Rotating weight reduction

Bauer112 · Oct 26, 2016

Cinno said:
A lot of questions and comments here. Let me clarify a few issues. The drive train components have mass but there rotational inertia is a function of there shape. So the jackshaft is heavy but it has a small diameter so its rotational inertia is small. The brake rotor is relatively light but its diameter is large so it has a relatively higher rotational inertia. These components are all part of the sled and have to be accelerated linearly by the track. So there are losses due to there rotational inertia and by the fact there base weight is sitting in the sled. The sled is accelerated by the track. The track has rotational inertia but because it has the sled sitting on it, the reflected track inertia back to the motor is track weight plus sled weight. You can't separate the two. If the track slips in the snow the track speed runs away as there is no weight attached. So reducing track weight has no more effect than reducing sled weight in other ways like lighter seats, a-arms, skids, etc.

Another thing about rotational inertia is you have to reflect it back to the motor to determine the effect it has on acceleration. The rotational inertia of the brake rotor maybe similar to the bottom gear in the chaincase but because of the bottom gear has a ~2.5 gear ratio its reflected inertia back to the top gear is 1/2.5 squared or only 0.16 as much as the top gear. So if you want to put some money into lowering rotational inertia you lower the brake rotor. I stand by my previous comments that most of the rotational inertia of the drive train components don't matter, including the track. The exception was that light weight crank Polaris added to the Axis, that was good.

I had a CMX (crazy mtn extreme) sled in the day and I was amazed by how they reduced sled weight. Its all in the details. You go after the big ones first then 100 small ones. CMX would cut off the exposed threads from every bolt on the sled. Many they redesigned to make smaller in diameter to reduce weight. Thinner tunnel metal. Hence the 20K price. Cinno

I am also an engineer but do not agree on some of your points. There is no way the that the track weight along with the other drive terrain components do not make much of a difference. I agree that the primary clutch and motor parts matter much more because they will make the clutch engage sooner but the motor still has to spin the track and other drive terrain components so if they are heavy it is no doubt going to slow down the engine acceleration by causing a larger load on the motor.

aksledjunkie · Oct 26, 2016

I think someone should just swap tracks and drivers and tell us. Don't give two hoots on what math calc is correct if the real world response isn't badass.

SRXSRULE · Oct 26, 2016

Cinno said:
I stand by my previous comments that most of the rotational inertia of the drive train components don't matter, including the track. Cinno

This statement couldnt be any further from the truth. You should do some testing to back up your math. you might be surprised. Eric

Cinno · Oct 26, 2016

Bauer112 said:
I am also an engineer but do not agree on some of your points. There is no way the that the track weight along with the other drive terrain components do not make much of a difference. I agree that the primary clutch and motor parts matter much more because they will make the clutch engage sooner but the motor still has to spin the track and other drive terrain components so if they are heavy it is no doubt going to slow down the engine acceleration by causing a larger load on the motor.

Everything makes a difference. Its just that the rotational inertia of the components from secondary clutch to the track driver is very small when compared to the sled and rider. Also the rotational inertia of the track cannot be separated from the overall sled weight. So reduce the track weight if you want but don't think by doing so your reducing track rotational inertia by some large amount because taking 6 lbs from the track is no different than taking out a gal. of gas.

mtncat1 · Oct 26, 2016

back when i had my zrt 800 we long tracked it put every conceivable light weight part on it made are own parts etc. then we punched the engine out to 1080 cc. it ran awesome . then my friend decided to make up a balance shaft delete which eliminated a huge amount of rotating weight inside the motor . the sled went from running great to having so much snap on acceleration it was almost violent . so when you get to the tipping point it can make a huge difference. it is a long trip though .

Bauer112 · Oct 26, 2016

Agree with SRXSRULE. Theres a lot more difference than just removing the same amount of weight in non-rotational mass.

couch · Oct 27, 2016

Bauer112 said:
I am also an engineer but do not agree on some of your points. There is no way the that the track weight along with the other drive terrain components do not make much of a difference. I agree that the primary clutch and motor parts matter much more because they will make the clutch engage sooner but the motor still has to spin the track and other drive terrain components so if they are heavy it is no doubt going to slow down the engine acceleration by causing a larger load on the motor.

Sounds like two economists:face-icon-small-dis

tdbaugha · Oct 27, 2016

Cinno said:
So reducing track weight has no more effect than reducing sled weight in other ways like lighter seats, a-arms, skids, etc.

I think everyone needs an explanation on what "effect" you are talking about. Pure rotational acceleration (ie idle to 8000RPM with 30mph track speed)? Sure maybe it doesn't have a huge effect.

But I think most people on here are talking about overall riding experience effect of lighter weight tracks. Tree riding, hill climbing, etc where you are constantly changing directions. That's where a 10 pound track weight reduction is going shine. 10 pounds spinning that large of radius at 40-50mph is going to make a massive difference in handling characteristics.

WyoPro · Oct 27, 2016

I'm an engineer as well, mechanical...while the other two are arguing over the math and an ounce here and there...I'll be out bashing trees and side hills with the help of a little boost and a camso 280. Cheers bitches.

tdbaugha · Oct 27, 2016

WyoPro said:
I'm an engineer as well, mechanical...while the other two are arguing over the math and an ounce here and there...I'll be out bashing trees and side hills with the help of a little boost and a camso 280. Cheers bitches.

