styles provide full independent suspension by hydraulic dampers and progressive rate coil springs. These give both a smoother ride and higher speeds than traditional types. They are also lighter. Race Bikes are the only style allowed to be used in races or qualifying heats. They are more compact and aerodynamic than a jog cart, have smaller seats that reduce weight, but require more athleticism for the driver to sit upon. Shafts may be of carbon fibre, aluminum, titanium, stainless steel or, less often, wood. Speed Carts have a similar design to race bikes in a single seat and stirrups for the driver, but have road-going tires, mud flaps, longer shafts and are heavier than race bikes. Some may have a limited form of suspension built into the seat. These are usually made of steel and are heavier than race bikes but much lighter than jog carts. When it rains, or the track has excessive moisture, trainers and drivers are required to put plastic mud flaps on the back of the wheels. All race bikes must comply with the relevant procedures and standards in order to be approved. In no way can a race bike have any component that will directly interfere with another horse or driver. Dog sulky Smaller sulkies are also used for dogs, both for racing and as transportation. The dog driving sulkies can be divided into two main types: Conventional two-shaft carts attaching to harness on either side of the dog or dogs. Single-shaft dorsal hitch carts, which attach to a single point on top of the dog's shoulders. A further distinction may be made between sulkies with the axles rigidly connected to the vehicle, and those with the axles insulated from the vehicle by springs and dampers. Those with springs and dampers may be further divided into single-axle sprung carts and "independent suspension" sprung and damped carts. Needless to say, those with independent suspension by coil springs and dampers tend to be both more expensive and smoother riding. Driving sulky construction materials run the full gamut from timber, through powder-coated steel tube, aluminum tube, and stainless steel tube. The very latest types (currently undergoing field tests in California) use nanotechnology-based stainless steels of prodigious strength-to-weight ratio. The great majority of driving sulkies available have the wheel axles rigidly affixed to the frames. This makes for a rough ride on anything but smooth surfaces such as pavement.