Goku gave the Dragon Team, composed of his many allies, a warning from the future. Three years after the time he got the warning, a pair of androids would emerge and kill most of them. The messenger, Trunks, the future son of Bulma and Vegeta, also warned that Goku would die before he made it to the battle due to inflammation of his heart's muscles, though Bulma in the future gave Trunks medicine that he took into the past and gave to Goku.
Once Bulma in the present day learned of the need to train, she got an idea. Capsule Corp was rich, so she budgeted some of their money to build a roller coaster that would kill normal people through G-force. Why would she make it? Because the Z Fighters, the fighters among the Dragon Team, were far stronger than ordinary humans and could use it to train.
She had a finite area to build the roller coaster in, so she ordered builders to make as long of a station platform as possible. After that, they started building a gentle slope up with a chain lift. After a certain point, though, the ride couldn't be supported on flat ground, so Bulma ordered landscaping to add a huge square of land (as in, it looked like a square when viewed from above) high enough up to increase how high the coaster could go. As they continued to build it higher, though, they were coming up on the limits of the area they were allowed to build in, so the coaster was made to turn.
The build left the elevated ground, so more landscaping meant it could go higher still. After making the roller coaster turn again, they seemed to reach a vertical limit of how high they could build it. Builders made the ride go from a gentle slope up to flat to a steep slope down. Continuing to build it down, they ran into the boundaries of where they were allowed to build again, which applied even when underground. Bulma ordered it to turn with a tight curve and keep going down with a steep slope. Soon after that, though, they had to use more gentle slopes down. Once they reached as far down as they were allowed to build, builders were ordered to make an S-bend to the right. After that was built, the next step was a banked curve leading to a small helix up, with a large helix up in the middle.
After Bulma was satisfied with how far up the helix went, she ordered it to straighten out and do three small over-banked curves to the left followed by a larger one also to the left. The next thing that she ordered the construction of was another small helix up. After it seemed to be around the same height as the previous one, Bulma said, "Now I want you to make a small over-banked curve to the left, a large one, and two more smaller ones. After that, build a helix up to the surface. I'll let you all figure out how to connect it to the station."
They obeyed and used a rather simple way to connect it. They also made the entrance of the ride look like a miniature replica of Kami's Lookout with fake Dragon Balls in the turnstiles for both the entrance and exit.
Bulma realized that she had no name for the ride, so she found Goku and asked, "Goku, what should I name something I've been working on?"
"What is it?" he asked.
"… It's something I made for you to train on. It's a roller coaster that I'm hoping pulls at least eighty times Earth gravity,"
"How about… Uh… I'm no good with names! Ask Gohan!"
Once Bulma found Gohan, she asked, "Can you come up with a name for a roller coaster I made for your father to train on?"
"How about… High Gravity? It uses intense G-force, right?"
"That's the idea! … But I haven't tested it yet. I'd rather not test it with people even if they could survive it."
Going back to it, Bulma ordered workers to put dummies that weighed as much as people in each of the seats… along with multiple cameras.
It took a while to get to the top of the ride, but as the roller coaster train descended, it started shaking. It was hard to tell what was happening once it got to the first helix up. Bulma wondered what kind of G-Force it could pull.
After it returned to the station, test results came back and its top speed was about a third of the speed of sound. Granted, Saiyans like Goku and his rival Vegeta could fly at speeds above Mach 5 and had extreme durability.
The maximum vertical G-force was ninety six G's, the minimum vertical G-force was negative four G's, and the maximum lateral G-force was ninety seven G's. Bulma started to worry that even Saiyans wouldn't be able to survive that even though her tests didn't tell her what the average G-force was or how long they'd experience the maximum and minimum G's.
Bulma was about to contact some of the Z Fighters to ask if they wanted to train by riding High Gravity, but deduced that Krillin, Oolong, Piccolo, Gohan, and Goku were busy. She wondered if they were fighting someone.
The next day, when Goku arrived, Bulma asked, "What were you doing yesterday?"
"Fighting Freeza's brother, Cooler,"
"He has a brother?"
"I didn't know either,"
"Wanna ride High Gravity? … I just realized how durable it must have been for the roller coaster train to survive the intense G-force, but I'm still worried it'll be dangerous to you. Either that or you'd destroy it by accident."
Goku decided to ride the roller coaster anyway. Bulma continued to worry that he'd either destroy or otherwise break the ride or die on it.
When the ride returned to the station, everything was somehow intact. She asked, "Are you okay? Do I need to convince one of our friends to get a Senzu Bean?"
"… I may have broken at least one bone. But it was fun!"
Where was Vegeta? He was training in a high gravity chamber that got up to a hundred times Earth gravity.