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.
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.
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.
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 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.
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.
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.