Tensioner Forces Analyzed

We were talking about this in a thread a while back and there are some who think the tensioner always puts the same amount of force on the belt no matter where it rests. There are also those who say the belt can be too tight in some cases. I thought it would be a good idea to measure these forces so we can find out exactly what we're talking about here. All my measurements were done with my cheap Harbor Freight torque wrench and are not meant to show the precise amount of force the tensioner places on the belt at any given point, only the relative force it takes to open the tensioner further.

The amount of force it takes to even move the tensioner off the fully closed stop was about 25ft/lbs. The 2nd measurement I took was right in the middle of the 2 little boxes, I assume that this is where the stock setup would rest. Torque here was 32ft/lbs:


The next measurement was beyond the limits of an unmodified tensioner, you will need to remove (notch) the middle of the stop on the tensioner to get your belt to rest at this point. A belt here is pretty hard to get on by yourself. The force measured was 42ft/lbs:


This last pic shows the tensioner at 52ft/lbs. It would be pretty much impossible to get a belt to rest at this point without taking some extreme measures during installation, unless you completely remove the stop:


The last measurement I got just before hitting the big end of the stop was about 60ft/lbs.

My current setup sits somewhere in between the 2nd and 3rd pic, around 47ft/lbs. It takes 2 people to get the belt on and is just about as tight as you can get a belt. We can then estimate from the above figures that the force the tensioner puts on my belt is approximately 50% higher than an OEM setup.

Awesome tech :bowdown:

this is why this site rocks :beer:

great info. Mine is probably sitting around the 45ft/lbs mark.

ttt:bowdown::beer:

Great info man, good experiment.

Excellent info, and it makes perfect sense too, since most torsion type springs increase tension as they are wound tighter, which is what is happening here. Good that you took the time to prove that, since I had read others argue that it is linear (I never believed that, myself).

Very nice JM! So can we extrapolate from the data that:

1) Notching the tensioner and adding a belt that takes advantage of this will lessen the probability of belt slip???

And/or:

2) Notching the tensioner with the concurrent increases in belt tension, will increase parasitic drag in the supercharger pulley system which will result in a graduated RWHP decrease as RPM's increase?

IronTerp said:

Very nice JM! So can we extrapolate from the data that:

1) Notching the tensioner and adding a belt that takes advantage of this will lessen the probability of belt slip???

And/or:

2) Notching the tensioner with the concurrent increases in belt tension, will increase parasitic drag in the supercharger pulley system which will result in a graduated RWHP decrease as RPM's increase?

Click to expand...

also possible bearing life loss. too bad there is not enough data to prove any of the affects.

IronTerp said:

2) Notching the tensioner with the concurrent increases in belt tension, will increase parasitic drag in the supercharger pulley system which will result in a graduated RWHP decrease as RPM's increase?

Click to expand...

TrickStang37 said:

also possible bearing life loss.

Click to expand...

Both of those are possible but to what degree is unclear. But at least we have a general idea of what kind of forces are at work.

My opinion is that a belt on any setup should be at 42ft/lb (pic 2) at minimum. From what I have read on here, and according to my own experiences, belt slip can occur at the OEM tension level... even if an aux idler and/or large upper pulley is used. This only places approx 30% more tension on the belt/pulleys which is likely within the tolerances of any bearings in the system. However it is reasonable to assume that additional stress will reduce the life of any component.

im glad your bring this up as i am pursuing possible belt slip issue. thanks for taking the time out to do this.

A notched tensioner (pics & info)

Stiegemeier said:

A notched tensioner will allow you to put a 1/2 inch shorter belt.

A simple and common sense way to improve torque and lessen belt slippage is to grind the pulley stop.
First move your water tank to the side and then remove center bolt. Grind the notch 3/4 back, replace bolt-then tank.

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I reposted this from Steggy's thread because I thought it was relevant to the discussion here. I find myself linking this pretty often for those who are having slip issues.

Great info!

On another note does anyone know if the GT500 tensioners fit the '03-'04 Cobras? Steeda now offers a billet aftermarket piece. I am wondering if that tensioning device offers greater ft/lbs.?

heres a great explanation of torsion springs

http://www.engineersedge.com/spring_torsion_calc.htm

Thanks for the info. Too many people take for granted what other people say. Only a few of us actually do the testing.

im gonna notch mine today

nice!

when i added my 2.8H kenne bell with a 3.5" pulley i had to notch mine as it didnt have enough room to fit the belt with the idler pulleys that would keep it tight... sucks when you get into the idler pulley setups and theres not a lot of belt sizes to choose from between 74-77"

racebronco2 said:

Thanks for the info. Too many people take for granted what other people say. Only a few of us actually do the testing.

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+1 You and a handful of others have actually provided the scientific data instead of hypothetical conjecture like most do on this site. :beer:

Very nice!Thanks.