This information is written from Tillett and taken from their website.
This is important as a good fit is essential for the protection of the driver and the handling of the kart. If a driver is loose in a seat he will pull to steer, pulling himself out of the kart, making it bounce at the slightest provocation.
To assess whether or not the size of a seat that you have in your possession is correct, sit the driver should in the seat and feel the gap over the leg bone, the hip bone and down the length of the ribs, from top to bottom. This must be done with any rib protection in place, but the race suit is not so important unless it has integral padding. The rib protector when worn tight, should not move inwards as the driver sits back in the seat. Once sat back in the seat, it should be difficult to squeeze your fingers between the seat, your ribs, hip and leg bones. It’s important that all the points are evenly pressured and no one element is either loose or pinching. If one area feels loose, some firm foam stuck between driver and seat is acceptable. If it is pinching you need a different size.
To assess the size without having a seat in your vicinity, we find that denim Jeans waist size, weight and height are good indicators. Coupled with information about the chest circumference and rib protector type. An over thick rib protector can make two sizes difference and leave you with very loose hips, so this is to be avoided (i.e. buy a P1) or accounted for with a WT wide top T11 seat.
Now you have a good fitting seat it is important to fit it correctly, half the weight of the kart moved 5 mm out of position will alter the balance. Over the years the average seat position has been going back in the chassis. And the average is now three cm further back than it was in the late nineties.
Please see our fitting guide PDF to help with this.