Page updated
February 2004

Get a Grip! - to improve road holding

"Camber";

"Caster";

"Toe-in";

"Toe-out";

"Anti-squat";

"Droop" -

So many options.
Where do you start?

Experienced racers use the numerous adjustment options to fine-tune the chassis - but first, it must 'work' properly - and be balanced.

Check out these links......

CHASSIS TWEAK

CHASSIS OPERATION

SHOCK ABSORBERS

SHOCK OIL

The testing process for each of these - and the solution - is very logical, and should lead to stable, predictable handling with 'factory settings'.

THEN, experiment with small changes to these primary adjustments (click on each underlined word for guidance).

RIDE HEIGHT

DROOP

CAMBER

SHOCK POSITIONS

The aim is to ensure the chassis can move freely - but in a controlled manner - so that the weight is moved to the correct corner at the correct time. Care - attempt one thing at a time, and observe effects. Buy or borrow a copy of "XXXmain" or similar

Southend - the world's longest pleasure pier

Chassis Tweak

What is it?
A twin-deck chassis takes its strength from the 'box' which is created by fixing the upright sections to the horizontal decks. If the screws are not tightened evenly, the 'box' section will not be 'square' resulting in a twisted (tweaked) chassis.

Remove the wheels and any other component which hangs below the chassis line.

Lay the chassis on a (perfectly) flat surface.
There should be no gaps between the chassis and the surface (unless caused by protruding screw heads), and the chassis should not 'rock' - either front to back or side to side.

Remove the shock absorbers and support the central chassis on blocks -
do the wishbones hang at the same angle from the chassis?
do the wishbones hang at the same distance above the perfectly level surface?
Measure if necessary.

How to correct a twisted chassis

Loosen all screws which form the 'box' shape.

Gently twist the chassis and re-test.

Repeat until the tweak is removed - or get a new chassis!

How to correct wishbones which do not hang evenly

Check the pivot points for obstructions or stiffness -

Remove obstructions

Lubricate pivots or 'shave' component faces to permit ease of movement.

Unscrew 'droop' screws completely.

Check construction of wishbone/wheel hub interface and adjust so that all are identical and in accordance with the instruction manual.

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Chassis Operation

What is it?
The independent suspension on all four wheels is designed/intended to enable the car to 'adjust' itself to different surface conditions.
As the car corners, the weight is transferred to the outside wheels and the car 'rolls' (but shouldn't turn over!).
If a wheel strikes a 'bump' or dip in the road surface, the wheel should be able to move up or down without adversely affecting the rest of the car.
To do this, the moving parts of the chassis must be able to move freely.
Some parts do not 'fit' properly from new. Others are badly assembled.
A simple visual and physical inspection will reveal any imperfections of movement.

How to correct it?

Carefully dismantle the section which does not move smoothly.

Test each component to identify the item (or items) requiring adjustment.

If a component is too tight in its housing, carefully remove excess material - file or 'shave' with a sharp knife.

Clean and lubricate pivot points.

Reassembly and ensure all components move freely under their own weight.

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Shock Absorbers

What could be the problem?
Shock absorbers connect the lower wishbone to a fixed point on the upper chassis.
If they are not exactly the same length - both fully extended, and at 'rest', the chassis wil not 'sit' evenly - and the previous attempts to 'balance' the chassis will be wasted!
Measure each length using a vernier guage or micrometer - preferably between the points at which the shock absorber is attached to the chassis.

How to remove variation
"Fully extended length"
is usually adjusted by moving the lower mounted lug on the shock absorber shaft (on the screw thread) - but check the car instruction manual.
The preferred length will depend on race conditions, but the instruction manual will quote a 'basic' length.
"Rest" length can only be tested by compressing the shock absorber (without the spring). Measure the length after the moveable section has 'sprung-back' - if it does.
The 'rest' length should be close to the fuly closed length. If it is not, the oil must be replaced - see below.

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Shock Oil

What could be the problem?
Shock absorbers contain oil in order to 'slow' or 'control' the movement of the spring fitted to the outside of the shock absorber.
Air mixed with the oil, too much oil, or too little oil will affect the rate and amount of movement of the piston within the shock body - and the amount by which the spring is controlled.
If the shock absorbers are not identical in their movement/damping effect, the independent suspension units will operate at different rates - and the car will be unbalanced.
To be effective, the spring rate and the rate of damping (control) from the oil should be matched. For example -

A light oil (40wt or lower) with 2 or more holes in the piston will have little effect on strong springs.

