It is difficult when children are growing quickly to keep them in the optimal position on their bike, and a poorly set up bike is no fun to ride. It’s not practical to buy a new bike every 6 months, so here is some advice to help parents/guardians balance cost with injury prevention, performance and comfort. There are entire books written on bike fit, so this is just a rough and ready guide. But if you follow these principles, you will do more good than harm. All of these principles apply to your own bike as well!
Bikes can be adjusted around their frames by changing stems, seat posts, handlebars, cranksets and saddles, and these relatively minor and inexpensive changes can make a big difference in the fit. Please bear the following points in mind to help your rider stay healthy and happy on their bike.
1. The most important objective of a bike fit is injury prevention. The most common cycling injury due to bike fit (as opposed to falling off) is knee trouble. That’s followed by lower & upper back, neck, hands/wrists and shouldersTo look after riders’ knees there are three simple things you can do. Given the rate at which children grow, you should check these regularly.
a) Ensure the rider’s saddle position (fore/aft) is adjusted so they are sitting toward the back of the saddle rather than on the nose. This is very important, as the soft tissue of the perineum is not designed to carry our weight. When they are in the right position on the saddle, check that their knee is over the pedal axle when their feet are level (3 o’clock & 9 o’clock). You can see in Figure 1 that a plumb bob dropped from the base of the knee should pass through the axle of the pedal. The saddle has horizontal rails, and you can slide it forward and back to change its position under the rider. If the rider’s knee is ahead of the axle, it pushes their knee cap back into their leg bones. If the rider’s knee is behind the axle, they are pushing out rather than down on the pedal stroke, and that puts extra strain on the knee tendons.
In some cases, the bike’s geometry will not enable you to get the rider’s knee in the optimal position even when the saddle is at the extreme forward position on the rails. In that case, you may need to buy another seat post which will position the saddle further forward. Most seat posts have saddle clamps that are “set-back”, but some seat posts have “in-line” clamps, and those will allow the rider to get further forward. (see Figure 4 – in-line and set-back seat posts) As the rider grows, you can go back to the original set-back post.
b) Ensure the rider’s saddle height allows them to extend their leg to an effective angle. We look for leg extension between 140 – 150 degrees, and refer to Figure 2 to see how it is measured. At Velocity Sports Cycling, we video the rider and use special software to measure the angle while they are riding. You can either buy a “goniometer”, like to one shown in the photo, or just approximate the proper angle. Please note: a) and b) affect each other, so you may need to iterate a couple times between them to get the final position.
c) Our feet do not all point perfectly straight ahead when we walk. They may toe-out or toe-in. If it’s a serious young rider, and they are using cleats, the rider’s cleats need to be positioned on their shoes to make their feet point in their natural orientation. That may be straight ahead or slightly toe-in or toe-out. So, determine whether your rider’s feet toe-in, toe-out or are straight ahead, and then adjust the rotation of the cleats on their cycling shoes to allow their foot to adopt the same position on the bike. Figure 3 shows two extreme examples, and you have to ensure you do not over-adjust and end up with the rider’s shoe rubbing against the crank arm.
2. The most common problem I found in the bike fittings we did at Slipstreamers club (a cycling club in west London for riders from 6 – 16 years old) was the rider’s reach from the saddle to the handlebars. This can affect their safety as well as cause back, shoulder, arm, wrist and saddle sore issues. In the effort to buy a frame that will last a while, many of the riders were on bikes too long for them. So, they were stretching too far to reach their handlebars. Often, they moved forward on the saddle to shorten the reach, and that moves their weight from their sit bones on the rear of the saddle to their soft tissues at the nose of the saddle, which as I said before is not a good thing. The riders also had straight arms, which doesn’t allow good control of the bike. You want their elbows to have a slight bend when their hands are on the hoods, and their shoulders should be relaxed.
Get their knee over the pedal axle first, and then change the reach to allow them to stay in the right position relative to the pedal axle and sitting on the right part of saddle. You don’t want them scooting forward on the saddle to reach the bars. Bring the bars back to them. You can effectively shorten the bike and reduce the reach by fitting a shorter stem. Figure 5 shows some different length stems, and they generally range from 30mm to 125mm in length. If the frame is relatively large for the child, then you should look at buying a shorter stem to reduce the reach. You can move to progressively longer stems as the child grows.
Finally, If you do want a detailed book on Bikefit, then I can recommend a book written by the head physio at British Cycling and Team Sky, Phil Burt. http://www.amazon.co.uk/Bike-Fit-Optimise-performance- avoidance/dp/1408190303 You can also visit my website http://velocitysportscycling.com/bike-fit/ for more practical information and bike and saddle fitting.