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Snow Science: Understanding Track Durometer

Snow Goer staff

Do you fully understand how your track’s durometer will affect the quality of your ride? Snow Goer’s own T.J. Krob, author of the Snowmobile Science column, provides a detailed background in the December 2017 issue of Snow Goer magazine. In every issue his columns provide a scientific or engineering background on something related to snowmobiles. To regularly read TJ’s column and many other reviews, comparisons and features in each issue, subscribe to Snow Goer magazine today.  

Track Durometer

The Ideal Setup Depends On The Intended Application

By T.J. Krob

As I write this, it’s a hot day in August under smoke-filled skies with not a drop of rain in weeks. The ground appears to be melting, with a steamy haze rising from it. Car tires zipping down the road on the asphalt must be near boiling – but how do those tires hold up to such abuse?

We’ve all done a few smoky burnouts, shredded some rubber off a dirt bike tire or even shredded a drive belt on a snowmobile – all things considered, those black rubbery traction devices we all enjoy abusing take a licking and keep on ticking. For snowmobilers, our connection to the ground, of course, is the mostly rubber track, and like tires and belts, it’s built for a specific application with measurable properties targeted for  specific applications. One of those  properties is durometer.

Durometer is an important measurement, but it is just one factor that goes into designing a track that is custom-matched to the intended purpose of a snowmobile.

The word “durometer” can be used in two contexts, with the first being the definition, “An instrument for measuring the degree of hardness of a material.”

Invented by Albert Ferdinand Shore, the durometer scale is a measure of how hard a material is – just like the Rockwell, Vickers and Brinell hardness scales used on metals – but most often the Shore reading is used for polymer type (“softer”) materials.

As with most hardness tests, the durometer test uses an indenter point pressed into a material. The depth that the point travels into the material is a function of how the material properties interact to resist penetration. Common Shore scales include A (softer material) and D (harder material) versions, which are distinguished by the shape of the indenter (a truncated cone point, versus a rounded cone point) as they impart varied levels of force into a material. The A classification pushes with a force of 1.8 pounds, while the D is a heavier, 10-pound force.

The second context can indicate a numeric result after performing the hardness test using the durometer, stating that, “X material has a durometer of 75A” – this indicates that the A scale was used with the A indenter tip during the test, and the result was 75 on the scale – a dimensionless quantity which is the international standard for measuring the hardness of rubber. For comparison, a rubber band passes the test with a durometer of 25A, a tire is 70A and an HDPE safety hard hat is 75D.

Track Durometer

What does this all mean when it comes to a snowmobile track and its construction? The answer lies in how the rubber will conform to the snowpack, or displace it from its original location.

A softer track (lower durometer) will be pliable and won’t dig into crusty snow – but is this soft track construction a good thing or a bad thing? A low-durometer track will flex to provide lift instead of digging or scooping a trench in deep, fluffy snow. On the other hand, if hard snow is encountered, the lugs will flop over and spin without catching enough traction to provide forward thrust.

Would the best answer be to have all tracks firm, because we riders want to move forward on the snow with the utmost grip possible? No, because that wouldn’t work well in powder.

Essentially there is no right answer – the makeup of the track under your snowmobile is entirely dependent on the application at hand. Snowmobile track designers target individual segments for which a rider might be using a track.

Trail riders with short lugs benefit from a firm lug to grab hard-packed trail conditions – and some of those lugs even come with a tiny ice pick in the body of the lug to dig at ice.

A crossover 50/50 sled might be ridden on a hard-packed trail for miles on a given day and see zero off-trail time; therefore the track shouldn’t be too tall in lug height and also firm enough to grab the snow, but not too hard to break off at high speed.

Mountain sleds are in a totally different class, where typically lug height wins. Tall paddles move massive amounts of snow under the tunnel to keep sleds climbing higher while staying atop even the fluffiest of powder – yet should grab when the mountain is glazed over after drifting solid.

T.J. Krob, author of Snowmobile Science

The durometer of the track in all of these cases needs to match the application to maximize performance.

A few additional factors for tracks include varied durometers throughout the lug, lug cross section and lug placement along the belting.

Lugs can be firm at the base and transition to being soft at the tip to provide good performance, or conversely use a softer compound at the base with a firm tip. Don’t confuse the firmness of the compound by just grabbing a 3.0-inch lug and flexing it over thinking the varied durometer is being felt – this is a function of the lug’s cross section. Having more material down low near the belting of the track means it has more support to resist deflection, while less thickness at the tip requires less force to flex since the hand (or snow) is exerting force through lesser cross-sectional area.

Placement of lugs also affects performance by how the snow will be broken up or set into flow under the path of travel. Depending on snow conditions and sled application, paddles that follow very closely in order might be missing traction on a snow surface if the lug in front of it has spun the snow out of the way – a staggered pattern can be employed to offset this.

Further out in left field are the Rocky Mountain Snowmobile Hillclimb Association (RMSHA) racers who benefit from screwing aggressive ice studs into each lug to provide traction on rocks and other debris.

Any way you ride, that big rubber belt under your snowmobile tunnel isn’t thrown under there haphazardly – it’s an integral part of how the machine performs for you, the rider. Any alteration in its construction could result in ruining a good ride, or transforming one into the best of the season.

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