Polaris RMK 800 snowbike

[Brett Kobernik]

I want to discuss tracks a little bit. This is one aspect that I don’t skimp on. I am not afraid to drop $800 on a new high end track. My current favorite is the Camso Conquer. There are a number of reasons I choose this track.

The first reason is traction in deep snow. I’ve run both the old Polaris (2.86" pitch) and the Conquer (3" pitch) on bikes. The Unicorn has a Polaris 163" cut down to 12.5" wide. There is no comparison. The Conquer’s paddles are .4” taller than these old Polaris tracks. I also think that the two rows of wider paddles compared to the four rows of narrower paddles is a better design for deep snow.

The Conquer is around 8 lbs lighter than the Polaris. That's always a bonus. Like I said before, my goal is for the bike to be 50 lbs lighter than the RMK sled. Loosing 8 lbs from just switching tracks means I really only need to loose another 42 lbs to meet my goal.

I need to run 6 tooth drivers to get the proper gearing ratios from the engine to the track. More on that later. If you want to run anti-ratchet 6 tooth drivers, I think Avid may be your only choice, which is what I run. The Avid's only come in a 3" pitch so I had to swap tracks anyway. Avid also makes a 2.86" pitch 6 tooth driver but not anti-ratchet.

One thing I will give to the Polaris track is that it is very "playful" and very easy to maneuver. It just spins too much if you're really looking for traction.

 

[Hawkster]

That polaris track gets more credit than it's worth , the skid is the savior . It is one of the worst tracks for leaving a trench . The other thing is the center windows , it's a old idea unless your a ditch pounder . Polaris knows this and temporarily did not have windows in front of the lugs .
It's always about the weight , when you cut the width down with a track with center open windows it will trench more compared to one without center windows in deep snow , your loosing the footprint .

Get ahead of the crowd and go with a monorail alpha skid , that track has claws because it flexes so much , constantly in contact .

The Riot/E-Z-Ryde skid would of been the perfect candidate to the Alpha skid but that would of been to much progress all at once .

 

[Brett Kobernik]

Drivetrain. Lots of **** going on here. This has taken me a while to get this dialed but I've used this basic set up for over a year now on the Unicorn. I'll go over each component in more detail when I finish them. This is an overview. I'll use the RMK secondary clutch and half of the RMK jackshaft. I've cut the RMK jackshaft in half and as dumb luck would have it, I can press a 1" keyed shaft into it and weld it. I use a 530 pitch 13 tooth Kawasaki Ninja sprocket welded to a 1" keyed collar which will get bolted onto the jackshaft. For the driveshaft, I've found I like the vintage Arctic Cat 1" hex type. Numerous sleds used this similar driveshaft for many years. I was able to find a splined collar that fits the spline on the driveshaft. With a little turning on a lathe, I am then able to press on a 530 pitch 21 tooth Harley Davidson sprocket and weld it. I use 1" self aligning go kart bearings and housings on the driveshaft.

The gear ratio is important. On the stock RMK, it's basically 2 to 1. The 13 and 21 tooth sprockets that I'm using aren't quite 2 to 1. This is why I use the smaller 6 tooth Avid drivers. The stock RMK are 8 I believe. So, even though my gearing is a bit taller, the smaller drivers compensate. For every single turn of the jackshaft, the track moves the same distance with both the stock RMK drivetrain and my cobbled together drivetrain.

 

[kanedog]

This project is wicked!

 

[fgauvin7]

If my memory is right 530 chain and #50 industrial chain have the same spec. So you could easily use a standard 13T 1in shaft sprocket that is already keyed. They are even available with hardened tooth depending on the manufacturer. Even your lower sprocket could be an industrial one machined to fit your hub and welded, only advantage, they cost almost nothing. (based on the fact that the Harley one was not laying in your garage already haha)

For My custom snowlike I use a YZ250 two stroke and its 520 chain, I just use some industrial sprocket and face them on the lathe to 0.220in thick.

But very nice project! Can't wait to see the end result

 

[Answ3r]

You can laser cut sprockets out of 1/4" CHT 100 and it works awesome. You need to face them and make a chamfer. You can also use 3/16" and only chamfer the edges. On a snowbike it last a LONG time. With 150+hp, you might need the whole width. Easy tip : McMaster allows you to download CAD files for the industrial sprockets and if you use Solidworks they are easy to modify for any tooth count you need

 

[Brett Kobernik]

If my memory is right 530 chain and #50 industrial chain have the same spec. So you could easily use a standard 13T 1in shaft sprocket that is already keyed. They are even available with hardened tooth depending on the manufacturer. Even your lower sprocket could be an industrial one machined to fit your hub and welded, only advantage, they cost almost nothing. (based on the fact that the Harley one was not laying in your garage already haha)

For My custom snowlike I use a YZ250 two stroke and its 520 chain, I just use some industrial sprocket and face them on the lathe to 0.220in thick.

But very nice project! Can't wait to see the end result

Yep, 530 and #50 are the same. I've used some industrial sprockets in the past. My experience was they didn't last long at all. Perhaps I just didn't purchase a high enough quality. I've found the motorcycle sprockets have excellent wear life. And they are cheap also. I think the Harley one was $25. Something like $12 for the Kawasaki.

 

[Brett Kobernik]

You can laser cut sprockets out of 1/4" CHT 100 and it works awesome. You need to face them and make a chamfer. You can also use 3/16" and only chamfer the edges. On a snowbike it last a LONG time. With 150+hp, you might need the whole width. Easy tip : McMaster allows you to download CAD files for the industrial sprockets and if you use Solidworks they are easy to modify for any tooth count you need

Great idea. The laser cutter is a little out of my league for right now. Diamond S here in Sanpete county has a water jet cutter. I'll check in with them on this idea. Those guys have been great coaches for me.

