Dear Ralph: My sled has a thumping noise and vibration that gets worse as I go faster. Where should I be looking for the...

Dear Ralph:
My sled has a thumping noise and vibration that gets worse as I go faster. Where should I be looking for the cause?
Clueless in Seattle

That’s a classic; usually this is caused by an “hour glassed” drive belt. This condition is usually brought about by burning the belt. You give it throttle, the track is stuck for whatever reason, the drive clutch squeezes the drive belt, the drive system does not turn, and the sheaves pretty much burn a notch into the belt, with the end shape looking like an hourglass.
From that point on, the sled will thump and vibrate. The worse the notch, the worse the thump. Sometimes you can’t see it all that easily, but the quick and easy way to verify is to install a new drive belt.
Most often the track will be frozen to the ground in the morning. Guy comes out and fires up his sled, warms it up, then hops on and gives it gas without verifying the track isn’t froze to the ground. The sled, when parked the night before, was warm and had some slush up in the suspension, even though the air temp was well below freezing. As it sits overnight, it freezes either to the ground or up in the suspension. This is why you want to lift and drop the rear of the sled to move the suspension around some, and this tends to loosen up the track as well. Making sure it is free before hopping on reduces the initial load.
This is why some riders will spin the track some in the morning, more so on colder mornings. Be very careful of ice flying out of the suspension, and never stand behind a sled that is spinning the track! You only want to break it loose, do NOT spin it at any kind of speed!
It can also happen when you’re stuck, especially in deep wet snow. Here the track acts as if it is frozen, as the load from all directions is great.
Bottom line; anytime you first engage the belt on a cold sled, you need to be aware of the potential of drive belt damage. You want to engage the drive clutch abruptly so as to break the track loose, but you must do so quickly or you will burn the belt. We’re talking maybe no more than one second to a second and a half. Or, if you ease into it too slowly, the sheaves will sit there and do a slow burn on that spot of the drive belt. Either way, you just wasted a $50 bill (or more).

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