Offset / back spacing again? HELP ME.

[Speed Dreamer]

I have a 1974 1303/super. I have Kerscher struts, and I'm waiting on the Tarox front disc brakes to come in the mail.

Now in I'm trying to figure out what is the correct offset for my custom wheels I'm going to order. I'm going to run a staggered setup. 17x7 on the front and 17 or 18 x 9 on the back.

Front: 17 x 7s with kerscher struts. The Tarox kit with vented disc increases track by 5mm over non vented kits similar to the ones sold by Topline, cb performance, ghia disc kit, etc. What would be the correct offset to keep these wheels with 205/40 tires under stock front fenders. I'm trying to do this without using a spacer, but don't mind using a small one if necessary. The offsets offered by the wheel maker are +22/+28/+35/+37/+43/+50. Food for thought phat73vw has a 73 1303 with 17 x 7 with +40mm et/offset, that fit. I have a 74 1303 and Jon at Topline informs me that my suspension is 10mm wider per side. My thoughts after reviewing this info, and checking out searches on offset /backspacing / is to go with either the +43 or +50 offset. ???? Help me.

Rear: 17 or 18 x 9s with offsets ranging from -1 to +50. I know I will have to order 2 or 3 inch wider fenders, and will purchase whichever ones I need to get 9 inch width wheels on the back. I'll be using either Topline or CB performance rear disc brake kit.

Zen and others, if I recall correctly have 8.5 and 9 inch Porsche wheels on the rear. Who has 17 x 9s, what are you using? wheel specs / brakes / fenders?

Zen, what rear brake kit were you using with the 2 inch spacer and 8.5 inch width wheels with I think a 55mm offset, and 3 inches wider fender. With the spacer attached doesn't that wheel have a backspacing of about +5 mm? If you visuallize the spacer as being a part of the wheel, for example if it was welded to the wheel. Am I correct in thinking that with the spacer attached to wheel, you actually decreased the offset to about +5mm.

Do the math using the Backspacing formula, or just visualize the wheels off the car, one wheel with spacer attached and one without, lying flat on the front and you are measuring the backspacing using the actual wheel. The wheel with the spacer would have the mounting surface moved by about 2 inches toward the back of the wheel. Almost giving it a zero offset going from 55mm to 5mm, decreasing the backspacing by 2 inches (50.8mm) The wheel without the spacer has backspace of 6.25 inches

??Using a 5.5 backspacing as acceptable for the rear. My thoughts when taking all this into consideration is to go with a offset of about +12mm (1/2 inch)

With a 9 inch width wheel, having a real wheel width of 10 inches, and a 5 inch center line, the +12 offset(1/2 inch), would put the backspacing at 5.5 inches; will this work? No scrubbing on the inside axles or bump stops, etc.???

Using tires in the range of 235 to 255 / 35s, do I need 2 or 3 inch fenders???

Does any of this make sense, or have I made a mountain out of a mole hill??

I have to sort of get this right the first time.

These wheels are made to order, paid in full in advance, with no refund. Please help me, all thoughts, advice, and ideas will be greatly appreciated.
Thank you all in advance.

[bugman78]

I have 17x7 42mm offset on my 78 super. I would go with the 50 offset. John told me after i already bought mine that i needed at least a 55mm to fit correctly and use no spacer. Mine rims come clear to the outside edge of the fenders. I am in the process off switching to the early strut suspension to make mine fit correctly!!

[Chris Percival]

Its a tough decision. The best thing to do would be to find someone with the most similar setup to you. Find out what wheel offset they use, then add 10mm to it to give yourself a bit of leeway. You can always add spacers, but you can't go the other way.