Latrobe Valley Triathlon Club

Bike Fitment Guide

Bike sizing guidance

Choosing the correct size of bike is essential if you are to feel comfortable when riding, it will also make the bike more efficient and easier to handle. Within this guide we will attempt to give basic guidance on choosing the most suitable frame size for you.

How do you choose the correct size bike?

A good starting point for you, a new cyclist, is to measure your inside leg which will determine your stand over height. The stand over height is the clearance between your crotch and the top tube of the bike when stood just in front of the saddle and comes into play when you need to dismount quickly; this is more likely on an MTB so we would suggest at least 1", but recommend 2". As well as your inside leg length you should also measure your height, this gives you two points of reference to help choose the size of bike from the bike size charts, which are located near the bottom of this page. Bike frame sizes usually increase in 2cm increments for road bikes and 2” increments for mountain bikes, so get someone else to measure you carefully as accurate measurements are essential.

How do you determine your reach?

As a bike seat tube length, or frame size increases so does the top tube and head tube length to keep the bike in proportion. So, if you find yourself in between sizes based on your height and inside leg length the upper body should be the deciding factor, because your reach to the bars will be affected. To help work out if you have a long or short reach you need to determine your ‘Ape Index’ this is your arm span (finger tip to finger tip) minus your height. If you have a positive 'Ape Index' (your arm span is greater than your height) then go for the larger of the two sizes. If you have a negative 'Ape Index' (your height is greater than your arm span) then go for the smaller of the two sizes.

Correct frame size

If you already have a bike that you are comfortable on you can measure the frame size. The two most commonly stated frame measurements are centre to top (a to c on the diagram to the right) or centre to centre (a to b). These measurements refer to the centre of the bottom bracket axle to either the top of the seat tube or middle of top tube in line with the seat tube. Other dimensions to take into account are the top tube and head tube, these determine your reach and your handlebar height. All dimensions have a certain amount of adjustment, saddle height and set back is adjusted via the seat post, handlebar reach is adjusted with stem lengths and saddle fore/aft adjustment, whilst handlebar height can be adjusted via spacers or different angle stems. But, it is essential you get the most appropriate frame size so that all these dimensions can be achieved. Manufacturers can state either measurement on your frame, so always check which they have used when viewing their range. When comparing frame sizes you should also take into account the style of frame, especially road bikes which come in traditional, semi compact or compact geometries

Glossary of frame terms

Seat tube: The frame tube that the seat post fits into and runs from the bottom bracket to the top tube and seat stays. Will often feature bottle cage bosses and have the front derailleur mechanism attached.
Top tube: The frame tube that runs from the head tube to the seat tube.
Head tube: The shortest frame tube on the bike which the forks steerer tube fit through
Seat stays: The two frame tubes which run from the top of the seat tube to the rear drop outs and chain stays.
Chain stays: The two frame tubes which run from the bottom bracket to the rear drop outs and seat stays.
Down tube: Usually the largest of all the frame tubes and runs from the bottom of the head tube to the bottom bracket. Will often feature gear cables and bottle cage bosses for fitting a bottle cage.
Forks: The two tubes that run through the head tube and attach to the front wheel.

How do you choose the correct size kids bike?

Choosing the correct size of bike is classed by age as opposed to height. Kids are still learning to ride, so it's important the bike you buy them fits correctly and is not too big! This is a common mistake in parents wanting to stretch out the life of a bike. The fact remains that when a child is learning, they often need to step forward off the saddle, so they straddle the top tube. It's important the frame isn't so big they risk hurting themselves when this happens.

You know your child best. If he/she is tallest in the class and is at the top of a size range, then go for the next wheel size up. If your child's age is in the middle of the range, try and resist the urge to 'upsize' them to get extra life out of the bike. This will be counter-productive to the childs enjoyment of learning to handle a bicycle.

