Alternator Belt Tension
 

I have just snapped another alternator belt. It seems to be a more common occurrence than I would like and I am not sure what I am doing wrong. I have all six shims in place as required. My new belt however, as with the last one, seems rather tight when two shims are used, but a bit loose when three shims are used. I have always been told it is bad to over tighten a belt so I went for this slightly looser three shim fit. This morning, in the dark with headlights and heater on, the belt was squealing a lot especially in first gear so I assume it was slipping. Looking at the remains, I think it overheated and eventually gave up!

IS it correct to go for the tighter two shim installation? Will the belt give a little during operation?

All belts should not be too tight or too loose, you might check if your pulleys are oil free and clean, too tight tension will always snap it off....

This is subjective (like anything based on "feel"), but proper belt tension results in a deflection of 1/4 - 1/2 inch, at the midpoint of the belt's longest run, when pushing on it with "reasonable" finger pressure (aka - a firm push, but not where you are grabbing something near the belt and pulling against it). 1/4 inch deflection will seem pretty tight but is just fine, and anything much over 1/2 will feel like the belt is flapping in the breeze.

On any belt I check/set, I get in there with a ruler in one hand and then push on the belt with one or two fingers of my other hand - works perfectly.

When you install a new v-belt, you want to recheck the tension after you have run it a bit. Not only will the new belt work down into the grooves as it runs, but the belt will stretch a wee bit.

Have you checked to see if the alternator spins freely, or if there is any play in the shaft, you could have a failing bearing. Also when you check for belt tension, turn the engine by hand a 1/4 turn, then check again, do that 3 or 4 times to make sure nothing is bent or running off centre.

http://forums.pelicanparts.com/suppo...s/beerchug.gif

And if all else fails, get yourself a belt tension tool.

My sanity check for belt tightness is this:
- belt slips if I turn the fan nut
- if I squeeze the belt with my fingers to make it tight and then turn fan nut, the motor grips the fan pulley enough that I can turn the motor over

Always use oe Porsche belt aftermarket belts are not sized properly and cause these typ of problems check pulley alignment and fan strap for proper torque
If all that looks ok also make sure your alternator bearings are good

Personally, I prefer the aftermarket Conti 10 x 725 belt to the Porsche Factory 9.5 x 725 belt my race car would normally use.

In the UK the 10 x 725 size seems to be more common than the 9.5 x 725, but you can get both. As the strength of the belt is in the fibres running around the outer edge, a wider edge might suggest a stronger belt? The profile is the same so it fits perfectly in the pulley groove.

Make sure you are using the right size belt for your engine. There are a couple of sizes and they are only different by a few mm's, but it makes all the difference in the world when shimming for tension.

i normally use that rule of thumb, but i found recently with an engine that was newly rebuilt with perfect compression, the belt would slip when the turning the fan nut, even when really really tight belt (less than 1/4 belt deflection) which had me really stumped, until i realised i just had really really good compression! So i now only rely on this specfic rule of thumb for an engine which has been broken in.

I normally also use the rule of thumb for ANY drive belt, on the longest run, turn the belt 90 degrees, if you can only turn it 90 degrees the belt tension is correct, if you can turn it more than 90 degrees, then it needs to be tighter.

Anyone else try that method?