Soft springs will not effectively if paired with heavy oil (50wt or higher) with only one hole in the pistons.
How to remove variation
The only solution is to re-fill the shock absorbers with fresh oil.
The existing oil will be contaminated, so new oil is essential, and the shock body must be cleaned carefully.
Refer to the car instruction manual - and ask for tips from somebody who has the same car.
Ensure that each piston has the same number of holes - and that the same weight of oil is used in each piston. If there is any doubt about which oil has been used previously - change them all!

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Ride Height

What is it?
The gap between the car chassis and the race surface - measured at the front and the rear of the car - but also at the four corners of the chassis which protrude between the wheels.
The amount of ride height depends on the track conditions, but should not be less than 5mm.
Ride height should either be the same at all 6 points (unless the front and rear points are not level with the other 4), or the gap at the rear should be greater by approximately 1mm.
More .......

How to adjust ride height?
Check the car manual instructions
Most cars use spacers above the spring - keep them safe, it may be necessary to change the ones used.
Some cars have threaded adjusters on the shock body.
Simpler to use in theory, but less easy to balance.
If the side-to-side ride height is different and the shock absorbers are of equal length examine the weight distribution of the moveable components (batteries, motors etc.) and consider the addition of weight to compensate for (say) heavy batteries.
DO NOT BE TEMPTED TO ADJUST THE RIDE HEIGHT ON ONE SIDE UNLESS IT IS ALREADY WRONG!

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Droop

What is it?
The term "droop" refers to the amount by which the chassis is permitted to 'lift' at each corner before the wheels lose contact with the ground - and therefore grip.
As with all significant adjustments, the amount of droop varies with the track surface and layout - as well as other features of the chassis such as spring rate shock absorber length and shock oil.
A reasonable starting point is 2mm droop at the front of the chassis and 3mm/4mm at the rear - but it must be identical on each side.
Click here for a definition of droop.

How to adjust droop?
Check the car manual instructions
Most cars have droop screws (see diagram) which enables fine adjustments to be made each side.
A perfectly flat surface is required, together with a reliable and accurate height guage.
The height guage should be 'stepped' in 1mm increases.
Push the guage under the car along the centre line at the front and the rear until it is supporting the car and the wheels can just rotate.
The difference between the measured ride height at rest and the height at which the wheels leave the ground is the droop!
Adjust as required.

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Camber

What is it?
"Camber" is the angle (in degrees) by which the wheels are inclined towards or away from the body of the car.
Click here for a diagrammatical definition of camber.
Negative Camber is desirable because as the car 'leans' through a corner, more of the tyre is kept on the track surface than if the camber was set to zero.
The most effective amount of camber will depend on track conditions and layout as well as shock absorber settings - and whether anti-roll bars are used.
A good starting point is 2 degrees negative camber on all wheels. It would be very unusual to reduce to less than 1 degree or increase to more than 3 degrees.

How to adjust camber?
Check the car manual instructions
Most cars have adjustable links between the top of the wheel hub and the chassis.
The method of adjustment varies, and some cars require the link to be removed in order to adjust the length of exposed screw. Otyers have 'turnbuckles' - which can be turned in situ.
Some cars require sections of the wheel hub to be replaced.
Essential tools include a perfectly flat surface on which to stand the car, and a simple, reliable and accurate guage - the simplest is a 'square' of plastic with each side set at a different angle to the 'vertical'.

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Shock Positions

What is it?
Most cars provide several fitting positions for the shock absorbers - sometimes as many as five at the top and two at the bottom.
In a near vertical position, the shock absorber will be responsive but 'harder'.
This could mean that the car is 'twitchy' and grip is lost suddenly.
In a 'flat' position, the shock absorber is less responsive but gets progressively harder.
This creates a smoother, more predictable, operation.
Choosing the best position will depend on track conditions and the other suspension settings - if there was an easy answer, there would be fewer options.
For example: A fast, smooth track layout will normally require the 'flat' position - but if a high speed chicane is included, the upright position will give better grip at an important section!
How to adjust shock position?
Check the car manual instructions
Shock mounting positions are normally screwed/bolted on to the chassis - so movement is a simple case of dismantling and re-positionning.
The choice of which lower shock position to use will depend on whether the suspension should be softened slightly (the inner position) or hardened (the outer position).
Yes, that does seem to contradict the 'vertical' rule. A simple reference to schoolday physics will confirm that more pressure is applied by the lower arm (therefore the track) if applied further from the pivot point!
However It is also worth noting that the distance between the mounting positions when the shock absorber is 'vertical' is a lot less than the distance when the shock absorber is 'flat'.
It may be necessary to increase or decrease the length of the shock absorber to retain the same ride height and droop.

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END