caster geometry and towing problems

[brider]

I read a layman's explanation somewhere, but I'll be darned if I can find it now.

What is the physical reason that an out-of-spec caster front axle will snap the wheels to one side when trying to flat-tow?

I understand the concept of "trail", the distance between where a vertical line thru the axle touches the ground, and a line thru the steering knuckle centerline touches the ground. When you add extended shackles with no caster shims, this distance decreases, but I can't physically picture how this encourages the front wheels to want to track to the side.

 

[e rock]

You just got part of it backwards.

The "line through the steering knuckle" (rotational axis) actually hits the ground in front of the vertical line in stock configuration. This means it has positive caster. Think about a shopping cart. The line through the rotational axis is way in front of a vertical line through the wheel.

When you add longer shackles in the front, it effectively tips the axle housing down, which reduces the "trail" as you called it. Once the caster goes negative, the contact of the tire on the ground is no longer forcing the tires to remain straight, and will tend to amplify any sideways directional input - going around a corner, hitting a rut, etc.

Hope that helps,
Eric

 

[zuluzeugma]

I like to think of it in bycycle/motorcycles terms. If you ride a bike normally, it tends to go in a straight line. However, if you try and peddle the bike backward, it has almost no stability. As a general rule of thumb: more positive caster the straighter you will go. more negative caster, the eaiser it is to steer.

 

[Downey]

All of the above is correct, and put into even simpler terms:
(a) When the pinion shaft is turned downward (angled towards ground), the vehicle has a tendency to track in a straight line. This is what makes your steering wheel return to center when coming out of a turn.
(b) When your pinion shaft is turned up (i.e. longer shackles on the front end of the front spring), the vehicle has a tendency to dart from lane to lane, and you are constantly steering back into your own lane. This is the situation that would not let your vehicle track properly when being towed!!!

 

[brider]

I think I get it now...

Thanks!

 

[74fjsemistock]

So I should drive in reverse?My junk walks with a 4" su lift and I haven't even put on the 1 1/2 " shackles.Maybe it's the 33x 12.50'sHijack over.

 

[zuluzeugma]

So I should drive in reverse?My junk walks with a 4" su lift and I haven't even put on the 1 1/2 " shackles.Maybe it's the 33x 12.50'sHijack over.

No, you should shim the axle or do a C&T