6 QUIZZES ON THE OPTIMAL RATIO OF YOUR KART

When it comes to choosing the right ratio, one more or less gear wheel tooth can really make a difference in terms of performance. From engine performance, to wear of the pinion, as well as the affect of the track’s grip on the choices to prepare a kart, thanks to Andrea Bossaglia, an IAME engineer, we have debunked some "paddock chatter" and deepened our knowledge in a field as articulated and fundamental as that of kart ratios.

1. THERE IS NO DIFFERENCE IN ENGINE PERFORMANCE USING TWO EQUAL RATIOS BUT WITH DIFFERENT GEAR WHEELS AND SPROCKETS

1. False: With the same ratio, it is the mass of the rotating parts (pinion, gear wheel and chain) that is different and affects engine performance. Let's compare, for example, the "pinion 10/gear wheel 89" and "pinion 9/gear wheel 80” pairs (where the numbers represent the number of teeth). With the same substantial ratio, the sprocket/gear wheel composition with fewer teeth causes the engine to behave as if the ratio is a little shorter, that is, the engine will have a minimum extra tendency to reach high revs quickly, thus obtaining more torque on the axle. In this case, acceleration will be preferred over top speed. The opposite is true for the pinion/gear wheel composition with more teeth: the ratio in this case will be slightly longer, bringing the engine to work at slightly lower speeds, preferring the top speed compared to acceleration coming out of a bend.

2.  TRACK CONDITIONS ARE A VARIABLE THAT SHOULD NOT TO BE TAKEN INTO ACCOUNT WHEN IT COMES TO CHOOSING THE RIGHT RATIO

False: Track conditions are a key variable when looking for the optimal ratio. In fact, if you take into account different levels of grip on the same track:

• Low levels of grip correspond to lower average speeds on bends, in these cases shorter ratios are preferred, obtaining more torque on the axle, otherwise the engine drops too much in rpm since it will find itself in an area of the bend where the engine torque is not very favorable.

• Shorter ratios are necessary even in situations of high grip since the torque necessary to optimally bring out the chassis from the bends is greater, gaining in smoothness.

• On the other hand, optimal levels of grip can involve the decision to lengthen the ratios. In fact, the speed drops less on bends and there is less demand for torque when exiting. In these cases long ratios will favour reaching higher top speeds on the straights.

         3. THE SMALLER THE PINION, THE MORE WEAR AND TEAR

 True: There are 2 main factors that affect the wear of the pinion. 1. With the same torque to be transmitted, the tangential force (which turns into the chain pull) applied on the pinion increases as its diameter decreases, which means that smaller sprockets are subject to greater forces. In the long run this compromises the durability and integrity of the component. 2. The number of teeth gripping the chain decreases as the diameter of the pinion decreases, so the teeth of the smaller sprockets have to withstand greater forces. While for the former case there is no solution, if not the continuous monitoring of the component and its preventive replacement when it is clearly worn, in the latter case it can be remedied by mounting a chain with a lower pitch. This will allow, with the same diameter of the pinion, having more teeth to grip and redistribute the pulling force of the chain.

4. IT IS ALWAYS PREFERABLE TO START FROM THE GEAR WHEEL WHEN MAKING RATIO ADJUSTMENTS

False: The choice of the right ratio is affected by many variables such as track conditions in terms of grip, engine delivery and set-up. This will involve an initial study of the optimal ratio that has been chosen, and then making adjustments based on the driver's sensations or responses to changes by the kart. What is therefore fundamental is the point of arrival in terms of optimal ratio and not the component that is to be modified, whether it is a gear wheel or a pinion. Suppose for example that for a given track you have selected a ratio of 7.272727, resulting from a pinion of 11 and a gear wheel of 80. If the driver feels it is slightly short, noticing a top speed that is too low on the straights compared to the vehicle’s potential, you can opt for a ratio of 7.1818, resulting from a pinion of 11 and a gear wheel da 79, or a ratio of 7.20 with pinion of 10 and gear wheel of 72 so even closer to the initial choice of 7.2727. The driver’s indications regarding the performance of the engine and not the process used to obtain a certain ratio is fundamental.

5. DIFFERENT COMPOUNDS INFLUENCE THE CHOICE OF THE RIGHT RATIO...

True: As we have already said, the grip of a kart on the track is a fundamental component when it comes to choosing the optimal ratio. The compound of the tyres therefore plays a key role. In fact, softer compounds, which reach the right temperature earlier and give higher levels of grip will be more able to favour higher cornering speeds, which can be exploited to use longer ratios, obtaining higher top speeds on the straights. On the contrary, harder compounds will result in shorter ratios, which is due to the fact that more torque is needed coming out of a bend, given the lower cornering speeds, and you can also benefit from the traction to make the tyres work better and let them reach the right temperature earlier.

6. IT IS ALWAYS ADVISABLE, FOR THE SAME RATIO, TO HAVE THE SMALLEST POSSIBLE GEAR WHEEL

True: In general, it is always preferable to mount the smaller gear wheel because at the same ratio the mass of the organs in lighter rotation will allow the engine to work at higher speeds. Furthermore, having a smaller gear wheel will reduce the possibility of hitting a curb when, for example, you cut across a chicane. It must be said, however, that you may find yourself in situations where this will not be optimal, preferring the composition with the largest gear wheel. If we take for example a ratio 6 this can be obtained by a: - Sprocket 12 and Gear wheel 72 - Sprocket 13 and Gear wheel 78 The second ratio will be slightly longer than the first and depending on the situations and variables at play on the track it will be more useful, if for example you need slightly higher top speeds.