How much progress had Van Halen made on their seventh studio album before parting ways with David Lee Roth in 1985, and how much of that work made its way onto 5150, the band's first album with singer Sammy Hagar?

Naturally, it depends who you ask. Nobody disputes that the band's original lineup had begun writing some songs for the follow-up to 1984's smash hit 1984 before personality conflicts and a dispute over Roth's 1985 covers EP Crazy From the Heat led to the singer's departure.

The band recruited Hagar, fresh off his own platinum sales success with VOA and the hit single "I Can't Drive 55," and began working together on what became 1986's six-times platinum 5150.

Roth countered by assembling a new band featuring Steve Vai and Billy Sheehan and releasing the platinum-selling Eat 'Em and Smile later that same year. He says his former bandmates got a head start on 5150 by using his work. "I wrote half of those melodies. Half of those vocal moves and everything were already done and on little demo tapes," Roth told Howard Stern in 1993.

In his 1997 autobiography (also titled Crazy From the Heat) he expanded upon this claim. "It had been easier for them to put a new album together because I had worked with Edward on half of that album already," he declared. "[They] brought in a new singer, went right to all those old tapes and started with that as their go point, so it was much easier for them to reconstruct."

In neither case did Roth specify which 5150 songs he worked on. The Van Halen Encyclopedia lists "Eat They Neighbor" as an "unfinished, unreleased original" song, one of three post-1984 tracks the book says Roth began vocal work on before leaving the band. It also notes that there is rumored to be an alternative version of 5150's "Summer Nights" with Roth-written lyrics, and states that Eddie Van Halen had just begun to write "Dreams" when Roth left the band.

READ MORE: Learn About 25 Unreleased Van Halen Tracks

The new-look Van Halen lineup agree that "Summer Nights" and 5150's opening track "Good Enough" were the first songs they ever jammed on together, although Hagar says he wrote the lyrics and melodies himself. "When I got there, they'd been up all night writing," Hagar recalled in his 2011 autobiography Red: My Uncensored Life in Rock. "They had what became 'Summer Nights' and what became 'Good Enough.' We started playing and the engineer Donn Landee recorded everything we did. I made up the first line on the spot - 'Summer nights and my radio.' It just popped into my head the first time I heard that riff. The rest of the song I just scatted my way through. I did the same thing with 'Good Enough.'"

"[Sammy] never heard it before," Eddie marveled in a February 1986 interview with Musician, saying that the band's first session with Hagar made him feel like he was walking on air. "He listened to each song a couple times, not even that, and he just came in and sang it."

Hear Van Halen Perform 'Summer Nights'

In his book Hagar also notes that Eddie and his brother, drummer Alex Van Halen "had a lot of music left over from what would have been the next Van Halen record before Roth split. They had a lot of semi-formed ideas when I walked in on it. I had to write all the lyrics and melodies. I worked on their jams, picked them apart, and made songs out of them."

Hagar described "Love Walks In" as "one of the first songs that we worked on together as a band" while introducing the song in the band's concert video Live Without a Net.

How Van Halen Wrote 'Best of Both Worlds'

The story behind the creation of one of 5150's most innovative and enduring tracks, "Best of Both Worlds," illustrates just how inspired Hagar was by his new bandmates. The Van Halen Encyclopedia reports that Eddie was in a hotel room in NYC when he came up with the main riff for the song and that he called Hagar at three in the morning - it doesn't specify which time zone - to play it for him.

In his book Hagar recounts coming up with the "Best of Both Worlds" lyrics on a drive home from the studio while listening to an instrumental demo of the song. "I didn't get what [Eddie] was doing [at first]. But, on the way home, I heard the rhythm of the thing, and I started singing in the car... it hit me hard, right when I was pulling in the garage. Bang. The chorus hit. I went in the shower, but I kept coming out to dry off and write some more lyrics on a notepad. The song came to me like a flood. ...I wrote the whole song while I was still taking the shower. I went in the next day and sang it. Everybody was blown away."

Watch Van Halen Perform 'Best of Both Worlds'

Van Halen Lineup Changes

Three different singers and two different bassists joined the Van Halen brothers over the years.

More From KSEN AM 1150