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I Effects of lug height on final gear ratio for a snowmobile...

  1. Apr 8, 2016 #1
    So, great site.
    I have a quick question that I think has a simple answer. On a snowmobile forum I visit there has an ongoing debate. The question is, all other factors being the same, does changing the lug height on a track, change the final drive ratio of the snowmobile? I think I know the answer but articulaying it correctly is difficult.
     
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  3. Apr 8, 2016 #2
    If you're driving on pavement, sure. The tips of the lugs, in contact with the pavement, are at a larger radius from the driving axle.

    If you are on snow, my opinion would be "probably a little bit." It would depend on where the effective center of force occurs on the snow-lug interface. If you are on hardpack riding on the lug tips, it would be the same as driving on pavement.

    On light, fluffy stuff I would expect the center of drive force to be anywhere from half way along the lug depth up to the body of the track. With the result that you would get between one half the expected drive ratio increase to no change at all.

    In the real world you probably wouldn't notice the difference. When you take into account the radius of the drive pulley plus the track thickness and lug depth, adding a little to the lug depth is going to be a small percentage change.
     
  4. Apr 9, 2016 #3

    sophiecentaur

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    You could get an idea of the different effective radius of the 'output' wheel by listening to the change in pitch in the engine over different surfaces or by measuring the speed over ground at a given number of revs. But there will be an added complication of slippage in very light snow because the snow will actually be moved backwards over the ground. The wheel revs will be higher than you 'd expect when there's slippage and you will be in the realms of Paddle Wheel motorboats.
    I would guess that finding an optimum would best be done by trial and error - if you have access to different drive wheels.
    Would it be possible for you to use a high speed film of the wheel's motion as it goes past a point? You could actually see (or imply) the part of the wheel that's stationary relative to the ground.
     
  5. Apr 9, 2016 #4
    Ok, so I should add another detail and see if your assertions stick. Say that for the sake of argument, there are two tracks, both solid. One is 1" thick and one is 1' thick. The drivers of the vehicle are the same diameter throughout the drive trains. Does the thickness of the track change the final drive ratio? if the argument that the radius of the track being larger makes the ratio change, wouldn't a track that is 100" long be a lower ratio than a track that is 150" long?
     
  6. Apr 9, 2016 #5

    sophiecentaur

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    I'm now visualising it right, I think. I was imagining the drive wheel actually digging into the snow to provide the drive. We never use those things in my part of the UK!!!
    We're talking about a tracked vehicle (durr).
    The speed of the drive wheel over the track will be the same as the speed of the vehicle over the ground (with no slip, the parts of the track in contact with the ground will be stationary relative to the ground). So I reckon that the only thing that governs the ratio is the radius of the drive. The thickness of the track may help or hinder the grip but, if there's no slip, it can't affect the ratio because there is no 'rotation' of the bottom section of the track (I assume).
    The actual length of the track is irrelevant to the ratio because the only bit of track that's transferring drive to the ground is the horizontal bit. It's the equivalent of driving over a strip of track that's been laid over the snow (basically). The details of which bits of the track are actually pushed against by the drive wheel in only relevant in as far as where the teeth of the drive wheel actually bear on the links of the track (i.e. deep into the teeth or near the tips) but I guess the wheel and track are always matched. The ratio depends on the number of teeth on the drive. Each rev will push you forward by the number of teeth times the pitch of the track.
    I've said the same thing is several ways. Dunno which one makes more sense to you. :smile:
    If I'm still talking BS then give me a photo of the vehicle.
     
  7. Apr 9, 2016 #6
    That is my assertion as well. There has been a long argued discussion on a site I participate in and now it has come down to placing wagers on who is correct. I'll wait for some more replies before I declare victory though.
     
  8. Apr 9, 2016 #7

    A.T.

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    Yes:

    129_0701_03_z%2b2007_yamaha_apex_rtx_snowmobile%2brear_suspension_track.jpg

    That is also my impression, unless the curved / rotating parts of the track transmit a substantial part of the driving force.
     
  9. Apr 9, 2016 #8

    sophiecentaur

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    I think you are right but it's not absolutely straightforward because of the bit around the wheel.
    Looking at images of tracks, it seems that the track is fairly flexible rubber and that the teeth will not, individually, produce much force on the snow. That means that the relevant radius in the system is where the rubber belt lies and not the length of the teeth. The majority of the traction force is from the teeth on the horizontal section and the force there is definitely the same as the force on the belt (same argument as previous).
     
  10. Apr 9, 2016 #9

    sophiecentaur

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    I have thought about that over my dinner. The fact is that the tangential forces from the rotating teeth are not acting horizontally and that's what counts in calculating the effective gearing. The horizontal component is, I think, the same as for all the other teeth so the 'velocity ratio' (i.e. the gearing) is the same all the way along.
    So the OP has to be correct about the teeth depth having no effect on the gearing.
    It would be easy to find out by turning the engine over manually and measuring the distance travelled with two sizes of track.
     
  11. Apr 9, 2016 #10
    Ooops! You are right.

    I hereby retract my post #2. Sorry folks.
     
  12. Apr 10, 2016 #11
    Can some articulate the differences between a drive wheel diameter determining gear ratio versus a drive system of a tracked vehicle with more than one rotating axis?
     
