MTB - Geometry and Options

This article discusses a mountain bike’s geometry, different materials, components and types. Understanding how your bike works will give you greater confidence in mastering the sport of mountain bike racing.
Created: July 2011
Revised: October 2012
Latest Feedback: October 2012

Frame Geometry

Choosing a mountain bike requires that you fully understand the required geometry or you could end up with a bike that is completely comfortable on the road, but totally awkward on off-road surfaces. The geometry of a mountain bike is completely geared towards cross-country, making it easier to handle the terrain, hazards, obstacles, climbs and descents.

The permanent features of a mountain bike lie in its frame. These are features you cannot change or replace, so if you don’t like what you’ve chosen, you have to essentially get a new bike. The seven critical components of the mountain bike’s frame are the headtube angle, the wheelbase, the chainstay length, the standover height, the bottom bracket height, the top tube length and the seat tube angle. Each is described in detail below:

Headtube Angle

The headtube angle determines how fast the bike can turn and how easily it can climb. The greater the angle between the headtube and the road, the greater the bike’s ability to turn quickly and climb with ease. However, the compromise is its stability, which reduces at greater speeds. Generally, mountain bikes have an extremely steep headtube angle of about 71 degrees, as opposed to downhill bikes.

Wheelbase

The wheelbase of a bike determines its manoeuvrability at the cost of stability. It is the distance between the axles and when the wheelbase is shorter, it offers more manoeuvrability. Mountain bikes typically have shorter wheelbases making them far more quick and springy. On-road bikes on the other hand, have longer wheelbases which improves their stability.

Chainstay Length

The chainstay length is directly related to the wheelbase and consequently shorter chainstays improve manoeuvrability but reduce stability. Short tight stays are essential to hop or jump over hazards and enhances mid-air control. All mountain bikes have short tight chainstay lengths, so the rider can easily handle obstacles on the way.

Standover Height

The standover height of a mountain bike is its inseam. Most mountain bikes have extremely low standover heights, as this reduces the chance of injury. The more you manoeuvre your bike over various types of terrain and obstacles; you increase the risk of getting hurt. Thankfully, most mountain bikes are designed to reduce this risk as they feature lower standover heights. Newer bikes relying on hydroforming technology have managed to obtain drastically low standover heights for added safety and protection.

Bottom Bracket Height

The bottom bracket height determines how easily the bike will turn around bends and corners. The lower the height, the smoother the bike will turn. However, when it comes to mountain bikes, clearance is also a concern. If the bottom bracket height is too low, your bike will end up coming into contact with rocks, stones and other hazards. Consider these factors carefully when choosing a mountain bike.

Top Tube Length

This is distance between the seat and the head tube and will also determine the length of the wheelbase. For mountain bikes as with other bikes, the top tube length will depend on the riders’ body structure. If you are tall, with long arms, you might be more comfortable with a longer top tube length. Conversely, if you are on the shorter side and if the top tube length is long, then you may have a very awkward ride.

Seat Tube Length

This is the angle the seat makes with the track. It determines where the rider will be located on the bike. A sharper angle enables the rider to generate more power by turning the cranks at a greater speed.

Bike Fit and Size

A successful race would depend upon how comfortable you are on your mountain bike. Here are a few aspects to consider.

Crank Length

The crank length would be determined by the length of your leg. Normally, this length is calculated as 18.5% of the total distance from the floor to the top of the femur, when you’re not wearing shoes. The femur starts at 12 to 15 cm below the hip bone. However, do remember than this calculation is just to give you a rough idea and your comfort should determine the final crank length.

Seat Height

At an ideal height, you won’t have to rock your hips in order to pedal, nor stretch your legs out to reach the lower part of the pedal. Your seat may be too high if you find yourself moving forward and sitting on the narrow portion, towards the front of the seat. It’s also important to remember to leave some bend in your knee when you pedal. If your leg is completely straight, then you’ll have to tilt your hips to ride the bike. This could be very uncomfortable and will impact your performance.

Seat Angle

Keep the seat angle almost horizontal. If the seat is nosed up too far, then it can affect your soft tissues causing discomfort and damage in the long run. However, if the angle is too far down, then you will tend to slide forward and the pressure on your hands will increase. This could lead to neck, shoulder and wrist problems as your hands will be bearing a lot of your weight.

