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Tech Tuesday: Wet Chassis Set-Up

Leaning to the outside while negotiating a bend helps increase the grip of the outside tyres

Some drivers love racing in the wet, others hate it. But it is also true that liking or disliking the wet depends a lot on what performance one has in these track conditions. Kart set-up is extremely important in wet weather and driving itself changes completely. Wet conditions set-up The main critical aspect of wet conditions is the extremely low grip of all four tyres. This leads to understeer entering bends and oversteer exiting them, with really low traction. Also braking becomes extremely difficult with frequent locking of the rear wheels. To help reduce all these effects, which persist even when rain tyres are fitted, we must really work on kart chassis set-up. First of all we must balance the chassis to give maximum front grip and maximum rear grip! A narrow rear end increases rear grip as does a wide front end for front grip. The front end can be be made really wide by fitting long front hubs. Now work on the front angles. Caster should be increased to the maximum so it gives incidence to the front tyres when entering a bend and reduces understeer. Camber should be set to zero with the driver sat in the kart. Finally, some toe-out gives some advantage. Sometimes, especially with not very tall drivers, it is important to lift the seat to raise the centre of gravity.

Edgar's Hyundri Super One MSA Series, MSA, KF2, PFI, RSF, Ben Barnicoat, ART.
Love it or Loathe it, At some point it’s going to rain, the right set up makes the world of difference.

This creates a greater momentum when running along a bend which helps increase grip on the outside tyres. Of course if you have to add weight to reach the minimum weight limit then add it as high as possible on the kart seat. Seat stays should be loosened to give more flexibility to the chassis and a stiff rear axle fitted. This last solution though, in my opinion, gives little advantage, since the speed and grip of the chassis are so low in wet track conditions that the forces acting on the chassis are also reduced. This means that the chassis flexes very little and the rear axle may not flex at all. As in dry track conditions, tyres are very important. Pressures should be much higher than for slicks on a dry track since in the latter conditions the tyres heat up much more. Be careful though that, when using rain tyres, all the tyre print is touching the ground and not just the centre section. You can check that after your first run. If you only run on the centre then the tyres won’t last as long and grip will of course be reduced. How to drive in wet track conditions As we have already started to see, in wet track conditions one important thing is to put a lot of load on the outside tyres, front and rear. This can be done by even eliminating some load from the inside tyres. So when driving in wet conditions the position of the driver’s body is extremely important. To give as much grip as possible always lean towards the side where you want to increase the grip. So when running along a bend lean to the outside.

Go on slicks they said…

An even more sophisticated movement of the body is to lean to the outside and towards the front when entering the bend and then lean backwards towards the outer rear tyre when exiting the bend. This will help reduce understeer entering the bend and also reduce the oversteer exiting it. Another important factor to consider is that in wet conditions it is much better to brake earlier and concentrate on the exit of the bend. Locking the wheels when braking can make you lose control, time and spin. Also, in general, never accelerate with the front wheels turned. Accelerate only when the front tyres are straight and parallel to rear tyres, or almost so. The most important thing though is to drive the kart on completely different lines from those used when track is dry. When the rubber laid down on on the track becomes wet it becomes extremely slippery so it is much better to find grip on wet but clean tarmac. The best line to follow is to run the bend all the way round the outside where rubber has not been deposited. In some bends another possibility is to go out wide when entering the bend and then to cut in to the inside. Finally, to help comfort and concentration, it is a good idea to buy a wet suit to keep your body dry and warm. All these points are just a start, you must gain experience of driving in these conditions and fine tune your kart’s set-up to optimise performance.


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Tech Tuesday: Kart Aerodynamics

An aspect often not estimated is the aerodynamic factor acting on a kart chassis and its driver. Since a tenth of a second is vital for a kart driver to win, we must consider that on particularly fast tracks and even more for fast 125 cc shifter gear chassis high speeds generate an important resistance of air on the kart. This is determined by the laws of physics that show how resistance to penetration in air is proportional to speed if such speed is minor to 100 km/m, but changes to a proportion to the square of speed for values over 100 km/h. Many kart chassis constructors are in fact now projecting front and side plastic bumpers also looking at good aerodynamic penetration.

