They were two below-the-radar signings for the 49ers that didn’t make headlines of any kind outside the Bay Area.

And, though one player is called Rock, he’s hardly a rock star, the front man with the microphone in his hand. He’s just one of the guys in the background ready to blend in, do what needs to be done and occasionally shine when he gets a chance to play a guitar solo.

But the addition of former Raider Rock Cartwright and the re-signing of C.J. Spillman were key moves, nonetheless, for the Niners; the kind of moves that can pay off time and again in subtle or surprising ways throughout the course of a 16-game NFL season.

Last year, the Niners’ special teams were as important as the offensive and defensive units in San Francisco’s success. As the 49ers went 13-3 in the regular season, won the NFC West and then advanced to the NFC Championship Game, it was the Niners who often won the battle of field position because of their kickoff and punt teams. Though place-kicker David Akers and punter Andy Lee were both Pro Bowlers, and returner Ted Ginn Jr. (also re-signed) was one of the best in the league, the guys around them were equally important on kick coverage and protection.

One of those players was Blake Costanzo, a free-spirit linebacker and special-teams ace who had 13 tackles on special teams in 2011 after coming over from the Browns, where he played for special teams coach Brad Seely -- now with the Niners -- considered one of the NFL’s best.

But Costanzo signed with the Chicago Bears in the offseason, leaving a gap that needed to be filled. That’s where Cartwright came in.

In March the 49ers signed Cartwright, a running back, to a one-year deal.

“Rock has been a standout special teams contributor who has also shown his versatility at the running back position,” said General Manager Trent Baalke, in announcing the signing. “He has all the intangibles that we look for in a 49er.”

Cartwright isn’t big, but is powerful at 5-foot-8 and 215 pounds. He played the past two seasons with the Raiders, starring on special teams. His most memorable play was a 35-yard pickup he made on a fake punt to help Oakland beat the Houston Texans 25-20 in October.

Before coming to the Raiders, Cartwright, 32, had played eight seasons for the Washington Redskins, the team that drafted him in the seventh round in 2002 out of Kansas State. For the Redskins his best season as a running back came in 2003 when he carried the ball 107 times for 411 yards and four TDs and caught 18 passes for 176 yards.

It’s been as a special teams contributor where Cartwright has made the biggest impact, however. He has 93 career tackles and also has returned 231 kicks for 5,450 yards (a 23.6-yard average), including one for a touchdown.

Cartwright also brings more to a team than just his play on the field. This past season he was honored for the second straight year with the Raiders’ Commitment to Excellence Award, voted by his teammates, given to the player who exemplifies hard work, commitment and enthusiasm.

Said Cartwright, when given the award: “I try to be accountable to everybody, my teammates, accountable to the community, to the organization, and I think that’s what this is about.”

Spillman, meanwhile, was signed to a three-year deal in February for a reported $6 million.

The 25-year-old safety played occasionally on defense, but was a key special teamer, as a gunner on punt coverage. According to Eric Branch of the San Francisco Chronicle, he led the team in special-teams tackles with 19 and was second on the team with 699 special teams points, as compiled by the Niners coaching staff.

The 6-foot, 196-pounder from Marshall originally was signed by the Chargers as a free agent in 2009, but then released in 2010 and picked up by San Francisco. He played 11 games for the 49ers in 2010 and 16 last season.

After signing his new deal, Spillman tweeted his excitement about staying with the team.

“Well Ladies and Gents looks as if I’m back in the Bay! Can’t wait to get back to work and get going. #NinerNation 2012 here we come!”