Force Measurement on a Snow Trike Aluminum Frame

"Skiing" with bicycle handlebars in front of you is nothing new. Over the years and decades, it has developed in different directions in different parts of the Alps, Scandinavia, North America, and elsewhere, with various forms of snow scoots and ski bikes. 

A snow scoot is the most simple design. It consists of two snowboards joined together, one after the other. A long vertical handle for steering is at the front of the front board. Without a seat, the 0snow scoot combines a street scooter and a snowboard.

A ski bike is an analog to a bicycle for downhill skiing. It has the frame and suspension like a bicycle but with skis instead of wheels. It may include a seat but has pegs in place of pedals.

And it’s all about speed! The actual velocity of snow scoots and ski bikes will depend on the steepness of the slope, the snow conditions, and the rider’s skill level. Advanced riders can reach speeds above 100 km/h (62 mph) or more on steep, well-prepared slopes.

How to brake a ski bike or a snow scoot depends on the specific model and design. However, like on any form of skis to make boards dig into the snow, creating drag is the most common way to slow down; either by plowing - the rider pushes the front of the vehicle down - or. by carving - the rider making sharp turns.

The snow trike

The “snow bicycling” scene has become quite lively In recent years, thanks to the Slovenian brand Hillstrike. Today, the company from Trbovlje is famous for its so-called "snow trike" devices.  Snow bikes that have one ski in front with a steering handle and two parallel skis on the back connected in a parallelogram which enables turning by tilting. Snow tricycles!

The company Hillstrike started in 2011 when the founder Matic Hribar, a passionate mountain biker and an all-around extreme sports enthusiast, had the idea of bringing mountain biking to the snow. The same year the first Hillstrike snow trike “prototype” was created.

The rear ski system can simulate the lower-body action of skiing. The center of gravity is like a mountain bike’s because the pedal system is where the crankset usually is. The advantage of this design is a parallelogram in the back with two skis as opposed to a single ski applied by the competition. This configuration provides a proper angle where the sidewalls of the skis can have a grip and add a carving effect instead of just sliding.

Riding the Hillstrike snow trike, you'll feel like riding a mountain bike on snow. You'll have control in turns and be able to master your drifts. The Hillstrike is stable enough to survive the wildest moves and cool tricks.

In the following years, Matic brought together a group of friends with the same passion for the sport and with different expertise. Together they have made a new prototype each year. What started as just fun developed into serious business in 2015 when Hillstrike d.o.o. was founded in Trbovlje. 

Measurements

The Hillstrike team's cooperation with Dewesoft goes way back to when this group of locals wanted to make a snow trike that would provide superior handling characteristics compared to a ski bike or a snow scoot. 

Hillstrike founder and product manager Matic Hribar says: “We first approached Dewesoft when designing our aluminium frame. Our trike frame is similar to a BMX bike frame. A lot of development research has been done on BMX bikes, and you can find data about the forces working on them. However, for a unique product such as our Hillstrike snow trike, it‘s not the case”. 

When developing their initial prototype, the Hillstrike team used Dewesoft devices to measure the forces exerted on the aluminium frame. The purpose of these tests was to ensure that the trike is not just easy to ride but also engineered to withstand the extremes of usage.

Matic states: “ We knew that if we wanted to know exactly what forces are exerted on the frame, we would have to measure them. And there is no better way than using Dewesoft measurement devices.” 
Three KRYPTON 3xSTG modules were mounted on a trike to measure chassis loads. Powered by a DS-BP2i battery pack, the modules acquired signals from half-bridge configured strain gauges. We recorded the bending and axial loads to learn how to optimize the chassis design, reducing its weight and securing safe use,

In later years, high-end mountain bike manufacturers have begun shifting towards carbon fibre frames to reduce weight and provide better performance. And the snow bike, in this aspect, slowly follows the trend. 

The frame is a critical ski bike component - any damage can compromise the structural integrity. Some may be prone to cracking or breaking if exposed to much stress or impact. Both aluminium and carbon frames have advantages and disadvantages. 

Aluminium frames are generally more affordable and known for their durability and strength. They are lightweight, handle rough terrain well, and are simple to repair and maintain.

On the other hand, carbon frames are more expensive than aluminium frames but preferred for their even lighter weight and stiffness. They can absorb vibrations well, making for a more comfortable ride. Carbon frames are less prone to corrosion and fatigue than aluminium frames but are harder to repair, and any damage can compromise the structural integrity.

The choice between aluminium and carbon frames for a ski bike or snow scoot ultimately depends on your budget, riding style, and personal preferences. Riders more concerned with durability and cost may prefer aluminium frames, while those prioritizing lightweight and stiffness prefer carbon frames.

Developing carbon fibre frames requires destructive testing to determine the breaking points of the head tube angle and the bending point of the seat support and foot pegs. 

Concerning safety, the foot pegs are the most critical part of the whole frame construction. On hard landings, as the rider is standing on the foot pegs, most of the force is absorbed by the rider’s feet. The foot peg mounting must be strong enough to provide sufficient support without breaking or bending. 

