Feature: Fitting Bicycles to People
In this article, long serving CAMWEST member Danny Hannan shares his views on how to fit bicycles to people. (Naturally, if something starts to hurt, something's not right. So stop riding, and perhaps consult your doctor).
Danny discusses fitting bicycles to people, frame designs, big wheels, small wheels, and suspensions.
He discusses and debunks a few myths, folk-law, and misconceptions.
Fitting a Bicycle to a Person
The most common complaint of cyclists who ride "conventional" bicycles is a sore bum — finding a comfortable seat. The second is sore, tired, or numb hands. The third is a stiff/tight neck and shoulders.
These are all symptomatic of problems that are much more related to the positioning of the seat and may or may not be related to the bicycle seat itself.
If you follow the procedure outlined below you should be able to get close to your optimum riding position — if your frame allows it.
Lets start with the seat itself.
People come in all shapes and sizes but the important parts to fit onto a bicycle seat are your sit bones (Ischium). The rear wide part of the bike seat must be wide enough to comfortably and easily accommodate your sit bones with at least 2cm spare on each side. Measurements of adult sit bone separation that I have taken range from 10cm to 14cm centres. For the widest case this requires a seat of some 18–20cm wide (they do exist).
To measure your sit bone separations use a hard surfaced chair topped by a sheet of white paper and a sheet of carbon paper, carbon side to the white paper. Sit on the chair only with your sit bones and sit heavily. This will leave a faint imprint of your sit bones on the paper. Mark the centres of your sit bone marks and measure the separation. You will need a seat with the main sitting area at least 4cm wider than your sit bones. Anything narrower will act a wedge separating your sit bones and causing pain. A seat that is too narrow will never be comfortable.
Also the rest of the seat needs to be of a shape that comfortably accommodates the soft area between your legs. This is different for different people and the only way to find out is by trial.
The softness of the seat is almost irrelevant to comfort. The size, shape, and position are far more important to comfort than softness. Firmer more supportive seats are often more comfortable for the long haul.
The importance of seat position
Fitting a bicycle to a person is prefaced on getting the seat position right for that person. If that is not achieved first, the rest is not relevant.
The seat position is quite complicated and is determined by a combination of factors.
The seat height is the longest measurement from the top of the pedal (at the furthest point from the seat) to the top of the seat and your requirement remains constant no matter what crank length you use.
If you use clip-in pedals, the position of the cleat will affect your seat height, as will the sole thicknesses of different shoes. Millimetres will make a difference.
There is a hollow just behind the centre of the ball of your foot (metatarsal/phalanges joints). The centre of the cleat should be located directly under this hollow and to maximise the stability of your foot on the pedal, the cleat should be located as far to the outside (lateral) edge of the shoe as possible. Some cycling shoes do not have the cleat position in the right place on the shoe for this to be achieved. So check that you can achieve this cleat position before you buy the shoes. There may be some minor adjustments (0.5–2mm) from this position as well as rotational adjustments to the cleats to allow the natural angle of each foot on the pedal (pronation) to be achieved. This places the ball of your foot 1–3cm in front of the pedal spindle.
Myth 1: Ball of the foot over the pedal spindle — Challenged
Your leg and foot length largely determines the height of your seat but two people with the same leg and foot lengths may require different seat heights due to differences in the ratio of lengths of different parts of their legs and feet. The old 109 formula (in-leg measurement hard into the crutch from the floor multiplied by 1.09) has serious shortcomings, but it's a good place to start. Then it's a matter of fine adjustments (2mm max at a time) up and/or down until you find the maximum seat height that you are able to pedal freely and comfortably without rocking your hips on the saddle. The seat hammering your crutch is an obvious indication that the seat is too high and pulling or tightness behind one or both knees indicates that the seat is too high but getting close. Most people have slight differences in their legs so this tightness may appear in one leg only but the seat is still too high for that leg.
Angle of Saddle tilt:
The saddle should be very close to horizontal and sometimes with the nose (front) slightly raised, but this will vary for different saddles, different people, and different handle bar heights. You will know if it's too high at the front due to discomfort and you will slide forward on the saddle if the nose is too low. A few saddles have a tension adjustment, which varies the amount of dish in the centre of the saddle. This may also need tightening if the saddle feels too high at the front but looks at about the right tilt.
Seat position fore and aft:
This is a most crucial adjustment for comfort and performance. With your pedals in the horizontal, 3 & 9 o'clock positions a vertical line from the front of your forward knee should intersect the instep of your forward foot. This vertical line from your knee will be behind your pedal spindle and possibly behind your pedal as well. The actual amount varies considerably between individuals and requires considerable trial and adjustment to find the right position.
