Odd shifting issue 105s

[zackola]

Hi,

I recently bought a Cannondale Synapse 5 w/ Shimano 105s on it and I'm having a kind of odd problem - when there is pressure on the back tire, I have great difficulty upshifting to the large ring in the front - Most times it doesn't take at all, sometimes if I continue holding the lever for 30 seconds or so it may eventually catch.

The weird part is that if I get off the bike, and flip it upside down, it shifts flawlessly! WTF?! I've brought it back to the place I purchased the bike 3 times now, but the techs there aren't allowed to actually take the bike out on the road to witness this (in NYC) - They put it up on a trainer where the back wheel was elevated, not touching the ground and it worked fine, so I guess it has something to do with the weight on the back wheel, but I'm no tech (yet) and wanted to see if anyone has some ideas for me.

I am aware that not all gear combinations are meant to be used and have asked people to make sure I am on the correct gears on the back hub when trying to upshift to the big ring up front.

For what it's worth this bike only has the 2 rings in the front, no granny gear.

Thanks in advance for any insight!

[Bill]

Will say this the one I am looking at is a very nice Cyclocross . Does yours look like this... http://www.cannondale.com/usa/usaeng/Products/Bikes/Road/Performance-Road/Synapse/Details/1258-0RAS5C_0RAS5T-Synapse-5 . If so I would think that it may be a sticky shifter or derailler. Those type of shifters I have never encountered. But I am sure someone here has.

[zackola]

Yup, that's the model.

[Bill]

Wandering and as I said I am not the best at this, did they check the biggest chainring to see if it was true (not bent). Also make sure it is manufacturer specs and not an aftermarket. Also see if they can check the chain line, professionally. The only other thing I can think of the crank spacing is off possibly. Something is not right, if they can not ride the bike see if they can ride along with you.

[dmath]

I had a very similar problem with triple chain ring set up using Shimano 105’s as well. I found that adjusting the barrel adjustment on the STI shifters fixed the problem. I just turned it about ½ of a turn counter clock wise to tighten the cable and it has worked great ever since. Hope this helps.

[Bill]

(06-20-2010, 10:15 AM)dmath Wrote:  I had a very similar problem with triple chain ring set up using Shimano 105’s as well. I found that adjusting the barrel adjustment on the STI shifters fixed the problem. I just turned it about ½ of a turn counter clock wise to tighten the cable and it has worked great ever since. Hope this helps.

I looked over the actual shifters and it does not show a barrel adjuster (that I could see). You know what just came to me is that sometimes people do not pedal the same when shifting, and to shift it has to be pedaled. Zack do you weigh different then the guys at the bike shop? If so try to find someone your weight to look at it.

[dmath]

If you look at the brake/shifter lever, you will see where the shifter cable goes in. At that location you will notice a plastic nut that covers where the cable goes in. That is the barrel adjuster. If you turn it counter clock wise it tightens the shifter cable, if you turn it clock wise it loosens the shifter cable cable.

[Bill]

Hmmm Ok thanks I really did not know that.

[zackola]

Thanks, I adjusted the high limit screw on the front derailleur 1/4 turn counter-clock wise and tightened the barrel adjuster a whole bunch (because it was super loose) and I think we are in business.

I have a feeling that the cable tension could be a bit higher, but I'm gonna put some mileage on this thing before adjusting it where it's clamped with the hex nut.

Thanks for your help everyone!

[dmath]

Awesome, glad it's working for you!