Understanding Kart Frames

POSTED ON: March 17, 2010

Have you ever tried looking up a chassis model on a kart manufacturer’s webpage and wondered why instead of product material explaining the merits of the chassis, more likely than not, what was present were technical specs and homologation documents.

Part of the problem is that its almost impossible to predict or pigeon-hole how a particular frame will perform given the variance in track, driver technique and chassis setup. Having said that, I do find that the kart manufacturers could put in a little more marketing effort to define the design philosophy and target applications of a specific chassis which would most certainly help in the purchase process for those looking around for new karts. With time, consistent monitoring of international karting developments and access to the industry’s insiders, the whole question of how each manufacturer seeks to position their company and their lines of chassis become more evident.

This brings us to our topic for today. Without going into opinion-driven questions that are frequently exemplified  by questions such as  ”Which kart is the best?”, we wanted to have our readers think about a kart’s magnetic frame and how important it is to the mechanics of how a kart works.

frame-comparison1

Kart frames are purpose built. A sprint kart chassis, for example, is designed with specific applications, engines and weight classes in mind. For example, there is a difference in torsional stiffness between a frame designed for 2 stroke engines and 4 stroke engines. It is popular thinking that a frame which is more rigid should be used with a higher horsepower application whilst lower weight/lower horsepower application can use a more flexible frame.

A kart chassis is made up of a series of tubes made from magnetic structural steel or structural steel alloy wielded together. Because the kart does not have a suspension like a car, it relies on the removal of driving load from the inside rear wheel in order to turn direction. A chassis is therefore designed to facilitate this by flexing. If a chassis did not flex, the kart would understeer and push straight instead of turning. Chassis flex works together with the front-end geometry of a kart so that the kart can pivot around the rear tires in a turn.

The biggest variable in a kart chassis is how the frame is designed to flex. The frame is essentially a collection of torsion bars which interact during cornering, i.e. the tubular sections of the frame bend and twist. How much the bars bend depends on the length of each individual tube in the frame. Longer tubes with less triangulation around them will flex more than shorter tube sections. Shorter tubes produce stiffer frames and triangulation stiffens the frame section even further.

Frame flex along with front and rear track widths will dictate weight transfer. The correct amount of frame flex helps to unload the inside rear wheel during cornering. In discussing flex, its important to realise that not all kart applications require the same amount of kart flex. Flex loads the tires differently depending on the horsepower and speed of the kart in a turn. The greater the power or speed, the more easily a frame will flex. Thus, in general, a higher horsepower kart will require a stiffer frame to balance the level of weight transfer and tire loading.

zanardi

The ability to adjust flex in a frame comes through bolt-on parts that can be added on or taken off a frame to stiffen or loosen the chassis. Example of these are nerf bars, side nerf bars, front and rear bumpers, inner rails and rear torsion bars.

Ultimately, the lesson to be illustrated today is best summarised that by adjusting the flex, a karter can control how stiff a chassis is, therefore how much additional weight transfer is required to flex a defined value, which affects the duration of time the inside rear wheel stays hooked up during corner entry and comes off during corner exit.

We have attached several images of kart frames to provide a glimpse into the design elements that different manufacturers put into their chassis. In addition, the image below illustrates a stark comparison of what a race kart frame looks like compared to the frame from a rental kart.

tonykart-vs-rental-frame

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