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Ron
01-24-2010, 09:33 PM
I've known for some time that spec's for Polaris drive clutch springs list finish rates with slightly different measurements. Polaris and Team calculate spring rates from a starting point of 2.5" to a finish rate at 1.19". Others such as SLP and EPI use 2.5" and 1.25". The .06" difference doesn't seem significant until you do the math. I was graphing some springs to visualize the intermediate loads and revised an SLP rate to an additional .06" of finish.
On the very popular SLP Blue/Pink Spring the finish rate changes from 340 to 350. And that calculation assumes that the spring rate is constant from start to finish-the rate likely increases as the spring gets shorter....this would make the adjusted finish number even higher.
Here's my math and the results (Please disagree if you see an error in my calculation)
SLP Blue/Pink is 140/340 so it changes 200#'s (340 - 140) in a distance of 1.25" (2.5" - 1.25") 200 divided by 1.25 is a change of 1.6#'s per .01" of travel. So .06" of additional travel would leave a finish rate 9.6#'s more-adjusted to the Polaris measurement. (6 X 1.6 = 9.6)
So the spring that you thought was a 140-340 may actually be a 140-350. I bring this up because it may contribute to issues at high altitude where you run lighter weights and have trouble getting full shift. It's a big step from the stock 800 spring at 140-330.
An easier way to recalculate the rates on springs with a 1.25" finish is to add 4.8% (6/125) to the change from starting rate to finish rate (340-140*4.8%= 9.6).

sled_guy
01-24-2010, 10:25 PM
Your assuming that the Polaris tolerances on springs is good. :)

With springs as close to each other on the starting and final tensions I've always figured the rate of change was most important. You never get to that final 1.25 or 1.19 inch of compression.

So the spring starts out just as stiff, but at any given spot in the shiftout it is a bit stiffer... but more importantly is that the rate is higher on that spring.

How that all figures in to shiftout, I don't know... and it's Sunday night and I'm not in to figuring and calculating. :)

sled_guy

volcano buster
01-24-2010, 10:34 PM
Ron,

I have a couple questions to go along with your points.

1) What is the static spring height with the clutch in the "open" position (engine off)?
2) What is the compressed height of the spring at WOT (max compression)?
3) I assume the difference of measurement 1 minus measurement 2 is considered the "travel" of the spring.
4) Is the travel equal to either the 2.5-1.19 or the 2.5-1.25?
5) If the answer to #1 is greater than 2.5", then what is the initial pressure of the spring when the clutch first starts to close?
6) Is the 1.19" the assumed width of the narrow side of the belt (inside/bottom)?

Ron
01-25-2010, 07:47 AM
Ron,

I have a couple questions to go along with your points.

1) What is the static spring height with the clutch in the "open" position (engine off)?
2) What is the compressed height of the spring at WOT (max compression)?
3) I assume the difference of measurement 1 minus measurement 2 is considered the "travel" of the spring.
4) Is the travel equal to either the 2.5-1.19 or the 2.5-1.25?
5) If the answer to #1 is greater than 2.5", then what is the initial pressure of the spring when the clutch first starts to close?
6) Is the 1.19" the assumed width of the narrow side of the belt (inside/bottom)?

I don't really know for sure, but each clutch is different and the measurements are only relative as they relate to tuning the clutch you own. Mfct'r tolerances also mean each spring may be slightly different too. So without an expensive device to measure spring rates it's not an exact science, maybe this is the point behind your questions.

1. Static spring height open-This will vary since springs will vary in length but also each clutch "open position" changes a little as belt side clearance is changed or with variances in clutch production one clutch to another.

2. Compressed Height at WOT-same answer as 1 above.

3&4. The measurement points (1.25 & 2.5) approximate the starting point and finish point of the clutch shift. The finish will change as the belt wears & with factors mentioned above.

5. It doesn't really matter since there is a variance anyway. We tune with what we have-including clutch tolerances, belt tolerances and spring variations. It would be helpful to know the wire diameter and spring length-Polaris gives them; Team gives spring rate; SLP and others give no additional info.

6. Is the 1.25 The narrow side of the belt-new belts are +/-, the 1115 is narrower than the 1080 so I guess that answer is no.

So is the argument that the slight difference in the finish point that SLP and Polaris use....is this significant? It is-to the extent that you are adding twice as much finish rate with the Blue Pink SLP spring (from stock) as you thought you were. This effects the spring rate throughout the shift, not just finish.
It's not significant when other factors such as belt width, clutch tolerances and spring tolerances are factored into the equation. To a certain extent we all start with a slightly different setup and this is just one more factor to consider when tuning. All of these factors are one more reason that identical sleds don't always perform the same.

800 XXX
01-25-2010, 09:35 AM
I am trying to make sense of these spring calculations... I know there are so many different setups working for guys... I found with stock primary spring there was not enough pressure, with Rons statement about the SLP blue/pink spring there is definitely some kind of logic!! I know weights also played a huge part in getting me dialed in on my primary but the spring tension with the blue/pink spring has definitely also given me a boost in shifting out and gaining the R's I was looking for. The math part of this is tough to understand but I'm sure it works somehow...

mountainhorse
12-28-2012, 01:11 PM
Bump

sledneck_03
12-23-2013, 10:47 AM
ok im looking for a equivalent for polaris 110/290, only thing close is a EPI 125/280.

by calculation its (280-125)*.048=7.44 so 125/287

going to give it a try with my mds weights at 1500 ft.

mountainhorse
02-16-2014, 01:50 AM
Great topic and great work Ron.


Bump.