Tsaari, it appears to be a good job so far on your bike part.
Keep in mind that without seeing a copy of the ~.blend file for your bike part, it's impossible to know exactly how you made the original mesh. I'll get the rude, rough part of the answer out of the way early: if you do not have a clean mesh, it might be necessary to start over. An image in wireframe shading, or posting a copy of your blend file to Blend exchange would be helpful in evaluating your object, and determining the best way to proceed.
I am not fundamentally opposed to retopologizing as recommended by Matthew Inglis, in his answer, but my own personal preference tends to reserve that to organic models like character heads, and tend not to use it on engineered objects, like bike parts. That said, until more information is available, it's not clear that retopology is needed.
Assuming that the underling mesh is reasonably "clean", copy your whole part, and save the original in another layer as a back-up. Next, in the copy, select the all vertices that lie in the groove area, and two rows of vertices on each side. Define a vertex group containing these vertices. Invert the selection, and hide (H key), everything but the vertex group. Now select the vertex group, and make a copy of the vertices. Use the original as a master. Using the copy, create the groove the way you want it, making sure that the outer vertices of the reconfigured groove are not moved. When the groove is done, you can unhide the rest of the part, delete the extraneous vertices, and merge the groove into your mesh, remembering to remove doubles.
Assuming the x axis is aligned with the long dimension of the cross section of the part, You part appears to be symmetrical around the x-z plane. If you wind up having to remake the part, I'd model one half, of the part, and use a mirror modifier to show the other half.