Rear Derailleur Capacity Limits
Garth
Deore RD-MT60 http://www.velobase.com/velobase.com/ViewComponent.aspx?ID=3f73f16c-1a7e-46c5-ab73-f21755e71b08
The capacity is rated at 38t. I'm using a 24/36/48 and 13/32 FW for 43
total. The issue I'm having is when in the 24t ring, and either the 28
or 32t cog, the pulley wheels are too close to the cogs. When spinning
forward, I don't notice it, but if I spin the pedal backwards, the
pulleys make a grinding noise, as they are right up against the cogs.
I have the angle screw at the max, but it helped only slightly with
the 28t cog, but not the 32. My chain length is proper.
Having this wide of range of gearing is new to me, so my first
assumption is that I've exceeded the capacity of the RD. If anyone
has an answer, I'd appreciate it.
CycloFiend
> Greetings, I finally got my Bombadil going, I'm using a vintage
> Deore RD-MT60
> http://www.velobase.com/velobase.com/ViewComponent.aspx?ID=3f73f16c-1a7e-46c5-
> ab73-f21755e71b08
>
> The capacity is rated at 38t. I'm using a 24/36/48 and 13/32 FW for 43
> total. The issue I'm having is when in the 24t ring, and either the 28
> or 32t cog, the pulley wheels are too close to the cogs. When spinning
> forward, I don't notice it, but if I spin the pedal backwards, the
> pulleys make a grinding noise, as they are right up against the cogs.
> I have the angle screw at the max, but it helped only slightly with
> the 28t cog, but not the 32. My chain length is proper.
"Angle Screw" = B-Tension adjuster?
http://www.sheldonbrown.com/gloss_ba-n.html
You might have some play in the derailleur itself. The parrallelogram of a
derailleur is held together by bushings, and so as they age and wear, they
tend to twist a bit and not hold the full range of motion they once did.
That derailleur is rated for 32 teeth max, so if it's having trouble with
the 28 too, I'd suspect something wrong with the travel, rather than the
B-Tension setting.
At the time that derailleur was designed, mtb rear spacing was 130 mm. That
changed to 135 (which you have on the Bombadil) in the early 90's.
You can get a pretty good idea if you remove your chain and put the bike in
the stand. Back off the limit screws and have someone shift the RD through
its range and you view from aft of the bicycle (in a stand is best). Check
the torque on the fixing bolt (mount bolt) for the derailleur.
If the RD doesn't line up with the largest sprockets when shifted without a
chain, then check your limit screws and your cable tension.
I'd suspect that you are at the edge of range with that derailleur, so
everything has to be set up right to start with.
If the RD does line up, check it again with the chain installed and look for
twisting in the cage or other signs of tired, worn bushings in the body.
Hope that helps,
- Jim
more -
http://www.sheldonbrown.com/derailer-adjustment.html
--
Jim Edgar
Cyclo...@earthlink.net
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William
but just in case. :)
I'm assuming there are zero lateral movement issues for this
derailleur
The thing that makes a rear derailleur run on top of your biggest cogs
is not directly a capacity issue. It's more of a max cog issue. Max
cog size in the documentation is just a guess anyway, because the
distance from the hub axle to the derailleur bolt is a big contributor
to how large a cog a derailleur can take. Getting this to stop
happening will be an iterative process. It's possible that the old B-
tension spring is just old and weak, and no amount of tightening the B-
tension screw will get it off the cog enough. It's possible that one
more link in the chain will enable that spring to lift the upper
jockey wheel off that biggest cog. Put the drivetrain in your 24/13
combo and see if it already sags. If it does sag, then yes it
actually may be a capacity issue, and you might not want to try a link
longer.
Some smart-aleck might tell you if it makes noise only when you pedal
backwards, then don't pedal backwards. :) Seriously though, that
action is causing a pull on the lower pulley which pivots the
derailleur forward. B-tension is supposed to counter that. Can you
determine if there is friction in the lower pulley, or is the chain
gummy? Lube in those areas might reduce that forward tug and get it
to stop happening.
You can find a newer more modern, more slanted parallelogram
derailleur for $40 or less easily and be done with it, too.
On Jul 28, 11:35 am, CycloFiend <cyclofi...@earthlink.net> wrote:
>
> > Greetings, I finally got my Bombadil going, I'm using a vintage
> > Deore RD-MT60
>
> Cyclofiend Bicycle Photo Galleries -http://www.cyclofiend.com
Horace
You said it was rated at 38t, and you have 43t total. The behavior you
describe is expected. Well, expected by me. Other people may have
differing ideas of how rear derailleur capacity is stated.
The top left section of this page discusses it:
http://www.sheldonbrown.com/derailleur.html
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Garth
great.
The bike has just been put together, a Bombadil. The SRAM PC-850 chain
is new, 114 links, 57 inches long, plus the power link.