Haha same here! And I'll do it with my chubbie stock track and a lot of boost! :rockon:

Bauer112 · Oct 27, 2016

Didn't mean to start an argument here, I was just looking at what my next purchase should be but until then I'm gonna rip it how it is. I agree the math doesn't mean much when you are riding the thing

WyoPro · Oct 27, 2016

On a more serious note...The ZRP lightweight kit is something I've been looking at. Is everybody having their clutch re-balanced at the same time when replacing the cover?

omernikski · Oct 27, 2016

I'm running the ZRP cover with the Ti bolt kit on my 685. I wouldn't say it was a night and day difference but it was a noticeable change throughout midrange. I added a lightweight rotor with magnesium to help shed a bit more rotating weight.

omernikski · Oct 27, 2016

Oh and I didn't have to rebalance my clutch. Bolt on and go

Dirty Steve · Oct 27, 2016

WyoPro said:
On a more serious note...The ZRP lightweight kit is something I've been looking at. Is everybody having their clutch re-balanced at the same time when replacing the cover?

Mine is in the hands of Indy Dan to be balanced. Also, the belt to sheave clearance was way out of spec from the factory, so that is being taken care of as well.

Sent from my XT1585 using Tapatalk

Cinno · Oct 27, 2016

tdbaugha said:
I think everyone needs an explanation on what "effect" you are talking about. Pure rotational acceleration (ie idle to 8000RPM with 30mph track speed)? Sure maybe it doesn't have a huge effect.

But I think most people on here are talking about overall riding experience effect of lighter weight tracks. Tree riding, hill climbing, etc where you are constantly changing directions. That's where a 10 pound track weight reduction is going shine. 10 pounds spinning that large of radius at 40-50mph is going to make a massive difference in handling characteristics.

Most would assume that reducing 10lbs from a 50lb track is a big change (20%) but it is actually 10 of 600 or 1.6% change. The track is pushing the sled and the track weight, the drive train doesn't know where the 10lbs is coming from. The "effect" is a change in acceleration rate of the sled. I don't think you will feel a 1.6% change.

The other assumption is the track is rotating at a large radius. It don't know what you consider "large" but the calculation for determining reflected inertia back to the motor starts with the diameter of the track driver (~6.5"). It does not matter how large of a diameter it gets to as it rotates around the suspension idlers and wheels. Cinno

tdbaugha · Oct 27, 2016

Cinno said:
Most would assume that reducing 10lbs from a 50lb track is a big change (20%) but it is actually 10 of 600 or 1.6% change. The track is pushing the sled and the track weight, the drive train doesn't know where the 10lbs is coming from. The "effect" is a change in acceleration rate of the sled. I don't think you will feel a 1.6% change.

The other assumption is the track is rotating at a large radius. It don't know what you consider "large" but the calculation for determining reflected inertia back to the motor starts with the diameter of the track driver (~6.5"). It does not matter how large of a diameter it gets to as it rotates around the suspension idlers and wheels. Cinno

I respectfully disagree. I assume your math is valid but there are missed points in vehicle dynamics.

I've ridden tons of sleds, lots of different tracks. Heavier/lighter tracks are most definitely noticeable whereas 10lbs stationary weight is not.

SRXSRULE · Oct 27, 2016

tdbaugha said:
I respectfully disagree. I assume your math is valid but there are missed points in vehicle dynamics.

I've ridden tons of sleds, lots of different tracks. Heavier/lighter tracks are most definitely noticeable whereas 10lbs stationary weight is not.

He isnt gonna get it. He is clearly missing several factors that come into play. Anyone that has been around motor sports understands that adding or removing 10lbs off a track is not even close to removing 10lbs of static weight off the vehicle.

Ive seen the data first hand that proves that wrong. I currently race a "Crate motor" class stock car, where you cant change anything on the engine. We test for hours and hours on the chassis dyno to find acceleration and HP to the ground. Some of those gains are made with lighter wheels and lighter tires. Guess what....A lighter track is doing the same thing! Eric

Dirty Steve · Oct 27, 2016

I was under the assumption that with reducing the weight of the rotating components like drive chain and track you are reducing the gyroscopic effect which makes the sled more, dare I say, "FLICKABLE"?

Maybe that's whats been said in engineering lingo?

I dunno....Caveman push throttle, make sled go up hill.

Either way, it's going to be a fun season.

Cinno · Oct 27, 2016

tdbaugha said:
I respectfully disagree. I assume your math is valid but there are missed points in vehicle dynamics.

I've ridden tons of sleds, lots of different tracks. Heavier/lighter tracks are most definitely noticeable whereas 10lbs stationary weight is not.

What did you notice what was different between the heavier/lighter track weight sleds? What were the snow conditions? What were the lug heights? I being rhetorical you don't have to answer.

About the only thing I might be missing (Vehicle dynamics!!) here is what happens if the track slips excessively and then grabs. Does it act like your popping the clutch on a car? If this occurs then a lower weight track will get to a higher speed quicker just before it grabs because the sled weight is uncoupled from the track. This effect seams like it would change with snow conditions and lug height.

Cinno

Rotating weight reduction

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