For this achievement you will need to drive with Timber in the Rusty Sidecar or any sidecar for 5 minutes. To unlock timber you must complete the side mission "Man Eat Dog World", this side mission will pop up as you are playing through the game quite early on. After completing this mission you will be able to have Timber on your roster. After unlocking Timber you have to head to a place where you can spawn in a vehicle, this can be done out the front of the Prosperity or at any liberated outpost in the garage section. When you select the garage head to land vehicles and the cheapest Sidecar is the Rusty Sidecar, once you have bought this just get inside the car and then press
and go to roster. Then hover over and click
on Timber, when he spawns he will automatically come and jump in your sidecar, now all you have to do is drive around for 5 minutes until the achievement pops.