HOW TO MAKE SURE YOUR WHEELS AND TYRES ARE THE RIGHT FIT FOR YOUR STREET MACHINE
AGREAT stance is the key to a goodlooking street machine. Taking a stock drivetrain and chassis, getting the height right and filling the guards with the right-size wheels and tyres is a sure way to transform your ride from amazing to exceptional.
The trick is to start with wheels that match your intended build style. For example, if you’re going with a nostalgia theme, 20-inch billets probably won’t look right. Once you have the wheel style sorted, it’s time to research tyres.
THE specifications of a tyre are stated on its sidewall. These number and letter combinations denote the speed rating, the diameter and the ratio of the sidewall height to the section width. Section width is the dimension across the fattest part of the tyre, not the tread width. So two tyres with the same section width from two different manufacturers may have quite different physical sizes.
There are 30 possible speed ratings for tyres, but the two key ones are the S rating (for passenger vehicles up to a speed of at least 180km/h) and the Z rating (for passenger vehicles up to a speed of at least 240km/h).
In addition, there are 238 possible load ratings. As a guide, a load rating of 100 represents an 800kg load capacity per tyre, while a load rating of 0 is 45kg per tyre. The sum of the load-carrying capacity of four tyres should be equal to the vehicle weight plus at least 50 per cent, to provide an appropriate safety margin for tyre fatigue under a variety of load conditions.
TO FIND the right tyres for your wheels, visit your tyre dealer and look at the tyre and rim manuals, or check data online. You’ll be able to research the range of tyres recommended for your rim sizes, and the height, tread width and section width for each option. We often have to talk modifiers out of, say, a 12-inchwide wheel where rules such as floating hub or lane-change test requirements would come into play; almost the same-width tyre can be fitted to a 10-inch wheel, where these technical issues won’t apply.
Choosing the right front tyres can be more difficult than the rears. You need to keep the turning wheel away from the mudguard, and you also need to have enough room for suspension travel. The best way to ascertain which front tyres are best for your street machine is to mount a wheel and tyre on the car and see how much room you have on full lock and bump. Then you can make a judgement based on the backspace and diameter of the mock-up combo. A rule of thumb is to have the overall width between the outside of one tyre to the outside of the opposite one at least 250mm (10in) narrower than the width between the inside edge of one guard to that of the opposite guard. A four-inch backspace on a seven-inch rim is common.
Rear fitment is easier. You can map your dimensions on the garage floor or draw a scale diagram. Drop a vertical from the inside of the outer guard and mark a line on the floor. Then mark the axle flange to the floor. Finally, drop a vertical from the inner guard to the floor. Then you’ll have all of the dimensions you need to work out the perfect wheel/tyre combination.
Again, experience has shown that you need about two fingers’ (50mm) clearance from the tyre sidewall to the guard lip, and to the inner panel. Remember that on cars where the rear guards overhang the wheels, you still need to be able to get the wheel on and off. There have been many awkward moments at the fitment stage, where the perfect wheel is about to be installed but can’t be!
WHICH WIDTH IS WHICH?
THERE’S a strange conflict in the way tyre specs and wheels specs are expressed, and it often gets the apprentice, the novice and sometimes the grey-haired into trouble. At some point after Karl Benz used the Mesopotamian wheel in his first car, it was decided that wheels would be identified by the diameter and width of the seat for the tyre’s bead. So a 17x8in wheel accepts a 17in-diameter tyre with a width that fits an eight-inch bead width.
The contradiction comes when wheel offset is measured. Wheel offset is measured from the outside of the rim, not the bead seat inside. An eight-inch rim, for example, will actually measure nine inches in overall width for a typical alloy unit. The backspace is measured from the wheel mounting face to the outside lip, not the inside bead. So where you might think that a centred eight-inch wheel would have a four-inch backspace, it is actually 4½ inches. Be aware of this, or you can easily end up over-track, or with your wheels half an inch closer to your guards than you planned. On a top-end car, it can be an expensive mistake.
Let’s do a sample calculation for seven-inch wheels with four inches of front- and backspace (eight inches of overall width) into a project car with known dimensions marked on the garage floor:
Distance between inside of guard lips 69in Outside wheel clearance (1in x 2) .......... – 2 Distance between outside of tyres ...... 67in Frontspace (4in x 2) ........................................ – 8 Flange-to-flange axle width ..................... 59in Backspace (4in x 2) ........................................ – 8 Distance between inside of tyres ........ 51in Inner guard clearance (1in x 2) ................ – 2 Max. width between inner guards ........ 49in You can use this to calculate any unknown element (usually the wheels and their backspaces) so you can decide what will work and then buy the right wheels and tyres, knowing that they will fit.
MOST of our discussion up to this point has been based on making wheels fit an existing suspension and body structure. But at the elite end of car building, this is not what happens. At this level, the wheels always come first, as the look of the car cannot be compromised Once the wheels are chosen, fabricators can then put the body at the height they want, fit the wheels into that body exactly where they want them, and then build the suspension to suit. Clearances, track measurements, floating hubs, lane-change tests, backspaces and other critical measurements all need to be worked out perfectly. This is the territory of the experienced constructor who knows exactly what they want and how to do it.
But while the elite method of car building is the reverse of what we’ve previously discussed, all of the calculations and processes remain the same.
TO MAKE low-profile tyres fit (while maintaining the same overall diameter of the original tyres and without resulting in excessive tyre width), the diameter of the wheel can be increased one, two or even three inches to compensate for the lower cross-section tyre height. This is called the ‘Inch-up Principle’, or ‘Plus One’. It achieves the desired performance improvements of low-profile tyres by providing a larger footprint, for quicker steering response, better cornering, faster braking and overall improved handling. PLUS ONE