Bestrider Pump Pro 20: Assembly Part 1

With all the components (new and also stock) of the Bestrider Pump Pro 20 kids bike in place, proper assembly can now take place!

As discussed in the earlier posts, some of the stock components are of good quality and will be reused. Examples include the wheelset, crankset and brake system.

On the other hand, other components which are not good enough will be upgraded. These include the handlebar, seat post and saddle, as well as the full drivetrain.

The stock Kenda Small Block Eight tires are more suited for light off-road riding, which is not needed here as the kids bike will be used on smooth paths only. Therefore, slick tires will be installed to reduce rolling resistance and also reduce the weight.

There are many tire choices for 20" 406 wheels, but slick ones that are wide and lightweight are not common. One of them is this Innova brand which I found, which have a claimed width of 37 mm (37-406).

Innova slick tires, 37-406 for 20" 406 wheels.

It has a folded bead which reduces the weight greatly compared to a steel wire bead. Weight is 222 grams per tire which is not that lightweight though.

One of the other tires which I considered is the Panaracer Minits Lite 32-406, which weighs just 167 grams per tire. However, the claimed width is just 32 mm, and from my experience, the actual tire width is always narrower than claimed. This makes them less suitable for the kids bike, as I prefer to install wider tires that will minimize the risk of the tire getting caught in cracks or drain grilles.

To match the new tires, I got Schwalbe SV6 inner tubes which weigh 89 grams each. No need for lightweight TPU inner tubes here.

The brown coloured side walls look good! Deore XT 11-36T 10 speed cassette has also been installed.

Weight Comparison

Stock Kenda tire: 369 grams each

Stock inner tube: 145 grams each

Stock weight per bike: 2 x (369 + 145) = 1028 grams

New Innova tire: 222 grams each

New Schwalbe SV6 inner tube: 89 grams each

New weight per bike: 2 x (222 + 89) = 622 grams

Just by swapping the tires and inner tube, over 400 grams of rotational weight can be saved! Also, changing from a knobbly tire to a slick tire will reduce rolling resistance significantly. In all, this change will greatly improve the ease of cycling by making the bike feel lighter and faster.

Another view of the stock wheelset, with new tires and cassette.

With a 1.85 mm spacer installed behind the cassette, this setup is correct, with the 11T sprocket protruding slightly above the freehub body, before tightening the lock ring.

However, actual tire width is measured to be only 32 mm wide, even though the tire is claimed to be 37 mm wide.

The rim internal width is already 20 mm, which is not considered narrow for 20 inch wheels. Therefore, the undersized tire width is not due to a narrow rim width. Rather, one trick that some tire manufacturers use is to mis-label their tire width.

In this case, even though the claimed tire width is 37 mm, the actual width is much narrower. This allows the tire manufacturer to use lesser material to make the tire, and thus claim a lower weight compared to other tires.

Not happy with this situation, so I will continue to look for wider lightweight slick tires.

Wheel diameter inclusive of tire is 470 mm.

With the wheelset settled, let's move to other parts of the bike assembly. The next focus is at the handlebar area, where the new carbon handlebar will be used.

Stock Tektro short reach brake levers, which are excellent for kids bikes. 178 grams per pair.

New lock on grips which are 95 mm instead of 130 mm long, which makes them more suited for kids bikes.

The Tektro short reach brake levers are good and will be reused, while the grips will be changed to a shorter lock-on type. At the same time, I decided to swap all the brake and shifter outer housing and also inner cables to a new set, to ensure that it is as smooth as can be. This took up quite a bit of time as I wanted to get the cable length just right.

Grips, brake levers and shifter installed on the new carbon handlebar, with ideal cable lengths.

Stock crankset with new chain ring bolts reinstalled onto the bike.

Brand new 10 speed cassette and rear derailleur installed onto the bike.

New saddle and carbon seat post installed onto the bike. I think this is the lowest position, after cutting the seat post.

The stock cable routing had the shifter cable routed on the right side of the bike, but I swapped it to the left side to run it alongside the rear brake cable.

First part of the bike assembly completed!

