TRUCK & BUS WORLD


REGROOVING PRINCIPLES

The regrooving operation consists of cutting into the tread of a tyre a tread pattern deeper than the original so as to prolong its useful life. Mileage is thus increased and costs per kilometre are reduced.

When regrooving it is of fundamental importance to pay attention to avoid exposing the upper belts of the tyre and to ensure that an adequate layer of rubber is left to protect them.

In this booklet Pirelli supplies specific instructions regarding the patterns to follow in the regrooving of their tyre treads and the recommended groove widths and depth of rubber to remove from the bottom of the original grooves.

The European (EEC) and American (DOT) normatives demand that tyres designed and constructed so as to be suitable for regrooving are marked on both sidewalls with the word REGROOVABLE

TECHNICAL DATA

The regrooving operation must only be carried out by specifically trained personnel. • Tyres worn to the point where the original tread pattern is invisible (even in a restricted area) cannot be regrooved as it is impossible to evaluate the depth of the remaining rubber.
• As it is possible that the tyre may be worn unevenly it is important to check that the tread has a residual minimum depth of at least 2-3 mm.

• Before beginning the regrooving operation the tyre must be examined carefully to check that it is in good condition and that it has not already been regrooved. A previous regrooving is immediately recognisable to a specialist operator: the grooves lose their sharp corners, especially in areas with zig-zag grooves and there are characteristic signs (undulations) left by the blade within the grooves.
If the tyre has already been regrooved the tread wear indicators (TWI), that is rubber bridges at the base of the tread, will have been removed during the regrooving operation, and the groove bottoms will be smooth.
Regrooving is inappropriate whenever the tread is scraped, cut or torn. Any tyre damage or repairs that have not been well executed must be rectified prior to regrooving.

• Before starting the regrooving operation all foreign bodies (that could damage the cutting tool) must be removed from the tread and the condition of the groove bottoms must be checked.

• The depth of the grooves must be measured at a number of points around the circumference of the tyre. The blade cutting depth must be adjusted in accordance with the minimum measured depth.

• The regrooving must be executed with the specific cutting tool (gouge) equipped with an electrically heated blade, and preferably in a well ventilated place.
It is advisable to select a blade profile corresponding to the regrooving width and the profile prescribed for that particular tyre.
In order to adjust the regrooving blade correctly add the minimum remaining tread depth figure (A) to the regrooving thickness (P) specified by the manufacturer for that tyre size and tread pattern.

• Fit the tyre onto a proper support.
Exert constant pressure and, following the prescribed pattern, cut the rubber at the bottom of the grooves following firstly the circumferential grooves and then the transverse slots where present After few centimeters measure the reegroving width and depth in order to check the correct choice and size of the blade.
For tyres with “Zero Degree Belts” complete the regrooving in the direction of rotation indicated by the equilateral triangle placed on the tyre sidewall.

• By carefully following the procedure described a thickness of rubber is left at the bottom of the groove that is sufficient to protect the upper belts of the tyre.
Under no circumstances should the regrooving blade be allowed to shave or notch the metal belts as this would render the tyre unsafe and could compromise subsequent remoulding.

Regrooving Graph