Balance Belt Idle Roller clearance

[bamalam]

I've got the balance belt tension to 3.5 using the porsche tool 9201.

At that tension if I have 0.5mm clearance between the idle roller and the belt below it there is almost 2mm of clearance between the idle roller and the belt above it, which it should deflect up to 1mm.

The only way I would get the proper clearances above and below the idle roller is to adjust the tension roller so the left side of the belt sits lower, but if I do that the belt tension is way out of spec.

Unless there's something I'm missing, it seems I have to choose between which clearance is more important, the 0.5mm below the idle roller or to touch the belt above it.

[v2rocket_aka944]

The 0.5mm clearance should be between the idler and the belt running above it. You need more clearance between the idler and the belt/gear below it. The tension shouldn’t be affected since the idler isn’t touching.

[bamalam]

Originally Posted by v2rocket_aka944

The 0.5mm clearance should be between the idler and the belt running above it. You need more clearance between the idler and the belt/gear below it. The tension shouldn’t be affected since the idler isn’t touching.

I was looking at this post with factory illustrations showing the 0.5 below and 0-1mm deflection above

https://forums.pelicanparts.com/porsche-924-944-968-technical-forum/421347-balance-belt-idler-misinformation.html,

[OK-944]

The idler pully is mounted on an eccentric cam - and by turing the cam either clockwise or counterclockwise…you will realize that it is possible to acquire two “more or less correct” roller to belt clearance readings, but only one of these will offer the correct readings at the appropriate belt tension.

My guess is that you should loosen the roller cam nut, move the roller over to the other "correct" position, and fine tune the tension and clearances at that position.

My interpretation of the correct clearances has always been .5mm between upper belt and pully, and 1mm below, and I’ve always done this and it works fine (Van Svenson has a good video illustrating this). Some folks actually use a piece of an old credit card as a feeler gauge to measure this upper distance…and some simply spin the idler manually and note that only the slightest finger pressure on the belt will stop the idler from spinning.

[wwdwgs]

On the Balance Belt, I think, the proper clearance is more important then the tension. And, of course, proper orientation of those thick cone-like washers. It's a little PITA to adjust the BB, but after 2-3 belt replacements, it becomes easier. OP, don't rush - be patient and persistent. Have a stimulating drink, not energizing one (like Red Bull). Watch some youtube videos posted by a German dude named Jurgen, who fixes the 944's with such an ease!
There is another "local" guy, who explains it very well: https://www.youtube.com/watch?v=iBxOboQNQ6U&pp=ygUcOTQ0IGJhbGFuY2UgYmVsdCByZXBsYWNlbWVudA%3D%3 D

[bamalam]

Originally Posted by OK-944

The idler pully is mounted on an eccentric cam - and by turing the cam either clockwise or counterclockwise…you will realize that it is possible to acquire two “more or less correct” roller to belt clearance readings, but only one of these will offer the correct readings at the appropriate belt tension.

This is my problem, even when the cam is loose and movable by hand, even at the edge of the cam the closest I can get is .5 from one belt and 2mm from the other.

The only way I can get it closer would be to tighten the BB tensioner, which would lower the top belt. But in order to get the top belt low enough for clearance to be in spec, the tension would be more than double the tension spec

[bamalam]

With the roller making contact with the upper belt, there's ~2mm below when it should be 0.5mm

[bamalam]

With the roller 0.5mm from the lower belt, there's a 2mm gap to the belt above it, which it should touch or deflect between 0-1mm

[wwdwgs]

Hm.. strange. I remember that I had some difficulty with this roller, but it worked out well. When I have similar frustrating situations, I just stop working on that part/area and come back in a few hours or next day. In this case, I would "rest", remove the belt completely, re-set it and start to play with tentioning.
And this is the correct roller (diameter), isn't it?

[7eighths]

Quite interested in this as about to put new belts on. Had a look at the manual and found this. The right hand dimension looks to be 0 to 1 mm on the workshop diagramme above.

[v2rocket_aka944]

thinking back the reason i would set the balance belt 0.5mm off the idler is that, if you get the idler too close to the lower b-shaft gear the belt can jam and shred teeth. that happened to me in maybe 2007? but it all seemed to fit and work better with the 0.5mm set above the idler.

[bamalam]

Originally Posted by v2rocket_aka944

thinking back the reason i would set the balance belt 0.5mm off the idler is that, if you get the idler too close to the lower b-shaft gear the belt can jam and shred teeth. that happened to me in maybe 2007? but it all seemed to fit and work better with the 0.5mm set above the idler.

I just decided to put it together with the 0.5mm clearance below and accept 2mm above.

Reason is I think the lower balance shaft has more chance of skipping a tooth than the upper, and that 0.5mm clearance is likely to prevent that.

Also, the upper belt cover is easy to remove with the rest of the engine together, and maybe after running it a bit the tension will change and I can readjust it. We'll see how it goes.

[Monkey Wrench]

with belt drives there is normally a shaft which drives the belt, and a another that is driven.
taking the rotation direction into account, you can then predict which side of the belt will be more slack. one segment has the mechanical tension due to the load it is transmitting , the other side is basically returning the belt and has less tension

when the engine is not running , during adjustment, both sides have equal tension unles therre is some sort of load on the belt. during operation , the side that has less tension ( the slack side) is going to "slap" more than the side with more tension.

taking that into account may help explain something about these clearance adjustments.

in general with belts , when you are adjusting it, the tension on the two sides of the belt will likely be equal because the belt is then not seeing its normal load

possibly other factors do apply tension even in a static state, like for example. if a shaft is locked in place and the valvetrain is applying tension to one side of the belt.

Im not familiar enought to advise wiht the particulars of this adjustment but this maybe a factor to put a bit of thought towards..

in some drives the tension can reverse when the load is changed, a belt drive may see a reversal of the normal slack and tension side.

easy example a belt is driven by a motor, the motor is switched off and now the machine it was powering which still has inertia is driving the motor , the belt tension may reverse while it is "winding down" towards a stop.

perhaps when you switch from accelleration ot decelleration you could see some such effects

also yes, as the above poster indicated , It may be important to be on the "correct side" of the eccentric. this is common with eccentric adjustments. in some cases this matters.

[djnolan]

the image says "if correct clearance cannot be obtained, turn pulley through 180 and try again"