Higher mathmatics - back spacing help?

[BigAl]

On my cargo trailer, my current tires are 205/75R15 on wheels with 3.5" of backspacing. There is 5.5inches from the wheel mounting surface to the side of the trailer.

I have a set of 235/85R16 tires. I want to mount them on 16X7 wheels but can only find wheels with 4" back spacing which is going the wrong way. I think I need to keep at least the original 3.5 of BS if not more? Would these tires rub on a 4" BS wheel? The current tire is 1.75" from the inside of the tire to the trailer side. Any formulas or calculators out there?

 

[java]

maybe these will help....

 

[lqdchkn]

So you said you have 1.75 inches of clearance between the tire and your trailer with current setup.

As I see it

new 235 wide tires are 9.25 inches wide (give or take for tire bubble)
old 205 wide tires are 8 inches wide (give or take for tire bubble)

Which is a difference of 1.25 inches, which would be about a .625 inch width increase on the inner and outer from what you have now. Which seems like it would fit if you have 1.75 inches of space on the inside.

 

[BigAl]

So you said you have 1.75 inches of clearance between the tire and your trailer with current setup.

As I see it

new 235 wide tires are 9.25 inches wide (give or take for tire bubble)
old 205 wide tires are 8 inches wide (give or take for tire bubble)

Which is a difference of 1.25 inches, which would be about a .625 inch width increase on the inner and outer from what you have now. Which seems like it would fit if you have 1.75 inches of space on the inside.

So if I loose another .5 by increasing the BS to 4, that would mean 1.125 increase, so I would be down to ~.5" of clearance
Thanks

 

[lqdchkn]

Backspacing is a measure of the wheel and doesn't determine your overall tire/wheel width in this case. So it's mostly irrelevant here with the measurements you've given. It does enlighten us to the overall mounting depth of the new wheel/tire combo, which is what I used to guestimate what you will have with the new setup from what you currently do have.

 

[navigator]

backspace is important in this application.
Backspace is the space from the rear of the rim to the "mounting plate".
less backspace pushes the tire further away from the fender.
the wider the tire, the less backspace you want.

 

[Grebby]

Surely Backspace is, as shown in the pic above, from the mounting face to the inside rim. So if you need the wheel tyre combo further out you decrease the backspace?

A backspace of 0 would bave the entire rim protruding outwards from the mounting face.

 

[summerprophet]

To do the calculations you require, we would need at minimum the width of your current wheels.

As reference though, if you went with 235 wide tires on your current wheels, it would reduce your inside clearance to 1.15" ((235mm-205mm)/2=15mm=0.6". I would not want to go less than an inch of clearance.

Soooo, unless your new wheels are significantly wider then your current setup, I would not increase your backspacing.

 

[BigAl]

To do the calculations you require, we would need at minimum the width of your current wheels.

As reference though, if you went with 235 wide tires on your current wheels, it would reduce your inside clearance to 1.15" ((235mm-205mm)/2=15mm=0.6". I would not want to go less than an inch of clearance.

Soooo, unless your new wheels are significantly wider then your current setup, I would not increase your backspacing.

I am looking at goint to a 16*7 wheel with 4" of BS. I think it will be too close to the trailer