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Swaps can take place early in the life of a bike as the proud new owner upgrades to new or used components of higher quality.
Swaps also can be made as the bike falls out of favor, or is being sold, where the higher quality components are traded for lower quality ones that the owner had onhand.
For example, the back of cranks are usually marked with crank arm length in mm, typically in the range of 165 to 185.
Unfortunately it is usually the same old story every weekend. If you classify yourself as a certain “type” you will not allow yourself to grow to your full potential ~you become stuck!
Fear can be a powerful suppressor, so be cognizant of any forming patterns that keep you from having love in your life because you are afraid of the unknown. If you don’t have the strength to get out of reoccurring heart breaking situations, then start observing some of your friends who may be stronger than you and follow their lead.
The most likely components to be original are the stem, handlebars, seatpost, and brakes.
The rear derailleur freewheel/cassette and chainwheels are probably the first to be changed on a bike.