A wheel must be strong enough to resist the many forces acting upon it but light enough to allow it to be steered. A wheel reduces friction by rolling and when 2 are used in conjunction with an axle allows the carrying of heavy load.
Load is most stable when it is within the area bounded by the wheels. Passengers tend to sit within that area as this provides the greatest comfort and protection for the passengers.
Your wheels are part of the overall styling of your car. Cleaning them makes your car look smarter. Try not to kerb your wheels as this can distort the wheel and create vibration when driving.
The wheel is made up of a hub at the centre of the wheel which is where the axle connects joining 2 wheels. A set of bearings will allow the wheels to rotate around the axle. The hub is then connected to the rim by either wires or spokes.
The wheel can be dished meaning that the hub and wires or spokes are set in a bit from the rim in order to protect them from damage. The rim is the outside edge of the wheel that holds the tyre.
The width of a tyre should never be greater than the width of the wheel. A tyre that is wider than the wheel will distort when cornering. The wheel being made of steel and the tyre being rubber.
Nor should a tyre should not be narrower than the wheel. It would will not flex properly resulting in excessive vibration and an uncomfortable ride.
Toe, Camber and Castor
This is how the wheel is positioned on the axle.
- Toe in is when the wheels point in towards each other.
- Toe out is when the wheels point away from each other.
- The more toe, the greater the wear to the tyre.
When the toe is pointing inwards the wheels are tending towards each other and this gives steering stability. So toe in is used on vehicles that are meant for public roads because it gives better stability and therefore safety.
Whereas when the toe is pointing outwards it makes the steering more responsive. Toe out tends to be used on racing vehicles because of the greater response.
- Camber is when the wheels are leaning relative to the vertical.
- The more a tyre leans from the vertical the greater the uneven wear to that tyre.
When the wheels lean into the engine this is called negative camber and gives a greater stability when cornering. The wheel leans in during the turn, giving greater grip allowing the turn to be taken faster. The greater the negative camber the more camber thrust that is generated. On even ground this is balanced out between the wheels, but on uneven or rough ground it gives shaky steering.
If the wheels lean out this is positive camber. This tends to be used on off road vehicles as it lessens the steering effort.
- Castor can add or subtract directional stability of your car’s steering.
- Too much castor and the vehicle is too hard to steer.
- Too little and there is a tendency to wander.
- Castor is the steering pivot point that is vertical through the center of the wheel.
- This is how the wheels are pushed for steering.
If the wheels are pushed forward from behind this is called positive castor. A bicycle with the front forks raked forward is a good example of this. This gives the bicycle stability. Positive castor is also known as trailing castor because the steering effect is behind the wheel (contact patch).
Negative castor is when the steering wheels are dragged from the front. Shopping trolleys are good examples of this. It gives the trolley maneuverability. This is also known as leading castor because the steering effect is in front of the wheel (contact patch).
A wheel larger than the standard size for your vehicle will mean the vehicle is going faster than the speedometer shows. Put simply this is because your speedometer is measuring how many times the wheels are turning. If the wheels are bigger than standard they will be travelling further and hence faster for a given number of turns.
Most vehicles will carry a spare wheel, but in order to create more usable space in the vehicle a much thinner spare is sometimes used. These are for emergencies only and are sometimes known as 40/40 wheels. This is a reference to the fact they should not be driven faster than 40 mph over a distance not greater than 40 miles. If your car has a space-saver wheel you must consult The Manufacturer’s Handbook for the correct use of it.
These are a major contribution to the safety and comfort of the vehicle. How much grip a tyre has is determined mainly by how much of the tyre is in contact with the road. This is known as the contact patch or sometimes the foot print.
Completely smooth tyres are known as `slicks` and are used for racing and are dangerous in wet weather. Tyres that are to be used on the road must be able to perform in all conditions.
In wet weather as the tyre rolls forward it squeezes the water from between the tyre and the road surface away from the contact patch. Eventually the tyre is unable to squeeze the water away and it rises up onto the water with the subsequent loss of control. In order to remove as much water as possible from under the tyre it has a tread.
The softer the rubber of a tyre is the greater its grip to the road surface. The downside of this is the softer a tyre is, the quicker it wears out. The rubber in a tyre will also age. The sign of this is cracking of the rubber which means the tyre itself has become weaker. Your spare tyre is especially prone to this and needs to be checked
The Contact Patch
All the forces of the vehicle are transmitted through the tyres. Steering and braking or accelerating are the forces applied to the tyres. The greater these forces are the greater the chance of loss of control.
While you cannot normally both accelerate and brake. You can both steer and accelerate or steer and brake if done with moderation. The more grip used for acceleration or braking the less grip available for steering.
The grip on each tyre will vary according to how and where your vehicle is being driven. Accelerating and braking will change to grip between the front and rear wheels. Steering will shift the grip to the side away from where you are steering.
The purpose of the tread is to remove water from the road surface to prevent the tyre from aquaplaning. The more tread you have the greater the amount of water that the tyres can squeeze away.
The UK minimum is 1.6 mm across ¾ of the width with the rest of the tread visible. Most car manufactures will recommend a minimum of 3mm. The more tread you have the better your stopping distances in the rain.
In order to do this the tyre has a series of groves creating the tread pattern. The parts of the tyre in contact with the road surface are called lugs and the more of these you have the better the grip in the dry. The parts not in contact with the road surface are called voids and the bigger these are the better your grip in the wet or adverse conditions.
The channels funnel the water away and create the tread pattern which will vary according to the purpose of the tyre. The much smaller groves across the width of the tyre are called snipes and help the tyre flex and push the water out from the channels off to the side.
The pattern of the tread will serve different purposes depending on the sort of driving you are expecting to do.
Components of a Tyre
Tyres may be cross ply (belted bias) or radial but for all practical purposes anything built after the 60`s will have radial tyres. Cross ply tend only to be fitted to classic cars and can be hard to obtain. In a cross ply the cords go diagonally in opposite directions from bead to bead. It has the disadvantage of a greater rolling resistance which means you use more fuel and less control at higher speeds due to the flex in the tyre wall.
In a radial tyre the cords go directly at right angles from bead to bead. The advantages of radial are a lesser rolling resistance which means you use less fuel and a smoother ride.
These are the differences between where a wheel is pointing and where it is going. This is caused by the contact patch distorting and the tyre wall deflecting.
The tyres generate more grip as the slip angles increase, till the grip starts to decrease and then is no longer available. Slip angles generate the cornering force of your vehicle.
If your vehicle was a 2 wheeled one you would get your cornering force by leaning into the corner. This is called camber thrust. Think motorcycle going round a corner
With a 4 wheeled vehicle it is obtained by the slip angles and to an extent by the angle of the wheels perpendicular to the road. Camber is when your vehicle wheels lean in or out of a corner. Camber thrust is the equal and opposite force.
Your best tyres should always be at the rear of your vehicle. The reason for this is that oversteer is harder to control than understeer. Oversteer is loss of traction at the rear and the back end spinning out whereas understeer is loss of traction at the front and the vehicle going wide. By having the best tyres at the rear, understeer is more likely than oversteer.
The correct pressures are important as this changes the profile of the contact patch. This in turn affects the handling, road noise and fuel consumption of your vehicle. Sometimes it is possible to fill your tyres with nitrogen rather than air. The nitrogen monocles’ are bigger which means your tyre will naturally deflate more slowly.
Syllabus For Learning To Drive – Emergency Stop
© Liam Greaney