Understanding Helix's by MrViper700
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 youíd like to pick up
a little bit of speed for racing your buddies in the field and at the same time
youíd 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 there are 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 itís a steady progression and is easier to tune for a novice clutch tuner. It can be made to work well with a particular clutch recipe. I use the word recipe, because thatís what it is, the helix is only 1 ingredient in this whole mix, all the ingredients have to work together for you to have cake in the end. You will find however that with a straight angle helix you canít do much more than 1 goal effectively. If you have not ever read a copy of Olav Aaenís 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, allot better than me. (Iím a 1 finger typist).
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 clutches faster, say we bump it up to a straight 50 degree helix. Whatís going to happen is the secondary clutch is going to up shift (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 a higher 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 over shift and bog. Over shifting is when you get the clutches 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.† 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 the tires but beginning to rapidly move, you then shift from 2nd gear to 4th gear............baaaaaaooooogggggg, what happened? the motor lost rpm and the car just dogged, itís still moving but itís not spinning the tires and rapidly accelerating like it was, RPMís are way low now, it will recover from this over shift 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 over shifts, to keep it from over shifting 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 top end speed up top, the rear clutch will be trying to backshift and the front clutch is trying to up shift still.
The rear clutch is torque sensitive, meaning when it senses a loss of torque from the engine it backshifts to keep the engine up in its power band. 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 up shifts much faster now, it is noticeably faster in a short drag race, but progressing thru the power band 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 up shift the clutches 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 up shift 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 youíre wasting less energy and transferring it thru the belt into the rest of the drive train of the sled. You can tell very easily if youíre wasting energy, itís 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 youíll be inclined to pick a smaller angled helix.
Using lighter weights and soft springs youíll be inclined to run a more aggressive helix.
There are 10,000 different 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 efficient set up, TESTING is the only way to get the correct set up your trying for!!!!!!!!!
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? †A stiffer secondary spring to grip the belt better?
Is it only the rear clutch getting warm? Sounds like the front clutch needs more weight to keep up shifting 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 youíre running a roller secondary clutch, you will find youíll need to run more secondary spring tension then that of a button secondary. Itís because the roller up shifts 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.
Thanks for the tip.† 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?
As long as itís not a giant rate increase yes, that will work
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