clawhammer72

My chain slipped off my chainring while riding today, lassoed my rear wheel, and tore off my rear derailleur.

Not knowing much about derailleurs, do I need to replace it with the exact make and model, or do I have some choices? Any recommendations?

It's a Shimano RD-MC38.




I'd also appreciate any tips in how to avoid my chain coming off willy nilly. It's not the first time this has happened. Last time, it bent my chainring.

Retro Grouch 

That's a mid level mountain bike rear derailleur. If it was my bike I'd get a Deore long arm rear derailleur as a replacement.

Just one chainring in the front? Is there a place to mount a front derailleur? If so, I'd install a front derailleur without the shifter cable and dial the limit screws so the derailleur straddles the chainring and doesn't rub the chain. That'll greatly minimize throwing the chain off the front.

clawhammer72

I've got three chainrings in front. Two are original to the bike and I added a third -- a granny gear which gets me up really steep inclines when I need it. I've thought about installing a front derailleur, but I don't know if it's possible to do and still use all my chainrings. I could go with just the large chainring, but I'd rather not if there are other options.

clawhammer72

PS. There is no setup for a front derailleur. I would have to manufacture/rig something. I'll post a picture tomorrow.

Retro, a second question. What makes one derailleur better than the other?

Retro Grouch 

How thin can you slice the baloney? Shimano offers 8 regular mountain bike groups from top of the line XTR to department store level Tourney, and they all function reasonably.

The difference is materials and manufacturing processes. The better ones weigh a tiny bit less and have tighter tolerences so they operate a bit more "crisply". As you move up the food chain the price goes up faster than the functionality. Top of the line XTR, for example, costs right at double the second from the top XT. The price difference at the lower levels isn't so pronounced. So far as performance goes, I can't notice any difference if I move up or down one group level but I can tell a difference if I move up or down two groups.

I suggested Deore for your application because it's similar to what you had before and because I think it looks and works a bit more crisply than the lower level offerings but costs just a few dollars more.

Jeff Wills

+1000 from me. I've worked with every grade of derailleur that Shimano makes over the years, and they all work well when new. Deore & Deore XT are at the sweet spot in the performance/price continuum.

In terms of the chain coming off, that's usually a result of the chain flopping around too much. According to this: https://linearrecumbent.com/linear_mach_iii.htm , a chain tube was an option. Does your bike have one?

VegasTriker

I have an earlier LWB version of this bike before Linear started using the 3X7 or 3X8 SACHS rear hub. It has a derailleur post that mounts on the frame and is separate from the bottom bracket holder. Maybe Mr. Bicycleman still has the part available. Unfortunately, OEM parts for the bike are $$$$. I crashed my 1997 Linear a time or two and slightly bent the USS bars. After learning that the bars were really expensive, I bought a second even older Linear just for parts. I've never had to use the parts yet but it makes a nice loaner bike for my friends. I paid just $200 for it so if I ever did need some of the more expensive parts it would be well worth it. I don't have any chain tubes on either bike and don't remember having the chain come off while I was riding the bike. You can make your own chain tube from 1/2 inch black irrigation tubing. An easy way to hold it in position is to glue velcro to the bike frame and to the outside of the tubing. You may need to add a spacer between the frame and the velcro to get the tube the right distance from the frame. My BikeE AT uses this system and I just replaced the velcro. It works very well on the BikeE.

clawhammer72

Vegas: Thanks. I like the velcro idea a lot. I'd like to see a picture of the derailleur post when you get a chance.

gnome

I'd contact Linear/Mr Bicycleman to see if he has any of the add on deraileur mounts left so you can mount a front derailluer to stop the chain derailment. He doesn't have many left. Hopefully I didn't buy the last one...

Another option is to purchase the return idler so that it takes some of the slack out of the chain. The chain tube addition would also be a good idea.

I'll take a picture of my Linear 2.0 with derailleur post when I get home.

Recumbomatic

https://www.nashbar.com/bikes/Product...1_10000_202432


Inexpensive, Shimano compatible, and it's gotten lots of good reviews.

gnome