Front-Back Position

Adjust your seat at the right distance behind the bottom bracket, so your peddling is more efficient. Normally, your kneecap should be just above the pedal spindle when the pedal cranks are horizontal. Avoid adjusting this seat position in order to reach the handlebars. If you need to adjust the bars, then do so by getting a handlebar stem with a different reach or choose a bike of a different frame size.

Frame Materials

The cost of the bike is directly linked to the material used to build its frame and how this material has been treated. Most mountain bikes are built from the following materials, which are high-tensile steel, steel alloys, aluminium, titanium and carbon fibre. Each is described below:

High-Tensile Steel

At the bottom of the spectrum, is the high tensile steel mountain bike made from a steel alloy mixture. Since it has a high carbon content, it is not as sturdy as other materials. Consequently, more of the metal is required to build the bike’s structure and this adds to its weight. Since it is fairly economical to produce, most of the lower end mountain bikes are made from this material, but keep in mind that the bike’s weight will hamper its manoeuvrability.

Steel Alloys

Most entry level bikes will have carbon steel tubing. Though this material is very heavy, it offers great inbuilt shock absorption to cushion your ride. Second only to titanium, it has an excellent strength-to-weight ratio and bike frames made of steel alloys are easy to repair. Chromoly steel gets its name from the chromium and molybdenum that is added to the steel to create this unique alloy. It is an extremely sturdy and reliable material for a bike’s frame. It is a material known for its durability and easy manoeuvrability.

Aluminium

The stiffest material is aluminium, which means that bikes made of this material are quite light. Since it is the stiffest, it is also the most prone to cracks, so if you use your bike too hard, be prepared for some irrecoverable damage. It is the ideal material to use for dual suspension bikes because its stiffness allows the frame to be as light as possible.

Titanium

Titanium is quickly becoming the material of choice when it comes to adventure racing; however, it is considered an expensive buy. Over recent years, it is being combined with other materials and thus the price has fallen to a more competitive level. Titanium weighs only 56% as much as steel and is equally strong. A light weight material is an important feature in an adventure race, where you may have to carry your bike on your shoulders and up or down steep inclines.

Carbon Fibre

This material is extremely customizable and allows the manufacturer to change the weave and the direction of carbon fibre strips in order to change the frame’s response. Therefore, by creating a frame with a vertical flex, they can make the ride smoother or design it with little lateral flex for tighter handling. Further, flexibility and stiffness can be changed on different planes of the same tube. Carbon fibre is much lighter than aluminium and yet much stronger for the weight. Overall, it is strong, light and stiff when compared to its titanium, steel or aluminium counterparts. Today, it is the most popular of the high-end choices for competitive adventure racers or XC riders.

Design - Choosing a Bike

Apart from the frame, which to a large extent determines the cost and weight of your bike, there are other design elements of a mountain bike that you have to carefully analyze before making a final choice.

Fully Rigid

These bikes don’t have a suspension in the front or at the rear, so your ride will be bumpier when compared to a full suspension or a hard tail bike. You won’t have a suspension to safeguard you from rocks and roots along the way. However, since there is no buffering effect, you may find more precision and responsiveness from a fully rigid bike. These bikes are more energy efficient, weighs less and picks up speed quickly when you’re on a climb. On the whole, a fully rigid bike is low maintenance but won’t offer much comfort when you’re on the trail.

Suspension

Many riders wonder whether they actually require suspension for their mountain bikes and the best answer is ‘why not’. You can always ride your bike without suspension and still have a thrilling ride. However, with suspension, you have the added advantages of greater control and comfort. While shock absorption is always the main benefit associated with suspension, the greater benefit is the enhanced control it offers. Your tyres will remain on the ground during bumps, allowing you to control the bike far better than a non-suspension bike.

Dual Suspension

A mountain bike with dual suspension is a bike that attempts to resist feedback during climbs. It provides a comfortable ride at all times even through hard brakes and is able to do this without substantially increasing the weight of your bike. Since this a tough combination to achieve, there are a variety of options available at varying technical levels. Deciding which bike to buy requires careful understanding of each model’s intricate technology and an assessment of your individual preferences. Are you willing to give up climbing efficiency for a smoother ride? Or are the bumps acceptable as long as the climb is easier?

