Early 80s stepfather/stepdaughter thriller

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Hi folks, looking for the title of a thriller that aired frequently on HBO roughly around 1984-85.
The premise is a mother remarries and her daughter begins suspecting the new husband(her stepfather) of bad intentions, likely thinking he only married her mother for the money.
She begins spying on and following him. One scene that has always stuck out is him buying her a 10-speed bike as a birthday present after realizing she’s been following him. I believe he says something along the lines of it’s fast or faster than walking, and I’m pretty sure one exact line is, “It has ten gears, you know”.
She eventually finds out her stepfather is having an affair with her best high school friend. He kills her friend by either stabbing or strangling her after the stepdaughter catches them having sex, and then attempts to kill her to silence her. He ends uo getting killed somehow and she survives, and I’m pretty sure her mother survives as well.
It was definitely made in the early 80s and the only details I remember about the actors are the stepfather looks like a cross between Ryan O’Neal and a less bulky Reb Brown(he wasn’t either actor), and the stepdaughter’s best friend was a pretty brunette with very big boobs. I had found the title maybe 20 years ago but have forgotten it since and can’t find it again. Thanks!

BlastHardcheese Asked question Dec 24, 2020