While approaching a red light I slow down and shift down to the middle chainring, 3rd cog from the left on the cassette.

Though, when the light turns green and I put some force on the pedals, the chain pops up to the 2nd cog, and the chain drops to the smallest chainring.

What can be the origin of this issue? I really hate it that this happens. It's already cost me a dropout and a set of jockey wheels and I'm not really into getting more things fixed.

Who can enlighten me?

In the back it could be too much cable tension on the rear derailleur- try turning the barrel adjuster clockwise 1/2 turn or so.
Or instead, it could be a bent derailleur hanger.
What's happening in the front seems very unusual but I suppose you may have not enough cable tension on the front derailleur.
Another possibility is excess frame flex which could be due to a cracked chainstay. Let's hope not.

Al

Chainstay is fine, no cracks or anything there.

The derailleur hanger has just been replaced by a brand new one. Shouldn't be an issue either.

I'll have a look at the front derailleur tomorrow, hope that will fix it.

If anyone has any other ideas, please let me know.

To get back to a certain death thread, I noticed that my derailleur looks bent a bit. As if it's bent towards the inside, which could explain the chain/cog flipping issue. I'll be having a better look at that, hope I've finally found my cause...

Grab your crankarms and push and pull on the arms. Does the crank move at all? If so, your bottom bracket is worn out or your crank to bottom bracket interface is damaged. Also check that your chainring bolts are tight. Any flex in that area could cause the chain to derail under tension.

Now this is an interesting one. To have front and rear mis-shift at the same time is very unusual, especially if the rear shifts completely to the 2nd/larger cog. Certainly rear derailleur adjustment alone would not cause that behavior. The chainwheel is fed off the rear cog, but merely shifting to inward in the rear should not normally cause the front to shift. I need to check on a few things.

1. I assume you shift to the If you shift to the middle/3rd combo just before you stop - correct?

2. Does the front or rear shift happen first, or are they simultaneous?

3. If you shift to the same combination while aready underway but at a slow speed and then accelerate does it do the same thing?

3. Any equipment changes, falls or other serious problems just before this started occurring?

4. When you check the rear derailleur do the following: Put the bike on a stand so that you can easily look from the rear derailleur toward the chainwheels. Shift to the 3rd largest cog (same as you are having problems with) but the large chainwheel in front. Check to see if the pulleys are parallel to the freewheel - not bent in or out. Then check to see if they are parallel to the center plane of the bike, that is not pointing to the left or right. Shift to the middle chainwheel and see if anything changes.

I too hope it's not a frame issue. The reaction of the frame certainly could be unpredictable enough to cause such odd behavior depending on where the problem is.

The front mis-shift has been fixed, didn't happen again after I setup the front derailleur.

1) I shift to middle/3rd combo, that's right.
2) Front hasn't mis-shifted anymore. Rear still mis-shifts by itself while I am putting force on the pedals.
3) At a slow speed, when shifting and accellerating, it does the very same thing.
4) I have had new wheels installed, with a same 9spd Tiagra cassette as was on the old rear wheel. I got into a crash a while ago, and the RD got bonked onto the road.
5) I will check during my lunch break this afternoon whether it's misaligned.

While riding and looking down towards the RD it somehow looks as if the bottom of the RD comes a slight bit more towards the chainstay than the top. A bit as if it's bent, not entirely sure how to explain what I'm seeing.

My bet is that you still have a bent-in hanger. I've experienced this a lot with mountain bike crashes on aluminum frames. That is, that the part of the frame where the hanger attaches is a bit bent, and still needs to be bent out. The bike shop has a tool that can realign this properly.

The hanger is a brand new one, was replaced after the crash. If anything should be bent, it should be the derailleur.

I wasn't clear. The place where the hanger attaches might also be bent. So might the derailleur.

I've been thinking about upgrading to 105 for a while. Might be a good time to just get a new 105 RD?

Maybe, but it won't help if your frame is still bent.
If you're considering more upgrades, De Vos between Haarlem and Amsterdam, has groupset prices (including installation) that are surprising. Something I'm considering for my own bike.

I've double checked the frame, and double checked it once more. There isn't any sign of deforming/bending at all. When putting in the rear wheel it falls exactly into centre, so I'm still betting on the RD.

De Vos looks interesting, not sure if I can spare 425 to get 105 installed yet though.

So I asked my wife to have a look at the derailleur. She's not into cycling or whatever, but she has a good eye. She did notice the derailleur was slightly off, so I basically gave her a list to take to the LBS tomorrow. She will be picking up some outer cable stoppers and some inner cable ends, and besides that she will be checking out the price on the RD-4500-GS. If it's below €30 she will be ordering one directly, if it's above, I will be ordering through the web to save some cents.

Finally figured out how the in-line barrel adjusters work *fail* so I decided to reinstate them and make sure I have the minor cable adjustments within handreach instead of having to get of the bike every time I need to make a small adjustment. So basically, that's why I need the outer cable stoppers

just be careful with those dudes.

^noted, thanks

My wife will be checking the LBS today for a new derailleur. Hopefully I'll save a bit and have a new one reinstalled later on this week.

Did notice the screws for my derailleur jockey wheels weren't too tight though, I did fasten them a bit.
Could this have any effect on my issue?

Not likely. Most top pulleys are designed to float sideways one mm or so to work properly with the cassette. That's the pulley which affects shifting the most.

Shouldn't be a problem then. I did notice there seems to be some float in the bottom of the derailleur cage. If I push it towards the iside it takes several mm's until it gives me some resistance, pulling it outside does the same.

Had my bike upside down today. Did a check on the RD/cassette and noticed that the RD is actually laying the chain on slightly sideways. So it's twisted inwards with a few degrees. Is that normal?

Next to that the B-tension screw is bent, possibly during the crash from earlier this year.

While holding and tensioning the RD manually I did notice that the shifting issues appeared less bad as it does when I'm on the road. Though still, putting sudden force on the pedals causes the chain to flip upwards, trying to reach the larger cog. I had the possibility to view from a bit closer, with the bike upside down, and noticed that it looks as if the chain is derailing and tries to reach up to a larger cog, failing to grab it and fall back down.
Can't be good for the cassette, can't be good for the RD and chain either.

I'll stick to very minor adjustments of the barrel adjuster and see how far I can get with that. If that still fails by the end of the week, I'm ordering a new RD.

Do all of your derailleur tuning with the bike right side up and the rear wheel fully seated in the dropouts.

I always do that. Just had it upside down to see a bit closer what was actually happening in the rear.

This morning something inside me just snapped. Riding up the Brienenoord (probably known by Metzinger and Botto) the rear just kept flipping over the cog, even after adjusting the barrel adjuster on the RD.

I'm done. I've ordered a new RD.