Changing the helix in your sled is a mild mod that can better suit certain things you want to change about your sleds personality. For instance youd like to pick up a little bit of speed for racing your buddies in the field and at the same time youd like it to be a little more responsive on and off the throttle in those tight twisty trails, well the helix is your answer. Before you just run out and buy one theres some things you might want to understand a little better.
A straight angle helix is just that, a straight angle from start to finish, it acts the same thru out its travel, as its a steady progression and is easier to tune for a novice clutch tuner. It can be made to work well with a paticular clutch recipe. I use the word recipe, because thats what it is, the helix is only 1 ingrediant in this whole mix, all the ingrediants have to work together for you to have cake in the end. You will find however that with a straight angle helix you cant do much more then 1 goal effectively. If you have not ever read a copy of Olav Aaens clutch tuning book, I urge you to purchase one and read it 2 times back to back. He explains what to look for in a straight shift curve, alot better then me.( im a 1 finger typer).
Lets say you wanted to improve the drag race set up of your srx or viper.
So you have a straight 47 (stock)from yamaha on it, well knowing that going steeper shifts the clutchs faster lets say we bump it up to a straight 50 degree helix. Whats going to happen is the secondary clutch is going to upshift(drop the belt in the secondary) at a faster rate, this allows the belt to rise in the front clutch rapidly, moving the trans into ahigher gear and grabbing more belt surface then when in low gear BUT providing less belt pressure now in the secondary clutch, so you have to increase your secondary spring tension to help control this shift, or it will simply overshift and bog. Overshifting is when you get the clutchs out ahead of the power curve of the motor. My example I use all the time is a 5.0 mustang car, v8 stick shift, lets say we rev it up to 5000rpm, dump the clutch while in 1st gear, it spins the tires and the rpm stays at 5000rpm but climbing rapidly, you shift into second(still holding the gas pedal to the floor), car is still spinning th etires but beginning to rapidly move, you then shift from 2nd gear to 4th gear............baaaaaaooooogggggg, what happend? the motor lost rpm and the car just dogged, its still moving but its not spinning the tires and rapidly accelarating like it was, rpms are way low now, it will recover from this overshift but it will be a long distance down the road before it climbs in rpm and mph.
This is exactly what happens to a snowmobile when it overshifts, to keep it from overshifting you must do 1 of 2 things, either use less starting angle on the helix or increase the rear secondary spring tension. The problem with using a stiff rear secondary spring to control the shift is that it may start off great but it will eat away your topend speed up top, the rear clutch will be trying to backshift and the front clutch is trying to upshift still.
The rear clutch is torq sensative, meaning when it senses a loss of torq from the engine it backshifts to keep the engine up in its powerband. So when this happens the front clutch will fight the rear clutch and the sled will just be very flat feeling and not pulling or gaining any mph or rpm for that matter. You only want to use as much rear secondary spring as needed to control the shift, not overpower it.
A multi angle helix lets you accomplish 2 things at 1 time, ALMOST like having your cake and eating it too. Using the same example we used before, we used a 50 degree start, the sled upshifts much faster now, it is noticeably faster in a short drag race, but progressing thru the powerband it has 1 problem, its flat feeling in the upper midrange and it loses rpm up at peak,slowing the sled down, and the sled also almost bogs and dies when you come into a tight turn from a full throttle blast down a straight away, the on and off again throttle response is gone from the sled. By using a multi angle helix we can correct this problem, by spreading the angles out we upshift the clutchs at the faster rate as desired, and then it will get into the center of the helix spread and we change those angles to a shallower numbers, this does 2 things, it SLOWS the upshift down at this point making sure the clutch's are not in front of the motors power curve, this also applies more pressure to the belt thru the secondary without increasing spring pressure!!!
Anytime you can hang onto the belt with more pressure in the front and rear clutch the sled will be faster simply because your wasting less energy and transferring it thru the belt into the rest of the drivetrain of the sled. You can tell very easily if your wasting energy, its called HEAT!!! slipping the belt creates heat from the friction!
What helix angles and spreads are going to be the best for you depends upon the weights you use in the front clutch, the type of riding you do, and overall sled set up,traction,motor mods, your rider weight, etc.
I like to tell people that if you use great big heavy weights and lots of primary spring tensions then youll be inclined to pick a smaller angled helix.
using lighter weights and soft springs youll be inclined to run a more aggressive helix.
There are 10,000 differnt ways to accomplish something when clutching, if you have 2 of the same sleds, same mods and they are both set up to go 100mph top speed, 1 will do better at something then the other one will, either faster getting to 100mph or a cooler more efficicent set up, TESTING is the only way to get the correct set up your trying for!!!!!!!!!
__________________
A straight angle helix is just that, a straight angle from start to finish, it acts the same thru out its travel, as its a steady progression and is easier to tune for a novice clutch tuner. It can be made to work well with a paticular clutch recipe. I use the word recipe, because thats what it is, the helix is only 1 ingrediant in this whole mix, all the ingrediants have to work together for you to have cake in the end. You will find however that with a straight angle helix you cant do much more then 1 goal effectively. If you have not ever read a copy of Olav Aaens clutch tuning book, I urge you to purchase one and read it 2 times back to back. He explains what to look for in a straight shift curve, alot better then me.( im a 1 finger typer).
