Helix effect on elevation changes?

[DKP]

So, I have a Piped 800 Axys. Works great up top, 5500-7000Ft, but when lower it over revs. I am a boondocker here in Revelstoke area. But lots of times, we are riding great trees in the sub 5500Ft elevation. Where, if I grab full throttle, she looses RPM. Not loading the pipe enough I think. Up top, she pulls great, no over rev and no "rah rah's" from the pipe. But also I have a pretty shallow helix, I feel. Presently, 66g series 10 weights, 120-330 spring. Helix is a 48 36 .36 I am 163 2.6 track, with Kurts gear down to 2.37. If I increse Helix angle a bit, will that lessen the lower elevation issues?

 

[PSI 800]

go with the kurts clutch kit and be happy. They work in conjuction with each other. I had both on my last sled and was really happy with the results. Ive ordered both for my new 850NA Poo

 

[DKP]

I'm trying to learn more as well. And I've already spent money on a few peoples "clutch kits". Only to be unhappy and have to change them. When sellers don't tell you helix numbers, you have no idea where you are or where to go, just that it doesn't work to your liking


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[xpturbo600]

To answer the title question directly, the helix has no effect. It only has one job...stall the primary shift based on the pressure (load) signal coming from the track. If you change elevation, it is best to assume track pressure signal remains constant. What changes is the barometric pressure, which increases mass flow across the motor as you drop elevation. I assume you have a base understanding of this fact. The problem this creates is that your motor makes more power at lower elevation, so your clutching target shifts, meaning helix angle needs to increase holding all other variables constant.

Keeping things simple, greater helix angle allows the primary to shift further out since the ramp angle reduces the amount of force the secondary spring can apply to the sheave. Think of this like it is softening the clamping force. Lower ramp angles stall the secondary shift by allowing greater spring force to be applied to the belt. Stalling the shift means the secondary is stronger, so the primary has to apply more force to open the secondary sheave. To increase primary clamping force mechanically, the tuner must either put more weight in the primary clutch holding rpm constant, or increase the engine rpm holding the weight constant. Spinning the clutch faster applies more force in accordance with the centrifugal force relation.

So, you are noticing the engine has more power at lower elevation. More power means it can pull higher rpms given the same load. To reduce rpm, you need to add weight to the primary or to reduce secondary shift stall, which puts more load on the primary via the belt. This approach assumes there is enough primary belt grip, so the primary does not start to slip now that it has more power. If the primary starts to slip, add more weight. Given this is a marginal adjustment, you could get at it with a helix angle change or by adding a bit more weight.

In practical terms, your clutch kit isn't designed for optimal operation at 5k feet, so you are near the limit at your highest elevation. That means you need to make a clutching change to encompass your riding elevations so your clutching remains efficient throughout your riding elevations. To do this, you need to clutch it so it is just off the limiter at the lowest elevation, roughly 250 to 300 rpms lower than the rev limit so over revs through the shift range don't hit the rev limiter. That will result in you pulling less rpms at higher elevations. But, that is fine so long as the performance is consistent and you don't spend your day constantly changing your clutching. Plus, you are only talking a couple thousand feet, so the change in power is manageable. If your power band is too skinny at high rpm, meaning your performance really drops off after a small loss in power, then you have an engine power efficiency issue, not a clutching issue. My recommendation is to add a gram or two in weight to each arm in the primary, or to take 2 to 3 degrees out of your helix angle and see what that nets you. Every sled is slightly different, so you need to test it.

Lastly, you note an inconsistency in your description. Top end remains fine, but there is rpm loss when chopping the throttle. If topend is good, then you need to evaluate the shift pattern dictated by ramp angles. This means you need the sled to back shift more when power drop occurs because the secondary isn't recovering from full shift. Given your 48 36 helix, you need more stall earlier in the ramp. Try something in the range of 44 36 or 46 36 to recover more backshift. It sounds like the clutching isn't happy as a collective combination, but that is a much longer conversation. Another approach is to find a primary spring with a slightly higher initial spring rate but the exact same final shift spring rate. For example, if you had a 120 - 300, you would try to find something like a 180 - 300. This would weaken the primary in relation to the secondary earlier in the shift, but still reach the same full shift ratio between primary and secondary. Increasing initial spring rate in the primary is like taking some weight out of the primary early in the shift range, but the finish rate is the same at 1:1 full shift.

 

[xpturbo600]

You could also try the same thing in the secondary (albeit a more direct and better approach). Increasing the initial spring rate in the secondary with the same finish rate at full shift will increase backshift when you drop throttle...what you are complaining about. But, both approaches (primary or secondary) will cause the primary to flare more when accelerating through the shift range. To address this issue, you compensate on the primary alone with ramp angle or how weight is distributed in the arm. To reduce primary flare in the mid range, shift more weight to the heal of the ramp and away from the tip. This puts more initial shift force and reduces full shift slightly, so you will need to add a bit more weight to settle full shift, but concentrate on putting it in the heal or middle of the weight.

