Polaris RMK 800 snowbike

[Brett Kobernik]

I’ve been fabricating snowbikes for a while now with increasing success. The next machine is going to use a YZ front end and a Polaris 800 RMK engine and rear end. The project is very much in its early stages but it’s standing on its own now. I don't name my machines, my friends do. My colleague thought this machine should be called Chimera. Here's the definition which I find quite appropriate:

1. (in Greek mythology) a fire-breathing female monster with a lion's head, a goat's body, and a serpent's tail.
2. a thing that is hoped or wished for but in fact is illusory or impossible to achieve.

My other friend called it a Yamalaris. Chimera the Yamalaris. Perfect.

 

[Brett Kobernik]

I’ve had a few folks inquire about what I’m up to so I’m going to document things here on Snowest. But first, I want to preface how I decided on the current project. It started with my old machine affectionately dubbed “The Unicorn” by the guys at Snowbike World. It started out as a KTM 520 with a 2012 Timbersled kit. It has been bastardized over the years with numerous engines. I scrapped the Timbersled kit and fabricated my own custom tunnel and drive train. This season, I blew up another engine and decided it was not worth rebuilding. I’ve done this too many times. It looked like The Unicorn was destined for the scrap yard.

In March, a buddy of mine acquired an old Powder Special and he wanted me to help diagnose some problems with it. I started looking at the engine and at one point I said “that might fit inside the dirtbike frame of my snowbike.” Long story short, we took some measurements and I decided what the hell, let’s give this a go. He sold it to me for $300 along with an additional tunnel cooler. Not knowing if this was really going to work, I slapped it together as quick as I could so I could to get it on the snow before the winter was done. Here’s a video of the thing running and the drive train turning in the shop.


The result was unbelievable. Wow. Before the thing was even totally dialed in I acquired a junked RMK for the next project. I’m convinced this is the way to go.

 

[TreewellDweller]

I’ve had a few folks inquire about what I’m up to so I’m going to document things here on Snowest. But first, I want to preface how I decided on the current project. It started with my old machine affectionately dubbed “The Unicorn” by the guys at Snowbike World. It started out as a KTM 520 with a 2012 Timbersled kit. It has been bastardized over the years with numerous engines. I scrapped the Timbersled kit and fabricated my own custom tunnel and drive train. This season, I blew up another engine and decided it was not worth rebuilding. I’ve done this too many times. It looked like The Unicorn was destined for the scrap yard.

In March, a buddy of mine acquired an old Powder Special and he wanted me to help diagnose some problems with it. I started looking at the engine and at one point I said “that might fit inside the dirtbike frame of my snowbike.” Long story short, we took some measurements and I decided what the hell, let’s give this a go. He sold it to me for $300 along with an additional tunnel cooler. Not knowing if this was really going to work, I slapped it together as quick as I could so I could to get it on the snow before the winter was done. Here’s a video of the thing running and the drive train turning in the shop.


The result was unbelievable. Wow. Before the thing was even totally dialed in I acquired a junked RMK for the next project. I’m convinced this is the way to go.

This is definitely cool. Anxious to see it when you are finished. Keep up the post as I like to follow along on these projects!!!!

 

[Brett Kobernik]

I am not trying to build a machine that mimics what a dirt bike feels like. I am building single ski sleds for big mountain travel in deep snow. I build what works for me. I don't claim it's right for everyone or it's the only way to go. My designs revolve around parts that are easy to come by. The other limitation is I want to fabricate everything with the tools I have. $$ is another limiting factor.

I work as an avalanche forecaster in central Utah and totally fell in love with the snowbikes because of their unmatched maneuverability. They bring me many more places than I can get to on a sled. The obvious problem for people like me is the conventional snowbikes don’t have any real power and don’t do well in deep snow. That’s not a problem anymore. One of the first tests of the new and improved Unicorn was in 2 feet of powder. That was all it took for me to realize the sled engine snowbike was for me. Game changer. Keep in mind that, again, this thing was hastily pieced together to get some initial testing done this winter, hence the salad bowl and dog dish clutch covers.

 

[Coldfinger]

Love the creativity.

 

[Brett Kobernik]

The new machine will closely follow The Unicorn since it’s pretty much tried and true. Like I said, the Unicorn has been bastardized and evolved over the years to its present bastard state. The first mod I did to it years ago was to try a longer track. I found a cheap used Polaris 163”. It was a bit longer than I wanted but I thought, what the hell? I was pleasantly surprised when I found out the bike would actually turn with that long of a track. The next thing I wanted was to get the rider position back over the track. Alan from Timbersled encouraged me to do this. The only way I could figure out to easily do this was to build a pull-back handlebar riser. I added some footpegs onto the tunnel. It worked like a charm and I’ve been running these ever since. This rider position was what allowed me to test the sled engine and CV clutches. It allowed for the width of the clutches by putting them forward of the rider. This is key in my opinion.

