Like grooving, how you sipe a tire is directly related to the track conditions. Sipes are by and large useless on wet, heavy tracks and make the most difference on a dry-slick track. They can be as close as 3/16-inch apart, but that takes a lot of skill if you don’t have the right tool. Getting them too close together can lead down the path of chunking, which we will get into a little bit later. So to keep our sipes parallel we relied on the Speedway Motors Mega Siper. This siper is machined out of billet aluminum and has 14 adjustable blades. Adjusting the blades is a snap by removing just two screws on either side. We chose a pattern that only used half of the blades because we knew that the track we were racing would retain some of its tackiness so we didn’t need as many sipes as we would have if the track went dry slick.
Another nice feature of the Mega Siper is that you can vary the distance between each sipe by changing the number of plates in between each blade. Plus, it uses standard utility knife blades that you can buy at your local hardware store.
Grinding is significantly different from grooving or siping, but no less important. Grinding knocks off the new surface of a fresh set of tires. That allows you to immediately utilize the good rubber from the drop of the green. Since you don’t have to wait for that top layer to wear off, grinding is particularly useful in short heat races where the tires don’t get much wear.
Grinding is also a valuable step in prepping tires that already have some laps on them. Not only will grinding remove the outside work hardened layer of rubber from a used tire, but you can also use a grinder to smooth out inconsistencies to return it to like new status. However, grinding is probably the trickiest of the three tire prepping processes.
To do it right for front tires, hold your grinder at a slight angle and grind from the outside of the tire to the inside. For the left sides, do the exact opposite. Since circle track cars are always turning left (we hope) forces are acting on the interior sidewall and trying to push out. Because of this, you should also grind outward from the inside edge of the tire. This helps to slightly round off the leading edge of the grooves so that they are less likely to be ripped off under side loading when the car is hooked up.
In a past article on the subject, CT found that many tire specialists prefer different tools for grinding. However, a popular option is a power sander with a variable speed control and replacable pads; use a slower speed for softer tires and faster speed for harder tires. Use pads between #36 and #40 grit.
Anytime you groove or sipe your tires you need to pay close attention to the depth and spacing of your cuts. Why? Simply put, if you don’t do it right you could cause your tires to chunk. Chunking is when small pieces of tire get ripped off of the tread. It’s costly, because a chunked tire is a junk tire.
Apart from running a tire that’s too soft on a high-grip track, here are a few items that will cause a tire to chunk:
• Too many grooves that weaken the tread blocks, reduce contact patch surface area and make the tire run hotter
• Cutting too many sipes through the tread pattern creating hard edges
Solving those issues is fairly easy, grinding will round off sharp groove edges to reduce the possibility of chunking. In addition, grinding your rear tires twice will help prevent chunks. Grind once laterally (across the tire) to prevent the tread from chunking under sideloading. A second grinding around the circumference of the tire will smooth out the leading edge of the treads so that they won’t chunk under acceleration.