kennyspec
Expert
Thinking of building a tubie in the future. Just wanted to get some ideas on whatworks and what doesnt. I will be using the nytro engine and the pro rmk front geometry. Assuming the chaincase center to centr distance could be custom or a belt drive, what would be the ideal placment of the drivers? How much lower should the centerline of the drivers be below or above the bottom of the bellypan? The lower they are the better the approach angle would be, correct? I assume the bigger the driver the better the aporoach angle is also and easier to turn the track around a larger driver but what is the downfall? With a larger driverit means the engine would have to be more forward in the chassis to allow for track to front bulkhead clearance. Whats the better tradeoff? A smaller driver and the engine moved to the rear of the sled a bit more or biger driver and a more forward engine? Any other critical measurments to be aware of? Distance from driver to the ski spindle mounting bolt?
Off Trail Mike
Gone Riding!
Kenny:
On the driver issue, I would stick with 8 tooth x 3 inch (or 2.86") or as close to 24" circumference as possible. When installing the AC skid into the Nytro, I quickly realized an 8" allows for more ground clearance while maintaining shallow approach angle and more importantly, suspensions seem to fit WAY better with 8 tooth drivers (I'm thinking of AC or Timbersled here). With 8 tooth you can move the skid closer to the driver without loosing too much ground clearance, and this buys you more axle adjuster room in the back of the skid. That's the problem I have with my AC skid in Nytro, it has about 4 turns of the adjuster screws to go from the front of the adjuster to the fully tensioned position (versus roughly 1/2 way back in a stock AC M with 8 tooth drivers). It's enough but barely, and had I moved the skid forward I would have lost even more ground clearance, so I chose the bare minimum adjuster position.
I would have the chaincase and brake (if so designed) to protrude maybe an inch below bellypan just to keep it close and easy to Armour/protect. It gets tougher to protect as you move it lower. You don't want your drivers/chaincase/brake to take direct hits.
On engine placement, its too bad we can't use a forward starter (like Doo/AC/Poo) as the Yami starter pushes the engine forward probably 2" give or take. Maybe its possible to re-position?? I wouldn't worry too much about driver size affecting engine position. We're talking 7/16" between 7 and 8 tooth drivers, and track lug height is just as big a engine placement issue as drivers. Maybe a super skinny bulkhead cooler would help.
Distance from driver to the ski spindle mounting bolt? Match Polaris RMK. They seem to have this dialed.
Others I would consider:
Ski Spindle Caster: 25 degrees and NO more. Any larger and the steering gets real heavy.
Ski Stance: Adjustable 37 to 35". Its super fun to be super narrow at times.
Belt Drive: I think its the way to go. Been on industrial equip for years and they plain work.
Good luck. I am always impressed by anyone who attempts a build. You ain't doing it because its easy!
OTM
On the driver issue, I would stick with 8 tooth x 3 inch (or 2.86") or as close to 24" circumference as possible. When installing the AC skid into the Nytro, I quickly realized an 8" allows for more ground clearance while maintaining shallow approach angle and more importantly, suspensions seem to fit WAY better with 8 tooth drivers (I'm thinking of AC or Timbersled here). With 8 tooth you can move the skid closer to the driver without loosing too much ground clearance, and this buys you more axle adjuster room in the back of the skid. That's the problem I have with my AC skid in Nytro, it has about 4 turns of the adjuster screws to go from the front of the adjuster to the fully tensioned position (versus roughly 1/2 way back in a stock AC M with 8 tooth drivers). It's enough but barely, and had I moved the skid forward I would have lost even more ground clearance, so I chose the bare minimum adjuster position.
I would have the chaincase and brake (if so designed) to protrude maybe an inch below bellypan just to keep it close and easy to Armour/protect. It gets tougher to protect as you move it lower. You don't want your drivers/chaincase/brake to take direct hits.
On engine placement, its too bad we can't use a forward starter (like Doo/AC/Poo) as the Yami starter pushes the engine forward probably 2" give or take. Maybe its possible to re-position?? I wouldn't worry too much about driver size affecting engine position. We're talking 7/16" between 7 and 8 tooth drivers, and track lug height is just as big a engine placement issue as drivers. Maybe a super skinny bulkhead cooler would help.
Distance from driver to the ski spindle mounting bolt? Match Polaris RMK. They seem to have this dialed.
