# Mtb derailleur vs road derailleur for cross bikes



## matanza

rear derailleurs are rear derailleurs right? Commuter bikes use them etc. Opinions???


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## rsroka23

10sp vs. 9spd
short cage (max 28t or so) vs. long cage (max 36t or so)
besides that, i would consider them close


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## jmoote

Road derailleurs are lighter and work with the cassettes used for cross. If you are running a mtb cassette or triple crank there is some argument for a mtb derailleur to get the cage length.


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## pretender

rsroka23 said:


> 10sp vs. 9spd


Derailleurs don't come with speeds.


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## backinthesaddle

Okay, an actual answer...

You can use a mountain bike r/d with a road or cross bike. It works fine.
You can use a 9 spd r/d with 10 speed shifters. It works fine.

As was said before, derailleurs don't have speeds.

I have used '9' speed Shimano 'mountain' derailleurs on several cross builds. Before I rebuilt my cross bike with a new group, I ran a XT medium cage on it for almost two years.

The other advantage of a mountain r/d is the fact that it's a bit tougher than a road r/d. If you ride your cross bike in the woods at all, the mountain unit is a smart choice. You can find short cage derailleurs too.

As for shifting, the differences are imperceptible. People who say that a mountain r/d shifts more slowly have no experience with the setup.


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## PeanutButterBreath

*One more opinion.*

The primary difference between road and MTB derailleurs is the geometry of the parallelogram and how it effects the travel of the upper pulley. Both are optimized to move the pulley, and thus the chain, as closely to the profile of the cassette as possible to ensure good shifting.

WRT your question, this means that a MTB der. can shift any road cassette, but a road der. may have trouble shifting above 30T or so before it moves the pulley too close to the cassette. The design of the frame's der. hanger has some influence on whether (or if) this intereference occurs.

Note that while MTB derailleurs can shift road cassettes, the un-optimzed path of pulley travel may degrade performance. Whether this is perceptible depends on many individual circumstances, and only the most discerning shifter would notice, IMO.

Road derailleurs are often available in short, medium and long cage versions. MTB versions are typically sold in medium and long, though short cage MTB derailleurs are not unheard of, usually at the highest component level. Cage length is only relevant to the maximum difference between the gear combo that requires the most chain length and the gear combo that requires the least. Optimal cage length is the shortest version that allows both, while being able to take up the maximum amount of chain slack (which exists in the latter combo).

Cage length does not change the size of the cassette that the derailleur can accommodate. Some people believe that shorter cages shift better. If so, this is only relevant to the use of a longer than necessary cage for a given configuration.

High-end road derailleurs may use more fragile parts in places such as the cage, but I would not expect this to matter unless you are considering Dura Ace <strike>or any of that cheapo weight weenie plastic crap SRAM sells</strike> Who else thinks puppies are cute?


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## Jwiffle

backinthesaddle is on the mark. If you need a longer cage, or something a little beefier, go with the mtn derailleur. You may need a longer cage if you're wanting a wider range of gears and running a mountain cassette (11-32 or similar). If you're running normal range of gears, and don't need any extra strength, you'll save some weight going with the road derailleur. A high end mtb derailleur will shift just as well as a high end road derailleur.


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## 88 rex

The problem is that you need a CX specific derailleur. 


All shimano stuff works with all shimano stuff.........road or mtn. Mix and match all you like.

SRAM road only works with SRAM road (and the new XX stuff). SRAM 9speed mountain only works with SRAM 9 speed mountain (X0 and lower).


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## pacificaslim

88 rex said:


> All shimano stuff works with all shimano stuff.........road or mtn. Mix and match all you like.


Not exactly. You can mix/match rear derailleurs with any shimano shifters since the cable pull is the same on all rear shifters. But the front derailleurs are specific to the shifters: i.e. mountain derailleur with mountain shifter, road derailleur with road shifters.


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## backinthesaddle

pacificaslim said:


> Not exactly. You can mix/match rear derailleurs with any shimano shifters since the cable pull is the same on all rear shifters. But the front derailleurs are specific to the shifters: i.e. mountain derailleur with mountain shifter, road derailleur with road shifters.


Bzzzt...wrong...

