A 29 inch bike also needs a 29 inch fork.
It is quite clear that you also need the fitting 29 inch suspension fork if you are riding a "29 inch mountain bike" with 29 inch wheels. Everything else would not be compatible. It will make your bike more comfortable if you use the correct suspension fork. Air springs and steel springs are the most common systems nowadays. High-quality forks provide many adjustment possibilities, such as compression damping, rebound and progressiveness. These can be used for a perfect adjustment to the trail and the riding style. Mountain bike forks with a travel of over 120 mm sometimes also offer a lowering function, which enables you to adjust the bike's geometry and often also the dampening for uphill or downhill rides. 29 inch suspension forks often are equipped with a thru axle in lieu of a quick release in order to increase the stability and stiffness and thus provide a more precise riding behaviour. The market also offers a small selection of upside-down suspension forks for 29" (in short USD). Due to their reverse designs, they are considered to be very innovative in this area. USD forks provide a slightly higher performance due to their smaller unsuspended mass and the better lubrication - than is the case with "conventional" forks. In our online shop for 29" suspension forks, you'll find a large selection of top brands and models. Tip: Here, you'll find our video contribution to the subject of suspension forks. Rainers Bike Talk