Oversized Tyres and Vehicle Roadworthiness

15mm rule only applies to passenger cars, not to 4WD type vehcles. (look a little further down the regs...)

Surely this can't be so .............. ??

So, DIO, what you are saying is that :
"ALL THE LAWS AND RULES APPLY EQUALLY TO EVERY-ONE ?"
By gosh ....... what a unique outlook !!!! Surely this can't be so .......

Most people I know [think that I'm crazy !!!] appear to be able to 'select' those laws that apply to them and dismiss those that don't .... unless those same laws are broken by some-one else with "differing selectivity."

Presumably the 'selective people' then wouldn't really mind if murder, rape, pillaging, theft and all other forms of illegality and bastardry could be selected by others as 'not really applying to them?'
Surely this can't be so .......

I remember once hearing an RAN Officer commmenting to another, when Church Services were about to commence on the foredeck of one of our Gray Funnel Line vessels. He had just noticed that a lot of us lower deck personnel were also lined up. He was non-plussed that we were using the same God as the Officers were!!!
Surely this can't be so .......

And yes, occassionally I do 'bend the rules/break the laws' myself so it's not a case of being 'holier than thou' .....
I just don't blame/point the finger at some-one else/cry at the results .................

as you say, DIO
it really is a matter of 'choose the behaviour - accept the consequences.'

Surely this can't be so ....... ???

Righto, chaps ...................... go for it !!

Such a pity the original poster didn't go straight for the National Code Of Practice document that has been discussed to death in every corner of the 4WDing world over the last year or so.

Search before reinventing a (square) wheel.

NCOP is now incorporated into every States local roadworthy information available on the internet. There really should have been no reason to come up with wrong answers to what are straightforward questions. One phonecall could have sorted what is/isn't allowed by the state authorities.

2" dia above the largest OE tyre size, 2" track width increase on solid axle vehicles, 1" on IFS+/-IRS, and nothing on McPherson strut. Max rim width of 8", max tyre width of 1.5 x OE tyre width.

Gary,

FYI I got the information straight from the Vicroads vehicle safety department, the guy at Vic Roads confirmed what is documented on the vicroads website. Do yourself a favour and contact vicroads. Just to be safe check the doco yourself on this link

http://www.vicroads.vic.gov.au/NR/rdonlyres/CC48FC10-4D57-4F04-82E1-3BD72EF88340/0/VSI8.pdf

Gary,

Maybe you should do some checking?

Although it is far more likely that you are indeed correct and the gentlemen at Vicroads, who gets paid to know is in fact incorrect.

"NCOP is now incorporated into every States local roadworthy information available on the internet" . .

Funny no mention on Vicroads website?? If it is show me the link.

Tell me when did Vicroads adopt this policy, and, if they did maybe you should inform Vicroads vehicle safety department, who seem to know nothing of it.

Finally it would have been easier to do a search on the "NCOP" if I new it existed, of course that would have been a waste of time anyway because it doesn't apply!

I wish I was as all knowing, all seeing as your goodself wouldn't it be wonderful. I can only dream.

Batman

They are interesting arguments. To put it in persective, 15mm is a 1.8% increase in tyre diameter for a large 4wd tyre, but its a whole lot more for a small vehicle.

The most common 4wd upsize we see is to move from 265/70R16 to 265/75R16 to allow for the fitting of tyres with a higher load rating, and aggressive tread. Its a 25mm or 3.2% increase. This usually corrects the speedo's inaccuracy. The gearing difference is not usually noticeable. Centre of gravity change is negligible. The tyre/wheel weight usually increases from 31kgs to 35kgs each and the tyre's rolling resistance increases, which will cost you at the bowser.

The positives from the change are increased traction, and the tyres can carry the vehicle's GVM with ease - reduced likelihood of puncture, and blowout on the highway. And wet weather grip is usually better. IMO, its a sensible choice.

I'd prefer to replace the current ridiculous "one size fits all vehicles" rule with a 3.5% limit to allow for sensible upsizing.

Now the braking argument???
Same size brakes; same vehicle mass; tyres rotate 3% slower.
Why would braking be affected???

Hi Phil

It makes a huge difference to the braking, a couple of ways to explain it. It takes more fuel to push these things so it also takes more energy to stop them. It is like a flywheel, the bigger and heavier they are the longer they will spin but make it harder to slow down, that's why flywheels on the back of your motor is a very precise art. I run 35" on my Cruiser (legal, engineered ) and the biggest thing I noticed was the brakes. If you could increase the rim and rotor diameter and move the callipers out it would overcome the problem.

I hope this makes sense as it blew me away how much different it made to my 4by.

Have fun

Craig

Years ago I had the good fortune to try under test conditions three different sets of tyres on a HZJ75 troopy.
We had a rolling road setup as well as a skidpan and test track to play with.
The troopy was initially fitted with near new 7.50 x 16 Roadgrippers (standard tyres), then a wider tyre of slightly smaller diameter and finally a much taller tyre (approx a 35" as it was the biggest that could be fitted to a troopy with 2" suspension lift.
Braking, accelerating, power and handling were evaluated both on the rolling road and the track by one very experienced professional driver using all sorts of fancy electronics to record the results. I then had a drive with each set of tyres afterthe testing was done, my seat of the pants impression was the same as that borne out by the tester and the equipment.
As expected the wider/shorter tyres gave the best acceleration and handling, the taller tyre totally destroyed any chance of 'acceleration' as well as making the vehicle fail repeatedly a simple braking test. They also did nothing for the handling as the troopy had a distinct top heavy feel.
The standard tyre size worked as Toyota designed it, the best all round compromise for that particular vehicle for most users.
We all modify our vehicles to suit our own personal uses but we change some things at our own risk.
As long as we are aware of any problem caused by our changes that is ok but what about the second and third buyer who is not aware of the degradation of the vehicle characteristics?