I ran a Dura-Ace triple f/d on my Surly 29er for two years and it worked perfectly. My brother is still riding it that way.


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## 88 rex

pacificaslim said:


> Not exactly. You can mix/match rear derailleurs with any shimano shifters since the cable pull is the same on all rear shifters. But the front derailleurs are specific to the shifters: i.e. mountain derailleur with mountain shifter, road derailleur with road shifters.


That may be true. I just know I run a SRAM Force front derailleur on my MTN bike with a SRAM X0 gripshift (and an XTR shadow rear with SRAM Attack gripshift for that). So if you were running a flat bar set-up for CX and were using gripshift, you could run whatever you wanted on the front. :thumbsup:


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## pacificaslim

SRAM fronts are all the same as far as cable pull requirement goes. 

But it's a straight up fact that shimano road and mountain front derailleur/shifters are different from each other. Can you make it work anyway? Apparently good enough for some people (backinthesaddle and brother) on some specific setups, but many other people encounter problems than can be solved by simply putting on the correct derailleur.


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## atpjunkie

*have ran*



backinthesaddle said:


> Bzzzt...wrong...
> 
> I ran a Dura-Ace triple f/d on my Surly 29er for two years and it worked perfectly. My brother is still riding it that way.


XT, XTR front derailleurs with STIs for years. Eliminates the need to use the roller for top cable routing
3 bikes all running top mount, top swing MTB, have used both STI and Bar Ends with nary a problem

longer cage means longer chain, means greater chance of a chain drop (long cages tend to bounce more)
If one chooses a MTB Rear Der, choose a short or Med Cage


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## Jwiffle

pacificaslim said:


> Not exactly. You can mix/match rear derailleurs with any shimano shifters since the cable pull is the same on all rear shifters. But the front derailleurs are specific to the shifters: i.e. mountain derailleur with mountain shifter, road derailleur with road shifters.


I don't think the front derailleurs are specific to the shifters. You can run a Shimano road shifter with a Shimano mountain derailleur.

However, front derailleurs are specific to seat tube angle. You can often get mountain fr der to work on a road bike and vice-versa, but not always at optimum performance. Seat angles on mtb bikes are usually slacker, and mtn front ders are designed around that.


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## atpjunkie

*I have found*



pacificaslim said:


> SRAM fronts are all the same as far as cable pull requirement goes.
> 
> But it's a straight up fact that shimano road and mountain front derailleur/shifters are different from each other. Can you make it work anyway? Apparently good enough for some people (backinthesaddle and brother) on some specific setups, but many other people encounter problems than can be solved by simply putting on the correct derailleur.


I get equal too, if not better performance using an MTB Top pull than I would using a road w/ bottom pull and relying on the cable pulley
and no it takes no special set up.


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## pacificaslim

Well, obviously you guys have managed to get away with it. But they are technically different. The road derailleurs require less cable movement for a given amount of derailleur movement. I suspect it is more critical on triple cranks. 

There is also the fact that the shape of the derailleur cage itself is different and the mountain cage is designed to fit on smaller chainrings.


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## old_fuji

Is anybody familiar with late 80's/early 90's Shimano Mountain LX stuff? I've got a FD and RD that I was thinking of throwing onto my new build (at least temporarily...until I can gets more cash). Originally, it had thumb shifters...would those derailleurs work with STI shifters, or am I just barking up the wrong tree?


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## Cyclo-phile

To the OP, let's restart. Is this a new build or a retrofit? Will you be using any parts that you already own? What do you want to use the bike for? Do you have an ergonomic preference for Shimano, Campy, or SRAM shifters?

The reason this matters comes down to two words: cable pull. Each company has selected their own proprietary leverage ratio within the shifter that their derailleurs are compatible with. That leverage ratio determines how far the derailleur moves with each shift.


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## old_fuji

old_fuji said:


> Is anybody familiar with late 80's/early 90's Shimano Mountain LX stuff? I've got a FD and RD that I was thinking of throwing onto my new build (at least temporarily...until I can gets more cash). Originally, it had thumb shifters...would those derailleurs work with STI shifters, or am I just barking up the wrong tree?


