The Frame upgrade lets the Drifter Bike take more damage before it breaks down. Frame I costs 450 credits at Trust level 1. A tougher frame means fewer breakdowns when an ambush catches you on the road.
Ambush camp snipers, tripwires and crashes take longer to wreck the bike. You spend less scrap on roadside repairs.
Frame I is cheap, so it is a good early buy if you crash often. Add later tiers once speed and fuel are covered.
Derelict Vehicles
Derelict Vehicles are the dead cars and trucks across Oregon, and nearly all of them can be searched. They give crafting parts, ammo, fuel and Medkits depending on the type. Loot them as you ride, and use them as cover.
Engine
The Engine upgrade raises the Drifter Bike's top speed and acceleration. Engine I costs 1,200 credits at Trust level 1, and higher tiers add durability. It is one of the first parts worth buying, since speed is how you escape Hordes.
Exhaust
The Exhaust upgrade quiets the Drifter Bike so you can ride near enemies without being spotted. Exhaust I costs 600 credits at Trust level 1. Buy it early if you like stealth approaches to camps and nests.
Forks
The Forks upgrade helps the Drifter Bike absorb damage. Like the frame and rear suspension, it means fewer stops to patch the bike with scrap. Buy it after the speed and fuel parts.