Think of the most important tool you need for your RC car both on and off track. If you answered the motor, tires or battery then you did not get it right. Perhaps, the mentioned objects have their own designated purpose and task for a remote controlled car but for the hobbyist itself, a pen and paper are two of the greatest tools ever.
Really, the pen and paper are the useful when dealing with RC cars. There is no joke here since listing down the numbers of your RC car is the subject; nothing else can do it better. Have you ever thought of experimenting before? If you have, surely you have also thought of writing down everything you discover, right? This is actually the same point why securing a written record of your RC car and its performance is very important.
Regardlessless RC cars, trucks, planes and buggies are customized or not; they all have individual standard measurement. Along the way, you will need to change some parts of your RC car to make it run faster and perform better. It can be the tires, the distance from the main body, the height and others. Elements such as the batteries, caster, running temperatures and many others are also important in the performance of your RC car.
If you still can not see why writing down these things are extremely important in having RC cars as a hobby; try to imagine a time you test-drive your RC car down a rough path you are not familiar with (granting you have not written down the numbers of your RC car and performance for that particular time of the day), when a huge rock came rolling down the hill and missed your RC car by centimeters. Pretend the rear part has been altered from its original state. How do you solve this problem?
The point is, anything that can significantly affect your RC car's performance is important in improving it. So, for the scenarioario given above (although we're not saying that it will happen), if you have put your entitlement RC car's numbers in to writing, it would not be such a big problem. All you have to do is turn to your record, and replace everything damaged according to what you have taken note of.