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Changing pulley size - what belt?
GreigM - 1/12/14 at 01:04 PM

Can anyone help me work out what belt size I need when changing an alternator pulley? My alternator recently gave up and have had trouble getting a replacement with the correct pulley, so am left with an option of an increased pulley size.

Current pulley is 67mm, new one will be 80mm. Current belt is 1305mm, so is there a simple way of figuring out how much longer I'd need the new belt?

There are assorted calculators online which work on the basis of 2 pulleys, and they all suggest around 20mm extra, but obviously I have the belt following a path, so it probably isn't that simple....or is it?

gremlin1234 - 1/12/14 at 01:13 PM

cant you transfer the pully from the old to the new?

loggyboy - 1/12/14 at 01:32 PM

without knowing the position of the pulleys in relation to each other (one being changed and 2 either side, inc all their diameters, are all thats needed) its impossible to be 100% accurate.
You can change then run a piece of string around them all and measure its length.

[Edited on 1-12-14 by loggyboy]

ragindave - 1/12/14 at 01:43 PM

I would simply cut the old belt run it round the pulleys and then measure the short fall

loggyboy - 1/12/14 at 02:01 PM

Is there not enough adjustment is whatever adjuster you have?

Trollyjack - 1/12/14 at 02:35 PM

it depends upon what amount of contact it has if its just two pulleys you have your belt needs to be 19.9mm longer so yes 20 mm is about right.
if three pulleys are involved you need to know deg of belt contact with the larger pulley. on 2 it would be around 180 on 3 it would be less

GreigM - 1/12/14 at 02:53 PM

Unfirtunately can't transfer the pulley from old to new - tried that and the offset is wrong, so the belt doesn't line up. I do have the old belt (had to cut it off) so can use that to measure shortfall, think thats probably the best way. Definitely not enough adjustment in the adjuster, it doesn't move a lot at all!

owelly - 1/12/14 at 03:13 PM

As stated, it's not possible to work it out on paper unless you know the circumference of each pulley the belt runs on, and how much of the new pulley has altered.
As Ragindave suggests, I have a long lump of cut belt that I use for sizing. Pass it round the pulleys and mark where it meets. Lay it out and measure it.

02GF74 - 1/12/14 at 08:11 PM

(circumference of new - circumference of old ) x overlap on the pulley plus a little bit extra due to angle of belt path changing.

is it v type or toothed? Be quicker to use a piece of string to measure the path then go to halford to buy a couple to see which fits best, keep the receipt and take back the ones that don't fit ..... t'is how I did it. The v type belt have the length encoded as part of the part number.

The Black Flash - 2/12/14 at 09:13 PM

quote:
Originally posted by 02GF74
(circumference of new - circumference of old ) x overlap on the pulley plus a little bit extra due to angle of belt path changing.

is it v type or toothed? Be quicker to use a piece of string to measure the path then go to halford to buy a couple to see which fits best, keep the receipt and take back the ones that don't fit ..... t'is how I did it. The v type belt have the length encoded as part of the part number.


So do the multi-rib types - I forget exactly how it works, but there's a standard for belt numbering which includes the length and number of ribs etc. So bit of string, then a google, and pick the size of belt which is nearest, all the manufacturers use the same numbering scheme.

DW100 - 2/12/14 at 11:21 PM

Yes poly V belts use a numbering system that is the number of ribs and the length in mm. So part no will be something like 6PK1092 (6 ribs 1092mm circumference)