twin i beam geometry mystery

[mat in tn]

its pouring here and I am in the middle of a tilt column restoration for lefty. it reminded me of one issue that I have seen time and time again. my own trucks too. I have what I think is a fair understanding of the geometry of these trucks and quirks, whether two or four wheel drive. however, the question is more about camber alignment after backing up. every van I have had along with every truck I have had, fifteen that I can think of off the top of my head, all gained a lot of negative camber when backing up just to have it straiten back out by moving forward. I am used to it, and it poses no problems but occasionally I have had some one ask "what's wrong with it". just curious how others have understood this.

[FuzzFace2]

I dont know about you but when I am in the drivers seat I cant see the wheels
When it is set to spec its for going forward so when backing up it fights and dose that to the suspension.
Thats my story and I am sticking to it
Dave ----

[mat in tn]

if you can't see the wheels when in the driver's seat then obviously your wipers still work

[Pete Whitstone]

I'll have to look for this effect, I've certainly never noticed it before.

If anything, I would think the front tires would go to a more positive camber angle. The logic being, the camber angle is dependent on suspension extension, and when you back up and hit the brakes, the rear squats and the front rises. And the front probably doesn't completely settle until you move the truck again and start bouncing the suspension around.

[mat in tn]

I never hit the brakes hard enough to lift the front. but I might try it. oh wait, then I will dump the cargo.

[mat in tn]

one other aspect that adds to this is larger wheels and tires. the bigger they are, the more exaggerated it seems to be.

[RenoHuskerDu]

Most front suspension geometry has anti-dive built into (in the rear too, where it does the opposite). This turns part of your braking effort into a lift vector. The goal is to keep the front end from diving too much when you brake. If you take off the wheels and just sit and study where the various pivots and mount points are, you can imagine how the mount points take the twist that's generated when you brake and actually use it to lift the front a bit.

The reverse of all this happens when you brake going backwards. All the anti-dive that the engineers implemented works in reverse and serves to jack the front end up.  And TTB under extension goes into positive camber. Which is what you see.

The amount of anti-lift factored into the back of some cars is impressive. I remember working on my handbrake turns in an older Volvo. Using only the back brakes, I could feel the back of the car squat with the braking effect. I've read that fancy multi-link suspensions such as on spendy yurrupean sedans also have anti-squat for acceleration forces. I don't know that our pickups, with leaf springs, have such sophisticated rear suspension geometry to do all this.

Inquiring minds are still puzzled how you manage to see it, while driving. Wife, helper, mirrored fence or wall?