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post #1 of (permalink) Old 12-21-1999, 08:39 PM
deepnsteeP
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a few ideas

Question for all of the snowmobile mechanics out there. I don't know if I'm missing something here, but by the Physics law of precession, the clutch would tend to pull out away from the engine. I was thinking of the possibility of having the clutching on the opposite end of the engine. I don't know if it would work any better, but it would take way smaller weights because the clutch would tend to move in on its own. It may cause less wear and tear and maintenance on the clutch.

Another little idea that I have been contemplating is the possibility of having a tertiary and quaternary clutches in the place of the gears or the belt system of the new CMX snowmobile. I thought that it may increase the low speed, hillclimbing power, while keeping the hi-speed on the trails. They wouldn't have to be very big or have a lot of movement to make a lot of difference in the gear ratio. I don't know how much horsepower it takes to operate clutches. That's my question, would it take too much horsepower to operate the four clutches to work very well?

just a couple of far out ideas
thanks for some response

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post #2 of (permalink) Old 12-24-1999, 07:30 PM
deepnsteeP
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Re: a few ideas

Come on guys, a little feedback here. I would appreciate it very much.

post #3 of (permalink) Old 12-25-1999, 11:14 AM
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Re: a few ideas

Hey Steep,
I have never seen any real data that says how many ponies a clutch "costs". I've seen
advertisements for "windage plates" for the Ski-Doo TRA secondary that say they pick up
something like 5 HP just by reducing the aerodynamic resistance.

Also - not trying to nit-pick just have a discussion - I believe the physics principle you are referring
to is the "right-hand rule" in regards to describing the moment of a force. The princple goes
something like this - curl your fingers in the direction of rotation and extend your thumb. Your
thumb will be pointing in the direction of the moment of the rotational force.

Precession, is a gyroscopic principle. Spin a gyroscope and balance it
vertically on your finger. The top of the gyroscope will start revolving in a circle. As the
gyroscope slows down the greater the diameter of the revolutions. The Earth itself
actually is a huge gyroscope and precesses in the same way. This is why the equinoxes very slowy
change - I cannot remember if they become earlier or later. It takes something like 22,000 years for
the Earth to make one precessional revolution.

Anyway, back to clutching. I expect that each clutch will make a respectable impact
on available HP. It's more rotating mask and frictional area (bearings, gears or whatever).I
think that the FAST Blade is going in the right direction by
eliminating the chaincase altogether and reducing rotating mass. Next in line
would be CMX and their belt system.

I too would like to see someone measure this. It could be done with one of those
Land and Sea Dynomite dynos. The thing is made to fit the end of a
crankshaft and I expect that with a little clever fabrication it could be adapted to
other parts of the sled. The problem here is how much HP will the pulleys
et al contribute to HP loss.

Just my .02 worth.

Regards,
Dan

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post #4 of (permalink) Old 12-25-1999, 01:06 PM
deepnsteeP
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Re: a few ideas

sorry about the misprint. I'm in high school and trying to remember some of the things that the teacher rambles about in class. I just thought that the lighter clutch would make some hp difference.

Also, the Blade isn't the first to eliminate the gearbox. My dad has a 1984 Polaris 440 SS that is a direct drive.

post #5 of (permalink) Old 12-25-1999, 03:37 PM
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Re: a few ideas

As Dan said, precession is the slow rotation of a spinning object and is a part of gyroscopic motion. Precessision is caused by an external moment being applied to the spinning object (out of axis with the spin). There are no axial forces involved.

There are actually no forces acting axially on the clutch (along the axis of spin), other than caused by snowmobile cornering forces. It doesn't matter whether the clutch is on the right or left side of the sled, the forces are effectively equal - depending on which direction you're turning. As you are turning there are gyroscopic forces on the engine bearings, but they are radially applied to the bearings.

The right hand rule is simply a method of defining a vector that relates to rotation, an applied torque (or moment), or anything else related to rotation (rotational velocity, acceleration, etc.). The rule is that your four fingers of the right hand are wrapped in the direction of the rotation when your thumb points in the direction of rotation. There are no axial forces along this vector, just rotation, etc. in the finger direction.

As far as efficiencies. A typical snowmobile transmission is between 75% and 90% efficient, depending on the gear ratio and clamping forces on the belt. What robs hp is bending the belt around a small radius (converts hp to heat - more at low and high gear ratios), belt slippage and creep, clamping forces create friction (with current designs the clamping loads are higher than required for the engine torque at high snowmobile speeds), and air drag on the clutches and belts. I can't remember the numbers for a chaincase off hand, but they are considerably more efficient (I believe 95-99% efficient). A constant ratio belt drive is somewhere between a variable belt clutch and a chain (depending on how well it's designed). One big advantage to running a belt instead of a chain is a belt removes much of the harshness and harmonics from the system and may be able to allow lighter shafts, etc. to be used (or less vibration for the driver, longer lasting parts, etc.).

A lighter clutch would reduce rotational inertia allowing faster accelerations, but won't effect (significantly) constant speed efficiency.

A snowmobile is quite an ineficient peice of equipment really. By the time the engine hp reaches the ground it is cut approximately in half. I'm sure if you hunt around a little you could find real numbers, but if you have a 100hp sled, you loose on the order of 10-25 hp in the transmission, 2-4 hp in the chaincase, and another 10-30 hp in the track drive, slide friction, bearings, track flex, etc. After that it takes a fair bit of hp to pack the snow down under the track and push the track lugs into the snow, so what you're left with moving the machine forwards is probably around 30% of the total engine hp for good snow conditions.

Keep in mind these numbers above are educated guesses. I haven't worked out the actual numbers. The transmission efficiency numbers are from some testing that was performed by AAEN Performance during the developement of some of their racing transmissions (they actually quoted 80% to 92%, but I believe a typical snowmobile is slightly worse).

Steve S.
http://home.off-road.com/~ovo/members/sshaw/sshaw.htm
post #6 of (permalink) Old 12-25-1999, 03:39 PM
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Re: a few ideas

The line that read:

"The rule is that your four fingers of the right hand are wrapped in the direction of the rotation when your thumb points in the direction of rotation."

Should read:

"The rule is that your four fingers of the right hand are wrapped in the direction of the rotation when your thumb points in the direction of the vector."

Steve S.
http://home.off-road.com/~ovo/members/sshaw/sshaw.htm
post #7 of (permalink) Old 12-26-1999, 05:46 PM
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Re: a few ideas

I read some imfo provided by Gates indicating belt power transfer efficiency can be altered significantly by adjusting belt speed. This would tell me that one of the things we sometimes error on particularily in the mountain sleds is we a lot of times gear down in the chain case which if our engine is running at peak rpm will result in a higher belt speed. Maybe we should only gear down to produce the desired results at initial takeoff and no more.

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