 

[jim]

Works great up top, 5500-7000Ft, but when lower it over revs. I am a boondocker here in Revelstoke area. But lots of times, we are riding great trees in the sub 5500Ft elevation. Where, if I grab full throttle, she looses RPM.

I'm a bit confused...you say when lower than 5500 it over revs, but then say it looses RPM sub 5500 feet.

Regardless, I'm assuming you over-rev a bit down low because your motor is making more HP and pulling more Rs.

As for RPMs, you can deal with that in a few ways. Yeah, you can mess with helix angles. You can also flush $100 dollar bills down your toilet for the same affect. Spring rates have a much better affect...the higher the rate at given compression, the more backshifting you get...quick on/off the pipe but might leave some track speed on the table. But a higher secondary spring rate will keep the Rs up when the clutch gets hot and sticky during long pulls and on/off throttle. If you like the trees, keep the secondary spring rate a bit higher...you are always on the pipe and predictable. And in the trees, where it sounds like you want to ride, staying on the pipe predictably is very important...can get dangerous if you get a bog or major RPM/HP drop.

True RPM adjustments should focus on the primary though. Basically less tip weight = more RPMs at target shift-out and power. Or you can add more spring rate and get more RPMs. Again, trying a bunch of weight combos can be similar to the $100 bill drill with helixes. But trying a different spring can be good. And I have found that if you have some higher spring rates in the primary and secondary, you will sacrifice a bit of track speed on full throttle long pulls, but you will always be on the pipe and very reactive track speed/performance. I always like to have the instant backshift/trackspeed as opposed to max track speed enabled by shifting out a bit more.

I have usually treated weights and helixes as getting things in the ballpark and then dial it in with springs. You are close...maybe try a different spring or 2. And also recognize that large elevation changes will change your HP by 20-30...and that's a big range to cover.

Don't want RPM fluctuations with elevation? Turbo w/ elevation boost compensate.

 

[Teth-Air]

I also read it that it is over-revving at low altitude so adding spring pressure makes no sense. I would see at what speed the over-revving takes place. He did say in the trees so likely pretty low speed so the initial ramp angle, spring pressures and helix angle would be in play. This then would be the area to make changes. A softer start pressure on a spring or a steeper start angle on a helix would let the shift take place easier and quicker to load the motor. Adjustable weights are nice because adding weight to the heel could be all that you need.

 

[Dynamo^Joe]

So, I have a Piped 800 Axys. Works great up top, 5500-7000Ft, but when lower it over revs. I am a boondocker here in Revelstoke area. But lots of times, we are riding great trees in the sub 5500Ft elevation. Where, if I grab full throttle, she looses RPM. Not loading the pipe enough I think. Up top, she pulls great, no over rev and no "rah rah's" from the pipe. But also I have a pretty shallow helix, I feel. Presently, 66g series 10 weights, 120-330 spring. Helix is a 48 36 .36 I am 163 2.6 track, with Kurts gear down to 2.37. If I increase Helix angle a bit, will that lessen the lower elevation issues?

Every 1000 feet you increase elevation, the engine loses 5hp.
5hp = approximate 200 rpms.
5hp = approximate 1 gram flyweight mass
200 rpms = approximate 1 gram flyweight mass

At 4500 feet, lets say the engine has 135hp.
5500 @ 130hp
6500 @ 125hp
7500 @ 120hp
@ etc....

A competent clutch setup should be able to run within 2000 feet elevation difference and be in +/ - 100 rpms of the power peak rpms
Example; engine power peak rpm @ 8000.
Estimate rpms @ elevations in a 2000 ft range
@ 4500 feet, full throttle, runs at 8100
@ 5500 feet, full throttle, runs at 8000
@ 6500 feet, full throttle, runs at 7900

IF you have flyweight mass to get power peak rpms at say 6500 feet, but you see 300 rpms over at 40~4500 feet, THEN your engine has estimated 15 more HP than the clutch is weighted for AT 4000ish feet; yes the engine will overrev. The engine is overpowering the flyweight grams (mass)

I would say leave the helix alone and change the start force of the primary spring. Remove the 120/xxx and go to a 100/xxx, or 90/xxx start force. Control the engine "speed up" by lowering the start force. This will also lower your engine speed at part throttle when running slow in the trees, etc.

