What do stone rejectors look like?

Take a look at many Bridgestone radials and you’ll see small, raised platforms evenly spaced along the floor of some tread grooves. Those are the stone rejectors. They’re small, but they play a big role in retreadability.

What’s their job?

To prevent stones from getting lodged in the grooves, then being driven through the undertread and belt layer.

The stone rejector platforms use extra rubber material to help keep stones from forcing their way through to the undertread.

What happens if stones reach the undertread?

If the stones pierce the undertread and get to the belt layer, they could harm the belts, exposing them to rust. This affects retreadability because it could easily destroy your casing.

How can you tell the difference between a stone rejector and wear bar? 

Stone rejector platforms are taller; their height depends on the original tread depth and application of the tire.

For example, the Bridgestone M726 EL drive radial – with up to 32/32" tread depth – uses stone rejectors that stand about 7/32" from the bottom of the grooves. Wear bar indicators stand 2/32" from the bottom of the groove.

While you can find hundreds of stone rejector platforms neatly lined up in a groove wall, wear bar indicators are placed in six, eight or ten places around the tire.

How can I find them?

Tread wear indicators run all the way across the tire surface so look for a symbol on the sidewall and follow it up and across the tread. Bridgestone uses either a triangle or the abbreviation “TWI” (for tread wear indicator) on the sidewall.

Before checking tread depth, make it a habit to locate the wear bars so you don’t mistakenly stab your tread depth gauge on top of a wear bar ... and lose 2/32nds of good wear.

Why are they called “rejectors”?

They’re called rejectors because that’s what they do; they help prevent stones from getting lodged in the bottom of the grooves, where they could get stuck and damage the belts.

And they’re in all tread grooves?

It depends on the tire design and application, but generally speaking, stones tend to grip more tenaciously in the center grooves and generally drop out of the outside grooves by themselves.

Why the difference?

As the tire makes contact with the road surface, the grooves close, trapping stones and small debris in the footprint. As the tire leaves the footprint, the grooves open and many of the small stones simply fall out.

When the tire flexes as it rolls into and out of contact with the road, the outer grooves are affected more than the center. It’s the additional flexing that often lets loose stones escape.

The dynamics in the center grooves are a bit different. The casing plays a big role in helping the tire retain its shape so the tread lies flat against the road for even wear.

Because of this, the center tread grooves open and close less, so if a stone gets caught, it tends to stay there. That’s why you’ll find stone rejector platforms in the center grooves of many Bridgestone radials.

And that’s all to prevent stone drilling?

Stone rejector platforms are just one innovation used in preventing stone drilling. To discourage stones from getting wedged in the first place, Bridgestone pays a lot of attention to the depth, shape and width of its grooves. They even consider on a case-by-case basis whether a groove wall should be sharply angled or gently rounded.

Super tough tread compounds also help.

How can tread compounds prevent stone drilling?

The cut- and chip-resistant compound that’s used on many of Bridgestone’s on/off-highway tires is super-durable and makes it harder for sharp stones to penetrate the belt layer.

Is there anything we can do to reduce stone drilling?

Make it a habit to inspect and remove trapped stones from your tires. They could fall out and never pose a problem, but, on the other hand, particularly stubborn debris could destroy the casing.

If you don’t already own one, go out and buy a tool designed for removing cotter pins. They’re between five and ten bucks. These tools do an excellent job of removing stones trapped in tread grooves. Some people use screwdrivers, awls or needle-nose pliers; we prefer cotter pin
removing tools.

A cotter pin tool does an excellent job of removing stones trapped in tread grooves.