31 replies to this topic

[Dr Terry]

That's why they general specify a maximum track dimension, not a maximum wheel width. It is the large offset which causes dramas, not just wide wheel rims.

 

Dr Terry

[Dr Terry]

just back on the 9" diff for a sec...the XW,XY GT HO's had a different wheel bearing to the "std run of the mill" 9" a HD version

That's is not quite correct, they were different, but they were not HD, they had even LESS load bearing capability, than the stock 9-inch or even a 6-cyl BW.

 

The HO's 31-spline axle shaft was thicker, so the axle bearing was the same width, the same outside diameter, but a larger inside diameter, than standard, so the balls ( their grooves) were slightly smaller.

 

Dr Terry

[Bigfella237]

Can you recomend anyone I should talk too?

 

Sorry, do you mean talk to about whether your rims are legal?

 

If a LX Torana has 18x8 with a 120mm backspace/108mm frontspace, on the front and 18x10 with a 100mm backspace/178mm Frontspace.

~
And has anyone ever managed to get this big rim combo engineered?

 

If you look at the offset in percentages it's a little easier...

 

Standard A9X Rim: 14" x 6" (152.4mm)
Standard Offset: +0.25" (+6.35mm) which gives us a

Standard Backspace of 3.75" (incl ½" flange) and a

Standard Frontspace of 3.25" (incl ½" flange) so the

Standard Backspace:Frontspace Ratio of an A9X rim = 55:45

 

The most you can increase wheel track is 25mm (roughly 1") over the A9X spec, which is 12.5mm (roughly ½" per side) so...

Maximum Offset (½" over std): -0.25" (-6.35mm)

Maximum Backspace 3.25" (incl ½" flange)

Maximum Frontspace 3.75" (incl ½" flange)

Maximum Backspace:Frontspace Ratio = 45:55

 

So assuming your hub-to-hub measurements are identical to an A9X then any rim that falls with a range of 55% to 45% backspace should be legal.

 

Your front wheels have a ratio of 120:108 or 53% backspace

Your rear wheels have a ratio of 100:178 or 36% backspace

 

So, there's a good chance your front wheels may be considered legal, but I very much doubt your rear wheels would be?

 

As I mentioned earlier there are several variables, which you would need to physically measure on your car to confirm. It's been mentioned that Hoppers brakes add slightly to the front hub-to-hub measurement, which reduces the amount of wiggle-room you have in the wheel offset, but you're close to the standard rim offset with the front anyway so you have a fair bit of room to play with.

 

Now the 9" diff throws you another curve-ball, the NCOP says you need to base your track measurements on the vehicle that the diff came out of the ford factory fitted to, less the amount the diff has been shortened, plus the 25mm allowance.

 

However I'm not sure what they'd use if the diff was completely custom built from scratch and has never been fitted to another vehicle? I'm guessing we just revert to the A9X track measurements?

[Rockoz]

Just a thought.

If you want wider wheels and track and different offset then you would need engineers approval.

If you want a wider rear track, wouldnt it be easier for an engineering approval if the diff was wider anyway?

You could get the allowable extra width for the new diff, and only need engineering for the diff conversion.

Would the shop doing the diff conversion give an engineering certificate when they do the work?

 

For example

 

Standard LX track 54 inches

Standard HQ track 60.2 inches

Thats 3 inches per side.

About the width of flares.

Edited by Rockoz, 03 March 2015 - 11:07 PM.

[Bigfella237]

It's a good thought Rob but wheel-bearing / axle load isn't the only consideration when dealing with rim-offset / wheel-track.

 

Adding track to the rear (especially if it makes the rear track wider than the front) will induce oversteer, making the tail prone to "breaking-out" halfway through corners, fun if you're a good catch but it gets hard on the nerves after a while!

 

Even if you can 'technically' get away with a much wider track measurement by using a wider diff from another model, an engineer should take into account the overall handling of the vehicle as a whole. Generally speaking, understeer (or "push" to our American cousins) is considered safer for Joe public than is oversteer, which is why most cars are designed in the factory with the front track wider than the rear.

 

There are other things for an engineer to consider with a diff conversion too, like if the physical size of the housing or a poorly designed jig changes the location of the control arm pivot points (and therefore the angles of the control arms) it will add or reduce "anti-squat". Too much anti-squat will make the tail of the car break-out as you're braking into a corner, not enough anti-squat will make it break loose when accelerating out of a corner.

[Rockoz]

What I was more looking at was say an A9X replica.

Use the wider diff to achieve the same track without the huge offset.

I had thought s along these lines when I had my Torana.

Pretty sure you would only have to move the outboard mounting point.

There would be no load issues on the bearings.

Warping due to welding would be minimal