Sunday, 22 September 2013

Frame Geometry 2

I have mentioned the importance of front-end frame geometry, but the rear of the bike is equally critical if a comfortable ride is to be had over a long distance. Key factors are seat tube (ST) angle and chain stay (CS) length.

Seat tube angle

The ST angle affects position fore and aft on the bike. A ST angle of 73 degrees is a typical compromise, however, some account must be taken of the purpose of the bike. When riding long distances top-heavy riders might require a shallower angle to bring their weight back off the handlebars, whereas bottom-heavy riders might require a slightly steeper angle for the opposite reason. Racing bikes, where comfort over long distances is less of a premium, will generally have a steeper angle which results in a more forward position for power sprinting. However, everything is a compromise, so the ideal ST angle will depend on rider physiology and the main type of riding that will be encountered.

Chainstay length

Longer chain stays give a 'springier' ride and give more clearance between the ST and wheel, allowing mudguards to be fitted, so this geometry is advantageous on touring bikes. Racing bikes have short chainsays because mudguards are not required and the more compact geometry improves handling and maximises power transmission to the rear wheel.


If you want to fit mudguards then there must be enough clearance between the rear wheel and the ST and the front wheel and fork crown, so the CS length and fork length must be adjusted accordingly, at the same time taking account of wheel and tyre size.

Wheel size

700c wheels have become the de facto standard on most road bikes, and 26" on mountain bikes. There is no particular reason for this other than historical accident and other sizes have been used on bikes in the past: 650b was used on French randonneuring bikes; and 650a on english 3-speeds. The aforementioned wheel sizes are actually diameters of the wheel with a particular tyre fitted. Standard sizes follow the ISO (formerly ETRTO) system which designates the wheel rim diameter as measured between the the bead seat, so a 700c wheel is actually ISO 622 with a 39 mm wide tyre (622 + 2 x 39 = 700). So, the actual sizes are:

700c = ISO 622 with 39 mm tyre designated 39-622

26" = ISO 559 with 50 mm tyre designated 50-559

650b = ISO 597 mm with 26 mm tyre designated 26-597

For a comprehensive treatment of this topic I recommend the late Sheldon Brown (Brown, 2013).


Brown, S. (2013). Tyre Sizing Systems. Available at [Accessed 22/092013].


Thursday, 19 September 2013


sat on

top of the world,

the woman who knitted

with yarn fashioned from fleeces

of the lonely sheep which grazed there;

where the weather station stood sentinel to

the wind blowing in across the margins of the land.

In that high place, where the snows had melted, she knitted


This was a real encounter I had after cycling to the top of a mountain in the Picos in northern Spain; there was an old woman knitting socks in this isolated spot, and I regret to this day that I didn't buy them from her when she tried to sell them to me.

Sunday, 15 September 2013

Breakfast Buttie

Oh bloody hell

this pan is hot,

my rindless backside

fresh from the fridge

is finding it searing

to say the least;

the bubble and pop of

my frying behind might

seem tasty to you but

I find that it brings out

the oil in my skin. Oh yes,

I know what's next

with your vain attempt

to butter me up, the ketchup

gives it away you see.

Don't eat me.



Cool summer mountain

above the line of houses,

sheets flap in the breeze

and Mam calls . . .

Monday, 9 September 2013

Frame Geometry 1

Frame geometry parameters (

When you set out to buy a bike you usually have some idea of the type you want; is it to be a mountain bike, a dropped handlebar racer or a folding bike for a commute. Sometimes this is all you need to know and apply these simple criteria at the shop. It gets trickier when you've been riding for a while and want a bike for a specific purpose, be it racing, randonneuring, cyclocross or extended tours; and this is where the subtleties of frame geometry come in.

The bicycle is an intrinsically simple yet brilliant design made of a frame, fork, two wheels, saddle, handlebars and drivetrain (pedals, cranks, chain and rings). How these parts are put together will affect the utility and handling of the bike, to the extent that getting it wrong will make for an unpleasant ride at best and dangerous at worse. The diagram above, rendered using BikeCad (BikeCad, 2013) shows the basic parameters of bike geometry.

At this point it is worth making a distinction between geometry and fit. Parameters which primarily affect bike geometry are:
  • bottom bracket (BB) height;
  • head tube (HT) and seat tube (ST) angles;
  • fork length (FL) and fork rake (FR);
  • trail (T);
  • wheel size (W);
  • chain stay (CS) length.

The parameters which affect bike fit (though it must be borne in mind that all these things interact) are mainly:

  • head tube (HT), top tube (TT), seat tube (ST) and down tube (DT) lengths;
  • saddle height (SH);
  • stem length (SL).

Bottom bracket height

BB height can vary between 24 to 31 cm, depending on what the bike is to be used for. Cyclocross and mountain bikes will require a high BB to give plenty of clearance, while a touring bike will require a low BB to give a low centre of gravity and a more stable ride. Hence, the starting point of bike geometry is often to choose the BB height, however, it is worth noting that this has consequences for subsequent angles and dimensions when building a lugged frame.

Head tube angle, fork rake and trail

This combination is the most imprtant aspect of geometry:

  • HT angle is the angle between the HT and the ground;
  • fork rake is the perpendicular distance between the straight upper part of the fork and the fork dropouts;
  • trail is the distance between the contact point of the wheel with the ground, directly below the fork dropouts, and the point on the ground projected in a straight line through the axis of the HT.

The HT angle and fork rake affect the steering characteristics of the bike because, between them, they determine the amount of trail. In order to grasp the concept of trail, first consider a bike with a perfectly straight fork in (one which has no rake). If the HT was perpendicular to the ground (a HT angle of 90 degrees) the point of contact of the wheel with the ground would be directly under the steering axis and there would be no trail. However, the HT is usually angled between 72 to 75 degrees so, in the scenario with our straight fork, as the HT angle decreases the contact point of the wheel directly below the dropouts falls behind the steering axis (the contact point projected through the HT) - this is the trail. So, as the HT angle decreases trail increases. The purpose of fork rake is to move the contact point of the wheel below the dropouts back towards the steering axis by curving the forks forward slightly, therby reducing the trail. Racing bikes tend to have steep (larger) HT angles and low trail because this makes the steering sharper and more responsive, but can also make the ride somewhat unforgiving because there is less flex in the more steeply angled fork. Touring bikes tend to have shallower HT angles and more trail because they are used at lower speeds and the steering does not need to be as responsive, and this results in a less harsh ride. Raking the fork reduces the trail, so the handling is actually influenced by both HT angle and rake.

It should be noted that complications arise when considering tyre size and whether the bike is to be loaded front of back, because this also affects the handling characteristics and the geometry must be adjusted to account for this. A particular case in point is the low trail geometry of French randonneur bikes which have large tyres and front racks and have geometry specifically designed for this to make handling as responsive as possible without being unstable. (Heine et al., 2007)

All that said, a HT angle of 73 degrees is a good compromise for the beginner in framebuilding using commercially available forks, and when there are no complicating factors with regard to large tyres and loaded panniers.


BikeCad (2013). Available at [accessed 07/092013].

Heine, J., Vande Kamp, M., and Wetmore, A., (2005). Bicycle Quarterly, 5(3), 42-47.