Originally Posted by
PhotoByBike
1) What wall thickness would you recommend for building 4130 recumbent bikes? Keep in mind the lengths of the tubing for different parts of the bike's geometry will be quite long compared to a diamond frame upright bike.
This depends on frame design (mono or multi-tube triangulation), wheelbase and CoM location. Beyond a given WB, mono tubes don't make much sense, not to mention that you'll be forced to go with .049" wall above 1.5" diameters (.035" is rare above 1.5" diameter in 4130). I need more frame layout specifics please.
Originally Posted by
PhotoByBike
2) Having never welded 4130 specifically, I'm assuming mild filler wire and C25 will work but I'd feel silly if I didn't ask. Is this a good combination for welding 4130, or would I require wire with a higher tensile strength and possibly a different gas mixture?
C25 is fine. Use ER70s-2 or s-6 filler. Do NOT use 4130 filler unless you plan to post heat treat the frame. Be advised, MIG on thin wall (much below .045") is not for the faint of heart. Yes, it can be done, but it's not easy or even recommended. The problem is lack of heat control while under the hood - unlike TIG or O/A where the user has full control when the situation approaches burn-through. It is further complicated when doing compound tube joints where consistent travel speed becomes difficult and tricky to maintain. I can MIG .035" in long horizontal passes, but add thin wall tubing joints and I'm reaching for TIG or gas.
Originally Posted by
PhotoByBike
Yes. The same ER70-s2 or s6 filler is used. To help reduce BB distortion, a gapless joint fit is highly recommended, then just lightly screw-in a pair of cheap, steel bearing cups prior to welding.
Originally Posted by
PhotoByBike
The last few questions are about Head Tubes, and perhaps this should go in a Sdifferent thread, but if I wanted to order tubing to use for a custom head tube for threadless forks, what size (ID) would I order?
Go here:
http://www.sheldonbrown.com/headsets.html
IIRC, the head tube ID for 1-1/8" steerer should be close to 34mm (or about 1.336-1.338").