I just got the 42110 and assembled bags 1 and 2. I tested everything before joining the front axle and rear axle, and the friction was quite low, not nearly high enough to get gears to skip or practically stall an PF XL motor. When I joined the two halves, the lower gears worked just fine, but there was slightly more friction generated than I expected. However, on the higher gears (3/lo and above) the friction increased way more than expected, rendering 4th/lo somewhat unusable and 4th/hi completely unusable. Strangely, all the gears work flawlessly if the car is pushed backward. Also, the rotation of the output of both the 4-speed and the 2-speed transmissions is uneven, the output of the 4-speed transmission being way more uneven than that of the 2-speed transmission. The friction certainly is not the sum of the front and rear axles together, it is far more. Also, it is worth noting that I modified the 3:1 gear ratio between the axles and central differential to a 1.667:1 ratio, as with the 3:1 ratio the problem was way worse, but not because of the gear ratio, but because both of the U-joints were out of phase. Any explanation for this strange behavior or tips on how to fix it are welcome.
10/10/19 update: I watched RacingBrick's video on this subject, and it turned out that PART of the problem was that my U-joints were out of phase. I fixed that problem and it is working a lot better, but the problem still persists, although to a lesser extent.
10/11/19 update: I switched the 1.667:1 ratio for the 3:1 ratio while making sure that the U-joints were in phase. The unevenness issue is virtually gone, but the skipping of gears still persists, but to a lesser extent. I can post another YouTube video if it helps.
10/11/19 update 2: I added bag 3 and both of the problems are back, though somewhat different. Before, the gears started skipping immediately, but now it works just fine for the first 5 to 10 feet, then starts to skip. I checked the U-joints and they're in phase. I'm very confused.
link to video:(https://www.youtube.com/watch?v=jNyK0ZDi-pE&feature=youtu.be)