Frame Damage Alert? Are Trainers Bad for Bike Frames?

You’ve spent thousands of dollars on your MTB. Winter is closing in, and you want to maintain your stamina through the winter so you’re thinking about a bike trainer.

But is it going to mess-up your bike? If ever wondered this you’re not alone. Let me help!

The Fear Behind Using Trainers

Twisting torque to the frame is the number one fear of using an indoor trainer. But is it justified?

When you ride, the entire bike can rock slightly from left to right and vice versa as you pedal and push downward on the pedals (translating the stress to the bottom bracket and into the bike frame).

However, when you consider having the rear wheel fixed via a bike trainer, the bike can no longer rock from side to side as you pedal. The trainer, built to hold your bike (and you) upright, forces the bike frame to absorb the flexing from side to side from your pedaling motion.

The most significant forces on your bike frame while using a trainer will apply when pedaling and when mounting or dismounting the bike/trainer.

So, Is Fear REALLY Justified?

Bikes will rarely become damaged when using a trainer as long as you do things properly. Improper setup is the number one cause of damage to bike frames when using a trainer, so it’s critical to set things up according to instructions.

Anatomy of a Mountain Bike Frame

Mountain bike frames come in a variety of configurations, and there are too many to get into here. Still, generally, we consider bikes to have two main triangles visually represented as sharing a single side, and the four outward points show the highest points of stress.

I’ll get more into this a little further down when discussing frame geometry, but generally, you should consider the four points of stress: the frame/fork connection point, where the bike seat post mounts, where the rear wheel mounts, and lastly, the bottom bracket.

How Trainers Actually Work

I just mentioned the general anatomy of a bike, being two triangles connected along one side. Installing your bike into a bike trainer means that you are adding a third triangle, albeit at a 90-degree angle, to the orientation of the bike frame’s two triangles, which orient from forward to backward along the line of the bike.

Think of a bike trainer as a form of training wheel that holds the bike up, as you can well imagine, and forces any side-to-side torque from pedaling, mounting, or dismounting the bike to transfer to the rear wheel connection point to the trainer (adding more strain to that specific tension point).

There are two types of trainers based on how they mount to your bike. The first is the type that connects to your existing rear axle via a cup and clamp mechanism. The second type requires the removal of the rear wheel and the installation of the trainer in its place.

Trainers that require you to keep your rear wheel installed on the bike are much noisier than their rear-wheel-removing counterparts. For more on this, check out my video, Best Budget Bike Trainer (Setup and Testing), where I show you the difference in noise level based on your tire tread.

Scientific Take on Frame Stress

If we’re going to talk about frame stress, then we’d better consider all the facts. Several factors affect our bike’s frame performance and stress.

1. Frame Stress and Strain
2. Frame Composition
3. Frame Geometry
4. Frame Butting

Frame Stress and Strain

Let’s start with where the frame typically experiences torque and twist stresses.

Bike frames suffer under two types of forces: stress and strain. It is the stresses and strains that determine the strength of a material. If we’re going to define them technically, then we’ll refer to West Virginia University’s descriptions:

• Stress – The internal distribution of forces within a body.
• Strain – The deformation of a body caused by an applied stress.

Frame Composition

Now for frame composition. In the world of bikes, there are four primary frame materials: steel, aluminum, titanium, and carbon fiber.

• Steel
• Aluminum
• Titanium
• Carbon Fiber

Comparing specific tensile strength and stiffness, we can see some really different abilities of each material. For example, if we compare carbon fiber to aluminum, we find that carbon fiber typically has around 230% of the tensile strength of its aluminum counterpart.

Definitions:

Yield Strength – Measured in megapascals, yield strength is the material property indicating the stress at which the material deforms irreversibly. In the case of carbon fiber, there is no yield strength because the material will not deform below its ultimate tensile strength.

Tensile Strength – Measured in megapascals, tensile strength measures a material’s resistance to being pulled apart.

Density – Measured in grams per cubic centimeter, density is the measure of how much mass is in a given volume.

Material	Yield Strength (MPa)	Ultimate Tensile Strength (MPa)	Density (g/cm3)
Aluminum alloy 6061-T6	241	300	2.7
Carbon Fiber	N/A	1,600 (laminates) or 3,450 for fibers alone	2.57
Steel, high-strength alloy ASTM A514	690	760	7.8
Titanium	100-225	246-370	4.506

As you can easily deduce, the greater the density, the heavier the material (Steel being the heaviest and carbon fiber the lightest). Furthermore, the ultimate tensile strength goes to carbon fiber, which is considerably more robust than the other materials.

How do these different materials compare regarding a bike trainer? Well, the higher the yield strength, the higher the stress the frame can take before it deforms. The ultimate tensile strength tells us how much force is needed to break the material. Lastly, density is directly related to the weight of the bike frame.

Therefore, we see that bikes with carbon fiber frames will generally withstand more stress than other materials.

Frame Geometry

Typical hardtail frames consist of two triangles: the main triangle and the rear triangle. Consider the image below:

As you can see in the image above, the rear triangle is tipped at the back by the rear wheel connection. On the main triangle, the opposite corner from the rear of the frame, the tip of the triangle holds the front fork and, therefore, also the front wheel and handlebars.

