than 20% for dunes or deep mud. Over a tire’s life-spawn, differences of as little as 0.5 of a PSIG can make a difference on it’s wear, and in racing tires the accuracy goes down to single tenths of a PSIG (with use of advanced instrumentation like tire temperature probes).
Effects of over-inflation
Over-inflation is better than under-inflation, in any day! Over-inflation does not cause the tire to blow up, unless you fill it well beyond it’s maximum capacity (which is well above the “maximum pressure” limit stated on the tire’s sidewall). An overinflated tire becomes stiffer and hence distorts less and generates less heat. It makes the tire recieve a concave shape, so it grips the road more with it’s center and not the shoulders.
However, modern radial tires won’t suffer from excessive wear due to this kind of inflation (although there is more wear than in normal driving), and will maintain higher grip because the tire will not bend so much under cornering. Within a range of about 10% of over-inflation, the tire will generate more grip than with proper inflation! The tires will react more instantly to driver’s inputs. They will also break traction more suddenly and with less feel, but will provide better control even during slides, and supply a more accurate feedback than under-inflated tires. If you drive the car hard, on race-tracks or while roaming winding roads, over-inflation can reduce wear.
While it is recommended to drive with the appropriate pressure recommended by the manufacturer, it’s advised to prefer over-inflation over under-inflation when in doubt, and even choosing to drive at all times with an addition 2-3 PSIG is very acceptable. If your tires have a problem with noise or wet grip, you might choose to experiment with such slight over-inflation to try and solve those problems. Overinflation is also used in performance driving, on winding mountain roads as on the race track, where it in fact reduces the wear. Do know, however, that over-inflation will increase tire wear on rugged surfaces (unlike on tarmac roads). Always remember than superflous pressure can be manually bled out at home.
Situations where you might be in doubt, other than hot tires, can also be when the car is loaded with luggage and/or passengers. Manufacturers recommend a different pressure for a fully loaded car and sometimes for a semi-loaded car. Even three adult passengers can be considered semi or even fully loaded. Some tires (snow, mud and all-terrain tires) will even be sensitive to the change of load formed by one passenger.
Modern tires are made to carry weights which exceed their own weight between 40 to almost 300 times their own weight (the highest value in achieved in aircrafts and very heavy trucks). The increase of weight increases the downforce that gives the tire more grip (based on the formula mv²/r = mgμ). So, if you have three adult passengers in the back, your back tires should have more grip, right? No, because the increased mass also increases the lateral force experienced by the tires, so the two effects should cancel each other out. However, the coefficient of friction is also reduced (slightly, though) under extra load, so over all there is a reduction of grip. The extra air pressure allows this effect to be canceled out so the grip level is overall increased
It can be beneficial to inflate tires with nitrogen. Nitrogen molecules escape through the innerliner at a much slower pace, and they don’t allow moist to enter the tire and improve heat desperse. However, nitrogen inflation is not crucial to your safety, and it’s possible to yield good results with normal air, by periodically replacing the air inside the tire with fresh air, to get rid of moist, dust and chemicals/rubber dust inside the tire. This is best done when the car is lifted and with a personal pump, but it is possible to bleed out about 70% of the pressure safely when the car is parked and leveld, and immediately reinflate at the gas station, once every six months. As a rule of thumb, by applying slight pressure against the valve, you will bleed out one PSIG for every ten seconds. This changes on each individual nuzzle, on the amount of pressure applied and the form of the object used to apply pressure.
Tire pressure can also be monitored via a RPSM system fitted unto the rims (very highly recommended! ), and through a less efficient system where changes in the air pressure are detected by the ABS sensors. It’s still important to periodically use a reliable gage for a manual check. To ensure proper inflation with the gas station digital pump, you might choose to inflate each tire twice. The system will make adjustments and reach a pressure closer to what is necessary.
