Bicycle Mechanics - What is this and what for?

Came across Shimano Tourney RD. Some of the models have this little cable roller pulley (see pic on top left). What does it do? All other RDs that I've seen don't have it. Even the top end XTRs. I'm puzzle why this little device is on a basic Shimano rd.

http://images.jensonusa.com/large/rd/rd293a00_____long_6sp.jpg

This is just a guess but it looks like the derailleur is designed for shift cable runs that come down the seatstay rather than along the chainstay.

Maybe to convert from SRAM shifters?

Assuming it's just a round pulley, there used to be (maybe still are) aftermarket versions of that pulley. It's kinda nice to avoid the cable loop out back there.

EDIT: Avid Thingamijig, that's the aftermarket one I was thinking of.

That is interesting that it's on a low-end derailer, perhaps that helps bike assembly quality where less skilled bike assemblers are involved.

In a typical derailleur the rear housing loop bends something over 90 degrees causing cable friction. Using a pulley to make the turn allows the rear loop to be almost straight, greatly (or so designers claim) rear loop cable friction. It's an idea that comes around every so often, along with seat stay routing for the same reason.

What's puzzling to me is why is this included on the low end RD only?

What's puzzling to me is why is this included on the low end RD only?

Perhaps it wouldn't sell well with the gram counters>.

I had a Shimano Nexave rear derailleur with the same sorta pulley. It came on a Trek 7700 hybrid. I have to admit, shifting was really smooth on it. I later transferred the derailleur on a Raleigh R800 road bike and matched it with a friction thumb shifter. Really Really smooth shifting. If you can find one on ebay. I say get it.

Maybe it is on low end derailleurs to compensate for the weaker strings used on them or lower quality pivots? You would think if it really does decrease cable friction by 20% that high end derailleurs would use it too. Could be a weight thing as a previous poster mentioned.

The Avid Thingamijig Got a lot of good reviews on MTBr.com. http://www.mtbr.com/cat/accessories/Extras/avid/rollamajig/PRD_351111_117crx.aspx

Might be worth checking out. Really cheap to test out too. I may do it just for the heck of it.

That looks like a really good idea. Could I get one of those pulleys and transplant it onto one of my derailers?

that thing can't weigh more than the big cable loop that it'll obviate, at least in the case of bikes with derailer cable routed along the top tube...

i recall almost buying a rollamajig, until i heard that cables can get contaminated rather quickly with its use. I believed that caveat then; now, i'm not so sure it'd have really been an issue. I'm still glad i never bought one, though.
Still, the pulley on the tourney rd is a similar concept, but it does have some differences when compared to the rollamajig. I think it's a neat detail, but it's hard to imagine myself buying a derailer with one of those strapped to it.

-rob

That looks like a really good idea. Could I get one of those pulleys and transplant it onto one of my derailers?
That's not realistically doable, as traditionally configured derailers wouldn't have any reasonably simple way of interfacing to the pulley. That is assuming you'd be able to source one as a spare part to begin with.
But if the general principle is attractive to you there are (at least) 3 options:
1) buy the Avid rollamajig
2) buy a Carumba Smooth Operator, a brake noodle eliminator
3) buy a Travel Agent, and set it up as a brake noodle eliminator.

No 1 will definitely slot right in against the Barrel Adjuster w/o any further fuss. Option 2,3 might require some tinkering.
The Smooth Operator is better sealed than the Travel Agent.

Ahh, Rollamajig, that's what it was called!
Chain Reaction has 'em (http://www.chainreactioncycles.com/Models.aspx?ModelID=1802)

Our newest patrol bike (Trek) has a similar set-up. Looks good to me; that old cable loop is a constant source of annoyance. Not only do they flex and eventually break, but they tend to get snagged on things.

What's puzzling to me is why is this included on the low end RD only?

Maybe they're just testing the waters. Back in the late 70's (when CB radios were the closest to a cell phone and before Americans ever heard of the Tour de France), I had a Royce Union "10 speed" with a Shimano Positron rear der. It was index shifting. As I recall, the index bumps were stamped on the der, not located in the shifters. This was quite novel in the world of friction shifting and am sure the pros and enthusiasts scoffed at Positron as gimmicky and unnecessary. I'm sure seasoned cyclists knew just how much to move their down-tube levers to match perfectly and probably took pride in this fine art. And laughed at rookies who had to constantly fiddle. Obviously, index shifting is a necessity today. Perhaps same with this cable routing system?

(I may start a new thread and see who remembers the Positron).

Maybe they're just testing the waters. ..I had a Royce Union "10 speed" with a Shimano Positron rear der. It was index shifting. As I recall, the index bumps were stamped on the der, not located in the shifters. ...am sure the pros and enthusiasts scoffed at Positron as gimmicky and unnecessary.

Well, I had a positron-equipped bike once, when I was very new to cycling, far too new to be aware of any nuances and style issues. I still disliked it because try as I might I couldn't get the actual shifting to correspond to the markings on the indicator by the shifter.

Well, seat stay cable routing has been quite popular on high end bikes if you ride off road so it's not just for that. Besides the place where a "Thingamajig" like pulley would aid the most to straighten up the cable routing would be on a road bike with chain stay cable routing.

Personally I think this is an example of reverse path development introduction. Often we see "better ideas" appear on the high end parts where folks will pay more to get a better product. Then as the years go by the new idea trickles down through the product line to the cheaper levels.

But as mentioned the weight weenies and traditionalists may not like the idea of their Durace or XTR derraileurs having some bit of flotsam on them that seems supurfluous. So instead they introduce what they think may be a good idea onto the lower end market derraileur to see if it stands up to user punishment and as an item that will make the bikes cleaner and more effective for shifting. If it works out well we may see the idea start to creep up the product line over the next few years.

Doing something like this isn't a case of experimenting with us but instead a case of seeing if it'll be accepted and if it works out over the long term of punsihment that users can so easily apply to it. Sort of the idea of Rapid Rise shifting came out some years back. Mind you look where that went.

Maybe they're just testing the waters. Back in the late 70's (when CB radios were the closest to a cell phone and before Americans ever heard of the Tour de France), I had a Royce Union "10 speed" with a Shimano Positron rear der. It was index shifting. As I recall, the index bumps were stamped on the der, not located in the shifters. This was quite novel in the world of friction shifting and am sure the pros and enthusiasts scoffed at Positron as gimmicky and unnecessary. I'm sure seasoned cyclists knew just how much to move their down-tube levers to match perfectly and probably took pride in this fine art. And laughed at rookies who had to constantly fiddle. Obviously, index shifting is a necessity today. Perhaps same with this cable routing system?

(I may start a new thread and see who remembers the Positron).

As in Commander Data?
:lol:

http://www.angryzenmaster.com/wp-content/uploads/2008/11/data.jpg

Reminds me of this product:http://www.jtekengineering.com/shiftmate.htm

So you can mix & match components.