The Traffic Accident Reconstruction Origin -ARnews-

Snowmobile speed response to archive thread

Michael V. Catrini (XCRtoyworx@AOL.com)
Thu, 29 Oct 1998 14:47:59 -0500 (EST)

I had stumbled across an earlier thread on this webpage while doing some net surfing and could not pass up the opportunity to respond. Being an avid snowmobiler, both recreational and race for over six years, I found the topic interesting

The discussion, as posed by Charles J. Berg, was of some ramp testing of snowmobiles last winter and the characteristic of the rear of the machine to drop dramatically after leaving the ramp.

There were a few responses as to the weight and CG of the rider and his(or her) affects on the attitude of the machine.

There was also a response by Charles with regards to the length of the track becoming a factor.

Although the rider is a factor, Charles was looking in more of the right direction. However, it is not so much the track length, as it is the movement of the track.

My original response was an e-mail sent to Gene K. Baxter (who's original response lead me to this sit). Following is a copy....

Subj: snowmobile speed
Date: 10/27/98 1:38:19 PM Eastern Standard Time
From: XCRtoyworx
To: baxter@inficad.com

Although it appears that this quotation of yours appears to be old...somehow i pulled this up while searching for snowmobile information. It quotes you as stating that the reason for the rear of a snowmobile to drop when it goes airborn as being the rider pushing himself backwards in preperation for the landing..Although this may contribute to the phenomenon..it is however not even close to the reason why...The answer lies in the rotational mass of the track that drives the snowmobile..Because the track is still rotating after leaving the ground, the torque from the rotation causes the rear of the snowmobile to pitch downward..It is this affect that allows the rider, if experienced and knowledgeable about this affect, to correct the attitude of the sled while airborn..Application of the throttle will add more downward pitch to the rear of the sled..Application of the brake will add downward pitch to the front of the sled..Thought you might like to know..

Gene was kind enough to reply and provide me with the web page address and also add some more of his own insight on my thoughts. His e-mail follows..

Subj: Re: snowmobile speed
Date: 10/28/98 11:35:07 PM Eastern Standard Time
From: baxter@forensicgroup.com (Gene K. Baxter)
To: XCRtoyworx@aol.com

Hi XCRtoyworx. Thanks for the clear explanation. That makes more sense than my thoughts regarding why the rear of the snowmobile may appear to drop when it goes airborne. In fact, building on your explanation, it then becomes apparent why the rear of their snowmobile always fell during their tests. As the snowmobile leaves the ramp, the sudden decrease in load on the track will cause the track to suddenly accelerate (assuming the throttle position remains constant) and, thereby, causing the rear of the snowmobile to drop, per your rotational explanation below. Good insight on your part. To be more precise, however, its not the torque from the rotation that causes the snowmobile to pitch downward, its the sudden change in rotational inertia of the track. Thanks for the insight, and for taking the time to explain it to me. Best regards, Gene.

Any more thoughts on this would be helpful as I am currently working on a new snowmobile suspension design..

Please e-mail me at XCRtoyworx@AOL.com

thanx again Gene..
Michael V. Catrini
XCRtoyworx@AOL.com

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