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Track Test Tech | The variations of the kart rear track width

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The variations of the rear track width

Rear track width (like front) is one of the most determinant factors in vehicle set-up. While the standard in competitive contexts is the maximum width (140 cm for KF/KZ), low grip and wet conditions make variations necessary. We stepped on track to see exactly how width changes condition driving performance

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What

Track test to see how the kart performs according to variations in rear track width

How

Test 1: rear track 136 cm
Test 2: rear track 140 cm

Where

At the Adria Karting Raceway in Adria (RO), Italy

Conditions

Well-kept and regualr-surface pavement with good, but not extreme, grip, after the Rok Cup Italia race weekend. To maximize the drive feel, we chose high-performance tires, specifically Vega White

Weather
26°C
IN THEORY
TRACK WIDTH UP FRONT

Front track width influences front-end grip (though, of course, it necessarily “works” in relation with rear track). The general rule is this: the wider you go, the greater the grip, especially in wide fast bends; the narrower you go, the more the kart under-steers and the more agile it becomes in changes of direction and tight bends.

TIRES AND GRIP

Variations in track width modify the rear tire footprint: a narrower track setting causes the axle to flex less (even less if it’s a stiff axle) and so causes a title in the footprint, with greater load on the outside; vice versa, a wider track causes the rear tires to do more “work” on the inside. The other key effect of a narrower track is greater rear-end bite. In high-grip track conditions and with soft tires this often causes the rear end to “bounce” sideways. To the contrary, a wider track gives you less rear grip, but allows the kart to slide in more uniform and progressive manner. Of course, on a slick track (without much rubber down) you’ll want a narrower setting: this will give you just the grip you need, without making the kart jumpy.

REAR TRACK WIDTH

Rear track width determines the amount of centrifugal force that discharges on the front outside tire. This discharge, in turn, determines a shift in the impact of vertical forces, causing the rear inside tire to lift more or less. Specifically: the wider the rear track, the higher the rear inside tire lifts off the ground and, therefore, the greater the weight that shifts to the front end of the kart, causing over-steering.
Another combined effect is given by the length of the axle segment that extends from the outside bearing to the wheel hub. Of course, the wider you set the rear track, the longer this segment gets and so the more it flexes, causing the entire axle to be softer. The result of this is lower grip for the rear load tire and, again, a tendency towards over-steering. The opposite happens with a narrower setting.

REAR TRACK 136 CM
53’’500
Set-up
Front width 5 notches
Camber Neutral
Caster Neutral
Rear height Low
Front height Neutral
What we expected

A 136 cm setting makes for a rather narrow track. So, with high-grip tires like Vega Whites and the rubbery track conditions we had (after a full weekend of racing), we were expecting excessive bite on the rear end, with understeering in corner-entry and side bouncing through curved sections.

Driving impressions

As expected, the rear end was tied down way too much with a heavy understeering effect. To counter this, we had to turn the steering wheel excessively for the entire duration of the bend and especially in the initial segment. Corner-entry also required a huge steering effort, due to insufficient rear end glide. In medium-fast bends, like the inside “L”, the rear-bite effect was so severe as to cause the chassis to bounce sideways.

Analysis

The graph generated by the AIM data acquisition system shows that in the hairpin before the S, the excessive rear-bite given by the136 cm setting (green line) demanded excessive braking and steering, to the point that the rear tires suddenly slipped away, causing a significant speed loss throughout the bend. On the other hand, the high grip helped at corner-exit. Not only, with the narrow setting making the kart very reactive and quick with direction changes, we were able to keep good speed through the S. On the last wide bend leading to the straight, instead, the excessive rear grip again made it hard to enter corner and keep in control through it, with a subsequent loss of speed in the first acceleration segment.

Tires

With the narrow 136 cm setting and a track with high grip conditions, our front tires were sliding away too much and not “working” as they should. And since they heated up more quickly than ususal over the first laps, it was impossible later on to set the pressure right (Vega recommends 0.78 – 0.82 bar). In the rear, instead, the kart’s excessive bite caused the tires to overheat and go well beyond the recommended driving pressure. These combined effects showed clearly from the look of the rear tire footprint at the end of our test: the surface was quite rough and ragged

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