Tubing choice for a 28.6 downtube on a lugged randonneur frame

First time posting in the forum, glad to be here.
Been building and repairing frames since 2013 in Minneapolis

I'm building a lugged Randonneuring frame with 'classic' diameter tubing for a friend and customer who weighs about 200lbs. He wants a 'supple'/reasonably flexible/comfortable frame. He also wants to be able to hop off the pavement and explore little dirt/gravel trails and such, carefully (not full on offroad with big drops or anything). The frame is spec'd for 650 x 42s, so there will be some rubber to soak up some front end stresses. The 1" steerer will also flex a bit protecting the downtube. He has another frame (an old Mariushi tourace T18) that he will continue to use for loaded touring, so this new bicycle will really be for lightly loaded trips (likely front bag only). I'm curious what folks out there would run for a downtube on this bike. Many of my customers so far have been looking for overbuilt touring and commuting frames, or they have been large enough frames that we needed to go for beefier tubes anyways. This seems like an opportunity to run a lighter downtube (like a .8.5.8 Columbus SL or something). I've worked as a bicycle mechanic for the last 15 years and I've seen so many broken down tubes I tend to find myself wanting to spec the downtube in something heavier like a 9.6.9 but I think i might be being over cautious here?

He's a careful/experienced rider, and he hasn't damaged the Mariushi Tour Ace T18 he's had for the last 10 years, and he rides that plenty, so maybe the .8.5.8 is fine? He's a bicycle mechanic and service manager and is aware of the variables of frame design, so he's not gonna ride this like it's a MTB. Trying to balance durability with performance...

Any thoughts folks have are appreciated.
Thanks!
-Al

Hey Al - welcome to V Place.

Please add a first and last name to all posts here on Frame Forum as per the user agreement.

Many thanks.

Will do from now on!
-Alex Heegaard-LeGros

I would go with at least .9 .6 .9 myself

I have had good luck on bikes for loaded touring with .7-.4-.7/.8-.5-.8, 28.6/31.7 TT/DT combination. I weigh about 200 pounds and ride kind of hard over less than greaT roads (when I feel like it). If I were using "standard" tubing I would go up to .8-.5-.8/.9-.6-.9.

The TT, DT and seatstays is where you build in your compliance; Chainstay and seatstay are where you get stiffness and the ability to hook up with the pavement/dirt.

Good luck!

jn