J Foam in a very fast tire when the track grip is high. It lends itself well to Primafelt carpet or when oil based additives are used. In these cases it may even be faster than T foam, although this lap time comes from a more aggressive handling car.

Specs:
Compound: 30 shore
Mounting pattern: standard 3 hole
Diameter: 1.87" or 47.50mm
Width: 1.5" or 38.09mm