What do do about stretched binder bolt lugs

[scott_tracy]

The plastic reflector bracket on the seat post is to stop it slipping down. It's been remarkably effective at that.

 

[Gtpulse]

Fairly sure that’s the wrong size seatpost for that bike. Should it not have been a 27.2mm from new? Just measured one of my frames with the same tubing and it’s a 27.2mm. I know you’ve already said you’ve had the bike from new etc but that looks like a too small diameter seatpost has caused that damage. Possible the seatpost has deformed to that diameter from over tightening but that’s really unusual. Sold from dealers it’s really not impossible that someone put the wrong size from new in that bike, seen it before!

I’d remove the seatpost, get those lugs straightened out and try a 27.2mm.

 

[Fourbikes]

Something like that happened to a Raleigh road bike that I had. The seat post although the correct size (27.2) was dented slightly, due to over tightening( it was a chromo steel one) which effectively made it too small. I replaced it with a different alloy one and that solved the problem.

 

[old_coyote_pedaller]

Whether it's the wrong diameter post or not, the bolt should not be tightened to the point it deforms the seat-tube. The lugs themselves won't be bent but the part of seat-tube they are brazed to so that the seat-tube is bent in to grip seatpost. Carefully bend them back till there is a parallel gap between them, making sure that seat-tube stays round at top. Also check slot in seat-tube that it too is parallel, not narrower at top as it probably will be if lugs are bent in so that they actually touch. You cannot effectively increase grip on seatpost if the lugs are touching each other, no wonder you've broken 2 bolts. That's way too much tension on bolt for that to happen. There's something wrong if you can prevent the seatpost slipping by using a plastic bracket tightened with a crosshead screw.

As Gtpulse said after straightening lugs try another post.

 

[CassidyAce]

If the seat post is just 0.2mm short of the diameter it should be, after straightening things out, it might be worth trying a Coke can shim around the current seat post and seeing if that's an improvement.

 

[scott_tracy]

Thanks all. A 27.2 mm post can be 'persuaded' into the lug but not beyond - it binds in the seat tube. I measured the outside diameter of the seat tube as 29.2mm, including paint. So with that I think the 27mm seat post is likely correctly sized if seat tube is 1mm thickness.

 

[old_coyote_pedaller]

Did you bend the lugs back before trying to fit the 27.2 mm post? If not it could be binding because seat-tube is crimped in.
I've bent out similar lugs on a bike by opening the gap between lugs with a screwdriver till you can fit in a really big screwdriver, jamming it into gap between lugs and levering from side to side, saves the holes in lugs from getting distorted if you use a big Allen key to do it.

You say you've had bike from new and the lugs have gradually been bent in over the years, when you first had bike did the seatpost slip down at all?
When did it start doing it? Recently or an ongoing problem? Before or after lugs were bent?

From what you say, the bike being new at time and the 27mm post was the one fitted I'd assume it is the correct post.

 

[scott_tracy]

Thanks O_c_p. It's always required a fairly tight seat bolt to stop the post slipping from as far as I can remember. I'm also fairly sure those lugs were angled rather than parallel from the beginning, they've just become more so. I did exactly as you described with a big screw driver and only levered the non-threaded lug through the central hole. I think the underlying issue is a flaw in the design of welded lugs: a bolt is designed to pull in a straight line but the lugs essentially rotate around the post as they come together so bending the bolt. I guess that's why the better seat post clamps have the bolt go into a threaded cylinder that can rotate to accommodate the decreasing radius of curve on tightening the clamp.
My plan is to see if I can source a suitable sized seatpost clamp before cutting and filing the bent lugs. I'm not going to miss the cable stop welded onto one of the lugs as I've converted the bike to v-brakes ( just so much better than the cantis it came with).

 

[xxnick1975]

As others have said, sounds like theres more to it than the design, as lets face it, loads and loads of bikes have the same set up and dont have this issue.

Sounds like the tubes deformed now after years of overtightening.

If you have a decent bike shop near you it might be worth getting them to check and run a 27.2 reemer down it if needed.

Definitely shouldnt be slipping, and should only need a bit of compression to hold

 

[scott_tracy]

As others have said, sounds like theres more to it than the design, as lets face it, loads and loads of bikes have the same set up and dont have this issue.

Sounds like the tubes deformed now after years of overtightening.

If you have a decent bike shop near you it might be worth getting them to check and run a 27.2 reemer down it if needed.

Definitely shouldnt be slipping, and should only need a bit of compression to hold

Thanks xxnick. That's not a bad idea if there's sufficient wall thickness in the seat tube.