United Trifold: R7000 Short Cage Rear Derailleur + Extra Chain Tensioner

After experimenting with a few different derailleur setups on the United Trifold earlier, I decided to switch back to an 11 speed road setup. This time, I will use a short cage (SS) 105 R7000 rear derailleur, instead of the mid cage version earlier. By doing so, this would solve the ground clearance issue.

Short cage 105 R7000 rear derailleur on the left, mid cage version on the right.

Looks similar from this viewing angle

There is some difference at this area, where the cage is attached to the body of the rear derailleur. This is due to the different large sprocket compatibility between the SS (max 30T) and GS (max 34T) version.

Comparing the mid and short cage derailleurs side by side.

Mid cage length is about 87 mm

Short cage length is about 72 mm

Mid cage weight is 226 grams

Short cage weight is a bit lighter at 220 grams.

Just changing to the short cage R7000 rear derailleur will not solve the slack chain issue, as previously experienced on the mid cage setup. Other chain guides that are used on Bromptons will not fit this United Trifold due to the frame or crankset interference.

Therefore, I got this chain tensioner that was commonly found on older Birdy bikes, before the bottom bracket-based chain guide was invented.

Small extension which will be attached to the rear derailleur cage, to manage the chain when the bike is folded.

This small chain tensioner weighs just 16 grams

At the top gear (smallest sprocket), adjust the chain tensioner so that it just clears the chain.

Adjust the chain tensioner angle so that it avoids the chain.

Plenty of ground clearance, with the cage at its lowest point.

As the chain is shifted inwards to the larger gears, the cage rotates upwards, together with the chain tensioner.

No issue with ground or tire clearance, with the short cage rear derailleur.

Chain at the lowest gear (largest sprocket). Chain length is barely enough, as shown by the super-stretched cage position.

Barely any chain engagement with the top guide pulley. Still able to shift though.

With this additional chain tensioner on the short cage R7000 rear derailleur, I am able to find a balance between good ground/tire clearance, and also sufficient chain management.

In the next post, I will summarize this derailleur drivetrain modification of the United Trifold.

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...

United Trifold: Saint Rear Derailleur and 10 Speed Cassette

Since the mid cage 105 R7000 rear derailleur is not suitable for the United Trifold, I had to find another set of components which will better suit the small wheeled bike.

The problem I faced previously was insufficient ground clearance, as the mid cage rear derailleur had a rather long cage. Therefore, I decided to get a rear derailleur with a really short cage, to solve this problem completely.

What I know is that the Saint or Zee rear derailleurs from the downhill groupsets have the shortest cage, which is why I decided to try the Saint rear derailleur on the United Trifold.

However, as the Saint rear derailleur is a 10 speed component, this means that I need to change the cassette and shifter to a compatible 10 speed type. For convenience's sake, I will continue to use the 11 speed chain first. The chain ring and crankset does not affect compatibility.

Saint RD-M820 rear derailleur

Super low profile! Barely sticks out from the side.

Really long inner link arm, as the cable pitch for MTB is longer than road. The high and low limit screws are located far apart from each other.

Super short cage! I forgot to measure the length, but it is really short. Definitely no problem with ground clearance.

As a MTB downhill component, it comes equipped with a clutch. Nice to have but not necessary.

Weighs a pretty chunky 271 grams, due to the extra robust construction for downhill riding.

Side by side comparison with the previous R7000 mid cage rear derailleur.

Big difference in cage set length and also inner link length.

Here is a 11-28T 10 speed cassette, similar to what I used many years ago on the Dahon Vitesse 2x10 speed setup.

105 5700 11-28T 10 speed cassette weighs 249 grams

Installed onto the wheelset with no issues. A 1.85 mm + 1 mm spacer is needed at the back of the cassette.

Looks good on the wheelset!

Next, I need a 10 speed MTB shifter, as it needs to be compatible to the 10 speed MTB Saint rear derailleur. A Tiagra 4600 road flat handlebar shifter is not compatible due to the different cable pitch.

Deore XT Trekking SL-T780 10 speed shifter, which I used many years ago on the Polygon Cozmic MTB.

Upon installation of the rear derailleur to the bike, I found that there was quite a bit of friction from the clutch unit, even though the clutch was turned off. Therefore, I opened up the clutch to take a look. Here is how you can service your clutch too!

How it looks with the internal clutch unit removed from the rear derailleur.

Here is the clutch unit. It can be serviced or replaced if it is faulty.

Then, I realized that the rear derailleur will function perfectly fine without the clutch unit. As I don't need the clutch function, I decided to just remove the clutch unit, allowing the rear derailleur cage to swing smoothly, and also reducing the shifting force.

Once the 10 speed shifter has been installed onto the bike, along with the other 10 speed components, I took the bike for a test ride!

A derailleur drivetrain looks good on the bike!

Lots of ground and tire clearance, due to the super short cage Saint rear derailleur.

The whole bike looks good in the sun. Purple and yellow colour combination is weird but really outstanding.

Went to the same step that caught the R7000 rear derailleur previously. No issue this time!

Good clearance for the rear derailleur cage.

The optical gear display on the shifter is good as well, due to the clearly defined gear indication.

Can clearly see which gear I am currently in, and how many more available on either end.

Based on the test ride, this 10 speed setup has passed with flying colours! However, I also need to check how well it works when it is folded. Due to the short cage, it is expected that the chain would become even more slack when the rear triangle is folded down.

When folded, the short cage barely takes up any slack chain.

Due to this, the chain becomes way too slack, as the chain is long enough to touch the ground already.

As the chain is way too slack when folded, it will touch the ground, which is not acceptable for me. Also, due to the short engagement between the chain and cassette when folded, the chain is likely to drop off the cassette when the bike is unfolded again.

Therefore, even though this 10 speed drivetrain rides really well, it is not good enough due to the poor chain management. I need to keep trying alternative ideas that will work better.