Condensed answer: An Ultegra chain can be integrated into a 105 drivetrain as long as it matches the number of speeds that the bike has. Otherwise, the shifting will be less than ideal, although it could work in certain conditions.
The Relation Between Gear Numbers and Chain Width
The width of a chain is closely related to the number of gears on a bike.
Bicycle chains have two widths – outer and inner. The outer width is the distance between the outer plates whereas the inner width is the distance between the inner plates.
The inner width of a chain changes little with each gear increase or decrease, but the outer width (total width), changes substantially.
Why?
The overall width of cassettes changes a little with each gear increase. This is done intentionally to make sure that a rear hub is compatible with a maximum number of cassettes.
To fit more cogs within roughly the same space, the distance between the cogs gets smaller. (The cogs get ever so slightly thinner too.)
The decreased spacing between the cogs requires a narrower chain with each gear increase. Otherwise, the chain will rub against the cogs.
The inner width has to change very little because the thickness of the cogs remains close to identical. Thus, the chain width decrease comes from using thinner outer plates.
The chain width change leaves us with two options:
- Use an Ultegra chain that matches the number of gears on the drivetrain.
- Use an Ultegra chain that is designed for 1 more speed than the drivetrain.
The second option is viable because the chain will be slim enough to fit and won’t cause rubbing. The other way, however, (using a chain designed for 1 less gear) will not work because the chain will be too wide and will cause rubbing.
The table below presents the inner and outer widths of 7 to 12-speed chains. As you can see, the outer width decreases notably while the inner width of 9/10/11/12-speed chains remains the same.
| Number of Speeds | Inner/Roller Width | Outer width |
| 7 | 2.38mm | 7.3mm (Shimano), 7.1mm (SRAM) |
| 8 | 2.38mm | 7.3mm (Shimano), 7.1mm (SRAM) |
| 9 | 2.18mm | 6.5-7mm |
| 10 | 2.18mm | 5.88-6mm |
| 11 | 2.18mm | 5.5-5.6mm |
| 12 | 2.18mm | 5.3mm |
What are the advantages of using an Ultegra chain in this case?
Ultegra components are one step above 105. That said, the quality of 105 is so good that a difference is rarely noticeable, especially in this case. Thus, the main advantage is simply availability.
In other words, if you need a chain, and an Ultegra model fits the criteria, you will have a viable option of very good quality.
