Veloria

Mixte frames come in two distinct types: One with twin lateral tubes running continuously past the seat tube to the rear dropout, and the other with a single top tube running to the seat tube, then splitting into thin twin tubes.

It is my understanding the single top tube model was adapted, because the twin lateral tubes have some disadvantages in terms of strength and other factors. Could someone advise, where I could find information regarding this issue? Were the twin lateral tube frames known to fail under some circumstances, like high speed or overloading?

This is not meant to start an argument regarding which anatomy is "better" or which is a "real mixte"; I am just looking for technical information, so that I can gain a better understanding of the designs. I have not been able to find literature on this.

Thanks in advance!

Exit.

The term "mixte" is used to specifically denote the frame design which uses two adjacent top-tubes. Frames with a single sloping top-tube are not mixtes. People sometimes use the term to describe any bike with step-through geometry, but that's a misnomer.

The logic behind the mixte design is that, because the two top-tubes connect directly to the two seatstays, it puts less stress on the seat-tube than standard step-through geometry.

Veloria

Exit --

When I say "single top tube", I do not mean the type of construction where the top tube simply stops at the seating tube. I mean specifically the type where the top tube splits after the seating tube, and those split tubes then reach the rear dropout. I think that Motobecane, Peugeot, and Schwinn all offered this design and still called it "mixte". The Rivendell Betty Foy, the Velo Orange Madame, and the Sweetpea Mixte are all examples of this frame structure.

My understanding was that "mixte" technically means any bicycle with a unisex construction, but that in common jargon it refers specifically to both frame types I described in the original post, the key being that the tubes in some way extend to the rear dropout.

Veloria

badmother --

Thanks for the reply. I have heard inconsistent info regarding whether a mixte construction is stronger than a Raleigh Lady's Sports type of construction.

Hmm... framebuilders forums. Duh! Thanks : )

mkeller234

Yeah, you are right. Both styles are still mixte frames because they both still have a extra set of stays. Good question, I am very curious to see what answers you get.

Exit.

Ohhh, yeah, pretty sure that's still considered a mixte.

miamijim

There are 3 ways to build one..

1. twin tubes running continuously from front to back.
2. 1 tube running from the head tube to seat tube with 2 seperate tubes running to the back giving the bike 3 rear stays on each side.
3. 1 tube that terminates at the seat tube.

While #2 may be considered a mixte its #1 that people want.

More info here: https://www.bikeforums.net/showpost.p...30&postcount=6

Veloria

Gosh, I was really hoping to avoid a debate on what constitutes a mixte (see the OP). What I was looking for, is technical info regarding why the "continuous twin tubes" construction is considered to be inherently weaker than the "single tube + rear twin split" construction. Instinctively, my thought was that the opposite would be the case (i.e. the continuous twin tubes ought to be stronger), but apparently that is not the case.

I was going to re-post this on the framebuilder forum, but I have a feeling that it might be a hot-button topic, so perhaps here I could get a more detached response by a neutral party.

miamijim

You cant get into one without getting into the other. When people buy 'mixtes' they tend to be looking for twin continuoues tubes front to back.

LWaB

Small diameter tubes have little torsional stiffness, even when paired.

Bionicycle

Mixtes belong to a group of bicycle frames known as Cross Frames. Cross frames came in to popularity in the very late 1800’s through about 1950 as a stronger built frame version than your average (for the day) diamond frame. Here is a link that is a good place to start learning about cross frames https://www.rijwiel.net/kruisf2e.htm

Both single top tube and dual top tube bikes that have the mixte seat/chain stays are (from what I have read) called Mixtes. I’ve always figured that the single top tube version was more of an answer to production cost issues, than it was to strength issues. A company could make a single top tube mixte much easier, and cheaper, but still retain the classic mixte lines; in profile anyway.

As far as strength of the frame… the design has been around about a hundred years, I truly think if there was a problem with the strength of the design we would be hearing much more than just rumors and hear say about it, and modern companies wouldn’t be building Mixtes to this day. Koga/Miyata makes a Touring/Trekking version of a Mixtie equipped for front and rear panniers. https://www.koga.com/de/index.asp.
I suspect if there were frame failures, of mixte frames, it was most likely due to the poor metal quality of the old frames, as with many old bikes.

Personally as far as frame flex, I’ve never noticed any more flex on my Mixte, than I ever have riding any diamond frame I’ve ever had, and I’ve been riding bikes for about thirty years. I wish someone could come up with some actual strength test data instead of just passing on information that someone else told them.

PDXaero

both mixtes as long as they have the third set of rear stays

Road Fan

Ok, terminology aside, I can't say if the twin lateral one is STRONGER than the single-tube or not, in terms of resistance to breakage or load-carrying capacity.

The twin lateral is less able than the single tube to resist the twist due to pedaling forces. When you push down, you cause two twists: one spins the crank around the BB axis, and this is desireable. The other causes the BB housing to rock; the side where you are pressing wants to go down and the side where the pedal is rising wants to move up, and this is not desirable. This tends to twist the BB and the seat tube with respect to the head tube, adn to twist the rear axle causing it to rock sideways in a really flexible frame. The fact that the two laterals are thin (of small diameter) and not tied solidly to the seat tube (that thin little bridge) means they do very little to resist this force and limit motion. The downtube is left to handle it mostly alone.

In contrast look at the single tube and for that matter the diamond frame. In both of these the upper tube has a diameter at least twice that of the lateral, so the resistance to this twist is at least four times (or is it eight times, keep forgetting the torsion equation!) that of the twin lateral, for a much stiffer frame in terms of resisting pedaling forces. Either of these designs should be a more efficient frame. This would especially be true if we compared traditional steel (any alloy) twin-lat with modern large diameter aluminum or modern big steel. The large diameter tubes will be much stiffer in torsion than the twin lats.