No, not that kind of areodynamics…

Aerodynamics on chassis

A kart with its driver has a very bad CX factor, which is the coefficient of aerodynamic penetration. This aspect, even though front area of kart and driver is limited, is determined by the total absence of front cover to driver and kart and by the turbulence that is generated behind the driver who represents nearly 45% of the front area of the entire system. So, for a start, a more compact position or maybe having the driver slightly lied down can give significant improvement to aerodynamic penetration and so also to performance.

Lateral bumpers

Side protections already give good effect since they cover rear tyres reducing turbulence and increasing penetration. So when widening rear carriage see if tyres are still sufficiently covered. Some companies like Tecno have studied both side and front bumpers to optimize fresh air flow towards the radiator obtaining better engine cooling.

The OTK M6 Bodywork was designed with areodynamics in mind due it its slippery profile

Front bumper

It has the function of course of  protecting from front collisions with other drivers or obstacles, but has also a good aerodynamic effect. First of all to work well the spoiler must be kept as low as possible to reduce turbulence of the air passing under the chassis. Such turbulence in fact would slow down the passing air which would “stick” to the chassis reducing its speed.

The air moving over the front bumper makes it work like a spoiler pushing it down and increasing front grip. Over 100 km/h such vertical force can be equal to 4 kg, but really under such speed the effect is limited, and becomes secondary respect to aerodynamic penetration.

Working in such way on the front bumper gives only positive effects: better aerodynamic penetration and small increase in front grip.

Base protection

Base protection is used to position the driver’s feet and to protect him from eventual stones coming from the track. The three surfaces of the front spoiler, the base protection and the inferior surface of the seat make a continuous surface. They must for this reason be on the same level so that any edge is avoided and air flow is free to pass smoothly under the kart. Some technicians say that a ground effect can be obtained on fast tracks curving slightly the base protection, but such work seems to be too complicated compared to the result.

PFPK1605GX160 -147
The ‘Big Nosed’ Prokarts have a large Nassau panel to divert air away from the drivers’ un-areodynamic legs

The driver

The driver has really, as already anticipated, a bad aerodynamic penetration. Feet and legs will direct air flow towards the chest of the driver and slow down the kart, or laterally if well positioned. The front number plate help to limit such effect and will have to be as wide as possible to cover the driver and also will have to be well positioned to connect itself to the legs.

The arms of the driver will have to be positioned well in contact with the body only moving the forearm to turn the steering wheel. This will also help a better aerodynamic penetration. An additional effect is that this position will send air flow towards the engine or radiator helping engine cooling.

Finally it is clear that small drivers (once more) are helped on an aerodynamic point of view. I have myself seen on the Parma track in Italy tall drivers loosing much of their advantage on the three long straights of the track.

Head down, Alfie Brookes tries to gap the field in the Honda Clubman class. - edit


With shifter gears kart it can be quite simple to see the effect of aerodynamic penetration by reaching a certain speed on a straight and pulling the clutch suddenly. The kart will go on running along and we can measure the speed of the kart (we must have a telemetry system that measures speed with a sensor on front wheels) after a certain number of metres (300 for example). Try this test with different configurations of the kart bumpers and driver positions. Of course testing must be done with exactly the same track conditions (possibly the same day) and no wind. If wind or grip of the track change results will be completely unusable since the effect of these two parameters are similar to the aerodynamic resistance.

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Tech Tuesday: Weight And Seat Positioning

Even though many parameters determine the grip on each tire, everything is finally led to the vertical force acting on each wheel. The greater such downward force is and the more the grip between the tire and the asphalt.


Centre of gravity

The centre of gravity is the point of a system (chassis plus engine plus driver) where one can consider all the weight concentrated. Once we define this in a kart, and generally such point is positioned in the stomach of the driver, we can calculate all the forces acting on the kart concentrated in the center of gravity. For example the weight of the kart can be concentrated in the centre of gravity as the sum of all vertical forces when the chassis is not moving (weights multiplied by the gravity acceleration). We will call “a” the longitudinal distance between the centre of gravity and the front carriage, and “b” the distance from the centre of gravity and the rear carriage. So we will be able to say, following the laws of the physical equilibrium, that the vertical forces acting on respectively on the rear tires (Fp) and the front tires (Ff) will be equal to:

Fp = Pvehicle * (a + b)/b,

Ff = Pvehicle * (a + b)/a,

where Pvehicle is the total weight of the kart (chassis, engine and driver) and:

Ff + Fp = Pvehicle.

