Grip is the resistance to the tyre/wheel sliding over the track suface. We have Lateral grip resisting the tyre sliding sideways under cornering forces, and Rotational grip resisting the tyre sliding over the track surface under traction (wheelspin) or braking (lockup).
The grip available at each wheel is dependent on a number of factors, including track surface and condition, tyre compound and temperature, but most importantly, the loading of the individual tyre against the track.ie. the harder the tyre surface is pressed into the track (loaded), the greater the grip, both lateral and rotational. Hence the importance of downforce in f1etc.
The combined grip of the two rear tyres gives us the rear grip, where rotational grip is a factor, whereas at the front, with no brakes or drive, rotational grip has no effect.
Unlike most driven vehicles a kart has no differential mechanism in the driven axle, so that both rear wheels on a kart always rotate at the same speed. In a corner, viewed from above, the inner rear wheel moves round a smaller radius than the outer, and if the two rears were both providing infinite rotational grip, the outer wheel would need to turn faster than the inner for the kart to change direction, but due to the lack of a differential this is not possible......
The solution is to unload (Release) the inside rear by transferring weight off the inside rear, and most of it will end up on the outside rear. The increased loading on the outside wheel (outside tyre pressed harder into track) increases the lateral and rotational grip of the outside rear whereas the unloading of the inside rear, usually to the extent that it is no longer in contact with the track, means that it is no longer providing any grip. The overall rear grip remains the same but we are no longer absorbing engine torque in a futile attempt to turn the rear wheels at different speeds.
So how do we induce this weight transfer off the inside rear?
The first situation is at turn in from straight ahead. The combination of caster and front wheels out causes the inner front wheel to move downwards relative to the chassis effectively lifting and unloading the inside rear allowing the kart to change direction. Increasing caster, increasing chassis torsional stiffness, narrowing rear track ,will all promote better turn in.
Once moving round the corner radius, the centrifugal effects will keep the inside rear unloaded. Weight transfer to the outside wheel will be promoted by raising the centre of gravity of the kart / driver combination, narrowing rear track, increasing caster.
So. You can't have too much grip. If you are 'bogging' in corners you have not adjusted the kart to give sufficient Release for the conditions, or have unrelated engine issues.
All 'Rubbishing' V welcome!!!
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Grip and Release by 'WDave' (08 Jul 2016 @ 8:01)
