mrsocal

Hello,

I just bought a used 2010 Trek 7.7 FX for $900. The bike looks like new and rides very nice however, I'm very disappointed as the rear gears are slipping (shifting up and down) and I noticed a significant scratch on the rear derailleur "105". It seems like the bike fell down on that side or was dragged causing this damage. I'm taking the bike this weekend to our LBS to check it out. I was wondering, even if they are able to adjust the gears, would it be better to replace the rear derailleur to avoid any other issues in the future or do I just ride it for a while and see what happens? Your thoughts please.
Thanks.

pursuance

On used bikes I've removed the RD hanger, make sure it's flat, then reinstall, then adjust the cable.
One can not get a proper RD adjustment with the bike upsidedown, so hang it by the seatpost if there is no workstand.

A new hanger can be bought.

dynaryder

Yeah,derailleur hangers usually take the brunt of the damage. Take a look at the bike from directly behind. The derailleur should be parallel to the frame. If it's angled inwards,then the hanger needs to be replaced. They're usually $15-25.

Note,the scratches may have nothing to do with your shifting issue. It could be the cable just needs adjusting or the chain and/or cassette is worn out. It could also be 'auto-shifting'. Does the rear der cable run under the bottom bracket? If so,try pulling on it on either side of the BB to move the cable back and forth through the tunnel it runs through. Dirt and other gunk can get in there and cause cable drag that can create the issue you're reporting. If that's the case,then the cable just needs some cleaning and lube.

Dunbar

My last bike (used road bike) had a bent derailleur hangar and scraped up rear derailleur. A bike shop should have the Park tool to check if it's bent, assuming it's not blatantly obvious by just looking at it. If they confirm the hangar is bent I would just replace the hangar. Once you bend them in a crash it will be a constant battle to keep them straight and they are not expensive. Once I had my hangar replaced I rode it for thousands of trouble-free miles. BTW, if you had inspected and test ridden the bike before buying it you could have discovered this issue before handing over the cash.

mrsocal

The bike shop said the rear derailleur hanger is bent although I didn't see it bent at all and they were very busy they couldn't work on it today. So I went to another bike shop, they didn't mention the bent hanger and adjusted the gears. It shifts better but it's not very smooth. I'm not sure who is right so I guess I'll just ride if for a bit and see what happens. Thank you.

mrsocal

The bike looked great and shifted okay. I rushed buying it as it was for half price. When I got home and noticed the slipping. Thanks.

mrsocal

Actually, the gears are not slipping any more, just a bit rough when shifting but no slipping for sure. Could the first lbs be correct and the hanger could be bent causing the rough shifting?

dynaryder

Bunch of things could be causing the shifting issue. The hanger could be bent slightly. Does the shifter cable route under the bottombracket(see my comment above)? The chain and/or cassette could also be worn. Or the cables could just need lubing.

Where are you located? Any bike kitchens or co-ops that could work with you to help you figure it out yourself?

mrsocal

Actually, I rode it today for a longer ride and it feels really good. I think the other bike shop did a good job. This is my first 7.7 FX and although couple of years old, it feels really good. Impressing acceleration and very smooth shifting. I tested every gear and they all shifted very well, the three in the front and the ten in the rear. Hopefully, it'll stay that way. Thanks again for all your help.

Wanderer

Keep in mind, that a bent hanger can prevent quiet shifting, and cause annoying improper shifting. It also make make noise when just riding, even slightly. If it's bent, get it repaired. It's a cheap repair, and will make a difference.

Ask them to show the alignment with it bent, to confirm. It will show on the tool/rim.

mrsocal

I'm sure you're right. I probably don't know how good the shifting of the 105 derailleur is, I'll take it back to replace the hanger. Thanks. Also, I wonder if I should replace the derailleur as well. The damage on it is so bad that the number 1 and 0 of the 105 is halfway gone with the scratch damage stretching upward. Wouldn't be safer to replace the derailleur and the hanger? Please advise. Thanks.

