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Idler Chat – Does Size Matter?




Spoiler: the answer is yes when you do the math.


At many races I attend, I get asked by casual observers what the advantage is to the larger idler wheels installed on our race sleds.  A superficial answer is that they look cool, but there are actual technical reasons and performance advantages to be had.

High performance snowmobiles pose some interesting engineering challenges.  Not the least of these is how to get a big 50-pound belt of frozen rubber track to accelerate and rotate as quickly as possible.


There are many forces at play fighting against the rotation of the track.  One is windage, or the air resistance created by the lugs or paddles in the track.  Another is the curving or bending force required to bend the track around the drivers and idlers in the front and rear of the suspension.  Another is the drag created by the rolling resistance of the idler wheels, both at the rear axle and along the rails.


While we don’t have exact measurements, it is not hard to understand that bending a track around a tighter radius (or smaller idler) requires more force than bending it around a larger one.  Anyone who has tried to bundle a track for shipping or storage knows that it is pretty easy to make gentle bends in a track while it lies on the shop floor but bending and bundling it tightly requires A LOT more force.


Tighter bends in a track also increase the windage or air drag effect of the track.  As the track bends around the rear axle idlers,  the tips of the lugs become further apart as they “fan out” when making the curve around the rear idlers.  The taller the lugs, the greater the increase in windage as they catch more air.  Speed tracks used in grass and ice racing will have very little windage from the rubber lugs due to the short lug height.  By comparison, a 1.75” paddle track used in loose snow will have significant windage.  Granted, the change in the windage effect due to the size of the idler wheels is small, but it all adds up.


Larger rear idler wheels can reduce both the force required to bend the track and the windage created by the lugs or paddles in the track.  This is because the larger idler wheels don’t require the track to bend as tightly during its rotation.


All idler wheels require a certain amount of force to make them turn or spin.  The forces that need to be overcome include the friction in the bearing, the force required to create inertia in the wheel (which increases with wheel weight), and the windage or air drag created by the spinning wheel.  These parasitic forces are a drain on the horsepower created by the snowmobile.  Just as it takes more force to spin an idler wheel faster than it does slower, it will drain more horsepower at higher speeds than it will at lower speeds.  But how can we make the idler wheels spin slower and use less horsepower?  By increasing their size.

At any given track speed, the larger circumference of a larger diameter wheel will result in it running at a reduced RPM.  How much less RPM?  The following chart shows some comparisons in idler RPM at an example 100 MPH.

 

Sled Speed (MPH)

Idler Diameter (inches)

Idler RPM

%

100

7.12

4703


100

8

4186

89%

100

9

3721

79%

100

10

3349

71%

100

5.5

6088

129%

 

At 100 MPH a typical 7.12” rear idler turns at about 4700 RPM.  At the same speed, a larger 8” wheel only spins about 4200 RPM as it doesn’t need to make as many rotations to keep up.  When expressed as a percentage, it only turns about 89-90% as fast.  A 2” increase in idler size results in an approximate 20% decrease in wheel RPM.  If we assume that the force required to spin a wheel is proportional to the speed at which we desire to spin it, then we can assume an approximate 10% decrease in horsepower robbing rotational drag at the wheel for each 1” increase in diameter in the idler size. While it can be argued that windage will also increase with wheel size, this increase is likely significantly less than the rotational friction in the bearing.


Are there other ways to reduce rotational drag in the idlers?  Yes, lower friction bearings can be a significant help.  And don’t forget about those smaller idler wheels along the rails.  While they may not see as heavy a load, look at their potential increase in rotational drag due to their smaller diameter and higher speed. Their higher speed makes paying attention to their bearings even more important.


So yes, larger idler wheels reduce parasitic drag in the driveline.  And yes, they look cool too.

 

 

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