How to Spot a Quality MTB Frame

Mountain Bike Frames

The frame is heart of a mountain bike. A mountain bike’s frame is what sets it apart from other bikes. It is the largest single component that makes up a bike and is usually the most expensive part of a bike.

The material that a frame is made with and the processing that the material went though influences a frame’s cost.

Aluminum is the most common material used for frames on quality entry-level, cross-country hardtail mountain bikes. Aluminum is what most mountain bike shoppers ask for, but there are also frames made with other materials, such as high-tensile steel, chro-moly steel, titanium, and carbon fiber. Processing of the material include heat treating, over sized diameters, and butting, where the tube wall is thicker at the ends and thinner in the middle to retain strength where needed but reduce overall weight.

Almost all the major bike companies mass-produce their mountain bike frames factories in China or Taiwan – albeit based on designs provided by the bike companies to the factories. While the frame’s origin shouldn’t be a major factor when it comes to quality, you should check each frame for workmanship and attention to details.

Here is a summary of some of the strengths and weaknesses of the popular materials used for today’s mountain bike frames:

High Tensile Steel:

High tensile steel, or carbon steel, is durable steel that is relatively cheap to produce. The steel is usually used for frames on low priced mountain bikes. Because of the high amount of carbon in the steel, high tensile steel is less rigid and weaker than the chromoly steel or the other materials used to make frames. To make the high tensile steel stiffer and strong enough for use in bicycle frames, more of the steel is needed to make a frame, which add weight to the frame.

Chromoly Steel:

Chromoly is a steel that uses chromium and molybdenum as additives in making the steel. Other names used for chromoly are chrome-moly, cromoly, CroMo or CrMo. You’re likely to see numbers such as 531, 631, 725 or 853, used to describe the formulation used to make the chromoly steel. Chromoly has been the material of choice for bicycle frames for many years until the arrival of aluminum.

Frames made of chromoly steel are more flexible than aluminum frames and has the ability to absorb some of the dumps along the road, making the ride softer and less stressful on the rider. A chromoly frame’s tubes will been noticeably smaller that aluminum tubes. To reduce weight, many chromoly frames uses double-butted tubings. Chromoly, as with other steels, will rust if not protected from the elements.

Chromoly frames are popular with hardtail mountain bikes. There are still a few bike companies offering high-end bike frames made from steel.

Aluminum:

Aluminum frames are light and fairly affordable. Now a day, you will find aluminum frames on most mountain bikes, even on entry-level bikes. Aluminum is lighter than chromoly, but not as strong. To strengthen aluminum frames, bike makers use larger, or over sized, tubes to build the frames. So an aluminum frame that is the same size as a chromoly frame may not be as light as you think.

You’ll see numbers, such as 6061 and 7005, used to indicate the grade and formulation of the aluminum.

Aluminum frames are also very stiff meaning most of the bumps picked up by the mountain bike on a trail will be transferred to the rider making the ride harsh and causing fatigue to the rider. Because an aluminum frame is stiffer than chromoly, it will fail before a chromoly frame used under the same conditions. Aluminum is very well suited for use for dual suspension frames. While aluminum doesn’t rust, it can degrade somewhat if exposed to the elements over a long period of time.

Titanium:

Chromoly Steel are still a rarity in mountain bikes. The characteristics of a titanium frame closely resemble that of a high-end chromoly frame. Titanium offers slightly more flex over chromoly, and does not rust. Even though prices have come down in recent years, titanium frames prices are still beyond the budget of a typical first time mountain bike buyer. The metal titanium is plentiful on earth, but the complex process to weld the tubes to make a frame drive up the cost of producing the frame and the retail price of the frame.

Titanium is usually alloyed with small amounts of vanadium and aluminum to give it better ride characteristics and weld ability. Titanium is even more malleable than chromoly, and has better fatigue and corrosion properties. Because of its similar characteristics to chromoly, titanium are suitable for hardtail mountain bike frames, just as chromoly, but is a too whippy for use in dual suspension frames.

Carbon Fiber:

Carbon fiber frames are very strong and stiff. They are less expensive than titanium frames but still more than most people would want to spend on their first mountain bike.

Carbon fiber bike frames are made by taking interwoven strips of carbon fibers and gluing them together with a strong epoxy. The strips are glued in alternating patterns to increase strength, much like arranging the directions of the wood grains in making ply woods. These strips can also be glued in particular patterns to point the direction of the carbon fiber strips, to change the response characteristics of the frame.
Carbon fiber is used in mostly for road bike frames, especially in time trail bikes, due to the frames’ lightweight, flexibility, and because frames can be custom molded into more aerodynamic shapes.

A weakness of a carbon fiber frame is that a frame can crack and fail if it hits an obstacle.

There is no perfect material for building a mountain bike frame. A frame’s material does affect the ride characteristics of a bike; but for most inexperienced riders, the difference between a chromoly frame and an aluminum frame will probably not be noticeable, except for difference in weight, until they’ve been riding for a while and have opportunities to ride different mountain bikes.

Besides the frame, the components attached to the frame will also affect the handling characteristics of a mountain bike.

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