Internet cycling forums can be a tough place to get actual information about bike design as they are generally filled with self-appointed ‘experts’ and those with a large axe to grind. But there was a time when one forum in particular played host to some of the most experienced and revered framebuilders on the planet – I’m referring to the long lost Serotta Forum. The Serotta Forum was a great place for a meeting of the minds when it came to frame design and I’m proud to have been a part of that process.
One thread from 2007 sticks out in my mind and it concerns chainstay stiffness and how different designs affect the way a bike feels and behaves. The original poster posed a question about how a given size and shape of chainstay is chosen by the builder and how the bike will perform as a result. Below is my response to the original question and a link to the entire thread should anyone want to go back and read the entire thread in context.
I thank the crew at the Paceline Forum for maintaining all the old Serotta Forum content – you guys do a great job.
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This is a favorite subject of mine.
I’ll say up front that I have no idea what bike you might like best. I’ll leave that alone.
First I’ll say something that you won’t hear from many builders…….curving or bending the stays into any configuration other than straight will make the stays flex more. There are very few absolutes in life but this is one of them. Curved stays flex more than the equivalent stay without the bend. They won’t flex a lot more……just a little. It depends on the type of bend and the degree of bend. One thing is for sure, the bent stay can never be stiffer due to it’s bend. The biggest reason to bend a stay is to allow for better clearances with tire and chainring. This is a bigger deal on mountain bikes than road but true nonetheless.
In any given material, stay diameter is the biggest factor in determining the stiffness. The bigger the diameter, the stiffer the stay. The problem is that if you make the stay bigger in diameter you run into clearance issues with both the chainrings and tire. So….. most stays are about the same in this regard. One can use huge stays but then you need big, deep dents to allow for the clearance room and the dents go a long way to negating the gains in stiffness made by the larger diameter stay.
There is a current trend in frame design to control BB flex with a big-arse downtube. This may feel like a good thing at first and it will certainly help with front derailleur rub but in the end it does not make for a stiffer bottom bracket. It’s often overlooked but it’s the chainstays that make the drivetrain stiff. A big downtube might be good for other reasons but it doesn’t do squat to make for a stiff drivetrain. A Slingshot frame is a great example of this. It’s simple when you think of it. The energy goes into the frame at the BB and goes to the rear wheel………and the only thing between the BB and the wheel is the chainstays. This is one of the big issues in designing a good Ti bike for a big or heavy rider. Titanium is much more flexible than steel. The way to make it as stiff as steel is to make the diameter much larger. But this has it’s own clearance issues. So they make the stays oval (making them stiff in the vertical direction and soft laterally…….not ideal) or they put in huge dents which has it’s own issues. So most Ti builders compensate for the wimpy stays with a huge downtube. And like I stated above this has little effect on true drivetrain stiffness. This is one of the reasons that many larger folks feel that Ti bikes don’t have the snap they are looking for and why it’s fallen out of favor with many racers.
When pedaling a bike out of the saddle chainstays are asked to do a number of different things. They undergo compression which is easy for almost any stay design to deal with as the loads are low. They undergo a torsional (twisting) load which most stays deal with fairly well regardless of shape. Larger diameter helps a good bit with the torsion. The other thing that happens to stays is that they see a lateral bending load as the BB is pushed from one side to the other. This is where ovalized stays can get in trouble. An oval tube has a major diameter and a minor diameter. There is a pretty good rule of thumb that addresses how an oval tube will flex compared to a round tube. The oval tube will flex about the same as a round tube that has the same diameter as either the major or minor diameters of the oval. In other words if you have an oval tube that is 30mm by 17mm in cross section it will flex about the same as a 30mm round tube in one direction and about the same as a 17mm tube in the other direction. So if you think of oval chainstays you in effect get the lateral stiffness of a rather small round tube.
For all the above reason I feel that a round stay is best. You get the most bang for the buck in every direction. You get good clearances with minimal denting and you get a nice stiff stay and drivetrain.
Wow…that’s more writing than I thought it would be. Thanks for sticking with me.
Dave