Myth 2: Front of knee above the pedal spindle — Challenged
Unfortunately most modern bicycles have very limited adjustments in this area and are made worse by steep seat tube angles of 74 and 75 degrees (to fit bicycles into smaller boxes to reduce transport costs). The ideal seat tube angle to fit most people is about 71 degrees. The movement rearward of the seat is about 1cm per degree for a small size frame and more for large sizes. This means that modern bicycle seats are at least 4cm too far forward for most people to achieve a comfortable seat position. Unfortunately seat tube angles are rather difficult and expensive to adjust on existing frames. The answer is a frame, custom built to fit your needs.
This seat position is also important to locate your centre of mass between the seat and the bottom bracket. By doing this, a large amount of weight is transferred from your hands and onto your pedals so that your hands, shoulders, and neck can relax and you get more power to the pedals. This also allows you to reach comfortably much further forward because it much reduces the weight that your hands, arms, and shoulders need to support.
This is determined by the purpose of the rider and by some anatomical factors. We need to assume that you require maximum efficiency, highest speed with least effort. If you don't require high efficiency, put your handlebars where they suit you. But I find the most efficient position is also the most comfortable and visa versa.
The height of the handlebars in relation to the seat is largely determined by a rider's flexibility.
Sit on the floor with your legs together straight out in front of you, knees straight.
- Highly flexible:
- Able to reach your wrists to, or passed your toes.
- Able to reach your fingers to your toes.
- Unable to reach your fingertips to your ankles.
Some rough guides of thumb:
- Highly flexible:
- handlebars 0–5cm below the seat height.
- handlebars 0–5cm above the seat height.
- handlebars over 5cm above the seat height.
Again it's trial and adjust to find what suits you best.
You can see from this that most bicycles are made for very flexible athletes or to the cheapest price possible.
It is your trunk or body length and your flexibility that determines the distance from the seat to the handlebars, handlebar extension. The longer your body and the more flexible the further away from the seat the handlebars will be. As a guide, pedal your bicycle with no hands and reach as far comfortably forward while pedalling, as you can. This is much easier and safer to do on a trainer. The position you adopt is close to the right handlebar extension. For dropped road style bars you should be able to ride comfortably in the drops with your elbows bent. If not, your seat and/or bars are not in the right position for you.
The most knowledgeable person in Sydney on these matters is Steve Hogg, at Pedal Pushers, Randwick.
He does an excellent biomechanical analysis and bicycle fitting service, it costs, and it's worth it.
BICYCLE FRAME DESIGN
The first and most important dimension is the seat tube angle. This is the key to getting a correct fit. People vary in their requirements of seat tube angles from 68 degrees (rare) through 71–72 degrees (most common) up to 75 degrees (rare). It's also the most difficult dimension to determine.
From here it's almost academic. The rest is a compromise of competing factors and simple measurement and arithmetic.
This is the maximum height of the top tube of your frame from the ground.
You will need at least 2–3cm and better still 5cm clearance over your top tube from your crutch when standing over your bike. Subtract the desired clearance amount from your in-leg measurement and that is your standover height.
Bottom bracket drop:
This is the distance that the centre of your bottom bracket (BB) is below the wheel axle line. The more BB drop the more stable your bike is but this conflicts with pedal ground clearance. The smaller the wheels the less BB drop or less pedal ground clearance. This is a strong case for larger diameter wheels.
Pedal ground clearance:
This is the shortest distance from the centre of your pedal spindle to the ground.
75mm is about the absolute minimum (pedals contact the ground too often) and for normal riding 100mm is more than adequate (pedals almost never touch the ground). For trail riding as much as 120mm ground clearance may be desired but I would not recommend it. Crank length also plays a part here. Tall or longer legged riders benefit from longer cranks but this reduces their ground clearance using the same BB drop.
How much of each?
700C wheels have a radius of about 350mm with 32 tyres; 345mm with 25 tyres, 355mm with 38 tyres and 26in wheels with 1.5 tyres have a radius of about 317mm. 75mm BB drop is standard and with 700Cx32 tyres and 175 cranks you have 100mm ground clearance. The maximum BB drop I would recommend with 175 cranks and 700Cx32 tyres is 85–90mm giving 90–85mm pedal ground clearance respectively. Shorter cranks can have a correspondingly lower BB. Smaller tyres or wheels need a correspondingly higher BB. It's a compromise for the method and purpose of your riding. Touring, commuting and recreational bicycles are more stable with a lower BBs and pedal ground clearance is not a big issue; criterion-racing bikes and off-road bikes need good pedal ground clearance. It's your choice of compromise: Purpose of the bicycle, size of wheels, BB drop, crank length, handlebar type and the list goes on.
