Weight Distribution - The Basics


Technical Report By Ray Munday

23 June 2009


Aside from tyres, weight distribution is perhaps the single most important factor that affects the handling of your car. This is especially true in 2wd.


What Is Weight Distribution?

Weight distribution refers to the amount of weight at the front and rear tyres when the car is sitting flat on the ground. It is usually referred to as a % of weight sitting on the front. It is a controlled by the where the centre of gravity (c.g.) sits between the front and rear wheels. The closer the c.g. is to one end of the car, the more weight sits over that end.


What Are Typical Values?

Typically a 2wd will have a weight distribution about 35% front / 65% rear, and a 4wd about 45% front / 55% rear.


How Do You Adjust Weight Distribution?

There are several ways to adjust weight distribution, and different cars have different options.

To achieve a more ‘forwards’ weight distribution, you would:
  • move the battery forwards (or other heavy components such as the motor or speed controller)
  • lengthen the wheelbase at the rear (move the rear hubs backwards)
  • shorten the wheelbase at the front (move the front hubs backwards)
  • add lead weight to the front of the car / remove lead weight from the rear of the car and vice versa for a more ‘rearwards’ weight distribution.
The effect of each change varies from car to car, but as a reference, here is the relative change for an RC10B4 with Lipo batteries:
  • moving battery from rear to front: 0.5%
  • adding 10g lead weight to the front bumper: 0.5%
  • removing 25g weight from rear corner pods: 0.3%
  • short wheelbase to long wheelbase: 0.3%
  • changing spur to 3teeth smaller: 0.1%

What Does The Weight Distribution Effect?

The weight distribution affects both the forward traction and the cornering balance of the car. It can also affect the jumping performance.

Forward traction:
When accelerating, having a more rearwards weight distribution means that the rear tyres have more load and therefore more grip. In a 2wd, this means more forward traction. If you go too far, the front wheels will lift off the ground and you will wheelstand (a problem in high traction / bumpy conditions).

Cornering balance:
More weight at the rear means that the rear tyres have to work harder to stop the rear from swinging around during the corner. If the weight is too far rearwards, the car will tend to rotate too much off power (the rear is like a pendulum). If the weight is too far forwards, the front tyres need to work harder through the corner and the front will tend to understeer (push) through the corner.

On power steering:
If the weight distribution is too far rearwards, there will be very little steering on power as the front tyres are hardly touching the ground. If it is too far forwards, the car will feel very loose on power.

Jumps:
More rearwards weight distribution will make the front sit higher over large smooth jumps, but can make the rear slap and bounce off sharp jumps. More forwards weight distribution may jump more nose down on big jumps but bottom out less on sharp jumps, keeping the nose higher.


When To Adjust?

Getting the balance between these is very important and the optimum weight distribution will vary from track to track and even round by round if the track conditions change. Different tyres may also need a different optimal weight distribution.

Typically:
  • Move weight to the front as the grip comes up, if the corners are more flowing and / or if the track is bumpy.
  • Move weight to the rear if the grip is low, if the corners are ‘square’ and / or if the track is smooth.
This adjustment has a greater effect on 2wd cars than 4wd. With 4wds, the rules are similar but with all four wheels driving there is less difference to forward traction when you change weight distribution.


Typical Keilor Settings

I typically run a more forwards weight distribution for Keilor as the grip is high, the layout is flowing and you need good on power steering. In my B4, I run battery forward with long wheelbase, 10g of lead weight at the front bumper and no lead weight at the rear. When it is really grippy, I will also add 10g of lead in front of the servo, but this can start to make the car a little slower through the tight corners.


Summary

Experiment with different weight distributions on a practice day or club day to see how it changes the performance of your car. Keep notes on track conditions and tyres. At big events, you may find you need to change this between rounds as the track condition changes!!

Here is a quick reference chart to stick in your pitbox:

Weight Forwards Weight Rearwards
Less forward traction More forward traction
More on power steering Less on power steering
Less chance of wheelstand More chance of wheelstand
Less mid corner steering More mid corner steering
Nose down smooth jumps Nose up smooth jumps
Nose up sharp jumps Nose down sharp jumps
Takes corners more smoothly Makes corners more ‘square’
Better for high grip Better for low grip
Better in bumps (accelerating) Worse in bumps (accelerating)
Worse in bumps (braking) Better in bumps (braking)