Offset / backspacing, scrub radius, tire size & axle width

[chainsaw]

Sorry if this isn't quite clear, but I'm trying to peck it out before heading off to work for the day...

Please correct me if I'm wrong, but as I understand it: Vehicles are factory engineered to have pretty much zero scrub radius. I don't know if it's any more important with unit bearing hubs vs spindles. But zero scrub (or as low as practical) seems like a good design goal when building up my Jeep, but you have to work with what you've got.

I think I understand the 3.75" backspacing recommendations usually given for stock axle rigs wanting to run 35's: It's going to keep the tire (mostly) out of the stuff on the chassis that you don't want it getting into.
That is based on the stock axle width. Depending on rim width and it's likely not giving the best scrub radius. Both the increased negative offset, and increased tire diameter are working against good scrub radius at the same time.

So what happens if one were to go to something like JK axles?
I'm assuming you'd need to relocate, or install new mounts to make it fit in a TJ, as I believe the mounting points are narrower. Iirc TJ axles are 60.5" WMS vs JK at 65.5"
Would it be practical to run wheels with 2.5" less backspacing on the JK axles (than what you would on the TJ axle to help with scrub radius? (While simultaneously trying to keep the outside of tire width down to maintain fender coverage?)

The same question might apply to Super Duty axles, but I suspect that you're getting into tire sizes where the desired track width is wider anyways...

With a spindle style axle you're going to want to keep a certain amount of rim width on the outside to try to protect the selectable hub. So the math might work out differently than with unit bearings.
Also, at some point I believe there's an "ideal ratio" of tire size, track width and wheelbase to take into account.

One more thought: Does anyone now when rim diameter, and backspacing become a problem to turning radius? (Or is it not worth discussing within the scope of a stock bodied vehicle?)

 

[Chris]

The better question would be why would you want to bother going through the effort to swap in JK axles?

What do you stand to gain from that? They aren't much stronger than TJ axles and the amount of work involved would be similar to swapping in Dana 60s.

I'll never understand why people think swapping in JK axles is an "upgrade".

You will need new mounts to fit JK or Super Duty axles.

Maybe you should start by telling us what your end goal is? Don't say 35" tires either.

 

[machoheadgames]

Generally speaking, RWD vehicles have some positive scrub radius and toe in, while FWD oriented vehicles have some negative scrub radius along with toe out. Very few vehicles have zero scrub radius or zero toe due to the tire squirm feel it creates and due to dynamic loading. Toe and scrub are related because of dynamic conditions. If you have positive scrub radius, then the tires will want to pull apart when driving straight. Toe in helps to combat this. So if you increase scrub radius, it's often a good idea to also increase toe in.

Alternatively, as you increase negative scrub, you would want more toe out to combat that.

If you are basing things off of the factory 30x9.50R15 + 15x8 stock wheel combo, a 35x12.5R15 on a 15x8 wheel with 3.75" BS actually adds about 25 mm of positive scrub radius. So really and truly, 3.75" is not "close to stock." It is close enough in that you can't really feel a handling difference, but it is not the same as stock. Personally I would increase toe angle a small bit if I ran 35's with 3.75" BS. I get why 3.75" BS is common - it's a common wheel selection choice and it's a known quantity that people have run with success, so I have no issues with it. But due to the added scrub, I would add a tiny bit of extra toe, or at least put it towards the greater end of the factory toe angle spec.

Scrub radius is really just related to the wheel pivoting around the kingpin axis, so wider axles should not affect this. What would affect this would be the specs of the vehicle the axle came from, so you'd want to compare the scrub radius of the JK in stock form with stock JK wheels and see where that lands vs the TJ axles with TJ wheels, and then extrapolate and adjust as necessary.

What a wider axle affects more is ackermann angle, which is the perfect differing angles of the front wheels (aka the inner tire turning sharper) to complete different arcs as the vehicle travels through the turn. Ackermann is determined by taking the center point of the rear axle, drawing a line from there through each kingpin axis, and making sure the tie rod hole intersects that imaginary line. There is a solid chance a wider axle will not jive with that on a TJ, however, like anything else, the differences are minor. The tires may chirp a bit on turns or something but it's not going to be that big a deal. To put it in perspective, you have plenty of vehicles with different wheelbases all using the same knuckles, and that would also ruin ackermann. So really it's not that big of a deal.