 

[Brett Kobernik]

For now, this old "hand me down" lathe is my main tool for my drivetrains. It's a toolmakers lathe from around the time of World War 2. The next step is to modify the Powder Special drive shaft so that it will accept my sprocket and bearings.

 

[kanedog]

For now, this old "hand me down" lathe is my main tool for my drivetrains. It's a toolmakers lathe from around the time of World War 2. The next step is to modify the Powder Special drive shaft so that it will accept my sprocket and bearings.

View attachment 346276

Nice long bed on her!

 

[Brett Kobernik]

Driveshaft:

• Circa 2000 Arctic Cat Powder Special 600 driveshaft
• 1" Go Kart axle bearings (UC205-16) and self aligning cassette
• Harley Davidson sprocket (JT Sprockets JTF989)
• Splined shaft coupler cut in half
• Avid 6 tooth anti ratchet drivers

The Cat driveshaft needs a little modifying. The first thing is to remove some of the hex on the sprocket end.


Same thing on the other side.


Then I shorten the shaft to the right length, drill a hole and tap it so I can use a bolt to retain the bearing.


The shaft is done. Next I cut the splined shaft coupler in half, turn it on the lathe to the correct diameter and press it into the sprocket. The sprocket is hardened so I use a stick welder set to DC reverse polarity with 7018 rod. This works well with no cracking and I haven't had one fail on the mountain yet. I welded the sprocket for the jackshaft at the same time.


Next, press the drivers on and the shaft is complete.


This installs Into the tunnel real easy. Easy assembly and disassembly are key for easy bearing changes every season or two.

 

[Hawkster]

Just a heads up , those shafts are notorious for snapping in two . They do not like to flex , something about the way they are built . Lots of unhappy campers out there when you start searching . Had two different machines in our group have it happen , first one was a learning curve second was easy . Drop the skid , remove the track , put skid back in and get out of the way .

 

[Brett Kobernik]

Thanks for the beta @Hawkster. Yeah, I've seen a few broken ones also. The ones I've seen were caused by failed bearings and the owner just kept riding unknowingly until the shaft broke. The other issue seems to stem from those old machines being so low that the drivers would hit rocks and stumps and bend the shaft. I'm not really sure what the best driveshafts are. I've had a Timbersled shaft break for no apparent reason. The shafts on the original RMK were defective also and it appears that this machine had one break. So far, I've had decent luck with this style I'm using. We'll see.

 

[Brett Kobernik]

Haven’t been able to work on this for a while but made a little progress today. Got the motor mounts configured and starting to get them installed. I’ll need to build an adapter plate for the engine next.

 

[Brett Kobernik]

Finally starting to get back to this project. I’m kind of all over the place with fabricating different things: motor mounts, jack shaft, pipe. Those are the three biggest hurdles In this project. Got the motor mounts mostly finished and I’m starting to work on the jack shaft, pipe, and getting a few other components in place. More details on each of those to come.

 

[Brett Kobernik]

The expansion chamber was one of the most challenging items to tackle on The Unicorn. I had no idea how I was going to build one that would work properly. After much deliberation I figured out a way that I could use the original expansion chamber by cutting it and changing its shape. This way I kept the stock size which is matched up properly for the engine. I’m a big fan of not reinventing the wheel.



I’m doing the same thing on the Yamalaris. Cutting the pipe in key locations allows me to swivel the sections around and weld them back together keeping the same size chamber but making it fit where I want.



Like I said, I’m kind of all over the place right now. The reason is that all these components need to fit with each other. I need to get their basic shapes and basic placement before finishing them. I have a good start on the jackshaft.

 

[Brett Kobernik]

Made good progress on the jackshaft over the last few days. Alignment is a big challenge. The secondary clutch on the jackshaft needs to align up to the primary clutch pretty dam good. Photo below: looking from the front of the engine.



Simultaneously, on the other end of the jackshaft, the drive gear needs to be in a good location so that the chain isn’t too tight or has too much slack. This whole process took a lot of my limited brain power to figure out. I’m using the Polaris stock jackshaft bearing behind the secondary clutch. I have it in place with one bolt in the photo below.



As I said earlier, the distance between the crankshaft and the jackshaft is important. It was 10 1/4” in the sled so I’m using that number. I pivoted the bearing on the single bolt holding it on until I had the correct spacing between the shafts.



Next I cut a piece of aluminum that will hold the bearing on the sprocket side.



This progress on the jackshaft was huge. I’m happy with how everything is lining up.

 

[Brett Kobernik]

Tackling the basic shape of the expansion chamber. Tricky business.

 

[Brett Kobernik]

That was a bitch. More joints than I expected. I used a wire feed welder, nothing fancy and not too pretty. I added a number of gusset plates on each joint hoping this will eliminate cracking. I’m going to coat this, then use heat wrap and then fashion a cover from the original aluminum cover.



The machine is progressing better than anticipated. I’m getting all the electrical components positioned. It’s a mess right now but it’s all going to tuck in real neatly just above the clutches. It looks like I’ll be able to use the stock wiring harnesses without any modifications. Plug and play hopefully.



I mounted the coils on the right hand side of the bike frame. Also, the exhaust valve solenoid is mounted on this side. So far, this is the only electrical component that I’ll have to extend the wires.

 

[Brett Kobernik]

I’m using the YZ twist throttle instead of the RMK thumb throttle. I needed to make a custom cable. I learned a little trick a few years ago for making cables. I use 1/4” steel rod for the ends. I drill a small hole and countersink it just a little.



Feed the cable through the hole and spread the cable open. Using a solder pot, dip the frayed end in the solder then sand it smooth.



I welded a short length of curved steel fuel line to the adjustment coupling so the cable housing doesn’t kink.