Road Legal Wheel Sizing

[Bazooka]

I was talking to me Engineer today about wheel size for my RA28.
When I told him that I plan to run 17" wheels he told me that we are only legally allowed to run a wheel 2" larger diameter then factory spec. This is something I have never heard of, being restricted to wheel diameter? I know about the tyre diameter and wheel/tyre width restrictions but nothing about wheel diameter.
I cannot find any reference to this in my searching or in the guidelines. I will ask my engineer to elaborate on this more when I speak to him next and maybe point me in the direction of the text stating this.
In the mean time has anyone ever head of this rule and been struck by it through engineering?

[fixeruperer]

yep, he is correct (as youd think he would be)

i dont know why there is the rule, there just is. you can only got 2" taller and so much wider. it used to be because of wheel track. but you can get wheels that dont add track and are wider/taller and they can still knock it back.

[masterofsinanju]

Just so everyone is playing with the same information

http://www.rta.nsw.gov.au/registrati...vsi09_rev4.pdf

So yeah, nothing about the wheel diameter explicitly, but limits on the wheel/tyre diameter.

In order to keep within the tyre limits for your 28" a 17" rim would have to have stupid low profile if you went wide tyres wouldn't it?

http://tire-size-conversion.com/tire-size-calculator/

Have a play around on there. According to information here on the forums, http://www.toymods.org.au/forums/tec...e-fitment.html your 28 would have been stock with 165SR14 - equating to 165/82/14, or 626.2mm rolling diameter.

so you are likely looking at 50 or lower profile tyres.

I can't see why you can't do it and would love to know what your gingerbeer has to say. But it's his signature you need, so I wouldn't go riling him up for the sake of going 17" instead of 15" or 16".

[oldcorollas]

50 profile is not that small... 215/45/17 is a very common size (Bridgestone S001's about $230 Kumhos sub $200)
16" is far less common in decent tyres?

NCOP
http://www.infrastructure.gov.au/roa..._2011%20v3.pdf

max width is 1.3x widest stock tyre? (page 22/23)
does give +- overall diameter, which limits tyre choice...

have another chat.
maybe other states have similar rule, or NSW used to?

[patrickleslie]

I dont have this on paper but based on a phonecall i made to Vehicle Standards last week, SA has a change in rolling diameter (I forget the exact spec), a maximum +-26mm track change, and the tread must be under the guard. You can go as big and as wide as you want, as long as the rims/tyres satisfy those three rules.

So this could theoretically mean that 18x12 is legal on an MA61 for example lol

[Daddio]

Keep within 10% of OEM rolling diamter and rims can be as big a diameter as the lowest profile tyres you can find to suit. Rim width is not limited BUT track increase is, so a lot of rim width as to go inwards towards the strut/shock which foils big widths....and incase you thinking of a narrowed back end with tubs, thats also a no-no.

[Grahama]

The 2" rule was proposed in NSW but a lot of car people (mainly organised 4x4 groups) rallied against it and it did not get implemented or was quickly removed.

There are limits to width - rule of thumb is that +1" in width is fine, +1.5" can be passed by an engineer. It gets difficult to understand if you're going over 8" wide, or maybe it only confuses me?

Do make sure your new tyre has the same diameter (or within 15mm) of the original, as per the tyre placard.
(essentially, you can fit 20" rims as long as you can find rubber bands for tyres that keep the same diameter)

[ToyTA22]

As Grahama mentioned, you should keep rolling tyre diameter close to standard. Otherwise you have speedo issuse to correct. Remember in NSW anyone has to be able to use your car in an ememrgency. I was informed of this when I wanted to fit brake bias pedal box to my former rally car. In otherwords any one should be able to use speedo as is.

[Bazooka]

I completely understand the rolling diameter and track width restrictions, this is not the problem. I can use a 17" wheel and tyre package that fits into this bracket easy. The problem is with the so called 2" wheel diameter rule, of which I have yet to find any evidence that it exists. I emailed my engineer about it but as yet no reply.
I believe it may have something to do with wheel loading that being a larger diameter wheel will have less load capability than a smaller wheel. However the load rating on each 17" wheel I intend to use is 680kg giving me more than enough envelope with the RA28. There seems to be very little information out there regarding wheel loading as it all concentrates on the capabilities of the tyre.

[masterofsinanju]

I agree that there is no explicit "2" rule", but the point remains that if your engineer won't sign off, then you have a pretty much de facto rule to comply with. If you can get your current guy to show you where the rule is and he is not willing to approve 17's, find another engineer maybe?

[Bazooka]

My engineer is a pretty decent guy and is very happy to help me work through the rules. He wont break them but has been very helpful with advise and suggestions on how to keep my car legal in all aspects. Although while on the phone to him he was not able to find the 2" rule written anywhere it seems the main reason behind it is axle loading. The larger the wheel the more load on the axle.
As I have not been able to find any information stating that any of my cars components ran 15" wheels in any factory spec (RN25 rear end and RT132 front axles) the way he suggested we tackle this was to find vehicles with the same (or similar) axles and weight that ran 15's. Basically if another car of similar design ran 15's then axle loading on this is not a problem. Also because there is such a big change to the sidewall height of the tyre he stated a lane change test would also be required to pass.

[dnegative]

Although while on the phone to him he was not able to find the 2" rule written anywhere it seems the main reason behind it is axle loading. The larger the wheel the more load on the axle

I don't understand, there is no 'rule' and the whole idea of engineering is to get someone qualified to determine if whatever your doing is safe.
It should be a straight up yes/no answer, sounds like he has no idea or is just lazy with the whole "if someone else has done it, must be safe" idea.

Engineers seem to be hit and miss these days, when I went and asked 3 or 4 of them a few questions I got a different and somewhat conflicting answers;
Some would say engine swap = bigger brakes while others would say if the weight doesnt change then no need and others would say if the performance is +10% then you need bigger brakes.
Some wouldn't touch any sort of custom brake setup, others were happy to take it for a spin around the block and sign it off etc
Some hated Japanese cars and some liked them.

[Bazooka]

Unfortunately it's never a simple yes or no answer. The problem I find is the ruling itself is never clear, it comes down to how the individual interprets this. This is why there is so much discrepancy between different engineers. It seems it is more of a fact that my engineer has heard of this rule in conversation rather than in writing so must assume that it exists. After speaking to him and going through the above motions he is now happy to give me the go ahead for 17's So if the ruling exists or not I have covered all bases.

[dnegative]

It should be, that's the whole idea of being a RMS certified engineer.
Customer A wants to do X, you take a look, run the numbers, apply your knowledge, experience and determine if its safe for road use.
That's why people are paying you a lot of money, I would be expecting sound answers, logic and some sort of backed up basis.