FD-7900 help

Discuss light weight issues concerning road bikes & parts.
Post Reply
PoorCyclist
Posts: 414
Joined: Wed Dec 07, 2011 1:26 am
Location: California's country side

by PoorCyclist

I have it setup 1mm above. Running SRAM red compact and PC-1091R chain. I don't have a barrel adjuster right now, just mark the cable to move it a bit and pinch it down again.

I keep reading it need slack, but I had it too slack and it will click into high but insufficient tension to reach limit. So i increased the tension (needed a 3rd hand tool), it will shift crisp 75% of the time but occasionally it would drag and hesistate to the big ring, still ridable just not as good as my 6700. At some point it seems the tension doesn't do much except make it stiffer to shift. Shimano docs says make it real slack and then tension the cable so in the big / big clears the cage by 0.5mm. And that's the right tension??

I read something here too about needing to angle the cage to clear the small/small, is that true?

If I move the derailleur lower than a penny width, would the cage/chain hit the big chainring teeth when shifting?

Did I pinch the cable correctly, there is a little hump that I go outside and around, then turn horizontal to sit in the groove. This is more obvious in the small ring.
Attachments
FD7900.jpg

User avatar
BeeSeeBee
Posts: 492
Joined: Mon Dec 21, 2009 6:00 am
Location: Bay Area, CA

by BeeSeeBee

Which shifters are you running? Sram? 6700? 7900?

by Weenie


PoorCyclist
Posts: 414
Joined: Wed Dec 07, 2011 1:26 am
Location: California's country side

by PoorCyclist

BeeSeeBee wrote:Which shifters are you running? Sram? 6700? 7900?


ST-7900

jasjas
Posts: 119
Joined: Sun Aug 23, 2009 10:15 am

by jasjas

http://techdocs.shimano.com/media/techd ... 651777.pdf" onclick="window.open(this.href);return false;


Didnt read anywhere that you set the cable to be slack, clearance can be less than the 0.5 you stated, you d never use large/large anyhow?

its difficult without an inline adjuster but i ve set my 7900 fd without one on a 595 and it works fine, maybe try setting the top screw adjuster so it over shifts slightly and then back it down and see if that improves things.

the 1mm clearance is just fine and u should nt need to angle the mech.

PoorCyclist
Posts: 414
Joined: Wed Dec 07, 2011 1:26 am
Location: California's country side

by PoorCyclist

I finally got it to work, it was a bit of trial and error. I angled the cage in and out, it was touching the crank arm at one point.

Once I got in the working zone and it was obvious, the chance of chattering and dragging when shifting up is very minimal now. Most of the time it catches on instantly.

On a compact crank I had to put the RD as low as possible (1mm and less) as the frame's braze-on tab allows.
The cage worked best parallel to the chain ring - for me anyway. I started with a lower tension, shifted up and pull on the cable to see if it it would still move the cage over, marked the cable and repeat. The correct tension can be set by hand in the small ring and NOT need a 3rd hand tool. I can see how a barrel adjuster is good for this, but I managed.
Then tightened bolts to specs.

I don't know if it was really so much better than 6700 front shfting, the 6700 is quite effortless too but it seems the 7900 can do away with even less effort to push the lever.

User avatar
ms6073
Posts: 2289
Joined: Fri Jul 23, 2004 8:24 pm
Location: Houston, Texas

by ms6073

Using Gore RideOn Professional deraileur cables on our bikes but when setup with a tight cable, my wife finds it difficult to shift cleanly into the large ring with 7900 shifter and deraileur. I found that if I setup the front deraileur cable with a bit of slack, during shifting she was not having to use all the force to overcome cable friction.
Michael - The Anaerobic Threshold is neither...

by Weenie


Post Reply