WHEEL OFFSETS AND CALIPER CLEARANCE REQUIREMENTS
One of the most common issues we
face when fitting bigger brakes are:
2/ Wheel offset / backspace considerations
3/ Wheels fouling on calipers
As a principle we at Hoppers Stoppers try to keep as close to original factory hub face position (track), so that if you have a larger wheel in a standard stud pattern you can fit our big brake kit without needing to buy new wheels.
However the logistics of getting the brackets and calipers safely bolted onto the stub axle can often mean we might have no choice but to move the hub position in or out a few millimeters.
This is not usually a problem unless you already have the tyres really close to the mudguards or suspension.
So if you have everything really close - call us first.
A couple of our kits that have a fairly large offset change are HR Holden and LC/LJ Torana, at 16mm out, and 55-68 Chev and Camaro, where we move out 28mm per side, so factor this into your wheel offsets. (Factory Camaro’s with discs moved 25mm from drums in any case so even Chev had to do it!)
STUD PATTERN CHANGES / OFFSET TERMINOLOGY
Lets first look at wheel terminology.
1/ WHEEL WIDTH
The width of a wheel is measured from bead to bead on a tyre where it sits inside the rim. eg a six inch rim with a ¼ inch of wall thickness on the sides of the rim would measure
6 ½ inches overall. The rated width of the wheel does not include the lips each side of the beads.
Some tyre retailers quote the wheel “backspace” but note this is not measured from the bead but from the hub face to the very edge of the rim including the metal thickness.
eg a 6 inch rim with the hub positioned right in the center would measure 3 ¼ inch backspace if it has a ¼ inch wall.
Most car makers set the center line of the wheel in from the hub face, inwards is called POSITIVE OFFSET, and outwards is called NEGATIVE OFFSET.
This is to get the scrub radius correct, which is important in keeping handling right.
In the 60’s and 70’s carmakers tended to set offset between 10 and 20mm in, ie 10P and 20P. Later cars often have greater offsets, typically 38P, 40P and 45P being common.
eg. a 6 inch rim is 152mm wide, and if set at 40P , would have the wheel center 40mm in from the hub face, being 40 from 76, therefore measuring 36mm from the outer bead to the hub face and 116 from the hub face to the inner bead.
So when you find it difficult to find the right wheel and decide to change stud pattern, remember that we probably designed to be close to original offset for the genuine stud pattern and that a very different offset might cause problems.
Compounding this is the fact that some wheel companies are only interested in selling later model wheels with 38 to 45P and these will probably come in too far and hit the strut or similar. We do make some special hub arrangements to cater for demand but be aware that just because you got the wheels cheap at a swap meet does not mean we can cater for them. Also remember that the little Jap car which came with 13 by 4 ½ wheels might not work out if you are trying to fit 18 by 7 and hoping we can make the brake kit in just the right offset.
Better to get the brakes fitted first and have the wheels made to suit.
4/ STUD PATTERN / THREAD TERMINOLOGY
Some Jap cars use the same 114.3mm by 5 stud patterns as Ford but have metric 12mm threads. If you are fitting a 9 inch diff you might want all your front studs in ½ inch UNF Ford threads but if fitting a Jap diff might want 12mm. Let us know what threads you want if it’s not obvious.
WHEEL SPOKE CLEARANCE
Bigger brakes by definition require bigger diameter rotors, and bracket designs sometimes dictate a shallower rotor hub section, placing the caliper closer to the wheel center/spokes.
Most 1950’s and 60’s steel wheels were designed for drum brakes and won’t clear even small disc brakes, let alone bigger discs. When we say you need a 15-inch wheel, we means a modern rim, which doesn’t step down before it meets the center. Old 14-inch centers attached to stepped-down rims could possibly still have issues with the caliper hitting the step.
Most of our PBR twin piston calipers come about level with the hub face, so if your center overhangs the hub face it’s probably going to hit.
Multi piece rims with a ring of nuts bolting the center to the rims may foul on the calipers. However they can often make the hub thicker and set the outer rim shallower and the inner rim deeper to compensate.
Check your wheels against our templates before deciding to order a brake kit.
SOFT LIP RIMS
These are another problem to be aware of. These are designed so that the billet wheel center is attached out at the bead part of the wheel on the biggest diameter, and the usual stepped down “well” is back doing nothing except hitting the caliper.
WHAT TO DO IF YOUR WHEELS DONT FIT?
1/ Wheel spacers?
In many cases an 8mm spacer will fix a problem, and usually the studs have enough thread to allow this. However the various state authorities generally forbid wheel spacers if not original equipment. Wheels coming loose have been seen so if you must run spacers check wheel nut tightness regularly. (Plus this moves your track out)
2/ Grinding calipers?
In a nutshell, simply don’t do it, no amount of grinding is safe.
In summary, when wheel clearances might be a problem, contact us for advice.