Street Machine - - TECH TORQUE -

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!


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

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