The first step when using a new chassis is definitely seat choice. In terms of shape, stiffness, and positioning it is the most important set up choice you can make to a new kart, affecting both performance and comfort of the driver. With the help of one of the most experienced companies specialising in kart and car seats, Tillett, we have started to analyze the parameters that can help obtain maximum results.
History of Tillett Racing Seats
Steve Tillett, the owner of the English firm, started karting in 1978 and from his first races using high grip tyres he soon understood the need to have best possible quality seats to avoid bruised ribs and possibly to improve performance. Since then, Tillett has become synonymous with high quality kart seats growing Year after year from one man operating in a garage, to a company comprising of six factory units totalling 14,000 square feet with a current workforce of 25 and a Tillett dealer in almost countries in the World.
As well working with glass fibre, Tillett also uses exotic composites such as carbon fibre and kevlar; offering lighter, strolnger and more versaitle products. Steve wasn’t going to stop at karts though, expanding into car racing, powerboats, ski bobs. Tillett seats have even helped Mel Clarke win an Olympic Silver Medal in Archery and have featured on the silver screen, in James Bond’s ice racer in Die Another Day, the tank in Goldeneye and the attack sub in GI Joe film.
Dimension and shape
The first parameter to take care of is the correct size of the seat. The fit is all about driver preference, some drivers perffering a tighter seat while other some room to breathe. The seat should however, be supportive and a general rule of thumb with the seat is to be able to slide a finger down between the driver and the seat.from the chest down to the hip of the driver. If the seat is too large. The consequent effect will be difficulty in driving since the body is moving left and right around corners, not being able to feel the kart. Also there is the possibility of the driver hurting his or herself through movement in the seat.
A good way to fill up the area between seat and body is to use a closed cell foam until the gap has gone. Don’t over-do it though as too much foam will reduce feeling in the kart. always buy a seat with the best fit possible to reduce use of foam this will reduce the feeliling in the chassis. Of course rib protectors, such as the Tillett R4 Ribtec, as well as protecting, help reduce part of the gap, depending on how thick each protector is.
Three distances need to be measured to fit a kart seat:
• Front to back in the chassis
• Angle or tilt of the seat
Most kart manufacturers will specify a reccommended seat height – measure from top edge of the seat down to rear axle surface and the distance between the extreme edge of the seat to the front (often two measures are used starting from left and then right), and the front tubing of the chassis. Tillett instead avoids this system as it is linked too much to the shape and size of the seat. Of course it can work well when measurements given by the kart manufacturer are the exact correct ones for the chassis and the seat they supply. However, depending on the driver’s weight and height such measures tend to vary.
Tillett suggests using the shortest distance from the back of the seat, from a point 2cm to the side of the spine depression, to the front surface of the axle. The average value of such dimension is 16.5cm, but can be within the range from 13.5 to 20cm. Tilt angle is generally given by the seat itself considering that Tillett seats often have a flat base which keeps the bodyweight of the driver as low as possible within the kart. The angle of the seat can still be varied though. Tillett recommends two angles: 63° from horizontal and 58° from horizontal. 58° helps lower the centre of gravity and helps taller drivers. They a height equivalent to 5mm protruding below the level of the main chassis tubes. This value of course can increase for small drivers, very low grip or wet conditions.
To adjust front to back position keeping height and inclination fixed, Tillett suggests testing on-track chassis behaviour avoiding narrow corners, where the steering system with castor and Ackerman angles will transfer weight from one side to the other and from one end to the other of the chassis. The best thing to do is to feel the balance of the kart with a base set-up along long fast curves, understanding if the chassis oversteers or understeers.Seat stays also have an effect on set-up. One seat stay per side helps lift and unweight the inside rear wheel, whilst pushing down on the outside wheel. Adding more stays per side will stiffen the rear of the chassis keeping it flatter along corners.
Tillett has sent us different stiffness seats that we will test and write about in the next issue. Different rigidities of the seats are obtained both using different kinds of glass fibre and resin and also different thicknesses. As the seat is part of the chassis, it determines the overall rigidity, especially of the rear of the kart. Generally Tillett recommends soft seats for low power karts, as these free up the chassis, stopping the inside rear wheel from dragging on the track and absorbing power (rear internal wheel lift earlier and at lower speeds). As the power of the engine increases the additional weight transfer of a slightly stiffer seat may prove advantageous (rear wheel not lifting too early and going back into position quickly for early acceleration).
However on some track surfaces or track conditions it may be better to achieve grip by putting as much of the tyre surface as possible in contact with the track, on others it may be that the only way to get grip is to increase the downward forces that act on the outside tyres. Only Testing can give the ultimate answer!
To help us with our work we used another interesting product from Tillett, the “T Board”. This instrument is made of glass fibre and carbon fibre and is incredibly strong and light and permits to measure and set precisely three dimensions of the seat: height, inclination and distance between rear axle and back of the seat. As anticipated some chassis manufacturers also specify the two distances from front right and left edges of the seat to the front tube of the chassis, as this can also be important to determine a possible rotation of the seat around its vertical axis and offset the weight of the engine positioned on the right with the driver.
The T Board must be positioned under the chassis and fastened by three clamps which are mounted on the lateral tubes and on the tube beneath the seat. This allows a plane of reference for the height of the seat. Using 2mm and 4mm spacers it is possible to set the minimum height of the seat. An additional instrument can be mounted on the T Board to set the inclination of the seat. It is a bent T shape instrument, which can be regulated in height and lateral length and has a beautiful carbon fibre protractor at its end, which gives back the measure of the angle of the seat. There are two channels for each arm of the T where the clamps can be easily mounted so that any obstruction to the clamps deriving from elements of the chassis such as tubes, brakes, engine mount, etc can be avoided.
Mounting the seat
Before starting to mount our first seat, the flexible T11VG, we measured the position of our old seat and in particular took care to write down the height, 16mm, equivalent to putting four rear sprockets beneath the seat which are each 4 mm thick. The distance between the rear axle and the back of the seat was 17cm with an angle measured by the protractor of 30°. This value must be subtracted from 90°, giving 60° of inclination of the back of the seat (driver back). What was also important to notice was that chassis was set at an average height amongst the three possibilities given on the Tony EVRR. This must be considered every time we measure the seat height, to avoid the seat touching the ground once we lower the chassis. The distance from one setting to the other on the chassis was 8mm. We set the chassis at the lower possible position. As the previous seat was already at minimum height we still needed 8mm more of height of the seat from the ground compared to before.
Since a high centre of gravity is a problem, we decided to increase the inclination to 55° as Tillett recommends. This bends the driver a little bit backward, reducing the height of his centre of gravity. The distance from the rear axle to the back of the seat was set at 15.4cm, which is a low value compared to an average of 16.5cm, but understandable considering the additional inclination given to the seat.