In short, people probably stress about these characteristics and details a bit too much. The main thing you don't want is your scrub radius to be so bad that you lock the steering when you hit the brakes due to the wheels traveling in too much of an arc (from the added scrub). 35x12.5 on 15x8 3.75" BS wheels is a known option that will not suffere from that, so it is good enough.

For what it's worth, the factory positive scrub radius on my truck was calculated at 20mm on 7" wheels and 15mm on 7.5" wide wheels. I would expect other vehicles like Jeeps are similar, so you are really probably going from 20ish mm stock, to 45ish mm on 35's with 3.75 BS.

I can't speak to the pertinent details of the JK axle swap. Personally if I did those, I would want to run whatever wheels put the tires back under the vehicle to the same degree that the stock axles do with stock or aftermarket wheels. My best guess is that if you ran JK axles, JK wheels would be the closest bet to make things "right" on a TJ.

 

[GoldenGorilla]

Agree with the above. Having run some 1.25" spacers briefly on my TJ Rubicon, you don't really feel the scrub radius difference driving at high speeds. You'll notice it more at low speed maneuvering where it feels like more resistance coming off center.

On the plus side, the TJ track width is really narrow for modern vehicles and on worn asphalt it rides a bit odd in the troughs worn by trucks. The wider track width results in it tracking a bit more naturally.

My other car is a pretty small sports car with very staggered wheels (235 front and 305 rear) and it's far worse for this trough issue. I feel like I really get kicked around on them if they're worn deep. Chains are required in winter here and it really chews up the right lane so the troughs can easily be an inch or deeper by spring.

 

[chainsaw]

The better question would be why would you want to bother going through the effort to swap in JK axles?

What do you stand to gain from that? They aren't much stronger than TJ axles and the amount of work involved would be similar to swapping in Dana 60s.

I'll never understand why people think swapping in JK axles is an "upgrade".

You will need new mounts to fit JK or Super Duty axles.

Maybe you should start by telling us what your end goal is? Don't say 35" tires either.

I used the JK axles as an example, more than anything. But it is something I'd be open to if it made sense.

As I mentioned in my Dumpster fire thread, my TJU need rebuilds on both axles, and I happen to have a variety of bit's laying about that might be suitable "upgrades". Living north of the 49th parallel means I pay roughly 50% more for everything. I explained the situation better over there, but the shorter version is trying to avoid redundant spending whenever practical. I don't really think of them as an "upgrade" so much as a lateral move with some possible limited benefits.

To answer your last, and arguably most important question:
I'm not new to wheeling, but this is my first Jeep. I live in an area that's not quite as dry as the southwestern states, and not quite as tall as stereotypical rocky mountain states. Within a few hours drive I could be wheeling wet-coast stuff, rocky mountains, or (ew) mud. immediately local to me is a plethora of forest service roads and terrain ranging from mild to wild. I also aspire to someday wheel places like Johnston Valley, Moab & Sand Hollow, among others. Ultimately I'd like a rig I could use to go fishing, and if I see a dry creek bed or rockfall on my way home, maybe take a turn and do some serious crawling. From my experience 35's or 37's on a stock-ish bodied rig can work well in these conditions with the right modifications, and still be a (solid ice cream ride too). I bought a TJU because I felt like it falls into roughly the right wheelbase, while still being narrower (for tight trails) than a JK or JL. I'm pretty mechanically inclined, but I live in a strata controlled property with a small one car garage, so I'll likely have to rely on friends and/or pay people to do my major fabrication projects.

In addition: I've already received a quote from a local provider to improve, regear and rebuild the stock axles, and it's knocking on $10k.
I've been quoted $5k for a TJ Rubicon front axle that would still (at minimum need regearing at roughly $1-1.5K), and I'd still nee to spend $4-5k on the rear
I can purchase a matched set of JKR axles for $5k (plus whatever it costs to convert them for TJ use...)
I have other parts available in my personal stock that could be used to cut costs (if it makes sense).

