Birdy AXS: Birdy Bike Frame and Handlepost

In the previous few posts, I have studied the stock setup of the Birdy City, followed by comparing the geometry and size, and finally dismantling the frameset. Now, I will study these individual frameset parts in more detail, by checking out the design and the weight.

Frameset parts laid out in an exploded view.

Birdy AXS: Introduction to Birdy City (Drivetrain Overview)

Here is a new folding bike for me to modify! It is the new Birdy City by Pacific Cycles, which is the entry level spec for Gen 3 Birdy folding bikes. Although there are many different component grades, all of them have the same frameset. Therefore, if I plan to change everything on the bike, might as well get the entry level model to save some money.

Many years ago, I tried the Birdy when it first came to Singapore, and I was very impressed with the suspension. However, I have never bought the bike as it was quite expensive compared to other folding bike options, plus the folded package always seemed awkward to me.

By chance, I happened to see a great offer by Mighty Velo, which had a good discount on some of the Birdy bikes. There was also an offer by Qoo10, which meant that when I combined the offers, there was a discount of more than SGD 700. This created a pretty irresistible deal, which is why I finally decided to buy the bike.

What's nice is that we can specify the colour of the bike frame for every spec, since the bike shop only assembles the chosen frame with the chosen component spec when you order it. It only takes a few days for them to queue and build up your bike, so the wait time is not a problem.

There are so many nice colours to choose from, but I finally chose this sky blue which looks really nice. In fact it looks very similar to the shade of blue used by Shimano.

Before I remove and modify any components on the stock Birdy City, I will first study it closely, and also do some test riding. This is to allow me to compare the ride feel and performance before and after the upgrades.

Completely stock Birdy City setup with 1x9 speed Sora.

United Trifold: Extra Short Handlepost Modification (Unsuccessful)

It's been a long while since I wrote about the United Trifold, but here I am to continue it. The whole bike has been modified from the stock 7 speed internal hub setup to a lightweight single speed setup.

One of the upgrades that I did was to change out the stock handlepost to a Fnhon double bolt type. At that point in time, I could only use a handlepost with a double bolt tall base to replace the original one, as the single bolt type is not compatible with the steerer tube.

The height was lowered by just 3 cm, which is not enough for me. Therefore, I continued to find ways to lower my handlebar position.

Then, I found that it was possible to use a single bolt handlepost with a shorter base, although some additional parts are required. These kind of handleposts are much more common, and comes in many more lengths. This would enable me to use a shorter handlepost to achieve a lower handlebar position.

The stock handlepost was 42 cm tall, while the Fnhon 3D forged handlepost is 39 cm tall. This time, I decided to get an even shorter 35 cm type.

Elosix 35 cm handlepost

Difference in height with the 39 cm handlepost is significant.

Due to a quirk in height definition (from folding joint), the 39 cm handlepost measures 42 cm including the base.

The 35 cm handlepost measures 37 cm including the base.

This means a 5 cm lower handlebar position if I switch to this new handlepost. Will it be too low? Only way to find out is to put it onto the bike and try.

90 grams lighter than the longer Fnhon handlepost (657 grams), and 200+ grams lighter than the stock purple handlepost.

The way to fit this handlepost with shorter base, is to use spacers to take up the difference in base height. I managed to find spacers with inner diameter of 28.6 mm (to match steerer tube) and outer diameter of around 50 mm to match the handlepost base. I was not quite sure which thickness would be most suitable, so I bought some of every thickness.

Spacers of various thickness!

The thicker spacers have a hollowed out section.

The spacer goes under the handlepost base, to take up the difference between this and the tall base handlepost.

Here is the amount of exposed steerer tube, too long for the short base handlepost to fit on. Note that this is the upgraded Litepro headset.

With the Litepro headset cover and without additional spacers, the steerer tube sticks out too far above the handlepost.

However, I found that using the spacer alone leaves a big gap at the side, even when it is pressed all the way down.

The inner ring on the Litepro headset cover is recessed, as compared to the spacer which is flush to the outer ring. That is why the spacer cannot fit directly on the headset compression ring.

Actually, the difference in base thickness is not that much. Only requires a 5 mm spacer.

The Litepro headset cover still has to be used, but with an additional 5 mm spacer on top. This height is just right.

5 mm spacer added under the handlepost. In hindsight, this is much easier than I expected. I did not have to go through all the trouble of getting the double bolt tall base handlepost.

With a much lowered handlebar position, it is now good for sporty riding!

Handlebar height from the ground is just 91 cm, even shorter than the Fnhon DB12 (95.5 cm).