Others I would consider:
Ski Spindle Caster: 25 degrees and NO more. Any larger and the steering gets real heavy.
Ski Stance: Adjustable 37 to 35". Its super fun to be super narrow at times.
Belt Drive: I think its the way to go. Been on industrial equip for years and they plain work.
Good luck. I am always impressed by anyone who attempts a build. You ain't doing it because its easy!
OTM
kennyspec
Expert
Thanks for the reply. Are you losing ground clearance as you move the skid forward as you have to mount it higher in the tunnell to keep the rails from going into the trackline? Seeing as how this will be a full custom build would a different "curve" to the front of the rails be beneficial? Cat has a small radius to the rails and the yamaha ones have a much larger radius. As for the front bulkhead cooler I was thinking of eliminating it alltogether and just adding more cooling in the tunnell. The bulkhead cooler could be replaced with a aluminum sheet and perhaps move the engine further back a bit.
Front geometry would be from the pro rmk so steering and castor would remain how they have it set up. I have about 200 miles on my rmk bulkhead on the nytro and it works great.
Front geometry would be from the pro rmk so steering and castor would remain how they have it set up. I have about 200 miles on my rmk bulkhead on the nytro and it works great.
kennyspec
Expert
I guess even if it was taken to tge extreme and the drivers were moved down so the bottom 3 or 4" stuck below the bellypan the approach angle probably wouldnt be reduced that much. The risk of taking out the drivers or chaincase out weighs the slightly shallower aporoach angle.
I am wondering if simple ribbed pullys and a belt could be used as a "chaincase" they hold up fine in some high horsepower automotive applications. Gearing would be a bit harder to change but it shouldnt have to be played around with that much. A tensioner should eliminate any problems of snapped belts due to any slack in them
I am wondering if simple ribbed pullys and a belt could be used as a "chaincase" they hold up fine in some high horsepower automotive applications. Gearing would be a bit harder to change but it shouldnt have to be played around with that much. A tensioner should eliminate any problems of snapped belts due to any slack in them
Nikolai
TY 4 Stroke God
Here's my take on things since I'll be building my tubie this summer from scratch.
- Whatever drive train you use, make sure it's at least 8.5" c to c. While the Yamaha Nytro/Apex/RX1/Vector drivetrain is bulletproof, the center to center distance of the shafts is only 7.5" which is too short. This makes for barely enough room to run a 3" paddle with 7-tooth drivers which are too small, especially if you are building from the ground up and have any option available. I'm planning on using a 8-tooth 3" pitch drivers, 9-tooth would be even better as long as the drivetrain you were using allowed you to gear low enough to compensate for the larger driver.
- As far as a drop and roll goes, I would D&R enough to give yourself around 3/4"-1" of track clearance with whatever track you plan on running. I've had drop and rolls before and they can start to handle poor as your move the driveshaft and suspension rearward, so I would only move it back the necessary amount. Keep in mind if you move the driveshaft too much, you'll need to reposition the rear suspension down and back as well.
- Rear suspension. I'm going to place mine as close to the drivers as possible to do two things. One is eliminate the need for an anti-stab kit, the other is to try and get the front skid shock as close to directly underneath me as possible. It also helps to keep your attack angle since snow can't push the track up. I feel these are the reasons RMK's get on top of the snow so well.
- For the front suspension I think I'm going to copy the XM Summit because they don't use any extra steering linkages, just a steering post direct to the tie rods. It'd be easy enough to purchase the front clip and use it build a jig, and it would allow for the same simplistic steering I have now but with proven factory ski-doo geometry.
I've been tossing are around drive train ideas for a long time, RMI, CMX belt drive, BDX Diamond Drive, Apex chaincase, ect. One idea I had was to use a custom steel jack shaft from Wahl Brothers with Yamaha splines on the clutch end to keep the Nytro secondary clutch and Polaris splines on the other end to use a C3 gear. I could set my own shaft centers this way, mostly likely 9" - 9.5" and have all the track clearance I want. The final center to center would be set by C3's sprocket choices and belt lengths. For the drive shaft, a BD Xtreme lightweight Proclimb shaft, the Poclimb brake caliper/bearing holder (lightened up as much as possible), BDX lightweight brake disc, and a C3 lower gear with a lightweight belt tensioner. Aside from the custom jackshaft, everything else would be off the shelf. If you/re building a tubie anyway, you'd only need small flangettes to hold the jackshaft eliminating the weight of a bearing plate (think Pro and CMX). Wilwood has a huge selection of ultra lightweight billet spot calipers that could be used with your own version of a driveshaft brake. Does the brake have to be hydraulic? I wonder if there's a strong enough mechanical caliper out there that would work? I could see significant weight savings (2 - 4 lbs) with a mountain bike brake lever, some kind of mechanical brake, and cable. Polaris Indy Lites used a mechanical brake with the rotor on the driveshaft but that was a 250 cc sled.
