The Shimano SM-BB70 [paid link] is a Hollowtech II bottom bracket designed for road bike cranksets. Although it does not utilize a direct cable mechanism itself, improper installation or related cable tension in the drivetrain can create issues that mimic bottom bracket problems. For bikes with external cable routing or internal guides passing near the bottom bracket shell, replacing the front or rear derailleur cable often requires working around the SM-BB70 [paid link]. This guide walks through the correct approach to cable replacement in such a setup, ensuring no interference or friction near the bottom bracket for optimal performance.
Tools Required
- 5mm Allen wrench [paid link]
- Cable cutters
- Pick or awl (for clearing cable stops)
- Torque wrench [paid link] (preset for 5–10 Nm range)
- New derailleur cable and housing
- Bottom bracket removal tool (Shimano TL-FC32 [paid link] or equivalent, if bracket removal is required)
- Grease and internal cable guide tube (if applicable)
Understanding Cable Routing Around the SM-BB70
Although the SM-BB70 [paid link] does not have a direct interaction with shifting cables, the front derailleur cable is often routed underneath the bottom bracket shell. This makes it critical to ensure that:
- The cable is free of contamination or rust where it passes beneath the shell.
- The cable housing isn’t rubbing against the shell or BB cups.
- The cable stop underneath the BB is clean and undamaged.
- No part of the cable interferes with the movement of the crankarms or chainrings.
Failing to address routing can lead to friction that impairs front derailleur performance and causes premature cable wear.
Preparation and Inspection Before Replacement
Start by shifting your chain to the smallest chainring and smallest rear cog to release tension. Loosen the pinch bolt on the derailleur (or use the barrel adjuster if needed) to slacken the cable. Next, inspect the bottom bracket shell area for any signs of wear, contamination, or tight bends in the cable housing.
If the bike features internal cable routing through or around the BB shell, removal of the SM-BB70 may be necessary. This gives full access to internal guide tubes and helps prevent cable drag.
Cable Removal and Housing Check
Once the old cable has been released from the derailleur, gently pull it through the frame. Pay close attention to the routing path under the SM-BB70. If housing or the internal guide tube has become kinked or dirty, replace them. Clean all cable stops and frame ports using a pick or awl to remove dirt or old grease.
It is critical to ensure no metal burrs or sharp edges are present near the BB shell that might damage the new cable over time. Smooth the shell area and apply grease where the cable or housing contacts the frame.
Installing the New Cable
Feed the new derailleur cable through the shifter and carefully route it down the downtube toward the BB shell. For externally routed frames, insert the cable into new housing sections cut to the appropriate length. For internally routed designs, use a guide tool or internal routing sleeve to navigate the cable past the bottom bracket and into the chainstay.
Ensure that the cable passes cleanly beneath the SM-BB70 shell (in external routing) or through a clean internal tube (if applicable). Avoid pinching the cable between the BB cups or routing it in a way that causes sharp angles or excessive bends.
Cable Tension and Fine Tuning
Once the cable is securely routed and the derailleur clamped, adjust cable tension using the barrel adjuster. Make small quarter-turn adjustments to achieve crisp, reliable shifting. Avoid overtightening, which can cause sluggish response or extra wear on both the cable and derailleur.
Test shift through all front derailleur positions while inspecting the cable’s path under the SM-BB70. Ensure there is no excessive movement or contact with the bottom bracket shell.
When Bottom Bracket Removal Is Necessary
In bikes with internal routing that enters or exits near the BB shell, accessing the cable channel may require bottom bracket removal. If this step is needed:
- Use the TL-FC32 [paid link] tool to remove the left and right cups of the SM-BB70.
- With the BB removed, inspect the cable tunnel or frame channel.
- Clean and re-lubricate the threads before re-installing the BB cups to 35–50 Nm torque.
Reinstalling the bottom bracket with attention to clean threads and proper alignment prevents future creaks and ensures smooth drivetrain performance.
Final Checks and Maintenance Tips
After replacing the cable:
- Recheck the derailleur limits.
- Ensure smooth actuation from the shifter.
- Wipe down the area around the SM-BB70 and inspect for cable wear points.
- Apply light lube to exposed cable sections.
Regular checks can prevent friction buildup, which may feel like bottom bracket drag but is often due to cable tension or routing errors.
FAQs About Replacing the Cable on Your SM-BB70
1. Does the SM-BB70 have a built-in cable guide?
No, the SM-BB70 is a bottom bracket and does not include an integrated cable guide. However, many frames route cables under it.
2. When should I replace cables routed under the bottom bracket?
Replace them if shifting performance drops, the cable is frayed, or there’s visible corrosion near the BB shell.
3. Can improper cable routing affect bottom bracket life?
Yes, poor routing may cause cable rub on the BB cup, leading to wear or contamination ingress.
4. Do I need to remove the SM-BB70 to replace internal cables?
Only if the frame’s cable routing goes directly through the BB shell. Otherwise, external routing usually doesn’t require BB removal.
5. How do I know if cable friction is affecting my shifting?
Sluggish front shifts or excessive shifter resistance often indicate cable friction, especially around the BB shell.
6. Should I re-grease the SM-BB70 when doing cable maintenance?
If the BB is removed during the process, re-greasing threads and inspecting bearings is recommended before reinstallation.