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When I graduated college, I was given a pretty nice bike: a 2009 Specialized Allez Elite(I think that's roughly correct. 105 parts all round, double front chainring, but I don't think the rear cassette is really 10 rings. Anyways...)

Only thing I've done to it is, after snapping one of the brifters internal components, I replaced both brifters with updated 105's.

Fast forward 12 years---I never really got used to the frame geometry (it's essentially a racing frame, I never got into racing and didn't know enough at the time to know what I wanted), and by now riding it is pretty uncomfortable to me. I want to change to a more comfortable touring style geometry, and the idea of disc brakes seems really attractive. Last time I took it out, it had severe trouble stopping.

The frame I had been looking at, till they stopped being available, was the Surly LHT. What initially caused me to look at surly is the sheer amount of braze-on's available (I want to add rims and a front rack to the bike). Now I'm not sure what I should look at. I would prefer to get disc brakes if it's possible. Anyways, to the point of the post:

If I move to a new frame (say, Surly Disc Trucker), how can I tell what parts I can carry over?

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    If you just want comfortable road biking, a touring bike like the LHT might be overkill - heavy, "slow" geometry... Perhaps look at an endurance road bike (like the Aethos), or even a gravel/allroad bike, which have a more comfortable but still agile geometry and are available in lightweight construction (but lack luggage capacity, obviously).
    – Erlkoenig
    Nov 26 '21 at 5:51
  • in regards to the disc trucker, I just realized today that the wheels are mounted in a completely different way to my current bikes, which means it wouldn't work with my skewer-mount roof rack. that's a deal breaker. the atheos is a carbon frame, which is definitely out. I want a steel frame, in part because I want something with significant weight capacity. I'm already over-capacity on my allez.
    – Faydey
    Nov 26 '21 at 7:56
  • What problems do you have with the geometry? Have you tried different stem lengths and angles? Different amounts of steering tube spacers under the stem? Keep in mind that the first few rides might be uncomfortable because you lack the core and neck muscles (and your brain is not used to using them). Modern rim brakes should work perfectly fine in dry conditions as long as the cables+housing are in good shape and you use good brake pads. You can use a rear rack with a carbon frame. The wheels (spokes) are most likely to break if you are heavy. Frames are usually plenty strong enough.
    – Michael
    Nov 26 '21 at 9:06
  • Would it not be a better idea to jsut sell your racer and buy a complete bike? You will probably find that tehre are not too many parts you can carry over, and some where it's possible but not the ebst idea. You will msot likely find a used bike in the same price range as your old one will collect.
    – Burki
    Dec 2 '21 at 11:43
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STIs: Compatible without issue if you want to stay with a drivetrain setup that's compatible with 2x10 Shimano road, which you probably don't.

Cranks: Not really compatible; will physically go on bike but the chainline will be wrong and compact double likely isn't what you want. LHTs have very long chainstays so the chainline imperfection may be inconsequential.

Wheels: Not compatible, no rotor mounts and the rear you have is 130 rather than 135mm spacing.

Derailleurs: Physically compatible, but only if you stick with 2x and a small cassette, so again probably not what you want.

Tires: What you have is almost certainly too narrow and road-ish for the kinds of wheels you'd want to invest in.

There are other things that transfer over but the money parts basically don't, so keeping what you have as a complete bike (potentially selling it) almost certainly makes more sense.

If you had to make use of the parts you have, or if the frame was toast or something, and build up an LHT with them, there's no getting around new wheels, but one thing you could do to make a somewhat functional and appropriately geared bike is use the cranks, STIs, and front derailleur (w/ shim to 28.6), then add an 11-36 10-speed cassette and an RD-M592 or other 9-speed mountain derailleur that can clear a 36, for a wide-range double type setup. (There are only a few that can do 11-36. Most can do 11-34, but 1:1 isn't ideal for loaded use.)

Of course, what you asked is how to know what components are compatible, not which ones are. In other words, how does one become someone that understands all necessary facets of bike component compatibility. That question is tricky and there are a lot of how-to-know-what-you-don't-know facets that prevent a simple answer, other than the obvious ones of "experience" or "be a working mechanic." You're asking about Surly and they go further than a lot of companies do by publishing their 'Frame Sheet' documents, which are geometry and compatibility guides for each frame they make. Those give everything needed for someone that knows the basics of what to look out for, but at level 1 you don't even have that yet. I don't have a better answer than to nerd out, stick with it, and ask when you get stuck. Sutherland's, Barnett's, and sheldonbrown.com used to be key resources, but they're all becoming of historical research or vintage repair use only at this point.

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Last time I took it out, it had severe trouble stopping.

One minor clarification first. One thing that many people don’t appreciate is that our brake and shift cables require periodic replacement. Dirt eventually intrudes into the housing. It is not a scam, it is an inherent property of the system. Is there an alternative? Yes, electronic shifting, where you put motors on the derailers and you actuate them with wired signals or wirelessly. That stuff works very nicely, but it is very expensive, and it is less likely to break but if it does you sometimes have no idea what went wrong and it can’t be fixed in the field.

Basically, if you changed the cables, you might be surprised. You can also benefit from upgrading the stock brake pads, sometimes. Kool Stop is often recommended. It is true that disc brakes have more stopping power than rim brakes, especially in the wet. However, most of us don’t need the extra stopping power, and most of us don’t ride in the wet that much, and good (and well maintained) rim brakes can stop well enough in the wet.

