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Blown Drive Belts

Tech Shorts January 10, 2006 0
One of the most unsettling events that happens when riding a snowmobile is that of blowing a drive belt. When it happens, the noise...

One of the most unsettling events that happens when riding a snowmobile is that of blowing a drive belt. When it happens, the noise and suddenness can shock and startle you. Often times, it is not known for certain a drive belt was the cause, as it can be mistaken for a blown motor or maybe a bearing in the drive system.

Most experienced riders have blown a belt when really into the throttle, and they know if they don’t respond immediately (get out of the throttle) there can be major consequences. Mainly, the engine is free to spin with no load, and can bump into the ignition’s rev limiter (if equipped with one).

Not getting out of the throttle quick enough can also cause the event to do more damage than a simple belt replacement. Often the speedo cable gets wrecked in the process, and sometimes crank seals can be damaged in lower engine case behind the primary clutch.

Sometimes these things happen even if the rider backs off as soon as they can. This is why many riders won’t leave a drive belt on their sled if there is any obvious tell-tale signs of imminent failure. The cost of being pro-active and installing a new belt can be far less than the damage caused by a blown belt that could have easily been avoided.

Riders should be aware of the conditions that cause extra heat to be generated. Mountain riders know this all too well, and are far more familiar with it that trail riders. Heavy pulling in deep or wet snow, high engine speeds and lower ground speeds, or low shift ratios, that slowly build heat in the clutches and drive belt.

Be aware of the signs that belt failure are imminent. At the first sign of clunking or thumping, back off. Often you will find a single or multiple cogs from the underside to be missing, and complete failure is only minutes away (usually). Hard engagement usually indicates a very worn (narrow) drive belt, and the flyweights have to travel further to get the clutch sheaves to engage and grab the belt. Edge cords starting to come out of the belt sidewall are not good either, but this poses less of a problem than missing cogs underneath.

Keeping your clutches clean and well maintained is one of the most important things you can do to maintain the peak performance of your sled. Again, mountain riders know this better than trail riders due to the demanding conditions the deep snow presents. All of the bushings wear, as do the contact surfaces that are transmitting all of the power from the engine to the track. Flyweights, rollers and bushings are the wear items in the primary (drive clutch), where bushings and buttons are the main wear items in the back (secondary) clutch.

And as strange as it may sound, replacing the spring in the primary clutch is a good idea to consider every time you’ve blown a belt. Not out on the trail, but when you get back to the shop. These springs (unless they’re titanium) suffer from the same heat-soaked conditions that weaken drive belts, and when they lose their spring rate the shift characteristics of the sled change, and performance suffers. When a sled doesn’t run right, the drive belt and clutching is usually one of the main causes.

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