United Trifold: Chain Tensioner from Bottom Bracket (Unsuccessful)

After changing to the 10 speed drivetrain as shown in the previous post, the United Trifold does not have any issue during riding. However, folding becomes a problem due to the short cage of the Saint rear derailleur, which creates a lot of chain slack when folded.

Therefore, I did more research to find out how to maintain tension on the chain when a trifold is folded.

There are additional chain guides that can be added to a trifold, so that it helps to tension the chain when it is folded. As shown below in the example, it wraps the chain around the chain ring, so that the chain takes a longer path from the chain ring to the rear derailleur when folded. This prevents the chain from becoming too slack.

Example showing how the additional chain guide around the chain ring will route and tension the chain when the trifold is folded.

Here is one example of the chain guide, which will be sandwiched between the bottom bracket and the frame.

It is made of a stamped piece of aluminium, and needs special shaping to avoid interference.

Weighs 38 grams

Thickness of the chain guide is around 3.1 mm. In this case it will act as the spacer of the drive side bottom bracket.

Unfortunately, I found that this chain guide interferes with the crank arm.

Apparently the bend on the chain guide is designed to avoid the crank arm shape on square taper cranksets, not the chunky Shimano Hollowtech crank arms.

No way to make it work in this case. I will not sacrifice the crankset for this chain guide.

On to Plan B. There is another chain guide which serves the same purpose, but it is designed differently. Let's see if it works.

Example of the alternate chain guide, shown on another bike. It is designed to avoid the chain stay and crank arms when folded.

Here is how it looks. Angles can be adjusted individually at the pivot points.

This part will be sandwiched underneath the drive side bottom bracket adapter, and also wrap around the bottom bracket shell.

In addition to the bolt, there is an additional pin to prevent rotation of the arm. Seems that angle adjustment is not possible after all.

This chain guide is a bit heavier at 60 grams.

The part that will rest under the bottom bracket adapter is about 2.4 mm thick.

The thickest part of this chain guide is 10 mm for extra stiffness.

Distance from centre of bottom bracket to the roller (which contacts the chain) is about 123 mm. This distance limits the size of the chain ring that you can use.

Internal diameter of the chain guide is 35 mm, which is slightly larger than the thread diameter of the English threaded BC 1.37" x 24 TPI bottom bracket threads.

Unfortunately, another interference problem. This chain guide does not fit the bottom bracket shell on the United Trifold.

This chain guide is designed to wrap around the bottom bracket, but it is too small for the bottom bracket shell on the United Trifold.

From my analysis, this chain guide is designed to fit around trifold with steel frames. The bottom bracket shell on a steel frame has relatively thin walls, as there is no need to make it thick for strength.

However, the United Trifold is an aluminium frame, which means it has an aluminium bottom bracket shell. Thus, the walls on the aluminium bottom bracket shell are thicker, and the outer diameter is too big for this chain guide. Yet another difference on this frame that causes problems with aftermarket modifications. If I try to grind away the interference on the chain guide, I think it will just break off as the material becomes too thin.

Therefore, both the existing solutions for a trifold chain guide does not fit on this bike, due to crankset or frame interference. I need to keep searching for solutions...