I've also recently thought about building a narrow Phazer width sled. Something along the lines of a 14.75"" wide tunnel with short driveshafts and a 162x15x3" cut down to 14" wide. I can't help but think how much fun a super lightweight narrow sled would be to ride with big HP. A 162x14 would have nearly the same flotation as my 155x15 but it would keep the skis down a lot better.
The biggest downside to building a chassis from scratch is the endless possibilities. I'm afraid I'll never be able to make up my mind and get started.
- Whatever drive train you use, make sure it's at least 8.5" c to c. While the Yamaha Nytro/Apex/RX1/Vector drivetrain is bulletproof, the center to center distance of the shafts is only 7.5" which is too short. This makes for barely enough room to run a 3" paddle with 7-tooth drivers which are too small, especially if you are building from the ground up and have any option available. I'm planning on using a 8-tooth 3" pitch drivers, 9-tooth would be even better as long as the drivetrain you were using allowed you to gear low enough to compensate for the larger driver.
- As far as a drop and roll goes, I would D&R enough to give yourself around 3/4"-1" of track clearance with whatever track you plan on running. I've had drop and rolls before and they can start to handle poor as your move the driveshaft and suspension rearward, so I would only move it back the necessary amount. Keep in mind if you move the driveshaft too much, you'll need to reposition the rear suspension down and back as well.
- Rear suspension. I'm going to place mine as close to the drivers as possible to do two things. One is eliminate the need for an anti-stab kit, the other is to try and get the front skid shock as close to directly underneath me as possible. It also helps to keep your attack angle since snow can't push the track up. I feel these are the reasons RMK's get on top of the snow so well.
- For the front suspension I think I'm going to copy the XM Summit because they don't use any extra steering linkages, just a steering post direct to the tie rods. It'd be easy enough to purchase the front clip and use it build a jig, and it would allow for the same simplistic steering I have now but with proven factory ski-doo geometry.
I've been tossing are around drive train ideas for a long time, RMI, CMX belt drive, BDX Diamond Drive, Apex chaincase, ect. One idea I had was to use a custom steel jack shaft from Wahl Brothers with Yamaha splines on the clutch end to keep the Nytro secondary clutch and Polaris splines on the other end to use a C3 gear. I could set my own shaft centers this way, mostly likely 9" - 9.5" and have all the track clearance I want. The final center to center would be set by C3's sprocket choices and belt lengths. For the drive shaft, a BD Xtreme lightweight Proclimb shaft, the Poclimb brake caliper/bearing holder (lightened up as much as possible), BDX lightweight brake disc, and a C3 lower gear with a lightweight belt tensioner. Aside from the custom jackshaft, everything else would be off the shelf. If you/re building a tubie anyway, you'd only need small flangettes to hold the jackshaft eliminating the weight of a bearing plate (think Pro and CMX). Wilwood has a huge selection of ultra lightweight billet spot calipers that could be used with your own version of a driveshaft brake. Does the brake have to be hydraulic? I wonder if there's a strong enough mechanical caliper out there that would work? I could see significant weight savings (2 - 4 lbs) with a mountain bike brake lever, some kind of mechanical brake, and cable. Polaris Indy Lites used a mechanical brake with the rotor on the driveshaft but that was a 250 cc sled.
I've also recently thought about building a narrow Phazer width sled. Something along the lines of a 14.75"" wide tunnel with short driveshafts and a 162x15x3" cut down to 14" wide. I can't help but think how much fun a super lightweight narrow sled would be to ride with big HP. A 162x14 would have nearly the same flotation as my 155x15 but it would keep the skis down a lot better.
The biggest downside to building a chassis from scratch is the endless possibilities. I'm afraid I'll never be able to make up my mind and get started.
