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What mountain bike frame size?

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When you plan to purchase a new bike frame or assembling your favorite bike many questions will arise in your mind such as What size bike frame do i need?  Where to find bike size guide? What length of stem should I use?  How to find correct handle bar position?

This article would help you to find answers for all the above questions.

Things to be noted down

Few important things you need to know before you go for bike frame sizing is that how seat tube and top tube length would change your riding style and how stem size and handlebar height helps you to accommodate well in the bike. You can also get help from your local bike shop to find correct bike frame size at a cost. You must also check the manufactures catalog and website for more information about the bike frame size.

Bike Frame Sizing

 

Old Bike Frames

In earlier days it is easy to tell the correct frame size, just stand over the frame and if you find one to two inch gap between the top tube and your crotch then that is the correct frame size. In those days in order to differentiate the bike frame sizes manufacturers use seat tube and head tube length. This was considered the only difference between frame sizes, whether the bike was a 19″ or a 25″. But the top tube length remains the same only the height of the top tube varies. Taller person go for 25″ and smaller person go for 19″. Since the top tube length remains the same the taller riders puts the handlebar far away by using longer reach stem to adjust the riding position in 25” frame.

New Bike Frames

Now the mountain bike frames are proportionally sized that means they have shorter top tube in smaller frame and longer top tube in larger frame and there is no need to change the stem size. Although this proportionally sized bike frame proved good improvement in comfort it has become little difficult to choose right frame size because the frames are available with more size.

Bike Frame Size

 

How to measure a Bike Frame?

Every manufacturer has listed their own frame sizing but generally all those bike frame sizes are measured by seat tube length that varies from 13” to 24”.

Simple calculation to find correct seat tube length for your height

(Inseam height (in inches) x 0.67) – 4 to 5 inches

For example (31 inch inseam x 0.67) – 4 = 17.44 inch

Importance of Top Tube Length Measurement and Stand-over Clearance

The seat tube length is no longer an important frame dimension only the top tube length measurement is an actual way for a rider to choose from the frames which are available with more sizes. The top tube length has becomes an important variable to consider when sizing the bike. The general rule of having good stand-over height is always applicable when finding a correct bike frame size. But today you will find a lot of sloping top tube length and stand-over height variation in small to large frames across brands. At the same time you will find sloping top tube frames smaller than its actual size and it is just an illusion. So, you must also include top tube length measurement along with the stand-over clearance as a criterion when finding the correct size.

Stand Over Clearence

 

Small, Medium, Large Frame Sizes

Most of the riders would think that they still get the same good fit although they can choose from several different bike frame sizes. Yes, it is true because handlebar stem and seatpost are readily available in the market with the different configuration and sizes. You can easily fit the frame according to your height. But still small, medium, large frame sizes with different top tube and seat tube length with variation in handlebar position and stem length would have some effect on your riding style.

Factors that Affects the Riding Style

A Frame with Longer Top Tube:

  • Usually a frame comes with sloping top tube and it does not look parallel to the ground. This sloping top tube eliminates bike frame sizing problem.
  • Longer top tube gives longer wheelbase and riders can use shorter reach stem.
  • A mountain bike frame which is built with such longer top tube normally gives less jittery handling with better shock absorption.
  • There is also a performance downside because the down tube will also be longer and in few cases it might bend slightly to make the frame finishing. This would hinder the acceleration while pedaling the bike.
  • Most advanced and branded bike frames have strategically welded frame design that gives good riding experience without losing the performance even though they have long top tube.

A Frame with Longer Seat Tube:

  • A longer seat tube will let you have a standard 110mm length stem with higher handlebars. This will give you enough space to carry accessories such as water bottles and pumps. But with longer seat tube it is difficult to make your handlebars as low as you want.
  • Many Factory built cross country frames as well as custom built frames comes with standard 110 mm length stem but many riders would shorten the stem to get good steering experience.
  • The height of the top tube will increase with the increase in seat tube length and reduce your stand-over height. It is essential to choose the frame with plenty of stand over clearance in case if you are trying to bail out of a bad situation.