I did talk to an LBS and they said that the Mountain LX stuff will work with modern brifters (I know how everyone loves that word).


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## atpjunkie

*couple fallacies here*



pacificaslim said:


> Well, obviously you guys have managed to get away with it. But they are technically different. The road derailleurs require less cable movement for a given amount of derailleur movement. I suspect it is more critical on triple cranks.
> 
> There is also the fact that the shape of the derailleur cage itself is different and the mountain cage is designed to fit on smaller chainrings.


an XTR Front Mech is designed for use with both a 48 tooth chainring and a triple crank
in fact Specialized's first SWorks cx bikes came with XTR cranks (came as a triple with the ability to go double) and XTR Front Mechs
Since 48T is common in cx it is a non-issue, even XT Mechs can be used because running a double you can get the thing to throw from a 48 or 46 to a 34 without any issue
A road shifter throws as much cable as an MTB pod, I think in fact their actuation ratios are the same 2:1 for Shimano products.


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## thechriswebb

I have to admit...

I took my road cassette and derailleur off (7700 Dura Ace RD and 11-26 Sram cassette) and put the Deore cassette and rear derailleur from my mountain bike on to try it out, and I am having a really, really hard time justifying putting the road stuff bike on.... The dip in shifting performance was not what I thought it was going to be, and I still have a 11 cog in the rear. I also admit that the 32 gear inches from the 39/32 combination that I can now access is pretty easy to get used to. I have a hill with a 20% grade that I have to ride up on my way home from work and I've been spinning up that thing FASTER than I ever went standing up and pushing the 39/23 or 39/26.....

Yeah, a guy at work tried to make fun of me last week when I rode up ("hey buddy, that's a MOUNTAIN BIKE rear you've got there in case you were wondering"). My non-flustered reaction kind of took the wind out of his sails, though. 

Anyway, I'm running a mountain bike RD and cassette and I'm going to have a hard time taking it off now. The extra speed I picked up by putting smaller gears on my bike has me itching for a compact crank now....

I'm using 10 speed Ultegra shifters, by the way.


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## atpjunkie

*it is why some of us luddites*



thechriswebb said:


> I have to admit...
> 
> I took my road cassette and derailleur off (7700 Dura Ace RD and 11-26 Sram cassette) and put the Deore cassette and rear derailleur from my mountain bike on to try it out, and I am having a really, really hard time justifying putting the road stuff bike on.... The dip in shifting performance was not what I thought it was going to be, and I still have a 11 cog in the rear. I also admit that the 32 gear inches from the 39/32 combination that I can now access is pretty easy to get used to. I have a hill with a 20% grade that I have to ride up on my way home from work and I've been spinning up that thing FASTER than I ever went standing up and pushing the 39/23 or 39/26.....
> 
> Yeah, a guy at work tried to make fun of me last week when I rode up ("hey buddy, that's a MOUNTAIN BIKE rear you've got there in case you were wondering"). My non-flustered reaction kind of took the wind out of his sails, though.
> 
> Anyway, I'm running a mountain bike RD and cassette and I'm going to have a hard time taking it off now. The extra speed I picked up by putting smaller gears on my bike has me itching for a compact crank now....
> 
> I'm using 10 speed Ultegra shifters, by the way.


love 8 speed because there are 11-28 and 11-30 cassettes, since 9 speed the choice option from road to MTB kind of sucked, from a 27 road to a 32 MTB


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## laffeaux

atpjunkie said:


> love 8 speed because there are 11-28 and 11-30 cassettes, since 9 speed the choice option from road to MTB kind of sucked, from a 27 road to a 32 MTB


An 11-34 (9-speed) is the same thing as an 11-30 (8-speed) with the addition of the extra 34 cog. And an 11-32 (9-speed) is exactly the same as an 11-28 (8-speed) with the added 32 cog. So there's no difference in gearing between and 8 and 9, other than the 9-speed gives you one extra gear on the low end.


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## atpjunkie

*I understand that*



laffeaux said:


> An 11-34 (9-speed) is the same thing as an 11-30 (8-speed) with the addition of the extra 34 cog. And an 11-32 (9-speed) is exactly the same as an 11-28 (8-speed) with the added 32 cog. So there's no difference in gearing between and 8 and 9, other than the 9-speed gives you one extra gear on the low end.


