How To Set Up And Measure For Correct Bike Size
- Information from Cycling Queensland
- Here is an extract from an article on the web site of Malcolm Firth, Senior Coach, Association of British Cycling Coaches
- Basis Systeme netzwerk contains a number of calculators including one that can help you find the right frame size for you.
Setting up your bike
Correct set-up on a bicycle will maximise a cyclist's energy efficiency, comfort and performance potential. Incorrect bike set-up can actually lead to muscle strains and overuse injuries in knees, ankles, hips and back. Correct set-up can be achieved by following these guidelines.
The Frame
A correctly sized frame will give you a balance between height and length, as well as between responsiveness and comfort. Factory-built bikes are made to a formula which suits the average shaped rider. People with long legs and short bodies (or the reverse) can find it difficult to obtain a stock frame which suits them. In this case, the rider should have his/her frame customised for them by a frame builder.
Measuring Frame Size
Dress in a pair of riding knicks and then socks with no shoes. Take your inseam measurement (crotch to floor). To do this, place a thick spined book between your legs, as if to use it as your saddle. Standing on a hard floor surface, face a wall and mark a line along the top of the book edge touching the wall. Stand up straight when marking. Using a tape measure, or ruler, measure the distance from the floor to the height of the mark. This is your inseam measurement (NB: take the measurement 4 or 5 times and average it). Now substitute the measurement into the following formula: o Road and Track Frame Size (cm) = inseam (cm) x 0.65 o Mountain Frame Size (cm) = inseam x 0.56 Road and track frame measurements are taken from the centre of the bottom bracket to the centre of the top tube. MTB frame size is measured from the centre of bottom bracket to the top of top tube. Please note, these measurements are a guide only and will form a good starting point. Variations will depend on shoe and cleat type.
Saddle Height
Overall seat height is perhaps the single most important measurement for determining correct positioning on the bike. Incorrect saddle height can result in a less efficient pedalling action. Being too low on the bike will cause you to bunch up on the bike and can restrict the full contraction of your leg muscles. Being too high on the bike will result in lost energy through rocking on the saddle and can cause over-stretching of the muscles. There are two methods by which the correct seat height can be determined: the Heel Method and the Calculated Method.
1. Heel Method
The easiest way to …. Is the heel method. In a normal riding position, turn the cranks until they line up parallel with the seat tube of the bike frame. Now, position the centre of the heel directly over the centre of the pedal axle. Adjust seat height up or down until leg appears fully extended. NB: If the hell of your shoe is thicker than the sole under the ball of your foot, you will need to lower the seat accordingly (eg. If heel is approximately 3mm thicker than the shoe at the ball of the foot, lower the seat by an extra 3mm.
2. Calculated Method
Use the same inseam measurement taken for frame sizing. Substitute the inseam measurement into the following formula: All on-road cycling (Clips & toe straps): Saddle height = 1.07% of your inseam = 1.07 x inseam (cm) All on-road cycling (Clipless Pedals – Time, Look, Shimano & clones) Saddle height = 109% of your inseam = 1.09 x inseam (cm)
Fore & Aft Seat Position
Adjusting position: Loosen seat bolt to slide seat back on its rails as far as it will go. Re-tighten seat bolt. Make a plumb bob – tie a key or squeeze a glob of "Blu-tak" to the end of a 1.5m long piece of string. On a level surface, lean the bike against the wall and sit in your normal riding position with feet at the 3o'clock- 9 o'clock position (ie. Cranks parallel to the ground). Have someone drop the plumb bob down from the tibial tuberosity on the forward knee (this is the small bump just below the knee-cap) to the floor in between the cranks and frame. Position the foot parallel to the floor. Adjust saddle position forward until plumb line falls directly through or just slightly (1-2 cm) behind the centre of the pedal axle. Re-tighten the seat bolt. Check to see that the saddle is perfectly level. Check this with a spirit level or lean bike against a brick wall and check against the brick lines (make sure floor is level).
Upper Body Position
Upper body extension is very much a personal thing. There are no rules for correct positioning - just general guidelines. The goal for most racing cycling is to find a healthy balance between comfort and aerodynamics.
1. Head Stem Length
Whilst in the racing position on your bike, looking directly forward, have someone drop a plumb line from the tip of your nose. Ideally, the line should fall directly through or within 1-2cm behind the centre of the point where the handlebars connect to the stem.
2. Head Stem Height
Again this depends on your height and frame size, however, a good guideline is for the top of your head stem to be 5-8cm below the top of your seat. Lowering the head stem even further will improve your aerodynamics, however, it may feel more uncomfortable to those who are less flexible in the lower back, hips and hamstrings – it's a personal decision.
