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Importance of Seat Tube Diameter?(3 posts)

Importance of Seat Tube Diameter?dlsmith003
Feb 28, 2003 6:08 PM
Hey everyone.......

I have a question to ask everyone in regards to seat tube diameter. How much importance does the seat tube's diameter play in the stiffness of the bottom bracket? I'm looking at having a bike built for me by a local builder, and I was curious on what seat tube diamter would be best. I currently own a Serotta CSi that has a 35mm seatube diameter. I was wondering if a 28.6 or 31.7 diameter seatube would make a difference in the bottom bracket's stiffness compared to a 35mm diameter? Thanks in adavnce!
re: Importance of Seat Tube Diameter?Rusty Coggs
Feb 28, 2003 8:34 PM
I think the CSI seattube is overkill.The downtube is often ovalized at the BB to incerase lateral stiffness,some more than others.28.6 can work if everything else is done right.There is more to it than just diameter.
Ask your frame builder firstrussw19
Feb 28, 2003 9:46 PM
No offence to those here on the board, but why ask us? Why not ask your frame builder instead. Hopefully he has seen you and talked to you and knows your size and riding style and will build you a frame accordingly. I have never built a frame, and I don't know that many of us here have either (I am sure someone here has, but most have not.)
The idea behind oversized tubes is that as the diameter of the tube increases, the lateral stiffness increases as well, so you can shave material from the tubes (via mechanical butting, or just making thinner walls) to make the tube lighter. It is what you notice first about aluminum frames.. the tubes are so oversized, but still light and stiff. There is a fine line that frame builders walk when they build bikes though. Too thin of a tube, and it's dented very easy and the strength is compromised. Too thick of a tube and your bike is a pig. How thin you can go is also determined by many factors such as rider size and strength, rider style, and frame material. You can use much thinner walls on aluminum tubes than steel. The Serotta CSi you are asking about is steel and therefore must have a very oversized tube set to be that strong. Otherwise it would not stand up to the back and forth swaying at the bottom bracket without becoming too brittle.

For good info on steel tubesets, check out Columbus's website and read about their Nivacrom and Foco tubesets.

And you also failed to mention what material you are having your frame built from. I am assuming steel since you brought up the CSi, but my advice is to consult your frame builder first. Newer generation steel frames are getting high tech and aren't that easy to work with. Stick with what your frame builder knows. Let them build you what they are good at, and if they don't build what you want, look for someone who does and let them do it.

I know that wasn't exactly what you asked, but I hope it makes a little more sense to you. You need to ask these questions to your frame builder and tell them exactly what you want your bike to ride like, then trust them to build it. Bike building is an art, and some builders are better artist than others. I hope you are using a local builder because you have seen their work and like it. If so, let them do their thing.