Some Factors in Bicycle Frame Tube Engineering

Recently, Yoshi posted a statement and question on 25Seven regarding my post about tuning ride characteristics.  I’m paraphrasing his comment here:  

“I want to know how you made the average…  and the relation of diameter and the thickness?”

Yoshi’s questions are important and fundamental for frame builders.  Without giving away any Seven Cycles secrets, I can explain some of the interesting basics.

In the above mentioned post, I talked about how we adjust the ride characteristics on each individual frame.  Here’s I’ll provide an overview of how we think about ride characteristics—at least as relates to tubing.  In essence, I’m talking about why we bother with tailoring the ride characteristics and why it’s worth the bother.

It’s pretty simple :P  We evaluate four basic categories of influence for tube engineering on each frame we design and build:

Make sure the frame rides well	We define this as ensuring the ride feels precisely the way the rider wants, whatever that might be.
Make sure it’ll ride forever	Most riders want a bike that’s going to last.  Not everyone seems to care, but most do.  At its simplest, we have two basic considerations for “forever”:    >>  Fatigue life:  We want to ensure that the tubing will handle all the stresses and loads it could possibly see in its lifetime.  This also gets into manufacturing processes—a topic for another time.  Suffice to say that the way in which the frame is produced has a significant impact on its service life.  So, the question isn’t simply, how big is the rider, or what type of big hit riding will the person be doing?  The method of manufacture plays an important role.    >>  Damage tolerance:  Impact resistance, abrasion resistance, notch resistance—all the “-istances”.  Basically, we want to ensure the frame will hold up to the bumps and bruises of daily use.
Make sure it’s optimized for the rider’s individual component choices	Component choice has a tremendous impact on tube set design:  disc brake or v-brake, for example.
Make the frame is as light as possible	Oh, yeah; this one.  “The lightest bike in the world, please.”  No problem, if we throw away the other three important considerations.  Again, as noted above with fatigue life, the manufacturing process plays a big role in how far one can push the desire for gram diet.

Most of these categories are fundamentally at odds with each other.  That’s what frame design is all about:  balancing opposing forces.  For example, light and durable are pretty much on opposite ends of the spectrum.  Fortunately, some details positively reinforce each other; optimization for components also helps ensure that the frame will be as light as possible. 

Stock frames compared to Seven’s design method

In fact, when designing stock bikes, engineers have to design for average not optimization.  That’s why, when you ride a bike that your friend said is awesome, you might find it too flexible, too harsh, or just plain wrong for some reason—what’s great for one person may not even be adequate for someone else. 

In a real world example, not too long ago we had two retailers visiting Seven Cycles, talking about their experience with a popular high-end carbon bike.  Both retailers were pretty much the same size, weight, strength, and ride capability.  One retailer thought the bike was too mushy.  The other retailer thought the bike was too stiff—that it skittered over rough roads.  Of course this is not entirely the frame’s fault but both bikes did have the same kit—the only kit the bike comes with—it’s stock, after all.

Of course, there’s not necessarily a right and wrong; that’s the beauty of preference.

It’s also the beauty of what Seven Cycles does.  Seven is not constrained by the parameters of averages—or even to the preferences of a professional racer.  Seven’s only constraint is the individual’s preferences.  And that’s not a constraint at all—it’s liberation:  the opportunity to deliver on precisely what the rider desires.  And, since we work directly with every customer, in conjunction with the retailer’s input, Seven can optimize this balance of opposing forces to the greatest degree possible.

Tomorrow, digging deeper into Yoshi’s question, I’ll move onto tube buckling limit; the lower threshold with which we begin any tube design—that’s where it all begins.