Cleat Position for Clipless Pedals
1. Position the cleat so that the pedal axle lines up directly underneath the ball of the foot. This is the point where you will get the greatest transfer of energy. 2. Provided you do not have any structural/alignment problems with your feet, they should point directly forward when in the pedals (parallel to the crank arms). This is called the "ZERO" position. Should problems exist with the feet DO NOT attempt to straighten this out. Set the cleats to accommodate this characteristic rather than correct it. It is believed by many that the locked-in position that occurs on the pedals, particularly with clipless pedals, is responsible for overuse injuries in the knee, ankle and hips. To avoid these injuries most new clipless pedals offer a floating cleat which allows a few degrees float in both sideways directions.
Malcolm Firth, Senior Coach, Association of British Cycling Coaches
".......The two main measurements required for a correctly sized road frame are the seat tube and top tube lengths. These, together with the seat tube angle are the parts of the frame most influenced by your body size. Top tube length is generally taken to be the horizontal distance from the centre line of the head tube to the centre line of the seat tube, whilst seat tube length is the straight line distance from the centre of the bottom bracket to the centre of the junction between the seat tube and the top tube.
To estimate the correct seat tube and top tube lengths for your road bike several measurements of your body and limbs will need to be taken. The first of these is your inside leg measurement. This is taken with you in a standing upright position, without shoes and with your feet about 25cm (10") apart. The measure is then taken vertically from floor to crutch (the point on the crutch where the measurement is taken from is the bony protuberance known as the ischial tuberosity - the lowest part of the pelvic bone - that you sit on). The seat tube length is calculated as being 2/3rds of your inside leg measurement. Thus, if your inside leg measurement was 84cm (33") the seat tube length would be 56cm (22").
The top tube length of your bike should be proportional to a combination of your trunk length and arm length. For these measurements you need to be seated with your back pressed firmly against an upright surface such as a wall. Trunk length is measured from the seat you are sat on to the top of your shoulder with you sitting fully upright. The forearm length is measured from the back of the elbow (with the back of the elbow pressed against the upright support and with the forearm horizontal) to the centre of your clenched fist. Total arm length is obtained with your arm straight out in front and measuring from the upright support to the centre of your clenched fist (keep your back firmly against the support). The measurements are used in the following equation: top tube length = 75.25% trunk length + 7.8% forearm length + 7% total arm length - 1cm (1/2"). The answer should be rounded out to the nearest 1/2cm (1/4").
For the seat tube angle you first of all need to measure your thigh and lower leg lengths. Thigh length is measured horizontally from the upright support to the front of your kneecap, whilst the lower leg length is measured from the top of your kneecap vertically to the ground. Then calculate the seat tube angle as: Cosine of 26.4% (thigh length / lower leg length). The answer should be taken to the nearest 0.5 degree.
It is worth remembering that the calculations used to estimate frame size are generalisations based on average figures calculated from measurements on a wide range of riders. Within those averages there are variations typical amongst a group of people. Therefore, no formula can give the perfect frame size and riding position for every rider. The final, and most important check will be for you to try out the position you have arrived at during several rides of increasing distance and severity. Only then can your riding position be given the fine tuning that will be needed to make it the best one for you........"
And a few of his comments on fitting TALL RIDERS.
"A few other observations. Tall riders generally have longer legs in relation to their body length than do average height riders. For example, I am currently coaching a young rider who is 6ft 3ins tall, whereas I am 5ft 9ins. As you can imagine, when we stand his head is much nearer the clouds than mine. Not so when we sit, there is less than 2ins difference in sitting height. Of this extra leg length in tall people, the thigh is often a greater proportion of the total than with average height people. This means that to get the knee above the centre of the pedal spindle when the cranks are horizontal, tall riders with very long legs usually have a shallow seat angle. In the case of your rider it sounds like the ratio of his thigh length to lower leg length is about 1.3 to 1, whereas the average height rider has a ratio of about 1.1 to 1 (see "Road Racing" by Hinault and Genzling). As a contrast, I once met a rider who was 6ft 4ins tall with a thigh to lower leg ratio of about 0.9 to 1. For him to get his knee over the pedal spindle he used a seat angle of 75 degrees.
Tall riders with long legs and short bodies can sometimes encounter problems with backache. For example, a combination of long thigh, shallow seat angle, short body and the handlebar stem well down in the head tube can mean a more acute angle between thigh and body when the leg is at the top of the pedal stroke. For some riders this can put excessive strain on the muscles on the back of the thigh that help drive the thigh downwards, leading to backache. I seem to remember Alexi Grewal (the American rider who won the 1984 Olympic road race) saying that he encountered this problem and only solved it when he went to a steeper seat angle than had been previously suggested from his body measurements. This just goes to illustrate that you cannot go by formulas alone when dealing with individual riders. They just help you get into the right ball park, after that you need to find out whereabouts in the ball park suits you best."