Dunbar

I don't think you need to replace the rear derailleur. Mine was pretty scrapped up and shifted fine. I wouldn't even have known it was bent if it didn't start rubbing the rear wheel spokes in the lowest gear (i.e., it otherwise shifted just fine.) If you want replace it for cosmetic reasons that's your call. I would buy one on ebay though and save some money. If you do replace the derailleur wait and do the hangar at the same time to save some money on labor.

mrsocal

Two reasons actually: I want to replace it to make sure that it's working well with no worries and also for cosmetic reasons. It doesn't look right with that damage on it. If I buy the derailleur on ebay, how can I make sure that I'm buying the correct part for my bike model year? Thanks.

dynaryder

Derailleurs aren't specific to bikes;it could be any road or MTB derailleur that's compatible with your shifters. It just has to be long or short cage,whatever you've got on there now,and the number of speeds has to match up(due to the chain width).

mrsocal

Thank you.

Wanderer

Given your statement, I would just have the dealer replace tbe hanger and derailleur at the same time. Alignment and adjustment will be part of the same job...... I'm like you, and would be happier with it done right, once and for all.

Meanwhile, ride it until they get the parts in.

martinus

The park tool der. Hanger straightener works really, really well... Down to a mm ! Donno what I think about taking the hangar off and checking it on a flat surface, it's a start but then what's next ? Adjusting it with a hammer ?have either shop show you how it works, both checking and adjusting... There is no real need to repl. the hanger, unless it's broken. Same goes for the rear der. If you want to repl. Various components for cosmetic reasons you are welcome to do so, just don't throw parts at it to fix a problem that may or may not exist. Which will get expensive, and if you dont know how well a 105 rear der. Works, than you are wasting your time and money. ( the answer is, as good as the shifter pod thats telling it what to do, and the cable connecting them to each other. Thats why everyone is asking about the cable and housing conditio. Rear Der. Are all the same in function, some are made from lighter material, but none have speeds assigned to them, that's what limit screws are for. ) This is where finding a good shop that you can trust comes into play ... Get a second opinion, if you must ... But be aware, bikes are easy to recognize Esp. If they have a certain issue, attached to them ... when it comes back for another issue, and the old issue was solved, Esp. With new parts, get ready to be treated differently.

dynaryder

Personally,I won't straighten a replaceable hanger,I'll just swap it. They're made of soft aluminum to bend and save the der and frame. If you bend them back,then the next hit it takes there's a good chance that instead of bending it'll break. Then instead of limping home with your shifting screwed up,you're pushing it home.

Wanderer

By repl the hanger now, you have the old one to throw in your tool kit, for emergency. And, there is only one way to have something look like new, once it has been scraped up. (Am I reading between the lines correctly?) A new 105 derailleur isn't going to break the bank, on a higher priced bike. MHO

You can also resell the old DR, or keep it for a spare. I use a lot of take offs, on neighborhood kids bikes (including grandchildren)

martinus

Soooo ... they are, made to break... but, since it didnt... replace it... because it might break, in the future ??? Oh boy ...

Wanderer

Aluminum is very weak, once bent, and should be replaced. Straighten, only in an emergency situation.

Yep, I would reflatten the old one, and save it in my tool bag, only for emergency use. I might even buy 2 while I am buying, just so I'd have one for later.

dynaryder

Yes.

Yep. Had a motorcycle tip over once and bent the clutch lever. My Dad said we could try to bend it back,but it might break. It did. Had just enough left to go to the shop and get a new one,but learned my lesson about bending aluminum.

mrsocal

That's what I'm leaning towards doing. I want to get it fixed right so that I don't worry about it. Thanks.

mrsocal

I took the bike in for a second opinion at a different bike shop, I asked to check my gears cause they are slipping. The mechanic just adjusted the gears and didn't say anything about the bent hanger. The damage on the derailleur is obvious, but he didn't even bring that up. I rode the bike and it shifted like butter. I was really impressed, I never had a 7.7 FX before. Is it possible that my LBS made a mistake? Then today I rode the bike for 20 miles, with no problem, only the middle rear gear is a bit sticky, but it shifts fine. So I stopped at a different bike shop (third bike shop) and asked to take a look at my rear derailleur and the hanger. The lead mechanic checked it out and said every thing is fine. Nothing wrong with it. So I was surprised again. So I guess for now, I'll change the bike shop and I'll ride the bike for a while then replace both the derailleur and hanger. Thank you.

mrsocal

Thank you all.