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Discussion Starter #1 (Edited)
Pulley belt size - demystify - Sheet ready

All info and sheet on my last post

So... I've made a quick search and found out that there is no general consensus in what regards belt sizes for different pulleys, and even for even sized pulleys.
Well, this sure ain't rocket science and I think a quick guide would avoid many mistakes on the time of choosing the correct size for our setup.

We have 3 changeable pulleys that will contribute to the correct final size of the belt:

- Supercharger pulley
- Crank Pulley
- Alternator Pulley

Having the original size of these 3 parts, and the original belt size, it would be a simple exercise.

So, if someone can provide me with these sizes I ask, then it will just be a matter of doing the math.

I will take the values Fisher once provided to calculate the Supercharger speed at a given RPM with different pulleys.

Stock Supercharger pulley size (mm) - 65,5
Stock Crank pulley size (mm) - 134,93
Alternator pulley size (mm) - ?? I will put 100, just for the example.
I will also consider that the original belt has 1350mm
So, with a simple 15% reduction on the supercharger pulley we would have about 9,82mm (65,5 x 0,15) to compensate for… So, the ideal belt size would be 1360mm.

With a 19% reduction on the supercharger pulley we would have about 12,44mm (65,5 x 0,19) to compensate for… So, the ideal belt size would be 1363mm.

Now a more complicated one… with a 17% reduction on the Supercharger, +5% on the Alternator, and + 2% on the Crank, we would have: -11,14mm + 5 mm + 2,7mm = -3,44mm; which results in 1350 + 3,44mm = 1354mm.
We also have to take in account that there are no belts per mm, so we must get the closest match.

Note, these values are (until someone provides me the accurate data) merely indicative.

Thanks in advance,
Citro
 

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So what you asking as I've had too much wine Citro ? ! I'd love to have a chart on my wall though with all the bloody belt sizes that would fit with diff'combo s . Mine is the complicated one with 1% crank and too pissed to do maths ! -lol
 

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Fwiw

Bear in mind that as well as the pulley sizes you also need to take into account how much around each pulley the belt fits, and this changes with the pulley size !!

I put together the attached spreadsheet when I tried to do this, but it only does a change in SC pulley and does NOT take account of the above variable.
 

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Discussion Starter #4
Bear in mind that as well as the pulley sizes you also need to take into account how much around each pulley the belt fits, and this changes with the pulley size !!

I put together the attached spreadsheet when I tried to do this, but it only does a change in SC pulley and does NOT take account of the above variable.
That's why I added the three pulleys that imply a change in the belt size, because I want to do what you did, but considering all three possibilities...crank, alternator and Supercharger.
You present a valid point, if I understand well what you're saying, you're telling me that despite the increase or decrease in size of a determined pulley in mm, the contact surface of the belt might not coincide with that size, right?
I guess those little details have to be taken in account, but must be "manually adjusted"... but if we start off with a solid base, it will all be easier.
 

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You present a valid point, if I understand well what you're saying, you're telling me that despite the increase or decrease in size of a determined pulley in mm, the contact surface of the belt might not coincide with that size, right?

YES in my spreadsheet I assume that the belt will grip 2/3 of the diameter of the SC pulley no matter what its size, in fact as it gets smaller less than 2/3 will be gripped.......

Good luck to you.
 

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Discussion Starter #6
YES in my spreadsheet I assume that the belt will grip 2/3 of the diameter of the SC pulley no matter what its size, in fact as it gets smaller less than 2/3 will be gripped.......

Good luck to you.
Thanks, Chris :)
I have a friend of mine who is going to measure the three pulleys, and then I'll post the results here.
I'm putting a K060539 for my new setup -17%(S/C) + 2%(Crank) + 5%(Alternator).
This one measures 1371mm inner circumference and 1385mm outer circumference.
I believe the value to consider is the inner circumference one.
 

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Discussion Starter #7
After gathering data from Fisher, chris.lamb, and Gate's site, I've made some basic calculations. I will then look deeper into this, but for starters we have belts from gates with the following inner circumferences approximate sizes in mm:

1353; 1364; 1371; 1389; 1400

Stock setup

Belt - 1380mm
S/C - 65.5mm
Crank - 134.9mm
Alternator - 49mm


Then, for a random setup (mine), we have:

S/C pulley reduction - 17% = 54.365 (-11.135)
Crank Pulley + 2% = 137.598 (+2.698)
Alternator + 5% = 51.45 (+2.45)

This means that the new belt has got to have: 1380 - 11.135 + 2.698 + 2.45 = 1374mm

So, the right belt for me would be the one with the closest value to 1374mm and that would be the K060539, with 1371mm.

