I plan on changing the setup on my road bike from a triple chain ring to a compact double chain ring. If it’s relevant, the current triple chain ring setup is a Shimano 105. Will it be necessary to install a shorter axle and/or an all new bottom bracket to make this change?
Rsapp

The bottom bracket is determined by the new crankset. Find out what it takes and you have your answer. My condolences on losing your triple.

Which 105 model? Square taper (1055/1056), Octalink (5503) or Hollowtech II (5603/5703)?

.
...as stated already above, you get the BB unit that is designated by the manufacturer of your new crankset.

If you're going from a triple to a compact double, depending on the gearing range you choose in your new crank chainrings, you might be able to get crisper shifting by going to a shorter cage rear derailleur and shortening your current chain (or the replacement you install with the new crank.)

Also as stated above, I'm at a loss to explain why this is the second thread in the last few days where someone is replacing a functional triple with a compact double crank. There must be some virus going around on Youtube. I really hope it does not get to the pandemic stage that turning perfectly good road bikes into fixies became. Those were bleak times. Not sure I'll ever completely get over that.

Want to sell your inferior triple crank?

Perhaps the people doing so don't understand that the compact double's predominance comes almost exclusively from one origin (in my opinion). Manufacturers realized that they could promote a bike with wide range gearing that cost them less to produce. Not only do they save the cost of a chainring, but by design there are fewer chainwheel sizes to deal with. Even the name was designed to be attractive - who doesn't want something more "Compact" that does the same thing as one that's more "bulky"? It looks like a real race bike, too, instead of one of those touring bikes that old people ride. Many people have not experienced the flexibility of using two easily shifted, closely spaced chainrings 95% of the time while still having a climbing gear (also closely spaced with the middle ring). So they don't know what they are losing by the jarring 16 tooth jump.

Yes, I know - a 50-34 is better than the standard 53-39 for hilly terrain and higher cadence (don't get me started on 53 tooth chainrings). But with the inane forced conversion to an 11 tooth small cog a 48-36 would would have made more sense. Unfortunately that's too "sissy," does not look sufficiently racer-like. I have seen recently advocacy of matching wider range cassettes with a "mid-compact" (52-36) crankset. Now you too can have BOTH a useless high gear and a semi-low gear, with the same 14 tooth jump.

Of course in this case we're talking about something worse - replacing an already in-place, presumably working triple. I don't see a good argument for doing so. A triple properly matched to one's ability and terrain should do exactly as I described above - it acts mostly as an easily shifted double. The complication of a triple is greatly exaggerated. Cyclists have toured for weeks and even months on triples that performed flawlessly.

Very well said CNY-BIKEMAN. I ride a and 80's Ciocc Designer 84 running a 52-42-30 up front and 12-25 9-Speed in the back that works perfectly for the local terrain and my level of conditioning. This combination produces ~16 distinct gearing combinations....yup, too much time with spreadsheets. There was a period where I considered converting this bike to a 50-34 compact double, primarily for a racier appearance. To get near the same range of gearing combinations, I would have needed a 12-28 10-Speed cassette. So.....new crankset, bottom bracket, shifters, cassette, chain, etc., all to "look racier". Not for me!

Dean

I actually went the opposite direction a few years ago, from perfectly functioning compact double to compact (I guess) triple (50-39-30). I kept my 12-27 cassette, got a slightly lower low gear, high end stayed the same. Kept my short cage derailleur, since I don't really care if I can use smaller than 23 rear on the granny ring. I mostly use the middle and large rings. It shifts better than the compact double ever did. My chains seem to last a little longer, maybe since I spend less time in the nearly cross chained gears (or maybe I'm just getting older and not pushing as hard!

Hollowtech the latest stuff you replace the spindle & the right arm , one piece , through external bearings..

SHHHHHHHHH....don't say the "H" word or he might appear!

When I bought my Airborne at age 51*, I spec'd it with an Ultegra triple after reading Joe Friel's book "Cycling Past 50". Seventeen years later, I'm glad I did. Though when the bike was new, I used the granny a lot more than I do now!
Steve

EDIT: *17 years ago!

Strong feelings about triple chain rings! So strong that I've decided that since I am definitely not getting younger and stronger, I'll forego the time and expense of the switch to a compact double.
Tsapp

The 30T low gear is more than "slightly" lower, it's 13% which is a significant step. Depending on which crank you now have, if yours has a separate 74 mm granny bolt circle, you could replace the current 30T chainring with as small as a 24T and get a really useful granny gear.

I have triples on all of my bikes with the two most ridden ones having FC-5703 cranks now geared 50/39/26 with the same 12x27 10-speed cassettes. That gives me a more than high enough high gear ( 112 gear-inches) with a true bail-out low gear (26 gear-inches) adequate for anything but loaded touring.

