Tandem Tom

So last night as I was drifting off to sleep I was pondering my touri bike frame and ideas. Wondered if I was a possibility to remove the top tube and substitute it with diagonal tube/tubes to create a mixte/step through frame?
As a disclaimer it might have been a late night snack that caused this idea!!

Andrew R Stewart 

Tom- It sure is possible and was done far more often way back in the day. I still see old kids coaster brake bikes with an odd TT location, slightly above what a common "ladies" bike would have but still far lower then a gent's. A couple of old timers told me that they use to cut and reinstall TTs on boys bikes to make girls ones. I've likely seen a dozen of these over the years and in the 5 different cities I've worked in. Andy

guy153

I don't see why not. But be careful about tube thickness. I use externally butted STs which are 1.2mm thick at the top. The ST is only 0.6mm in the middle which is therefore a much weaker as a place to attach another tube. So you would want to add a sleeve.

Then there's the design where you use two long thin tubes all the way back to the dropouts which also might be interesting. I don't know if those are usually attached a bit to the ST as they go past it. But you might get away without a sleeve in that case as it's not bearing as much load.

Andrew R Stewart 

The twin sloping tubed design that guy153 mentions is what the classic and once common mixte frames used. The addition of the stays, extending rearward past the ST to the rear drop outs, is to remove the bending forces that a "ladies" frame will otherwise have. It is odd to ride a bike with a BB drop around 15cm, so low that pedals nearly hit the road when even going straight. Andy

Doug Fattic 

I haven't built a lot of step through type of frames but my recommendation would also be to use twin laterals going from the head tube back to the dropouts as a top tube. Years ago when we were experimenting with frame types for our charity bikes in Ukraine, I made a step through with a straight very sloping top tube. It was a poor choice. The frame did not have enough stiffness and pedaling action caused the bicycle to sway. This is why step throughs with a single top tube are curved to attach higher up on the seat tube. I have a few curved top tubes if you want to go that route. However again I'm going to suggest (along with Andy) the superior design is twin laterals.

unterhausen

I think the superior design is a large top tube and laterals back to the dropouts. I have never tested this theory though. I keep thinking about making my wife a bike and that's the design I'm going to be using. The advantage of twin laterals for a retrofit is that it's easier to attach at the seat tube.

I made 3 mixtes when I was at Trek, I always wondered what happened to them. Two of them were for Bevil Hogg's daughters. I always figured they were the first step throughs ever made there.

Andrew R Stewart 

Like this. A small bike based on 559 tires. Andy

unterhausen

I really like that color

guy153

A beautiful bike. Please don't take this is a criticism, but aren't the extra SS going to just stiffen up the rear triangle, which was a normal rear triangle to start with? If we wanted to stiffen something up it would be the front triangle. Since the TT attaches quite high it should be OK anyway. But the design with two lateral tubes all the way from the HT might give more stiffness as the front "triangle" is now a sort of tetrahedron.

Andrew R Stewart 

Maybe... but the rider of this bike is only 105lbs and really likes how she fits the bike and how it rides. No comments about "stiffness", either too much or too little. My goal was to create a life bike for a rider who is getting older and has less flexibility herself, not some trendy design that I can show off The color was her choice, same as her Subaru.

I'll end with my usual comments about how we sometimes dwell too much on a bike's numbers and less on it's application. And that tires and wheels likely deflect more then any frame does, especially when the tires are 85psi 1.25s. Andy

unterhausen

If you don't have the rear mixte stays, there is a real danger the seat tube will fail at the top tube. I think that's the main concern, not stiffness.

Andrew R Stewart 

Exactly. The forces that run through the HT will push back on the TT, where ever that is. On a "ladies" frame that point is down on the lower half of the ST. The ST will see bending forces, hinged at the TT/ST joint.

So it is about stiffness but not in the way we usually think of. Andy (who thinks of bubble gum as a frame material. With enough of it the strength and stiffness will be enough...)