4 bar rearend trail

[25jrjr]

Refer to the caster section at the bottom of the page. I would like to hear an explanation how positive caster doesn't lengthen wheelbase.


http://www.car-engineer.com/suspensi...icle-behavior/

[rsawyers9]

On a dirt modified, you cannot adjust the lower control arm. So to increase positive caster, you have to adjust the upper balljoint toward the rear of the car, effectively shortening the wheelbase by a small amount.

[25jrjr]

On a dirt modified, you cannot adjust the lower control arm. So to increase positive caster, you have to adjust the upper balljoint toward the rear of the car, effectively shortening the wheelbase by a small amount.

Since the upper and lower balljoints are connected by the spindle, if you tip the top back how can the bottom not move forward a slight amount.

What about Ford style lowers?

[JustAddDirt]

25jrjr
that link you posted, in the diagram, you need to look at which way forward is ( it is listed in black to the left of the illustration). I think you are confused.

also read what you posted above., and think about it. the spindle is 2-3" above the centerline of ball joint, so if you move the upper ball joint rearward the spindle snout goes back as well (shortening wheelbase). If the lower a arm never moves, how can anything tip forward?



I work with SAI (Steering Axis Inclination) angles daily, and build the computer software that calculates it if you vehicle has been in a collision.
I know what I am talking about here.

[RDBracer]

positive caster moves spindle back this is true but the contact patch of the tire moves forward since the piviting point is the lower ball joint.

[25jrjr]

positive caster moves spindle back this is true but the contact patch of the tire moves forward since the piviting point is the lower ball joint.

yep, always been my thinking too!

[dave41]

haha, positive caster makes the wheel base shorter.

your way of checking each side is a complete waste of time, all your measuring is the wheel base you can do that wheel to wheel with a tape measure, that side to side measurement has nothing to do with a rearend and lead or trail.

if you are on here I don't see your post

[JustAddDirt]

no, the tire contact patch is directly under spindle pin, but the steering axis moves forward of tire contact patch, as positive castor is increased.

[25jrjr]

no, the tire contact patch is directly under spindle pin, but the steering axis moves forward of tire contact patch, as positive castor is increased.

Can you post a diagram? Im from Missouri!

[billetbirdcage]

Can you post a diagram? Im from Missouri!

[JustAddDirt]

thanks billet, I tried to post, but got blocked by work security police.

[25jrjr]

[QUOTE=JustAddDirt;2110978 If the lower a arm never moves, how can anything tip forward?

So if it doesn't move forward or backward, why does the wheelbase change?

[JustAddDirt]

That was stated that way because the lower a arm has no length adjustment.once bolted in it cannot be adjusted.
When more castor is needed the upper ball joint / a-arm is used for the adjustment and it is adjusted rearward for the increase in castor, thus shortens wheelbase.
Look at diagram and were it says + castor. Picture that going rearward, and the spindle pin has to go rearward. As well. The more castor. The more it will shorten wheelbase.

[25jrjr]

I've always considered the steering axis as the contact patch.Its pretty negligible with lower pin heights isn't it?

Its true....you can teach an old dog new tricks! Thank you sir!

[Lizardracing]

According to the diagram above. If you were to move the upper BJ back on the horizontal plane (or less than the BJ to pin radius) the pin will move back but if you could move the BJ along the radius of the pin it stays put. So the real answer is the wheel base may or may not change as it depends on where the BJ is moved to.