There is still some more work to be done to complete the bike assembly, but there is nothing much to be concerned about.

Bestrider Pump Pro 20: Crankset and Chain Ring

On a kids bike, the choice of crankset is very important. The crank arm length, saddle height, and bottom bracket height are all related to each other. If the dimensions are not specified properly, the pedaling ergonomics will be completely wrong, making it uncomfortable for cycling.

If you look at the different kids bikes, there are many different crank arm lengths. From the 90 mm found on the Early Rider Seeker 14 bike, to the 102 mm found on the Bestrider Pump Pro 16, it varies quite a lot across kids bikes. Generally, the crank arm length gets longer for larger kids bikes, until it stabilizes at about 170 mm for adult bikes.

On the Bestrider Pump Pro 20, the crank arm length is 127 mm. I am not sure how all these crank arm lengths are derived, but I know that at least for Bestrider bikes, these are carefully considered and chosen.

Generally, when kids ride bikes, they want to be able to put their foot on the ground when stopped. This means that the saddle height needs to be low enough. However, in order to ensure proper leg extension during pedaling, the saddle height cannot be too low relative to the crankset.

If the bottom bracket is lowered, then the crank arm length also needs to be reduced to ensure sufficient ground clearance. Therefore, all these inter-related dimensions have to be considered together, to ensure correct pedaling ergonomics.

The stock crank arm length is 127 mm which seems like a suitable length, so I will continue to use this crankset. Let's take a closer look at the crankset and the chain ring.

Right crank arm weighs 336 grams, which is pretty decent. Only slightly lighter than Ultegra R8100 right crank arm, even though it is much shorter.

Left crank arm is 180 grams which is also quite decent. Ultegra R8100 left crank arm is 193 grams.

So far I am very happy with the crank arm weights, as they are decently light and look good. However, the same can't be said for the chain ring...

32T chain ring weighs 137 grams! This is very heavy given the small size, as it is made of steel instead of aluminium.

I tried to find replacement aluminium chain rings that are lighter, but could not find any suitable ones, due to the unusual Bolt Circle Diameter (BCD) and non-symmetric 4 arm of the crankset.

Stock steel chain ring bolts weigh 19 grams. Will be replaced by aluminium ones.

Plastic crank arm fixing bolt, 4 grams. Different in design from Shimano type.

Stock chain ring thickness is 3.6 mm, which is pretty thick and thus heavy. This is due to the narrow-wide teeth to improve chain retention.

Need to choose a chain ring bolt length that is long enough to pass through the chain ring completely, and be supported by the crank arm holes as far as possible.

4 sets (male + female) of aluminium chain ring bolts only weigh 7 grams!

Crankset modified with my signature colourful chain ring bolts! I use the same chain ring bolt colours on my other front single bikes as well.

Next step is to replace the stock bottom bracket. Although there is no problem with the stock Hollowtech bottom bracket, I decided to replace it with a Shimano SM-BBR60 since I have a few of them lying around unused.

On the stock setup, quite a few bottom bracket spacers are used. Since the frame bottom bracket width is a standard 68 mm, while the bottom bracket adapters are a standard thickness (around 11 mm), the usage of spacers imply that the crank set spindle is longer than usual. Normally, if a road crankset is used, spacers are not required.

Spindle length of stock kids bike crankset is about 116 mm (rounded up).

For reference, the spindle length of a standard Shimano road double crankset is 110 mm (rounded up).

This means that the spindle of the kids bike crankset is about 6 mm longer than a standard Shimano road crankset. Therefore, assuming the same frame bottom bracket width and same bottom bracket, about 6 mm of spacers is required.

After experimenting with a few different combinations, I found that 5 mm of spacers is sufficient, while ensuring that the crank arm fixing bolt can be installed without bottoming out.

5 mm of bottom bracket spacers, made by combining two 2.5 mm spacers.

SM-BBR60 bottom bracket with 5 mm of spacers weigh 77 grams.

Since I have two 2.5 mm spacers, I can balance them out by installing one on each side of the frame.

New bottom bracket installed, with a 2.5 mm spacer on each side of the frame. Looking good!

