Wheel backspace

[aggie79]

Before I purchase my wheels and tires, I would appreciate some feedback as to if I have figured things correctly.

I am looking at either a 205/75R15 tire on a 15" x 6" wheel or a 225/75R15 tire on a 15" x 7" wheel. The question is what backspace should I get.

Here's some details. I purchased a set of Grant's fenders. There's about 10 5/8" distance from the teardrop sidewall to the innermost lip of the wheelwell arch. The distance from the sidewall to the hub face is 6 1/8". I'm planning to keep about 1/2" clearance between the widest part of the tire and the lip of the fender.

I mocked up a fender to a sidewall, and measuring for the tires, this is what I came up with:

For the 225 tire, approximately 4" of the 7" wheel width would need to be on the sidewall side of the hubface. For the 205 tire, approximately 3" of the 6" wheel would need to be on the sidewall side.

Are the 4" and 3" measurements the backspace I should try to find for my wheels?

Thanks, Tom

P.S. I have read the stickies on wheel offset and backspacing, but still can't get my mind around the terminology. And the last time I tried to do any math on my build - figuring out the length of the a-frame tongue - I had to re-work the frame.

[hugh]

If I"m wrong someone will correct me but here goes. We will keep this simple. Axle mounted on trailer, no wheel assy on axle. Measure the distance from the wheel mounting surface or in other words the face of the hub the wheel bolts to , to the side of your trailer your fender will be bolted to. I am assuming your fender will be bolted flush to the trailer side.
Lets say the distance is 4 inches. so if you buy a wheel with 3 inches of backspacing the inside edge of the rim will have 1 inch of clearance to the side of the trailer. You should also be aware the some tires have a bit of a sidewall bulge depending on the tire pressure so you want to allow for that also. Backspacing only refers to the distance the rim is offset to the inside.

[aggie79]

Thanks Hugh,
That is what I was thinking. I appreciate your confirmation.
Tom