hhliao

I posted the same thread under the frame builder forum, but would like to get some feedback from C&Vers as well.

I recently purchased a 3rensho frame from a fellow BFer in decent condition. As I started handing components on the frame, I noticed that the middle section of the top tube seemed to be sagging a little bit. I put a straight edge on the top tube, and, sure enough, the middle of the top tube was about 2 mm lower than the end points of the top tube. It was essentially a very gradual and gentle curve in the top tube. The frame seemed otherwise straight with no sign of being previously crashed. Since it is a petite frame, I was even thinking that the original builder was trying to create a lower standover height this way. On the other hand, 2 mm contributes nothing to standover height for all intents and purposes.

My questions is: Is this something that I need to worry about? If it is, is there a simple way to fix this other than a complete tube replacement. Thanks a lot for you insight.

cb400bill

Pics?

rhm

When you ride a bike, the top tube is under compression; and not a lot of it (that's why it can be thinner than the seat tube and down tube). So it doesn't sound to me like it would be a safety issue, assuming there is no (other?) sign of damage.

It's hard to imagine how the tube could have got bent, even if the builder intended it this way. Just out of curiosity, have you laid your straight edge on the bottom of the top tube, or the sides? Do they all show the same bend?

RFC

hhliao

@cb400bill I will try to take some photos this evening. We will see how holding a straight edge in on hand while taking a photo with another turns out. I attach some distant side views here.

@rhm When I put the straight edge on the sides of the top tube, it lines up perfectly. On the bottom side of the tube, it shows that the tube is curving upward at the two ends. Therefore, I think the curve is only in the vertical plane. I cannot identify other signs of damage from the frame.

@RFC The curve is definitely not as obvious as the Fuji TT bike.

hhliao

@RFC As a side note, how do you embed picture in the post? I use the insert image button and enter the URL of the photo, but apparently that didn't work in my previous post. Thanks.

RFC

I use Photobucket, but I'm sure the other cloud services use the same code, they have to.

For Photobucket, I hold the cursor to the photo, which displays the menu. Then copy the [IMG] code and paste it into your post. Give it another try. And double check to make sure you are copying the right code.

hhliao

Thanks a lot. This works!

rhm

Huh. You know, I think I can see it in the photos! Out of curiosity, have you checked the downtube and seat tube with the straight edge? I could easily persuade myself they're a little curvaceous as well.

hhliao

The seat tube is pretty straight. The downtube is not as straight as the seat tube but far better than the top tube.
This is such a letdown. I spent a small fortune (by my standards of course, I am poor) and thought I found the perfect frame for my wife.

rhm

Oh, I wouldn't worry about it. Does it ride straight? Unless it's ridiculously unstable, she can ride it. We don't even need to tell her about the frame's special features!

Scrockern8r

^^ Yup.
As long as things are lined up well and it passes the String Method test. You are good to go.

Bianchigirll 

@RFC The curve is definitely not as obvious as the Fuji TT bike.

the Fuji has a curved TT?

Hilarystone

Top tubes often seem prone to be slightly curved - I've seen two frames recently of really nice manufacture which had bowed top tubes. My suspicion is that it happens when the top tube is cut a smidgeon long ... and when built is under compression which results in the bowing.

hhliao

Thank you all for your insights. I am more confident about the structural integrity of the frame now. I will build it up and test ride it to see if it tracks straight.

ricohman

I've had a bike with a slight bend in the top tube. And I know it wasn't crashed as I bought it that way. Actually my dad bought it that way.
And it was fixed with two pieces of wood and a hydraulic ram. Took my dad and the neighbor about 5 minutes to make it perfect and since the neighbor was a machinist nothing less than perfect would do.
A month later I hit a car with that bike and then they welded it back together not so concerned about it being perfect.
So yes, it can be made straight again if it really bugs you.

hhliao

Thanks. It's good to know that it can be done. From the posts in this thread, it sounds like the curve is not going to affect the function of the bike. I am just glad to know that I don't HAVE to deal with it. One day when I can afford a respray, I will probably ask a professional frame builder to do something about it.

sailorbenjamin

Have you told her yet?

hhliao

She's not in the country right now. I am leaning towards pretending that it doesn't exist. We will see if she finds out or not.

noglider 

Pretending is lying, so don't do that. If she notices and asks, tell her you had the same concern and then were comforted by others and learned that it doesn't matter. That's honest, and it's good news, too.

unterhausen

that doesn't seem possible, if the TT is long, the seat tube angle will be a little more relaxed. I mentioned a number of possibilities in the thread in framebuilding if anyone wants to go take a look. Steel tubes are not straight as they come from the tube manufacturers. Framebuilders have their own limits as to how bowed the tubes can be before they send them back. Generally, a framebuilder will pick an orientation so that the bow makes as little difference in the function of the bike as possible.

On the subject of telling the prospective rider, I doubt she or anyone else will notice

hhliao

That's exactly what I meant when I said "pretending that it doesn't exist".
I am not even sure how one can blatantly lie in this kind of situation. Maybe something like this: "Honey, welcome home, I've got you a new bike. Now, look at that top tube, have you any seen anything as straight as that?"

luker

often a top tube, especially one on an expensive bike, will get a little bow when someone repeatedly uses it for a stool, at a rest stop or what not.

Don't ask me how I know.