gitaneusa.com :: View topic

garage sale GT

Is it possible a 1980 Super Vitus 971 Criterium required a 26.6 seatpost rather than a 26.4 because of 0.7mm thick 28mm tubing? (28-0.7-0.7=26.6)

Is it possible the lug rotated backward a bit and left a tiny bulge in the top rear of the seat tube, around the slot, because of the extra clearance?

Is it safe to try to bend or clamp it back after getting the correct seatpost or should the frame be condemned?

I don't know how thick the paint is and couldn't measure inside the seat tube anyway.

The other possibility is that Super Vitus 971 was rated for less than 220lb. I believe it was, but it may have been in order to have adequate rigidity, not to prevent failure, and I am hoping the failure was due to the wrong post. Please help if you can.

sandranian · Site Admin

Yes. With Gitane, anything is certainly possible.

garage sale GT

Do you have any idea whether it might be safe to use provided I install a seatpost which fits the seat tube and perhaps add a clamp around the bulged part?

scozim

Can you provide a photo of the "bulge" on the bike?

verktyg

Metric diameter Super Vitus 971 and 983 tubing had 0.6mm wall thickness at the top of the seat tubes. Super Vitus 980 was 0.5mm thick.

The proper size seatpost for SV 971 and 983 frames was 26.6mm but...

All three of my 1984 Gitanes built with SV 983 (a Super Corse and 2 Tour de France models) had undersized 26.4mm seatposts fitted.

On many frames the inside of the seat lugs were warped out of round from the heat of brazing, especially from attaching the seat stays. Also the seat tube inside the lug could bulge from excess heat.

On a properly built frame, the seat lug should be rounded out, reamed to size and honed to remove any burrs inside the tube.

The seat tubes on most production bikes and even many top name frames were rarely preped before painting. Sometimes the only use of a reamer was during assembly to remove excess paint overspray inside the tube.

It was quicker to just use an undersized seatpost than fix the problem.

In your situation, it sounds like the seatpost was set too high. Seatposts need to have at least 3" (75mm) of insertion into the seat tube to prevent what happened to your frame. Deeper is better for heavier riders on thin tubing frames.

If you need the seat higher than standard seatposts are designed for, you may need a larger frame.

These picture show what can happen with an undersized seatpost.

This was my first 1984 Tour de France. Notice how the "ears" on the seat lug are almost touching from overtightening.

Notice the crack in the lug from overtightening on this frame.

Here's some pointers for preping a seat tube. This is a new frame.

An inexpensive brake cylinder brush hone to clean up the inside of the seat tube. It the hone is small enough it can even be used on the fork steering tube.

or a standard brake cylinder hone...

A smooth surface inside the seat tube can make adjusting the seat height much easier. Sometimes a change in height as little as 1mm can make a big difference in ride comfort.

In your situation, a competent LBS or frame builder with the proper tools and knowhow can probably round out the bulge in your seat tube plus round out the seat lug and ream the inside of the seat tube to take a 26.6mm seat tube.

Just reaming to size may thin the tubing to much if the lug is out of round so rounding is more important than reaming.

The biggest concern would be if there were any cracks in the seat tube caused by the bulge. Super Vitus tubing is at least as tough as Reynolds and Columbus tubing so that may not be an issue depending on the amount of bulge.

If you can't get the seat tube round below the lug, get one of the extra long MTB seatposts and insert it well below the bulge.

Send pictures...

garage sale GT

Thank you for the help. I have already pulled the seatpost up and pulled it toward the stem as hard as I could, then removed it, so the slight bulge doesn't show anymore.

I am concerned, however, that I may have weakened the brazing of the seatstays to the lug.

There was no cracking. The seat clamp notch was square edged so I gently rounded the corners with a small, round chainsaw file.

I hoped it would be enough to gently dress all the sharp edges as verktyg showed, then to firmly install a post which was the size of the inside of the tubes and a few inches too long, and put the clamp bolt back in. What could a frame builder do besides force the parts around a round mandrel which had the tubing's i.d.?

garage sale GT

[/img]
These are after I straightened it out a bit. However, near the bottom of the slot, there formerly was a gap between the slot and the seatpost of about a millimeter, and the top of the clamp was squeezed together too much like the red bike in Verktyg's pictures.

Rather than removing metal, even if I had a small brake cylinder hone or expandable reamer, I am convinced it would be kinder to the frame to just spread the clamp a bit, and tap in a greased 26.6 post.

I was mainly concerned if anyone had had the brazing around the stays crack, or if the top of a 971 post would crack after some very mild corrections.