Tandem Tom

Well it has been over a year since I took a frame building course with Doug Fattic. And I thought enjoy riding my bike.
​So now I am getting the itch to build a frame set for my wife. She currently rides a 1981 Miyata 310 Mixte which I converted to 650b.
I was just perusing Frame Builders Supply and looking at the lugs and had a question. The HT lug, that houses the twin tube diagonal downtubes, states for 14mm tube. Taking a look around it does not seem like an size that is available. Also what wall thickness would be appropriate?
Next question! On the Miyata where the twin downtimes meet the seat tube they are brazed at that point. I see a lug that is a bit like a "stand off" that looks kinda neat. It appears that the 14 mm tube sit in a cradle which I assume can be silver brazed?
So any advice or suggestions would be! Doug I will probably be picking your brain!
Thanks!

himespau 

Sounds like an awesome project. I hope you post pictures here.

Andrew R Stewart 

The "mixte" frame I built used a standard 1" top tube instead of twin small diameter tubes between the seat tube and the head tube. A third set of stays went from that ST/TT joint to the seat stays just above the drop outs. I chose this because of the stiffer front end a 1" tube produces and could source a lug set designed for this.

If you're going to go the twin tube route you might consider US 4130 9/16" x .035. Of course you'd have to hog out the upper HT lug sockets to fit. Consider going lugless or bilam for that upper HT joint. I also extended the upper HT lug with a sleeve to get even higher bars.

Take care with the 3rd stays and the chain run. Many production mixtes have some RH 3rd stay manipulation to allow the chain to clear it. That's why I attached my 3rd stays higher up. A simple pair of stand off tubed could span the gap between the 3rd stays and the seat tube, think of a brake bridge like connector. Also study the cable routing paths and the foot clearances around the rear brake.

This bike was based of 559x1.5" wheels and tires and fits the 5' 1" tall rider well. Andy

Nessism

I wonder if 1/2" tubing could be used instead of 14mm? Maybe fabricate a couple of shims?

Sluggo

How about stainless? Find the Right Plastic and Metal Materials | Online Metals 14 mm available with 1mm or 2mm walls. But kind of spendy. And after a little more research, not very strong. Here is some metric 1020: CARBON ALLOY ROUND TUBE 42CrMo4 | Metric Metal, but I think you would be better off using .5" chro mo and adapting to fill the extra space.

I have also been musing about building a mixte frame. Do you all have any opinions about downtube wall thickness? With a diamond frame, you could go real light for a small light person, but since the triangulation is somewhat compromised by the mixte configuration, how heavy would be appropriate? Seems to me like good old .9/.6/.9 would be safe, but could you go lighter?

rellis

14mm equals 9/16" (14.28mm anyway). You can get 9/16" 4130 tubing in .035 and other wall thicknesses.

https://www.aircraftspruce.com/catal...tubing_un1.php

https://www.wicksaircraft.com/shop/4130-round-tubing/

Lots of frames were made and referred to as mixte's in the 1970's made with the twin top tube.

unterhausen

I wouldn't use 14mm -- too stout. Either shim the sockets or blacksmith them to fit 1/2"

bulgie 

Don't forget to do something to make the right-side midstay clear the chain. Not only while pedaling, but also while shifting — the chain moves around a bit randomly while shifting. So leave a little extra room, unless gouging up the paint is acceptable.

The chain clearance is usually done one of three ways:

1. Bending the stay outward at the chain clearance spot. Maybe bend both stays the same to look symmetrical, but you only need to bend the one on the right.
2. Indenting the inner face of the stay at the clearance spot, kinda like what trad framebuilders did on the chainstay, for horizontal dropouts of the tab-in-a-slot type. Or for a cruder version, look at an "Electro-forged" Schwinn (Varsity et al.)
3. Mounting the midstay to the seatstay a little ways up, away from the dropout. The easiest but not my fave aesthetically.

You can also use two or more of those ways in combination, so as to not have to do any one of them as much.

Using a straight untapered tube has a couple downsides compared to a tapered stay. One is it's just uglier at the dropout end, the other is the chain clearance problem is made just a little tougher to work out. If you want twin mixte stays all the way to the headtube, long stays with a taper used to be available, but they'll be hard to find now. I would probably use the slenderest tapered seatstays I can find, and splice them at some point to lengthen them, with the same diameter tube as the top end of the tapered tube. It won't be too hard to splice a 9/16" straight tube to a 14 mm tapered seatstay. Use a short piece of smaller tube inside the splice as a sort-of lug. If interested, ask for advice on how to ensure braze penetration in that kind of joint — there are some tricks to it.

Of course Andy's way, with a single 1" toptube, lets you use whatever midstays you want, without any splicing. Without any numbers to prove it, I assume the 1" TT makes a stronger and more rigid frame, but some people just want the look of the twin-stays instead of a single TT.

Mark B

Tandem Tom

I really appreciate the replies! This will be my resource as I get ready. One thing that helps me is my wife's current Miyata I can see some of the details.

Tandem Tom

I really appreciate the replies! This will be my resource as I get ready. One thing that helps me is my wife's current Miyata I can see some of the details.

Doug Fattic 

Mark's recommendation is a good one to make a really long seat stay by using a regular 14mm one attached to a section of 9/16" tubing with probably .035" walls. In that case I use my lathe to make the short splice that will fit inside of both of those tubes. Often the splice has to have 2 outer diameters (different on each end) to just fit inside the ID of the 2 sections being brazed together. There are some tricks involved and it can take some effort to keep the 2 sections straight when they are being brazed together.

Having said that if you are using the bridge thing that brazes onto the seat tube and has arms that reach out to attach to the seat stays, those arms are a bit short so the seat stays have to be bent so they are spread far enough apart to reach the dropouts that are 130 or better yet 135mm apart.

I'm not positive of this and I am contradicting my buddy Andy, but I think twin laterals are better than a 1" diagonal top tube in your case. Andy made his frame for a smaller rider so there is less need to worry about the sway that can happen if the top tube is attached too low. Twin lateral mixte frames are more difficult to build so production companies wouldn't have bothered unless it was a superior design.

If you notice step through bikes with single tube sloping top tubes, they are typically bent so at 1st the top tube is almost parallel with the down tube but then it curves to meet the seat tube higher up. I have some curved top tubes designed for this purpose that came from Miele when they went bust in Toronto. I'll share. In that case you will be fillet brazing the frame. I would still recommend attaching the 2nd set of seat stays from the seat tube to somewhere near the dropouts like Andy did. Step through frames without that extra support can be pretty whippy.

Lots of other good advice already shared. One of my other students is making a mixte for his wife and has taken his time to collect materials and figure out the design. I'll pass along contact information.

Tandem Tom

Thanks Doug! Maybe another class is in my future!!!😁😁

Andrew R Stewart 

I'll add one more aspect of mixres VS drop TT "ladies" bikes. When the TT attaches onto the ST below the seat stays there's a bending stress that the ST sees. When the rear wheel hits a bump the seat stays will try to push the top of the ST forwards. The TT/ST joint becomes a fulcrum. There are many "ladies" bikes that have bent STs and thus interestingly changed geometry. Having a third set of stays that braces the TT/ST joint makes bending STs pretty much a non issue. Andy

unterhausen

I am thinking about building my wife an ebike since step-through ebikes are hard to get from my lbs until next year. I probably would do something like what Andy did, but I have some curved tubing I probably would use for the top tube.