Geometry / Sizing
What is it?
Essentially it is the angles and measurements that determine the shape and size of a frame.
What do I need to know about geometry and sizing?
The key thing to understand is that geometries vary across different brands and models, so test riding is often the most effective way determine bike fit and feel. That said if you get your bike professionally fitted in a bike shop, then you don’t have to know much at all – leave it to the experts!
Knowledge is power
Getting a grasp of the basics will go a long way in determining the bike and style that is right for you, though this is mostly relevant to intermediate and advanced riders, it can also helpful for keen beginners.
Wheel base is the measurement between the two wheel axles. A longer wheel base provides stability but sacrifices manoeuvrability. By the same token a shorter wheel base provides more responsive handling but is less stable at higher speeds. Larger wheel bases are often found on down-hill mountain bikes as stability at speed is fundamental whereas a shorter wheel-base is often found on cross country or enduro bikes (where the terrain is more technical and manoeuvrability is key).
The headtube angle is the angle at which the headtube meets the ground. The more relaxed the angle the slower the bike will turn but the more stable it will be. A steeper headtube angle will decrease stability but provide more responsive and snappy turning.
Seat Tube Angle
The seat tube angle is measured the same way as the headtube angle. Changes in the angle size will determine how aggressive (hips forward) or relaxed the riding position is. Aggressive riding positions place the rider vertically over the bottom bracket resulting in great pedalling power and in some cases better aerodynamics.
Top Tube Length
The length from the centre of the head tube to the centre seat tube in a straight line is called the top tube length. Technically however this is not the actual top tube length as the slope and curve of the tube itself is not measured. Changes in the top tube length can lead to a similar change in the wheel base length. The main effect the top tube length has on a rider is the stretch required to reach the handlebars from the seat tube, people with larger arms spans generally opt for larger top tube sizes.
Chain stays can be found on the rear triangle of a bike frame. The length of the chainstays determines the bikes ability to turn as well as pop and mono. For this reason short chain stays are found on dirt jump bikes. The length of the chainstays also directly affects the wheel base size of the bike.
Bottom Bracket Height
The height of the bottom bracket has a direct impact on cornering; the lower the bottom bracket the lower the centre of gravity and the easier the cornering. The drawback to lowering the bottom bracket height is a lower crank clearance, which is particularly important for mountain biking where undulating terrain requires higher crank clearances to pedal without obstruction.
Mountain bikes can come in a range of different materials each has its own pros and cons and it’s important to factor in purpose and budget when considering a new purchase.
High Tensile Steel
Once upon a time nearly all bikes were made from this steel. It’s strong but heavy. Most major brands no longer offer bikes that come in this material and brands that do usually only use it on lower end budget models. If price is an issue this material is the lightest on the wallet.
Chromoly is an alloy made from chromium and molybdenum. The result is a lighter stronger material when compared to high tensile steel that offers reasonable flex. Chromoly is often found on BMX bikes and some dirt jump bikes as its impact resistance is high whilst relatively light.
Aluminium frames are lighter than chomoly frames but not quite as strong. Alloy frames are generally moderately priced and are frequently used in most beginner and intermediate road and mountain bikes. Advances in technology (such as hydro-forming) have improved strength and cost effectiveness of the material, making it one of the most common materials currently on the market.
Light, strong and very flexible. Titanium frames are lighter and stronger than steel frames but are infamously hard to weld resulting in some frames having a reputation for broken joins. Despite their bank emptying ability titanium frames are still highly sought after due to their superior ability to flex and snap back.
This is a synthetic material comprised of thin strands of carbon twisted together to form yarn which can then be woven and placed in a mould and coated in a stiff resin to hold shape (like papier-mâché). Carbon fibre is nearly 5 times stronger than steel and 2/3 lighter. The downside to the technological delight is its price and its resistance to impact damage. High end cross country mountain bikes often feature carbon fibre frames.
Naturally as time goes on advancements are made in frame technology, most of these are breakthroughs in the materials used and advancements in the actual construction of the frame. Some highlights include:
Put simply is the process of thickening the tubing where it needs to be thickest and thinning it where it is not needed. Originally all butting was external but almost all bikes today will have internal butting.
- *Double Butting *
As the tube is created, extra material is added internally to each end of the tube. By doing this the overall tube wall thickness can be reduced resulting in a lighter weight tube.
- Triple Butting
Triple butting is achieved by stepping down the material at each end of the tube. This means the butting starts out in the standard, double-butted manner but then is thinned before stepping down again to the normal tube wall thickness again resulting in a lighter tube.
- *Double Butting *
TIG welding uses the same material for the bond as the frame it's easy to achieve a strong join and is relatively inexpensive.
One Piece Construction
The advancement of carbon fibre has allowed manufacturers to construct a one piece frame, and the science seems simple. No joins = no joins to break. Though that is an over simplification, the outcome is for a slightly stronger more rigid frame.
And for the history buffs...
All terrain riding can be roughly traced back to the 1950’s in the form of “Velo Cross.” Though this was little more than riding road bikes on dirt roads its likeness to present day mountain biking is uncanny.
Geoff Apps a British motorbike trials rider is often accredited with being one of the first individuals to develop a custom built lightweight mountain bike frame using 27” Nokian snow tyres, the bikes where sold in England under Cleland Cycles until the mid 1980’s.
The birth of modern day mountain biking is unofficially recognized as starting somewhere in the mid to late 70’s in the United States. Riders Crested Butte, Cupertino and Colorado are thought to be among the first modifying existing bikes often cruisers to all terrain bikes. The Schwinn Excelsior was the acclaimed frame of choice due to its geometry and rigidity. Tyres where changed, motorbike/bmx handlebars where slapped on and a crude drive train implemented. Before long, riders where hurtling down mountain fire trails on their “klunkers” in an almost identical fashion to today's single track riding mountain biking.
It wasn't until the mid 80’s that bike manufacturers, whom at the time where solely focused on road bikes where swayed on the idea of mountain biking and released the first purpose built all terrain bicycle shortly afterwards. A partnership between Tom Ritchy, Gary Fisher and Charlie Kelly released the first mountain bike under the brand Mountain Bike which later dissolved and became Gary Fisher Mountain Bikes which in turn was sold to Trek Bicycle Corp in 1991.
1990 – 2009 saw unprecedented growth of the mountain biking genre into a globally recognized action sport. The sport encapsulates a large portion of the bicycle market and is still growing. Somewhere in this period 29ers snuck into the market in the form of a single speed ‘oversized BMX’ used for dirt jumping. 650B’s (todays 27.5’s) had a shot a jumping into the market but never took on. Early 00’s saw the mass introduction of full/dual suspension mountain bikes to the consumer market, some funky design went on during this period as geometries where re-hashed to accommodate the extra bounce.