top speed reduced..

[grader]

i agree with you arteeex, but my reference to track rpm is still correct. if you think of a track spinning around 2 points, drivers and rear idlers, these are the points that rob energy as the track is being forced to change direction 180 degrees. put a mark on the 121 track and calculate its revolutions for any given distance. do the same for a longer track. the marked point on the shorter track did more passes around the drivers, and lost more energy doing it.

 

[ranger1]

Grader, as long as the idlers & drivers are the same size on both the 121 & 136 sleds, wouldn't they all rotate the exact same amount on any given equal distance even though the short track itself is making more full revolutions? Just a thought

 

[John R]

Here is an interesting fact about track speed. When your sled is travelling 100 MPH. The track on the ground is not moving reletive to the ground,(it leaves it's mark), therefore track ground speed is 0 MPH. At the same time the track in the tunnel is travelling twice the sled speed (200 MPH) as it gets to the front where it once again comes in contact with the ground. Just like your feet when walking, one foot on the ground,(not moving) the other foot travelling twice your walking speed to get forward.

 

[John R]

Should have been relative.

 

[Blue Dave]

John,

Welcome to TY! Please add your location to your profile. Also, note that you can edit your posts so you do not need to make a spelling correction in an additional post.

The diameter of the drivers on the track drive shaft determine the track speed relative to the drive shaft RPM. If you do not change the driver diameter when going to a longer track then the track still moves the same distance with each drive axle revolution.

Therefore with all else being equal a longer track will make less full revolutions per minute at any given speed but the "speed" of the track (linear inches per second / feet per minute / miles per hour) will be the same regardless of the overall length of the track.

If your track is 100% hooked up with the ground (zero slippage) I agree with you that the track speed relative to the ground is zero. Any track speed relative to the ground is slippage.

However I disagree with your other comment. Specifically that if the sled is going 100 mph (with zero track slippage relative to the ground) then the track speed is 100 mph (not 200 mph) relative to the slide rails, the tunnel, or any other fixed point along it's travel.

The longer tracks have more rotating mass, more surface area of friction with the slide rails, and usually taller lugs which act like a fan trying to move air in the tunnel. It is for these reasons that long tracks usually will give you less top vehicle speed than short tracks. I am only talking about absolute top vehicle speed. The maximum vehicle speed reached within a given distance is a different story. Depending on the distance from a dead stop to the finish line, a long track in the correct conditions can sometimes achieve a higher vehicle speed (and less elapsed time) over a given distance because of the increased traction (less slippage relative to the ground) that the longer track and taller lugs can provide.

 

[John R]

Dave, I realize the track speed is a constant 100 MPH and relative to the slide rails is 100 MPH, however relative to the ground the track in the tunnel is moving forward at 200 MPH GROUND speed. The tunnel is moving forward at 100 MPH and the track is moving forward in the tunnel, therefore moving forward faster than the tunnel. (ground speed). Ask a physics teacher. John.

 

[copo427ss]

Lets slow it down a bit. If there is 0% slippage and the machine was traveling at 5mph, and you were walking next to the sled and could see the top of the track. You would be keeping pace with the top of the track at 5mph. Now if there was any slippage at all then the track would be going faster or slower than 5 mph. At 0% slippage the track cannot be moving faster than the machine it's propelling. John walk next to a bulldozer, excavator, tank, ect. You will walk at the same speed as the top of the track is moving and the machine is going. They are both going in the same direction. So at 0% slippage neither is faster than the other.

 

[Dano]

I think someone is getting mixed up on where the track is driven from and put in relation of a car tire. When driven at the bottom, hence the cogs of track, it doesn't matter if your track is 120" or 180", you just need to overcome weight and resistance. To power a circumference at the center as a tire, then your difference in speed is effected by the difference in driven center axle of wheel to outer surface. Change driver size on a sled and speed is effected. Change track length and speed is the same, but compensation has to come from somewhere for extra weight/lug resistance when dealing with same hp. But in theory, adding lug height can give you more top speed as it increases distance from drivers to riding surface given that track was on a hard surface. Those gains are quickly overcomed by rolling resistance.


