to analyse a situation in which a very long gear ratio is employed. To make the comparison clearer, I will refer to the gear ratios I used in the first two circuits, based on the experience gained with the ROK GP 125 cc single-speed engine (Produced by Vortex, the Engine Division of OTK Kart Group, editor’s note) that I use in the ROK Cup one-brand championship, in which CKR takes part with its racing team. As for the Atalanta Motorsport Park, however, I will rely on the information provided to me directly by the track manager, who is also an official dealer of the CKR chassis and fully knows the type of gears used. This approach, in my opinion, is essential to offer an analysis based on concrete and comparable data.

9 How to orient yourself when you have to make adjustments?

Based on the track and your experience, you learn to recognize the right moment to intervene on the gear ratio and optimise its effectiveness. The advice is always to proceed in small steps: during the roughing phase, variations of 0.05 points can be made on the overall ratio (a value obtained by dividing the number of teeth of the gear wheel by the number of teeth of the sprocket, so as to obtain a numerical index that represents the actual gear ratio, editor’s note), for example, going from 1.65 to 1.60, then to 1.55 and so on. Once the correct work area has been identified, it is possible to further refine the ratio with even smaller variations, even in the order of thousandths of a point: a level of precision that only the most experienced drivers can perceive and fully exploit. In the case of a Shifter kart, for example, you can start from a 15/25 configuration (1.667 ratio) and lengthen slightly to an 18/29: a small step towards a longer transmission. If this solution is too extreme, there are several intermediate alternatives, such as 16/26, 14/23 or 17/28, which give slightly shorter ratios than the 18/29, but still longer than the initial configuration. A further option, more refined and suitable for more experienced drivers, is represented by the 18/30. While generating a similar ratio to that of the 15/25, the use of a larger gear wheel results in a greater flywheel effect. The result is that the engine reaches maximum speed slightly later, i.e. a few metres later, offering a different and more gradual progression response.

10 Are there tools that can help you find the Ideal ratio?

Yes, there are very useful tools to identify the ideal gear ratio. In the present day, the main one is undoubtedly data acquisition. Most devices on the market today offer fundamental channels that allow you to collect extremely useful information. Among the most valuable data to analyse is the histogram of the percentage of engine speed utilization on a given track. For example, if the histogram shows that the engine works mainly between 12,500 and 14,500 rpm, with a limiter at 15,000 rpm, it means that the engine remains in the upper part of the optimal range: in this case it may be advantageous to choose longer ratios to maximize top speed. If, on the other hand, the average rpm is concentrated between 11,000 and 12,000 rpm, therefore in a lower range, it will be advisable to adopt shorter ratios to facilitate acceleration and keep the engine in more efficient revs. A practical example of how the analysis of the average engine speeds can influence the choice of the ratio comes from the comparison between the South Garda Karting in Lonato del Garda (Italy, 120 km from Milan) and the Franciacorta Karting Track in Castrezzato (Italy, 80 km from Milan). At first glance, the two tracks look similar. It is no coincidence that, in 2020, at the opening of Franciacorta, many drivers adopted ratios almost identical to those used in Lonato, mounting gear wheels with just one or two fewer teeth. Over time, however, it has emerged that the differences between the two layouts are substantial. Lonato has three restart points concentrated in the last sector, while Franciacorta stands out for fast bends and much smoother sections. The histogram of the percentage of use of the engine revs confirmed these differences. At Lonato, the engine works more often at medium revs (about 11,000 rpm), while at Franciacorta it tends to remain more stable in the high end (about 13,300 rpm). The result is a significant difference in the gear ratio: Nowadays, for a single-speed kart, a 12/79 in Lonato and a 12/74 in Franciacorta are commonly used. A variation of five teeth, which would hardly be noticed without data analysis.

Over-revving can be a wake-up call to monitor, but it should be interpreted in the context of the track and the general behaviour of the kart

11 Can over-revving also be an element to use to determine the right ratio?

Over-revving is not a determining parameter to establish the ideal gear ratio, since there are tracks where the limiter is reached and others where this does not happen, without this necessarily indicating a ratio error. However, staying over-revved for too long, especially on the straights, is not positive, neither in terms of performance, or for the protection of the engine. For example, on a 180-metre straight where speeds reach 140 km/h, driving the last 40 metres with the engine constantly at the limiter is not optimal. As long as the engine remains over-revving for 2-3 seconds, the situation can be acceptable; however, beyond this threshold the effect becomes counterproductive, because you enter an area where the engine no longer pushes and the wear of the engine is also accelerated.

12 If you are undecided about which is the best path to take, whether to work on shorter or longer ratios, what should you do?

When it is not clear whether it is worth lengthening or shortening the ratio, the stopwatch is the most reliable judge. The best method is to analyse the split times sector by sector: if a longer ratio significantly improves the fast sector and involves only a slight loss in the guiding one, the overall balance will still be advantageous. For example, if you gain 6 tenths in the fast and lose only 3 in the mixed, the choice proves to be effective. It is important to always evaluate the effect of each variation in all sectors of the track, without relying only on the feeling of driving or the maximum value of the laps achieved.

As Angelo Parrilla (founder of DAP, historic manufacturer of chassis and engines for racing karts, editor’s note) recalled: "With karting the goal is always the same: always install the longest possible ratio”.

13 What common mistakes do beginners make when choosing the ratio?

The most common mistake beginners make when choosing the ratio is to give in to the temptation of the easiest solution, which is to shorten the gear to get more thrust out of bends. In reality, this is often a shortcut that hides other more relevant problems, such as a poor driving style, an unoptimized set-up or incorrect weight distribution. The priority should be to optimise driving and setup: only once a good overall balance has been achieved does it make sense to start working on the gear ratio with precision, perhaps varying by one or two teeth on the gear wheel or, in the finest cases, with half a tooth. The most dangerous mistake is to systematically make the gear ratio too short, a "conservative" choice that risks limiting the performance of the kart rather than improving it, especially in terms of travel speed and exploitation of the optimal engine range. As Angelo Parrilla (founder of DAP, historic manufacturer of chassis and engines for racing karts, editor’s note) recalled: "With karting the goal is always the same: always install the longest possible ratio”.