Helix effect on elevation changes?

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?
 
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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Oct 8, 2009
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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.
 
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Oct 8, 2009
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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

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Nov 26, 2007
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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

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Nov 27, 2007
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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.
 
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