With 5 mm of spacers, the spindle will not protrude from the left side crank arm, allowing the crank arm fixing bolt to be fixed properly.

Here is the stock crank arm fixing bolt! With a tool pattern that is different from others that I have used before.

Fits a Size 6 Allen Key, but the plastic hexagon hole rounds off too easily even when applying very little torque. Due to this slippage, I am unable to tighten the crank arm fixing bolt properly for bearing pre-load.

Taking out a standard Shimano road crank arm fixing bolt for comparison, to see if it can be a substitute.

Different length is OK, but the different thread diameter means it cannot be used.

The thread diameter on this crank arm fixing bolt is about 18 mm, which is different from the 15 mm or 20 mm types that I have.

As a replacement, I got these M18 aluminium crank arm fixing bolts. I could not decide on a colour so I got both blue and black.

Problem is, the new aluminium crank arm fixing bolts do not fit, as the flange diameter is larger than the stock plastic flange diameter. I could grind it away if I wanted to make it fit...

Comparing the different flange diameters. This is not a dimension that is specified, so I would not have known until I have them in hand to compare.

The original plastic crank arm fixing bolt has a flange diameter of 25.7 mm. Any larger and it will not fit inside the left crank arm.

In the end, I decided to get the special tool to fix the original plastic crank arm fixing bolt. I found that this crankset is made by Prowheel, and uses the Prowheel bolt and tool.

Prowheel tool for crank arm fixing bolt.

The tool fits nicely! Now I am able to tighten the crank arm fixing bolt properly.

Key takeaways from this crankset study:

1) Spindle length is longer than usual, requiring the use of bottom bracket spacers.

2) Stock chain ring is thick and made of steel, making it unusually heavy.

3) Crank arm BCD and 4 arm angle is unusual and a replacement chain ring is not available.

4) Crank arm fixing bolt uses the Prowheel standard, and a Prowheel tool is recommended.

Overall, the items I swapped out from the stock setup would be the bottom bracket and spacers, as well as the chain ring bolts. The other parts shown here will be reused as they are of good quality and in good condition.

Bestrider Pump Pro 20: 10 Speed Drivetrain

When I was deciding on the drivetrain to use on the Bestrider Pump Pro 20, there are many considerations, since this is a kids bike. The stock drivetrain has a 8 speed semi-Shimano drivetrain, which is actually pretty good for kids bikes. However, I wanted something nicer than the 8 speed drivetrain.

On the previous Bestrider Pump Pro 16 kids bike, I took extra effort to make sure that the 3 speed drivetrain is easy to use even for kids. It was a unique, one-of-a-kind 3 speed drivetrain for 16 inch kids bikes. Now, on this new 20 inch kids bike, the shifters will be changed from the Revoshifter (good for small hands) to a more standard trigger shifter.

One of the prerequisite to operate a trigger shifter is a hand size that is big enough and strong enough to push the shifting levers. Usually, this only becomes feasible from the age of 6 onwards.

Another important feature for kids is the Optical Gear Display (OGD) on the shifter. This helps them to know which gear they are in, and understand how to shift gears depending on the terrain. Problem is, many high end flat handlebar shifters (such as 11 speed road or MTB shifters) do not come with OGD. Whereas for other shifters such as the Sora 9 speed shifter, it has an OGD but no numbers on it, which is basically pointless. The stock Shimano Altus SL-M310 8 speed shifter is actually good because it has gear numbers clearly shown on the OGD.

If possible, the shifting force and shifting stroke for kids should be minimized, to make it easier for their small hands and short fingers to push the shifting levers. From what I know, to have shorter shifting stroke, a high end shifter such as Deore XT or XTR will be ideal. At the same time, the shifting force will also be lower as the construction uses sealed bearings inside for smoother operation.

Thus, I was looking for the best shifter that fulfills all these conditions:

1) Light shifting force

2) Short shifting stroke

3) Optical Gear Display with numbers

The ideal candidate for this is thus the Deore XT SL-T780 Trekking 10 Speed Shifter. This model has been out of production for many years, but I still have it from old stock. It was first used on the Polygon Cozmic CX3.0 MTB, then the United Trifold when I was experimenting with multi-speed drivetrains.