Crank length is only critical for high-level competition riders, but a reliable guide is people with seat heights of up to about 80cm do better on 165mm cranks and seat heights about 80–90cm, 170mm cranks and seat heights over about 90cm, 175mm cranks. There is a fair amount of grey area as well. You can get up to 180mm cranks and some road bike brands come in 2.5mm steps as well but these are getting scarce. If you change your crank lengths all of your, seat height, seat setback and handlebar position will need to change.
The size of your frame or the length of your seat tube is then roughly determined by subtracting the distance of your BB from the ground, from your standover height. That's rough but if you haven't yet got a frame to measure, from; the centre of the BB to the centre of the top tube along the seat tube it's a close estimate. You can get more technical and include trig functions to do an exact calculation if you are mathematically literate.
This is the distance from the centre of the BB to the centre of the rear axle. To maximise the handling qualities of a bicycle the weight should be distributed 50/50 front and rear. This is not achievable but an effort should be made to maximise it. The taller the seat height, the more laid back the seat tube and the lower the BB the longer the centre-rear measurement needs to be. This places the rear wheel further behind the rider getting better weight distribution front and rear. The back of the seat should be well in front of the rear axle. This may require a centre-rear measurement of over 500mm in the more extreme cases. Modern bicycles are built with a short centre-rear measurement to fit into a smaller box.
This is the distance from the centre of your BB to the centre of your front axle. This determines the position of your front wheel. It should be calculated for 700C wheels even if you are using smaller wheels. Using 700C wheels sets the bike up with the correct length frame for the rider to be comfortable but you can crib a bit if you are using smaller wheels. It is determined by the clearance you need from the toe of your shoes to the front wheel or front mudguard at the closest point. It's much better if your toe and front wheel do not overlap.
This is for a taller person and is an example only.
For 700Cx35 tyre with mudguards allow 365mm for the wheel, 175mm for the cranks and how ever far your foot over hangs the front from the pedal spindle, say 100mm. Centre-front is 365+175+100=640mm.
Steering set up
Head tube angle, range 69 to 74 degrees. The 69 degrees is stable at speed and in straight lines but gives slow handling, the 74 degrees gives very quick nimble handling. Both can be stabilised or quickened up by varying the rake (forward bend) of the forks. Increasing the rake quickens the handling and decreasing the rake slows or stabilises the handling. The steeper the head tube angle (74°) the less fork rake you need and the lower the angle of the head tube (69°) the more rake you need. I normally recommend about 72 degrees and 45–60mm rake, 45mm rake for high stability and 60mm rake for reasonably nimble handling. There are ranges of combinations that will give reasonably similar results. The determining factor is the purpose of the bike. For the more technically minded the head angle and fork rake determine the trail that should be about 45–65mm, 45mm being nimble handling, 65mm being more stable.
Once you have made all these decisions, the down tube connects the bottom of the head tube to the bottom bracket and the top tube connects the top of the head tube to the top of the seat tube. The lengths of these tubes are determined by the placement of the parts of the bike that are needed to provide a correct fit for YOU. You can follow this guide and have any shape frame; compact, BMX, mountain or conventional frame, you like and it will still fit YOU.
Myth 3: The length of the top tube has a relationship to the length of your forearm — Challenged
If you leave the steering tube long it will be easy to adjust your handlebars to the correct height for you, then trim what is not needed.
Choices are steel or aluminium more commonly and titanium or carbon fibre less commonly.
The only difference you can make to the weight of a bicycle that cannot be replaced is the frame. The frame makes up from as little as 10–12% of the weight of a bicycle. What you fit to the frame has much more effect to the overall weight of the bicycle than does the frame itself.
Myth 4: Aluminium frame bicycles are lighter — Challenged
The differences are what you want the bicycle for and how much you want to spend.
The reliability of steel frames makes them much more suitable for people who weigh 100kg and more.
Currently it is cheaper to mass-produce bicycle frames in aluminium but steel is easier for the construction of custom-built frames.
What are the advantages and disadvantages?
Many people think that aluminium frames are lighter than steel frames. But comparing like with like, a high quality steel frame and a high quality aluminium frame both with a lifetime warranty, there would be a very small difference in weight. The steel frame could possibly be the lighter.
