Trimming what exactly with new tires/wheels ?

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

Mar 27, 2003
I've narrowed in on a few wheels ... both 16x10 , 6 on 5.5 etc .

The M/T Classic II is -12.5mm offset I would think if the backspace is 4.5 ... which should work fine as some of the guys here have stated .

The other wheel is a -33mm offset . So roughly 4/5th inches further extension outwards from the well .

If I go with a wheel , like this , which is 16x10 but has an offset less than -12.5mm ( ie:-33mm ) , then what can I anticipate needing triming ? I understand you don't have the benefit of seeing how my light/med OME lift sits on the truck , but can this be answered in terms of general rules , or trends ..

IOW's , as I extend my 315's on 16x10's further out from the wheel well via an increasingly negative offset , what will my tire begin colliding with upon full turns ?

I'm considering that it might be prudent to take along my grinder to the tire shop when I have these wheels fitted ...

no pics either :flipoff2: . They're only 16" , but maybe a tad bright ... :D
I'm not sure I would go to 10"s with a 35x12.50 which is what a 315/75/16 basically is, you have a lot of rim sticking out if you are going to wheel it in rocks. And a narrower rim holds the tire better when airing down. On my 91 with J springs I'm running 15x8s with 35x12.50 MTs. They have 3.25" backspacing and I really really like the wider stance and I like that nothing really rubs. I hit my mudflaps occasionally, but other than that I'm pretty clear. I was looking at the AR 589s for my 94 in a 16x8 with 4" backspacing. A bit wider than stock but not as much as my 91 :(
The standard rim is 8" wide and a back spacing of 3.25". Now to keep the center in a wider wheel you add have the width change to the backspacing. So 2" increase in width adds 1" to backspacing or a final 4.25" of back spacing. Now with only an addition .25" your looking at being closer in the vehicle and more back spacing even in further. Although the trimming will be easy since you will be working from in the vehicle :D.

Disclaimer: My memory has been faulty lately with several senior moments, you might want to wait for someone to verify my explanation.
Stock LC wheels are not 3.25" backspacing. I believe they are 4.25" backspacing. They are 16x8s on 93-97s. Backspacing is measured from the edge of the back of the rim to mounting face of the rim. Less backspacing pushes the wheels out further giving the vehicle a wider stance, more backspacing brings the wheel closer to inner fender narrowing the stance. For running big tires I would not want to run anything more numerically than stock 4.25". I would want to run a smaller numerically backspaced wheels to push the tires away from the body. You can go too far. I believe a 2.75" backspaced wheel would push the tire out so far you would hit the outside edge of the fender or flare if you have them still when you articulate in. But a 2.75" backspaced wheel is not an off the shelf wheel, so I guess we don't need to worry about that too much.

Offset is different than backspacing. Wheels of different widths can have the same offset but different backspacing. Some wheels are negative offset, some are positive, some are zero.

AR 589 numbers for comparison:
size bolt pattern backspace offset weight rating

16x7 6 on 5.5 4.24" +8mm 3040 lb
16x8 6 on 5.5 4" -11mm 2100lb
16x10 6 on 5.5 4.5" -24mm 2100 lb
17x8 6 on 5.5 4.5" -24mm 2100 lb
;) All this I am familiar with ...

I was just having difficulties visualizing where the tires would start rubbing at full locks based on the 16x10 with offsets starting at -12.5mm and moving up thru -25mm and on to -33mm .

Ultra has a few nice wheels ... they're 16x10's are also in the neighborhood of -25mm .

So might I need to trim :

  • The bumper ?
  • Aft / inside aspect of flare ?
  • Inner aspect of wheel well ?

You may have addressed this earlier, but why are you going to 10-inch rims?
I think probably because all the specs list anywhere is the offset for most of the newer wheels. It irritates me as all I want to know is the backspacing.
Am I missing something ? I'm comparing apples only right , so aren't backspacing and offset two sides of the same coin ?? Two complementing figures > ?

For ex. a 10" wheel with a BS of 5" has a 0 offset , correct ?