Deore XT SL-T780 Trekking Shifter for 10 speed MTB drivetrains. Rare item nowadays!

I like that each gear position is accurately indicated by a silver line.

Premium aluminium lever. It has Instant Release, 2-Way Release, and Multi-Release features, which is not necessary.

Weighs 126 grams without the shifter inner cable.

Since I have decided to go with this 10 speed shifter, I have to find the other 10 speed components to match the shifter. This includes a 10 speed cassette, 10 speed MTB rear derailleur, and a 10 speed chain. It is harder to find high end 10 speed MTB components nowadays, since MTB drivetrains have mostly moved to 11 and 12 speeds a few years ago.

Therefore, once again I need to look towards older models for high end 10 speed components. 10 speed MTB components are available at a few levels, such as Deore, SLX and Deore XT. I chose to get a Deore XT cassette to save a bit of weight, and to have a more corrosion resistant cassette.

Deore XT CS-M771-10, which is a 10 speed cassette. I got the 11-36T cassette for a wider gear range, and to be compatible to the rear derailleur as shown below.

Nice and shiny new cassette! I like the sand blasted finish which gives it a premium look.

Larger sprockets are 36T, 32T, 28T and 24T, which are nice gear steps.

Being a Deore XT cassette, the largest 6 sprockets are mounted on 2 aluminium spiders to save weight.

Close up look at the larger sprocket set, with 28-32-36T sprockets.

Next 3 sprockets are also mounted on a spider to save weight.

Entire 11-36T 10 speed cassette weighs 308 grams, which is pretty good.

In order to match the 10 speed shifter that I have chosen, I need to find a compatible 10 speed rear derailleur. As the shifter is a MTB / Trekking type of shifter, I need to use a MTB or Trekking 10 speed rear derailleur, due to the cable pull ratio.

If I just get a standard 10 speed MTB rear derailleur, the cage will be too long for small wheel bikes. A 10 speed road rear derailleur such as Tiagra or the older 105 is not compatible, which is a shame since those road rear derailleurs have short cages which are ideal for small wheel bikes.

Using my previous experience with the United Trifold, I know that there are short cage 10 speed MTB rear derailleurs, such as the Shimano Saint or Zee. I no longer have the Saint rear derailleur, so I had to buy a new one. Decided to get the Zee rear derailleur instead, and managed to find a brand new unit even though it has already been discontinued.

Shimano Zee RD-M640-SS Short Cage 10 speed rear derailleur. There are two specifications, and I needed the one with a larger 36T rear sprocket compatibility to match the 11-36T cassette.

Brand new Zee rear derailleur! Shadow construction for a low side profile.

Uses a stamped inner link, instead of an aluminium version on the Saint rear derailleur.

It is also equipped with a one-way clutch with on-off switch, to minimize chain slap on bumpy terrain.

Check out the super short cage! Only compatible with front single drivetrains.

Cage length is only 50 mm from pulley to pulley, which is way shorter than even Dura-Ace and Ultegra road rear derailleurs.

Weight is 267 grams, which is pretty chunky for a short cage. This is due to the beefy construction to make the derailleur more resistant to knocks and bumps.

I realized that there is no need for the one-way clutch feature, since this kids bike is not going off-road on bumpy terrain. Same as what I did for the Saint rear derailleur on the United Trifold, I also removed the clutch unit.

The one-way clutch parts can be removed, leaving this area empty. It will still work perfectly fine, just without the clutch feature.

Clutch parts removed from the Zee rear derailleur.

After removing the clutch parts, about 16 grams is saved. Weight is reduced to 251 grams.

Main purpose of removing the clutch is not to save weight, although that is a nice bonus. Rather, the main objective is to reduce the shifting force. As the clutch inadvertently provides resistance during inwards shifting, this translates to a higher shifting force. By removing the clutch, the rear derailleur cage is able to rotate more freely, with just the spring force and no clutch resistance. The result is lighter shifting force at the shifter, which is the main objective.