Nikolai
TY 4 Stroke God
I hope you don't mind if I clutter your thread with some of my ideas.
Here's a couple pics of the Indy Lite caliper, they are $30 brand new on ebay. Would it be enough to stop a 400 lb sled? Doubtful, but it looks light haha
The backing plate for it.
Here's a couple pics of the Indy Lite caliper, they are $30 brand new on ebay. Would it be enough to stop a 400 lb sled? Doubtful, but it looks light haha
The backing plate for it.
kennyspec
Expert
Some good info in this thread.
If you look at the powerhouse customs sleds really close, they use the pro geometry but the tie rods are mounted about 1/2" higher on the spindles. This is because they also mounted the tie rods directly to the steering post in the Center of the sled. I wonder how that affects bump steer and the steering system. Must work pretty good if they are using it on their sleds.
If you look at the powerhouse customs sleds really close, they use the pro geometry but the tie rods are mounted about 1/2" higher on the spindles. This is because they also mounted the tie rods directly to the steering post in the Center of the sled. I wonder how that affects bump steer and the steering system. Must work pretty good if they are using it on their sleds.
Sportsterdanne
TY 4 Stroke Guru
For a pure powder sled i dont think a perfect front suspension geometry is needed, thats why i think something along the Ski doo freestyle would work ok. Its a simple and probably lightweight design.
kennyspec
Expert
I was just using the pro geometry because that's what I have to model off of. 90% of my riding is just fields and ditchbanks. I go out to the mountains every second year. Perfect handling isn't extremely important as a 156" isn't going to ride like its on rails anyway.
I wonder how hard it would be to extend a apex chain case to 8.5" centers? I do have a tig. It would be nice to keep the reverse. I not going to be too concerned with saving every ounce of weight.
I wonder how hard it would be to extend a apex chain case to 8.5" centers? I do have a tig. It would be nice to keep the reverse. I not going to be too concerned with saving every ounce of weight.
Off Trail Mike
Gone Riding!
kennyspec said:
Bang on Kenny. You loose ground clearance as you move the skid forward to keep the rails out of the trackline. The radius of rail curve doesn't matter very much. Its the approach angle that drives everything. The manufacturers claim 18 degrees, but Yami, AC and Timbersled (ice age rails) are all 21 degrees relative to the ground (I measured all of them), and I don't see going much flatter than 21 as you really start to lose a lot of ground clearance.
You are right on the tight radius of the AC rail bends up front, but I find that skid really pops up on top easily so I am not convinced longer radius is the way to go.....maybe others can share their experience on this. I kept my wheels up front at the bend and my hyfax wear in that area is almost zero after 1500 miles, so the design seems to work. The Ice Age rails have a longer radius, but I don't have enough miles on them for a meaningful comparison just yet.
I keep hoping that Yami gets us a new motor to work with. I'd love to do a custom sled, but I'm not sure I want to do a build around the Nytro motor. Its good, but not great in the weight department. With the stub shaft on the crank, the balancer arrangement, overhead cam chain drive, alternator, one way clutch and internal starter, I think you could easily shave 10lbs off the engine without doing much at all.
I think a turbo 750cc would be a good 180hp powerplant @ 9000rpm on 91 octane.
Who knows, maybe we will see something down the road.
OTM
Nikolai
TY 4 Stroke God
kennyspec said:
Good to know. I wonder if there's much of a weight difference between the two front ends? The XM spindles look lighter but the a-arms look about equal. Chassis design would be simpler with the XM since the a-arms are parallel to each unlike Polaris, the front tube work would be very similar to front of my sled which is only a couple tubes. Pro on the other hand is proven to be one of the best platforms there is.
Mike, there is a member on snowest who was on the Yami hill climb team for awhile and pulled the counter balance weights out. It's about a 1.7 lb loss, all rotating mass. I PM'd him a few months ago and he hadn't had any issues yet. If I could get 180 naturally aspirated HP out of my Nytro with engine work and exhaust I'd sell the turbo. 180 HP is plenty for our elevation and losing the turbo and counter balance parts would drop almost 20 lbs, maybe more. That would make a 380 lb dry 4-stroke entirely possible. Just not sure on the longevity of running without the counter balance weight. You could bolt on Excell's new Viper exhaust which side exits, and then you could run a regullar depth tunnel.
