Gearing down

[ZRrrr]

The age old question with far too many parameters to answer as a generic response of "yes" or "no".

So often I hear the terms "track speed" and "efficiency" thrown around by people when they are comparing apples to oranges in most cases.

Efficiency - what efficiency? Temperature? belt slippage?

Are you actually measuring track speed in the same conditions, same parameters? Measuring track speed one day with a full tank of fuel in 3 feet of powder and hot clutches and another day with no fuel, the hood off and hardpack are night and day comparisons.

To truly compare what track speed does for your sled in a given situation, you need to take different sets of gears out to the hill with you and try it out. Some sleds with heavy weights and clean sheaves will grip a belt really well in powder and spin the track all day without heating up. That same sled might slip the belt like crazy on harder snow if it can't spin the track.

I have worked with the gearing on sleds for years. They are better now, but most mountain sleds still come stock with gearing too tall for its intended application. The original Summits and RMK's had flatlander gearing in them! The old 1994 583's would pull upwards of 90 mph on the forestry roads! Talk about slippage when they were hillclimbing.....

Drop a few teeth on the gears and give the motor the mechanical advantage, the belt will likely shift out farther in the clutches and keep the RPM's in the optimum power range with less slippage and less heat generation.

Slipping clutches are an exponential failure. As the clutch starts to slip, it generates heat in the sheaves and belt. The more heat it generates, the more it starts to slip.....the more it slips, the more heat it generates....and further down the spiral it goes.

Anyway, that's just my $0.02.:d

THANK YOU!!

However, my thanks come with a caveate that gearing down is not always the answer. Working with clutching and a few other simple changes may mean better performance out of stock or taller gearing. Lots of variables to consider and lots of trial and error.

 

[mb1]

Efficiency - what efficiency? Temperature? belt slippage?

Yes, efficiency = ( Power output / Power input ) So in your clutches, slippage will generate heat, which is wasted energy.

 

[Lost]

The brand of sled has a big effect on when to gear down or up. Poo seems to work better at or near overdrive but Doo has a dislike for overdive. I gear my Doo just to stay just out of overdrive in the conditions that I am tuning for. You will spend a LOT of time clutch tuning to get it right (if you ever get it right) now I just buy the Joey kit gear down a bit and spend a lot more time riding than tuning. just my $0.02

 

[Summitric]

I'm with ya lost...put in joey's clutch kit and drop 1 tooth and awaaaaaay we go. Joey's kits are already mountain tested and put through the thrasher.... Not alway 100%, but dam close to perfect.

 

[Dr. Meatman]

awesome thread....should take out some of this information and make it a sticky....45 is about the trackspeed i run up a hill I geared down and as stated before was able to make it up with clutching. IMO maintaining a higher trackspeed is just as important if not more than attaining trackspeed... the higher you go the less oxygen, warmer temps and higher belt temp all factor into the equation.

 

[Ancient Sledder]

When you are talking about track speed is that the speed of the sled?

It is how fast the sled's track is turning when we are climbing a hill in the bottomless powder. My track speed is 45 mph, but the sled is probably only really climbing at 20 to 25 mph. That's until I get stuck,LOL. Then I still have 45 mph track speed but no ground speed. PS,I run 19-44 with a 2 inch 151 track.

 

[99summitx]

If you were to change just one sproket which one would it be, decrease small top or increase large bottom? I am running 21/43 with 8 tooth driver.

 

[teeroy]

If you were to change just one sproket which one would it be, decrease small top or increase large bottom? I am running 21/43 with 8 tooth driver.

I would add a couple to the bottom if you want to gear down, a 19T is a very small gear for the chain to wrap around and prolly not as efficient. I'm running 19/45, going to a 21T for this season.

 

[Lost]

I agree with Teeroy 19 is getting a littke skinny for that HP.

 

[Modman]

I would add a couple to the bottom if you want to gear down, a 19T is a very small gear for the chain to wrap around and prolly not as efficient. I'm running 19/45, going to a 21T for this season.

Sometimes going bigger on the bottom isn't an option as there won't be enough clearance in the case. But you have a good point about the efficiency, as well as increased wear on the sprocket and chain with a tight radius.

Goodwin is the only one I know of making really big bottom gears for the XP's. Up to 49T on the bottom.

Efficiency wise, sometimes it makes more sense to drop a tooth or two on the top sprocket instead of increasing friction by pulling more chain around in the case if you have to go up a chain size or two (if you increase the bottom sprocket). Not sure what the rotating mass difference between a 72 vs 74 link chain and the friction co-efficients of both.

Economically, I think it makes more sense to drop a tooth on the top, unless you are looking for a bigger gearing change than just a couple MPH. If you have the length in the current chain, I would say go bigger on the bottom to get the most efficient driveline but...$85 US for an XP bottom sprocket....!!!! Ouch!
Goodwin Performance, Inc. - Snowmobiles, Snowcross, High Performance Clutches, Suspension, Service & More!

 

[Summitric]

Stock gearing in most ski-doo rev summits is 19 - 45 ..... Think their engineers didn't take into account wear etc?? Just putting it out there for ya to ponder....

 

[teeroy]

Stock gearing in most ski-doo rev summits is 19 - 45 ..... Think their engineers didn't take into account wear etc?? Just putting it out there for ya to ponder....

I think it's exactly the reason why they do it....in the least it would cost you a chain every few seasons. for the gas and go rider, possibly a chaincase, gears, chain, tensioner, labor....etc, etc....:d

 

[Paininthe#$%]

Does anyone out there gear down for tail riding? most machines will go over or close to 100 MPH is that really necessary....

 

[Rob1334]

Does anyone out there gear down for tail riding? most machines will go over or close to 100 MPH is that really necessary....

Yes.

Haha, jk. My RMK had an amazing top end when riding the flats, but never topped it out, and to me that was just wasted speed. I decided to gear it down to improve the sled in the speeds I will utilize and notice it the most. 19/42 for me