121" Track on a Sportfire

[wycrumb]

Hi Deere Guys,

Long time no posting for me but here is what I am currently working on.
My main rider is a ’80 Sportfire. I did the Liquifire track conversion a few years ago and it worked well but the track I put in it was nearly shot and it is not easy to find a good one at a decent price.

There are however many tracks out there that are 121” 2.52” Pitch like all modern sleds with a standard track have come with for years. I found a ’94 Polaris Super Sport to donate a 121” x 15” x ½” track. I also looked at the drive shaft but it is a longer 1” shaft than the Deere 1.125” shaft. The suspension from the Polaris was pretty well beat up so I found a suspension from a 1995 XCR 440 that is almost identical to the Super Sport Suspension. I also pulled the Polaris gears and chaincase seal, both will work on the Sportfire.

I visited my local snowmobile junk yard so I could try find an option for a drive shaft. I went into the small building where they exclusively store hundreds of drive shafts. Deere axle in one hand and caliper in the other I started looking. None of the axles looked to have the correct length, spline and drive sprockets so I just started looking for drive sprockets that would work on the Deere axle shaft. I knew going in there that Deere was an oddball using a 1.125” shaft so I didn’t hold out much hope. That was until I found a 99 MXZ 600 drive axle that was 1.105”. I made them a deal on it, took it home, took measurements, pressed the drivers off the shafts and pressed the MXZ drivers onto the Deere shaft. They fit very snug but did not show any signs of fatigue while pressing. I got them located properly on the shaft and installed the drive axle with the 121” track. Even though the MXZ drivers are 7” dia. And the Deere are about 6” they still fit in the tunnel with room to rotate. Everything fit very well.

I rebuilt the XCR suspension, doing some reinforcement welding on one part that was broken in the past on both Polaris suspensions. I replaced all 12 bearings, new hyfax, cleaned and greased everything else and new hardware wherever possible.

I geared the sled down in the chaincase to account for the larder diameter drivers. I looked up a Polaris Indy Trail 440 knowing that they used 7” drivers on the axle, and the motor was the same size as the Sportfire. The Indy Trail used a 21 tooth top gear and 39 tooth bottom gear. John Deere used the same sized gears in one of its sleds, I am thinking it is a 340 Trailfire since I parted one out and have both those sizes to use on this sled.

Lastly I mounted the new skid in the Sportfire. I ended up using the John Deere 3/4" shafts that the trailing arm pivots on and that bolts inside the tunnel because the Deere tunnel is wider than the Polaris tunnel. Put the Polaris suspension next to the old Deere suspension and limited the front arm down so it was close to the same height as the Deere skid. This would get me close to right on the amount of ski pressure needed. I then used the original hole in the tunnel for the front mounting, and I drilled out the rivet beside the original back mounting hole, this way I was still using the tunnel reinforcement plate on the back side of the tunnel. I plugged the original mounting hole in the rear with a nut and bolt so the reinforcement plate would be stronger.

The track tightened up nicely and it runs and drives great! No snow yet to really get some testing time on it but it looks like a few minor adjustments to ski pressure and I will off and running!

Hope this thread helps any of you who want your sled to be a more reliable daily rider!



Parts - '94 Polaris Super Sport



Sportfire with old track and skid still installed



Drive Shaft In Progress



Drive sprocket on the shaft, still needs to be pressed farther on.



Splined End of Deere Shaft



Deere Shaft Size



MXZ Shaft Size



New MXZ Driver



Sportfire Driver



Deere Gears



Gear Options



Finished Shaft



Chaincase



Chain Tensioner



Chaincase with guts installed, I ended up using a larger 21tooth top gear so the chain didn't have to be tightened nearly that far.



Drive Shaft / Track installed



Clearance between track and tunnel



Chaincase Seal



Rebuilt Suspension




Reinforced Bracket that always seem to break on this type of suspension.



Rear Mounting, the left bolt is the rivet drilled out for mounting the suspension, the right bolt fills the old hole.





Complete

[JDT]

Looks like you did a real good job there Wyatt.

Any idea what the weight difference is with the updated skid?

[ljm]

Awsome job. I've been wanting to do that to my 81 LF.

So if I understand correctly. You used the same front mounting hole. The rear you drilled the rivet out and used the hole from where the rivit was?

How much wider is the Deere tunnel?

I'm curious about how it rides. Hopefully you get snow soon and can give us a report.

Thanks for the excellent detail in your post.

Lloyd

[wycrumb]

Awsome job. I've been wanting to do that to my 81 LF.

So if I understand correctly. You used the same front mounting hole. The rear you drilled the rivet out and used the hole from where the rivit was?

How much wider is the Deere tunnel?

I'm curious about how it rides. Hopefully you get snow soon and can give us a report.

Thanks for the excellent detail in your post.

Lloyd

Lloyd,

You are right I used the original front mounting hole, I drilled out the rivet on the front one by accident thinking the suspension would have to be mounted there that is why there are two bolts in the front. For the rear mounting hole I drilled out the rivet and used the new hole where the rivet was, again the second bolt in the rear is just plugging the hole for strength.

The tunnel was about 1/2" to 3/4" wider on the Deere, I used the deere shafts and some washers to the suspension can't slide from side to side on the shafts.

Looks like you did a real good job there Wyatt.

Any idea what the weight difference is with the updated skid?

Todd,

I didn't weight the skids but I know the Polaris skid is a little heavier than the original Deere. The XCR suspension I used has 12 boggie wheels which add weight, the suspension from the Super Sport had less wheels so it might be a little lighter. The track I am guessing are pretty similar in weight.

Thanks for the nice comments guys! I enjoy have well laid out instructions and information like this so when I can give it back I do my best! Plus I will have this documented so I know what my sled has in it when it come time to service it in the future.

[JDT]

You said you used a larger dia 21T upper gear so the chain would have to be tightend so much.
But if the gear is larger in diameter would it not have more teeth?
I can see your concern as in the photo the chain looks uncomfortable.

Do you know the HP rating of the mill in the donor 440 Polaris? Is it not more than the Sporty 440?
You know, twin carbs and all.

I Know what you are talking about with your warning about raising the rear too high.
The sled will take on some wild handling charactoristics at speed.
I did a similar upgrade on an old Scorpion Mark II a couple of years ago and after the short test ride I lowered 2 inches in the rear.
I also needed a new windshield and that's all I am saying about that.

[wycrumb]

Todd,
Here is what is going on, I chose the 21 tooth top gear for three reasons.

1. The chain just didn't look right with the 17 tooth top gear and I didn't have a slightly shorter chain

2. The Polaris Indy 440 fan that I looked up used the 21 tooth top gear and a 39 tooth bottom which is what I have in it now.
I am not sure of the HP difference but I am sure my sporty is light and the t7 motor in it is pretty quick with the powerjet carb.

3. One thing I have not mentioned is that after taking the pics of the the gears I have I opened the Sportfire's chaincase. I found a 22 tooth top gear which told me that using the 21 tooth would give me a little more bottom end, enough, I am not yet sure.... hopefully.

The Polaris Super Sport was running a 488 Fan with twin carbs and it was using a 35t bottom and 17t top for a gear ratio of 2.05:1
The Sportfire had a 39t bottom and 22t top for a ratio of 1.77:1
Right now I have a 39t bottom and a 21t top for a ratio of 1.857:1

Looks like if I have to I could put the Super Sport setup in my Sporty for a lower gear ratio.

[Matt]

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