Need some help calculating serpentine belt length

[Joel K]

Hi,

I need a custom belt since I swapped the location of the alternator and compressor on an LT4.

The blue tape measures 70.5” and it goes around the tensioner(tensioner is not engaged and relaxed). Wondering how much length I need to subtract for the real belt with tensioner engaged.

Thanks for the help.

Here is a pic...

 

[Kurt Hoffman]

To measure for mine I held the tensioner at mid adjustment, then pulled a string around the path. Result left the tensioner a little before halfway so I ordered another belt 10mm shorter and that came out perfect so I would suggest taking your measurements with the tensioner at 3/4 if using my method.

 

[Joel K]

To measure for mine I held the tensioner at mid adjustment, then pulled a string around the path. Result left the tensioner a little before halfway so I ordered another belt 10mm shorter and that came out perfect so I would suggest taking your measurements with the tensioner at 3/4 if using my method.

Thanks Kurt, appreciate the help.

 

[Joel K]

I think I lucked out, this engine uses a dual sided v-rib belt and I measured 67.75” and I found a 67.5” belt made by Dayco. There does not appear to be a huge variety of dual sided belts to choose from.

 

[Joel K]

Actually the Dayco belt is 67.25” and I found a Gates belt which is 67.37” so I ordered the gates belt. Although I think it will be too small I’ll give it a try. A lesson learned, I did not realize the belt line needed a dual sided belt and there are not many choices. I should have mocked up the plywood version of the alternator bracket with the belt before I fabricated it out of aluminum.

Definitely not looking forward to remaking that bracket. Maybe I can find a smaller diameter pulley to put on the tensioner. But for now I’ll give the gates belt a try.

 

[Joel K]

Well, I think I lucked out. After measuring with a string, turns out since all the pulleys are grooved the string sat in the groove channel and that measurement wound up being quite a bit short. The 67.37” belt was way too short so I ordered the next size larger double sided belt at 70.15”.

Seems like it will work. I had to retract the tensioner all the way and push the belt onto the compressor pulley. There was really no slack when putting it on and was quite snug. I took it off and put it on again and it seemed to fit a bit better. I provided a close up of the tensioner stopper. To me it doesn’t seam too small a gap, but would like feedback from more experienced builders. Since the next larger belt is 1.3” larger I don’t think that will work.

Here is the accessory layout.... showing the type of tensioner...

Here is a close up of the tensioner gap....

Thanks for the help.

 

[Kurt Hoffman]

Joel,

Sorry I lead you astray with my string method. Looks like you have it figured out. I do not have any experience with the LT style tensioner, the LS is very different and is tighter the further you turn/move it (radial spring design). The LT tensioner appears to be setup to apply a more linear force on the belt. If the belt "feels" tight I would think you'll be fine (for example, is it difficult to turn the alternator pulley by hand?).

 

[Rich Kruger]

Sometimes I've seen quite a bit of bounce at the tensioner from the combined out of roundness of the driven pulleys ( there are no perfect circles or straight lines in the universe, but we come damn close). Other engines I've witnessed had virtually no tensioner ocillation ( almost perfect circles!) Only time will tell if yours will be ok, the stop not hitting its mount. You can purchase or maybe borrow a belt tension measurement tool to see if its too tight which might cause premature component failure. General rule of thumb is about one inch of deflection up and down with moderate hand strength. My two cents is that it looks fine to me and the high state of tension will help with the amount of wrap you have on the alternator which looks to be about the minimal amount you would want. In other words job well done and move on to the next problem.

 

[Joel K]

Joel,

Sorry I lead you astray with my string method. Looks like you have it figured out. I do not have any experience with the LT style tensioner, the LS is very different and is tighter the further you turn/move it (radial spring design). The LT tensioner appears to be setup to apply a more linear force on the belt. If the belt "feels" tight I would think you'll be fine (for example, is it difficult to turn the alternator pulley by hand?).

Kurt, no worries. I really appreciate your help and I am just happy I found a belt that looks like it fits. To your point, it appears very tight and I found a picture of this tensioner with the stock belt and the gap with the stock setup seems just a little wider so I think I am in good shape.

 

[Joel K]

Sometimes I've seen quite a bit of bounce at the tensioner from the combined out of roundness of the driven pulleys ( there are no perfect circles or straight lines in the universe, but we come damn close). Other engines I've witnessed had virtually no tensioner ocillation ( almost perfect circles!) Only time will tell if yours will be ok, the stop not hitting its mount. You can purchase or maybe borrow a belt tension measurement tool to see if its too tight which might cause premature component failure. General rule of thumb is about one inch of deflection up and down with moderate hand strength. My two cents is that it looks fine to me and the high state of tension will help with the amount of wrap you have on the alternator which looks to be about the minimal amount you would want. In other words job well done and move on to the next problem.

Thanks Rich, appreciate the extra detail. I am still going to laser align the custom compressor and alternator pulleys/brackets before install. I definitely have some deflection, but would be a nice thing to do to measure the deflection to rule out any issues.

 

[Mesa]

There are belt tension measuring devices out there....reminds me that I need to order one. Interesting that what looks to be your water pump pulley is grooved. Most LS pumps are not and run as yours does on the 'back' side of the belt, maybe yours is a thinner surface area than other LS applications.