rear der. compatibility

[KC]

I have a Shimano RD-M750 for which the cage just got badly bent. The
rest of the drivetrain is same as the RD: ca. 2001, "mega9" XT for most,
but the shifters are XTR... for whatever that's worth.

Can I use an RD-M761? How would it differ from my 750? Would I need to
swap out anything else (e.g. chain?)

Within Shimano's line(s) what's generally the difference between say, a
rd-m750 and an rd-m751? Could I use an rd-m751? How would that differ
from my rd-m750?

Thanks a ton for any advice... Incidentally, if anyone here is a vendor
and wants to offer me a good deal on a comparable rear dérailleur, shoot
me an email (I live in Los Angeles, CA).
-KC

[Hank]

Any Shimano MTB RD will work.You would not need to swap out any other
parts unless they are damaged as well.

RD-M751 doesn't have the 750's rubber cable housing boot. 761 has the
cool-looking 3D logo on the outer plate but is not significantly
different in function.

[KC]

Thanks. The Shimano web site is surprisingly void of explanations of
their different products (or how they differ from each other, that is.)

What are the benefits of a long cage over a short cage? I think a 751
has a longer cage than my 750... not sure though.

Also, any idea what Shimano means in reference to GS and SGS designs?

One more thing... Any opinions on whether I should take the thing in and
have it bent back & re-aligned? I wonder how time intensive (i.e. how
much $$$) that task would be for a decent mechanic. I can find the 751
for $65-ish and the 761 for $60-ish.

Thanks again,
KC

[Hank]

A decent mechanic wouldn't try to bend it back. Once it's bent, an
attempt to bend it back will likely break it. But it's probably a good
idea to have a shop check the alignment of your dropout and hanger.

Both the 750 and 751 came in GS and SGS models, and the cage lengths
were the same in each revision.

GS=medium cage, 33T total capacity
SGS=long cage, 45T total capacity

So subtract your little cog from your big cog, then your little
chainring from your big chainring, and add the two together. If it's
less than 33, you can use the GS. Either way, you can use the SGS. The
only advantages to the GS are weight and ground clearance.

FWIW, most 27-speed MTBs can't handle the GS. (32-11)+(44-22)=44. The
GS is for downhill and freeride bikes with double cranks.

That Shimano.com does not explain these differences is not surprising,
since they don't make the 750, 751, or 761 anymore. They barely make
the 771. They're really pushing the 772, the low-profile "Shadow"
version, which puts most of the RD's bulk between the wheel and the
plane of the dropout, rather than outside of the hanger. You might
want to look into that one, since it could help prevent the same thing
happening to you again.

That "Shadow" mechanism is also available in the less-expensive SLX
group, RD-M662-SGS, brand new for 2009.

[Hank]

On Sep 29, 6:18 pm, Hank <h...@wirtznet.net> wrote:

>
> FWIW, most 27-speed MTBs can't handle the GS. (32-11)+(44-22)=44. The
> GS is for downhill and freeride bikes with double cranks.

I really suck at math. (32-11)+(44-22)=43, duh.

[kc]

Hank, thanks for the explanation. Much appreciated. However, I'm still
not clear on the difference between the 750 and the 751, if both were
available in GS and SGS. Is it really just that the 751 lacked the
rubber cable housing boot? That's a pretty minor "feature" to create a
whole new model number...

> So subtract your little cog from your big cog, then your little
> chainring from your big chainring, and add the two together. If it's
> less than 33, you can use the GS. Either way, you can use the SGS. The
> only advantages to the GS are weight and ground clearance.
>
> FWIW, most 27-speed MTBs can't handle the GS. (32-11)+(44-22)=44. The
> GS is for downhill and freeride bikes with double cranks.
>
> That Shimano.com does not explain these differences is not surprising,
> since they don't make the 750, 751, or 761 anymore. They barely make
> the 771. They're really pushing the 772, the low-profile "Shadow"
> version, which puts most of the RD's bulk between the wheel and the
> plane of the dropout, rather than outside of the hanger. You might
> want to look into that one, since it could help prevent the same thing
> happening to you again.

Actually, what happened to me was stupidity, not a riding accident. I'm
pretty sure I bent the RD putting the bike into my truck bed with less
care than it deserved. The "Shadow" model looks nice, but also
expensive. I can find a new 751 or 761 for $60/65.

> That "Shadow" mechanism is also available in the less-expensive SLX
> group, RD-M662-SGS, brand new for 2009.

Hmmm... now I have to ponder the benefits of (or lack thereof) old XT
over new SLX??? THANKS! ;^)

-KC

[Hank]

On Sep 29, 9:40 pm, kc <kc_n...@sonic.net> wrote:

> Hank, thanks for the explanation.  Much appreciated.  However, I'm still
> not clear on the difference between the 750 and the 751, if both were
> available in GS and SGS.  Is it really just that the 751 lacked the
> rubber cable housing boot?  That's a pretty minor "feature" to create a
> whole new model number...
>

The last digit there is for very minor revisions. Compare an Ultegra
6500 front hub to a 6501. It's nigh impossible to tell the difference.
I know I can't.

[kc]

Figures. :-/

One more Shimano-decyphering question...

It appears I bent my chain too (when I tried to ride not knowing I had a
bent RD, the chain kinked).

Difference(s) between a CN-HG53, 73, and 93? (Shimano's site lists them
as the Deore, Deore LX, and Deore XT chains respectively, but they are
all "super-narrow 9-speed hyperglide" chains, and all weigh ~304g for
116 links. Local mega store has them for $9.95, $18.95, and $29.95,
respectively. The specs say "same thing" the prices have me scratching
my head...

Thanks,
KC

[Hank]

Progressively prettier finishes. Get the cheapest one you can stand to
look at,

[Qui si parla Campagnolo]

I think you need to make sure the derailleur hanger is also straight.
If whatever bent up the cage, it 'may' have bent the der hanger also.

[KC]

I did check that, it *looks* straight, but I don't have a way of
verifying it. Is there a quick/dirty method I can do? I don't have any
tools for testing & adjusting hanger straightness.

Thanks.
-KC

[Hank]

Most shops will have a tool to check it. Usually the Park DAG-1:
http://www.parktool.com/products/detail.asp?cat=48&item=DAG-1

[Qui si parla Campagnolo]

Nothing quick, the derailleur hanger needs to be parallel to the plane
of the rear wheel..so the rear wheel needs to be true as well. Trundle
down to the bike shop, the competent one, and have them check it.