We are making an approximation considering the weight of the kart a force.

Leaning can als help you look cooler, not just distribute weight…

Effects of weight distribution variation

So the variation of the position of the seat of the chassis along the longitudinal length of the kart varies the parameters “a” and “b”, which means that also weight distribution of the kart varies between front and rear tires. Generally the longitudinal movement of the seat has a maximum value of around 4-5 cm, but a few centimetres determine great differences in weight distribution. For example the distance between front and rear carriage is generally in karts around 104 cm. With front weight equal to 40% and rear equal to 60% the centre of gravity is 41.6 cm from the rear axle and 61.4 cm from front carriage. If we move the seat 2 cm to the front of the chassis distribution will be 42% on front tires and 58% on rear tires. Such variation of 4% of the weight could appear a small quantity, but can really determine great difference in kart performance. If we move the seat of the chassis towards the front carriage the vertical force on front tires will increase and the force on rear tires will decrease. This will automatically determine an increase of front tire grip and a reduction of rear tire grip. So moving the seat to the front increases oversteer and moving it to the back increases understeer. The variation of the two parameters “a” and “b” will not vary directly shifting the seat. In fact the movement of the seat will determine a movement “c” of the centre of gravity as follows:

c = Pdriver/Pvehicle*x,

where “x” is the movement of the seat.

So it is extremely simple to setup the basic grip on the four wheels just by the right longitudinal positioning of the seat of the chassis. After such positioning the other parameters of the chassis will vary the forces on the four wheels for fine tuning.

leaning too much can also hinder your driving…

Generally weight distribution must be regulated to have 60% of the weight on the rear tires and 40% on front tires. It is also true that every chassis has its own particular regulations for what concerns weight distribution. Ask your chassis builder or shop for the right values of weight distribution between front and rear tires. Such parameter is really too important to be mistaken. In addition to this we must consider that weight distribution should be equal between right and left tires on the front and on the rear. Because of the engine positioned on the right hand side, the seat will be slightly shifted to the left hand side of the chassis. This is of great importance especially when braking before a curve, since the balance of the chassis will be perfect, and the kart more stable in such phase, only if left and right wheels will have the same grip acting on them and the centre of gravity will be in a central position respect to the wheels.

Also the driver, especially in wet track conditions, can move inside the seat and vary weight distribution. For example moving your body forward when entering a corner helps front grip, and moving the body backwards when exiting a corner helps traction. This, also thanks to lower speed of the kart, has great effect on wet track conditions.

Next issue we will proceed consider weight distribution on the four wheels varying centre of gravity height, which will also determine great effects when running along a curve.

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Tech Tuesday: Rear axle choice

The Rear axle is one of the main elements to influence chassis setup. The stiffness of the axle determines behaviour of rear part of the chassis, making the kart oversteer or understeer, and changes strongly the balances of forces acting on the chassis.

1As we know in fact kart behaviour along a curve is based on the possibility for the internal tire to lift off the ground since there is not differential system on rear axle. Now a softer axle bends more and quicker than a stiff one. This means that a kart can reduce grip on rear tires, and increase oversteer with a softer rear axle and vice versa with a stiff one. Very simply, but always considering there must be a balance in kart setup between front and rear stiffness, increasing rear axle hardness gives better grip on rear tires, but difficulty in entering curves, because of too high grip on rear tires compared to front tires. The opposite happens having softer axles.

But which are the parameters that vary rear axle stiffness? They are three, material, thickness of tube wall, diameter of the tube. Along the years chassis have shifted from having full tubes of small diameters (25 mm) to having hallow tubes as rear axles with increasing diameters from 30 mm, to 40 mm, and in recent days to 50 mm. What happens is that a tube which bends with an elastic deformation returns to its initial straight form after the kart exits the curve. This return to initial shape can happen with different speed and promptness.  A stiffer axle is quicker in returning to its straight shape, and this determines faster reaction of the kart in having the internal tire back on the ground when exiting the curve. The two rear tires touching the ground give more grip and the possibility to accelerate after the curve. So material of the rear axle determines different setup of the kart. Some materials are surely better then others in their elastic capacity to regain initial straight shape. Much effort is being done by kart constructors to find materials with better elastic characteristics. Thickness of the tube also acts on axle stiffness R with the following formula:

The material, wall thickness and diameter of an axle all affect set-up

R is proportional to: a * (d22 – d12), where d2 is the Bigger diameter axle gives surely a better look to the chassis. But the practical difference is that for physical reasons, which we will not go deep into, a bigger diameter axle is more reactive than a smaller diameter one. So the shift to larger diameter axles has had the aim of increasing reaction of the chassis exiting curves and giving the possibility of accelerating earlier.

