Because SCS decided to manage the lower part of the exhaust as a chassis.
The entire exhaust is divided into chassis part and accessory part. My exhaust stacks needs some of the lower part of the exhaust. That's why I had to create a custom chassis.
http://pete379jp.s69.xrea.com/image/steam/Peterbilt_389_Pack_details_exhaust.jpg

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[M1] - Back of the cab/sleeper lights turn on tail lights, tinted rear/side (sleeper) window, removed toolboxes and frame deck plate, and added chrome sleeper shock cover.
[M2] - Removed back of cab/sleeper lights, tinted rear/side (sleeper) window, removed toolboxes and frame deck plate, removed chrome handrails from back of the cab/sleeper, added painted sleeper shock cover, and custom front fenders.

*mod version 1.31
Low Roof Sleeper [V]
It is just added an accessory slot for Cabin Deflector (Chrome Turbo Wing).


(Only 6x4 chassis available. Sorry to 6x2 chassis and liftable axle lovers.)
[V] - Removed lower part of the exhaust (for my exhaust). The other part is exactly the same as the vanilla 6x4 chassis.
[M] - Removed lower part of the exhaust (for my exhaust), removed rear end assy (for my mudflaps), and added chrome frame cover.
[MC] - [M] (see above) plus painted chassis frame and suspension.

My custom chassis [MC] does not support metallic colors. It actually works without any problems, but texture scale of the flake noise doesn't match to the chassis, so it doesn't look beautiful.

*mod version 1.31
Stretched Short/Long Chassis
I added "stretched chassis" to my custom chassis [M] and [MC] series.
Wheelbase of my stretched long chassis is 300 inches (762 cm). 43 inches (110 cm) longer than vanilla long chassis. If it is more longer than this, it may hard to drive for the gameplay.
My stretched short chassis is exactly the same length as a vanilla long chassis.


These accessories are dedicated to my custom chassis.


The headlights of the Grill Classic (grill73 and grill73b) do not have a high-beam light mask. When the headlights are off (for example daytime), flashing the headlights does not make the reflective housing of the headlights glow.


These accessories are dedicated to my custom chassis.


There are three reasons to omit the mount parts of Full fenders and Hoop fenders.
1. Because it is a blind mount, it is not very noticeable and I decided that I should omit it.
2. Minimize the decline in graphic performance by suppressing the number of polygons.
3. It is a pain in the neck...


[M] - Exactly the same as vanilla models, but they have an addon accessory slot.
[MC1] - [M] (see above) plus removed steps (Day Cab), twin fuel tank straps (Low Roof Sleeper and Ultra Cab Sleeper).
[MC2] - [MC1] plus painted fuel tank hangers.
[MC3] - [MC2] plus added center back panel (Low Roof Sleeper and Ultra Cab Sleeper).


Each of them has two identical items, but one is a black front window frame and the other is chrome.


My custom cabins, some chassis, and some my accessories support paint jobs, but there is a note below.
Some in-game paint jobs don't look better because the location of UV map of my Mudguards and the game's Cabin Deflector has overlapped. Because I forgot to consider the use of the Cabin Deflector...