Another classic example:

S/C pulley reduction - 17% = 54.365 (-11.135)

This means that the new belt has got to have: 1380 - 11.135 = ~1369mm

Again we should go to the closest value to 1369mm and that would be the K060539, with 1371mm.

Another classic example:

S/C pulley reduction - 19% = 53.055 (-12.445)

This means that the new belt has got to have: 1380 - 12.445 = ~1367mm

Again we should go to the closest value to 1367mm and that would be the K060535, with 1364mm. Here we might remember that we are talking about a difference of 1mm in choosing between the K060535 and K060539 (1367-1371 = -4mm); (1367-1364 = 3mm)
One is looser, and the other tightened, this should also enter the equation.

So, we are talking of barely noticeable differences, such small variances that are not accompanied by a wider range of belt sizes. And if you consider that the math done here should also take in account the contact surface of the pulleys, then the differences would be even more marginal.

I will complete this tomorrow with more data.

Thanks for your time.
 

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Discussion Starter #8
Guys, it's taking me a little bit longer to compile the whole Data... I hope to have it ready on Sunday :)
 

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Discussion Starter #9
So... after analyzing the data, here we go... unfortunately I'll have to start by telling you guys that the results of the analysis didn't go as I thought they would, mainly due to the lack of offer on belt sizes... but, let's get it started, shall we?

Some relevant Notes:

I could not find any reference to the sizes of Contitech belts (OEM for stock Cooper S and JCW).

I used Gates, Contitech and Goodyear belts.

I assumed the original size of the Cooper S belt is 1380mm, like Fisher wrote. And so, if they present it like 6K1388 and is in fact a 1380mm one, I guesstimated the same length difference with the 6K1374, and assumed it was a 1366mm one. But, as I don't like uncertainties, I only considered as valid options the Goodyear and the Gates belts.

I saw sizes from 1366mm to 1383mm (as optimum belt size) on both ends, i.e. the shortest and longest belts needed for extreme setups.

I used belt sizes from 1353mm to 1380mm. I only used the 1353mm one because Fisher wrote in one post that he used it, as per data gathered, in theory, no setup requires such a short belt.

According to Fisher, these are the exact sizes for a stock setup:

Belt - 1380mm
S/C - 65.5mm
Crank - 134.9mm
Alternator - 49mm

If you have any other brand of belts, or know relevant sizes that are not shown on the Excel Sheet in attach, please let me know and I'll modify all that is necessary, and re-upload the Excel Sheet.

You can find all the information I could gather regarding the belts in the Sheet.

Well, I don't expect my Excel Sheet to be a "must follow", I expect it to be a good starting point to demystify the pulley belt sizes and its correct use. Then, with others experience (practice) and the sheet's theory, I hope we can achieve something that might serve as a practical guide.

This said, all comments are welcome, and I am counting with your help to make this work.

Thanks for all who made it possible to build the Sheet.

Instructions:

Select your Crank; Supercharger; and Alternator pulley sizes under the belt's brand of your preference "Gates" or "Goodyear", and then see your current setup belt optimum size, and the recommend belt for your application.

Enjoy!

Citro
 

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In the name of Fun
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Do I need something else on my PC to make this work as can't seem to use it Citro ?
 

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Yep but it said read only - anyhow I'll try it now - cheers Tim
 

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guys im hgihgly confused... i check my belt on my car currently running a 17% and when i ordered it i got a 1353 BELT with it.. it fits fine a little tight but it fits fine......

is this normal?

im switching to a 19% so what do i need to ask Ambroz to send me it with to have the optimal belt size cuz when i had ordered a 17% i had asked him to pass me 3 extra belts and i got 1353's..... i izz worried
 

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Discussion Starter #16
Hi Ale,

I'm using the 1360mm for the 17%. If you're going to put the 19% one on, maybe the 1353mm will fit just fine :)
 

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i am running a 20%, same belt as the 17% should be fine

the wheel dia is smaller but your using same shaft position, and only part of the edge, so the numbers involved are not that big in the end

also runing ambroz alt pully

you will enjoy the 19% in that cool air ale
 

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i hope so!! its getting colder by the day here can't wait to get the new ic and the pulley.. if only i could get a reach of fisher so he can send me his paypal details!!

Al, whats your maxrev at with the 20% ? Is the kmt alt. pulley same as oem? does the alt. start sufering above 17% pulleys? basically is there anythign i should think of changing whilst fittign the 19% ?
 

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Just read through this and the 2 charts, still confused. Lohen sent me what appears to be the 15% belt for my 17% (realised this as its slipped off and i got stranded!!!!)

So rather than fucking about and waiting for them to send me stuff, i was going to go to motorfactors but need to know exactly what im asking for?
 
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