BELT SLIP (for the up-teenth time)

DrFishbone

SCCoA Member
As if you you hadn't heard enough of this already....:rolleyes:

First off, the reason I ask is because of the threads I've seen on here, I'm not convinced that everyone that thinks they have fixed their slipping issues actually have. So, let's talk belt slip one more time.

When using at least a 5% SC pulley and a 10% jackshaft pulley. Of you guys that either have or are running this combo [and know what beltslip feels/looks like]: what belt have you had success with (if any)?

I have MP springs, and the tensioner is [now] clocked so that there is room for travel when the belt stretches under load. Pulleys are in decent shape.

The stock-sized Dayco belt slips with this combo (but works fine with a 5% and a 5%).

Also, this belt slips: brand new 38 1/8" Gates GREEN belt- the heavy duty one (fits very snuggly!) Take a look at the pictures....I think it slips because of one or both of these reasons: 1) the belt ribs are shallow and/or 2) the material is more fibrous than rubbery (probably has to do with the intended purpose of the belt - fleet / heavy duty applications) the last two pictures are of a Gates micro-v (7-rib). It looks like one built like the 7-rib would work for the jack-to-sc pulley, but before I go order and buy another belt, I wanted to be sure...:rolleyes: So, what lengths or this belt (assuming it's a good one) will actually fit? down to 38" maybe?
 

Attachments

  • 100_1912.jpg
    100_1912.jpg
    411.5 KB · Views: 351
  • 100_1914.jpg
    100_1914.jpg
    401.4 KB · Views: 354
  • 100_1916.jpg
    100_1916.jpg
    401.4 KB · Views: 350
  • 100_1917.jpg
    100_1917.jpg
    410.8 KB · Views: 316
I have a 38.5" belt on with a 5% blower and stock J/S pulley. Doesn't seem like I get much, if any, slip. But then again I only got like maybe 300 miles after I put the belt on before my trans decided to go south.
 
I beleive a lot of belt slip is caused by the pulley ribs are pointed vs flat at their peak. I have bought pulleys where the ribs are actually a point. If you look at any belt they are flat in the valleys. I had to remove a pulley and actually file the ribs a little flat to conform to the belt. When I did that the slip disappeared. I beleive the pulleys design and engineered is more important than the belt type. Just my thoughts

Ken
 
magnum powers pullies have the peaks on the groves machined flat, so that the belt better engages into the such that the sides grip the sides of the groves. In particular as it stretches.

The key to reducing belt slip is to make sure your tensioner does not become unloaded as the belt slips. As we change pulley sizes, we loose optimum belt length for the stock tensioner range. What this will cause is that as the belt stretches (and they all stretch under load) the tensioner plays out to keep tension on the belt until the tensioner hits it's stop and can't continue to apply the pressure. This unloads the tension on the belt causing the pulley to slip.

The key is to clock the tensioner and then remove the stop, effectively loading the belt sooner, and increasing the distance that the loading occurs.
 
magnum powers pullies have the peaks on the groves machined flat, so that the belt better engages into the such that the sides grip the sides of the groves. In particular as it stretches.

The key to reducing belt slip is to make sure your tensioner does not become unloaded as the belt slips. As we change pulley sizes, we loose optimum belt length for the stock tensioner range. What this will cause is that as the belt stretches (and they all stretch under load) the tensioner plays out to keep tension on the belt until the tensioner hits it's stop and can't continue to apply the pressure. This unloads the tension on the belt causing the pulley to slip.

The key is to clock the tensioner and then remove the stop, effectively loading the belt sooner, and increasing the distance that the loading occurs.

We shot a video on our dyno (doing 1/4 mile runs) about a year ago to document this problem (the tensioner movement you just described). The results we found were shocking. I never knew it moved so much at full throttle.

Check out the video here: http://www.youtube.com/watch?v=WqyT8dYzAkM
 
I beleive a lot of belt slip is caused by the pulley ribs are pointed vs flat at their peak. I have bought pulleys where the ribs are actually a point. If you look at any belt they are flat in the valleys. I had to remove a pulley and actually file the ribs a little flat to conform to the belt. When I did that the slip disappeared. I beleive the pulleys design and engineered is more important than the belt type. Just my thoughts

Ken

That's what I'm thinking Ken. I'd like to find a belt that fits the pulley better first, but will try filing the pulley ribs down if I still can't find the right one. The ribs are so pointed on mine, I bet I'll have to file them though.

