Repairing an old Trek

[Basil10]

Hi all

I wonder if anyone can help with this.

I have a (90's?) Trek 800 Mountain Track Sport with a bent derailleur hanger. Can a replacement hanger still be bought for these or should I try to straighten it? Thanks

 

[Cycleops]

Hi Basil and to the forum.

I also had an old Trek MTB which had a bent hanger. You should be able to get a replacement but you'll have to identify it visually. I think SJS cycles have quite a few so try there first.

 

[figbat]

You’re lucky, the hanger on my 1997 Trek 830 Mountain Track is part of the frame - I had to tweak that VEEEERY gently to get it square again.

 

[HobbesOnTour]

I'm far from an expert in such matters but I've had 2 Trek 800s and neither had a seperate hanger.

 

[weareHKR]

 

[CanucksTraveller]

Take the hanger off so you can see it on its own, and look online at anywhere that sells spares, you should be able to see one that's the same shape. They're not usually unique, and several bikes from several manufacturers will use the same template. SJS as mentioned, heck even Halfords might have one that fits.

 

[DCBassman]

Trek 800s don't have a separate hanger. It will need careful straightening. Might be an LBS job, unless very handy with a big adjustable...
Definitely a 'be careful' job.

 

[SkipdiverJohn]

The 90's steel Trek frames I've had experience of did not have separate removeable gear mech hangers, they were an extension of the rear dropouts.
Fortunately these Trek frames are (generally cro-moly) steel, not easy-crack aluminium alloy, so a bent one should be able to be carefully straightened. The important thing is to work out in what way it's bent, and by how much. You only want to unbend something that's been accidentally bent once, not get it wrong and have to attempt it more times.

 

[Reynard]

The 90's steel Trek frames I've had experience of did not have separate removeable gear mech hangers, they were an extension of the rear dropouts.
Fortunately these Trek frames are (generally cro-moly) steel, not easy-crack aluminium alloy, so a bent one should be able to be carefully straightened. The important thing is to work out in what way it's bent, and by how much. You only want to unbend something that's been accidentally bent once, not get it wrong and have to attempt it more times.

That's because repeated bending work-hardens the steel. It then becomes brittle due to the change in the metal's crystalline structure, and will break when a load is applied.

But if it's done carefully and with thought, then there should be no issues.

 

[SkipdiverJohn]

That's because repeated bending work-hardens the steel. It then becomes brittle due to the change in the metal's crystalline structure, and will break when a load is applied.

There speaks someone who understands engineering! 👍 When you cold bend metal, you are taking the material beyond it's yield point, which is why it stays bent and doesn't just spring back again when the applied force is removed. Each time you re-bend it, you weaken it a bit more, and if you do it repeatedly the material will crack and fail. You can tell permanent changes are happening to the material, because if you bend it quickly enough it gets hot. If the distortion was purely elastic then the energy input would be returned kinetically once the applied force was removed.

 

[Reynard]

There speaks someone who understands engineering! 👍 When you cold bend metal, you are taking the material beyond it's yield point, which is why it stays bent and doesn't just spring back again when the applied force is removed. Each time you re-bend it, you weaken it a bit more, and if you do it repeatedly the material will crack and fail. You can tell permanent changes are happening to the material, because if you bend it quickly enough it gets hot. If the distortion was purely elastic then the energy input would be returned kinetically once the applied force was removed.

We've probably all done this with a paperclip at some point, repeatedly bending it backwards and forwards.

The more you bend it, the harder it becomes to bend until it suddenly lets go. But you can also see the colour changing while you are doing this - the area under stress becomes much more dull, which is, in part, indicative of the structural changes in the metal and the heat generated by those changes.

Of course, the heat is generated because the conversion of one form of energy to another is never perfect.

 

[Basil10]

Thanks for all the replies. I assumed it was removable/replacable but it sounds like it's going to need bending back to as close to normal as I can. It's only a cheap old thing but I do want to do a good job.

Maybe it's worth removing the derailleur and putting it in a vice. I'll have a careful go anyway and will let you know what happens. Thanks again