Tandem Tom

We're they any mixtes that had the twin laterals brazed to the side of the head tube vs. the type with the lug?
Thanks

unterhausen

I think when I made some mixties at Trek, I cut down a head tube lug and brazed them in where the head tube would normally go. I always wondered what happened to those bikes.

Andrew R Stewart 

I have a vague memory of a mixte frame with the twin laterals passing along the sides of the HT and having a flat plate type treatment that wrapped around the HT. Much like a classic wrap around seat stay style we ooh and ahh about.

I would think one would want to keep the laterals fairly close together as they travel from the HT to the ST and then flair out to clear the ST/tire and the chain. BTW it is the chain clearance when the smaller cogs are engaged that can be a challenge to work around. My solution was to have the laterals join with the lower end of the seat stays just above the dropouts, not have the laterals join with the dropouts. Andy

duanedr 

I seem to remember some old ones had the tubes end protruding forward of the HT and maybe even with lights embedded in the ends. I don't remember them being high end bikes - probably from the 70's era. I don't think I could name a MFR though.

unterhausen

Side tack twin top tubes were pretty common on cruisers at one time. Look for pictures of a bike with a fake gas tank.

bulgie 

Twin mixte tubes all the way to the HT is mostly a cost-cutting thing IMHO. The best mixtes, like from Rene Herse or Peter Weigle, have a single inch (or 26 mm) round TT. The mid stays only reach a little past the ST, attaching to the TT just forward enough to give good tire width.

Andrew R Stewart 

Sometimes when I climb hills stand and sway the bike side to side ("honking", not "dancing" for me these days). I end up with sweat stains on my top tube right behind the upper stack. and grease on my thighs. I know that I would not want a wider front end of my bike. But for a rider who never stands or swings the bike enough for leg/frame contact this twin lateral along the side of the HT could work. Andy

Tandem Tom

Thanks for these replies! Budgie,would not twin laterals create more of a "truss",Hey my background is carpentry/woodworking, than a single TT?

bulgie 

Maybe Budgie (whoever that is) will respond for himself, but for me, I'd say the truss you speak of is in a plane perpendicular to the one where the stresses are applied. Stresses in the plane of the truss are negligible.

But for twisting, the moment of inertia of a 1" tube is much higher than for two 9/16" tubes that weigh the same. To the point where the mixte tubes offer practically no twisting resistance, so you're relying only on the DT for the frame's resistance to twisting. So the frame with the 1" TT can be built lighter for the same reassuring feeling of wheels that stay in the same plane.

Obviously millions of frames have been built with the twin tubes, and they last and ride OK, but they're generally not very light. It's not a huge difference in the grand scheme, but I like chasing marginal gains — it's one of the things you can do on an expensive boutique bike that the mass-producers can't afford.

Tandem Tom

Budgie, spell check converted you to a bird!
Thanks for the reply. So a bit more of my idea. My current touring bike, Surly LHT, is great but I would like a mixte style for easy on/off. So the if my DT was say oversize would that make it even more stiff and resistant twisting? Then the laterals that come up from the rear DO area would be brazed along side the DT?

base2 

Tandemtom You may have seen this project I have going on: Base2's Gravel Mountain Mixte Project

For me the torsional stiffness was a huge non-negotiable priority but I wanted to be true to the classic mixte design. I didn't want it to be a "sport" or an "anglais" or any one of the more common (and cheaper to build) kinds like shown above on account of concessions are/we're made from an engineering perspective to satisfy manufacturing considerations.

A berceau would've indeed provided a lower step, and I did consider it. Ultimately I decided the loss of a straight path of triangulation from the drop outs to the head tube mattered in terms of engineering philosophy and the classic mixte truss of triangles was the path to achieving the goal. "Vertical compliance" () for rough terrain (the use case that inspired the mixte's original form) and my own personal requirement of enough torsional stiffness to inspire confidence in high speed or technical descents.

What was decided upon was 16mm 0.8mm wall cro-mo seat stays, sleeved and spliced at the seat tube lug. Then a 1&⅛ to 1&½ tapered head tube with a large-ish 38mm diameter bi-ovalized downtube and extra long rear center to promote a more 50/50 weight distribution to build in room for heavy loads on panniers. The longer span of wheelbase ought to help in the very compliance department and the lowered seatpost collar ought to allow more seatpost flex for shock & vibration.

The frame is in paint as I write this and it may be several months until completion. But until then, perhaps this may shake a few ideas loose?

Base2

bulgie 

yeah a 1-1/2" DT should provide so much torsional rigidity by itself that it doesn't matter what you do for a "toptube" -- for torsion anyway. those twin stays will be more than adequate for the remaining stresses, like just holding the bike up without folding. You've probably seen the cheap town bikes that just use an enormous DT with no TT at all, and those hold up just fine, though maybe not for radical shredding and hospital air.

Oh and I want to add, I said Rene Herse used a 26 mm round TT, but since I wrote that I've seen exxamples of that brand using twin stays all the way to the HT also. Here's an example, from 1947:



Pretty!