Belt blows at 120kmh

Perhaps you are running out of gear if your shifting all the way out at WOT? 57/63's may not be good for much more then that ballpark, i dont really know for sure, i know i never really liked to run my m8 up around there for very long at all because of that fear. Unless something is off, and your flipping the belt over/blowing it apart, you may need to test a little taller gear like 60/60 or split the difference. Hope you get er figured, good luck!

Limit the secondary and try a spring with a higher finish rate (320 or 340) on the primary.

I bet you are running out of gear and your belt is climbing right out of the sheaves and then blows.

Klimbing Kitty said:

I bet you are running out of gear and your belt is climbing right out of the sheaves and then blows.

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Exactly!!! My M7BB does the exact same thing. No high speed runs for us!!

Check your belt deflection. Also if your belt and clutches are cool you could gear up to the 60/60.

Check your center to center as well.

What's happening is the secondary keeps shifting up and the primary stays put (so to say), until way to much belt slack is there. Then when you back off a bit or hit a bump, boom, belt starts to wrap and pulls deeper into the secondary. When you first start to play with the tortional set-up, it is a good idea to make a stop spacer (before the belt will go past the sheave bottom) for your secondary. It's somewhere between .8 and 1 inch depending on your kit or pieces you put together. Take it out when your done and enjoy the rest of your top end.
The clutches are not in balance. Often it is just too much weight in the primary, But for you, IMHO it is all the components. Too much weight, too light a finish on your primary spring and too much finish on your helix (remember it's just an opinion).
Gearing is just gearing and if your clutches are that unbalanced, change in gearing will just move the belt blowing spot up or down in MPH.

Geo

I am with Geo on this.


Can you post a picture of the primary and secondary sheaves?

I have had my M6 up to 153kph and not blown a belt with 57/63 gears and stock drivers.

Don.

Bearcat said:

I don't have pics available at the moment. I'll try to take some.

I've forgot to mention one important thing: I'm still using stock clutch. It's 8.5" instead of 8.25". This combined with no stop spacer in secondary is a belt killer combination - right? This causes primary can pull the belt too deep (down into helix) and cause belt blow?

I'm wondering all this info. Seems many people are saying primary spring is too light from the end. But if I put stiffer spring or reduce primary weight - rpms will get higher and engine will overrev? How I can get rpms down if I put stiffer spring or less weight?

I'm thinking aloud now - correct me if I'm wrong As geo said belt snaps when it gets loose and tight again. Less helix angle would cause secondary doesn't react so fast bumbs etc - this means engine will overrev and keep the belt tight? But how to prevent engine overrevving? I'm little lost here.

I really love the 48 initial angle. It pulls warms straight from bottom up. Should I try 46/36p or 48/36p helix?

If you can run 153 kmh with 57/63 gears - there's really no need for longer gears!

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153kmh no problem and not getting the belt into the helix. IF the belt is not slipping down there it will still ride on the helix and not tear up. The only belt blow i had was with a polaris belt with a 10" secondary and light finish spring rate in the secondary.

Pictures of the sheaves tell the whole story.

The 8.5" will only shift out with a four or four and a half coil spring. Five coil springs will bind way before 1.375".

What I have done is adjust the secondary first for the gears and track size then bring the primary to it. I am betting on to low of a spring rate in the higher shift of the secondary. Black marks down around 5.75"s in diameter in the secondary.

Don.

You added 2 .060 shims to the offset when going back to the 036 and 10" right?

I really like the 036 belt it is a good belt but likes heat.

Check your motor mounts take a very good look at them.

Don.

Don, is the 10.4 to 10 difference in offset .060 or is that just what it likes. Dont want to highjack but going from 10 to 10.4.

Bearcat after looking at your secondary you have the classic low pressure in over drive. The torsion system is bad for this and hard to find the sweet spot. It can be done.