Fixing Stem length

  • Proper stem positioning and length would remove excessive pressure on the wrist and make easier to endure the upper body soreness.
  • Riders having disproportionate leg to torso sizes will get benefit from changing the length of the stem to achieve comfortable riding position.
  • Using a longer stem would stretch you far and gives more control to the front wheel but the result would be slow steering performance.
  • Using shorter stem gives upright riding position and moves the riders towards the center of the bike. Riders need to slightly bend their elbow when riding in upright position.
  • Freeride/DH bike normally have stem length range from 25mm to 55mm+ and cross country bike have stem length range from 70mm to 130mm.

Adjusting Handlebar Height

  • Placing correct Handle bar height either by adjusting the bar height too low or high can improve your riding posture and comfort level.
  • Depending on your body flexibility you can achieve comfortable back angle without pain in your neck.
  • Use spacers to raise or lower the height of the bar and stem relative to the frame. or otherwise you can buy riser bar or riser stem to adjust the height.
  • Handlebar type often referred to riser bar or narrower bar.
  • Narrower, straighter-bar setup 22-23in (56-58cm) range is used for cross country bikes, much useful for long stretches of pedaling.
  • Freeride/DH riders normally choose wider bar in order to achieve more control over the bike.

Long torso and short legs Syndrome and Vice-versa

Riders having long torso and short legs must choose a large frame for cross country or a trail bike. Shorten the stem a little and move the seat little forward to have comfortable position in larger frames. You can also get an acceptable cockpit and top tube length in big bikes such as Freeride/DH bikes without fixing long stem. Mostly medium sized frames from Freeride/DH bikes would better fit for you. if you have Height 5’6″ and 30″ inseam wants to ride a All mountain bike with 5” travel then medium sized frame would give you longer wheelbase and more stability. Normally All mountain bikes have steeper frame angles and enables the riders to enjoy both climb and descent. Larger frame with short stem would be much suitable for somebody having shorter torso than legs.

Choosing Bike frame based on your Riding Style

If you prefer to ride long travel full suspension bike over rough terrain choose the smallest possible frame with maximum clearance for safe ride – as long as the bike does not cramp you inside the cockpit and you can still stretch your legs. If you preferably like to sit upright like riding a cross country/XC bikes choose bigger frame that still provide you enough stand over clearance.

Bike Size Guide

Two important measurements such as rider’s height and inseam size play a major role in finding correct bike frame sizes. The following table is just a rough guide to know where you fit.

Bike Size Your Inseam Length Rider’s Height Rider’s Height (cm)
13″ – 14″ 25” – 27” 5′ 0″ – 5′ 2″ 152 – 158
15″ – 16″ 27″ – 29″ 5′ 2″ – 5′ 6″ 158 – 168
17″ – 18″ 29″ – 31″ 5′ 6″ – 5′ 10″ 168 – 178
19″ – 20″ 31″ – 33″ 5′ 10″ – 6′ 1″ 178 – 185
21″ – 22″ 33″ – 35″ 6′ 1″ -6′ 5″ 185 – 193
23″ – 24″ 35″ and up 6′ 4″ – 6′ 6″ 193 – 198

Just sit on the bike and go for a test ride. You can also find your comfort level by just sitting on the bike and pedaling backward.

Four things you need to notice when you hop on the bike:

  • You must not feel too tall and stretched far or too short and cramped inside the cockpit.
  • You must also be able to control the bike when you face the bad situation. This means you must be able to easily bail out from the pedal and stand on your feet flat on the ground.
  • Many riders think that stand over clearance is not important and it is meant only for beginners, this idea is totally wrong.
  • Make sure you also concentrate on stand over clearance when choosing correct bike frame size and this will save you from causing major injury when things go wrong.
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