cooker

I'm going to get a bike shop (the authorized BF dealer) to look at it, but would love to hear some advance opinions. I don't know if I hit a rock or if my limit screw worked loose, but in any event yesterday on a dark shaded trail, my rear derailler arm snagged the spokes and badly bent the derailleur hanger part of my Bike Friday steel frame, even pulling the actual dropout partially open. It's a thinner part than on a full sized bike, and I'm worried that if it is simply bent back into alignment it will be seriously weakened.

Is there a way to tell if it is safe? Can you heat-treat it to restore its strength or anything line that?

My guess is that if it is too far gone they can order a new rear triangle since it simply attaches to the rest of the frame by a hinge. Or would it make more sense to somehow weld it?

Pictures tomorrow. Comments welcome. Thanks

DMF 

I wouldn't f* with it. Replace if possible.


(You might also want to consider a dork disk.)

Asi

I understand the the hanger is built into the drop (not removable)? Is it from steel? (the dropout)

If so try a heatgun on about 600 degrees Celsius (not above, you will mess the heat treatment that it had) and straighten it with a 10mm x 1mm bolt (an axle from a wheel is the best tool) combined with a crescent wrench to get some leverage also to straighten the drop also. If it breaks then replace the dropout (see below)

You say something about pulling the drop? Like the drop is pulled out of the frame? If so then the best thing is to replace the drop with a new straight one and will require some brazing/welding. Ask a frame builder to do this job.

I have a russian crap bike with the hanger built into the drop and the entire dropout section was 3mm thick. It was bent on several occasions (the hanger and the drop from topview was like a paperclip.. skewed) and unbent by force (with no heating). It is still in good functioning order after 40 years with 130mm OLD instead of 120mm (without cold setting only a bit of force when installing the wheel + a mushroom nut) - so it's a mess but it didn't break

cooker

Thanks. Sorry no pictures - dropped it off at the shop today. The guy who eyeballed it said he thought it could be straightened but they're waiting for a more experienced guy to look at it tomorrow. I'll know more by Thursday.

I'm actually a fan of dork disks - I would never take one off a bike just for this reason, but sadly it didn't come with one. I'm still puzzled about the cause - I wasn't shifting down at the moment of impact as far as I recall and the shifting had been fine for 15 km up to that point, but I was on a trail with a lot of shade wearing sunglasses and there were some tree branches down so maybe I hit something. With the 20" wheels the derailleur is pretty close to the ground.

JohnDThompson 

I'm not really familiar with Bike Friday construction, but in general, quality steel dropouts are forged from mild steel specifically because this makes them amenable to reforming in the event of accidents. There will be some work-hardening if you bend it back into proper alignment, but unless you make a habit out of bending your dropouts into crazy shapes you should be ok.

cooker

Here's a sketch. The seat stay and chain stay are formed from a curved tube, and the combined dropout/derailleur hanger is like a fin welded on to the back of that. The arch at the top of the dropout (red arrow) is a bit thin and when the derailluer hanger was wrenched backwards (pink arrow), that's where it bent. I wonder if that arch will still be strong enough after it is bent back into shape.

DMF 

Oooo. You're right to worry. Let us know what the guy says, and how this turns out.


Chances are you got a branch through the chain and into the spokes.

davidad

If it isn't cracked in the center of the bend and it's steel it can be straightened.

DMF 

Straightening it might crack it.

I'm tempted to recommend heat, but at this point I'm out of my depth WRT to working the alloys used on bicycles. Framebuilders forum might be a better place to ask this.

cooker

They fixed it!!

The guy at the desk when I picked it up says they waited for the best tech to come in and do it and that he thought it wasn't too badly damaged. Not sure if I overreacted, or if the steel is just very good and the tech guy very skilled, or if I will suffer later if it fails at some point. I don't ride in a style that would put me at risk of serious injury if my back wheel locks up.

They put the old derailler back on too, which surprised me as the first guy who looked at it said that would have to go. The repair guy wasn't there and the desk guy didn't know it they had replaced any parts, like the jockey arm of the derailler. It looks perfectly lined up and worked fine on a 10 km/6 mile ride home, and the steel of the dropout looks completely normal with just some light tool marks on the paint to suggest it has been deformed and reset. Mind you I didn't remove the quick release for a more thorough look

I don't know if DMF was being serious or sarcastic in suggesting a dork disk, but I am tempted to go back and get one. It was an unlikely accident in the first place, and will probably never happen again, but if it did I wouldn't want to bend the steel back into alignment a second time, and would feel pretty stupid!

Thanks for all your support!