Straight Gauge Stays (Aircraft Spruce)

Does anyone have any experience using the 4130 tubing Aircraft Spruce sells for stays?

I’m looking at using the 5/8"x.035" or 3/4"x0.035" material for seat and chain stays.

Any input would be appreciated.

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All my bikes use seat stays from ACS

  • 5/8 035 for enduro
  • 1/2 035 for xc / bike packing
  • 3/8 058 for xc / gravel

It’s all I use. They sell Plymouth pro moly. Its nice tubing and bends well.


Agree with all of the above responses from @anon68659156 and @wzrd . I use a mix of 3/8" and 1/2" tubing at 0.049 or 0.035 wall thickness depending on the customer and the application.


Thanks for the info. I was worried 5/8 035 was going to be on the edge, but you all using 1/2 and 3/8 is inspiring. I may go with the 5/8 035 for the chain stays and 3/8 035 on the seat stays.

The “baseline” CS and SS we use for our steel mountain bikes is:

Chainstay - .75x.035in (19.05x.9mm)
Seatstay - .625x.035in (15.9x.9mm)

We use this chainstay because the slight taper gives more room at the dropout for welding:

That being said, there is no baseline because every design is different! Tube diameter and butting are all over the place in the frame building world. I wish I had more time and resources to test it.


I have also had great results with the pro-Moly tubing in the .035 wall thickness for stays: 3/4” chain stay, 1/2” or 5/8” seat stay. If you look up the spec on the material it has excellent yield strength. Have only run 5/8” chain stays on a kids bike so far.

@Daniel_Y what specifically do you want to test?

This is probably enough for an entirely different thread! But I would love to collect some real-world data on the relationship between the tube spec (diameter and thickness) and frame stiffness for different geometries.

A while back I did a hacky FEA parameter sweep across tube sizes and frame sizes, looking at front triangle torsional stiffness:

We use this as a baseline for making decisions for tube specs and different riders. However, it would be great to back it up with some lab data.

Other things to test:

  • rear triangle stiffness
  • to SS brace or not SS brace
  • fatigue life
  • life cycle testing of different joining methods
  • Validity of ISO standards

I just listened to the SU&BB episode with Rob English. Some great insight into structure and tubes. He said he’s yet to go too small on seat stays, and is currently using 5/16” (I couldn’t understand the gauge). I’m assuming that’s a road bike.

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He also mentions testing deflection with and without a seat stay bridge, and finding no significant difference.


Both size/shape of seatstays and bridge/no bridge make zero difference in my experience.

A lot of my customers freak out if there’s no bridge, though. So I just put them on.



I have a fatigue machine and could probably test a bunch of this stuff. It should work for doing static load deflection comparison too. Could add some strain gauges also.

I’m working on comparing joining methods for main tube/head tube currently. The main tubes are an “experimental” steel, but I plan to do some more standard materials also. I’ll work on getting a post started at some point to share.


What is SU&BB? A podcast?


A podcast that Joe at Cobra does.

Shut Up & Build Bikes.