Here's a copy of his reply to me
Nikolai
TY 4 Stroke God
I was wrong on the Apex chaincase centers, it's only 7" (6.875 to be exact).
I read on Snowest that the older Ski-Doo RT1000 chaincases are 9" C to C and hold up to big power. If you wanted to retain off the shelf parts that would be a good option. There would be plenty of room to run a 3" track with 8-tooth drivers. They look pretty lightweight.
Sorry for the huge pictures, I robbed them off ebay.
I read on Snowest that the older Ski-Doo RT1000 chaincases are 9" C to C and hold up to big power. If you wanted to retain off the shelf parts that would be a good option. There would be plenty of room to run a 3" track with 8-tooth drivers. They look pretty lightweight.
Sorry for the huge pictures, I robbed them off ebay.
Nikolai
TY 4 Stroke God
After doing a little research, it appears that the Yamaha Viper and Proclimb jackshafts are the same on the gear end, meaning you could put PC gears on a Viper. After considering all my options, I think I finally settled on a drive system that should be simple, relatively light, and under $1,500. You get a strong driveshaft with the brake mounted on it, efficiency of a belt drive, and retain the Yamaha clutch. The entire setup would be relatively clean and easy to fabricate since you're essentially only mounting four bearing holders to the chassis. You could also get whatever length belt you wanted from C3 and choose your own center to center distance.
BDX Proclimb driveshaft $120
BDX Proclimb brake rotor $90
Avid 9-tooth 3" pitch drivers x2 $260
complete, new Proclimb brake caliper from cat $230 (less on ebay but then your starting out with used bearings and brake pads)
Yamaha Viper jackshaft $200 (its not on the microfiche yet, but the Nytro jackshaft is $200 so I'll assume the Viper will be similar)
Bearing flangettes x3 $25
C3 Proclimb upper gear $130
C3 Proclimb lower gear $220
C3 Gates belt $130
C3 tensioner $100 (I'm assuming Kevin will sell just the bare tensioner by itself since their cover would not be needed)
For around $1,500 you'd have yourself a complete belt drive, made up of entirely off the shelf parts. From reading on Snowest, the C3 belt drive on the Proclimbs sounds like it's bullet proof and it'd be that much better on a rigid tube chassis. Center to center could be whatever you want on the shafts. I want to run 9-tooth drivers so I'll probably shoot for 9" centers.
Just some food for thought.
BDX Proclimb driveshaft $120
BDX Proclimb brake rotor $90
Avid 9-tooth 3" pitch drivers x2 $260
complete, new Proclimb brake caliper from cat $230 (less on ebay but then your starting out with used bearings and brake pads)
Yamaha Viper jackshaft $200 (its not on the microfiche yet, but the Nytro jackshaft is $200 so I'll assume the Viper will be similar)
Bearing flangettes x3 $25
C3 Proclimb upper gear $130
C3 Proclimb lower gear $220
C3 Gates belt $130
C3 tensioner $100 (I'm assuming Kevin will sell just the bare tensioner by itself since their cover would not be needed)
For around $1,500 you'd have yourself a complete belt drive, made up of entirely off the shelf parts. From reading on Snowest, the C3 belt drive on the Proclimbs sounds like it's bullet proof and it'd be that much better on a rigid tube chassis. Center to center could be whatever you want on the shafts. I want to run 9-tooth drivers so I'll probably shoot for 9" centers.
Just some food for thought.
Off Trail Mike
Gone Riding!
Caleb,
What are the advantage of a 9 tooth over 8? Less ratcheting and skid closer to driver (avoid stabbing) I can see, but that's a 12% gear up or 12% more torque being put through that drive shaft at any given speed.
I didn't look at the PC/Viper drive shaft, but assuming its not a hex like the Nytro. That said, I have never heard of a drive shaft stripping either.
Just curious.
OTM
What are the advantage of a 9 tooth over 8? Less ratcheting and skid closer to driver (avoid stabbing) I can see, but that's a 12% gear up or 12% more torque being put through that drive shaft at any given speed.
I didn't look at the PC/Viper drive shaft, but assuming its not a hex like the Nytro. That said, I have never heard of a drive shaft stripping either.
Just curious.
OTM
Nikolai
TY 4 Stroke God
I like the larger drivers simply for less rolling resistance. That's really the only reason I want to use 9's.
