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AgitatedPancake
05-04-2006, 04:03 PM
I am one of those guys that doesnt know alot about one thing, but knows alittle bit about everything. As im sure most of you know with the stock wheels and tires, the kingpin angle points to the ground directly in the center of the tread of the tire you are running. That makes it so when you turn the wheels they pivot perfectly on that center point. That is the perfect position and has minimal stress on steering components. When you go to a larger tire on the stock rim, the kingpin angle stays the same but higher up so it lands further out. It would probably contact near the outside edge of the tire, or further. This makes it so when you turn your steering wheel you are literally dragging/sliding the tire across the ground instead of just pivoting, because that point is not centered. My question is, for say a 33" tire what is the optimal backspacing on a rim to bring the ground contact point of the kingpin angle back towards teh center of the tire, as to relieve unneeded stress on the steering components? If you dont want to actually do the math, could you possibly tell me what the WJ's kingpin angle is so I could do all of the calculations for myself? Thanks, Greg.

PS This is not necessarily going to be implimented just something I want to figure out.

LouisianaZJ
05-04-2006, 04:26 PM
just run whatever backspacing clears the tires....
any additional stress is going to be minimal compared to the stress created by the heavier tires

doyll
05-04-2006, 04:48 PM
The farther out the wheel and tire are, the more strain is put on the ball joints, spindle and wheel bearing. Bigger tires still center on stock wheels and do not change the centerline at all. Everything is exactly in same place it was with smaller tire. The reason for less backspacing is the need for more clearance for tire in inner fenderwell, frame, control arms, etc. Running less backspacing increases the load on everything and is only compounded by bigger heavier tires.

That being said, I'm thinking of getting less backspacing on my wheels too. ;)

AgitatedPancake
05-04-2006, 05:00 PM
I know wheels with less backspacing are used for clearance, this is just one of those things that I was thinking about. Doyll, the kingpin angle stays the same but with the hub further off the ground, it contacts the ground further out, so it does effectively change the center. Like I said im not going to go to diff backspaced wheels because of this, just thinking out loud.

J B
05-04-2006, 05:09 PM
The farther out the wheel and tire are, the more strain is put on the ball joints, spindle and wheel bearing. Bigger tires still center on stock wheels and do not change the centerline at all. Everything is exactly in same place it was with smaller tire. The reason for less backspacing is the need for more clearance for tire in inner fenderwell, frame, control arms, etc. Running less backspacing increases the load on everything and is only compounded by bigger heavier tires.

That being said, I'm thinking of getting less backspacing on my wheels too. ;)

You're misunderstanding what he's saying.

The concept he's describing is called the scrub radius. The scrub radius is defined as the distance to the left or right between the centerline of the tire and the point where an imaginary line drawn through the kingpin axis hits the pavement. If the scrub radius is zero, that's the best case because the steering effort is minimized: the tire simply turns on its centerline axis (notwithsanding the effects of caster, which you're going to have no matter what).

Minimizing the scrub radius is the whole reason for the upper ball joint being inboard of the lower one.

The tire has to rotate about the point where the kingpin axis hits the ground, so if the scrub radius is anything but zero, you're actually swinging the whole tire around that point instead of simply turning it on an axis. That takes much more steering effort.

if you keep the same wheel, the scrub radius changes if you get a bigger tire. The tire centerline is in the same place, but the kingpin axis hits the ground at a spot more outboard than when the tire was stock. To keep the scrub radius at zero, you use a wheel with less backspacing to move the tire outboard and shrink the scrub radius.

All of that said, I don't know anyone who has ever even looked at this before. I'm going to consider it in my D44 buildup, but it'll be a compromise between the scrub radius and the other backspacing concerns that we normally have to think about.

AgitatedPancake
05-04-2006, 05:17 PM
Thanks for hte info J B. If anyone could get me the kingpin angle id be glad to do the calculations for the stock setup vs a lifted setup and what you would need.

doyll
05-04-2006, 06:01 PM
Okay J B got that.. Sorry for the missunderstanding AgitatedPancake.

Okay AgiPan, (whole name is too big ;)) I tried my FSM and it has nothing for kingpin angle but the caster spec. Nothing for SIA... Guess that be it. It would be interesting to see how much difference and what it's effect would be.

Now I'm curious ... Is the kinpin SIA different on same model axle for different tire fitment? Like a D44 running 235/75/15s vs one running 265/75/16s.. I have modified the SIA on cars by moving the strut in at top (camber) and then adjusting the spindle to bring wheel/tire camber back to change the footprint of tire on the track. Getting everything to work together can be a real PITA.... Trying to get the best track feel, handling and traction..

J B
05-04-2006, 09:17 PM
Now I'm curious ... Is the kinpin SIA different on same model axle for different tire fitment? Like a D44 running 235/75/15s vs one running 265/75/16s..

I seriously doubt it. It won't be THAT noticeable with relatively small tire diameter changes. Certainly not enough to warrant different inner Cs for the same axle.

My guess is that any major difference in factory tire sizes might be accomodated by differently backspaced wheels, but that's just a guess. But there is rarely any major difference in tire sizes.

sailbrooks
05-05-2006, 12:47 AM
So Agipan..... you want to calculate the difference B/t scrub radius of a lifted WJ and stock one.......Why don't you just go pull your hub, shaft and ball joints and measure the angle relative to vertical. Sounds like too much work for me. But from there you could measure the scrub radius at various lift heights and derive your force results from that. Keep in mind that if you are doing any kind of torque or force calculation you are going to have to factor the differences of the suface area and friction coefficients of whatever tire models you choose. You have fun with that........I finished my last final yesterday. :drinkers:

AgitatedPancake
05-05-2006, 01:47 AM
So Agipan..... you want to calculate the difference B/t scrub radius of a lifted WJ and stock one.......Why don't you just go pull your hub, shaft and ball joints and measure the angle relative to vertical. Sounds like too much work for me. But from there you could measure the scrub radius at various lift heights and derive your force results from that. Keep in mind that if you are doing any kind of torque or force calculation you are going to have to factor the differences of the suface area and friction coefficients of whatever tire models you choose. You have fun with that........I finished my last final yesterday. :drinkers:
My calculations at how far off (in distance) the tire was off from the correct position hahaha. Force, and all that can go @#*(&$@(*&*(. I've read serious stuff about suspensions, force sucks and is WAY above my head.

*EDIT*

You guys can call me AP, its my abbreviation alot of places lol.

BlaineWasHere
05-05-2006, 09:20 AM
Not to hi-jcak this thread but I am going to anyway. I understand that less back spacing puts more strain on everything. The question I'm asking is HOW much more strain. I have 3.75" of BS? am I going to ware everything out super fast and not make it worth my time to run that BS?

J B
05-05-2006, 10:30 AM
Not to hi-jcak this thread but I am going to anyway. I understand that less back spacing puts more strain on everything. The question I'm asking is HOW much more strain. I have 3.75" of BS? am I going to ware everything out super fast and not make it worth my time to run that BS?

Less backspacing is harder on bearings and balljoints. You'll likely see it more with the hub bearings than anything else. Those things suck enough on a stock vehicle.