We simulated and recorded drop tests with a high-speed camera, DS-CAM-1100m to analyze the force required to deform the ski trike structure.

Measuring with the help of a high-speed camera, we dropped weights in different height increments on the back of the seat and observed when the shock was sufficient to bend the frame. The results obtained by this experiment provided the designers with a reference force that would be present if a rider would hit the seat on a rough landing.

Jumping and the consequent rough landings also result in high forces on the front forks. The head tube is the most common failure point on a regular bicycle - and the snow trike is no different. Our testing determined the number of carbon fibre fabric layers required to withstand similar or greater maximum forces than its aluminium counterpart. 

For this test, we rigidly mounted the frame onto a welding table and made a jig that would gradually increase the forward force on the forks. The result provided valuable insight into the force levels needed to bend, crack, or break the head tube welds.

Knowing the induced force that the standard aluminium frame can withstand, the designers of the carbon fibre frame can now adjust the model to be sufficiently strong without adding unnecessary weight.

Next development step

The idea of further development is now to focus on the handling or riding characteristics. To initiate this, we made preliminary measurements to have data on how the snow trike behaves on the slopes. 

We performed the last step of our ride characteristics testing at Krvavec Ski Center - on the closed course prepared for the first international ski bike race held in Slovenia. With the help of an IMU1 unit coupled to a KRYPTON CPU, we wanted to measure parameters such as cornering angle, corner force, and cornering speed. 

The conceptual measurements provided us with some insight into the basics of handling. Nonetheless, we need more detailed measurements. We need a better understanding of the relations between slope angle, cornering angle, and handlebar rotation. Instead of an IMU1, we will use a NAVION-I2 device, providing additional slip angle measurements with an extra angle sensor for the handlebar rotation.

Conclusion

We did our preliminary ride characteristics testing the day before the International Skibike Race at Krvavec. I, the testing engineer, even took part in the race filled with the confidence provided by our measured data. Unfortunately, riding is more than good design. On my way down the slope, I made a bad judgment, followed by a crash, which prevented me from taking the podium.

“I’m surprised by the results that we acquired from the preliminary handling measurements,” says Hillstrike manager Matic Hribar: “These results give us an insight into which measurements are feasible and what will be the environmental requirements if we want to empirically compare the riding of our snow trike with snow bikes and snow scoots.”   

Over the years, Dewesoft has been happy to be part of developing many innovative products. Specially, we are proud of the local companies in the Zasavje region that try to bring something new and different to the market.

Dewesoft's technology and expertise in data acquisition have been instrumental in developing advanced measurement and analysis systems for snow strikes. Systems that enable riders and coaches to track performance metrics, such as speed, acceleration, and power output, crucial for optimizing performance. 

In this case, our measurements validated the Hillstrike product concept. In the future, Hillstrike wishes to equip different types of ski bikes with Dewesoft measurement equipment to gain more insight into how the snow trike behaves in the snow. The empirical data show a winner in a straight line and cornering speed but also provide a clear understanding of the path Hillstrike can choose in further product development.

Overall, the growth of the snow bicycling scene is a testament to the creativity and ingenuity of companies like Hillstrike. They have helped transform an obscure activity into a thriving and exciting sport. 

The ski bike race

On Saturday, March 4, the first weekend of March, the 1st International Skibike Race competition was the highlight of the three-day skiing event in Krvavec, Slovenia. Skibikes, snow scoots, and snow trikes competed for European Cup points.

The Krvavec ski center is in the Kamnik and Savinja Alps at an elevation from 1480 to 1971 meters. It’s in the heart of Slovenia - just 25 km away from Ljubljana, the capital of Slovenia. You access the ski center by gondola, foot, or car - but don’t forget winter tires!

A narrow gorge, long and winding, leads from the Ljubljana plain to the lower gondola station. This Saturday, the parking lots and the roadsides are packed with cars. You may end up parking a long walk away from the station. Luckily a free bus service can pick you up a bring you to your “stairway to heaven.”

Blinding sunshine and glittering white snow. Amazing views of the green Ljubljana plain below and nearby rising mountains. Thousands of weekend visitors of all ages populate the slopes and crowd the outdoor dining areas. Speed and sunglasses, picnic and pop music - freedom and relaxation on the top of the world!

Thirty competitors from four countries competed at the Krvavec SkiBike Championship Euro Series 2023, joining two ski biking events following the rules of giant slalom. The ski biking events attracted skiers and riders from Austria, Croatia, Germany, Greece, France, Italy, Switzerland, Turkey, the USA, and Slovenia. 

The winner was Austrian Sebastian Bichler, who was faster than the Slovenians, Matjaž Urtelj in second place, and Miha Glavič in third.

Sebastian Bichler is a professional ski bike rider with several speed records. In 2017, Sebastian set the world record for the fastest ski bike speed at 138.752 km/h (86.129 mph) on a modified snow bike at the Vars Speed Challenge in France. In 2018, he broke his record by reaching a top speed of 145.082 km/h (90.187 mph) on a specially modified ski bike at the same event.