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Belt Tensioner Tech

Belt tensioners are very misunderstood or maybe a better word, if it’s a word is under-understood. A belt tensioner has a lot of jobs. And when you add a supercharger to the mix more yet.

Let’s start off by making it clear that the stock/OEM serpentine belt system was not designed to run a supercharger in any way shape or form. So right from the start we have that going against us. Don’t lose sight of the fact that the supercharger is not supposed to be there. If you look at most OEM supercharged cars they have a separate belt to run the supercharger with a wider belt and a beefy tensioner.

The Tensioners Job

The belt tensioner has a lot of jobs. The main and simplest one is to provide enough pressure to keep the belt tight so it doesn’t slip. It is also responsible for taking up the slack in the belt as it stretches under load. Lastly the tensioner is supposed to not allow the belt to go “solid” or tight (more on this below). That is a lot of jobs for something that is so simple, and it gets a whole lot more complicated when a supercharger is added to the mix.

Spring pressure

The tensioner has a spring in it, that is what provides the tension/pressure on the belt. The spring is a “progressive” rate spring. I could spend pages outlining the difference between progressive and liner rate springs, but I won’t. If you want to understand the difference in detail a quick search will get you all sorts of information. The short version is that with a progressive rate spring the further the spring is compressed the more the rate (stiffness) goes up. For a tensioner this is good, for something like a valve spring, not good. The OEM tensioner was designed to cope with the stock accessories, not a supercharger and the stock accessories. In simple terms think of the accessories and supercharger as “weight”. The more “weight” you have the stiffer the spring needs to be. Clearly when adding a supercharger you need a stiffer spring. But why?

There are two reasons why you need more spring pressure. The first one is simple. Because you are adding a supercharger, which “weighs” a lot more than the accessories the belt needs more traction/pressure or it will slip on the supercharger pulley. The second reason is more complex.

When the motor and supercharger change speeds dramatically in relation to each other, like during a gear change, hitting the rev limiter or spin/hooking the tires the belt wants to go “tight”. The rotors in the supercharger represent quite a bit of weight (much more than the spinning parts in the accessories) and a lot of kinetic energy. When you make let’s say a gear change from 2nd to 3rd at redline the engine drops about 2,000rpm. The motor is directly hooked to the transmission, rear end and ultimately tires which means that the RPM’s drop instantly. The tires contact with the road works like a brake for the motor. The supercharger on the other hand is spinning at 13,100-18,000 rpm and is being slowed down to 8,700-13,600rpm during that gear change. The “problem” is that the only thing slowing the blower down is the motor, which it is connected to it by the belt. Well, that belt isn’t terribly happy about slowing the blower down and what happens is that the belt goes tight, real tight, and tries to slam the tensioner off of its maximum travel stop. This is a problem, a couple of big problems.

The belt can/will slam the tensioner off of its maximum travel stop so hard that it will bend the tensioner arm. Take a look at your tensioner………how hard would you have to hit that arm with a hammer to bend it? Pretty hard huh? I have even seen tensioners ripped off the front engine cover! That’s hard! The first and most dramatic problem with the tensioner bending is that the pulley is now at an angle and the belt comes off. You can put it right back on. But it will come right back off again as soon as you start the car. You’re stranded. I’ve also heard of belts breaking but I have never seen it. Most people at this point put a tensioner with a stronger arm on, it doesn’t bend anymore and all is good, or is it? The answer is no, it is not “all good”, far from it. The only thing a stronger tensioner arm does is make it so you aren’t stranded on the side of the road from bending, which is a good start. But it doesn’t solve the problem.

What is the problem?

The problem is that when you change over to a tensioner with a stronger arm and don’t increase the spring pressure or travel (more on this later) you have only put a band aid on things. You still have a big problem and that is the tensioner slamming into its maximum travel stop. All of the force that it took to bend that tensioner arm is still there, the arm is just strong enough not to bend. Now all of that force is transferred to the pulleys and crankshaft snout. Would you hit the end of your crankshaft with a hammer hard enough to bend a tensioner? Nope, that would be crazy. But that is exactly what happens when you bottom a tensioner. And it gets worse. The snout of the crank goes through the oil pump (it drives the oil pump). Whack the crankshaft hard enough and the oil pump gears shatter……and you just bought a motor. This is not uncommon, there are a lot of smoked motors out there because of shattered oil pump gears. So many that there are super high dollar aftermarket billet steel oil pump gears available.