But it is not only stiffness of the axle to act and change setup. We know that rear carriage width also acts on rear tire grip. The wider the carriage, the lower the grip. First of all this is due to the fact that a longer rear axle, and a wider spacing between rear tires, permits greater deformation and bending of rear axle along curves. It is somehow equivalent to having a softer tube. On the other hand another effect is that rear carriage width increase makes the rear tires lift with more difficulty. Rear carriage is flatter on the ground and even though rear grip decreases, eventual sliding of rear tires is smooth and uniform, easily controllable. If we reduce rear carriage width rear axle becomes stiffer, grip increases, but rear carriage is less stable. Internal tire lifts without rear axle bending much. What happens is that external rear tire blocks itself laterally on the ground instead of sliding. But as soon as lateral force is too strong rear external tire suddenly looses grip along the curve, with fast lateral slides of the rear of the kart, which becomes much more nervous and difficult to control. With high drivers this effect is even more evident since centre of gravity is positioned higher and momentum on kart increases. That is why high kart drivers usually tend to have wider rear and front carriages.

Finally now we know which parameters act on rear axle stiffness, amongst these usually the type of material is indicated by a lettering on carved on the axle. Also we know how stiffness acts on kart setup. Finally we also have seen that rear carriage width acts on rear axle flexibility and on general chassis behaviour along a curve.

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Tech Tuesday: Choosing the right Karting seat

The seat and its supports are often underestimated by chassis tuners and drivers, but on the contrary such elements are a fundamental part of the chassis and determine a lot of the stiffness characteristics of karts.

Seats come in many different brands, stiffnesses and materials, Giving each one unique characteristics

Various seats and supports

The structure of the seat and its connections to the chassis generate a system which varies sensibly the chassis stiffness. Moreover the material of the seat can vary and so also the stiffness can vary. Glass fibre, Kevlar, carbon or mixtures of these elements, with also different thicknesses of the layers, can generate variations in the torsional flexibility of the chassis in its rear part, since the seat itself will change its stiffness.

For example a carbon fibre seat is, with the same thickness, three times as stiff compared to a glass fibre one. A Kevlar seat is on the other hand much more elastic if compared to a glass fibre one. Costs are also to be considered and, as usual, carbon fibre seats cost at least double a glass fibre one.

With the use of supports it is possible to preload the chassis on one or both sides. Such load will be positioned on the rear axle bearings. Such preload will be obtained by using length of supports of just a few mm superior to the distance between the holes in the seat, where the support is screwed, and the lateral axle bearings. Supports can be bent and twisted together to obtain something very similar to a spring. Straight supports are extremely stiff. In fact a straight support works in compression on the rear axle where it is linked. Such supports are very stiff in compression. On the other hand the same support bent, works in flexion, so has a much higher elasticity and, as said before, works well as a spring, deforming when a sudden bump is hit by a rear tyre, but still creates downward force when it is needed to increase rear grip.

World's biggest kart seat T7 OMG
Proof that your dad can have a go in your kart, the “I won’t fit” excuse just wont cut it here…

Effects of supports on chassis behaviour

The stiffer the seat and its supports and the faster will be the diagonal load transfer to the wheels. In fact the weight of the driver whose effect is also increased by the centrifugal lateral force when running along a bend, is transferred to the rear tyres through the seat and its supports. If no supports are present the chassis will first bend and then only part of the weight will transfer to the rear tyres.

Usually it is best for better performance and faster reaction of the kart to have a quick weight transfer, which means good seat and supports stiffness. On the other hand a very stiff seat and supports system does not absorb any irregularity of track surface. We must not forget that if initially caster of the front internal tyre transfers load to the rear external tyre, it gives back load to front wheels as soon as it is the only rear tyre that touches the ground.