How it looks with the shorter handlepost and lower handlebar position.

2-bolt type of clamp for a more secure clamping and also lower weight, since a quick release clamp is not useful for me.

So far, the modification is not an issue, every part can go on without any problem. However, during test riding, I noticed a strange phenomenon.

When riding with both hands on the handlebar, the bike steering is OK. However, once I take one hand off the handlebar, the steering becomes quite unstable. It becomes very sensitive, and it is nearly impossible to ride in a straight line when only using one hand on the handlebar. Once the handlebar starts turning to one side, it tends to keep turning, and does not self center to the straight position.

This is quite strange as I have not rode a bike like this before. To find out what was going on, I swapped back to the previous handlepost to compare. The stability became normal again, once the longer handlepost was used.

After more testing, I think I know what is the reason. When I changed to a shorter handlepost, the effective steering offset between the handlebar and the steerer tube became shorter as well. This is not an official term, just something that I use to describe this effect.

Imagine a horizontal line between the centre of the clamp to the ruler. A shorter handlepost will have a shorter horizontal distance.

This phenomenon is similar to changing to a shorter stem on a road bike or MTB. A shorter stem makes steering more sensitive, which is what happened when I changed to a shorter handlepost (with shorter effective steering offset).

Seems that there is a limit as to how much shorter I can make the handlepost, without affecting the steering stability too much. I tried adding a stem extension to put the handlebar in front of the handlepost clamp, to improve the stability. Although the steering stability is improved, there was too much flex in the handlebar and handlepost, due to the additional joints, so I decided against this idea.

In the end, I decided to change back to the Fnhon handlepost, which offers a good balance between steering stability, handlebar height and also weight. This modification to a shorter handlepost was not successful, but I did learn about how handlepost height affects steering stability.

United Trifold: Assembly Begins

In my case, I am basically building a United Trifold from scratch, since I decided to upgrade nearly every part of the bike. From the bare frameset, I added the crankset, wheelset, seat post and handlebar, plus a totally new single speed drivetrain.

By doing so, I learnt a lot about this bike, both good and bad points. And this is just the start of the bike modification, there is lots more to come! It's definitely not the most cost effective way to build a bike, but it is fun. Besides, I bought this bike not because I really need it (I have many other bikes), but because I was curious to find out more about this type of trifold bikes. I used to have a Brompton M6R, but the upgrades were mainly cosmetic and functional, and not a total teardown like this one.

With my dream wheelset and basic drivetrain installed.

New carbon handlebar and Ergon grips installed, on the new Fnhon handlepost.

Single speed setup, with just one rear sprocket, and a new chain tensioner.

52T Stone narrow-wide chain ring on the non-series RS510 crankset.

Installing cables on a folding bike is always tricky, as you need to ensure that the cable slack is enough to enable folding and steering, and yet not leave too much slack as it can get in the way during folding. Sometimes it takes a lot of measurements, plus trial and error to make it work optimally.

Measuring the weight of the two brake inner cables before installation. There is no shifter cable for this single speed setup.

After cutting the excess cable off, here is the weight removed. This gives a total inner cable weight of 35 grams that is on the bike.

I added this SM-CB90 Brake Cable Adjuster to the brake outer casings, to release more cable when I need to open up the brake calipers more, during wheel removal.

I love the new 3D forged handlepost, with no welding at the joints. Also check out the new Litepro headset.

Here is how the handlebar looks with the brake cables connected! Super clean with no shifter cables.

It was tricky to find a suitable place for the inline SM-CB90 brake cable adjuster, as it needs to avoid areas with sharp bends, and also avoid being near the frame to avoid knocking sounds during riding or leaving scratches on it.

Litepro A62 seat post, with a new lightweight carbon saddle.

Here is the original Tektro caliper brakes, conditioned to work smoothly, and with new Ultegra brake pads.

Also using the stock Tektro brake calipers at the rear. Smooth cable routing is one of the factors for good braking performance.

Love the look of the wheelset, with the unique paired spoke pattern. Looks just as good as the Kinetix Pro wheelset on the Dahon MuEX.

Rear drivetrain completed! Drivetrain efficiency is at good as it gets, with a near-perfect chain line and no internal hub drag.

The ideal chain length is when the two pulleys are in line with the hub axle. (or is it the chain tensioner pivot?)

Single speed drivetrain, with one 14T sprocket on the standard freehub body.

The bike is almost ready for a test ride! There is a huge weight saving from the stock setup, it will be revealed in the next post.

