WHEELS, TUBS AND TYRES - THE FACTS!
One of the first things on any self-respecting car guys ‘to-do list’ is to fit bigger wheels and tyres under the rear guards – usually followed by a huge reduction in the width of the fronts. At the height of the Pro-Street movement, we were seeing huge Mickey Thompson rubber squeezed under the reworked quarters of everything from early Holden’s and Ford’s to classic Chev’s, Studebakers and even a VW or two. Whilst it bought the drag racing look to the street, this practice was often frowned upon by the authorities unless you had an engineer who was, lets say, colourful in his description of the mods made to the rear section of the chassis.
Generally, this requires the rear section of the chassis and floor to be cut away and remanufactured. Broadly speaking that’s a no-no with the rego authorities. Welding up a rear clip from square or chrome-moly tube isn’t ever going to pass a rego check and with the new, much wider wheel wells filled with 15-inch wide wheels and 22-inch wide rubber you might as well put a sticker across the back window of your car stating “C’mon defect me, I dare you!”
TROY SAYS: Full wheel tubs are generally not accepted as the requirement is with a full wheel tub that the chassis rail is moved to accommodate much larger wheel and tyres, most of the time larger than legally allowed, and with proportions that do not meet the allowed 70%. When major structural changes like full wheel tubs are fitted to a vehicle the vehicle would be considered to lose its identity and the vehicle can become a ICV (individually constructed vehicle) and as such would have to comply with all the current ADR requirements. Other areas affected by major chassis modifications such as full wheel tubs, is the vehicles torsional strength. Also its ability to stand up to the rigors of day to day driving and handling.
There’s a world of difference in full and minitubs, both legally and with the amount of work needed to gain the result. When you minitub a vehicle, you open the wheel wells (tubs) up to the chassis rail which requires no modifications to be made structurally. This allows more clearance for a larger wheel and tyre combination to be fitted to the rear of the vehicle without any chassis modifications whatsoever.
With some vehicles, where the shock absorbers mount through the wheel arch, you may have to opt for a pair of coil-over shocks relocated to a new fabricated position, which will require an engineers report to remain legal.
TROY SAYS: As a general rule mini tubs are OK, the rule has always been for mini tubs that the chassis rail is not modified, however there are some things that need to be addressed when fitting mini tubs, the requirements for the vehicle to maintain the original or better torsional rigidity, normally, when mini tubs are fitted depending on the vehicle, the strength of the vehicle can be affected, the fitment of the wheel tubs may require the vehicle to undergo a beaming and torsion test to make sure that there is no effect to either of these areas of the vehicle. NCOP6 states that; if inner wheel tubs are fitted;
1. The vehicle structure is not weakened.
2. Seat anchorages or seatbelt anchorages are not weakened.
3. The wheels or tyres do not protrude beyond the bodywork of the vehicle when viewed from above.
4. The wheels or tyres do not contact any part of the vehicle’s steering or suspension, brake lines or bodywork for the full range of suspension movement.
5. The maximum and minimum allowable track is not exceeded.
6. The vehicle’s ground clearance remains within legal limits.
It is also worth noting that the National code of practice does not include vehicles that are complied to ADR69 (full frontal impact protection) or ADR73 (offset frontal impact protection). These vehicles would have to undergo extensive testing to verify the structural integrity of the vehicle.
WHEELS & TYRES
Of course opening up the wheel wells will mean much bigger wheels and tyres to fill the newly opened area. The authorities have strict guidelines for the limits on the size of the wheels and tyres you try to fit under the rear end. With that in mind, there’s a ratio that needs to be applied when working out the minimum width for the front. Basically, if you plan to run a 15x12-inch rim with a 15-inch wide Mickey Thompson tyre on the back and 15x3-inch rims on the front with 145 Michelins, you’re in for a rude surprise when the engineer talks to you about the do’s and the “yeah, that’s not going to happen’s”.
TROY SAYS: It is important to note that the largest wheel size that can be used on a passenger vehicle in NSW is 10 inches wide, with that the wheel/tyre proportion is 70% e.g if you use a 10 inch wheel on the rear the largest wheel you can use on the front is 8 inches and the narrowest wheel you can use is 7 inches wide, legally we can't use 15 inch wide wheels any more with 5 inch front runners. it is also important to note that we can only fit wheels that widen/narrow the track width of the vehicle by 25mm. The dimension used for track width is the vehicles original track, not the donor vehicle that the differential has come from. Wheel spacers must never be used, if the chosen wheel and tyre combination requires the fitment of wheel spacers, the offset of the wheels is wrong and a different offset wheel must be used, or in some cases a completely different wheel. It is also important to note that the wheels must be located on the wheel hubs, hub locators can be used and these come in various sizes to cater for differing wheel hub diameters. For vehicles which comply with ADR24, (tyre and rim selection) wheel track width is maintained within the limits set out, and the diameter of the wheel/ tyre combination should be maintained to within 7% of original, if this is not maintained the vehicle would have to be tested to ADR18 (instrumentation) to maintain accuracy of the speedometer. Tyre load limits must be maintained for all commercial type vehicles.