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Joined: Wed Nov 07, 2018 4:35 am
Posts: 69
Vehicle: 87 Samurai

Post Posted: Wed Oct 09, 2019 12:56 am 
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Been doing some reading on Alignment specs, and how a vehicle with modded suspension can sometimes require different specs than the same vehicle that is unmolested and still has the original tire size. There is much alignment info and forum-chat out there for roadsters, race cars and such, and even some for [newer ] Jeeps and trucks, but very little for our Sj13 Samurai's. So......I'm trying to port the general theories of such, over to our zuk application.

Of interest, is the complex interplay between caster, tire diameter, mechanical-trail, camber, scrub radius and relative camber changes when making turns [ on road ], and how it effects the handling characteristics of our Samurai.

Tire diameter, caster angle [ and thus trail ], toe in / out, and scrub radius to a degree are all adjustable with swapping parts or adding wheel spacers or wheels with different widths and offsets. The Steering Axis Inclination [ SAI ] is not easily adjustable on the Samurai...…..having a solid axle, steering knuckles and king pins.

So my questions relate to, why we alter the caster angle after making a suspension mod, change to ride height, changing tire diameter, adding suspension up-travel, track width, etc, etc. What exactly are we wanting to achieve with alignment spec changes, post mods? In my mind, are we wanting to "get back to" or maintain the good road handling characteristics that may have been lost with said mods. This means.....no death wobble, correct return-to-center steering, appropriate negative / positive camber for cornering, correct toe in / out for straight-line stability and yet have decent "turn in" characteristics without wandering, light but not wandering steering, etc.

When we mod, we often go to a larger tire diameter. This mod alone changes the trail alignment spec, making it longer. For example.....going from a 205 / 75 / 15 tire to a 31" diameter tire almost doubles the trail length spec. Does this change alone, noticeably affect the handling / steering as discussed? Adding positive caster does the same thing....increase the trail length spec.

What is the significance of "trail"? Well, from what I understand, not enough trail makes for flighty, light steering that has a propensity for steering wander. Too much trail can initiate "death wobble" oscillations, and magnify any other component deficits that lean toward causing death wobble. Correct amount of trail will provide stable steering, good return to center, without causing death wobble or overly heavy steering. In theory anyway.

So what is the "correct" amount of trail for our Samurai's? Is it the amount that was originally engineered into the car, with a caster spec of 3.5 deg positive, and 205 / 75 / 15 tires?
By my calculations, oem trail is about .86 inches. Not sure if this is correct or not tho. I have not found the actual trail spec published, but rather had to use a calculator to try to figure it out;

https://www.rbracing-rsr.com/rakeandtrail.html

If trail spec is changed with tire diameter and caster angle......then the longer trail [ from larger tires ] can be brought back into spec by reducing caster angle.
IS this the whole point to a "correct" caster angle? to bring the trail into spec?

This is where it gets complex.....the interplay between caster angle, camber pos / neg in turns, scrub radius being a "magnifier" of characteristics, suspension up travel and how camber changes with compression, etc.

What are your thots on Trail spec, your caster angle, tire diameter and why you run the specs / settings that you do?

This is an interesting link showing the different specs between different cars.
http://www.car-engineer.com/suspension- ... -behavior/

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Joined: Thu Feb 26, 2009 11:00 pm
Posts: 11193
Location: Melbourne

Post Posted: Wed Oct 09, 2019 8:00 am 
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Suzukis have inadequate caster because they were designed with manual steering- less caster = lighter steering at low speed.
When converting to power steering adding more caster is desirable.

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Joined: Wed Nov 07, 2018 4:35 am
Posts: 69
Vehicle: 87 Samurai

Post Posted: Thu Oct 10, 2019 1:48 am 
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Gwagensteve wrote:
Suzukis have inadequate caster because they were designed with manual steering- less caster = lighter steering at low speed.
When converting to power steering adding more caster is desirable.



Yes, I've read that in other forums from many other guys, and it holds true for many other older cars and trucks, not just our zuks.

Still looking for your thots.....
Do we adjust caster angle to achieve a desirable trail spec, or, is mechanical trail just a product of setting the caster angle?
I'm not considering caster angle as how it relates to pinon angle and driveshaft angle.....this topic is about steering and handling.

Caster talk is a funny topic...…… ask 10 different average wheeler enthusiasts what they set their 4x4's caster angle at, and you'll get 10 different answers, but nobody seems to know or divulge their rationale to back up their angle.

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