SANTA CLARA (AP) — Rock Cartwright never got to say a proper goodbye to his mother eight years ago.

She never told him when doctors discovered a thin film covering her heart that restricted blood flow and ultimately caused her untimely death, fearing her youngest son would be worried sick. That was her nature, and also his.

By the time Cartwright received word from a cousin that his mom was in the hospital in his hometown of Conroe, Texas, on Memorial Day 2004, he was too late. She was gone at just 46.

"There was nothing they could do about it," he said Tuesday. "It kind of happened unexpectedly."

To this day, the 49ers special teams standout and running back keeps Johnetta Cartwright close each time he steps onto the football field — and that hasn't changed as he changes teams for the third time in his 11-year NFL career.

The 32-year-old Cartwright has a picture of her on his phone and looks at it before every game. He wears a pendant with his initials on one side and her full name and the years she lived on the other.

"I hate that my mom had to leave at such an early age," he said. "It's life. She's in a better place now. I definitely feel she's always with me. She watches down on me."

Cartwright will be filling a big special teams void after the spring departure of Blake Costanzo, who signed a two-year contract with the Chicago Bears.

Ask Cartwright and he will say, over and over again, that he is willing to do whatever the 49ers need to make another run at a Super Bowl that was right within reach last season.

Perhaps that's why he is a perfect fit in coach Jim Harbaugh's unselfish, team-first system that worked so well last year. Cartwright has 228 career carries for 956 yards and six touchdowns, with another two catches for scores.

It was his late maternal grandmother, Betty, who gave him the nickname Rock when he was 9 and a budding baseball player. He stopped growing in high school — he stands 5-foot-8 — and switched from linebacker to running back.

"My grandmother gave me the nickname Rock Baby when I was younger because I was kind of clumsy," Cartwright recalled. "Then all my buddies, they just started calling me Rock. It kind of fits because I didn't grow any. I'm 5-8 and it fits me well. I'm fine with it, I enjoy it. It kind of stuck. There you have it. It doesn't bother me. It's cool with me. I wasn't going to shed that. My grandma started it. She was really special to me. It's a great thing to have."

With the Niners, Cartwright doesn't have to adjust to a big cross-country move this time as he did following his first eight seasons with the Washington Redskins. He played the past two years with the Oakland Raiders.

Harbaugh and the 49ers (No. 4 in the AP Pro32) are counting on Cartwright filling a key spot for an opportunistic unit that developed its own quirky identity last season — not to mention its own theme song in Future's "Tony Montana."

"I think it gets the opposing team kind of frightened a little bit," Cartwright said with a smile.

And Cartwright might just get more carries on offense in the final two exhibition games than he figured to before Brandon Jacobs injured his left knee in Saturday night's preseason loss at Houston.

"I know I'm able to run the football at this level. I know I'm able to play special teams at this level, at a high level," Cartwright said. "Any time I'm able to carry the ball, I'm always trying to show I can still play at a high level."

Cartwright has a fan in three-time Pro Bowl running back Frank Gore, that's for sure.

"He's a hard worker. He's very smart," Gore said. "He's a great guy in the locker room, in the running back room. He's been around a lot of great backs and I ask him a lot of questions about the other great backs, what they do and what they didn't do, pick his head. I'm happy he's over here. Whatever role coach wants him to do he goes 100 percent."

Yet Cartwright isn't counting on a roster spot just yet. He is living in a hotel less than a mile from team headquarters, his belongings in a storage unit in Oakland until he knows whether he'll be sticking around for the season.

"I've never gone into a year saying, 'I'm going to make the team,'" he said.

Cartwright has developed that humble, cautious approach along the way — in part because of what he went through losing his mom.

He had just spent time with her the previous days as he prepared to return to Washington, D.C., for a minicamp that day in 2004. They had just gone out to eat at her favorite chicken restaurant, and the day before she passed Cartwright sat on her lap on the porch and brought out her favorite soda.

"The day before we were doing things that were saying goodbye, but I didn't know it at the time," he said.

His mom needed to stop smoking and start eating better, but she "didn't really abide by the rules. That kind of scared me." Cartwright has taken charge of his health and is clear of heart disease symptoms at this stage — and he's determined to be a positive example for his daughters, 12-year-old Brianna and 3-year-old Jaida. Brianna remembers her grandma, and even Jaida asks questions when Cartwright takes them along to visit her gravesite whenever he is home.

Cartwright is now playing at a fit 209 pounds, down from his high of 265 in junior college and the 237 he weighed at the NFL combine before being selected by the Redskins in the seventh round of the 2002 draft out of Kansas State.

"Self-discipline plays a part in everything. I don't take anything for granted," Cartwright said. "I don't take the next day for granted. That's why I go out and try to play so hard, because you never know when that opportunity isn't going to be there anymore. You have to seize the moment."

Notes: LB Aldon Smith, who injured his right hip in the team's first exhibition game against Minnesota on Aug. 10, isn't ruling himself out of Saturday's third preseason game at Denver. "At the rate I'm going, who knows?"... RB LaMichael James was back in full pads for Tuesday afternoon's practice after injuring his left ankle in Saturday night's preseason loss at Houston. ...CB Chris Culliver was not on the field for the start of afternoon practice for the second straight day. He did take part in Tuesday morning's walk-through session.