Kart seat positioning is a difficult but important job. The driver is half the weight of the vehicle and if their weight is out of position the kart will be unbalanced and difficult to set up. Many manufacturers will give you a set of dimensions, which can be confusing unless you are using the exact shape and size of the seat that was used to get their positional information in the first place. Therefore, to get the correct balance in the chassis, it is important to understand where the drivers back is in relation to the rear axle. These instructions are to help with attaining the most common position for the average 70kg driver. Sometimes a compromise position will have to be used for smaller or larger drivers.
To accurately fit a kart seat, place a flat sheet of plywood, or something similar, on your kart stand, then space the chassis tubes from the wood to give you the correct ground clearance. (For ultimate accuracy use the Tillett T Board TRACK fitting jig. Please see below for information.) 5mm is usually the maximum dimension that you can set the base of the seat below the tubes with a modern chassis. If you do not have this low point information, set your kart up on a very flat piece of ground with the correct tyre pressures. Take a note of the distance the seat protrudes below the chassis tubes when you have around 15mm clearance between the floor and the seat base. Next place the seat back on the chassis sitting on the wooden board or the T Board track fitting jig. Held like this it is stable and easy to hold in the ideal position. (Tip: A weight placed inside the seat will help keep it stable and upright.) As most seats have a flat on the bottom it is usually best to use the set angle provided by this.
Using both the height of the seat flat and the important “axle to drivers back” dimension shown in the diagram below, you can put any seat in any kart and the driver will always be exactly in the same position in relation to the rear axle. The current average seating position for a 70kg 1.80m tall driver is 15 cm, whereas with a KZ chassis the dimension would be further back at 13.5cm. A shorter junior would use around 18cm.
Be aware that historic karts always needed the seat set further away from the axle. Pre-2005 chassis with a 40mm diameter axle used around a 21.5cm gap and karts aged from 2005 to 2009 with a 50mm axle approximately 18 cm. If the kart has a 30 or 40mm axle you will need to compensate for this by changing the gap dimension accordingly. Please note that bolting a substantial amount of lead weight to the back of the seat can make a difference to the seat position. It forces you to position the seat further forward to achieve the same balance.
With the seat in position, check that all seat stays are parallel to the composite. If the flat metal tabs are set at a different angle, bend them with a large adjustable spanner until they exactly match the angle of the composite. They do not have to be bent to be close to the composite and the gap can be filled with nylon or aluminium spacers without a performance penalty. However, do not use rubber spacers, not only will this allow uncontrolled flexing which will break the edges, but the bolt will be effectively loose, acting like a saw, moving in and out as the seat twists, cutting a slot and allowing the seat to drop.
To mark the holes, place a blob of paint on the end of a long bolt and pass it through the four main stays spotting all four holes without moving the seat. If there is a gap between the seat and the stay, pass the bolt through the tab at 90° to the seat stay. Drill all four holes accurately and when fitting the bolts use the correct number of spacers to keep the composite from being twisted out of its naturally moulded shape. Make sure to maintain the seat in the desired position, then tighten the bolts until very tight. If you use a flexible VG or Vti seat in an adult kart without any extra seat stays only using the four main chassis stays. On the main two upper stays it is advised to pop rivet aluminium support plates against the composite.
The seat is then in place and you can then fit any extra seat stays and lead weight required. Weight bolted on the sides and under the front between the legs should be slightly spaced away from the seat with nylon washers. This helps the seat flex around the rigid lead and stops breakages. Also bend the lead in an arc under the front, this is done so as not to pull the composite out of its naturally moulded shape. If water pipes and data cables are to be fitted, make sure that any hole drilled for a cable tie is more than 5mm away from the edge. Holes drilled too close to the edge can cause a crack.