Legal wheel width advice for eventual turbo NB

[yellow4]

Hey all, newbie to the forum and the MX-5 world in general. *wave*

I'm looking at new wheels and need some advice on what width to go for while keeping them legal in NSW. A turbo will be on the radar eventually (nothing crazy, maybe 150 kw ballpark), so I'd like to buy wheels I can still use with the extra power down the track.

So from what I understand on an NB8A I can go up to 15x7 without certification and 15x7.5 with. Will 15x7 with grippy 205s be okay for regular road use on a moderately boosted NB? Or should I go for 7.5s and wider tyres?

Cheers,
Yellow

[smy0003]

Welcome!
If you haven't found it yet, have a read of this:
http://www.rms.nsw.gov.au/documents/roa ... 9-rev4.pdf

The upshot is no more than than an inch bigger than standard, however, that standard may refer to the largest wheel fitted to an NB, which was the 7" wide racing hart form the SE turbo.
Track is the trickier bit, because the max increase is 25mm, so the wheels can poke out no more than 12.5mm from standard. That's not a lot of wiggle room. So you could theoretically run an 8" wide wheel, but it would have to have an offset of +40 to be legal. Buddy club make an 8" wide wheel in +38(?) offset, which is legal enough.

[NitroDann]

15x7 will be absolutely fine, and engineerable.

[rascal]

Track is the trickier bit, because the max increase is 25mm, so the wheels can poke out no more than 12.5mm from standard. That's not a lot of wiggle room. So you could theoretically run an 8" wide wheel, but it would have to have an offset of +40 to be legal. Buddy club make an 8" wide wheel in +38(?) offset, which is legal enough.

Track is unaffected by wheel width, if factory offset is +40 then you can legally run +28 offset rims, in a 15x8 (ie 1" wider than widest factory (Racing Hart 7s) and 12.5mm extra offset.)
Enkei now make an RPF1 in 15x8 +28 offset, which may or may not be coincidence..

[smy0003]

Track is affected by wheel width. Offset is taken from rim centre, so width increases potrude both sides of the offset point.
An 8" 0 offset wheel (potrudes 4" inboard and 4" outbaord) will poke out a whole inch more than an 6" 0 offset wheel (potrudes 3" inboard and 3" outboard).

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[rascal]

Track is affected by wheel width. Offset is taken from rim centre, so width increases potrude both sides of the offset point.
An 8" 0 offset wheel (potrudes 4" inboard and 4" outbaord) will poke out a whole inch more than an 6" 0 offset wheel (potrudes 3" inboard and 3" outboard).

No its not..

Track is the measurement from rim centreline to rim centreline. The width of the rim makes no difference to the measurement. Offset is the only thing that changes track. (assuming you aren't modifying suspension arms, pickup points, etc)

Your 6" & 8" examples above would both have exactly the same track measurement, as the offset hasnt changed.
The distance from both outside edge of the wheels would change, but this isnt how track is measured.

The legal requirement to only vary track by 25mm is 100% related to rim offset, and 0% related to rim width.

[NitroDann]

Not according to the legislation.

[rascal]

Track is the measurement from rim centreline to rim centreline. The width of the rim makes no difference to the measurement. Offset is the only thing that changes track. (assuming you aren't modifying suspension arms, pickup points, etc)

eg

[smy0003]

Hmmmm I always assumed vicroads at least measured it from the outside rim lip.
Sorry rascal, didn't mean to get my wired crossed like that.

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[yellow4]

Thanks all. Wasn't sure if the SE wheels would count in the legislation. But I'm a minimalist, so 15x7 it is!

[rascal]

all good...

Just its a common thing that pops up on forums, and causes confusion, so just wanted to make sure it was corrected before it runs off and becomes gospel as forums are prone to do...
Below is screengrab of pg3 from the Vicroads literature VSI26 which also shows they measure from the tyres centre..