Measuring wheels takes some patience and a good micrometer (caliper). The Rim may be 15mm in outside diameter (OD), mimicking a 15inch dia rim, and the center rib/nub/nib may be 17mm and 5mm wide. Thus, a typical tire with a 1mm thick casing will be 19mm trued (17+1+1), a meatier tire may be 18 or 19mm when mounted and untrued. Some manufacturers spec their wheels by the rim, many spec at the rib/nub/nib. In general for a lot of our series we run wheels with a Rim OD of 15, 16, or 17. 17s push the limits because they result in tire OD of 20mm and are hard to fit inside a wheel-well and they raise the CG, plus they result in a “higher” (hotter, faster) gear ratio [on our site is a spreadsheet that calculates the effect of wheel/tire size on Actual Gear Ratio]. A larger final OD also raises the chassis off the track and significantly reduces the already meager downforce we get from the motor mags.
Your tire is too small (tight) in Diameter for the wheel if it crowns ‘convex’ on the center; the nib is stretching the center up and pulling in the sidewalls. Your tire is too small in Width when it crowns ‘concave’ at the sidewall, producing devil-horns with a sunken center. This can also happen if the nib is too wide for your tire’s internal channel. For example CB Design tends to have wider ribs than others (they are popular with people who use super stretchy Urethane tires like PaulGage).
A loose tire is as bad as too-tight a tire because it will slip as you accelerate out of a corner; and your glue will break fairly easy. A tight tire requires no, or less glue, which reduces yet another factor that can go wrong. Once glue breaks the result is like a slipping drive (crown, spur) gear or heavily worn tires.
Sometimes you have to check the sidewall wording (the Slot.It markings) because with some wheel tire combos (esp with F30s), if the label faces out the tire is 0.5 to 1mm ‘skinny’ from the outside rim, (this is because of the positioning of the rib groove inside). Flip the tire and it fits perfect in that case.
Unfortunately the vendors are inconsistent on sidewall labels. A ’30’ on a Slot.It indicates it is a SIPT28/F30. A ’22’ means it is one of three 22-shore Slot.It tires (an F22 and two sizes of N22). A ’15’ = F15 Slot.It. A tire with just Slot.It on the sidewall hints it is a ‘P6’ which works good on Carrera and Scaley Plastic tracks [and in some unique weather situations with a low power motor it is acceptable on our track (but not as good as a 30)]. BRM 30s have specific part-#s on the sidewall but no Shore values. NSR Supergrips have a dull finish like F30s, whereas Ultragrips have a slightly shiny finish because they are a formula that includes some urethane (but Ultragrips dull with use and truing). NSRs do not have part#s or Shore values on the sidewall.
Note that Shore 22 and 15 tires are more likely to split when CA/superglue is used on them (it dries them out and cracks them).
For rear tires wide is typically better; in general the tires we use come in 10mm, 11, <span style=”text-decoration: underline;”>12mm</span> widths. 14mm widths are reserved for F1 type wheels, wont fit under most wheel wells and are Illegal in most of our series. Our F1 races typically spec NSR F1 tires.
Look at Dave’s (Butch) pit box for how he organizes and labels his spare tires; he clips the label from the sack and places it at the bottom of the cell where his tires are at. Other guys do the same with ‘tire tubes’ (basically pill bottles).
Break out your notebook and get advice from the various guys on what tires to use, and the specific part #s to use for certain series and motor/gear combos.
"... get on your bad motor scooter and ride!"
Sammy with Montrose