Here is the deal and I guess I am sounding like a broken record on this. When you take a rubber band and stretch it the band thins. This happens to the belts in all CVT clutches. Softer belts more than harder belts. Now the DDrive is totally different than the chain case by putting much more load to the clutch. This is why the larger diameter helix in these systems. Ok ever one who trys to pull more over drive gets these black marks down about 5"s diameter in the secondary. The belt looses its grip down here because of the low belt surface area and major feed back from the drive system. The torque rises at this point that the spring and helix are trying to pinch more and the belt is thinning. This thinning then needs less off set or the fixed sheaves pushed closer together to keep the belt pinched. Once you do this you will hold the belt there and mph more.

IF you look close you will see where the belt is slipping at the top of the secondary by nice polishing then the black mark at about 5"s diameter. This shows how narrow the shift torque is. The 036 is really the best belt for these sleds if your not going to mess with a different off set. The 036 will not thin as much under load but is hard. This hard belt will not sick as much load to the sheave and will not thin as much because the torque will not over load this belt. The narrow torque shift fits this belt perfect.

Ok so now we have the different compound belts and they tend to stick more. On my M6 with the 10" secondary I run the polairs IQ race belt 1104. It put so much load to the clutches I had to move the secondary in as far as I could but then wanted more over drive. I then cut .060 off the back side of the secondary to move it in even more. This let me pinch the belt between the two clutches by moving the two fixed sheaves closer. If I didn't do this the 1104 would thin at high torque and slip then blow the belt in seconds at 120km+. The heat from the belt slip would vault so fast most can't tell that it is the off set doing this.

Now back to the 10" to 10.4" with out changing the center to center the belt is longer to run it. The belt is longer like a longer rubber band and gets a bigger contact patch in the clutches at 1-1. This extra torque feed back to the longer belt (more surface contact) makes it thin more than the shorter belt (less surface contact) that thinning tends to slip more and the longer belt needs the fixed sheaves closer together. This is why the different off set bars.

The off set bar is just a guide for the average bear. If you spin the sled up to or more than 85mph you are pulling into this bad range (stock offset bar) for the belt to start thinning and loosing grip at that 5" diameter in the secondary. If your happy with 1-1 than there is no need to change but to widen your shift ratio you have to move the secondary into the motor more closing up the space between the two fixed sheaves in the clutch system.

036 needs more off set than the 044 meaning the 044 needs the secondary moved in more.

046 needs more off set than the 060 meaning the 060 needs the secondary moved in more.

All this means is the 036 will slip more then the 044. The extra stick of the 044 belt lets it thin more under high torque. Same between the 046 and 060.

Trust me on this. Keep moving the secondary in .030 at a time till it stops MPH more. Then go back one shim.

As the belt breaks in you have to move the secondary in. I MEAN YOU HAVE TO MOVE THE SECONDARY IN FOR THE FACT THE BELT IS NOW THINNER. YOU WILL BLOW THE BELT WITH A THINNER BELT THAT STICKS BETTER IN ONLY 100-500KM'S ON THE BELT if your pulling more than the average guy in clutch shift. The belts are (going off the top of my head correct me if i am wrong) 30 degrees totally new. The sheaves in the clutches are 28 degrees so the belt will thin 2 degrees as it breaks in.

Real world torque feed back is totally different than the computer is telling cat. BUT cat is not trying to pull more over drive like most of us tuners tend to with mods and clutch work.

So in short I am telling you that if you see these black marks down in at 5" diameters or your blowing belts keep moving the secondary in .030 at a time till the heat is gone. Heck I have added windage plates to my secondary just to add heat now to my secondary.

NOTE: picture and the same black marks at 5"s cupped out in the sheaves. Full shift attained on 8.5" primary and 036 with out cutting the back of 10". NO shims behind the secondary running right on the bearing.

Don.

Bearcat, just keep dropping wieght 'till you overrev. It'll probably drop rpm for a while then when your close in weight it'll start to rise. Change back to the orange or yellow- white.

THEN, play with the secondary. It'll be an easier learning curve.

Geo