Legal Question in Intellectual Property in Illinois

First, no one can answer the question without specific information on the part and the manufacturer, so take this answer as general information.

To answer the first question, ask yourself is there anything really special about this part other than it fits. For example, even in a patented machine, a screw that serves no purpose other than to act as a fastener like every other screw is not protected by the patent. Even though the screw may be non-standard, it does not fall under the protection of the patent. On the other had, another small piece that is a critical element in what makes the patent unique may be covered by the patent.

Even if the product can be made without infringing the patent, you may be infringing the trademark of the bicycle manufacturer if you sell the part and identify it as made for that bike. The general test for trademark infringement is whether there is a likelihood that a potential buyer of the product would be confused as to its origin.

As for whether you should be concerned that the bike manufacturer might take your idea and produce the part themselves - absolutely. There is no protection for your idea unless you get them to sign a non-disclosure/non-circumvention agreement, which they are not likely going to agree to. On the positive side, many manufacturers have no interest in selling spare parts because they don't have the right channels and there simply isn't enough money in it for them. If they sell their bikes to Walmart, Walmart may sell common accessories and repair kits but carrying individual parts would never meet the volume and inventory turnover required by that type of store.