Kids Bike Components PART 1 - selecting Framesets and Cranksets

We all want our kids to be as comfortable and confident on their bikes as possible. If speed is their goal, they will only go quickly when they are! It's not rocket-science, but it is a reasonably in-depth subject and so many different components influencing it. So if you are planning to build a new bike or you want to improve their existing bike, here is Part 1 of 3 on the basics of correct component selection to achieve the best bike fit...

• Frameset
• Crankset
• Handlebars & Stem
• Saddle & Seat Post
• Gear Shifters & Brake Levers
• Pedals & Cleats
• Wheels, Tyres & Gearing

Part 1: Frameset and Crankset

The combination of all the key components listed above, define the location of the 3 contact points for any rider: their feet, bottom and hands. By selecting the correct components, you can achieve the best bike fit for your rider which will maximise their comfort, confidence and power.

(It's worth reading this, in conjunction with our advice on BikeFit and what you are trying to achieve with the correct components - CLICK HERE). So let’s take a look at each of the key components in turn, starting with;

Frameset

It might seem trivial, but it all starts with the frameset, no amount of clever components will make up for a frame that is too big, or too small. Why are adult XS frames no good?

Generally, small adults are short in the leg and long in the torso, hence the vast majority of adult XS bikes may have a low-standover, but they will also have a long top-tube. This is a real problem for children, as they are usually long in the leg and short in the torso, due to the order in which their bodies grow. Whilst they can stand over the top-tube, they end up in a permanent ‘time-trialling’ position, due to the length of the top-tube, heavily compromising comfort, confidence and often power too.

We only stock the unique HUPcc range of short top-tube framesets (beware of 'similar yellow' framesets that have 51cm top-tubes for 7 year olds!?!?). The HUP evo, straatrace, azure and enduro frames are the only brand with sizes and the correct geometry specific to youth and small adult riders - CLICK HERE for Framesets.

With the correct frame size identified, component selection is relatively easy. Get the frame size wrong, it will be a constant problem, namely pain in the small of the back and the hamstrings from over-stretching. Not all manufactures supply precise sizing data, we do for each model - listing a minimum inside leg dimension and a typical height range.

Top Tips: carefully check a framesets 'Standover Height' & 'Top-Tube Length' before buying! Sizing Guide Here...

Crankset

Children have narrow hips, so it’s important to take this into account when selecting a suitable crank and bottom bracket (the BB is the main crank bearing). Cranks with a narrow Q-factor (width between the inside of each crank arm) achieve as straight a leg as is possible whilst pedalling. There is more to consider however, as whilst the length of the bottom bracket axle affects the Q-factor, it also affects the chainline (alignment of the chainrings, to the cassette and the resultant angulation of the chain itself). Ideally, you want to line the chainring up with the middle cassette sprocket, for an equal chain angle at either end of the cassette, to maximise durability and smooth operation of the rear derailleur.

BB selection is a balance between both considerations - Q-Factor and Chainline. Often just inboard of the the middle sprocket is good to achieve the minimum Q-factor, however the style of riding has an influence. If the bike will see plenty of climbing, then a chainline slightly inboard of the centre sprocket would be fine, but for a road bike seeing lots of fast pedalling in high gears, a chainline slightly outboard of the centre sprocket would be sensible.

Just like adults, the crank length is very important for children. Cranks designed for adults tend to start from 165mm and upwards. As a general rule, for power and comfort you need to be sizing a riders crank length at approx 10% of their height. So, with the shortest adult cranks at 165mm long, riders tend to be in the U14’s before they are suitable.

U8-U12’s need child-specific cranks and often the answer comes from the BMX world, where manufacturers tend to produce cranks in a range of lengths. We tend to work on a formula of 10% +5mm, to allow for future growth. For example, we would usually specify a 145mm long crank for a 140cm tall U12 rider. 

Something to bare in mind is the Bottom Bracket height of the frameset, the distance from the floor to the centre of the Bottom Bracket. In general, off-road bikes will have a higher bottom bracket height than a road bike. Therefore, its common to see a child on the correct length cranks and correct saddle height, but they still can’t touch the floor whilst seated. Certainly in racing circles with experienced riders, this is very common. It’s not recommended for general cycling with inexperienced cyclists!

Another important consideration for cranks, is the BCD layout (chainring bolts layout). Typical road cranks are 110mm bcd, 5-bolt. Typical MTB cranks are either 104mm bcd or 94mm bcd 4-bolt. However, the important consideration for kids is to be able to fit the correct size chainrings for their cycling discipline, especially if riding on larger 700c wheels. Historically the problem with the widely available 110mm bcd 5-bolt cranks from the BMX world has been the limit of the smallest chainring you can fit it is 34T, too big for most U8-U12 riders in Cyclocross and MTB in particular. To solve this, we developed our own #HUPcc Kids Cranks using a narrow Q-factor (road derived) design, but using a 104mm bcd 4-bolt layout, allowing the fitment of 30T chainrings (30T, 32T, 34T, 36T & 38T chainrings) so it can be used to create the correct gearing for all age groups and cycling disciplines.

We also have modified SRAM Apex cranks, made-to-order for your rider, with a narrow Q-factor and lightweight hollow tech axle for double chainring set-ups, which fits modern 86mm wide bottom brackets often found on full carbon framesets - CLICK HERE..

Top Tips: Crank Length (cm) = Height (cm)/10 + 0.5cm. Use as narrow a Q-factor as the chainline will take. We use HUPcc cranks with 107.5mm bottom brackets for smaller riders and 113.5mm bottom brackets for larger riders.

(It's worth reading this, in conjunction with our advice on BikeFit and what you are trying to achieve with the correct components - CLICK HERE)