Melissa - there are several signs of belt slip. The one that I think always appears over time is the magic belt dust. You would notice it on the parts around the supercharger snout, maybe the water pump, inside the sc pulley lip, etc. Another, if you have belt slip pretty bad, it a noticable drop in boost likely around 4k+ RPM. On the track this past weekend, with the green belt and 15%OD, when I hit about 4k in 2nd gear, I lost enough power to drop the nose of the car. :rolleyes: I only put up with one run with that combo and switched back to 10% OD with the old torn-up belt - and did much better. There are some other signs, but I think these are probably the most prominent. Also, I hadn't thought to use the QH to monitor it (thanks AGAIN Chris V.), but you can use the MAF readings from a run and see the MAFV drop when you have belt slip. Obviously, if the sc slows down, you'll be pulling in less air.

I think the ticket is simply to find the right belt (deep grooves, rubbery composition), make sure the ribs on the pulley match the belt closely, upgrade the springs and make sure your tensioner can travel all it needs to.
 
Look at the video Bob posted. That tensioner moved a good 3/4". Granted that was a 6 rib belt and it's longer than your SC belt, but having dyno'd literally hundreds of SC's, I have had plenty of occasion to watch the tensioner movement on the SC and most people are quite wrong about how much they think their tensioner moves. It probably moves more than you think.

As I also said before, you have to be careful about reclocking the tensioner and I would not remove the limiter. The limiter should be set so that it prevents the belt from touching itself. Reclocking the tensioner is a hold over idea from when there were no heavy duty springs. With heavy duty springs there is generally no need to reclock.

Then with respect to the belt engagement, looking for belts to match your pulleys is backwards. Fix your pulleys. MP nips the tips off the ribs based on results from lots of R&D done by Charles and Kevin. Kevin has been through 100's of belts and several sets of pulleys with every modification you can think of. Blasting lightly helps, but mostly it tears up the belts which drastically shortens their lifespan. The truncated ribs do a great job of increasing surface tension on the sides of the V's were friction is most useful. If the pulley ribs are able to bottom out into the belt for any reason then all traction is instantly lost.

How do I know when I have belt slip or not? I spend a lot of time on the dyno and I don't trust "oh I felt this" or "oh I saw that". I datalog boost & HP, so there is no question when I encounter belt slip on someone's car. Keep in mind that belt slip is a nebulous term. I have datalogged supercharger rpm vs. crankshaft rpm and determined that even on the best pulley setups and even at part throttle there is belt slip. In fact with multi-rib pulleys and belts traction is always in a state of flux. But if you are experiencing belt slip that results in a visible loss of boost on your boost gauge then you have an extreme problem which is not just a function of one belt brand vs. another. You have a tensioner, pulley, and/or belt length problem.
 
Thanks for the comments Dave.

Then with respect to the belt engagement, looking for belts to match your pulleys is backwards. Fix your pulleys.

You're right...I was looking at this backwards. Even the deep rib belts bottom out on the sharp pulley ribs...I should cut the pulley ribs down first, then judge the belts that I have.

The truncated ribs do a great job of increasing surface tension on the sides of the V's were friction is most useful. If the pulley ribs are able to bottom out into the belt for any reason then all traction is instantly lost.

That's exactly what my thoughts were on the problem. So, as you suggested, I think I'll try filing the ribs down first, and try the HD Gates belt that was slipping again. With more contact, it may not slip enough to cause a problem. If so, maybe then would be the time to look at a more rubbery textured belt (Gates green stripe, non-HD). :cool:
 
what you need to do all depends on how close to ideal the belt is that you get. Due to the variety of pulley combinations, the ideal belt is different for each combination, and none of them are necessarily belt lengths commonly stocked.

Finding the right belt can be extremely frustrating.
 
Filed down the pulley ribs on the sc and js pulleys using the Kevin Leitem method :p

Belts look to fit better...we'll see how it does sometime this weekend.
 
Alright, seemed like filing the pulleys did help...the belt held without slip until about 5000rpm on several cool saturday night runs. :cool:

SO...