Where the two triangles share a side, we again have opposite corners of each triangle, sharing points of stress, the seat tube at the top, and the bottom bracket at the bottom.

If we consider all four points mentioned, the triangle points at the front (fork), back (rear wheel), top (seat post), and bottom (bottom bracket), then we can see the four points of stress for a bike frame. When fixed to a trainer, the rear part of the frame takes an unusual amount of stress as compared with freely riding outside.

Frame Butting

Frame butting in mountain bike frame construction refers to the process of varying the wall thickness of the frame tubes. Manufacturers use this technique to optimize strength and reduce weight.

In a butted tube, the walls are thicker at the ends where stress is higher, particularly near the welded joints, and thinner in the middle where less strength is required. The result of frame butting is beneficial to decreasing weight and providing robust frame strength at the points of welds or higher stress, which is essential for mountain biking, where the balance between strength and weight is critical.

Types of Butting

• Single-Butted: One end of the tube is thicker. These are less common in modern mountain bikes.
• Double-Butted: Both ends of the tube are thicker than the middle. It is the most common type of mountain bike frame construction.
• Triple-Butted: The tube has three different wall thicknesses, offering an even better strength-to-weight ratio.

How a frame manufacturing process uses frame butting will generally increase stiffness with triple-butted versus single-butted. However, single-butted is more prone to cracking, breaking, and deformation at stress points compared to higher thicknesses. Therefore, only the cheapest of bikes might have single-butted frames. In contrast, most bikes use double-butted, and high-end ones typically use double or triple, depending on the material type.

The amount of butting is often directly related to the amount of stress each point can take. Therefore, on a bike trainer, if the rear frame is double or triple-butted, it is less likely to suffer a stress failure than single-butted.

Closing Tips for Safe Trainer Use

There are two primary causes of problems with using an indoor trainer with your bike:

1. Improper installation
2. Sweat

The number one reason for frame damage to a bike using an indoor trainer is typically due to improper installation (either too loose or too tight at the rear wheel/frame connection point.

Sweat is (surprising to some) the second thing likely to damage your bike. Think about it this way: when you’re riding indoors, ideally, you are working out. The fact that you are working out means you are going to sweat. Remember, you won’t have the cooling breeze you would have riding outside, so it tends to be a sweatier occasion than if you go for a ride outside.

Sweat works on metal and alloys like salt water – if left to its own devices, it will corrode your bike alloys over time. Carbon fiber can even suffer from this fate. It was shown that seawater (similar to salty sweat in many ways) would degrade the ultimate tensile strength and the elastic modulus of carbon fiber. In other words, it degrades carbon fiber’s stiffness. (Source: 6)

Keeping these issues with trainer setup and use, here are seven tips to make using a trainer better (for you and your bike).

• Keep your chain clean with a chain cleaner.
• Sweaty handlebars can start to smell and corrode. Use a sweat catcher like a towel or something to cover the frame and handlebar stem area.
• Ensure you set the correct tightening for your rear wheel attachment to the trainer. Try using a torque wrench if you need specific torque for the trainer to rear of frame installation.
• Using the proper hardware for your bike/trainer is essential.
• Rotate the front tire if using tubeless so the sealant doesn’t settle too much in one spot.
• Pick up a set of tires specifically for use on your trainer if using the rear-wheel-on type of trainer.

Article Sources

1. “Science behind the Sport | Frames.” 2023. Wvu.edu. 2023. https://sciencebehindthesport.wvu.edu/cycling/frames.
2. “Carbon Fiber vs Aluminum.” 2020. Dragon Plate. DragonPlate. 2020. https://dragonplate.com/carbon-fiber-vs-aluminum#:~:text=For%20example%2C%20different%20carbon%20fiber,in%20reduction%20of%20other%20properties.&text=This%20chart%20shows%20that%20carbon,1.71%20times%20that%20of%20aluminum..
3. ‌Sayed Abolfazl Mirdehghan. 2021. “Fibrous Polymeric Composites.” Elsevier EBooks, January, 1–58. https://doi.org/10.1016/b978-0-12-824381-7.00012-3.
4. ‌Wikipedia Contributors. 2023. “Ultimate Tensile Strength.” Wikipedia. Wikimedia Foundation. September 5, 2023. https://en.wikipedia.org/wiki/Ultimate_tensile_strength.
5. ‌Wikipedia Contributors. 2023. “Titanium.” Wikipedia. Wikimedia Foundation. September 29, 2023. https://en.wikipedia.org/wiki/Titanium.
6. ‌“8 TH INTERNATIONAL CONFERENCE on COMPOSITE MATERIALS.” n.d. https://www.iccm-central.org/Proceedings/ICCM18proceedings/data/2.%20Oral%20Presentation/Aug25%28Thursday%29/Th37%20ONR%20SESSIONS%20-%20Research%20in%20Composites%20Materials%20and%20Sandwich%20Structures%28S.O.%20Yapa%20D.%20S.%20Raj/Th37-3-AF1538.pdf.