This is not a manual on how to modify your car but instead focusses on the safety considerations involved in modifying your car.
The first thing to consider before buying a car and modifying it is: Would you be better off buying a better car in the first place? After taking time to consider your needs and the relative costs in terms of time and money you might find that you would be better off just buying a more suitable car.
If you do choose to modify a car there are two considerations that affect its performance. There is handling and there is power. You can improve the handling without improving the power but you should not improve the power without improving the handling. Also, power is either improved or it isn't and as I said before this isn't a mechanical manual, therefore the focus will be on handling. Note that you should check with your local authority on what modifications are allowed.
The handling of a car is the manner in which it will go around a corner: its steerability and its stickability (a combination of body roll and adhesion).
The easiest and least expensive way to improve handling and steering of a car is to pump the tyres harder. However this only counts for so much.
Wide wheels give the motorist the opportunity to improve both the appearance and the handling of the car at relatively low cost. But people have been known to carry the wide wheels modification to extremes. Inappropriately wide wheels can cause problems.
A car, as it comes from the manufacturer, has a roll/slip factor built into it. The standard wheels and tyres are wide enough to provide reasonable traction under normal circumstances. If the car is pushed beyond reasonable limits the tyres will lose their adhesion and the car will slide. A safety factor, believe it or not, because if it didn't slide it would roll. Changing the tyres for a type which gives better adhesion brings the slip factor down a little but not so much as to greatly reduce the roll factor. The fitting of wheels not more than 3 cm wider than standard wheels has a similar effect, even greater with wider, better tyres.
Beyond this, however, the situation can become dangerous. Too much grip is now provided by the wheel/tyre combination and the car may roll before it slides. Certainly the cornering capacity of the car has improved but the ultimate result of overdoing it is much more disasterous. Besides, the body roll of the car has been increased to the point where it is less comfortable to ride in.
Other popular methods of effecting handling modifications are by lowering the car and fitting stiffer suspension. If the car is lowered and the suspension not stiffened nor the wheels changed there is a chance under severe cornering that the suspension will 'bottom'. When this happens the springs and/or shock absorbers reach the limit of their travel and the sudden loss of suspension movement will cause an equally sudden and unexpected slide- one which will be hard to control.
On the other hand, if the car is lowered and the wheels widened but the suspension otherwise left standard, then there is a reasonable chance that the wheels will rub on the springs or the guards when the car is cornered hard. This will increase the chances of a blow-out.
If the car is lowered and the springs and/or shock absorbers exchanged for stiffer units but the wheels and tyres left unchanged, the slip factor will be increased to a dangerous level and the car will want to slide at every corner.
To bring about the improvement that most drivers want, a little of all three modifications should be carried out. For example: wheels widened by 3 cm (and the tyres replaced with ones suited to the new rim size); body lowered on to the suspension to a maximum of 3 cm provided no fouling will occur; and stronger double-acting shock absorbers fitted. In most cases, however, te costs of such modifications are prohibitive for the average driver, so he does one or the other and the end result is a car which does not handle as well as when it left the production line.
Finally, there is another point that should be considered in relation to wide wheels. The suspension of a car is designed to carry a certain amount of upsprung weight. The widening of a set of standard steel wheels imposes on that suspension more strain than it was originally intended to carry, with corresponding increases in wear on spring mountings, wheel bearings and so on. If a set of wider wheels is contemplated it is advisable to wait until you can afford a set of good quality lightweight wheels such as those case from various alloys of magnesium or aluminium. As it happens they are also considerably stronger than steel wheels.