  13. Apr 10, 2016 #12

    sophiecentaur

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    If you mean more than one drive wheel, the drives must have the same peripheral speed (they are locked together by the track) and I would think there would have to be a differential between the two drives, to share the drive without 'contention'. If you had one large drive wheel and one small drive wheel, both driving the track then you would need to have the right gears to make those wheel speeds right. The overall 'gear ratio' - in terms of linear metres per engine rev would be the same for both wheels.
    Does that answer your question? I wasn't sure what you meant.
     
  14. Apr 11, 2016 #13
    No, not really but it is a good point that further reinforces my assertion. My question is how do you articulate the relationship between a drive system with one axis (ie Wheeled vehicle) versus a drive system consisting of 2 or more rotating axis (tracked vehicle)?
     
  15. Apr 11, 2016 #14

    sophiecentaur

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    HAHA. I thought that's what I just did.If you have two drive wheels of different diameters then the gears before each one must ensure that the peripheral speed of both wheels is the same.
    Note: If you want to specify the 'gear ratio' for a system with a linear drive (i.e. a track), you can't express it as n:m where n and m are turns of a shaft. You have to specify it in terms of turns of the input shaft for a given linear distance.
    Whether some, one or all axles are driven, the revs of all wheels have to (will) follow the rule. and the fact that they are bearing on a different bit of the track makes no difference. They could be side by side and it still would make not difference because all that counts is distance per rev has to be the same for all.
    OR . . . .are you wanting a statement about how the Power to each drive wheel is shared? That depends entirely on how they are driven and it's also very relevant to 4WD cars, where the front and back wheels may not have equal torque but their speeds have to be the same (natch). I can't see any point in things being that clever on a tracked vehicle because you cannot get one slipping but the other not. I made the point earlier that the two drives would probably need a differential drive of some sort to avoid stress on the drive and track is, for instance, there is some stretch in the track..
    PS could you restate the question with a different word from "articulate" because I am not sure what you mean precisely.
     
  16. Apr 11, 2016 #15
    If you mean the ratio of engine RPM to road speed then yes, changing the thickness of the track will change the ratio.

    Look at where the center of the drive sprocket axle is in relation to the surface of the track. You can calculate the tangential velocity (essentially vehicle speed if you ignore track-to-road slip) of any point from the axle out. The tangential velocity is the rate of rotation times the radius from center of rotation. So for a given RPM the tangential velocity increases as the radius increases. If the surface of the track gets farther from the axle (thicker track) then the tangential velocity increases. Increasing tangential velocity relative to engine speed says the drive ratio has changed. It's just like putting larger diameter wheels and tires on a car, the vehicle will travel farther per axle revolution.

    However, when talking about vehicles (e.g., cars, trucks, motorcycles) tire diameter is not included in quoted final drive ratios, just the ratio of engine RPM to output RPM. If you change wheel and tire diameters significantly you need to have your speedometer recalibrated or risk getting speeding tickets.
     
  17. Apr 11, 2016 #16

    A.T.

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    Ok, let's have a look:

    how-snowmobile-works.gif

    Looks like the drive sprocket is nowhere near the ground.

    What is this radius, for the straight part of the track that contacts with the road? Where is the center of rotation for that linearly moving part of the track?
     
  18. Apr 11, 2016 #17

    sophiecentaur

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    I'm glad some more people have joined this thread.
    You would need to justify that.
    As far as I can see, where it is in contact with the road (where it counts) the top and bottom surfaces of the track are moving at the same speed. The periphery of the drive wheel is the thing that sets the speed of the inner part of the track that it's in contact with it. If the inner part of the track does not compress or stretch then that is what determines the speed of inner and outer surfaces on the straight section, in contact with the road. The pictures of tracks (Google has dozens and dozens of them) seem to have a continuous inner belt, in contact with the drive wheel and any added thickness of track consists of articulated transverse treads. These treads radiate outwards when going around a curve or drive wheel but are parallel along the straight section. So the distance per drive wheel rev is independent of the thickness of the track.
    I can see that an alternative design of track could change things - for instance if the inner surface of the track had teeth that compress longitudinally as they go over a wheel - but I don't think that is the case on a snowmobile.
     
    Last edited: Apr 11, 2016
  19. Apr 11, 2016 #18
    Justify? Now you've got me smiling. Let's get that out of the way. How are you defining final drive ratio? How are the people in your discussion defining this?

    Where it's in contact with the ground it isn't moving at all.

    Now, do you agree that in circular motion tangential velocity equals angular velocity times radius? If not, justify.
     
    Last edited: Apr 11, 2016
  20. Apr 11, 2016 #19
    In response to A.T., above, I'm talking about the distance from the axle center to the surface of the track, not the road or ground. The straight part of the track is completely irrelevant. (Got me smiling with that one). Note that where the track is on the ground it isn't moving, whether or not the vehicle is moving.
     
  21. Apr 11, 2016 #20

    jbriggs444

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    I expect that a real track will have a fabric belt embedded within to sustain the tension load. This belt will likely be near the inner surface of the track. It is the speed of this fabric belt relative to the drive wheel's rotational speed that is crucial. Accordingly, it is the distance from the axle center to the fabric belt that matters.
     
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