Wednesday, October 20, 2021

United Trifold: Chain Tensioner Upgrade

Even though I am converting the United Trifold from the stock 7 speed internal hub to a simple single speed drivetrain, I will still need a chain tensioner to manage the chain.

The chain tensioner is needed to ensure that the chain does not become slack during riding, or during folding. For standard derailleur drivetrains, the cage of the rear derailleur acts as the chain tensioner to manage the chain.

I already have the new front single crankset, and also a brand new wheelset for conversion. I can actually use the same stock chain tensioner that came with the bike, since the function is the same. Upgrading to aftermarket chain tensioners will not modify or affect the function, except the aesthetics.

Stock chain tensioner, made almost entirely of plastic, is about 186 grams. This looks almost the same as the Brompton type.

On the Brompton M6R which I had previously, I did not change the stock chain tensioner. It was not a component that I felt needed to be upgraded, unlike other parts such as the hinge clamps, Ezy wheels or grips.

Back of stock chain tensioner. This squarish profile matches with the shape of the rear dropout area.

However, I soon found that the stock chain tensioner could not be used with 11 speed chains, as the pulley teeth are too wide. They are suitable for single speed or 7/8 speed chains, but not 11 speed chains. I wanted to use an 11 speed chain because it will then be similar to my other bikes, plus it will be a bit lighter as well. Also, a narrower 11 speed chain will fit better on the narrow wide chain ring to prevent chain drop.

Therefore, I had no choice but to change to an aftermarket chain tensioner. Most of them look the same, regardless of the brand Litepro or others. There are a few different designs by H&H and other boutique Brompton brands, but those are quite rare and much more expensive.

In my case, for a single speed drivetrain, I basically only need the chain tensioning function. The sliding pulley is not necessary to perform shifting, unlike on multi-speed Bromptons with a few external sprockets.

Litepro chain tensioner, made of aluminium but with many thin and hollowed out sections. Weighs only 129 grams, quite a lot lighter than the stock resin chain tensioner.

At this point, I am not sure if the chain tensioner will fit without any modifications or not. I have heard that if you change to this Litepro chain tensioner with the stock 7 speed internal hub, the chain line of the internal hub sprocket and the chain tensioner does not match up, causing chain drop issues.

While studying the United Trifold, I found that the rear hub OLD (Over Locknut Distance) is quite strange. The Nexus Inter-7 hub has an OLD of 130 mm, but when I measured the bare frame, I found that the frame OLD is 135 mm.

When assembled, the axle nuts on both sides are used to squeeze the rear triangle, so that the frame is compressed to make the 135 mm OLD frame clamp the 130 mm OLD hub. This is a bad design, as the frame is deformed quite a lot.

I believe that United makes all the Trifold bikes with a rear OLD of 135 mm, to suit all their different models of 5 speed, 7 speed and 11 speed drivetrains. Since the Alfine 11 speed hub has an OLD of 135 mm, all other specifications will just have to use the same frame, even though they are technically not compatible.

Since most rim brake wheels have a rear OLD of 130 mm, there will be a 5 mm difference in hub width. I can get a rim brake wheelset with a 135 mm OLD rear hub (disc brake hub), but it will not be of the special spoke lacing design which I wanted.

As I will change the rear hub fixing method from nutted type to quick release type, it is not possible to compress the frame with a QR axle. It is not strong enough to do that, nor is it safe to apply so much stress on the QR axle.

Therefore, I need to increase the rear hub OLD from 130 mm to 135 mm. From my prior experience modifying the Dahon Boardwalk and Dahon Vitesse, I knew that there is a hub adapter than can do exactly this.

Hub adapter that is fixed onto the end of the hub, to extend it by 5 mm.

This end of the adapter is placed over the rear hub axle.

Weighs just 3 grams, which is insignificant.

This hub adapter can be placed on either the drive side or the non-drive side, to increase the OLD from 130 to 135 mm.

Normally, it will be placed on the non-drive side, so as not to affect the position of the cassette relative to the rear derailleur hanger. In my case, since it is just a single speed sprocket, either the drive or non-drive side is OK.

Hub adapter placed on the non-drive side of the hub (bike is shown upside down).

After adding the hub adapter, the OLD of the hub fits snugly into the rear triangle. There is some clamping effect, as the hub OLD is a bit wider than the opening of the frame. This is OK as it helps retain the wheel inside the frame, after I remove the QR axle.

Now, the next part is the secret to converting the rear hub from nutted type to QR type. Remember that even with a QR axle, I will still need to add the chain tensioner. The original position of the chain tensioner is to be clamped on the outside of the frame, using a second nut.

However, I have never seen a QR axle with a chain tensioner added on top of it. How will it be done?

My idea is to use a long QR axle, and use a second QR nut to clamp both the chain tensioner and the frame against the rear hub to secure them.

This is the first QR nut, which will clamp the rear hub lightly inside the frame.

A long QR axle is needed, which gives me enough thread length to put on a second QR nut later. This QR axle is from a MTB wheelset which requires a longer QR axle.

Steel QR axle for the rear wheel weighs 65 grams

Lightweight titanium QR axle for the front wheel is just 22 grams.

After placing the first QR nut on the QR axle, the chain tensioner is then placed over it. When the QR lever is closed, this first QR nut should only clamp the frame lightly. The majority of the clamping force should come from the second QR nut, which will be located on the outside of the chain tensioner.

I had to select a small QR nut, so that the chain tensioner can go over it. This QR nut also acts to help centralize the chain tensioner on the QR axle, replacing the original nut on the 7 speed internal hub axle.

Placing the chain tensioner over the first QR nut. The nut needs to be small and short enough, so that it does not protrude from the chain tensioner.

Finally, use the original large washer, together with a second QR nut to secure the chain tensioner on the frame.

The first QR nut (hidden inside) needs to be set so that it only lightly touches the frame when the QR lever is closed. The second QR nut (shown outside) needs to be tight, so that when the QR lever is closed, it clamps the chain tensioner and also the frame strongly against the hub axle.

This setup works surprisingly well, and allows the chain tensioner to be added to a conventional QR axle.

An alternative way would be to just use one QR nut on the inside. This QR nut would be set to clamp strongly against the frame. Then, after adding the chain tensioner, use a M5 nut or something similar to clamp the chain tensioner against the frame, using the thread of the QR axle. This allows the chain tensioner to be removed without loosening the QR axle. However, a tool is also needed to make sure that the M5 nut can secure the chain tensioner to the frame without it self-loosening.

With this chain tensioner change and QR axle setup, the bike is ready for a front single speed drivetrain! The next step would be to determine the position of the single rear sprocket, to best match the chain line of the front chain ring.

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