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The one part nobody can see from outside, but it still plays a huge role in the car performance. First step is about the gear differential.
First of all, I start with replacing the kit case with the graphite one, as it provides better heat resistance and makes the diff work more consistent through the runs. The kit cases work all really well, but for the top notch performance and durability, the graphite version is a great option.
#324955-G
#324912-G
I then apply grease on the outdrives, put them in the diff case with the o-ring inserted. When I insert the big shims and pins, I first apply a drop of oil inside the diff, before inserting the first bevel gear. Then I put in more oil and then insert the optional alu diff pin with ONLY 2 gears. I am using the optional gears with o-rings, that work partly as a self locking diff.
#335081
#335031
Make sure not to overfill the diff, so fill it up just above the diff pin.
When you close the differential, make sure to tighten the screws in a cross order, as shown on the picture for proper function.
Quick & easy gear diff tip:
2 gears = more rotation
4 gears = more stability and grip
  
Part 2 - the shock absorbers in our XB2'26 build series.
When taking all parts out of the box and cutting out the white plastics from the mould tree, it is highly recommended to check the edges of these parts, and preferably sand them down with using a fine sand paper or brick like I do. This will ensure perfect fitting in the shock bodies of all those parts and therefore a proper shock function.
One quite common mistake while building these cars can be the pivot balls - make sure to separate the shock cap balls from the shock end balls. Even though they look similar, they have different shape. See the picture to recognize which balls belong where, or properly see it in the manual.
For my current carpet setup, I use the +2 shock ends in the front and +4 in the rear. The shock lengths I use is 21mm in front and 25.5 in the rear. Check my current setups on the setup forum to match the geometry with these shock lengths. Note that different shock length is needed when changing the front/rear hub height for example.
When all this is done and the shocks are assembled, it is time to fill them up with some silicone oil. My current setup for 2-hole 1.6-1.8 config is the 300-300 oils from the kit and for 3-hole 1.5-1.6 config it'd be 400-300 oils.
For holding the shocks while filling them up I use the HUDY car stand or the designated Shock holder:
#108160 or #109801
Closing of the shocks is step-by-step explained in the manual. I fill the shocks a bit over the top, put a drop of oil also into the cap, then tightening the cap onto the shock body. Press the shock rod in to allow the excess oil to bleed out. After this, I put the shock screw in the cap and I pull the shock in and out approx 6-10 times. Leaving the shock rod pulled out, then remove the screw again and press the shock rod in again to remove all excess oil left. Finally insert the screw back in the cap.
I always double check to have the same amount of rebound on the shocks on both sides. Then put the spring on with the spring cups - for the front I use -2 and for the rear -6 spring cups, to get to my desired ride-height with no problem.
Quick & easy shock tips:Softer springs -> more mechanical grip, better bump absorption on rough tracks, smoother landings, but the car can feel lazy and roll too much in corners.Stiffer springs -> sharper response, more stable in fast corners and jumps, but less overall grip.Softer oil or more/bigger piston holes -> shocks move faster -> more grip, more forgiving car, but body roll increases and the car may bottom out more easily.Thicker oil or fewer/smaller piston holes -> shocks move slower -> car feels more stable, precise, and "flat" through corners, but can lose traction on bumps.On high-grip tracks (carpet, glued dirt) -> go stiffer for more stability.On low-grip / dusty tracks -> go softer for more grip.
   
The rear transmission is what drives the XB2 forward, so let's take a look at what I do when building it on the '26 kit
On high grip, and especially carpet, I like to replace the kit gearbox with the graphite mixture one. It reduces flex on the rear end, that adds more stability to the car. But the material is also more heat resistant, so it helps with keeping the temp of all the moving parts more stable, especially while being close to the motor.
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And yes - I choose to use the 3G front motor gearbox for carpet here in Europe, as it gives me more rotation. I would suggest the 5G for very high grip conditions, or if you are looking for smoother and more easy-to-drive car.
5G conversion: #329005
5G Graphite Gearbox: #323026-G
25T gears for 5G (special resistant): #324225-G
For carpet, I go with the kit setting of the diff height, that means 1dot up (dot facing down, diff up), as can be seen in the picture.
When inserting the diff into the gearbox, always apply some grease to prevent any damage on the gears.
IMPORTANT: Always put the diff inserts in the same height on both sides to prevent any damage to the parts and for proper function.
I use the included ventilated slipper plates with the kit yellow slipper pads, this combo works very well on most of the conditions we run on.
Quick & easy tips for the rear tranny:
3G = more rear grip, more rotation, more aggressive
5G = more flat, more corner speed, more calm and easy to drive
Diff higher = more corner speed, less grip
Diff lower = more rear grip
(Also depends on the ride height of the car)
  
On a 2WD car, the driven part is the rear drivetrain, together with the rear suspension assembly - Steps 4&5.
Let’s dive into the build steps and take a closer look at some key details.
Starting with the rear shock tower, I use the "kit" (initial) settings for the wing height, that being the upper holes. This has been found a standard base for almost all tracks we go to.
Higher wing - more grip and high speed stability
Lower wing - more steering and rotation, less grip.
Toe - mounting the chassis brace and suspension holders on the car, I most commonly use 1° or 1.5° of rear toe-in. RF position (C block) up&out as well the RR position (D block) being up&out. This setting gives the car more stability I found, and is my go-to base setting for high grip tracks.
I recommend installing the shock adapters on the carpet kit (separate bag in the kit) before mounting the arms on the chassis, as this way it is much easier to access. I use the two inner arm holes, with the arrow on the adapters pointing out from the center of the car.
Now onto Part 5 - rear drivetrain:
Alu link holder has been updated to become even more rigid than ever before. Now, the titanium shock stand-offs are also included in the kit!
I switch the kit graphite hubs for the optional alu ones, as these provide better stability on high grip tracks. Nevertheless the included graphite is a great start for all racers no matter what level.
Make sure to properly check the inserts in the hubs, so the front matches the rear, therefore the pin goes straight through the hub, and not under an angle. My start position is pin out&up (hub in/low).
Hub higher (pin / outter arm pos. lower) - more stability
Hub lower (pin / outter arm pos. higher) - more grip and rotation.
Last important notes on the rear end - make sure the collar on the sway bar is tight well and centered, to prevent any bind, and assure proper function.
My initial shock position is the same as marked in the manual.
For more corner speed and softer susp. - lay the rear shocks down
For more support on the rear susp. - stand the shocks up
  
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