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Ovalized seat tube?

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Ovalized seat tube?

Old 09-27-13, 05:53 PM
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Ovalized seat tube?

Lately I've been thinking the angle of my seatpost looked just slightly bent backwards and even the seat tube itself near the middle appears a bit warped. A lot of bikes look like this to me so I know there's an optical illusion, at least for my perspective and this may be the case. However, I was running my hand up and down the seat tube and it's bulged out from the intersection of top-tube and seat stays down about 2" or so. Is this a common design feature to reinforce the end of the seat tube or is this what is referred to as ovalized?

The reason I'm so concerned is that I had to bail on a turn and I road off a normal curb onto pavement. I weigh 400 lbs and I thought I raised up and let my knees and elbows and feet take as much of the shock as possible but in the heat of the moment I can't say for certain if I was completely off the saddle.

Thoughts? Suggestions?

The frame is 4130 cro-moly, double-butted and from memory I believe the seat tube was about 1.6mm thick at the opening.

Edit: I went back and zoomed in on some photos of the bike and it appears that the bulge near the top of the seat tube is a design feature, so I'm feeling better about that. But, I still think the seat tube looks bent at about the level of the front derailleur.

Here's a photo I took after the curb incident but the angle isn't the greatest for determining straight lines.

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Last edited by corwin1968; 09-27-13 at 06:01 PM.
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Old 09-27-13, 06:16 PM
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I'm a little surprised that I can see it from your picture, but that looks like an externally butted seat tube to me
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Old 09-27-13, 06:31 PM
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Placing a straight edge along the ST at a few points around it will help you see whether it's bent or not. If it's externally butted then the gaps will be consistent with each placement of the straight edge. Andy.
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Old 09-30-13, 06:08 PM
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adjustable reamer and next larger diameter seat post and you can ream the seat tube, round .

I took my Bridgestone RB1 out .2 mm to use 72.2mm seat posts ..[ but it was not ovalized ]

did you measure the ID with a Digital caliper , that is a better indication than a picture of the whole bike.
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Old 09-30-13, 06:35 PM
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Actually, I just freaked out when I discovered the bulged out seat tube but I now realize that it's just an external butt. The area I'm concerned about is that area where the FD is clamped. However, I often think seat tubes on many bikes look bent at this location. There is some kind of optical illusion for me.

I'm feeling better about the whole thing. If I get really concerned I'll just haul it to my LBS. I can remove the FD but I'm not confident I can put it back on correctly.

The listed frame weight is 5.06 lbs (without fork) and my cheap digital kitchen scale weighed it about 4.7-4.8 lbs so I'm assuming the denotes a fairly stout set of tubes. Does that sound correct? It's generic 4130 Cro-mo.
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Old 09-30-13, 06:50 PM
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While I agree that one measurement is worth a thousand assumptions the problem with measuring seat tube is this. The length of the typical caliper's jaws are too short to get past the heat affected zone where both warping could have occurred during the building and also where the binder slot is cut. This top section is very often not really round and can benefit from some (repeat SOME) reaming if done prior to the binder slot being cut. (Who here has caught a reamer blade on the slot, I have)? But the real post sizing issue if found below the top/lug section. This is where the ST is not distorted by the heat of welding/brazing but also the wall thickness can be far less then the top section. This is why reaming is really only a correction for distortion at the thick top section and not a smart thing to do below this portion. A common ST wall thickness for classic steel bikes is .6mm and can be as little as .4mm on some fancy stupidlight bikes. So reaming out .2mm might reduce the ST's strength by a minimum of 33%. I would strongly suggest a pause to reflect on this possibility before proceeding.

How well this area of the frame is built is a point of pride by many builders. Reducing the heat induced distortion a goal. Depending on the build sequence this area can see three or more heat cycles. The main frame tubes get joined, the binder gets attached, the stay tops get capped and then attached to the sides or back of the ST. Then we need a stress riser, I mean a binder slot cut. I have had three of my own frame fail over my 35+ years of building. Two were cracks in the ST just below the lug and behind the seat stay tops. I have had the opportunity to reach inside many hundreds (one of the benefits of being a shop wrench all my life) seat tubes and many (steel) frames have a slight bulge just where my two had cracked.

External butted St's (like the OP might have) help reduce this as does the quick production welding that is common in the Asian factories. But, again, I would really think hard about using a reamer for up sizing a seat post. Now a Flex Hone is another thing... Andy.
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Old 09-30-13, 06:57 PM
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Originally Posted by corwin1968
Actually, I just freaked out when I discovered the bulged out seat tube but I now realize that it's just an external butt. The area I'm concerned about is that area where the FD is clamped. However, I often think seat tubes on many bikes look bent at this location. There is some kind of optical illusion for me.

I'm feeling better about the whole thing. If I get really concerned I'll just haul it to my LBS. I can remove the FD but I'm not confident I can put it back on correctly.

The listed frame weight is 5.06 lbs (without fork) and my cheap digital kitchen scale weighed it about 4.7-4.8 lbs so I'm assuming the denotes a fairly stout set of tubes. Does that sound correct? It's generic 4130 Cro-mo.
5 lbs is a fairly common weight for frames built with wall thicknesses of .9 or 1mm in the thick sections and .6mm in the thin ones. Today this is considered stout. Back in the day this was middle of the road. Today stupidlight is more like 3lbs for the frame. Andy.
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