Wheels, tyres & offset

[Guy]

I'm getting my (first rod project) F100 ready for rego and want to fit new wheels (Astro Supremes from the States, anyone seen em here?). My (ignorant) question is, what does offset/backspace mean? I know I want 15x7 & 15x8 rims, but the company lists these sizes with different offset and backspace menasurements. It also lists these sizes as 'Reverse' with even bigger offset figures. How do these measurements affect what I end up with? Also, does anyone know where I can find radials with a skinny whitewall to fit these rims? I'm confused! Thanks!

[Pep]

I'm getting my (first rod project) F100 ready for rego and want to fit new wheels (Astro Supremes from the States, anyone seen em here?). My (ignorant) question is, what does offset/backspace mean? I know I want 15x7 & 15x8 rims, but the company lists these sizes with different offset and backspace menasurements. It also lists these sizes as 'Reverse' with even bigger offset figures. How do these measurements affect what I end up with? Also, does anyone know where I can find radials with a skinny whitewall to fit these rims? I'm confused! Thanks!

OK...backspace and off set are the same thing, except most wheel manufacturers talk in back space Which is the distance from the wheel flange ( the side that mates with the brake drum/disk) to the outer lip of the wheel ( the very edge ) This can be moved when manufacturing to compensate for diff track ( a wider diff will mean a bigger back space ). The way you measure how much rim width and back space you need is to take a plumb bob and measure from the drum face to the inside inner guard. Then you drop the plum bob from the wheel arch from (where it curves) then measure from the brake drum face again to the string. Now substract a half inch from each side for tyres up to 28 inches, and about
3/4 inch from each side for taller tyres. Thats to compensate for tyre roll when cornering. Now add the two measurements you have and thats the rim width to use with the back space ( which is the first measurement you made minus the tyre roll figure. ) Sounds confusing, but if you go to dragway.com.au they have a neat explanation.

[Carps]

Good advice Pep, you can also use a straightedge if you don't have a plumbob.

One thing to double check is that the brake druns are properly seated on the axle flange and not at a slight angle. Shouldn't make too much difference but it can lead to having not enough outer clearance and too much inner clearance if anything is not perfectly square when you start measuring.

Another good idea, is to aquire a tyre of the size you'd like to use and try it under the wheelarch to make sure it's going to fit. Then when measuring backspace for the wheels you'll already know the rim widt and can make adjustments to ensure the wheel fits the wheel opening correctly. If the opening has room for a ten inch wheel for example, and you only use an eight, making the back spacing as close to neat as you can will leave the wheel sitting too far under the fenders to look 'right'.

Of course the final look is up to you, so the only rule is realy to measure twice, check, then measure again before welding or other irreversible work is started.

[fc ute]

hey guy, have you looked at the older style 5 spoke dragway wheels, they are very close to astro supremes and would be a lot cheaper and they are everywhere
craig

[choco]

I had a helluva time with my wheels. I chose American Racing Salt Lake Specials, 15 x 8 and 15 x 7 for my 36 Plymouth Coupe. I had centrelines, 15 x 10, but they were a little too wide at the rear and I didn't want to widen the rear fenders. The wheels came with zero offset, but they scraped the inner fender. I had to get one inch offset built in, which meant returning the wheels, machining the centres out and welding them back in about one inch further out than stock. I put them on and they still touched a tiny smidgen. So a few different tyre combinations were tried. I settled on a set of tyres that fit correctly and didn't touch, but I needed a couple of 5mm spacers to keep the tyre away from the inner fender and still fit well under the outer fender. They fill the wheel wells perfectly, but the clearance is very tight. I fitted a panhard bar so stop sideways movement around corners, but the real test will come later when I finally hit the road. The whole exercise meant that I have the widest possible wheel/tyre combination that won't foul or scrape when cornering or bouncing and will fill the entire wheel well. If it doesn't work out, I shall then go back to the 15 x 10 zero offset and widen the rear fenders.
It is fairly critical that you get this right, simply because the wrong combination will give you dog kennels or, heaven forbid, that "crunch-crunch" noise every time you hit a pot-hole or that "grind-grind" noise when you turn a corner.

[Guy]

hey guy, have you looked at the older style 5 spoke dragway wheels, they are very close to astro supremes and would be a lot cheaper and they are everywhere
craig

Mate, I have ... but finding the Astros has now become an obsession; and they do have that unique 'dished' look that set them apart. I'm gonna hold out for the real thing (or give up and go for plain red steelies).