I used Park's formula for chain length http://www.parktool.com/repair/readhowto.asp?id=26
With 18.3" chainstays, 48t and 24t rings, , I end up with 55.6 inches
for chain length, according to this "formula".
The chain doesn't sag at all in the small/small combo.
I thought maybe the chain was too short/long, but after measuring my
road bike, I don't think so.
My road bike has 18" stays with 26t and 48t rings with the same chain
length and it works fine. It has an XT RD-M737 RD. Unknown capacity,
though the cages are the same length.
I'm thinking it's just the RD isn't up to the capacity at this point.
Garth
close, it was only when I was lubing my chain and pedaling backwards
did I notice this.
I suspect there is no harm being done, but I'll likely replace it
anyways when I can get a new one.
This is the first time I've encountered this issue though.
CycloFiend
> The RD is nearly new. Wear isn't an issue. Maybe it's just not so
> great.
(snipped)
> The chain doesn't sag at all in the small/small combo.
Then, I'd add a link pair (mebbe two) and see what happens. It sounds like
it's at the edge of its comfort, and I'd personally trade away that
small/small setting. A little slack there won't be too much of an issue,
generally.
- Jim
--
Jim Edgar
Cyclo...@earthlink.net
Cyclofiend Bicycle Photo Galleries - http://www.cyclofiend.com
Current Classics - Cross Bikes
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Gallery updates now appear here - http://cyclofiend.blogspot.com
"Maybe a bike, once discarded, pines away year after year for the first hand
that steered it, and as it grows old it dreams, in its bike way, of the
young roads."
-- Robert McCammon, "Boy's Life"
Angus
By looking at the picture of the derailleur the upper pivot and upper
jockey pulley are in different places. Look at the picture and
imagine a shorter chain, it would rotate the cage counter-clockwise,
pulling the upper jockey pulley away from the cog.
Angus
On Jul 28, 7:39 pm, CycloFiend <cyclofi...@earthlink.net> wrote:
>
>
>
> > The RD is nearly new. Wear isn't an issue. Maybe it's just not so
> > great.
> (snipped)
> > The chain doesn't sag at all in the small/small combo.
>
> Then, I'd add a link pair (mebbe two) and see what happens. It sounds like
> it's at the edge of its comfort, and I'd personally trade away that
> small/small setting. A little slack there won't be too much of an issue,
> generally.
>
> - Jim
>
> --
> Jim Edgar
>
> Cyclofiend Bicycle Photo Galleries -http://www.cyclofiend.com
> Singlespeed - Working Bikes
>
William
derailleur
CycloFiend
> This must be "if Jim is wrong day"
Y'know.... I'm wrong _every_ day.
In fact, I strive to be wrong every day.
Somedays I do six wrong things before breakfast...
> By looking at the picture of the derailleur the upper pivot and upper
> jockey pulley are in different places. Look at the picture and
> imagine a shorter chain, it would rotate the cage counter-clockwise,
> pulling the upper jockey pulley away from the cog.
I just keep thinking that the issue isn't really the overall chain length as
much as it is side load on the derailluer. My suggestion was to lessen
tension from the chain while the derailleur is at its most extended
position.
I kinda lost track if the OP mentioned that he'd checked derailleur travel
with no chain installed (been workng/engineering today/this evening). If
the derailleur swings through the proper range without a chain, and keeps
enough spacing from the cogs, then it might be worth tweaking the chain
length to see if that helps.
Should look like this -
http://www.parktool.com/images_inc/repair_help/der_llimit.gif
from:
http://www.parktool.com/repair/readhowto.asp?id=64
- J
--
Jim Edgar
Cyclo...@earthlink.net
Garth
Jim, I checked the RD travel with no chain on, and it hits the 28t cog
despite the B screw all the way in, at the highest tension.
I'm not sure what value the Park Tool idea is though, as on my road
bike, the XT RD will hit the 32t cog with the chain off, but with the
chain on it's not too close, and it shifts fine.
I'm using all 114 included links of a SRAM chain, BTW. As far as I
know, the Bombadil doesn't need longer than a stock chain.
Rene Sterental
reversing the B screw to get some extra separation...
That's what I did on the Bombadil first and now the Atlantis to be
able to run a rear 12-36 with an XT Rapid Rise RD.
René
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Ken Freeman
chain is IMO the main way of rotating the cage so the jockey wheel
position os farther from the cog. Then you have to see if operation
over the rest of the range is acceptable. If it's not, you might need
a different dérailleur.
A B screw adjustment might help.
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Garth
for the bike. Wow .... how modern derailleurs have improved. The
spring action is super light, the pulley cage is 2cm. longer than the
vintage Deore was,allowing for a 45t capacity, up from the 38t of the
Deore. Needless to say, it handles everything just fine. Plenty of
clearance. I notice easier shifting in friction.