I have a decent understanding of suspension fundamentals, but it's been a minute.
It was a though that floated around my head at 4am, and I though had some merit & I wanted to throw it out there to see what feedback I could get.

machoheadgames has given a little more to think about...

 

[chainsaw]

Generally speaking, RWD vehicles have some positive scrub radius and toe in, while FWD oriented vehicles have some negative scrub radius along with toe out. Very few vehicles have zero scrub radius or zero toe due to the tire squirm feel it creates and due to dynamic loading. Toe and scrub are related because of dynamic conditions. If you have positive scrub radius, then the tires will want to pull apart when driving straight. Toe in helps to combat this. So if you increase scrub radius, it's often a good idea to also increase toe in.

Alternatively, as you increase negative scrub, you would want more toe out to combat that.

First off: Thank-you for your detailed response!
Second: I sounds like you either do race car stuff, or seriously sporty car stuff...

If you are basing things off of the factory 30x9.50R15 + 15x8 stock wheel combo, a 35x12.5R15 on a 15x8 wheel with 3.75" BS actually adds about 25 mm of positive scrub radius. So really and truly, 3.75" is not "close to stock." It is close enough in that you can't really feel a handling difference, but it is not the same as stock. Personally I would increase toe angle a small bit if I ran 35's with 3.75" BS. I get why 3.75" BS is common - it's a common wheel selection choice and it's a known quantity that people have run with success, so I have no issues with it. But due to the added scrub, I would add a tiny bit of extra toe, or at least put it towards the greater end of the factory toe angle spec.

Thanks for the numbers, and the recommendations!

Scrub radius is really just related to the wheel pivoting around the kingpin axis, so wider axles should not affect this. What would affect this would be the specs of the vehicle the axle came from, so you'd want to compare the scrub radius of the JK in stock form with stock JK wheels and see where that lands vs the TJ axles with TJ wheels, and then extrapolate and adjust as necessary.

What a wider axle affects more is ackermann angle, which is the perfect differing angles of the front wheels (aka the inner tire turning sharper) to complete different arcs as the vehicle travels through the turn. Ackermann is determined by taking the center point of the rear axle, drawing a line from there through each kingpin axis, and making sure the tie rod hole intersects that imaginary line. There is a solid chance a wider axle will not jive with that on a TJ, however, like anything else, the differences are minor. The tires may chirp a bit on turns or something but it's not going to be that big a deal. To put it in perspective, you have plenty of vehicles with different wheelbases all using the same knuckles, and that would also ruin ackermann. So really it's not that big of a deal.

I forgot about Ackerman. I get it won't really make a huge difference, but is there a "point of practicality" limit that should be considered at all?
I need to keep the tires covered where I live, so at some point the available width of fender will come to bear on this concept anyway.

In short, people probably stress about these characteristics and details a bit too much. The main thing you don't want is your scrub radius to be so bad that you lock the steering when you hit the brakes due to the wheels traveling in too much of an arc (from the added scrub). 35x12.5 on 15x8 3.75" BS wheels is a known option that will not suffere from that, so it is good enough.

For what it's worth, the factory positive scrub radius on my truck was calculated at 20mm on 7" wheels and 15mm on 7.5" wide wheels. I would expect other vehicles like Jeeps are similar, so you are really probably going from 20ish mm stock, to 45ish mm on 35's with 3.75 BS.

I can't speak to the pertinent details of the JK axle swap. Personally if I did those, I would want to run whatever wheels put the tires back under the vehicle to the same degree that the stock axles do with stock or aftermarket wheels. My best guess is that if you ran JK axles, JK wheels would be the closest bet to make things "right" on a TJ.

Thanks again!
I actually watched a 4wd IFS GM truck nearly literally pull the front suspension apart due to horrible massive offset wheels overstressing the front steering (and subsequently balljoints & CV shafts).
I thought it was something interesting to contemplate, and I suspected someone had probably already done at least a reasonable amount of the math. Appreciate your feedback.

 

[chainsaw]

Agree with the above. Having run some 1.25" spacers briefly on my TJ Rubicon, you don't really feel the scrub radius difference driving at high speeds. You'll notice it more at low speed maneuvering where it feels like more resistance coming off center.

