Why are pedals and bottom brackets reverse-threaded?

I'll explain bottom brackets just for simplicitys sake. Any other similar things would be able to be explained about the same. Looking at the LH thread on a bottom bracket cup you need to look at how the crank moves, and how the bearings move.

As you pedal forward the bearings load is transferred to the bottom bracket cup. The bearings actually move in reverse, opposite the movement of the crank. As the load is increased on the crank arm a similar load is placed on the bearings and onto the bb cup. In simple terms, the bearings are trying to loosen the cup as you pedal forward.

If the cup was a right hand thread the bearings would be forcing the cup to turn counterclockwise which happens to be the same direction that you remove the cup. A LH thread on the drive side cup eliminates the problem BUT if the cup is installed without care the movement will actually get the cup so tight that it is almost impossible to remove it if necessary.

There was a very popular carbon fiber frame/bike brand that grossly underestimated the LH force of this action which resulted in the aluminum threads bonded to the frame to actually come loose from the carbon, destroying the frame. It was corrected in a hurry.

Italian threaded bikes still use 2 RH thread bottom bracket cups which causes some consternation for amateurs especially when they are removing the cups or when a customer returns a bike with a messed up bottom bracket because it was not installed correctly and with great care.

Personally, I believe the LH/RH revelation is to reduce failures and legal challenges from poorly installed product.

A long time ago, Chrysler/Dodge/Plymouth used to have LH lugs on the left hand side of the vehicle for the same reason.

Bottom Bracket Reverse Thread

things are threaded so that the normal use of them won't cause them to unscrew. If the normal motion is in the direction that would also be used to unscrew the thing, it is normal to use reverse threading to avoid that problem. No one wants a thing to unscrew from normal use, after all.

If you ride long enough the part is going to unscrew itself. If the rotational direction tightens the part it will never come lose.

Pedals are, so that the action of pedaling doesn't make them come loose.

it is only one side

so they won;t unscrew with all the pedaling

wle

thx for the answers everyone <3