Strength of leather at different spi

[DrewdkAUS]

I hope that I can explain this well enough.

I am looking into new stitching/pricking irons and need a bit of help. I will be using a wide range of leathers and thicknesses and a making wide range of articles

As far as I know, different leathers have different strengths within the fibres,the smaller the hole the stronger the whole seam will be.  The smaller the spi the weaker the seam will be. 

Is there a point where the spi starts to weaken the leather or overall strength of the article.

E.g.. If I were to sew 2 pieces of 2mm veg tan at 10spi as opposed to 8 spi using the same method(pricking iron with awl or stitching iron) would it make the overall strength weaker?

Sorry if I didn't explain myself real well. 

[billybopp]

Yes.

[KingsCountyLeather]

While billybopp is right, you were probably looking for a bit more...

You've explained it very well yourself in the question. Making more holes closer to each other will weaken the article.

[DrewdkAUS]

I know it's all variable but is there a rule of thumb as to what spi would give you the strongest stitch.

E.g. 4mm veg tan would stay the strongest at 5spi. 

3mm veg tan at 7spi.

Something like that?

I realise not every article will need the strength and that looks play a large part but I want to buy one set of irons that will last me most of my leather working days.

[Mattsbagger]

I'd say you will need 2 sets just to be safe. I've found I either use my 8spi or my 6spi. And I either use 1mm or .8mm Tiger thread. I have 10spi but don't use them. Looks to close for me.