There are 3 types of crank to hub interfaces used in unicycles. Cottered, Square Taper, and Splined.
This is the oldest type of crank/axle interface that is still used. The axles and crank holes are round. The axles have a groove in them. A wedge, called a cotter, slips in there to keep the crank still.
The cotters are thin little pieces of metal that taper. They wear out and have to be replaced. Sometimes they shear in half, causing the crank to spin freely about the axle, which can be fairly catastrophic for the rider.
Basically, this interface is obsolete. It is weak and requires a lot of maintenance.
This was the standard in the bike industry for many years and is still the standard in the unicycle industry. Many bikes still use this as well. The axle is a square peice of metal that tapers (thinner on the outside). This allows you to press a crank (which has the same taper on the inside) on to the axle by using a bolt. This system is reasonably strong. Problems exist in the maintenance. The cranks cannot be removed very often, since each installation removes material from the axle. This will eventually wear down your axle. On bikes it screws up your chainline. Also, if someone rides on a loose crank, the axle and crank turn into round useless things never to be useful again. If installed properly the first time, this is a good system. Here is a quote from John Childs on his method of crank installation:
If you don’t want to buy a torque wrench, you should take the cranks and hub to a bike shop and let them do it. At the very least take it to someone who has a feel for installing them and can get close to the amount of torque needed.
This system will serve you very well if properly installed and maintained. However, even if maintained well, it has strength limits due to the narrow axle.
A more recent development in the bike industry and even moreso in unicycles, splined axles are the best interface available. The axle has grooves on it. The inside of the crank has a matching pattern (although different companies make different patterns, because unicyclists are dumb and don’t use ISIS yet) and they slide together. A nut pulls the crank onto the grooves, and the crank and axle sit together snugly. This system has numerous advantages:
Strength: Stronger than square taper designs. Splined cranks have larger diameter axles, which add strength.
Good interface: The crank and axle have much surface area contacting each other, so the interface is very strong.
Ease of maintenance/removal: Splined cranks are very easy to remove or replace, provided you have the right tools. Unlike square-taper designs, the crank does not draw further onto the axle with every installation, so, if maintained well, splined axles can last a very long time.
Some differences between various types of unicycle cranks:
Spline count: This is simply the number of grooves on the axle. Generally, the more the better, but a higher spline count does not ensure a better product.
Pinch bolts: Some splined cranks have a pinch bolt in addition to the main axle bolt. The pinch bolt makes the crank clamp onto the axle. This makes crank removal and installation very easy, and keeps axle wear minimal.
Splined cranks should remain lubricated on their axle. A liberal application of copper anti-sieze on occasion will fill the gaps between the crank and axle, preventing the annoying squeaking often assosciated with splined cranks.
For the spline interface, it is best to use a lubricant with body to fill gaps. However, it is also important to use a lubricant that will not get sqeezed out of the splines when you tighten them. Anti-sieze fits both criteria, and is available at pep-boys for $2. A $2 tube will last you several years.
I always lube up my pinch bolt threads with anti-sieze and then tighten them as much as I can. I slip a pipe or something over the end to increase leverage. The lubricant prevents damage to the threads and allows you to get the bolt tighter.
they’re a brand of crank arms, that you don’t really need unless you’re really really good at unicycling. they’re almost impossible to break, and have a lifetime warranty, and cost $100 for the crank arms and $320 with the hub.
i havn’t messed w/ my splined cranks yt, but i know that getting a hammer , putting something that fits through the spokes and won’t bend then knocking the crap out of it works tho and if you want to make it perty put a piece of wood between the something that fits in the spokes (a long bolt perhaps… or maybe an extension) and the crank arm itself