On the plus side, the TJ track width is really narrow for modern vehicles and on worn asphalt it rides a bit odd in the troughs worn by trucks. The wider track width results in it tracking a bit more naturally.

My other car is a pretty small sports car with very staggered wheels (235 front and 305 rear) and it's far worse for this trough issue. I feel like I really get kicked around on them if they're worn deep. Chains are required in winter here and it really chews up the right lane so the troughs can easily be an inch or deeper by spring.

Thanks for the additional insights!
I'd almost guess Miata, but the 305 sounds a bit big....

 

[machoheadgames]

Second: I sounds like you either do race car stuff, or seriously sporty car stuff...

Lol I don’t really, just a matter of having studied the concepts. I’ve run a number of different wheel and tire combos on vehicles over the years and while I prefer stock wheels and close to stock size tires in almost every application, I have found that it takes a really poor setup to get to the point where it makes a noticeable difference.

I forgot about Ackerman. I get it won't really make a huge difference, but is there a "point of practicality" limit that should be considered at all?
I need to keep the tires covered where I live, so at some point the available width of fender will come to bear on this concept anyway.

If keeping under the fenders is the priority, I’d say you’re probably best off sticking with stock axles and running a relatively deep wheel and skinnier tire. 6” flares are a thing (but a bit undesirable IMO) which adds some wiggle room but you’ll probably want to stay around 4.5” BS to keep tires under that. Honestly I’d just focus on tire placement and not worry too much about Ackerman, scrub or toe. I don’t think you can screw up the scrub with JK axles because you’d need deeper than stock JK wheels or extra tall tires. 35’s on stock wheels probably wouldn’t even put JK stuff to negative scrub if I had to guess. I’d imagine your bigger issue will be keeping a 5” wider axle underneath the jeep with large tires, regardless of wheel. I don’t think the JK wheels will even pull that off since they’re not too much deeper than TJ stock wheels.

I actually watched a 4wd IFS GM truck nearly literally pull the front suspension apart due to horrible massive offset wheels overstressing the front steering (and subsequently balljoints & CV shafts).
I thought it was something interesting to contemplate, and I suspected someone had probably already done at least a reasonable amount of the math. Appreciate your feedback.

Lol, yeah positive scrub really likes to tug on tie rod ends…definitely no good for anything when the wheels are way out there like that.

 

[GoldenGorilla]

As I mentioned in my Dumpster fire thread, my TJU need rebuilds on both axles, and I happen to have a variety of bit's laying about that might be suitable "upgrades". Living north of the 49th parallel means I pay roughly 50% more for everything. I explained the situation better over there, but the shorter version is trying to avoid redundant spending whenever practical. I don't really think of them as an "upgrade" so much as a lateral move with some possible limited benefits.

The context is helpful. Those prices are looney. I'll see myself out

A fair number of people have done JK axle swaps on TJs. They're doing it here for other reasons, often LCG builds, but also because the steering setup is stronger and designed for crossover, not the inverted Y of earlier SFA Jeeps. Not sure what those guys do about track back alignment to avoid bump steer. The amount of work it takes to move the spring perches, put on the control arm mounts and everything, I can't see that being any cheaper at a shop than refurbing a TJ axle. If you can do the cutting and welding, maybe it makes sense. The axles are 5" wider total than TJ axles and there are 7" stock style fenders you can use which take care of the coverage. I think you already know the hubs on JKs are 5x5" while the TJs are 5x4.5" so you'll need different wheels, but not because of backspacing. The JK 2 door and TJ 2 door wheelbase is very close. I don't think Ackerman or scrub radius would cause any issue as Jeep designed them to work for both JK variants.

 

[Negreac]

There are a number of reasons someone would want to use wider axles than the OE 60.5" TJs.

1. More room for larger tires and outboarding suspension components
2. Wider track width may fit the type of wheeling you are doing better
3. There are more options available for JK outers including stronger knuckles, high steer crossover setups, locking hubs, brake upgrades, etc.