I just don't want or need a 32 or 34, I don't want the extra gear. You can shift an 11-28 or 11-30 with short or med. cage rear derailleurs. For a 32 or 34 you are better served with a long cage and then suffer all the chain issues/ of a long cage. Plus you have wider gaps in your gear spread.
For 9 speed, what I would like is a 11 or 12-30


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## Cyclo-phile

atpjunkie said:


> I just don't want or need a 32 or 34, I don't want the extra gear. You can shift an 11-28 or 11-30 with short or med. cage rear derailleurs. For a 32 or 34 you are better served with a long cage and then suffer all the chain issues/ of a long cage. Plus you have wider gaps in your gear spread.
> For 9 speed, what I would like is a 11 or 12-30


A med. cage works fine on an 11-34 as long as you only have two chainrings. SRAM rates their med. cage derailleurs for 37 teeth of range, which equates to an 11-34 cassette and 36/50 or 39/53 chainrings. I imagine Shimano has a similar range for their med. cages.


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## adam_mac84

Digging up an old thread that i found on google... My wife's new build has 11-34 cassette and Sora RD. With the B screw all the way in, it gets into the 34 but definately makes some noise as the pulley is so close to the cassette... If i pull the deraileur 'back' a little bit, it opens up and noise goes away... Any problem with finding a machined bolt that is a hair longer to make up this space?

50/34 front crank FWIW, and this problem is in the small ring


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## Kram

PeanutButterBreath said:


> Who else thinks puppies are cute?


I like turtles.


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## atpjunkie

*swap the cassette out*



adam_mac84 said:


> Digging up an old thread that i found on google... My wife's new build has 11-34 cassette and Sora RD. With the B screw all the way in, it gets into the 34 but definately makes some noise as the pulley is so close to the cassette... If i pull the deraileur 'back' a little bit, it opens up and noise goes away... Any problem with finding a machined bolt that is a hair longer to make up this space?
> 
> 50/34 front crank FWIW, and this problem is in the small ring


for a 32 and you'll be fine. 1-1 is not really needed IMHO


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## PastorPaully

I'm thinking about doing a MTB derailed on my TCX as I upgrade the drivetrain. The main reason I like it is that you have more cassette options. However, I am still in the research phase.


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## Erik_A

I love this XT short cage for cross: Shimano Deore XT derailleur (M750 SS)

RD-M750 circa 1999
Stated 28t max, but 30t is fine on most frames

If anyone has one and wants to trade me for a my like new Shimano Long Cage rear derailleur please let me know.


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## atpjunkie

Erik_A said:


> I love this XT short cage for cross: Shimano Deore XT derailleur (M750 SS)
> 
> RD-M750 circa 1999
> Stated 28t max, but 30t is fine on most frames
> 
> If anyone has one and wants to trade me for a my like new Shimano Long Cage rear derailleur please let me know.


eric, you know you can swap cages right? A friend of mine used to make XTRegras by putting Ultegra cages on XTR mechs. Next time someone destroys a rear der. if the cage is in good shape ask for it


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## headloss

PastorPaully said:


> I'm thinking about doing a MTB derailed on my TCX as I upgrade the drivetrain. The main reason I like it is that you have more cassette options. However, I am still in the research phase.


Just be careful not to get a 10 speed MTB RD (dynasys), they changed the pull-ratio and they won't work with Shimano STI.
Wiggle has a good deal on the 9 speed XT in medium or long cage for $64. wiggle.com | Shimano XT M772 9 Speed Rear Derailleur | Derailleurs Rear - MTB That's the list price for the current Deore version.

If you are using SRAM 10, it's not an issue.


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## Erik_A

atpjunkie said:


> eric, you know you can swap cages right? A friend of mine used to make XTRegras by putting Ultegra cages on XTR mechs. Next time someone destroys a rear der. if the cage is in good shape ask for it


Awesome advice! I need to Google this.


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## atpjunkie

Erik_A said:


> Awesome advice! I need to Google this.


he's a poster here MShaw. You could look under his name or PM him


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