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Building diff and rims to achieve track width for engineering

LH LX UC A9X

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#1 Torana_Boy

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Posted 02 November 2018 - 11:12 AM

I’m looking to get my Uc Torana engineered with a VN Borg Warner.
I want to have the diff shortened 30mm (15mm each side to retain shock mount) so later I can get custom 15” GTS rims made and get max dish.

What i’m Struggling to understand is what offset and back spacing will I require to maintain wheel track (my engineer is allowing me to use LH - UC or A9X rear track).

I’m going to try placing my 14” gts rims on a stock UC diff (studs removed) and measure from Center to center of each rim, repeat this for stock 15x7 gts rims to compare then subtract 30mm from that measurement and see if it falls within the stock and max track width (stock wheel track is 25mm less than max length).

Is there a easier way I can do this?

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#2 Bigfella237

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Posted 02 November 2018 - 11:52 AM

Your engineer should know this already but, if using an axle from a different make/model, the Torana specs are no longer relevant.

 

From NCOP11-Secton-LS-Tyres-Rims-Suspension-Steering-Wheel-Track (download here):

NCOP11-Secton-LS-Tyres-Rims-Suspension-S

 

So you need to find out the rear track measurement from the vehicle your donor diff came from, then subtract however much it was shortened by, then you're allowed to go up 25mm from there.

 

The whole point of this rule is to maintain the factory original weight distribution across the wheel bearing & axle so you're not over-stressing them, the wider the wheel the more leverage it has.

 

You can of course reverse the calculations and make the diff width suit the Torana specs, which will maintain the original handling characteristics (specifically the tendency of the vehicle toward over or under steer), but the rules don't specify that, and it sounds like you have some limitations on how much you can remove?

 



#3 Bigfella237

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Posted 02 November 2018 - 12:01 PM

To simplify the above (and how the NCOP should have been worded), find out the offset of a VN Commodore rim then add (or subtract depending on whether it's a positive or negative offset) 12.5mm and that's your maximum offset you can use.

 

But as always, get your engineer's approval before doing anything!


Edited by Bigfella237, 02 November 2018 - 12:03 PM.





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