Rough Start to 105 Upgrade? What to Check

Yes, a rough start to a 105 upgrade is usually caused by a few fixable issues—most often a misaligned derailleur, incorrect cable tension, or a chain that’s too long or too short. This guide explains what to check first, how to fix the most common problems, and when the issue is actually a compatibility or installation mistake rather than a fault in the groupset itself.

Formula

The ideal chain length for a standard 1x or 2x setup is approximated by the formula:

Chain length (links) = 2 × (chainstay length in inches) + (largest front chainring teeth + largest rear sprocket teeth) / 2

Where:

• Chainstay length is the distance from the center of the bottom bracket to the center of the rear axle, typically 16–17 inches on a road bike.
• Largest front chainring teeth is the count on the big ring (e.g., 50 for a 50/34 compact).
• Largest rear sprocket teeth is the count on the biggest cog (e.g., 28 for an 11-28 cassette).

The formula assumes a standard rear derailleur with a cage length that matches your cassette range. For mountain bikes or very wide-range cassettes (like 11-42), you may need to add 2–4 links to account for the derailleur’s required wrap capacity.

Worked Example

Using the default values from the calculator:

• Chainstay length = 16.5 inches (we use 12.7 mm per half link, so 2 × 16.5 × 25.4 = 838.2 mm)
• Largest chainring = 50 teeth
• Largest rear sprocket = 28 teeth

Ideal chain length = (838.2 mm + (50 + 28) × 12.7 mm) / 12.7 mm per link = (838.2 + 990.6) / 12.7 = 1828.8 / 12.7 = 144 links (rounded down).

If your current chain is 110 links, you’re far too short—which will cause a rough, grinding start and can damage the drivetrain. The calculator shows a difference of 34 links, meaning you need a longer chain.

Background: The 105 Groupset and Its Common Upgrade Issues

Shimano’s 105 is a mid-tier road groupset that’s been around since the 1980s. The latest version (R7100, introduced in 2022) is 12-speed and shares much of the technology with Ultegra and Dura-Ace. When people upgrade to 105, they usually replace the entire groupset or just the shifters and derailleurs. The most common “rough start” complaints are:

• Chain skipping under load when pedaling hard from a stop.
• Derailleur not shifting smoothly into the smallest or largest cogs.
• Excessive noise, clicking, or a feeling of “gritty” pedaling.
• Chain dropping off the chainring when shifting to the big ring.

These issues are rarely due to a defective groupset. They almost always stem from installation errors, worn components, or compatibility mismatches.

How a 105 Upgrade Works: Key Components and Setup

Upgrading to 105 involves replacing the shifters, derailleurs, cassette, chain, and often the crankset. The system relies on precise cable tension and derailleur alignment. Here’s what each part does:

• Shifters: The brake/shift levers pull or release the cable that moves the derailleur.
• Front derailleur: Moves the chain between chainrings. Its height, angle, and limit screws are critical.
• Rear derailleur: Moves the chain across the cassette. Its B-tension screw and limit screws must be set correctly.
• Chain: Must be the correct length and width for the cassette and chainrings.
• Cassette: Must match the freehub body (Shimano 12-speed uses the HG hub, but 12-speed road cassettes require the new HG+ spline).

If you’re upgrading an older bike, you also need to check that your wheels are compatible with the new cassette. Many 11-speed hubs can accept 12-speed road cassettes, but some older wheels may not.

Common Problems and How to Fix Them

Here are the most frequent causes of a rough start after a 105 upgrade, in order of likelihood:

1. Incorrect cable tension: If the rear derailleur doesn’t shift crisply, the cable is either too loose or too tight. Shift to the smallest cog, then turn the barrel adjuster counterclockwise to increase tension, one click at a time, until the chain moves to the next cog cleanly.
2. Derailleur alignment: The rear derailleur hanger may be bent. Use a derailleur hanger alignment tool (or have a shop check it). A bent hanger causes poor shifting and a rough feel even with perfect cable tension.
3. Chain length: As the calculator shows, an incorrect chain length causes skipping and noise. A chain that’s too short can damage the rear derailleur when you shift to the big-big combination. A chain that’s too long will sag and cause poor shifting.
4. Front derailleur setup: The front derailleur must be parallel to the chainrings and positioned so its outer plate is 1–2 mm above the big ring teeth. If it’s too high or low, the chain will rub or fail to shift.
5. Worn cassette or chain: If you reused an old cassette, the new chain may skip on worn teeth. Always replace the cassette with the chain when upgrading.
6. Freehub body compatibility: Some 12-speed Shimano cassettes require a specific freehub body (HG+). If your hub is older, the cassette may not sit properly, causing a rough, noisy ride.