Hard Tail

These bikes have a front suspension but no rear suspension. Some riders may add a bike suspension fork as an upgrade, to improve comfort. However, most bikes these days come with a standard suspension fork. A hard tail bike normally weighs a few kilograms less than a bike with full suspension. Apart from this advantage, you won’t waste energy during sprints. Further, you can improve your pedalling technique without that bouncing feeling, which is commonly associated with full suspension bikes. A hard tail bike also offers a shock-absorbing characteristic when high quality materials are used for the chainstay and the seatstay. It is definitely more durable, as it does not have a rear suspension or pivots to maintain. Hard tails are also very versatile, so manufacturers are able to offer better frames and specially designed tubes to improve ride quality. This is the most popular choice when it comes to adventure racing.

Soft Tail

A soft tail mountain bike is a cross between a full suspension bike and a hard tail bike. It has a little amount of rear suspension which is activated by the flex of the frame, rather than a rear pivoting linkage. It will have an elastomer which is placed in line with the seat stays. There are no moving parts on a soft tail bike other than the elastomer so it has a very basic design. It maintains power delivery and pedalling efficiency because of its strong chainstay and is much lighter than an average full suspension bike.

29er

29ers have taken off across the world and have popped up in all the Australian bike stores, with all the big brands now offering a 29er model, but expect to pay more!

The 29er features a larger diameter wheel, than the standard 26'' we've come to understand as standard on mountain bikes. The 29er wheel size is actually the same as a road bike, and many hybrids - being 700c, but seeing the 29" wheel on serious MTBs is really only just taking off this season in Australia, perhaps due to some world-class racers winning on them recently - guys and gals!

These big wheeled bikes certainly give the advantage of better roll over ability, but tend to suit taller riders (over 175cm). For petite ladies smaller than 165cm, an off-the-shelf model may not be possible to ride, as the whole frame will be too big and you'll have trouble getting the right stand-over height. However, with that said, there have been many small women carve up the world circuit on the new 29ers but who knows what sort of concept bikes and modifications have been done to improve and customise their fit.

But if you're tall, chances are you'll love what the 29er can do for you right out of the box. Converts to these big-wheelers seem to love their ability to make riding bumpy trails much easier, which in turn will give you both a comfort and a confidence boost, make you ride faster than you could with a 26" over the same terrain, and overall make your riding more enjoyable no matter what level of skills you have.

In terms of geometry, the 29er will feel more stable than a 26" mostly due to the longer wheelbase, but there is also a lower centre of gravity for the rider in comparison to the bike, which makes your ride feel more stable.

Wheels

For a mountain bike, the wheels play a vital role in the bike’s handling ability. Since they undergo substantial pressure during the ride, it is imperative that they are made from the strongest materials. At the same time the weight of the tyres are of equal importance, since rotational weight is more than 3 times that of static weight. This means that even if you buy a fairly economical bike with less sophisticated materials, investing in a set of lightweight tyres can improve your performance drastically.

Finally, think about going with the new tubeless tyres, especially if you're looking to reduce weight. Some wheels will be compatible, some not. Tubeless tyres have been used in the triathlon racing scene for many years and now we're seeing the MTB racers using them too. That's because the tubeless tyres are more resistant to punctures from pinching and thorns and can run on lower (softer) tyre pressures. Properly fitted and sealed, a tubeless tyre won't need inflating frequently, in fact some say months! But of course, just like their triathlon counterparts, tubeless tyres are very difficult to fit so if you get a flat on a ride the idea is to reseal with an aerosol silicon sealant and a CO2 cartridge, but you really still should carry a spare tyre - which negates everything because a tyre (unlike a tube) is bulky and heavy!

Choosing a mountain bike requires a careful assessment of all the options and varieties out there in the market. Your mountain biking skills, proficiency, technique and exposure as well as the chosen terrain, should play a vital role in your purchase decision.

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How To Choose A Mountain Bike, Buying A Mountain Bike, Mtb Buyers Guide, Buying A Mtb, Beginner Mtb, Mtb Frame Geometry, Mtb Parts, Mtb Bike Fit, Hard Tail, Dual Suspension, Soft Tail, Mtb 29Er

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