Just playing around with wheel offsets in preparation for getting a 9" made for the GTS.

The only HK wheel track measurements I can find are:

14x5 rim - 57.12".

D70 tyre (and hence 14x6" rim) - 58.3".

Can anyone confirm that these are correct, and if they are is the 58.3" track for D70/6" with or without wheel spacers?

I suspect it is WITH wheel spacers (as in car with radius rods), not for say a V8 auto HK with D70 tyres. If so how thick are the wheel spacers? 1/8"?

This would also mean that a HK Rally rim has most of the whole extra 1" of width outwards compared to a 5" rim? Close to the same backspace between 5" and 6" rims?


All I'm trying to do is figure out what width to make the 9" with two goals in mind:

1. Staying close to the original PUBLISHED wheel track (for Engineering purposes). Although the Rodtech front end increases the front track by about 20mm from memory.
2. Having the 15x6 (or possibly 15x7) Pontiac Rally wheels fit with something like 215/65/15 or 205/65/15 tyres (https://www.antiquetyres.com.au/node/77).

Trying to get hold of an original HK 6" rim and fit an old 205/70/14 to it (205/70/14 almost exactly the same size as 205/65/15) and fit that on the banjo in the car now to see what I need to do diff wise, taking into account the backspace variation between the HK rim and the Pontiac rim. Ideally I'd get the diff width to also work with the 15x7 Pontiac rims, so I can run wider rear rubber later if I choose, but I guess I'd have to not compromise the rear track too much as it'll normally have the same 15x6" rims all around. The other variable is that track increase with the Rodtech front end, probably best to make the track the same front and rear I guess when the same rims and tyres are fitted front and rear?

Edit - just found the published HK tracks (from HK Engineering Technical Specifications):

HK - 57.12" front and rear.
HK with Rally Rims - 58.12" front and rear.
HK with radius rods and rally rims - 58.40" rear only (GMH must have rounded this up as AFAIK the wheel spacers add 0.25" to the track (1/8 per side), so it should be 58.37mm. HT-HG is 58.38" as published, and these AFAIK have the extra 1/8" built into the rims so no spacers needed).

If I'm doing my maths right this means the HK 5" rim and the HK rally (6") rim have the same backspace. I can find the HK 6" rim offset quoted as 1.19" (I assume this means positive 1.19" or +30.2mm). Which I guess means the HK 5" rim is 1.69" offset (+42.9mm)? HT 6" rims lists as 1.06" offset, or just over 0.125" less than the HK 6" rim.

Also found the HT-HG data:

14x5" - 1.19" offset. Which given the HT's standard front and rear wheel track of 58.12" with 5" rims, and the fact that 58.12" is exactly 1" wider track than HK with 5" rims means that the HK 5" rim must be 1/2" more offset than the HT 5" rim, which correlates what I figured out before, ie HK 5" rim is 1.69" offset.

SO IN SUMMARY:

HK 5" wheel track - 57.12". 5" wheel offset 1.69".
HK 6" wheel track - 58.12". 6" wheel offset 1.19" (same backspace as 5" rim).
HK 6" rear wheel track with radius rods - 58.40" (published GMH, should really be 58.37" or 58.38" I reckon).

HT-G 5" wheel track - 58.12". 5" wheel offset 1.19" (exactly 1/2" less than HK).
HT-HG 6" wheel track - 58.38mm. 6" wheel offset 1.06" (0.125" less than HK, and the same as HK rear with 6" wheels and radius rods)(different backspace to HT-HG 5" rim)

Edited by user Wednesday, 1 August 2018 4:29:14 PM(UTC)  | Reason: Not specified

Hi Byron Don't know if this helps and you probably already know. If you are going to use a 10 bolt HK T G housing and good axles the bearing to flange measurement is approx 22 to 25mm less then std axles and just use HQ backing plates to step the drum the same amount.

Disclaimer: For most here this won't be of great interest, but for those doing a Rodtech front end in a HK-HG this will be of great interest.


Talked to Rodtech again, and been working on wheel offsets to suit. Here are some facts for those interested.