The lightest aluminium racing frames are a little over 1kg and top-end steel frames are about 0.5kg heavier, however the aluminium frames only have 1–2 year warranties and the steel frames have 10 years to lifetime warranties.
Why the differences?
Aluminium alloys fatigue quickly when flexed. Therefore aluminium frames have to be made inherently stiffer so that they don't flex. This is achieved by using larger diameter tubes and larger diameter tubes are heavier. They provide excellent power transmission to the back wheel with little loss through frame flex but the down side is the ride is much harsher, they don't flex.
Frame steel alloys are particularly resistant to fatigue and so the tubes can be much smaller diameter. A good steel frame weighs less than 2kg, has a pleasant ride quality because the frame flexes and absorbs some of the road shock, and has high reliability and is repairable.
If you were a high-level racing cyclist the 0.5kg weight reduction and the added power transmission of an aluminium frame would count, but if you were a touring cyclist, the comfort and lifetime warranty of a steel frame would be far more important.
Titanium in particular and carbon fibre to a lesser extent potentially has the best of both worlds but at a much higher price.
WHEEL SIZE AND CENTRE OF GRAVITY
To illustrate the point let's consider a wheeled platform that has a constant ground clearance of 200mm and the only thing we change is the diameter of the wheels.
With 100mm diameter wheels the load bed is 100mm above the wheels and the platform is inherently unstable. With 400mm diameter wheels the load bed is at the axle level and the platform is still unstable but is more stable than with 100mm diameter wheels. With 800mm diameter wheels the load bed is 200mm below the axles and the platform is very stable.
Myth 5: A low centre of gravity has no relationship to the height of the axles — Challenged
This has application to bicycles. The more of the load that is below the axles the more stable the bicycle. The larger the wheels are, the greater the drop of the load below the axles.
So why do we use smaller wheels on bicycles?
The smaller the diameter of the wheels, the stronger, lighter, stiffer, the less space they take up and the lower the air resistance. This is very applicable to folding or travel bicycles but has little scientific argument elsewhere.
Larger wheels have a lower rolling resistance, a better ride quality for two reasons and provide a more stable bicycle. The larger diameter wheel has a slower rise and fall over bumps and dips than a smaller wheel giving less road shock and has more radial wheel flex and so absorbs more road shock. The poor ride quality of smaller wheels is why many small-wheel bicycles have suspension.
700C or the old 27in wheels can be built adequately strong for any rider or even tandems so the strength argument has little weight. They do contain more material than a smaller wheel and so are heavier.
In short, a smaller lighter wheel will accelerate quicker than a larger heavier wheel but not by a large amount because the physics of the higher rotational velocity works against the smaller wheel.
For travel and folding bicycles little wheels are great but for other bicycles the bigger the wheels are, the better (within reason).
Firstly, for any suspension to be effective the sprung weight — rider, frame load etc must be very heavy compared to the unsprung weight — anything that is rigidly in contact with the road surface. For this reason the suspension and the wheels must be as light as possible in comparison to the load that they carry.
Though suspension is very advantageous in off-road situations, the necessity of suspension on many modern bicycles is a result of the mass production of cheap very stiff aluminium frames that have poor ride qualities. The need for the added weight of the suspension defeats the reason of the lightweight aluminium frame in the first place.
The best suspension, because it is the lowest weight, is your pneumatic tyres. Lightweight large section low-pressure tyres provide a very comfortable ride. The problem is the compromise; they don't always roll as easily as high-pressure tyres and are heavier and slower to push up hills.
Suspension bicycles can also compromise power transfer. The changing weight of the rider as they pedal can cause the suspension to bounce or bob. This wastes power. The more sophisticated suspension units largely reduce this bounce, but at a price.
Suspension bicycles have very limited capacity to attach racks or carry luggage. Any weight attached to the suspension units will rapidly reduce the effectiveness of the suspension because it increases the unsprung weight. Most suspension bikes are not designed to carry loads.
The effectiveness of suspension seat posts is not great for two reasons. The changing seat height reduces your pedalling efficiency and because the unsprung weight (the whole bicycle) to sprung weight ratio is poor.
To buy a lightweight stiff aluminium frame bicycle with suspension is basically an oxymoron. The suspension removes any power transmission advantage of the stiff frame and the added weight of the suspension removes the weight advantage. Suspension bicycles only have advantages in off-road use.
A lightweight steel frame and forks, with 700C wheels and 35–38 tyres would have better ride qualities than many suspension bicycles, have good rack and luggage carrying capacity, be much lighter to cycle up hills and have much better pedalling efficiency, a much better solution for most riders.