It’s important to keep the nuzzle caps on. If they get lost, the nuzzle will pick up dust and dirt, which will apply pressure against the valve to lead to a slow but constant lost of air pressure. It’s also important to realise that these rules apply for all FIVE tires, reserve tire included. It losses air just as much as all other tires, and it is important for it to be properly inflated. It’s actually better to inflate it with a few additional PSIG, due to the lost of air over time. Some cars have a thin fifth wheel (due to reason based on cost, size and weight) that is usually inflated in a very high pressure (often the maximal possible pressure). Also remember than reserve tires placed on the rear bumper or under the hood, wear out just as much as the tires on the wheels. Have it replaced periodically.
Another note: Tire pressure cannot be judged visualy in modern radial tires. They have a reinforced sidewall and they lose air pressure at a slow, unnoticed rate. The result is that even when the tire is under-inflated by 50% (which increases milleage by 10%, tire wear by 70%, pollution by 45%, dry grip by 30% and wet grip by 70%!), will be bearely noticable when not moving. Kicking, pushing and looking at the tire is not going to show any difference and the naked eye of a skilled individual will at best notice that the shoulders are slightly crammed unto the road. While moving, however, and especially while cornering at a conservative speed, the tire’s sidewall wil distort extremlly.
The simple dial gauge works by use of a flexible pipe that is forced to erection when the air pressure runs through it. This effects a wire that turns a set of gears that rotate the clock. The clock operates better in it’s midrange and when the resistance to it’s movement is reduced by use of oil filling. The problem is with the multiple mechanical parts, especially the gearing. Some dial gauges are shaped like “cams” which involve a more complex gearing system, which is far more suspicious for misreading and malfunctions. Do not trust such a gauge as it can be off by 3 to 18 PSIG!
A pencil gauge is by far more simple. If is simply formed by a chamber with a sprung piston on one side and a one-side valve (normally a scharder valve) which pushes a bar that specifies the correct inflation. These are less accurate than electronic or dial gauges, but are more reliable than public gauges and will not be knocked out of callibration by blows. A good pencil gauge can be as accurate as 0.5 of a PSIG.
The air pump itself is a reciprocating Air compressor which uses a series of pistons to pump the air from the chamber to the tire. There is a lost of efficiency through the operation of the pistons as they become worn, of the operation of the electric engine that operates them; the air regulator can be knocked out of alignment. There is a leak of air from the edges of the hose (which can often be heared very audibly). The pumps have been found none-accurate by checks in the US and UK, and they are subsequentally adjusted in advance so that they fill “too much” air so they will result in overinflation rather than under-inflation, if they do miss the right amount of air.
While it is possible to purchase two pairs of tires that share similar treats, it’s highly advised to purchase a foursome of identical tires. These tires should of course fit the rims of the car, and the index of load and speed relevant for the car. Modern radial tires have a steel belt that becomes twisted in the direction of the steering angle (based on the wheel alignment), so it cannot be rotated around like old tires.
The solution is to take the tire place it on a different rim while keeping it rolling in the same direction. The tires should be “rotated” this way once every eight to fifteen thousand kilometers (rotate it at 10,000 kilometers as an average), so the front-left tire is moved to the rear-right rim and vice versa. This cancels out the wear caused by the alignment of the front wheels and the application of forces through the front in a front-wheel driven car, as well as by the features or normal urban driving where the right side tends to meet curbs and tighter cornering efforts. The rotation is particularly relevant where the car is front-wheel driven, powerfull and where the driving style is not very smooth or gentle. Driving in towns and winding roads also accelerates wear and requires a replacement at 10,000 (if not at a mere 6000!) instead of 15,000.
When tires get worn, the popular advice is to place the good tires on the rear. The front wheels of the car tilt when you turn the wheel and they rotate you into corners. The rear wheels have to keep the car in line with the front wheels. Worn rear wheels, resulting in a slide (or blow-out) will make the car slide out and spin around (oversteer). If the front wheels are worn and slide (or blow-out) they would slide forward and out of the corner, known as understeer. This kind of slide is easily felt through the steering, and is more naturally recovered from by slowing down or even turning the