Yesterday I got to looking at it again. Since, in my case, I swapped a 5% OD JS pulley for a 10% (both supercoupe performance pulleys), the SC-side diameter was the same between the two pulleys...but....the crankshaft side was smaller...hmmm... ;)

Got to looking around and found that the crank-to-jackshaft belt was slipping on the last test run. I found that the reason it was slipping was because the tensioner was bottoming out (only had about 1/4" of travel!!!), so I de-nippled that idler arm as well, clocked the tensioner a little further and gave it a spin.

No belt slip anymore. :cool:

I am using a 42.5" Gates belt (stock length I believe) for the crank-to-jack...and it is kind of long with the 10% SCP jackshaft pulley. The best solution would be to get a slightly shorter belt (maybe 42"), but I was able to clock the tensioner just enough to clear everything and give the tensioner about 1/2" to travel before hitting the stop. This way, as opposed to getting a shorter belt, I get more belt wrap..we'll see how it does long-term though. It might be best to get a shorter belt for the JS-to-crank as well.

So in conclusion - it looks like the ugly Gates HD (greenback) belts will work for the supercharger, if pulley ribs are not too sharp-peaked. While initially there was evidence of the SC belt slipping, the bigger problem with my setup turned out to be the lower tensioner bottoming out, letting the 7-rib slip on my SCP 10% JS pulley.

Anyone experiencing belt slip should check the tensioners first. Check to make sure your belt/pulley combo does not yield a tensioner that is maybe .... < 1/2" from it's stop. I would think that the target range would be in the middle of the stops, or on the higher side if anything. As the video that Bob posted shows, the tensioners do move alot! (you can observe this on your own car by cracking the throttle and watching, or better yet, on the dyno)

Thanks for the help gentlemen.:cool:
 
Last edited:
Awesome. This is stuff that I was pointing to although I had completely forgotten about the crankshaft side of the SCP pulley. The way it was dropping boost dramatically at a specific rpm I pretty much knew it had to be a tensioner problem. On the lower tensioner you don't have to worry about the belt touching itself due to over extension of the arm, so you can get away with any amount of clocking or cut the tab off like you mentioned.

The other thing you didn't come out and say specifically, is that the belt length you are using on the blower is shorter than any other belt I've seen. I didn't even know you could get a 385 belt. That will help out people who are running a 10% pulley on a stock JS.
 
The clocking limit on the lower tensioner (at least with my dayco pullies), ends up being the JS pulley itself. :eek: I was barely able to clock it far enough to not touch the JS pulley when unloaded - even though I don't guess this would be severely detrimental.

Yes, the SC belt is a shorty! In order to even get it on the car, I had to cut my lower A/C mounting tab off the accessory bracket to let the tensioner swing back further. Even with this extra swing, you really have to roll the belt onto the sc pulley. If a car has A/C it would be very difficult, if not impossible to use this belt.
 
The clocking limit on the lower tensioner (at least with my dayco pullies), ends up being the JS pulley itself. :eek: I was barely able to clock it far enough to not touch the JS pulley when unloaded - even though I don't guess this would be severely detrimental.

Yes, the SC belt is a shorty! In order to even get it on the car, I had to cut my lower A/C mounting tab off the accessory bracket to let the tensioner swing back further. Even with this extra swing, you really have to roll the belt onto the sc pulley. If a car has A/C it would be very difficult, if not impossible to use this belt.

I also tried a 385 a few years back when I still had A/C, and I was also using stock J/S and a 10% blower pulley. I could not get the belt on for the life of me, even with a friend helping. It was one of those "so close yet so far" things. Just to confirm this. :)
 
I also tried a 385 a few years back when I still had A/C, and I was also using stock J/S and a 10% blower pulley. I could not get the belt on for the life of me, even with a friend helping. It was one of those "so close yet so far" things. Just to confirm this. :)

what i did was losen the supercharger bolts and lift the back of the blower to allow the nose to lower and allow room to get the smaller belt on. once over the lips the blower can be bolted back down tight
 
when my oldest son and i would take his 90 automatic to the track and the boost would fall off at higher rpm, i would just do a bit of belt dressing. but dont over do it. ;)
 
what i did was losen the supercharger bolts and lift the back of the blower to allow the nose to lower and allow room to get the smaller belt on. once over the lips the blower can be bolted back down tight

That's a good idea Kevin. I'll have to remember that one.
 
Back
Top