General Cycling Discussion - Chainwheel changes

I am planing to change chainwheels, (the granny at best) from a 30 to a 26 or 24T. The front derailluer os a Shimano 105. The rings are 52-42-30. What are the limits on the 105?

Greyhead

I swapped the 30 on my wife's 50-39-30 for a 24t granny. You will want a chain keeper in any event, but based on our experience I would go with the 26t; a 42t to 24t seems like an awfully big shift. The 2008 version of the Shimano 105 FD did handle the 24T, but you could definitely drop the chain. Depending up your usage and your cassette you might consider a wider range cassette, at least up to what your RD will handle.

The limiting factor will be the rear derailleur's capacity. A 105 triple FD should be able to handle 24 teeth difference. That's a 52/42/28T. More is possible, but it may depend on your seat tube angle and the cassette you're running. If you go beyond the derailleur's capacity, the chain will drag on the bottom of the cage. One of my recumbents is running a 30/44/56 with no problems.

I made such a change. Don't recall how many teeth on the original chain ring, but we went to 24t first, and couldn't keep the chain on while downshifting. Went up to 26t, and a wide ratio cassette. It worked fine. There is a really big jump from middle to small chain ring, though. My perception is that shifting the FD one time is now like dropping two cogs on the cassette. It used to be two. Still, if you have some really steep hills, it's a solution.

The only problem has been that if you are in the small chain ring, you need to be in towards the largest cog in the rear. If you use the smallest cog, the jockey wheel is so far rearward that the the chain sort of doubles upon itself, and the chain is actually rubbing against another part of the chain. I can't say if this is common to all such modifications, or only mine.

The max teeth change recommended on a shimano front triple derailler is 22t. This can be stretched to 24- just about- but any more than that and you will have to be perfct in the set up of the derailler.

And a warning- Make certain that the chain ring you get is the same width and alignment as the one you are taking off. I fitted an "Incompatible" width on the Tandem once and the chain used to jam between the inner and middle rings.

But a member in a group I ride with has 50/39/24 fitted to his bike. He does not have any change problems- but he is carefull on changing without pressure on the pedals and derrailler alignment.

And if you run into R.Derailler cog problems as Nermal mentioned- You may have to change the rear derailler to a long cage form.

My Shimano XTR FD is rated for a max. size of 44 - 48T chainring. I installed it anyways on my then-new Ultegra 52-39-30T crankset. It works flawlessly. My advice is to try it regardless of what the specs may say. I have see many other examples of Shimano specs being way off.

I know I'll be pounced on for saying this - but I'm just suggesting people to try. Then publish the results - Yay or Nay.