PeterCNX

I am trying to figure out if installing a new cassette will work with my dérailleur before buying a cassette and finding out it will not.

At present I have an 11-34 cassette with a Shimano Claris RD-2400. The bike is a 2 x 8.

I wish to install an 11-42 cassette, 8 speed.

I looked at the technical information but do not understand what I see. Apparently the dérailleur has a short and a medium cage version. I cannot tell how you know which since the markings do not include SS or GS but mine is probably the medium cage and the pulley centers are 85mm apart.

Model 1：RD-2400-SS
Cage Length: Short Cage
Compatible with: 8 Speed
Low sprocket_Max: 32T
Low sprocket_Min: 25T
Max. front difference: 16T

Model 2：RD-R2000-GS
Cage Length: Medium Cage
Compatible with: 8 Speed
Low sprocket_Max: 32T
Low sprocket_Min: 25T
Max. front difference: 20T

If your comment is nothing doing, then what should I look for in searching for a new dérailleur?

Bill Kapaun

See the spec sheet attached-
Largest cog is 32T, although you can usually fudge a couple more.
42T? No way.
Another critical spec is your chain wrap capacity.
37 or 42T in the SS & GS 2400 versions.
That means-
(Largest cog-smallest cog) +(Largest ring-smallest ring) cannot exceed those numbers.
An 11-42 cassette is already using 31T of wrap + the difference between your rings.

There are probably some 9 speed "mountain" RDER's that may work.
What's the difference between your rings?

PeterCNX

The chain rings are 50-34 so a difference of 16.

.... and thanks for your reply, it helps me understand and the attached file is a very nice summary.

bboy314

I have used 9 speed Shimano Deore and XT Shadow style derailleurs with a 42t low cog, technically I think they’re supposed to max out at 36 but it’s worked for me. These would also index with an 8 speed setup.

PeterCNX

That is very interesting .......... so perhaps I just need to broaden my view and look for a 9 speed RD with the approximate range.

FastJake

The Wolf Tooth RoadLink allows you to use larger cogs. But you will definitely also need a MTB rear derailer to have any hope of using a 42T cog. Be aware that Shimano 10S and 11S MTB rear derailers use a different cable pull and will not index with your 8S shifters.

Have you considered smaller chainrings?

PeterCNX

A hanger ... well I had not thought of that at all. But new shifters to use new cable method ........... this is moving towards difficult ground.

I did initially consider installing a smaller chain ring but decided to leave them alone. My reason was that this is likely to have similar dérailleur challenges and so I thought the cassette may be easier. For example I cannot change the 50 ring since it appears integrated into the spider / axle. So it would be one ring only. Perhaps I should go back and consider the smaller ring since that is a very easy way to find out what will happen.

There are no easy fixes with the bicycle industry .............

cyccommute 

Those “maximum capacity” specifications are conservative. The largest low gear cog the derailer can accommodate is somewhat more difficult to get around but the maximum capacity can be exceeded. Your proposed maximum capacity is 47. I run an 11-36 cassette with a 44/34/20 crank on several bikes. That’s a gear capacity of 49. I’m also using that with both SRAM and Shimano 9 speed and 10 speed systems. I’ve even used chainrings up to 48 teeth with the 20 tooth low which increases the gear capacity to over 53 teeth.

I am using long cage mountain bike derailers. The SRAM XO I have says that the maximum chain wrap (aka gear capacity) is 45. I use Shimano XTR RD-M952 which has a chain wrap of 43. That’s the one that I pushed to over 53 teeth. I do use a Wolf Tooth Road Link to get it to handle the 36 tooth low gear. My point is to experiment to see if exceeding the stated chain wrap is possible.

This rear derailer should do the job. You can also find many used 9 speed mountain derailers on Ebay that will do the job as well.

70sSanO

There are a few options for Claris 2400. Since it uses the older cable pull and RD ratio of 1.7:1, you can use up to a 9 speed mtb RD.

You might consider a sub-compact crank with 46-30 chainrings. You may need a slightly older road triple FD, for the 30t to 46t shift… but maybe not. With a 30t you might get away with a 36t cog. A 30/36 ratio is slightly higher than a 34/42. You still need an SGS long cage RD regardless.

I am running a Wolftooth RoadLink on my mtb’s to run a 40t cog. But keep in mind that a RD hanger extension “generally” causes a RD to not shift as well on smaller cogs. I didn’t notice any shifting issues because my smallest cogs are 13t. And some other users don’t notice slow shifting.

John

cyccommute 

I’m running a road link on my road bike and a touring bike. I’ve haven’t noticed any problems with shifting on smaller cogs either when upshifting or downshifting. On the stand and on the road, shifts are equally crisp in all the gears. All of my bikes have 11 tooth cogs for the highs.

PeterCNX

Could I please ask a bit of a basic question. The size of the dérailleur cage has been mentioned in places. I guess that I should consider finding a large cage but I am not totally sure why. There seem to be conflicts of interpretation.

For example if the role of the cage is to manage chain tension on the system with a span of gears then if there were only one chain ring then only a short cage would ever be needed in such cases. That seems to suggest that if you have a significant difference between the sizes of chain rings (whether two or three gears) then a large cage would be the best option. Is that right? If it is then are medium cage sizes really necessary?

Sadly that makes me think of a related question .... the pulleys - they can be the same size or different sizes. But why?

FastJake

Yeah, you got it. The cage length determines the spread of gearing the derailer can cope with. So, you could use a long cage on everything. But in many cases you'd have a cage that's longer and slightly heavier than necessary. Medium cage is just another option between short and long.

Sheldon goes into more detail on derailer capacity: https://www.sheldonbrown.com/gloss_ca-g.html#capacity

I think the larger pulleys is mostly a marketing thing to make them look different. But someone can correct me on that.

PeterCNX

Thank you for that and for the comprehensive link.

Kapusta

No not necessarily. The chain wrap needed is determined by the difference between the largest and smallest rings PLUS the difference between the largest and smallest cassette cogs. So with a wide range cassette, you may still need a med or long cage RD. You just need ti crunch the actual numbers.

PeterCNX

and that as they say is a good point .....