Moving on, in principle, you can move all your current 105 parts to the new bike save the brakes. Many people tend to recommend that if you’re going disc brakes, they might as well be hydraulic. I tend to agree, but cable-actuated hydraulic disc brakes exist, and my understanding is that they are quite good. Whatever the case, if you are going with cable disc brakes, you would want good compressionless brake housing for them. If hydraulic brakes, you’d need new hydraulic levers. At the 105 level, those would inherently be 11 speed, so you’d need at the very minimum a new rear derailer, chain, and cassette. The front derailer should work acceptably, and the crankset is very likely to work even if it doesn’t shift quite as well as a fully stock 11s setup. For the front derailer, there may be a complication. FDs come for braze on mounts, or they come in 3 clamp sizes (31.8, 28.6, and 34.9). I have a feeling your Specialized is 31.8 and the Surly may be 28.6. You’d need to double check this. if braze on, you can get a clamp adapter for any of the 3 sizes, and mount the braze on FD to the clamp. This only works with round seat tubes, but I assume that both bikes are round.

Speaking of chain and cassette, by now, I would expect yours to be worn out, so you might want new ones anyway.

You won’t be able to move your wheels over, as they don’t have disc rotor mounts. They may be too lightly built for a bike loaded with cargo anyway.

The handlebars, stem, and seatpost should be able to fit. However, for the stem, the Surly is likely to have a different reach and stack than the Specialized, and you’re likely to want a different length stem.

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To answer this in a general sense for future readers, here are the "compatibility points" that I can think of.

Note that if you have an older French bike, all bets are off. Those French—it's like they have a different standard for everything. And there are outliers for non-French bikes too.

Wheels

  • Diameter: Most common diameters are 622 mm (aka 700C, 29", 28"), 584 mm (aka 650B, 27.5"), 559 mm (aka 26": older mountain bikes). Some bikes with disc brakes, especially gravel bikes, let you switch between 622 mm and 584 mm
  • Tire width: Frame clearance can be an issue.
  • Mounting technology: quick-release (or solid axle) vs through axle. There are adapters that let you install a TA wheel on a QR bike, but not the other way.
  • Quick-release over-locknut distance (OLD)
    • With quick-release wheels, nearly all front wheels should have the same OLD, but rear wheels can have OLDs of 135 mm (mountain bikes), 130 mm (road bikes), 126 mm (older road bikes), 120 mm (track bikes, some old 5-speeds).
  • Through axles
    • Diameter: most road & gravel bikes use 12 mm front and rear. Occasionally you'll see 15 mm.
    • Threading: there are three thread pitches that vary by manufacturer.
    • OLD: nearly all road and gravel bikes use 142 mm rear, 100 mm front. There are a few that have 135 mm in the rear (from when disc brakes on road bikes were new and designs were still evolving). There is a "road boost" standard that is 110 mm in front and 148 in back, but this is uncommon.

Brakes

Rim brakes

There are a few different mounting styles for brakes that will limit compatibility.

  • Cantilever/V-brake posts
  • U-brake/para-pull posts
  • Center-mounted sidepulls/centerpulls
  • Direct-mount sidepulls (uncommon, found on some racing bikes)

Disc brakes

There are 3 different mounts for disc calipers. You can find adapters to convert one caliper to a different mount.

  • Flat mount
    • Rear flat mounts require mounting bolts specially sized to the depth of the mounting points
  • Post mount
  • IS mount:always requires an adapter—no one makes IS-mount calipers
  • There are also a number of disc rotor sizes, and positioning the caliper to fit the rotor may require an adapter to offset the caliper.

Contact points

There are some high-end road bikes that use integrated handlebar/stems, and special headsets for internal cable routing.

  • Stem attachment: quill vs threadless.
  • Handlebar ferrule diameter (for stem compatibility):
    • With modern bikes: usually 31.7 mm (often indicated as 31.8 mm). Sometimes 35 mm
    • Quill stems: generally 26 mm ferrules, except for older Cinelli, which had 26.4 mm.
  • Steerer tube diameter (for stem compatibility at the other end):
    • Threadless: Almost always 1 1/8"
    • Quill: Almost always 1"
  • Seatpost:
    • Modern road bikes most often have 27.2 mm seatpost diameters, but 11 different seatposts diameters have been used.
    • Some modern road bikes have airfoil-shaped seatposts that are not compatible with other bikes.
  • Pedals: Almost all pedals use the same 9/16" threading, with the left pedal being reverse-threaded. One-piece cranks (found on cheaper bikes) take pedals with 1/2" threading.

Drivetrain

  • Bottom bracket: There are numerous different bottom-bracket standards. The most common is the British or ISO threaded bottom bracket, but this cannot be assumed. Some manufacturers have proprietary bottom-bracket types.
  • Front derailleur mounting: Clamp-on vs braze-on. If clamp-on, your derailleur needs to have the right clamp diameter for your seat tube. Also, some bikes are designed for 1× setups only
  • Shifter actuation: electronic vs mechanical. Some high-end road frames do not have accommodations for mechanical shifter cables; some have adapters for specific routing.
  • Chainring count/size: Chainrings can interfere with the adjacent chainstay, which may limit the number or size of chainrings you can mount on a given frame.

Note: I am not delving into compatibility between different component groups, which is a complex subject of its own. Just compatibility between components and frames.

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    +1 for dissing the French... I heard on youtube "nobody copies the French, and the French copy nobody"
    – Criggie
    Dec 1 '21 at 1:52
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The best way is dimensioning all the pieces you can. Remember to differenciate inner and outer diameters. Also bear in mind the torque you should apply to each element, it's important not to overcome its breaking index because it could produce you injuries in the future. It's also a good option to go to an specialist, the problem is that they will take your money from a thing that you can also do

Good luck!

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