Hey guys, recently finished my first technic model being the McLaren P1. It's great, and I decided to motorize it however experiencing some problems. When the wheels are not touching the ground, there is no clicking/hesitation in moving forward from the rear motor that connects to the differential gear(s). However as soon as I place it on the ground and try to accelerate, I get intermittent and often clicking. I'm thinking the problem could be the motor isn't strong enough for the weight of the car(as the gearbox is maintained along with the piston engine) or maybe that main gear has to be changed..Any ideas? I had to rework some of the rear clam in order to fit the motor and still keep the right ride height...

I've also changed to a proportional motor instead of instant acceleration but that hasn't helped as much too.

 

If it's with the gears, I wanted to know how I could increase the torque or power for the car to actually go forward without any clicking noises..It can't even get over carpets without clicking and barely operates fine with a flat surface(All these operations are in Neutral Gear).

 

 

Here is the link showing all the issues; https://docs.google.com/document/d/1RRaNe1Wn7IPpqE2EsSx4JtDdFY7sslA-bRdLolljFNw/edit?usp=sharing

 

Simplest way to move your model with a single L motor would be to replace rear hubs with planetary ones. But you have to realize that you're loosing a lot of power in the gearbox. 

Don't make a motorized LEGO car with a gearbox. It went wrong for me and even if you try your best, it's too impractical. I highly suggest you remove the gearbox if possible.

Lego motors are likely to be not enough, and you will need something to shift the gears like a servo. I suggest you remove the gearbox altogether at first. The bonus point is that you get to remove some weight.
These models are very heavy, even a couple large motors will only be enough for a crawl. The 42039 set weighs 2 kg and needs 2 buggy motors to move at atleast some speed (according to youtube). That`s two of the fastest lego motors. Mine barely moves with 1 XL motor. The Mclaren is 7 kg as reported by some stores, and motorising it pushes the limit of what is possible with Lego. Even if you can get enough motors, the shafts will not handle this. And you will be making it heavier and heavier with more motors and batteries. Of course, there are aftermarket metal and carbon parts, and i saw people reinforce Lego universal joints and bushes using carbon fiber on top of them. But here you may throw hungreds of dollars towards motors, batteries, parts and still have the powertrain click or break somewhere or the car still barely crawl.  
So i have some ideas:
- use an rc car/plane motor, reciever, servo and battery. This powertrain will be light and fast, but I only tried it for small cars, about 1 kg. Ofc the gears and shafts will take a beating and may not withstand the torque required to move this car at a proper speed.
- use an appropriate scale rc car. Replace the shell with the Lego Mclaren body. This will go fast and you will not have to care about something in the drivetrain breaking.