Time Trial (TT)/Triathlon bike positioning

Time Trial and Triathlon bikes are slightly different with regards to sizing, the aim of a TT/Tri frame is to get a flat back and reduce your aerodynamic drag. A TT or Tri bike will have a slightly steeper seat tube angle; this moves your saddle position forward so the angle between your torso and thighs is not reduced excessively. The reason for this is because if your thighs come up too high you will lose power, so by moving the saddle position forward and your bars being lower you do not lose any power, but gain an aerodynamic benefit. The diagram (right) shows the two positions, notice how the TT/Tri position results in lower front end whilst retaining the angle between thigh and torso, you will also see how the point of contact with the saddle is further forward while the reach to the handlebars is more or less the same.

Everybody keeps asking, “should I get a Triathlon Bike or a Road Bike?” Well, here I’ll explain (and show) the differences, and attempt to convey the benefits and drawbacks of one versus the other. In following posts I’ll help you walk through the actual bike buying experience.

There are three major differences between a triathlon bike and a road bike. One difference, the frame geometry, is what actually determines the literal definition of a Triathlon Bike versus a Road Bike. The other two, aero bars and wheel size, are also a part of the whole equation.

The true deliniator between a “Triathlon Bike” and a “Road Bike” is in the frame geometry. The frame is the core of a bicycle and the main part to which all of the other components are fitted. The frame geometry refers to the angles and shapes of the frame’s tubing. An image from a similar article on the Bike Sport Michigan website is helpful in showing this difference:

This standard frame design is referred to as a diamond frame. As you can see, each of these frames is comprised of two triangles sharing a common tube–the seat tube. The angle of the seat tube differentiates a Triathlon Bike from a Road Bike. The image above clearly shows the difference between a standard Road Bike, which typically has a 72°-73° seat tube angle, and a Triathlon Bike, which will have a seat tube angle of 76°-78°.

What effect does the seat tube angle have on a rider? Well, the steeper the seat tube angle, the more effort is exerted by the cyclists hamstrings compared to his or her quadriceps. That means a Road Bike allows you to use your bigger muscles (quadriceps) more and therefore you can exert more power relative to a Triathlon Bike. It also means, when you get off the bike, your quadriceps are more tired. No big deal if you are done for the day. Big deal if you still have to do some running.

To make up for the reduction in power that a steeper seat tube creates, a Triathlon Bike will typically have one or both of the following compensating features: aerobars and smaller wheels.

Aerobars serve two major purposes. First, they put the cyclist in a more aerodynamic position. This drastically reduces wind resistance at higher speeds. Second, they allow the cyclist to ride in a more restful position. Aerobars are almost a necessity for any triathlete. It’s important to note, however, that aerobars can be installed on nearly any type of bike by using clip-ons.

The main difference between built-on and clip-on aerobars is the positioning of the shifters. You will have bar-end shifters on built-on aerobars, as shown in the picture below. Notice the shift levers at the end of the aerobars and the brake levers at the end of the handlebars. This is designed for racing efficiency; you do not have to come out of the aero position to shift. This also means that you cannot brake and shift from the same position. Riding with this style of aerobar is not for beginners. Built-on aerobars now come standard on most mid- to high-end triathlon bikes.

The following picture depicts clip-on aerobars. As you can see, these mount directly to your existing handlebars and allow you to ride in the same aero position as with built-on aerobars. The main difference you’ll find with clip-ons is that they do not have bar-end shifters. You shift and brake from the standard position on your handlebars.

Learning to ride in the aero position is a bit tricky. You give up quite a bit of control with regard to both maneuverability and functionality (shifting and braking), but the results typically worth the effort.

If you decide to put clip-ons on a standard geometry road bike, you’ll probably need to adjust your seat forward and reduce your stem length. Otherwise, you will be much too stretched out, resulting in reduced power and increased fatigue, and negating any of the benefits of the aero position.

First, if you’re learning something from this post, you’re probably not fine-tuning for the 12th leg of the Tour de France. In other words, you don’t have a different bike, helmet, shoes, sun glasses, etc. for each ride you take. If this is the case, and you have the choice, I suggest sticking to 700c wheels.

So there you have it. You now understand the fundamental differences between triathlon and road bikes.