The details
- The RadRunner 2 ships on the Kenda K-Rad, 20 x 3.3, and Rad Power sells that exact tire as the replacement part.
- The OEM version includes Kenda's K-Shield puncture-resistant liner and a reflective sidewall stripe, both worth keeping on a commuter that carries passengers and cargo.
- The 3.3 inch width is nonstandard: most 20 inch tires stop well short of it, and true fat tires start around 4 inches. That narrows the drop-in options considerably.
- Beyond Rad's own store, the K-Rad in this size is stocked by Kenda's shop and general bike retailers, so an out-of-stock at Rad is not a dead end.
If you want something other than stock
Some owners do run nearby widths, but it is a measure-first project, not a click-and-hope one:
- Clearance: fenders, fork, frame stays, and the passenger-rack hardware all sit close to a 3.3 inch tire. A wider tire can rub, and a narrower one changes the bike's ride height and feel.
- Rim width: the RadRunner's wide rims are made for a wide tire. A much narrower tire on the same rim gives a squared-off profile and vague handling.
- Rating: stick to tires marketed as ebike rated or with a reinforced casing. This is a heavy bike, often loaded, and lightweight BMX-style 20 inch tires are not built for it.
- Tubes: match the tube to the tire size range. The stock setup uses tubes sized for the 3.3 width, and undersized tubes stretch thin and puncture more easily.
Which variant this covers
This answer covers the RadRunner 2. Rad's K-Rad listing covers multiple RadRunner models, and the original RadRunner rolled on the same size, but Rad revises components between generations, so confirm your model appears in the compatibility list on the part listing before ordering. Your current tire's sidewall also states its exact size.
The trap to avoid
Assuming 20 x 3.0 or 20 x 4.0 is close enough. The 4 inch fat sizes generally will not clear the RadRunner's fenders and frame, and 3.0 tires sit noticeably differently on the wide rim. The second trap is buying a K-Rad in one of its other widths: Kenda makes this tread in several 20 inch sizes for BMX use, and the narrow versions are a different tire in everything but name. Check the sidewall size on the listing, not just the model name.
What owners actually do
The overwhelming pattern in Rad owner groups is replacing like for like: same K-Rad, same size, keeping the K-Shield liner. Owners who ride over debris-heavy streets often add tire liners or sealant-filled tubes rather than switching tires, since the puncture protection options in this odd size are limited. A common maintenance note: many flats attributed to the tire were actually chronic underinflation, so check pressure against the sidewall range regularly on a bike this heavy.