Backspacing for 285/75/16

[UKTJ]

I am looking at putting 285/75/16’s on my TJ once the lift is complete.  This is roughly speaking 33”x11.5”.

 

My understanding is that without spacers the widest I can go on stock wheels with 5.25” of backspace is 10.5”.  On that basis I am looking at using a wheel with 4.5” of backspace.  In my mind all the extra backspace is on the inside side of the wheel, but only half the extra tire width is, so this combination will work.

 

Is my logic sound?

[digger]

An interesting size, I'll have to look and see if they are available on 15".

[V]

My steel wheels on my '93 XJ are 15x8 AR-767 with 4.75" backspace. This was a popular size kept in stock by UK importers for YJ and TJ last century for 33x10.50 tyres, they had no idea if they would be OK on a XJ with 33x12.50 so I bought one to try first.

 

My TJ had 265/75R16 BFG MT for a while with stock wheels and flares. The tyres were covered by the flares. Your 285/75R16 are 20mm wider, so on the same wheels they will need 10mm of flare coverage. I can't remember how close tyre coverage was, but you could buy one 285/75R16 tyre and have it fitted to your spare wheel for static testing on the front and rear axles. Mount the new tyre on another stock 16" TJ/XJ wheel and you don't have to worry about driving without a spare. If the extra spare wheel matches what you already have, you can go to six tyre rotation with two spares at some point.

[UKTJ]

This is proving a real head scratcher for me.  I have not found a wheel in the UK I am particularly keen on with the amount of backspacing I would ideally like (4.5”).  I have looked at options from the US, but the shipping costs make that a very expensive option at the moment.

 

I have therefore been mulling over using wheel spacers.  Ideally I need to space the wheels out by somewhere between 1/2” and 3/4”, but as far as I can see that seems a spacer size that is not really catered for.  There are spacer rings that just slot on to the studs up to about 1/2” or there are spacers that fix to the studs and then provide a new stud starting at about 1”.  I am guessing this is driven by the length of the studs and the space needed to counter bore into the spacer for a nut.

 

As I understand it, adding a spacer pushes the wheel out further from the hub and therefore increases leverage.  Add to that I am putting on a significantly heavier tyre than stock.  My concern is the forces being applied to the outer axle shafts and ball joints will significantly increase the risk of failure.

 

Does using a spacer materially increase this risk?  Or am I being unnecessarily concerned?

 

The alternative I am thinking about is using a 265/75/16, which at about 10.5” will need only a tiny amount of spacing if any at all.

[V]

With spacers, you can go up to 1/4" with the wheel studs you already have. Some Jeeps have shorter studs that don't have sufficient thread engagement, these studs should be pressed out and replaced with longer studs.

 

There is nothing you can do between 1/4" and 1-1/4" with a spacer, only a different wheel will get you in this range.

 

From 1-1/4" to 2", studded spacers and wheel adaptors are available. I think there are TUV approved products on the market now. I use Spidertrax adapters on my Ford 8.8 axle with 35" tyres. They have earned a 'Rubicon' badge with my Jeep.

 

Wheels with the correct offset are the way to go. They will be stronger and eliminate the extra work removing studded spacers. I have run 265/75R16 BFG on a 60th Anniversary TJ with 4.11 gears and a SkyJacker 2.5" lift and it's a great combination. I am fairly certain, I had to fit 1/4" thick disc spacers on the front to regain some turning circle.

 

From a reliability perspective, the stock unit bearing on the front axle is only designed to cope with a narrow tyre. The bearings are small and too close to each other. With a 10.5" tyre you will probably not notice reduced unit bearing life. With a 12.5" wide tyre you are likely to. The WJ/WG and up have a slightly bigger unit bearing but it is still small. If you want reliability, spindles are the way to go. There are kits for 5x4.5" PCD but these are only good enough up to 35", for 35" and over 5x5.5" PCD or 6 or 8 stud kits eliminate the unit bearing weakness. You have to replace a large number of unit bearings to break even on the cost of a spindle conversion which is why a lot of owners stay with unit bearings.

 

I have run, and I am still running 27-spline OEM shafts with 33x12.50R15 tyres on my older XJ. I have gone through countless unit bearings and universal joints, but I have yet to snap an axle shaft on that vehicle. I drive carefully off road and my Jeep is rarely in 2wd (it has a NV242 transfer case) so the 'weak' D35 rear axle doesn't get all of the engine torque on high traction tarmac. Truetracs at each end have also helped to reduce the likelihood of shaft failure. I think I have eked out as much reliability as I can from the 'economy' specification of the original equipment.

 

My TJ was OEM 27-spline shafts and unit bearings on 265/75R16 and it was fine too. You have no worries using 2wd with a D44 rear.

 

[UKTJ]

Thanks, yet again, for such a comprehensive reply @V

 

Sounds like my gut instinct was boadly correct and spacers will create additional potential weakness on top of the impact of bigger tyres.

 

I have seen spacer rings up to 13mm (1/2"), but there are warnings on the website about making sure studs are long enough to accomodate them, which I doubt.  I have also just come across a hub centric 20mm bolt on space, but I have also seen some suggestions that with a spacer that small the studs may need to be ground down.  So neither sounds ideal.

 

I can use my existing tyres for a while, there is nothing wrong with them, so I will keep looking at wheel options I think.  Maybe I need to open up the search and look at black ones, so far I have excluded those.

[Fourpot]

One other thing to consider is the movement of the front wheels when turning. With wheels with small backspace or with spacers, your wheels are further out (obviously -  that's what you want), but the fronts also move through a much wider arc from lock to lock. Your average car's centre of steering rotation is the middle of the tyre - ball joints right inside the wheel. With my tyres (265/70/r16s) and an offset of 4mm (rim centre is 4mm further out than the hub face, the outer wheel goes forwards quite a lot and the inner wheel goes back the same amount, as the ball joints are hardly at all inside the wheel. Add in turning and compressing the suspension at the same time, and you may need more room than you think.

NB -  I know nothing about TJs