A couple of years ago, Shimano dropped the separate granny ring bosses on their upper line triple road cranks (105 and Tiagra) and uses a "tripilizer" middle chainring drilled with a 92 mm BCD so a 30T chainring is the smallest that will fit.

The granny is a real stump-puller. It was interesting to notice over the first few years I had the bike how I would use it less and less as I rode the same moderately hilly stretches in my area. Now I try to avoid using it, but it gives me a lot of confidence on steep hills to know it's there if I need it... and there are times that I do. As I get older, I expect I'll go back to using it more.
Steve

I, for one, think the OP [MENTION=145941]tsappenfield[/MENTION] should go ahead and try it. Maybe you'll like the compact crank. If you don't, then you're out a little more time to put the triple back on and you've learned something.

You can take my 3x9 triples from my cold, dead hands. I always shake my head as the innocents struggle with wide-stepped compacts here in the Rockies. They shoe-horn an 11-36 cassette on a road bike and hate the 15% steps between gears instead of running a sweet triple with a 6%~8% step, 14~28 cassette. WTF, how does that make sense? And it looks stupid too. Now they want us to go 1x12 with a 10-40 cassette....

All the "One-by" bikes I saw at InterBike last year looked like they had a buzz-saw blade mounted on their rear wheels. Sad!
Steve

EDIT: Here's a bike I rode at the Outdoor Demo. A great-riding bike, but IMO a stupid-looking drivetrain. I think a front triple looks better. Notice how the biggest couple of sprockets are black so they don't give the "lumber-mill" effect so much.

The OP would be out money as well, and it appears has already learned something from the responses.

He would only be out what he couldn't recoup from selling the parts he doesn't use.

While I agree with you on many of the merits of triple cranks, there are a lot of people out there that love compacts, and aren't switching back.

I'll have to take your word for it, as I've not seen that many people with triples convert to a compact. It seems to be mostly people with standard cranksets who want lower gears without a triple conversion. I can certainly understand that from an enconomic perspective. I don't see anything at all to "love" about losing a chainring.

Where the triple chainline puts the middle chainring, is the line that runs between the 2 chainrings of a double..

And yet we see many questions as to how best to convert from a double to a 1X. Makes no sense to me and my stock question is “did front derailers and shifters suddenly start to suck ?”. I can understand a 1X on. Mt. bike where you find that with a huge range of 11 cogs on the rear, you just don’t use 3 chainrings much, so want it simple.

Well, no I’m not buying that either. I LIKE having a lot of gears and a triple gives me that, especially useful when the cogs are closely spaced so you don’t get huge jumps. Possibly more important on a road bike where terrain is not constantly changing and where 1 tooth jumps help maintain steady cadence.

I pondered gearing a bit as I spec’d a Di2 group and had the option of the GS derailer, which will allow an 11-32. That option lets me take my carbon bike into some hilly terrain, but as I already have a “hill” bike with a 12-25 9 spd., plus a 26-38-52 triple, Ive no need for that.

Coming from a background of commuting on a vintage-mtb-converted-to-hybrid, then cargo bike, and living in a hilly place, i totally support triples. Never ran out of gearing high or low.

I even converted my only road bike from double 42-52 to 22-32-44. Doesn't make sense if i only ever used the first 7 cogs with the 42 chainring.

Oh, then how about 1x conversion? Notice that many 1x set ups use something like a 44 chainring. Well, my 3x has the same 44 chainring and two smaller chainrings to choose from.

Oh, and 1x isn't new or innovative or cuttin edge either, many low performance market and commuter bikes still come with 1x6 or 1x7 set up.

When I purchased my road bike ten years ago, a triple chainring seemed logical based on my age and type of riding I was doing. Like an earlier poster, I found that over the decade, I was using the "granny gear" less and less. I also started doing Senior Games racing, so I thought that a compact double as on my time-trial bike would be the way to go. That's the background and thinking behind the change. But as stated earlier, because of the Oak Hill beast of a hill (personal) and many other reasons as pointed out in this thread, I'm staying with the triple.
Tsapp

tsappenfield,

I have just changed from a triple to a Shimano compact double 50~34T. YES, I had to change the BB. Reason was that my chain line was way to far outboard. Measurement from center seat tube was 57mm and a road bike should be close to 43mm.

Go to the Shimano site, select your type bike, type in your crankset, scroll down the table and it will give you the axle length for proper BB chain ring spacing.

2017-2018 SHIMANO Product Information Web

I needed a UN-55 110mm BB. I Amazoned it, installed it, and my chainring spacing is spot on 43.5mm and my drive chain runs good in all rear cogs.