The stiffness of seat and supports permits the rear part of the chassis not to flex too much, so we will have a good lift of the internal rear tyre during a curve. If the chassis bends too much instead, both rear tyres will keep touching the ground generating great under-steer.

Even when youve done with your seat they can be useful

On the right side of the chassis under the seat we always have two tubes, that are not only connected by the welded tubes, but also by the engine mounts. This determines the fact that the right side of the chassis is stiffer then the left one.

Acting on the seat supports we balance stiffness on the softer left side. If we use an old softened seat, with holes too big for the screws, we will have a low performance chassis. There is nothing worse then a chassis with its seat moving all around. It is also useless to try to set up the chassis with all sorts of regulations, if the area where most of the weight is concentrated deforms easily in any direction.

It is for this reason extremely important to well fix the seat to the chassis with well dimensioned holes, possibly hardened by metal rings, maybe glued to the seat with some resin.

Only when grip is extremely low (dirty or wet tracks) we can use rubber rings that absorb all the forces without loosing connection between the seat and its supports. The best solution, even though more complex, is to have the correct seat for different track conditions. Also some sponge can be used to increase comfort, but with no excess, otherwise we would loose the feeling between our body and the seat-chassis. In fact it is always with your “bottom” that you feel your chassis and understand how to drive it.


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Tech Tuesday – Pistons

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Tech Tuesday: Pistons

Pistons are elements that lay an important role in engine performance and reliability. They in fact act as valves on the gas produced by mixture burning in the combustion chamber. These gases at high temperature and pressure have a very high quantity of energy that can be transformed to mechanical energy through the con-rod and crank-shaft system. This system is moved thanks to the capacity of the piston in sealing the volumes over and under it in the cylinder and crank-case. Also in this passage mechanical energy of the piston expressed through a longitudinal movement is transformed in rotational mechanical energy. All this happens with great forces acting on the piston (mechanical stress) that are added to the high temperatures that the piston reaches in contact with the burned gases (thermal stress).

Roys DD2 Piston

Piston movement

Pistons move longitudinally in the cylinder, but their speed changes constantly in a sinusoidal way. This means the piston starting for example at top dead corner (TDC) has a speed equal to zero. Then it accelerates quickly reaching top speed when the con-rod is at 90° respect to the crank-shaft arm. After that speed decreases again and is null at bottom dead corner (BDC). This quickly changing speed indicates strong accelerations (both negative, deceleration, and positive). Any acceleration generates a force called inertial force. Inertia is the capacity of every body to oppose itself to acceleration (negative and positive). The opposition is the inertial force. Inertia increases with weight of the element and the square value of otational speed of the crank-shaft. Maximum negative and positive inertial forces are at TDC and BDC.

In addition to the longitudinal movement along the cylinder axis he piston has orthogonal movements that are determined by the lateral forces generated by the inclination of the con-rod respect to the cylinder longitudinal axis. Since the diameter of the piston is slightly smaller than the one of the cylinder, the piston can move sideways inside the cylinder. When the piston is moving upwards it is pressing on one side of the cylinder liner. When it passes the TDC and comes down the piston presses on the opposite side of the cylinder liner. This is due to the fact that the con-rod is inclined on the opposite side.


Total forces on the piston

In addition to the inertial forces acting on the piston, the head of the piston has to deal with another mechanical stress, such as the high pressure coming from the burned gases in the combustion chamber, and a thermal stress as the temperatures transmitted from these same gases. In particular in two stroke engines a cycle is completed in 360° of the crank-shaft, which means the piston has less time to cool down respect to the situation of a four stroke engine. In two-stroke competition engines such as kart engines the highest temperatures are reached at the centre of the piston head and can reach 400°C (752°F) in air cooled engines and 360°C (680°F) in water cooled engines.

Heat transmitted from the burned gases in the combustion and expansion phases to the piston head is then given to the cylinder through the ring(s). From 30% to 60% of the total heat energy is transmitted this way and helps cooling the piston. Piston temperature increases around 3°C every 100 revs/min and also 15° every bar of increases of medium pressure in the combustion chamber. Also combustion timing and compression ratio influence the temperature of the piston.


All these factors stress strongly all the structure of the piston and this is why materials and shape are well designed to obtain a very light and strong/resistant component. Aluminium for control and limitation of deformation given by high temperatures and additional elements added to the piston are key for obtaining a good result. Shapes also have changed a lot during the years, but finally they seem to be today very similar to one another especially in two-stroke competition engines for karts.

