53/39 to 50/36.. braze on front D

[amator]

Just by eyeballing, how much more would the front D have to be shifted down from a traditional 53/39 to a 50/36 compact crank.
I have a front braze on Tab.

[thwart]

About this much... more or less. :D

Seriously, although folks say that with compacts and braze-ons you may not be able to go low enough, I personally haven't seen that happen. YMMV.

[DarrenCT]

About this much... more or less. :D

Seriously, although folks say that with compacts and braze-ons you may not be able to go low enough, I personally haven't seen that happen. YMMV.

i agree w/ Tom

thats the word on the street atmo

[amator]

About this much... more or less. :D

Seriously, although folks say that with compacts and braze-ons you may not be able to go low enough, I personally haven't seen that happen. YMMV.

Sorry, U havent seen a compact crank fitted successfully on a braze on or.....

[rustychisel]

The answer, generally, is about 2~3mm. YMMV

[thwart]

Sorry, U havent seen a compact crank fitted successfully on a braze on or.....
... it works well in the bikes I've built. No problems as mentioned above... so far.

[Ray]

I wouldn't worry too much about a 50. I've had problems with 48 and 46 big rings and throwing the chain over the top of the ring, but with 48 its a very occasional thing and with 50 it shouldn't be a problem. I'm sure it all depends on the placement of the tab, though. Likely more of a problem on a frame more than a few years old. I'd guess (hope?) that frames that have come out since compact cranks got popular would have enough adjustment to deal with 'em.

-Ray

[Dave]

The correct answer is 6mm lower. The radius is proportional to the change in circumference divided by 2 x Pi. Three teeth subtracts 1.5 inch from the diameter, so divide 1.5 inch by 6.28 and you get .239 inch or 6mm.

[amator]

Test fit shows around 3-4mm with chorus 07 UT 50/36 and corresponding compact front D.
By the way i heard the 08 front D for chorus and record can be used with both standard 53/39 and compact

[mschol17]

The correct answer is 6mm lower. The radius is proportional to the change in diameter divided by 2 x Pi. Three teeth subtracts 1.5 inch from the diameter, so divide 1.5 inch by 6.28 and you get .239 inch or 6mm.


I think you mean circumference

[MilanoTom]

About this much... more or less. :D

Seriously, although folks say that with compacts and braze-ons you may not be able to go low enough, I personally haven't seen that happen. YMMV.

I've seen it, but it was on an older (pre-compact era) frame. In order to get the FD low enough, I had to take a small round file (a chainsaw sharpening file did the trick) and lower the slot a bit.

A second option is to use a triple FD. The inside of the cage hangs low to accomodate a small inner ring, so it'll keep a chain from falling off when shifting to a 34.

Tom

[Dave]

I think you mean circumference, and you need to divide your answer by 2.

Yes, I did mean circumference, but the circumference of a circle is Pi times diameter or Pi times twice the radius.

To figure the change in the chainring radius, which is the amount that the FD would have to come down, you take the change in circumference and divide by (2 x Pi).

The answer is still 1.5 inches divided by 6.28.

[fixednwinter]

With a standard geometry (horizontal top tube style) frame (Colnago Master X-Light), I can get the front derailleur down low enough without modification. This is using an FSA C-16 derailleur, which is for compact cranks (50/36). The cranks are FSA SLK with outboard bearings.

Even with careful adjustment, however, I find it doesn't quite shift as well as my setup with an inexpensive 105 clamp-on front derailleur and a Shimano R700 crankset (50/34) on my Hampsten. I don't know whether it's the additional stiffness that the clamp-on provides, or maybe there's something about the Shimano rings (always thought it was hype, but...). Bottomline - shifting under load from the small ring to the large on the clamp-on Shimano 105/R700 combination is much better than the FSA braze-on C-16/SLK setup.

[thwart]

I've also found, as you have, that sometimes a standard double FD will outperform a "compact specific" FD... Campy's first attempt at this for example :crap:

I've yet to experience chain drop on a compact with a standard double FD... knock on wood. And upshifting seems at least OK, or usually better.

[mschol17]

Yes, I did mean circumference, but the circumference of a circle is Pi times diameter or Pi times twice the radius.

To figure the change in the chainring radius, which is the amount that the FD would have to come down, you take the change in circumference and divide by (2 x Pi).

The answer is still 1.5 inches divided by 6.28.


You are of course correct. I'm an experimental physicist; we don't worry so much about factors of two. :)

[fixednwinter]

I've also found, as you have, that sometimes a standard double FD will outperform a "compact specific" FD... Campy's first attempt at this for example :crap:

I've yet to experience chain drop on a compact with a standard double FD... knock on wood. And upshifting seems at least OK, or usually better.

So far, no chain dump on the standard double clamp-on. It shifts flawlessly.

Although I mentioned the upshift issue specifically, it's interesting that you mentioned chain dumping - I've done that a few times on the braze-on, and currently run a Third Eye on my 'Nago.