There were two things that have held me back in the past from trying a sled engine/clutch build. First, the width of the primary clutch sticking out from the machine. The Arctic Cat was $300 and I figured it would be worth it even if it didn’t work. Turns out it’s more than acceptable.

The second thing I always thought would be hard to overcome was how to deal with the pipe and expansion chamber. Well, after committing to the project, it came together much easier than I thought. I basically just cut the stock Arctic Cat pipe into pieces that were 90 degree elbows and reconfigured them like a plumbing job to fit the bike.


So that’s the preface to the current project. A lot of it has been trial and error. I’ve had a good amount of dumb luck like the fact that the Arctic Cat (actually Suzuki) engine fit in the dirt bike frame without any significant modifications. There have been lots of little things also that sped up the test of this bike like finding a small dirtbike muffler and figuring out that the original Timbersled chaincase cover would fit upside down and on the opposite side of the tunnel.

Now onto the next build which I'll take my time with over the summer.

 

[Brett Kobernik]

Polaris RMK 800, 155 track, $400. Engine needs lots of help. Looks like the driveshaft may have broke and been replaced. The inside of the tunnel was a bit beat up and the front arm on the rear suspension was tweaked. For the price, I was ecstatic. “One man’s trash is another man’s treasure”. I stared at it on the trailer for a week or more speculating what I was going to do. I finally decided that I would use the entire tunnel, rear suspension, tank and seat in stock form. I’m not going to narrow this tunnel or track. I’m going to run it as is. From my experience, I feel confident the wide track will be fine for a snowbike.



I dumped it in my shop and started removing parts. As I did, I started thinking that maybe I could use the bulkhead and keep the engine where it is. I quickly realized this won’t work for my design. First, the bulkhead is way too low. For a snowbike, I figure you need at least 12” of clearance between the bottom of the frame and the ground. The Unicorn has 13.5” of clearance. The second thing I noticed is that the engine is shifted off to the rider’s left side of the sled so that the primary clutch sticks out past the tunnel. This wouldn’t do. The bulkhead had to go.



Before tearing the engine out and removing the bulkhead, two CRUCIAL measurements were needed. First, a measurement of how much the secondary clutch is offset from the primary clutch. This can be adjusted once the jackshaft is in place but you need to be close to start.



The second measurement and probably the most important is the distance between the primary and the jackshaft. This needs to be dead on.


I removed the bulkhead which was not all that easy. Freaking glued on. Next I stared at the handlebars and frame that attaches to the tunnel. I tried to envision using them by connecting a linkage arm from the stock handlebars up to the triple clamps on the dirt bike frame. Similar to the Snowhawk design. I soon scrapped this as well. Much more simple to just use a pull-back riser off the dirt bike triple clamps.

 

[Brett Kobernik]

Meanwhile, I continued to make adjustments to the Unicorn and it was performing better and better! I haven't been this excited about snowbikes since I first rode one. All of a sudden, the Unicorn has about 90 horsepower and there's no shifting! It didn't gain any weight either. If anything, it may have lost a few lbs. Totally different machine.

 

[Brett Kobernik]

Here’s the pile of stuff that I removed from the sled. Bulkhead, steering parts, A-arms, skis, rails, etc. The pile weighed 80 lbs.


What surprised me was how heavy the plastic was. 30 lbs! The headlight/hood weighed 10 lbs alone! That’s about the same as the aluminum bulkhead.


All together, I removed 110 to 120 lbs of parts. Now obviously I need to add a bunch of weight back onto the machine but this gives me a target that I’ll try to beat.

 

[Brett Kobernik]

Oh yeah, I almost forgot about the stock muffler which I won’t be reusing. It weighs over 10 lbs. this brings the weight of all the removed parts up to right about 130 lbs.

 

[couch]

Love this!!!!!!
Was thinking of doing a similar build - more power and ease of a cv for a non bike rider.....
Take a yz forks and front portion of frame and basterdize a 144 /155 pro or a yz frame + sled bike kit but move track further forward so ski to skid distance is much less ....
Look forward to seeing more!!!!!

Sent from my LG-H873 using Tapatalk

 

[Brett Kobernik]

Now that everything that I’m not going to use has been removed, it’s time to start getting ready to fabricate and assemble. The next thing I needed to do was figure out what to do about the dirt bike frame. It was obvious that I was not going to be able to just bolt it onto the tunnel. The frame would need to be heavily modified. I chopped it to pieces. I started stacking things up on blocks to get rough estimates of where everything needs to live. One of the key things I wanted to do was to get the secondary clutch lined up to the inside of the tunnel. This meant I had to run the jackshaft over the top of the throttle bodies rather than below like Polaris had it.