We will see in the next issue exactly how pistons are built and how the selection of the materials is made of a series of elements that help make a resistant component with also good thermal characteristics that help reduce friction, increase performance, reliability and durability of the piston and the entire engine.

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Tech Tuesday: Rotax Max Jetting and carb setup


If you have an EVO, you need to read our Rotax Max EVO article.

There are several different brands of software available like the Jet-Tech Max program that can really help save a lot of time when you go to the track. However the software needs to be calibrated to your engine and carb. That is when you need to have a strategy and become very familiar with the signs that indicate the correct rotax max jetting. These signs can be the plug colour, the dryness and colour of the piston crown and the exhaust header. If the main jet is correct, the plug will be light brown in colour and the piston crown will be dry with a dark brown hue. The engine should also be reaching maximum revs at the end of the longest straight. Using the base settings on a software such as the Jet-Tech Max program will get you close to the correct jet. It then takes several practice sessions to find the correct jet and calibrate the software to the engine.

To choose the correct setting for the needle takes a little effort from the driver who really needs to give good, consise feedback to the mechanic mechanic and needs to be objective and honest about how the engine is performing. It also requires  disicipline not to waste a valuable carb development session just to chase someone else around the track in the heat of battle, try and get yourself some space. As the needle has most influence between 1/4 and 3/4 throttle, the driver needs to focus on how the engine is performing out of the corners. It’s best to keep it simple, pick one corner on the track and get a feeling for how the engine pulls out of that corner. Getting the fastest exit on the best line is not the point of this test, it is to try and optimise the needle setting so the engine pulls strongly and cleanly down the straight.

Jetting affects the amount of fuel mixing with the air

It is a good idea to start rich by lifting the needle up to its highest position. Because the two stroke engine is not very efficient lower down in the revs due to the design of the pipe, the cylinder can be insufficiently scavenged which means a fresh charge of air and fuel is then mixed with exhaust gases. To help overcome this problem the engines typically run a lot of ignition advance to help get the air/fuel mixture burning a little earlier in this dirty environment. If you lift your needle up so you can richen the mixture and make best use of all that ignition advance you may well feel quite good gains in bottom end torque.

By repeating the procedure of going out and feeling how the engine pulls from 1/4 to 3/4 throttle, and then coming in and changing the needle position, the driver can get a good feel for what setting is best. The driver could also try accelerating from different throttle positions on the straight, say from 1/2 to 3/4 throttle, to help evaluate how each needle position effects the performance. To confirm what works the driver could then focus on his line through a portion of track and use split times to see what setting is quickest.

A short note on needles: the K27 and K98 needles have slightly different profiles. The K98 has a steeper taper which will have the effect of richening the mixture ever so slightly above 1/4 throttle. It would take a good driver to notice but the difference would become apparent on a dyno.

Clip Positions on the K98 Needle

You may ask why does anyone need jetting software if they can just go out and feel how the kart is performing? After all that is what every karter did before laptops were common at the track. The truth is that setting up a carb correctly can often be very time consuming and sometimes frustrating for everybody, not just the beginners. What jetting software does is act as a time saving instrument that once you have put the hard work into calibrating the software to your engine and carb allows you to go to the track, plug in the figures from a weather station to the computer and be very very close if not dead on the correct jetting immediately.

The beauty of a program like Jet-Tech Max is that it will take into account things like your choice of floats and idle jets and has features that allow you to easily fine tune to a high degree how close the software will be to the actual needs of the engine. One of these features is called Flow Bench which is a quite advanced adjustment that, according to the developers of the software, is best left alone until the driver and mechanic are quite experienced and confident with how well matched the software is to the engine, as it is a very fine adjustment. It is also possible to set the program up so that you have database settings for several different engines and carburettors, even if they are spread among the range of Rotax classes.

One other feature of the software that adds value is the sheer amount of supporting information included in the package, such as a wide range of chassis setup guides from the manufacturers, as well as driving tips, carb preparation guides written by the developers of the software and official Dellorto and Rotax guides to the engine and carburettor. If you can absorb all that valuable information and put it in to practice then you are well on the way to be a winner!

Alternatively, You can also use this chart to help with your jetting


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