the action of a lever, a rigid bar that pivots about one point and that is used to move an object at a second point by a force applied at a third (point).
|"Old School" rigid fork|
Old style forks without suspension correction could tuck the wheel up nearer to the fork crown, minimizing the length, (or lever), of the fork blades. This gave the fork better strength against frontal impacts, and with longer head tubes to help dissipate the energy from those impacts, forks could be svelte, pretty, and still be strong.
But that started to change when those pesky suspension devices were introduced to mountain bikes in the late 80's. Most designs used a telescopic design where the two fork legs were divided in half, a lower part, called the "fork lowers", (amongst other names), and the upper part that connected to the crown, called the stanchions. The stanchions slide into the lowers when a trail obstacle is encountered.
That meant that the fork legs had to get longer to allow for the travel of the suspension, so the tire wouldn't bottom out on the crown. Early suspension forks had a whopping 63mm of travel. Then they moved to 80mm, then an unheard of 100mm of travel.
With each incremental increase in travel, a longer fork was necessary. Remember- a longer fork means a longer lever, which means more stresses on the frame and fork crown. At first, suspension designers thought we'd have to go the dual crown route to ensure safe usage for longer travel forks, but materials technology applied to the problems have kept 26 inch forks largely a single crown affair even up to this day.
That all changes with 29 inch wheels, which makes the axle to crown measurements even longer for a given fork travel. Compound this by the intentions of designers to use the shortest possible head tube, and you can see why it has become harder to find a long, rigid fork. The forces at work become so great that to make a fork safe, the fork gets heavy, or too complex to manufacture to get them to pass the required testing procedures.
|2013 Specialized "Chisel" fork. 480mm A to C|
However; some forks are out there that do have a longer "lever", (or axle to crown measurement), that will keep your bike's intended geometry correct. These typically are carbon fiber, because it has a strength to weight ratio and capabilities to be produced at weights and costs that buyers are willing to accept.
Trouble is almost all are 1 1/8th" to 1 1/5" "tapered" steer tube types because they need that design for the extra strength required to make those long, lovely legs work. You folks with straight 1 1/8th steer tube bikes know it is hard to find these longer legged beauties in carbon, and steel ones are not easy to track down either. When you do, they are more often than not really heavy. Well, that and stiff.
Kinda defeats the purposes of getting a rigid fork in the first place.
With 100mm travel hard tails being more and more the norm for 29"ers, it seems that the rigid fork fans will be harder pressed to find rigid fork nirvana. It is nice to see Whiskey Parts and Specialized coming out with these newer choices for the future, but there are slim pickens for the 100mm travel hard tail bikes right now.
And it is all due to leverage.