STD HK-HG hub to hub or disc to disc (front and rear are designed to be the same) is 1535mm according to Rodtech. My calcs. tell me that for a published track of 58.12" for a HK with 6" rims or HT with 5" rims, which is 1476.248mm. If you add the wheel offsets to this (1.19" each side for both HK 6" or HT 5") you get 1536.7mm. Pretty close.


The stock Rodtech front end with XF Falcon rotors and VS Commodore calipers is 1510mm so 26.7mm narrower than stock.
The optional BA-BF Falcon rotors with AUII calipers is 1525mm wide so 11.7mm narrower than stock.

So there is some minor leeway to play with, for me I will have 5.85mm per wheel extra available (11.7mm total) to me that I thought I didn't have.

So my front track calculations for the 15x6 Pontiac Rally rims with 3.875" backspace (0.375" or 9.525mm offset) are: 1525 - (2 x 9.525) = 1505.95mm. This is 29.7mm greater track than a HK with 6" rims (58.12", 1476.248mm track), so hopefully the Engineer will allow me to use the HT with 6" wheel track (58.38", 1482.852mm), which gives me a track increase over the HT of 23.098mm which is less than the allowed 25mm. This also means my front wheels will sit basically 11.5mm further outwards than a HT 6" wheel does for the same front camber setting. I reckon this will be OK as the front tyres are going to be 205/65/15 which are all but the same size as an ER70H14 which was an optional tyre on a HT with 6" rims.

Rear track is easier as I can narrow the diff. If I'm running the 15x6 rims on the back, I just make the diff 11.7mm narrower than standard and the track remains the same as the front. What I'm tempted to do though is use a pair of Pontiac 15x7 Rallye rims on the back with 215/65/15 tyres, these have 4.25" backspace (0.25", 6.35mm offset). So to keep the 1505.95mm track to match the front, if I use the standard diff width of 1536.7 with the 6.35mm offset each side I am at 1524mm which is 18.05mm too wide, so the diff needs to be shorter by 18.05mm, so basically 9mm a side. Using a comparison calculator tells me that this 15x7 wheel compared to a HT 6" wheel will sit 1.7mm closer to the body and 23.7mm closer to the wheel arch - a little bit too much for comfort for me. If I shorten the diff by 20mm per side, compared to the HT 6" rim the 15x7 rim sits basically exactly 12.7mm (1/2") either side of the HT 6" rim. I'm pretty sure a HT-HG 6" rimmed coupe can take another 1/2" of rim closer to the wheel arch. Compared to a HK 5" or 6" rim the 15x7 rim with 20mm less diff per side sits only 8.7mm closer to the inside body - looking at my car with 5" rims on it now there is heaps of room. With the diff now 20mm per side shorter the 15x6 Pontiac rim with 3.875" backspace sits within 1mm of where a HK 6" rim sits.
With the 20mm per side narrower diff the rear wheel tracks will be:

15x6 Pontiac rim (3.875" backspace): 1477.65mm (compared to the front track with the Pontiac 6" rim of 1505.95mm) - sits right between the HT 6" and HT 6" tracks of 1476.25 and 1482.85mm so still legal if I wanted to do this at any stage but not optimal. I do have the option of using 0.25" (1/8" per side) of wheel spacer if I need to in this instance.

15x7 Pontiac rim (4.25" backspace): 1484mm, just above the original rear track of HT 6". This works out to be a narrower track in the rear by 21.95mm but the tyres are wider, and if the calcs above are clear in your head the outside edge of the front 15x6 rims sit just about level with the outside edges of the rear 15x7 rims, just the rears sit in further on the inside. As detailed above the 15x6 on the Rodtech front end stick out 11.5mm further than a HT 6" rim, and the 15x7 on the 40mm total narrower diff stick out 12.7mm further than a HT 6" rim does.

Hopfully this makes sense if you are attempting something similar!

Edited by user Friday, 3 August 2018 7:36:26 AM(UTC)  | Reason: spelling error

That all makes sense. I'm amazed how many errors crop up in published literature.

I suppose they have so many separate departments & some are more adept at arithmetic than others.

Dr Terry