Derailleur question
 

hello
I have a 2017 giant TCR advanced 2, it's bone stock with 105 components and I was wondering how do find out which Derailleur I have. I'm interested in possibly changing out my cassette for an 11-32 for a trip to the mountains next spring. stock is an 11-28 I found this https://www.westernbikeworks.com/pro...ear-derailleur, which says I can run standard or up to 11-32. so I can swap out my cassettes when needed. but it got me thinking that I really don't even know what my stock cassette can accommodate. after inquiring at my lbs they just recommended that I just get stronger and save the money.
any advice?
thanks
robert

Chain wrap
Longer derailleur arms can take up more chain, to handle the difference between the small chainring - smallest sprocket and the large chainring - largest sprocket. If the arm is too short, in the small-small combo, the bottom returning chain from the chainring will rub against the wrapped chain on the top pulley. A slight amount of rubbing isn't too bad, since this is an unusual cross-chained combo that's rarely used.

Top pulley
And there needs to be clearance between the top pulley and the largest sprocket so that the chain can shift on and off that sprocket. This gap depends on your bike frame's design, too, so some stock bikes can handle an oversized cassette, larger than the manufacturer recommends.

Radius, measuring from the center of the sprocket to the center of the pin of a chain on it:
28 tooth: 2.23 inches
32 tooth: 2.55 inches
The difference is .32 inches, or 8mm.

You can look at your derailleur when it's in the small chainring - 28 tooth sprocket to see if the gap appears large enough to accomodate a 32.

Gave it a try
I took a chance and purchased a 11-32 cassette for my Ultegra bike. Installed, the 32 sprocket has very little clearance for the top pulley. It looks like it wouldn't work reliably. But it's been shifting perfectly, no problems. And the derailleur arm is long enough that there's no chain wrap problems. The bike had a 11-28 when it was new, and the rear cage measures about 58mm center-to-center on the pulleys.

I had the bike on a repair stand, and shifted to the third largest sprocket and the small front ring. Then, turning the cranks slowly and carefully, I shifted two clicks to the largest sprocket -- good. Then did the same procedure with the large chainring-large sprocket -- still good.

Critical: on the large-large combo, I checked that the chain still had some angle at the pulleys -- the chain wasn't pulled out straight, the pulleys were still taking up plenty of slack. And on the small-small combo, there was a good gap between the returning chain and the top pulley.

Finally, a few "parking lot" riding tests, shifting carefully.



I like the 32. I'll even use it on moderate grades of 5%, so I can spin at a faster cadence. And I can stay seated on a 10% grade instead of having to stand up.

You may need a longer chain if yours is at minimum length for the 28T cog. The chain MUST be long enough to shift into the big cog/big chainwheel combination without strain.

thanks
rob

Shimano components have part numbers on them. A rear derailleur part number would be like RD-5800.
Look on the back side. You will probably need a rag to wipe off the crud and a flashlight to see the numbers. Some are molded in the black bits and are a bit hard to read.

Some shimano parts have the same part number but they still have different specs.
On the shimano site, there are 2 version of the current 105 rear der with the same part number.
One has a 28t max cog, the other 32t. The diff would be the length of the cage. "Long cage" and "Short Cage" are relative terms, not exact measurements. They vary by brand/model/years. It is pretty obvious which is which went you see them side by side.

I put a mtb der, cog and a longer chain on my road bike when I went to ride in Colorado. That 39x32 worked pretty well for the up hills for a flatlander. I realized I only used the 39x32, the 39x28 and the 53x11. Wheeee. IMO, the road climbs aren't that steep, just long and at high altitude. Gear down and don't try to keep up with the locals who are used to the thinner air.

Cost? A cassette and chain aren't that much and you will be replacing them every couple of years anyhow. Buy the lock ring removal tool and chain whip so you can change them out yourself.

thanks
Cost? A cassette and chain aren't that much and you will be replacing them every couple of years anyhow. Buy the lock ring removal tool and chain whip so you can change them out yourself.[/QUOTE]

I have the tools to do the job and when I rode in Colorado I stayed away from the really big climbs because I knew my lungs would explode. I enjoyed myself immensely. I'm planning a trip to ride 6 gap and 3 state 3 mtn, Ga and Tn. the climbs are big and tough for a flatlander but the altitude isn't bad at all. I would just like to make a good go at it instead of a weak lame attempt. everyone I've talked to locally tells me how tough it is but also how much fun it is.
thanks
rob

I was just able to check my derailleur and it's an RD5800 and from center to center pins it's 58mm. so is that a small, medium?

short. 28 tooth rear max., depending on front ring difference.

RD5800 is indeed short cage. 11-30T will be pushing it.