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Front mech chain ring gap

Posted August 06, 2015 10:39PM by Skye in the Cycling Forum

Skye
Skye Skye Nott
@TheStaminist   Skye Nott
Location: Vancouver, Canada
Joined: 3 years ago   Posts: 516

My braze-on type front derailleur (SRAM Force) isn't matching the curve of the big chain ring (50T) very well. It's probably the main cause of the chain jumping off the big ring when I shift up on front from low on the cassette, something I want to fix asap. The high/outer screw is already right on the edge where any lower and it won't shift up in any gear.

Any suggestions on ways to change the angle of the front mech? It's a single bolt braze on; maybe a shim at the top of the back, and the bottom of the front, to maintain some sort of clamping angle. Sounds fiddly. Grinding down the internal face and the external curved washer would be more secure but is obviously a one-way modification.

Maybe it's the angle of the head tube? This frame originally had a standard crankset so maybe the braze-on is angled for a larger 53 tooth chain ring. Come to think of it, the braze-on mount is bolted to the frame, shimming that might be a better option. I could also bend the mounting tab in a vice, but I don't suppose I can find a replacement anywhere.

Maybe I should just convert/upgrade to a yaw-type front derailleur?

Webmaster - Staminist.com

Attachments

force front derailleur gap.jpg 64.6 KB open | download
force front derailleur gap.jpg
trek 5200 front derailleur mount tab.jpg 49.5 KB open | download
trek 5200 front derailleur mount tab.jpg

Skye
Skye Skye Nott
@TheStaminist   Skye Nott
Location: Vancouver, Canada
Joined: 3 years ago   Posts: 516

Did some googling and it seems this is a common problem on Trek frames before compacts became popular. At first I was looking around to see if they made compact specific FDs but they are extremely uncommon. Ordering one of these, hopefully it will give a slightly different angle to the FD mount. Trek part number 253438

Attachments

trek fd hanger compact 253438.jpg 22 KB open | download
trek fd hanger compact 253438.jpg

Skye
Skye Skye Nott
@TheStaminist   Skye Nott
Location: Vancouver, Canada
Joined: 3 years ago   Posts: 516

Update:

Got the new Trek front mech hanger 253438 yesterday (stamped "C" for compact), unfortunately the only difference seems to be the mounting slot is extended lower. Otherwise the angle is the same when viewed from the side, so the derailleur to chain ring gap is nearly the same. Compared to the original photo above, the gap to the rear of the derailleur cage has been reduced maybe 4-5mm which is better than nothing. Maybe there's some natural variation in the machining process, or the old hanger got knocked around.

While I was replacing the hanger, I also replaced my SRAM Force 10 speed big chain ring which was fairly worn. I checked the chain wear using my wear gauge, and it's not even close to the 0.75% wear mark, so I don't think the chain is the problem.

This combination seems to have fixed the problem - the chain no longer jumps off the outside of the big ring, when shifting up on the front & at the same time being on a high gear on the back, at least on the bike stand. Going to take it for a test ride now to confirm.

Webmaster - Staminist.com

Attachments

Trek front hanger compact 253438 vs standard.jpg 38.4 KB open | download
Trek front hanger compact 253438 vs standard.jpg
SRAM Force 50T chain ring powerglide replace.jpg 87.1 KB open | download
SRAM Force 50T chain ring powerglide replace.jpg

Skye
Skye Skye Nott
@TheStaminist   Skye Nott
Location: Vancouver, Canada
Joined: 3 years ago   Posts: 516

Not a single chain dropped. Woohoo!

Ride by Skye Nott 7
Vancouver, Canada 26 10
Connecting the dots
Aug 20, 2015
4:59 pm

812 m
73.4 km

3:03:36
24.0 km/h

 
133 W avg




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