Craig and Traveller,
Nice explanations. So I guess the bottom line is that its the extra tyre weight rather than diameter that affects the braking. I don't know the weight of the 35's but I'd expect that most are a lot heavier than the weight of the Grand Treks or Bridgestones that come on our vehicles. The vehicle manufacturers love lightweight tyres - it improves the fuel consumption as well as the braking.

It woud be nice to quantify the difference with stopping differences. Maybe one of the mags might get interested in the issue.

Hi Phil

It is mainly weight but do not discard the extra diameter, if you have two wheels one 1mtr and one 2mtr both doing the same revolutions and measured the speed on the outside you would find it is about 4 to 5 times faster. Now if you remember the brakes are in the same place on both and you need more braking effort in the first instance you will find the first instance is way bigger than it was before. I know 100kph is that speed no matter what size tyres you have on, but that first instance is definitely longer.

If you look at race cars from go carts to F1, they do anything they can to make the brakes as big if not bigger than the tyres, their is a good reason for this.

Have fun

Craig

Hi Phil,
In regards to braking issue, mate ,you said so yourself,same vehicle weight same brakes,BUT bigger wheels/tyres turn slower at the same speed as smaller wheels and if you asked anybody about the affect of acceleration of a 4by after they have put biggie's on and they will say that it has dropped noticeably,same applies for slowing down!
Stopping distance will probably stay the same but the brakes will work a lot harder
because of the leverage affect

Norm C, The only problem you have with your Hilux with your tyres (even if the National Standard is ever adopted) load ratings on 31-10.5 tyres are under the Toyota specifications. The only tyres around that size that complies with the load rating is 16inch tyres some are smaller and some are larger like a 265/75/16. I'm looking at 265/75/16, braking wise I might change the disks to vented/grooved type to add to the performance of the brakes.
For the insurance company to void a claim then have to prove a modification caused or contributed to a crash, haven't noticed any real braking problems but if a blow-out occurred and the tyres didn't match the load rating I think that would be a big problem.

You are right hound, but on the new Hilux there is conflicting information. The owners manual says load rating for tyres is 112. The tyre placard on the door frame says the sum of the load carrying capacity of the tyres on each axle is to be not less than 1,600 KG (rear - the highest).

The range of 112 tyres available to fit the Hilux is quite small, with none in the 31 inch size. My 31 inch tyres are load rated 109, which is 1030 KG per tyre (112 load rating is 1120 KG per tyre). 1030 times 2 (two tyres on the axle) is 2060 KG, which is well over the 1600 KG minimum as per the tyre placard.

Before I bought the 31s, I rang Qld transport. I'll admit, I did not explain the full circumstance. I just asked where I (or they) should look to get the tyre load rating for my vehicle. The answer was ' it will be on the tyre information placard which will be on the frame of the drivers door. Good enough for me. On that basis, I believe my tyres are legal. It was then just a matter of am I prepared to use tyres that are rated to carry 90 KG less than what the owners manual suggests, though well above the tyre placard requirment. After due consideration, the answer was yes, so they were fitted.

Funnily enough, my tyres are LT construction. Some of the few 112 rated tyres that I could have fitted (smaller than 31 inch) are passenger construction.

One of the things I am meticulous about is increasing tyre pressure, particularly on the rear when fully loaded. I go up to 46 PSI on bitumen when fully for a trip.

I'm not claiming that my solution is perfect (what is?), but I believe it is both legal and safe. I've done about 35,000 KM on the tyres so far including GRR, Mitchell Plateau, Cape Leveque and Bungle Bungles. They have performed well, are wearing well and I've had no punctures.

It's even an offence for a tyre service fit illegal tyres according to our tyre services,
apparently in the event of an accident they leave themselves open to prosecution or litigation.

Glad to see someone with sense. It is illegal for to fit a tyre of a lower load limit than the tyre placard states. Regardless of the 15mm rule on oversize. Some dealers do, either don't know the law or don't care about their customers, or just to make a sale do as customer wants. Either way left themselves open to litigation.
Cheers
Charlie

NormC, It certainly is a grey area especially with the hilux as the hilux's standards tyres seem to have a higher load rating than the place card sticker. The regs in Victoria do say the load rating only has to be equal to the place card, so your tyres would be legal in terms of load rating.

Toyo tyres has a good page indicating legal requirements in all states, not sure on how to do a link so paste the address below into a browser.

http://www.toyo.com.au/TechInfoPDFs/Alternative%20Wheels%20&%20Tyres%20%20-%20Regulations.pdf

As for an increase in Diameter, the reg's refer to tyre size for that vehicle. The Hilux is made in Thailand and imported to all countries, but has many different tyre sizes and combinations even though the car is identical with same brakes and weights etc.