How to Choose the Right Parts for Your Upgrade

Before buying a 105 groupset, check these compatibility points:

• Number of speeds: The R7100 is 12-speed. If your bike has 11-speed wheels, you may need a new freehub body or a different cassette.
• Brake type: 105 is available in rim brake and disc brake versions. Disc brake versions need flat-mount calipers and 160mm rotors (or 140mm with adapters).
• Crank length: Choose a crank length that matches your old one (typically 170mm, 172.5mm, or 175mm).
• Bottom bracket: The 105 crankset uses a 24mm spindle. You need a bottom bracket that matches your frame’s standard (BSA, PF86, BB386, etc.).
• Cassette range: 105 is available with cassette options like 11-28, 11-30, and 11-34. Make sure your rear derailleur’s cage length supports the range (short cage for 11-28, medium for 11-30/34).

Costs and Where to Save

A complete 105 R7100 groupset (shifters, derailleurs, crankset, cassette, chain, brakes) typically costs between $600 and $900 if bought aftermarket. Installing it yourself saves labor costs, but you’ll need a few special tools: a cassette lockring tool, a chain whip, a torque wrench, and cable cutters. If you’re not comfortable with derailleur setup, a bike shop can install and adjust it for around $100–$200 in labor.

To save money:

• Buy the groupset as a full kit—it’s often cheaper than buying parts separately.
• Reuse your existing crankset if it’s compatible (e.g., if you’re upgrading from 11-speed 105, the crank may work with 12-speed if you change the chainrings).
• Shop for last year’s model (R7000 is 11-speed and often discounted, but it’s not 12-speed).

Safety and Maintenance Tips

After installation, do a thorough safety check before riding:

• Verify that the chain doesn’t fall off the chainrings when shifting under load—if it does, adjust the front derailleur limit screws.
• Check that the rear derailleur doesn’t shift into the spokes. This is caused by the low limit screw being set too far inward.
• Make sure all bolts are torqued to spec (usually 5–7 Nm for derailleur bolts, 12–14 Nm for cranks).
• Lubricate the chain after installation and wipe off excess.

If you experience a rough start that persists after these adjustments, it could be a sign of a bent derailleur hanger or a damaged freehub—both require professional inspection.

Frequently Asked Questions

Why is my new 105 drivetrain skipping when I pedal hard?

Skipping under load usually means the chain is worn or the cassette is worn. If both are new, check the chain length—it may be too short. Also, verify that the rear derailleur’s B-tension screw is set correctly. If the screw is too tight, the upper pulley wheel is too close to the cassette, causing the chain to bind on the cogs.

Can I use an 11-speed cassette with 12-speed 105 shifters?

No. 12-speed shifters have different cable pull ratios and indexing, so they won’t work with 11-speed cassettes. You must use a 12-speed cassette and chain. Some aftermarket companies make adapters, but they’re not reliable and are not recommended.

How do I know if my chain is too long or too short?

Use the calculator above. A quick check: shift to the largest chainring and the largest rear cog. The rear derailleur should be at its most extended position, but the chain should not be taut. If the derailleur is fully extended and the chain is still tight, it’s too short. If the chain sags significantly when in the small-small combination, it’s too long.

Do I need to replace the bottom bracket when upgrading to 105?

Only if your current bottom bracket is not compatible with the 105 crankset’s 24mm spindle. For example, if your bike uses a GXP or BB30 bottom bracket, you’ll need to swap it for a Shimano-compatible one. If you’re upgrading from an older Shimano groupset, the bottom bracket may be fine.

Why does my front derailleur rub in the big ring?

This is usually a cable tension issue or a limit screw problem. Shift to the big ring and tighten the high limit screw until the chain just clears the outer plate. Then adjust the cable tension so that the derailleur moves slightly inward when you shift to the small ring. If the rub persists, check that the derailleur is parallel to the chainrings.

Is a rough start a sign of a defective groupset?

Rarely. Shimano 105 is a well-made product, and most issues are installation-related. If you’ve checked all the adjustments and the problem persists, it could be a bent derailleur hanger or a faulty component—but that’s uncommon. A bike shop can diagnose it quickly.

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in t | Current chain length (links) | links | 110 | 90 | 130
in s | Rear sprocket teeth (largest) | teeth | 28 | 11 | 52
in f | Chainring teeth (largest) | teeth | 50 | 22 | 60
out ideal | Ideal chain length (links) | links | (2 * 12.7 + s + f) / 2 | 0
out diff | Difference from current | links | ideal - t | 0