Alignment geometry

[StG58]

Trying to wrap my head around alignment geometry for the front of our TJ's. Right now it's making my head hurt more than anything.

Caster, Camber, Scrub Radius... What happens to the geometry when we lift, change tire and wheel sizes, wheel offset and pinion angle. What do we have to live with and what can we adjust / fix.

Messing about with this stuff effects drive-ability and safety.

Anyone want to try to explain this stuff to me (us)?

I can start out with what I think I know and go from there.

Toe in. The front of the wheels are set so that the fronts is slightly closer together that the back.1/16" to 1/8" of toe in is good, and we can set this in the drive way. Jerry has a good write up on this.

Caster. The steering axis angle in relation to the center of the tire...it would seem that changing the pinion angle would change this.

Camber. The lean of the tires in or out. It is what it is...

Scrub radius. The distance in or out of the center of the tire contact patch from the line where the kingpin angle intersects the road. It seems that tire size and wheel width / offset would effect this. Can we use this to compensate for lost caster? It seems that when the scrub radius gets extremly negative, the TJ wants to "wander" going down the road.

Pinion angle. The angle of the front pinion in relation to the driveshaft / transfer case. We know this matters for driveshaft type and alignment.

Edited to add: Then there's bump (roll) steer and roll center, though roll center seems to be a rear suspension geometry thing. Bump steer (both front wheels go up) doesn't seem to effect solid front axles though roll steer (one front wheel goes up and one goes down) does.

 

[Stinger]

I think you pretty well understand it. Are you looking for confirmation of that?

 

[Jerry Bransford]

Looks like you have most of that down really well, I will only add a comment on bump steer. Solid axles are indeed affected by bump steer when the steering geometry is incorrect.. like if a dropped PItman arm has been erroneously installed when the driver's side track bar mount has not also been dropped a corresponding amount by an aftermarket dropped mounting bracket. The Pitman arm and driver's side track bar mount must match. Both must either be non-dropped as the factory installed them, or both must be dropped via aftermarket parts. Mixing a dropped and non-dropped Pitman arm or track bar mount together is a recipe for bump steer.

The easiest definition of bump steer I know is when the steering is forced left/right in response to the front axle moving up/down over a bump or dip in the road. Certainly the TJ can become a victim of bump steer if the steering geometry has been jacked up by throwing bad choices of steering parts willy-nilly into the fray.

 

[StG58]

Actually, I'm looking for how all this interrelates and what we can do and what we shouldn't do. Kind of put all of it into one big ball of wax, so to speak.

The thing that lead me down this particular rabbit hole was scrub radius. When you go to taller tires, that moves the steering axis out. If you go with less back spacing, that moves the center of the contact patch out. So far so good. But how far is too far for a particular tire diameter? 3.5 backspacing vs. let's say 4.5" backspacing vs. the stock 5.5" backspacing. I don't have enough hard numbers to calculate that. AND I don't know at what point it becomes an issue. Everything that I've read says that any tire 33" in diameter and under is fine with stock geometry, but no mention is made of what the backspacing is for that "all is fine" call.

I know for a fact that putting to wide a tire on to wide a rim with a lot less backspacing will cause a vehicle to want to wander. That's a really bad thing in a short, wide rig like a TJ. The Cj's had this problem starting at about 33x12.50's on 10" rims (don't remember the backspacing, but it wasn't much of a difference). They would throw you all over the road in certain circumstances.

 

[xenocryst]

Holy crap! Quality post! I knew alignment geometry was important, but I've never seen it laid out like this. Not everyone here knows how to tinker on their Jeep (yet!), and this kind of info is great. Thanks!

The illustrations really help. Where did they come from?

 

[billiebob]

A bit more on the bump steer. Our Wranglers come with that Y rod steering geometry which actually reduces the bump steer effect since both tires toe outwards on a big bump. They do it equally and together balancing the bump steer effect. A common change is to a one piece tie rod which now means both tires toe the same way on a big bump. Especially if the Jeep is lifted without a drop pitman arm to run parallel to the one piece tie rod.

RE the tall tire, less backspace dilemma.... To an extent by going taller and reducing backspace you might keep the scrub radius close to stock. But at the extremes of 10" rims and 3" backspace with low profile wide street treads the results are probably not what was planned. Most of the mods for Wranglers are pretty well thought out. But going beyond 33s on 8"rims definitely requires research to keep the handling close to stock. Going to 35s, 37s, means the "stock" handling is gonna change.... more. And even the best research and design can result in surprises. But if you are doing it for specific off road goals? As long as it is still comfortable on the street those surprises are probably a non issue.

You can pretty much do anything you want if you can keep all the criteria in the pictures on the first post close to stock. If you do that and go to 35s, 37s the BIG change will be an increase in unsprung weight and the ability of springs/shocks controlling that mass increase on such a small, light vehicle.