United Trifold: Handlepost Upgrade

As you may already know, I like to modify my bikes, to make it suit my requirements. The United Trifold bike is no exception.

From the start, I had already planned to upgrade the handlepost, due to various reasons. Although the stock handlepost works well, I did not like the purple colour. It just does not look good in my opinion, although others may have a different opinion. The purple frame is fine for me, but not the long handlepost with a big purple shaft.

Upon receiving the bike, another reason I wanted to upgrade was to lower the height of the handlebar. As on my other folding bikes, I tend to use a short handlepost, as I want the handlebar height to be lower for a more sporty riding position. From the Dahon Boardwalk to the Fnhon DB11, I have always been using shorter-than-average handleposts.

I do not like high handlebars that force me to sit upright and pedal, as it feels very inefficient. I guess I prefer the sporty riding position of drop bars, which I am very used to.

Before changing to a new handlepost, let's check out the stock handlepost first.

Stock purple handlepost on the United Trifold. I was surprised to see a double bolt clamp at the handlepost base!

Most handleposts we see on Dahon-style folding bikes have a handlepost with just a single bolt at the base, which means a shorter clamping height. The downside of this design is lower handlepost stiffness, as the handlepost only clamps onto the steerer tube with a short section.

I first saw the double bolt design on Tern Physis handleposts, which are highly rated for the incredible stiffness and strength. I know this in detail as I have installed the Tern Physis 3D handlepost on the Dahon Boardwalk. Lots more details there if you would like to know more.

This means that the stock handlepost on the United Trifold is actually of a good design, having a tall base to improve the handlepost stiffness. However, the length is still too much for me, so I need a shorter handlepost.

However, it is very hard to find a double bolt handlepost to replace the stock handlepost! I first came across the double bolt Fnhon handleposts when I was building the Fnhon DB11, but they are hard to find nowadays. Apparently, the cost of production for these 3D forged handleposts is too high, and the production has been discontinued. It is a pity as they are really good handleposts, with high stiffness, low weight and nice appearance with no welds.

Why can't I use a single bolt handlepost to replace the stock double bolt handlepost? Because with a shorter clamping section on the single bolt handlepost, the steerer tube would be too long. I can add a  thick spacer under the handlepost to solve this, but the handlepost stiffness would still be poorer.

Weight of stock United Trifold handlepost is 783 grams. This is considered heavy. Imagine if your road bike has a stem that weighs 700+ grams instead of 100+ grams!

After much searching on Taobao, I finally found a double bolt Fnhon handlepost, which is from existing stock. Basically once its gone, they won't be making any more of it. It is twice as expensive as the conventional single bolt type, but it is well worth the money. Just look at the pictures below, and also from the previous comparison when I was building the Fnhon DB11.

New handlepost in sandblasted matte black

Double bolt clamp at the base! Even the bolts and hardware are black in colour.

Weighs 657 grams, quite a lot lighter than the stock handlepost.

New handlepost is 39 cm in length, while the stock handlepost is 42 cm in length. Just a 3 cm difference.

I wanted an even shorter handlepost, such as 35 or 37 cm, but they are not available. Therefore, this new handlepost is only 3 cm shorter than the stock one. A small difference but hopefully noticeable.

Both are double bolt clamp designs, as can be seen from the tall base.

Fnhon handlepost has a red coloured button to release the safety catch, while the stock handlepost has a sliding latch. Both are nice.

Position of clamps are different. Fnhon handlepost has the bolts at the side, while the purple handlepost has the bolts directly under the clamp. Not sure if there is any actual difference in function.

Here is the main difference. Fnhon handlepost is 3D forged as one piece with no welds at the joint, unlike the other one.

Handlebar clamp designs are almost the same. Both open up fully if you need to insert a curved or riser handlebar.

Not exactly the same, but close enough.

Internal clamp design is different. The Fnhon design has the handlepost fully hollow, without a "wall" like the purple handlepost.

The guide for the shaft/bolt are located at different positions. I'm not sure if there is any noticeable difference in quality, strength or durability. Probably just a different way of achieving the same function.

Upon closer look, they are quite different internally. Same clamp concept, but different layout.

As seen earlier, the slot for the double bolt clamping are located differently.

The handlepost should be a straight swap, since both are double bolt types with a tall base. By swapping, the handlebar height will be lowered by 3 cm. I would have preferred a lower handlebar height, but I can't find a shorter double bolt handlepost.

However, before I installed the new handlepost, I decided to study the headset first. I was checking if there is a possibility to change to a better headset, as the stock headset looks quite inferior. Check out the details in the next post.