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 76 mm 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.