 

[DKP]

I also read it that it is over-revving at low altitude so adding spring pressure makes no sense. I would see at what speed the over-revving takes place. He did say in the trees so likely pretty low speed so the initial ramp angle, spring pressures and helix angle would be in play. This then would be the area to make changes. A softer start pressure on a spring or a steeper start angle on a helix would let the shift take place easier and quicker to load the motor. Adjustable weights are nice because adding weight to the heel could be all that you need.

So when you say steeper start on helix. That is higher number? If I am a 48-36 now go to a 50-36? I can't get lower initial spring pressure with the same final force that I have. Not down to like 100 like DJ states at the end of his post. I'm sure my kit wasn't not thought out well. And I'm hesitant to drop a whole lot more cash on someone else's kit, only to have to change it too because of the mods. I have a bunch of different springs and ramps as it is now. And adjustable ramps.


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[DKP]

I'm a bit confused...you say when lower than 5500 it over revs, but then say it looses RPM sub 5500 feet.

What I am meaning is at 5500 I can be 3/4 throttle and it will be a bit over rev. But if I go full throttle it drops rpm. Sometimes 400+. Now gain 1000ft and it holds rpm at full throttle. This is in the trees. Just my combo is way off and trying to get a handle on it. Had it rocking in the spring. Was beating 850s on hard pack hills. But ya. That was hard pack. Once we get pow again. It's all out the window. It's like in the pow my pipe not heating enough when boondocking and mud throttle range. Give a stab of throttle. Sometimes it hit rpm. Sometimes bark and be 4-500 low. If on the throttle lots and keeping pipe hot, this was less. I also installed the stock packing and heat shield on my pipe this spring. To help hold heat. Haven't tried that in the deep yet tho. I kinda think my secondary isn't loading enough down low speed. It's snappy and the rpm climbs great. Nice and responsive.


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[Teth-Air]

So when you say steeper start on helix. That is higher number? If I am a 48-36 now go to a 50-36? I can't get lower initial spring pressure with the same final force that I have. Not down to like 100 like DJ states at the end of his post. I'm sure my kit wasn't not thought out well. And I'm hesitant to drop a whole lot more cash on someone else's kit, only to have to change it too because of the mods. I have a bunch of different springs and ramps as it is now. And adjustable ramps.


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Yes, higher number is steeper and a steeper helix shifts up faster which will load the motor more and slow its rpm's. But if you have adjustable weights, just add more weight to the heel as it costs much less. What spring are you running?

 

[Teth-Air]

What I am meaning is at 5500 I can be 3/4 throttle and it will be a bit over rev. But if I go full throttle it drops rpm. Sometimes 400+. Now gain 1000ft and it holds rpm at full throttle. This is in the trees. Just my combo is way off and trying to get a handle on it. Had it rocking in the spring. Was beating 850s on hard pack hills. But ya. That was hard pack. Once we get pow again. It's all out the window. It's like in the pow my pipe not heating enough when boondocking and mud throttle range. Give a stab of throttle. Sometimes it hit rpm. Sometimes bark and be 4-500 low. If on the throttle lots and keeping pipe hot, this was less. I also installed the stock packing and heat shield on my pipe this spring. To help hold heat. Haven't tried that in the deep yet tho. I kinda think my secondary isn't loading enough down low speed. It's snappy and the rpm climbs great. Nice and responsive.


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Or your clutch is sticking at the 1/2 way point. Check the weights, rollers and bushings.

 

[DKP]

Clutch is not sticking, I am getting full shift. And Pri spring is 120/310 Sec is 140/260 at present. Looking at my notes I made before changing sec spring It was a 140/240 with same pri spring and my notes are "Great low to midrange. Running 8100. Should be 8400 with pipe. Snappy response down low, with no perceptable trenching. Heavier powder snow" This was around 6000-7000' At Blue Lake. I remember, If I dropped some weight to get RPM, at that elevation, it seemed to not load up right. Slow roll on throttle would pull RPM to 8400. But if stab the throttle open, it was hard to explain. I feel, not heating pipe enough and it would be down on RPM with a stab to WOT. Sometimes even a burble, or cough out the exh. Like what DJ calls the "rah-Rahs' That's why I was thinking possibly the helix just wrong for my application. I know that if an engine isnt loaded enough, then it wont make pipe heat and RPM. Adding more weight, just drops top RPM. I think I am already full on the heel IIRC.

