What's best for threading rod?

The Bolt Supply House (or similar name - can't remember for sure) has threaded rod in grades 5 and 8 as well as stainless. Their stuff is vastly superior to hardware stuff. Unless you are really hurting for strength, a grade 5 should be plenty. While fine thread is, in fact stronger, I'd say it's overkill for a roof rack.

Ted

Let me amplify what Ted said: the hardware store rod is WIMPY stuff. I haven't run a proper tensile test on it but in real applications it stretches easily. I would NEVER use it for critical applications.

Fine thread is stronger than coarse thread since the root diameter (the bottom of the threads) is larger on fine thread. Usually about 15% difference, depends on the size.

Cheers.

Ted Edwards wrote:

What sort of roof rack is this and what kind of vehichle? Do you plan to use it to lift the vehichle?

My calculations show effective thread area of 3/8" NC thread as thread_Tarea .375

0.0775 sq.in. which gives 20Ksi stress with a load of 1550 lbs. 20Ksi is way below the yield for grade 5.

Ted

Be sure you fasten the angle to the shell in several places and use good sized washers on the inside. I'd be far more concerned with the fiberglass being damaged than the bolts failing.

How long are these "rods"? Have you considered using grade 5 bolts and welding the heads to whatever? Easier than welding the rods. I'd consider three substanial tacks on each of three of the bolt head flats.

Ted

Get the threaded rod - it will come in handy for other things. I paid about Cdn$1.50 for a 3' length of grade 5 1/4-20. The 3/8 will be more but it's still pretty cheap.

Ted