No one knows for absolute 100% certainty what causes oil pump gears to shatter. Like most things mechanical there could be multiple causes. But, a large portion of motors with shattered oil pump gears have a supercharger and a tensioner that will bottom. We have never heard of shattered oil pump gears on a blown car running a tensioner that can’t bottom. It’s probably happened, but it’s rare.

So how is this solved? Partially it is solved with more spring pressure. A LOT more spring pressure. It took us getting a Gates belt tension gauge and doing a lot of checking before we got an idea how much spring pressure was really needed. When we built the first Frankentensioner we thought that we probably had too much spring pressure and would wear the pulley bearings out faster. But we didn’t care, worn out pulley bearings were much more preferable to bent tensioners and shattered oil pumps. Ends up that having double the spring tension (two tensioners) was just right according to the Gates and Dayco belt engineers. Yes, the tension is right at the top of the range, but just fine. We found every OEM and aftermarket tensioner that we tested (we did a lot of testing) didn’t have enough belt tension. About half of the aftermarket tensioners had more tension that stock, but still not nearly as much as you would want.

Tensioner travel

The most overlooked thing about belt tensioners is the travel. Travel is how far the tensioner arm will swing between zero tension (no belt on it) to bottomed out. The travel is critical to a correctly running belt system. The stock travel was never meant to cope with the much higher loads a supercharger puts on the belt.

Belts stretch a lot more than you would imagine. And the longer the belt, the more it will stretch. A 100” belt will stretch twice as much as a 50” belt. A stock belt is about 100” long. A supercharger belt is about 130” long. That’s 30% longer. Rough math says that right off the bat you need 30% more travel to deal with the extra belt stretch.

So how is travel connected to belt stretch?

Before you put a belt on your car the tensioner is sitting on its minimum travel stop (MTS). You put a pry bar/cheater bar on the tensioner, swing it down some and put your belt on. Note how the tensioner with the belt on is no longer on its MTS. It is somewhere in between the MTS and maximum travel stop. The reason you can’t have it on the MTS is twofold. Firstly because of the springs progressive nature there is very little pressure on the belt so it will slip. You need to have some preload (get into the spring more) for there to be enough pressure to keep the belt tight. The second reason is when the belt is loaded by the supercharger it stretches the belt and the belt gets loose. You need the tensioner to be far enough into its travel that when the belt stretches it doesn’t put the tensioner so close to the MTS that it loosens up and slips. Just set things up so the tensioner is further into the travel then, it will solve the problem. Right? Wrong. You also need to have enough travel in the other direction to deal with gear changes, etc that want to slam the tensioner off of its maximum travel stop. This is where travel becomes very important.

You need enough travel so that the belt won’t go loose under acceleration and enough travel where it won’t go tight (hit the maximum travel stop) during gear changes, banging the rev limiter and spin/hooking the tires. Tensioners with stock travel do not have enough to do both. You either get a tensioner that hits the maximum travel stop, loosens up under acceleration, or both. There is no way to cheat not having enough travel.

Conclusion

Just because you’re not chucking belts or bending tensioners doesn’t mean your tensioner is working correctly. You could still be slamming into the maximum travel stop (you probably are), experiencing a little belt slip during WOT/high RPM’s, or both.

We have never ever seen a tensioner that is strong enough, has enough spring pressure and enough travel aside from ours. Never.

Would we like to offer a big beefy awesome looking tensioner with enough spring pressure and travel? We sure would. Are we capable of designing and producing a tensioner like that? Absolutely. Is the market big enough for two super high dollar billet tensioners (there is already a high $$ billet tensioner out there)? Probably not. How many people can really afford to plop down $400 or so on a tensioner? Not as many as can’t. So we offer a solution that solves all of the problems other tensioners don’t, and for very little money. Yeah. It’s not pretty. But at the end of the day what do you want, a pretty tensioner? Or one that works? What is more important, how a tensioner that you can hardly see looks? Or having maximum boost and not beating up your crankshaft/oil pump gears?

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