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[Teth-Air]

Clutch is not sticking, I am getting full shift. And Pri spring is 120/310 Sec is 140/260 at present. Looking at my notes I made before changing sec spring It was a 140/240 with same pri spring and my notes are "Great low to midrange. Running 8100. Should be 8400 with pipe. Snappy response down low, with no perceptable trenching. Heavier powder snow" This was around 6000-7000' At Blue Lake. I remember, If I dropped some weight to get RPM, at that elevation, it seemed to not load up right. Slow roll on throttle would pull RPM to 8400. But if stab the throttle open, it was hard to explain. I feel, not heating pipe enough and it would be down on RPM with a stab to WOT. Sometimes even a burble, or cough out the exh. Like what DJ calls the "rah-Rahs' That's why I was thinking possibly the helix just wrong for my application. I know that if an engine isnt loaded enough, then it wont make pipe heat and RPM. Adding more weight, just drops top RPM. I think I am already full on the heel IIRC.

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Dalton | Polaris Snowmobile Products DPPS

Dalton | Polaris Snowmobile Products DPPS

daltonindustries.com

This is my "go to" spring. As you can see it has one of the biggest spreads of any spring on the market. A full 205 lbs.

Dalton Black/Grey

DPPS-B/GY

100 lb

305 lb

 

[Dynamo^Joe]

Clutch is not sticking, I am getting full shift.

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Ermm, yer getting full shift. Ok, what is your top & bottom gear, and track drive sprocket? Example; 19-49 w/x driver gives 74mph at full shift. So then with yer gearing gives x mph at full shift....are you getting that track speed in a hard pull, in deep snow?

 

[DKP]

Won't get full shift in deep snow, not with a N/A sled you wont unless geared crazy crazy low. On hard snow I get full shift, Belt runs right to edge of clutch.

 

[Dynamo^Joe]

@DKP "have piped 800 axys" & "Slow roll on throttle would pull RPM to 8400" & "I think I am already full on the heel IIRC."

Umm, what primary clutch cam arm are you using? Is there a link to a picture from the vendor somewhere, where I can see the picture of the cam arm from the side?
Want to see what you say "heel", what does "heel" mean. Like....where have you placed the composites on the cam arm.

 

[DKP]

@DKP "have piped 800 axys" & "Slow roll on throttle would pull RPM to 8400" & "I think I am already full on the heel IIRC."

Umm, what primary clutch cam arm are you using? Is there a link to a picture from the vendor somewhere, where I can see the picture of the cam arm from the side?
Want to see what you say "heel", what does "heel" mean. Like....where have you placed the composites on the cam arm.

Typical Polaris -10 series ramps. The SPI ones which use round magnets to adjust weight. Heel=base, close to the pivot. I know it's not a skidoo, but you should know that DJ.


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[Dynamo^Joe]

All im going is by your words...
-full on the heel
-Where, if I grab full throttle, she looses RPM
-Give a stab of throttle. Sometimes it hit rpm. Sometimes bark and be 4-500 low
-Running 8100. Should be 8400 with pipe.

I dont know what or where you have the composites inside the weight.
I'll assume you have all the weight in hole #1.

This season, go out for your typical riding day as-it-is right now [if the composites are in hole 1]. Get your sea legs with riding again and find the deficiencies again, the things you did not like of the engine speed's conduct.
Then a second day, go out, same thing and get about 1/3 of the riding day in and stop. Move all the composites to position 3. Go ride. See the difference between 1 and 3 and record your new deficiencies.
You might find wanting to put weight in 2, or 2 and 3 or split it between 1,2,&3.

you must, you must, you must write a track speed down where you see the deficiency you want to clean up/solve/fix...
Aaen says you need 2 data inputs, 1 rpms, and 2 track speed. Without the track speed, you cant identify where any part in your clutches are being used in relative to your throttle input.
Example; "where i grab full throttle, she loses rpms - happened at estimated 38mph track speed.
With that track speed, you can estimate very close to how much spring force is being used, which helix angle is being used, where the spider roller on the cam arm [flyweight] is.

• If you dont measure it, you can't see it.

Hmmph, your issue(s) is why I just like to stick to old fashioned Aaen tuning; with stock OEM or fairly stock profiles, and not have composite weights at all. I'd rather have a selection of springs with different start and final forces to nail down a calibration.
To me you have an engine power shape problem. Its like having a dart board hangin by a rope, swinging like a pendulum and you're trying to hit the bullseye; basically trying to chase your engine power shape. The power shape is moving around and you're trying to chase it with clutch tuning. Id say go back to stock as possible, then nail down your clutch calibration; then change to the aftermarket exhaust parts.

Aaen says you need 2 data inputs, 1 rpms, and 2 track speed. Without the track speed, you cant identify where any part in your clutches are being used in relative to your throttle input.

 

[PSI 800]

Spent a lot of time reading and re-reading Aaens clutch tuning book and they are still a bit of a mystery to me. There is a lot of knowledge in there to try and remember. Clutches are like carbs, some guys just have em figured and the rest of us fumble around for awhile. Sounds like Dynamo Joe is 1 of those guys, I would do what he suggests.