The wheels of a skateboard are usually made of polyurethane, and come in many different sizes and shapes to suit different types of skating. Larger diameters (55–85 mm) roll faster, and move more easily over cracks in pavement and are better for transition skateboarding. Smaller diameters (48–54 mm) keep the board closer to the ground, require less force to accelerate and produce a lower center of gravity which allows for a better response time, but also make for a slower top speed and are better for street skateboarding. Wheels also are available in a variety of hardnesses usually measured on the Shore durometer "A" scale. Again like car tires, wheels range from the very soft (about Shore A 75) to the very hard (about Shore A 101). As the A scale stops at 100, any wheels labeled 101A or higher are harder, but do not use the appropriate durometer scale. Some wheel manufacturers now use the "B" or "D" scales, which have a larger and more accurate range of hardness. Modern street skaters prefer medium-sized wheels (usually 51–54 mm), as small wheels with lighter trucks can make tricks like kickflips and other flip tricks easier by keeping the center of gravity of the skateboard closer to the deck, thus making the deck easier to spin. Street wheels are harder (A 100/A 101). Vertical ramp or "vert" skating requires larger wheels (usually 55–65 mm), as it involves higher speeds. Vert wheels are also usually slightly softer (A 98/ A 99), allowing them to maintain high speed on ramps without sliding. Slalom skating requires even larger wheels (60–75 mm) to sustain the highest speeds possible. They also need to be soft and have better grip to make the tight and frequent turns in slalom racing. Even larger wheels are used in longboarding and downhill skateboarding. Sizes range from 65 mm to 100 mm. These extreme sizes of wheels almost always have cores of hard plastic that can be made thinner and lighter than a solid polyurethane wheel. They are often used by skateboard videographers as well, as the large soft wheels allow for smooth and easy movement over any terrain.
Skateboards are for people who want to try something new and extreme.  If you have not given it a shot and you want to challenge yourself then you should try skateboarding.  It is a fact that there are certain risks that go along with skateboarding.  However, skateboarding can teach you to balance and it is also a great way of losing weight.  Skateboarding can also help you in making friends while you are in the park.  With skateboarding, you can learn perseverance and build confidence.
The top part of the truck is screwed to the deck and is called the baseplate, and beneath it is the hanger. The axle runs through the hanger. Between the baseplate and the hanger are bushings, also rubbers or grommets, that provide the cushion mechanism for turning the skateboard. The bushings cushion the truck when it turns. The stiffer the bushings, the more resistant the skateboard is to turning. The softer the bushings, the easier it is to turn. Bushings come in varying shapes and urethane formulas as well as durometers, which may affect turning, rebound and durability. A bolt called a kingpin holds these parts together and fits inside the bushings. Thus by tightening or loosening the kingpin nut, the trucks can be adjusted loosely for better turning and tighter for more stability (useful when landing tricks). Standard kingpin nut size is 3/8" - 24tpi. The position of the hanger respect to the baseplate is also determined by the pivot, a rod that slots into the corresponding seat in the baseplate. The pivot stops the hanger from rotating around the kingpin. The pivot must allow some movement around the bushings and therefore is not a perfect fit. The space between the pivot and its seat in the baseplate is filled by a pivot cup, a plastic part that will take most of the wear and tear of the pivot and assist in centering the hanger needs to be lubricated every so often.

To get higher ollies, I would suggesting resting your back foot on the very end of the tail and the front foot in the middle of the deck to allow for more slide. Popping as hard as you can and catching your board in the groove of the nose with you ankle practically resting in the groove will level the board out in the air. after this initial movement slide you feet back to the bolts on the tail and nose to ensure a clean landing. 
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