Similarly ,

  • 4.5 BS = .5" OS or 12.5 mm
  • 4.0 BS = 1" OS or 25mm
  • o BS would equal 5" OS or 125mm :D

Like an ant walking the length of a ruler , the measurements on either side of it will always add to the static length of that ruler ; assuming zero ant length ;)

Therefore , if you know what the offset (OS) is , you most certainly know the backspacing . They are related , though they may not appear as the ant analogy all the time . This is so with a positive BS .

Simply : BS - OS = (wheel width)/2


Now I bet you're thinking this doesn't quite add up , right ? [ remember I asked , 'am I missing something?' ] Well if you are you'd be correct ! I began to apply my theory to genericfj80's data , and I kept getting wrong answers ... I started to dig a little deeper , and realized that The goofball that decided on wheel spec. measuring had done an odd thing in terms of his conventions . BS is the distance from the inside rim surface to the backside of the wheel mounting surface. However this is NOT so when it comes to wheel 'width' . The wheel width is the distance between the outside edges of the bead seat. Herein lies the ugly discrepency in my logic . To correct for this we must discover what the difference is between the half width and the ( BS - OS ) . Lets try with Gen's data :

16x7 6 on 5.5 4.24" +8mm 3040 lb
16x8 6 on 5.5 4" -11mm 2100lb
16x10 6 on 5.5 4.5" -24mm 2100 lb
17x8 6 on 5.5 4.5" -24mm 2100 lb

16 x 7" wheel : (4.24*25-8)/25-3.5 = 0.42
16 x 8" wheel : (4.0*25+11)/25-4.0 = 0.44
16 x 10" wheel : (4.5*25+24)/25-5.0 = 0.46
16 x 8" wheel : (4.5*25+24)/25-4.0 = 1.21 ** Increasing wheel diameter seems to result in somewhat abberant results ; though on the whole they are acceptable . This measurement represents the fatness and flare of the lip in the picture above .

So what ?! The bottom line is BS and OS are related ... it's not that I am focused on it alone , but as a trend , I appreciate that as the OS moves in a certain direction that the BS will behave predictably which is important information if all you have at hand is the OS .

I'm still interested in hearing ideas and experiences on what will need modification as the BS and OS decrease ...

Thanx :beer:
Simply : BS - OS = (wheel width)/2

I have read people state that the backspacing on the OEM 16x8 wheel is 4.25 and 4.5. From the above, shouldn't the backspacing on the OEM wheel be 4 inches?

Also, anyone know what the "J" or "JJ" means after the width spec, i.e. 16x8JJ?

If your going to carry out your calculations to two decimal places I would at least use 25.4 as the conversion factor from inches to millimeters.

I take it your are trying to get all the calculations to equal the same amount?

The difference could be the result of slightly different bead styles, lip sizes or in the case of a stamped steel wheel the material thickness.
This is seriously fawked ... I asked a simple question .

Tell ya what ... grudgel thy brains no more about it !

The stock OEM alloys and steel wheels for the FZJ80's are all 4.5" backspacing (zero offset.)
I was told by a wheel manufacturer that all steel wheels have zero offset.

[quote author=Tyler link=board=2;threadid=8539;start=msg73231#msg73231 date=1070954786]
This is seriously fawked ... I asked a simple question .
Tell ya what ... grudgel thy brains no more about it !

Can someone pass me the butter? :flipoff2:

Tyler, to be candid dude, I'm not sure how much you'll hit where. I know you are looking mainly at street use, so max flex shouldn't be much issue. You may want to lower your bump stops and add some in the front. In the rear, I think it would be all flare and even with the flare off, you may have to grind and/or cut off the rim of the fender (the piece that L's back towards the frame). On the front, since you're going further out, it may only be similiar, but if you have not pushed your axle and further forward, you may have issues on the rear of the wheelwell. :flipoff2:

Maybe these won't be issues since you're only talking 35's, but those are the areas of concern with going to 38's (well, among a few other). :flipoff2:

:flipoff2:Now :flipoff2: shut :flipoff2: up :flipoff2: and :flipoff2: pass :flipoff2: the :flipoff2: frickin :flipoff2: butter. :flipoff2:

Users who are viewing this thread

Top Bottom