He was 70 years old.

Brad confirmed the news in an emotional post ... saying, "My dad will always be my hero. He was quiet and understated, but that didn’t change the impact he had on me or that I watched him have on everyone who knew him."

"I am forever grateful for what I learned from this man, and I will remember him every day."

Brad's brother, Brian, also expressed his sorrow ... saying, "Got the chance to go with my dad a couple times this year to watch him drive his car."

"We never thought we would be here still after 2 1/2 years of fighting this. Still losing him isn’t easy knowing it’s coming, but I’m so glad I took the opportunity when I had the chance to do it."

Bob and his brother, Ron, were introduced to racing as teens when they served as crew members for their father, John.

Bob went on to have a decorated career -- he won 24 races in the ARCA series, including his championship run in 1989 ... ranking him eighth on the all-time series wins list.

Keselowski's dominance in the sport spanned over a decade -- from 1985-1996 -- and he finished top three in the points for a remarkable 7 consecutive years.

By 1995, Bob had taken his talents to the Camping World Truck series, where he was a driver and co-owned the K Automotive team, winning his lone race in 1977 at Richmond.

NASCAR released a statement saying ... "Bob Keselowski was a true racer whose determination and love of the sport embodied the NASCAR and ARCA spirit."

"He worked tirelessly to build his family-owned team, doing whatever it took to compete. Our thoughts are with the Keselowski family during this difficult time."