Lets say you wanted to improve the drag race set up of your srx or viper.
So you have a straight 47 (stock)from yamaha on it, well knowing that going steeper shifts the clutchs faster lets say we bump it up to a straight 50 degree helix. Whats going to happen is the secondary clutch is going to upshift(drop the belt in the secondary) at a faster rate, this allows the belt to rise in the front clutch rapidly, moving the trans into ahigher gear and grabbing more belt surface then when in low gear BUT providing less belt pressure now in the secondary clutch, so you have to increase your secondary spring tension to help control this shift, or it will simply overshift and bog. Overshifting is when you get the clutchs out ahead of the power curve of the motor. My example I use all the time is a 5.0 mustang car, v8 stick shift, lets say we rev it up to 5000rpm, dump the clutch while in 1st gear, it spins the tires and the rpm stays at 5000rpm but climbing rapidly, you shift into second(still holding the gas pedal to the floor), car is still spinning th etires but beginning to rapidly move, you then shift from 2nd gear to 4th gear............baaaaaaooooogggggg, what happend? the motor lost rpm and the car just dogged, its still moving but its not spinning the tires and rapidly accelarating like it was, rpms are way low now, it will recover from this overshift but it will be a long distance down the road before it climbs in rpm and mph.
This is exactly what happens to a snowmobile when it overshifts, to keep it from overshifting you must do 1 of 2 things, either use less starting angle on the helix or increase the rear secondary spring tension. The problem with using a stiff rear secondary spring to control the shift is that it may start off great but it will eat away your topend speed up top, the rear clutch will be trying to backshift and the front clutch is trying to upshift still.
The rear clutch is torq sensative, meaning when it senses a loss of torq from the engine it backshifts to keep the engine up in its powerband. So when this happens the front clutch will fight the rear clutch and the sled will just be very flat feeling and not pulling or gaining any mph or rpm for that matter. You only want to use as much rear secondary spring as needed to control the shift, not overpower it.
A multi angle helix lets you accomplish 2 things at 1 time, ALMOST like having your cake and eating it too. Using the same example we used before, we used a 50 degree start, the sled upshifts much faster now, it is noticeably faster in a short drag race, but progressing thru the powerband it has 1 problem, its flat feeling in the upper midrange and it loses rpm up at peak,slowing the sled down, and the sled also almost bogs and dies when you come into a tight turn from a full throttle blast down a straight away, the on and off again throttle response is gone from the sled. By using a multi angle helix we can correct this problem, by spreading the angles out we upshift the clutchs at the faster rate as desired, and then it will get into the center of the helix spread and we change those angles to a shallower numbers, this does 2 things, it SLOWS the upshift down at this point making sure the clutch's are not in front of the motors power curve, this also applies more pressure to the belt thru the secondary without increasing spring pressure!!!
Anytime you can hang onto the belt with more pressure in the front and rear clutch the sled will be faster simply because your wasting less energy and transferring it thru the belt into the rest of the drivetrain of the sled. You can tell very easily if your wasting energy, its called HEAT!!! slipping the belt creates heat from the friction!
What helix angles and spreads are going to be the best for you depends upon the weights you use in the front clutch, the type of riding you do, and overall sled set up,traction,motor mods, your rider weight, etc.
I like to tell people that if you use great big heavy weights and lots of primary spring tensions then youll be inclined to pick a smaller angled helix.
using lighter weights and soft springs youll be inclined to run a more aggressive helix.
There are 10,000 differnt ways to accomplish something when clutching, if you have 2 of the same sleds, same mods and they are both set up to go 100mph top speed, 1 will do better at something then the other one will, either faster getting to 100mph or a cooler more efficicent set up, TESTING is the only way to get the correct set up your trying for!!!!!!!!!
__________________
blkmax600
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Very good reading....thank you!
ttt for tuner time
shortstop20
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So let's say you're using a multi-angle helix(50/34) and your secondary is getting a little hot. Backshift seems fine. No noticeable bog or over shifting of the clutches when going from a stop to 85 MPH WOT. Secondary doesn't seem to get hot when going slow only going 50-60+ MPH. Acceleration seems to die off a little after 50-60 MPH. What would you suggest? Stiffer secondary spring to grip the belt better?
Its only the rear clutch getting warm? sounds like the front clutch needs more weight to keep upshifting the belt. If you just keep adding rear spring tension what will happen is it will really become hot because it will want to backshift all the time and start to fight the front clutch, however if your running a roller secondary clutch, you will find youll need to run more secondary spring tension then that of a button secondary. its because the roller upshifts faster and backshifts faster from the lack of friction on the roller to helix ramp, and this will require MORE tension to control the shift.
shortstop20
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Thanks for the tip. So lets say I throw 2 grams larger weights in the primary and then it pulls my RPM down to less than desired peak RPM, then I would want to change to a primary spring with a higher finish rate on it in order to get my peak RPM back to where it's supposed to be, correct?
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as long as its not a giant rate increase yes, that will work
shortstop20
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Well, I'm guessing the 2 grams larger weights will lower my RPM's by 100, maybe 200 RPM's at most. I figure I will start with 10 lbs. higher finish rate and if that doesn't work try 20 lbs. higher finish rate. I really need to get my tach calibrated sometime, it's pretty much like shooting a gun in the dark otherwise.
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