The reduced BB twist relative to the head tube also means better aligmment between the rear axle and the front axle.

Is the twist energy lost or stored/returned? That's way another debate. Since there have been a lot of great racing performances on flexy frames such as the aluminum Vitus that an Irish rider (grrr Sean Yates?) used to win the TDF, it really might get returned.

This all comes from Tony Oliver's excellent book on frame design, "Touring Bikes."

Stonger, I don't know. More efficient? Probably.

RobbieTunes

First I've seen a single top tube mixte, but I think they're all cool.

Blue Order

No, that's not right. A mixte frame has a lateral tube (not a top tube) extending from the head tube to the rear dropout. Doesn't matter if it's single tube or twin tube, they're both a mixte frame.

A step though frame is different- it has a sloping "top tube" that ends at the seat tube.

unterhausen

Road Fan has the basic idea. Bending moments are resisted by a quantity that includes the cube of the diameter. So double the diameter, you get 8 times the resistance. Triple the diameter, get 27 times the resistance. Use two tube instead of one, get double the resistance. However, you do get credit for moving the two mixte tubes away from the neutral axis of the bike. Don't make me go get my statics book. I hate to even start analyzing why a frame feels whippy under touring loads.

mkeller234

That Motobecane looks like a whole new twist. A single top tube that splits before it reaches the seat tube, very interesting! Or are my eyes lying to me?

alicestrong

My 77 Moto Grand Touring mixte has the same frame construction.

Vitus tubing...

It's also "golden"...

Veloria

Thank you Road Fan and others! I think I am getting the idea and now have the terminology for further research. It is hard to look things up when you do not even know what terms to look for. This is very helpful.

Re the Motobecane mixtes: the Mirage and Grand Touring had the continuous twin stays; the Super Touring and Grand Jubilé had the single top tub diverging into twin stays. Here is my darling, sweet Marianne with continuous twin lateral stays.

To follow up on the strength issue: My reason for asking the question in the first place, was that several respected frame-builders have now told me that they do not (and will not) make the "classic" type of mixte with the continuous twin lateral stays, because that construction is not as effective as the single+split construction. They made it clear that it was not an issue of cost cutting, but a quality issue. If the female frame is to meet the same standards as the male frame, it could not be done with the classic twin lateral construction.

Blue Order

My '76 Grand Touring mixte has the single top tube diverging into twin stays...

Rob van Der Plas recommends against using a mixte for loaded touring, on the grounds that a loaded mixte is unstable at higher speeds. I wonder if the single + split construction solves that problem?

I will have to post a pic of Ms. Blue Order's Bianchi mixte-- an even stronger construction than the Motobecane pictured above.

Veloria

I had a feeling that someone would prove me wrong as soon as I said that!


This is in line with what others have told me. According to those who produce the single + split construction mixtes, it does solve the problem. Thanks for the link to that book; I'll check it out!


Yes, please post it!

Doohickie

Twin lateral tube frames do not more easily fail. A single, larger diameter, tube is structurally more efficient, meaning it can carry more load with less weight. It's not that the twin top tube of a mixte frame will fail, it's that it ends up being a little heavier. If you think of the old (1960-70 era) Schwinns, they had smaller diameter tubes, but to carry the load they needed to be thicker-walled tubes so they tended to be much heavier, about 40 lb. for the whole bike, versus less than 30 lb. for bikes in the 1980s that had just slightly larger diameter (but thinner walled) frame tubes. Larger diameter + thinner walls = lighter for the same strength than smaller diameter + thicker walls.

They are also heavier because they have three sets of stays coming from the rear lug, versus two for most bikes. That third pair of stays costs something in terms of weight.

If you look thorugh bike catalogs, you'll see that mixte frame bikes are slightly heavier than their diamond frame counterparts.

Veloria

Hey Doohickie, nice to see you here.

I had not realised that mixtes tend to be heavier than the diamond frame counterparts. So what you are saying then, is that modern builders do not produce mixte frames with twin lateral stays, because of the weight these would have to be in order to be as structurally efficient as the men's frames? That had not occurred to me, because that was not how they phrased their objections. Interesting idea.

One thing I've noticed, is that Vanilla -- which does make a mixte with twin lateral stays -- calls their mixte a "cruiser" (Cybelle's Cruiser). To me, the term "cruiser" suggests something that is less capable than, say, a "road bike" or "touring bike", and I wonder whether this designation is related to the same issues.

bikemeister

OK, I'd like to throw my opinion into this thread.
I always thought "mixte" came from the idea that ONE frame style could be used by both men and women - it was a "mix" of the two frame styles.
I love the Peugeot mixte frames - too bad they're not made anymore.
Just wish somebody would make them in oversize aluminum tubing for today's cruiser/hybrid market!

Road Fan

If anything it's very minor partial credit. The two laterals are not usually tied in a way where they transfer significant torsional stress to each other. There is a benefit in terms of longitudinal whip. But that's not strength, either. Imagine if you took two drinking straws and tied them together with pipe cleaners, making it look like a ladder. Then grab the ends of the ladder and see how easily it twists. There is no inherent torsional bracing in that structure, beyond the twist resistance of the two tubes. In order to get good torsional stiffness from those skinny tubes, you'd have to craft them into a large-diameter three dimensional structure, like a space frame or a Tensegrity Beam. But it would then just be a lumped approximation of a tube of similar diameter.

No prob with the statics book, I got one.