Dan

 

[John R]

Dave, sorry I must have been having a senior moment, you are absolutely right. My appologies for going so far off topic. John.

 

[John R]

Sorry Dave, but I am back again and I stand by my original statement. When a tracked vehicle is crossing a field at 5 MPH and the upper track is not moving faster than the machine, it is on a trailer. If the upper track and machine are both going 5MPH. the track would never get to the front.
Try this simple experiment. Lay a can or jar on it's side on a table, place your fingers on the can and move your fingers, hand, wrist and arm forward rolling the can forward without slippage, you will find that the can goes only half as far and half as fast as your arm. The can is the rear axle of your sled, and your arm is the track.

 

[Blue Dave]

Mmm...... interesting. You made me think again on this one! I was actually about to agree with you before Copo 427 ss chimed in and got me thinking some more with his post.

However after thinking some more I agree with you since we are talking about the speed of the track relative to the GROUND (not the skid frame of the sled).

The track speed relative to the skid frame (top or bottom) is the same as the sled's ground speed if there is no track slippage. That is easy for everyone to understand and I do not think that there is any argument with that.

However I never thought about the track speed relative to the ground. I agree with you that with zero track slippage the speed of the bottom of the track relative to the ground is zero. Since the top of the track is travelling at the same speed but in an opposite direction I would have to agree with you.

The difference between the top and bottom of the track is twice the track speed. Imagine two objects travelling away from each other in opposite directions each travelling at 10 mph. The distance between them will increase at a rate of 20 mph.

It is easy to visualize this with your trailer scenario. First imagine the sled on a jack stand on a stationary trailer with the track spinning at 10 mph. The bottom of the track has a speed of 10 mph relative to the ground in one direction and the top of the track has the same speed relative to the ground but in the opposite direction.

Now imagine that the trailer is moving in the same direction that the sled is pointed with a trailer ground speed of 10 mph. Now the bottom of the track has zero speed relative to the ground and the top of the track has a speed of 20 mph relative to the ground.

 

[John R]

Thanks, Dave, I am not crazy afterall. Bring on the snow! John.

 

[deweysgt]

by simply extending the rails to 136" without making the proper changes to the chassis (i.e. a true conversion to Attak configuration) is one of the biggest problems. Been there, done that

 

[ZapeX2]

lol.... you guys are making my head hurt....

 

[Sled Dog]

think about it this way, the longer the track, the lower the rpm of that track at any given speed. it has its limits of course, but i lost nothing from 121 to 136, and next to nothing from 136 to 144. 2 strokes are another story as my buds with ski doos cant pull the same speeds when going even to a 136. must be a torque thing.

This isn't true with a track drive. The angular displacement of the drivers is transferred to the track therefore the track length doesn't matter. For example, ten revolutions of the drivers will move any track length the same distance forward. Without slippage, a 121" track is moving at the same velocity as a 163 track at a given speed; the shorter track will make more revolutions but it isn't moving any faster. Regardless of the track length, there's still the same number of inches in a mile that the track has to cover.

The inertia of the longer track will effect the top-end speed because it takes more energy to accelerate the additional mass.

X2

The driver size dictates the speed as it covers so many inches per revoution. The track merely follows what the driver speed is dictating same as your top gear on a chain case. One thing to also remember is that the force is applied to the top side of the track the track when moving forward on the ground is the slack side. If you think of your chaincase which works the exact same way the chain being the track the slack side is where the tensioner picks up the slack. On your sled the suspension compresses on the top under acceleration which is why you dip then as the sled picks up speed and there is less load on the top the springs / shocks expand and take up the slack. For those of you who have experienced driver ratcheting on your track know that your speed is slowed but your drivers were still going faster than the track.