4 bar rearend trail

[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.