Direct Drive Chassis Power Limitations

[JoeRainville]

Hi Guys,

This FAQ is posted in response to question about the "Direct Drive" (DD) sleds ability to handle higher power. This includes all 78-82 Spitfires and all 82-84 Snow and Sprintfires. All these sleds were 340cc with hp from about 28 to 32.

The problem with transmitting high hp through a direct drive chassis has mostly to do with those little cute track drive sprockets and their interaction at the track contact point.

The DD sleds get their "chain case" gear reduction by using a larger diameter driven on a common shaft with smaller diameter (and circumference) drive sprockets with a tight 2" drive lug pitch, with either a 7 or 8 tooth sprocket.

Here starts the challanges. Dispite loosing about 40 or so parts, the DD sleds are not the most efficient design for high torque transmission. First, the larger diameter secondary makes the belt leave the primary at a steaper angle, and this reduces contact area where you need it most. Look a the drive clutch on most Sprintfires. The belt has typically cut groves into it from slippage.

Then, the track has to make a very sharp bend around those tinny sprockets. That eats up power. To offset the potential power loss, Deere was smart. They used a thinner cross section track so it would bend easier, and they did use a larger rear idler on the 78-82 Spitfires to offset some of the rolling losses. Unfortunately, even with the larger diameter sprockets on a Trailfire and Sportfire (80-81), that thinner track didn't take the 440cc motors torque as well as the rubber track form the LF and 82-84 Sports.

The real limiting factor is the small contact area from the drive sprockets to the track. There are not that many teeth in contact, even with a drive lug pitch tightened up. Also, the lack of a center sprocket and center rear idler also helps to limit the power that little chassis can transmit. Since Deere choose the center 'cracked' cleats, the track can wiggle around more than if it had standard straight cleats. I believe Rich posted that the Trailfire track on an 80's LF would "Balloon" at high speeds due to the lack of center support.

So this begs the question, why didn't Deere use a stronger rubber track? The main answer if efficiency. Polaris did use a rubber track on the Cutlass, SS and Star sleds. According to SnoGoer, even the 440cc SS couldn't get out of its own way at sub zero temps because the track was so stiff. A thicker track takes more power to bend, and it only gets stiffer as it gets colder, reducing drive line efficency much more than a thinner track would.

I do believe Deere tried an 80's Liquifire track on a DD sled, and verified the loss of drive efficiency. However, I've had several discussions with folks on casting new sprockets with a 3.29 drive lug pitch on a new hex shaft to covert DD sleds to the LF track. A less efficient track is better than no track, or cutting up the sled for the Polaris rubber one with different drive lug and slider centers. So far cost has kept this from a reality. I also spoke to Kimpex, and for $300,000 they would love to make me a mold for a new rubber track with a 2" pitch. Uhh, no thanks.

So, in closing, I feel the limiting factors from least to most important:
1) Drive belt contact
2) Thin, weaker cleated track
3) Small contact area with the drive sprockets

That's my story, and I'm sticking to it...
-Rainville


B)

[JRC ETD]

OK remember most drivers just know how to work zippers, that is the extent of my engineering ability...but we did know that the tunnel design of Polaris sleds allowed larger diameter drivers, less bending of the track at the drive shaft! This allowed greater gear selections and clutch set-ups for performance. The Deere sleds bent the track harder over the smaller drivers robbing power and speed and limiting clutch adjustment for performance. So we did try using the DD secondary to create a larger circumference and more torq on the conventional jack shaft race sleds, but ended up with so many other variations that we didn't have time to solve the experiment. Many say Deere had less power than the TXl ect., Not true, the experienced crew at Roseau had a broad power band in the engine, bigger circumference drivers, low weight mass, effienct and adjustable clutches, and less tensil steel in the Yokahama track...thus a easy rolling sleds with effient engines and power transmission. These issues were addressed and eliminated very well with the second generation sleds. Now is were Dick Teal, Lee Hardesty, Stan Hayes and Jack Hoffman are supposed to enter, I'll go back to these stuck zippers! Jon

[JoeRainville]

Hi Guys,

I wrote that FAQ based on observation of the DD sleds and a little engineering and racing background as to why the DD sleds were limited to 340cc. I would love to here the Horicon Engineers side to the story. Please guys, feel free to add to and/or correct anything I said above!

Thanks for reading,
-Joe Rainville