Long tracking an Indy Evo 121

Bushwacker1

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Nov 27, 2008
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We bought 2019 Evo's last year. I long tracked one last year with a 146 track and a pro suspension. I looked at the 2020 Evo Rmk and as Polaris stopped short of offering a true Rmk version and the one I long tracked was very close to what they are offering. The 2020 RMK in my opinion would more be described as an SKS. I decided to use some parts that I have acquired to long track the other one for a trip over Christmas.
My swap meet season netted me an Indy 550 144 rear skid, a 155 Pro Rmk skid, a 141 cobra 1.6" track, and some gripper skis. I did some measuring on my project sled 141 Pro Rmk on the rear axle placement with the 141" track tensioned and found that the Polaris Indy 550 144" rails could be used with a 141" track with some minor modifications. I decided to go with 7 tooth drivers to get some more room for the shorter track. The 7 tooth drivers did cause some issues in the end but more on that later.
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Polaris Indy 550 144" rear skid. These come with 8.5" rear wheels. I believe the front and rear arms are the same as the Pro except the transfer rods on the rear shock.

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Cut out the rear axle slot and moved it forward as far as possible. It could go a little farther forward but some work on the axle adjuster would need some modifications. 20191219_120302.jpg

Marked out the back of the rail and cut it off for some more clearance to deal with the wheels moved forward. Installed some new Hyfax and trimmed them to fit this cut. 20191218_154120.jpg

Pro Rmk arms and shocks. 20191219_192643.jpg

Cleaned out all the pivot shafts and gave them some fresh grease. Assembled into the 144 rails with a new set of scratchers. Removed all the outboard idler wheels.

I took some measurements from a 2014 Pro Rmk for skid placement. I have attached a sketch. All measurements were taken from the drive shaft centerline and the top of the tunnel. The EVO 121 front arm mount was .125" farther back and .25" farther down than the Pro. As they were so close to where they need to be I went with the existing front arm hole in the EVO and made adjustments to the rear arm mount point to go along with the front arm placement in highlight. I have posted other Pro measurements in other posts and I believe these ones are the most accurate as I found some inconsistencies in my prior measurements. 20191218_154029.jpg
Found that Yamaha used 3" pitch drivers on a hex shaft. These will fit the Polaris hex shaft the Evo comes with. I wanted to keep the Polaris shaft intact so I used an Indy edge shaft and cut it shorter and machined the end to accept the spedo gear and turned the end to 1" to accept a locking collar type bearing.
I have attached a sketch for shortening and machining the edge shaft to fit the evo. 20191220_020632.jpg

The Evo front arm uses 7/16" bolts to attach to the tunnel. The Pro Rmk used M10 x 1.50 bolts. As the tunnel holes are too big some hat bushings were made to take up the space. Some longer bolts were also required due to the bushing thickness. 20191220_020647.jpg
I had some Pro Rmk rear drop brackets for the rear arm mount. To get the arm the correct distance from the tunnel some spacer plates were needed. These were bolted on so they can easily be removed if wanted. 20191218_154157.jpg
141 Cobra 1.6 Track.

Components were weighed as I went.
121 Evo Skid 58.4
141 Pro Skid 42.3
Pounds saved 16.1

Evo drive shaft 2.52 Pitch 7.4
Reworked drive shaft 3" pitch 7 tooth 7.0
Pounds saved .4

141 cobra 1.6 39.8
121 Evo track 31.9
Pounds gained 7.9

Gripper skis 12.2
Evo skis 11.0
Pounds gained 1.2

Rear arm mount brackets ( forgot to weigh) Pounds gained ?

Calculated weight saved on long track conversion is 7.4 pounds.

I weighed this sled before and after. This is a non electric start sled. Gas was drained out. Oil tank was filled. There was a spare belt and a rear seat storage bag installed on the sled.

Stock sled weighed 418.5
141 sled weighed 413.5
Weight saved 5 pounds

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Rear snow flap looked to be close with the 141. It did get sucked up once while ridding but some reforming in the field took care of that and it did not get sucked in again.

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Here is the o crap moment. Got it all done and was going to adjust the track and found it to be very hard to turn. After some looking I realized the 7 tooth drivers are not large enough to let the track clear the bump out in the chain case and the track was dragging on it. I then remembered reading about this long ago when the Pro first came out and guys were trying to install larger lug tracks. They were removing the chain case and cutting it and rewelding it with more clearance. There is enough room to put 8 tooth drivers in but a new track will have next to no room for adjustment and will be difficult to install. I finished this the same day we were leaving and did not have time to build an 8 tooth drive shaft so I took the quickest fix I could think of and just trimmed about 3/8" off the side of the track. Not the best solution but it did not delay our trip.
 

Attachments

  • Pro Rmk Skid into Indy Evo.pdf
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  • Hex drive shaft mods.pdf
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Last edited:

Bushwacker1

Well-known member
Lifetime Membership
Nov 27, 2008
282
169
43
Wisconsin
My son rode this sled on our trip to the Snowy range over Christmas. We had some good conditions with most days being able to find untracked powder as deep as a foot. We ran some off trail and also ran some untouched logging roads to some nice play areas. The running boards need to be upgraded and the rider position is terrible for off trail. I sent my son out front to have first track on a lot of the logging roads and he and the sled did very well. He tipped it over countless times trying to cut it up in the meadows and struggled to find that balance point while ridding on one ski. As this is always a rookie move I believe a forward rider position would have helped him, and upgrading to a Pro front suspension would also help him find that center of balance much easier. I was actually impressed with the way this sled performed considering the track is only a 1.6 lug. I rode it for a short time busting out a logging road to a play area that has a good steady climb and it did a good job getting through but when getting back on the Pro with a 2.4 track you can really tell the difference in track grip. The rider position reminded me of riding my 2004 edge and I found myself with a Knee on the seat more than a full standing position. It was fun to ride knowing it will get you thru if you truly apply the needed rider input required. Thinking about the finished weight at 413 pounds it would be nice to get that down sub 400 especially for smaller lighter new ridders. A Pro front end and shocks, tube type drive shaft and pro taper bars would be the next step. Running boards would most likely add or be a wash. I am not sure if the Pro RMK over structure and steering would clear all the 550 items under the hood and if not it may be why Polaris did not include it on their 2020 RMK Evo. The last item that would pull some weight would be a quick drive back plate and belt drive but getting the gear ratio would require aftermarket or even custom built sprockets to get to the same gear ratio (2.59 to 1) needed for the 550 engines available power. A 2.25 Powerclaw 141 would also be a good upgrade for busting powder over 1.5 feet deep. This would add around 6 pounds but would compare in weight to the 146 Hurricane ( I installed one in our other Evo I set up for my daughter) and I believe the 141 power claw 2.25 would out perform the 146 hurricane 1.75 in deep powder.

Now for my comments for the Polaris marketing folks reading this. If a true sub 400 lb EVO Pro RMK with forward rider position and Rmk running boards would have been available for 2020 I would have bit the bullet and bought one or even two of them this year in spite of just buying two of the trail sled versions last year (the trail version hit the nail on the head). It did not make much sense for me to buy the current RMK offering when just adding a longer track and installing a pro skid to what I already have gets me to the same point and still wishing for what could have been. Timing is everything these days and as my kids are getting older and bigger they are both approaching the point of an upgrade from a fan cooled mountain sled. A sub 400 pound (say 375) True Pro RMK in the 80 to 100 hp water-cooled engine (how about 1/2 of an 800 or 850 fuel injected engine) would be on my shopping list for next year.
 
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