Obviously the JK 44 swap is common due to availability and some improved strength over factory TJ axles but there are many aftermarket axle providers selling TJ drop in axles with varying widths with JK Cs as an option as well. If you are building a TJ that currently has stock Dana 30/35 combo and are looking to buy once cry once pre-built D44s are a great option. And if you are doing that it is fair to ask if you should also go for wider WMS-WMS and/or JK Cs for the reasons mentioned above.

On scrub radius, axle width will not affect as mentioned before. The contributing factors are as follows:

1. Steering Axis Inclination (aka King Pin Inclination). This is set by the outer Cs and not adjustable. On a Dana 44 it is 12.5 degrees

2. Distance between Steering Axis and the WMS (Wheel Mounting Surface). Often This is also generally static since it is determined by the geometry of of the knuckles and the unit bearing assembly. As far as I am aware no aftermarket offers varying geometry here. The only exception would be if you are installing locking hubs, which depending on the kit can push out the WMS by up to 7/8".
3. Wheel offset. Given everything else remains constant a more positive wheel offset will pull the wheel in further to the vehicle and reduce the scrub radius
4. Tire Diameter. Given everything else remains constant a larger tire will reduce scrub radius.

Generally speaking ~1" of positive scrub radius is a good target and from my understanding aligned with what stock TJs and JKs came with in OE form.

Here is a great short video explaining Scrub Radius:

And another where they cover Scrub Radius briefly:

Here is also a diagram I made comparing OE TJ Rubicon Wheels/Tires to Hutchinson Rock Monsters with 37s that further shows the geometry. In this particular case the 0.89" reduction in offset (increased SR) along with the 6.5" larger tire diameter (reduced SR) yielded a net ~0.2" increase in SR. If you wanted to maintain the 1" SR on 37s you would want to find a -0.2" (~-6mm) offset wheel (on a 8.5" wheel that is 4.5" of backspacing). For a 35" the SR would be ~1.3" so ~-0.1" (~-3mm) offset (on a 8.5" wheel that is 4.75" backspacing). For other wheel widths offset remains the same but you would add 0.25" backspacing for every 0.5" of wheel width.

In theory a 35" tire on an 8" wheel with 3.75" (-18mm OS) backspacing would have almost 1.6" scrub radius. I think the reason why it is common is cause people sacrifice scrub radius for tire clearance. This is where a wider axle can help since it will give you the additional clearance without the need to sacrifice scrub radius.

 

[chainsaw]

Thanks!

One of the idea's I'm contemplating, is modifying the late 70's F250 Dana 44 I have in my shop. I haven't yet learned CAD, but
I might be able to rig up at least a rough idea of where things would land with what I have in mind. I'd probably either cut it down to 80's-ish waggoneer width, or aim for somewhere in the 62-63" range. Obviously waggy width would provide easier axle shaft availability. I might dig up Mr N's old site to try to figure out if there's a "mixed combo" of stock length inner shafts that could get me close to the 62-63" setup. I have 1/2 ton hubs I could use and that would accommodate the beadlocks I have too....

 

[Negreac]

One caveat on my post above. I don't believe all Dana 44 variants have a 12.5 degrees steering angle inclination. Best to check your specific axle before doing the calculations.

 

[Negreac]

Thanks!

One of the idea's I'm contemplating, is modifying the late 70's F250 Dana 44 I have in my shop. I haven't yet learned CAD, but
I might be able to rig up at least a rough idea of where things would land with what I have in mind. I'd probably either cut it down to 80's-ish waggoneer width, or aim for somewhere in the 62-63" range. Obviously waggy width would provide easier axle shaft availability. I might dig up Mr N's old site to try to figure out if there's a "mixed combo" of stock length inner shafts that could get me close to the 62-63" setup. I have 1/2 ton hubs I could use and that would accommodate the beadlocks I have too....

Mocking it up in CAD would be the best if you have the tools, the models and the skills. For most of us that isn't such an easy option.

 

[chainsaw]

Mocking it up in CAD would be the best if you have the tools, the models and the skills. For most of us that isn't such an easy option.

I'm one of those for whom it "isn't such an easy option". Unless you're referring to Cardboard Aided Design!