After numerous days of taking measurements, adjusting things and lots of pondering, this is what I came up with for the basic frame.



I will mount the engine so that the flywheel cover (pictured left) and the primary clutch (pictured right) stick out equal amounts on each side of the machine. This actually will offset the cylinders to the lookers left side of the machine. This is 5 to 6 inches different from the Polaris stock position where the engine was offset to the lookers right in the sled. I’m very pleased with this positioning. The amount the flywheel and clutch stick out is very acceptable to me. The overall width will be 24 inches.

 

[Brett Kobernik]

Love this!!!!!!
Was thinking of doing a similar build - more power and ease of a cv for a non bike rider.....
Take a yz forks and front portion of frame and basterdize a 144 /155 pro or a yz frame + sled bike kit but move track further forward so ski to skid distance is much less ....
Look forward to seeing more!!!!!

Sent from my LG-H873 using Tapatalk

Yeah man, you're on it. I've ridden motorcycles all my life but I still find shifting the snowbikes cumbersome in a number of situations like steep sidehilling when I have my foot off the peg and the peg and shift lever are buried deep in the snow.

And yes, I was able to get the ski closer to the track by about 6 inches compared to the Unicorn. This thing is gonna be sick.

 

[eric82930]

I love your low budget builds. And getting them on the snow asap to test before making it look good is my philosophy too. I have one custom build that's going on 3 years of changes and I still haven't painted the frame because I know I will end up hacking it apart again to make it better or lighter when I have more time.

On the Polaris it looks like you have the version that uses the front tunnel cooler you can probably save weight and get rid of that too .there was a few years were Polaris didn't even install that cooler on the 163 pro. it may even allow you to move the motor closer to the track which is pretty critical and I love how you put the Jack shaft over the throttle bodies like Ski-Doo all great ideas to get the center of mass where it needs to be.

Also I'm curious on your thoughts on the exo sled? Other than cost and weight what other things different for your avy forcasting usage? If your math is right you should be way lighter and have no running boards to drag.

 

[Brett Kobernik]

I love your low budget builds. And getting them on the snow asap to test before making it look good is my philosophy too. I have one custom build that's going on 3 years of changes and I still haven't painted the frame because I know I will end up hacking it apart again to make it better or lighter when I have more time.

On the Polaris it looks like you have the version that uses the front tunnel cooler you can probably save weight and get rid of that too .there was a few years were Polaris didn't even install that cooler on the 163 pro. it may even allow you to move the motor closer to the track which is pretty critical and I love how you put the Jack shaft over the throttle bodies like Ski-Doo all great ideas to get the center of mass where it needs to be.

Also I'm curious on your thoughts on the exo sled? Other than cost and weight what other things different for your avy forcasting usage? If your math is right you should be way lighter and have no running boards to drag.

Thanks man, you and I have some similar philosophies.

Yeah, that front tunnel cooler made me scratch my head. Ultimately I decided to keep it although I did chop off the bottom 4" of it. What I liked about it is that it is a sturdy piece of aluminum that I bolted my frame to. I figured it would really tie the frame and tunnel together nicely.

The Exo Sled caught my attention briefly. Very briefly. There are a number of reasons I decided it wouldn't be for me. The first thing that could possibly be overcome are the rails. Sled rails are WAY too wide in my opinion to put your legs around to touch the ground. Your legs basically need to come down right next to the tunnel. The next thing is the width of the body. It's pushing 4 feet. Too wide for me. The last thing that you mentioned is weight. When I'm finished I'm hoping to be 50 lbs lighter than the stock RMK.

Good thoughts, I appreciate the comments.

 

[d1100t]

Nice build.
I think you've done a very good job.
Do you have a pic of the clutch side that shows how much higher the drive clutch is compared to the drive shaft?

 

[Brett Kobernik]

The engine is not mounted yet so things may change slightly but it looks like the jackshaft will end up about 9 inches higher than the crankshaft.



I'll use a motor mounts from the RMK. Only one of this style is used in each machine. I obviously had one of them from the donor machine. I found 3 more on eBay.

 

[d1100t]

Thanks
So it kinda looks like the track drive shaft will be about 4" lower than the drive clutch??

 

[Brett Kobernik]

Thanks
So it kinda looks like the track drive shaft will be about 4" lower than the drive clutch??

Yeah, I'd say that's about right. Maybe a bit more.

 

[NEWDUCK]

Awesome builds! Plus you could always add in a turbo later .