What Size?

By Chris Juden. This accompanies the Petite Bike Test article.

This test has given me the opportunity to check a few theories of mine about how to fit bicycles to people. There are quite a lot of formulae around, published in various books etc, but few of them attempt to deal with the extremes. And as I've said before: they seem a bit too keen to fit people whatever equipment happens to be on the shelf in most shops, rather than empowering the customer to demand that something different be ordered.

The first problem is to measure yourself. Of course I had to ask all the petite test riders to do this, and although I'm sure they really tried to get it right, I'm afraid that their measurements did not correspond very well with the bikes they found most comfortable. Fortunately I had set up a visit to our Castleton base from Martin Series, who represents the Bioracer fitting system in the UK. A number of shops up and down the country have one of these systems, with which you can be accurately measured and then have a frame of whatever type you require - time-trialing, ATB or tourer etc. - designed for you by an 'expert system' computer program. The frame can either be built by Bioracer in Holland or to their pattern by a local framebuilder.

Anyway, with a view to finding out what their system would prescribe for our little lot, Martin brought along his mobile rider measuring system and made a set of measurements. I'm afraid I can't yet tell you what the computer has made of them (perhaps we've broken it!) but I can tell you that these measurements look a lot more consistent than the self-measuremnts.

Surprisingly, the most error-prone figure was the apparently simple measurement of inside leg, which was under-estimated by as much as 4cm. This also threw out the torso length - arm length was generally quite accurately measured. In order to remove the uncertainty (and the discomfort!) from this measurement I propose the following method - you'll need a helper.

1 Stand in bare feet on a hard floor, back to the wall, and get someone to measure your height: H by marking the wall using a large book or LP record etc placed vertically on your head and against the wall.

2 Turn to face the wall and measure the height T to the top of your sternum by a similar method. The sternum is the breast-bone. The ends of your collar bones almost meet on top of it, making a little notch into which you can fit the corner of the LP etc.

3 With your arm down by your side, make a fist - fingers inwards so that the back of your hand is in line with the arm - and have your helper measure your arm length A from the middle-finger knuckle up to the acromion. This is a fancy name for the bony tip of the shoulder.

4 Now sit on the floor with your back to the wall and have your sitting height S measured.

Leg length L = H - S. This gives a leg length about 2cm greater than the maximum found by other measuring methods, e.g. the Bioracer, but anyone can thus obtain consistent results without special equipment. L equals the maximum height of frame (HT) you will be physically able to stand over, but a frame this high would be inconvenient and sometimes painful to use. The maximum I would recommend for a road touring bike is: HT = L - 5 (cm).
Frame height isn't the same as the commonly quoted frame size, which unfortunately is a very poor guide to the actual size of person a frame might fit - since the size that is right for you depends very much upon what kind of bike it is. Very approximately, touring frames are usually about 25 to 28cm less than L, ATBs another 5 to 10cm smaller still.
We've already established that crank length should be between 20% and 21% of L - keeping towards the lower figure if you've got dodgy knees or the higher one for big feet.

Body length B = T - L and arm length A we already have. I add these two together to give a combined arm and body length C. This can be used as an approximate guide to the optimum reach (RE) measured horizontally between the centres of handlebar and seatpin. My advice for a touring bike with dropped handlebars is: RE = C x P where P is between 50% and 52% for women and 51% to 53% for men. If flat (ATB-style) handlebars are used then P should be increased by about 5%.

I'm afraid I can't be more precise than that. Suffice to say that less flexible riders (especially those with back problems) who like to have their handlebars level with or higher than the saddle should use the lower figure to calculate reach; while those who prefer and are physically able to adopt a more streamlined position, with the bars significantly lower than the saddle, may use the higher percentage. Seat-tube angle and the type of seatpin used - where it places the saddle: behind, on or in front of its centreline - has an influence upon where exactly you sit and affects reach.

To measure the reach of your own bike, first measure how far your handlebars are above the ground (at the level of the centre of the bar where it goes through the stem) and make a mark on the front of your seatpin at the same height - raise the saddle if necessary so as to expose a bare section of seatpin. Measure from the back of the bar (close by the stem) to the mark on the front of the seatpin, and add 27mm.

The difference between A and B has a bearing upon handlebar height for riders who seek the most eficient riding position. A is usually greater than B, and placing the bars below the saddle by a distance approximating to (A - B) seems to be about right.

Remember what I said about frame fitting formulae. They are nothing more than an approximate guide: somewhere to start from if buying a first bike or an indication of the direction in which to start changing things if you're not happy with your current riding position. If you are happy with a riding position nothing like that given by these formulae: don't worry. If you've got used to it and it works that's all that matters.

Other pages in the Petite Bike Test: