I'm getting a disc tab welded on my titanium bike. (Something that's a bit trickier than just approaching a pal or neighbour who welds steel, believe me.)
When I spoke to the shop, they said that I should be sure to remove everything on the bike.
Me and a friend got to thinking about this. Why would that be a stipulation? First we thought it was because the heat of the welding would affect the attached components.
Then we realized that this is absurd. Okay, it is one thing to weld something onto the chainstays with the rear wheel attached but there should be no problem with leaving the front derailleur on, or the headset, or the front wheel. Even the crank, I should think. Heat doesn't transfer very far along a tube during welding.
Then we thought: it's because the shop doesn't want to be responsible. And the bike shop wants convenience. I.e., it's just easier to work on a frame that doesn't have anything attached to it.
Anyone have an answer?