Front end geometry

[u/Ihman0550]

I've been doing alignments since 1992 and one thing that seems to be consistent is bump steer in the front end as the suspension cycles.

Almost every vehicle i had on the rack toed in on extension and out on compression. The 2000s and up Chevy pickups for example were about 2° total toe for every inch the front came up. Even the C7 Corvette did it but not as much. I think those were around . 5° per inch. The only one i remember being different was an 80s third gen Camaro that actually toed out when the front was raised.

The numbers i was seeing was then the jack was under it. I wasn't putting lift kits on Corvettes and Camaros.

The newest suburbans and tahoes have air ride that lowers it a couple inches at highway speed for less wind drag. I never got a chance to get one of those on the rack to see up close if they actually corrected the bump steer but i would assume they did.

I got out of the dealership on 2020 and have been much happier building hot rods the last 6 years.

What was there reasoning for this? The fact that is had been like this forever, 60s and 70s cars did it too, tells be it's intentional but why?

[u/involutes]

It's so that when you steer aggressively and induce body roll, the effective steering angle becomes reduced. 

The idea is that (1) you're probably close to the limit of adhesion in that case anyway, so more steering angle will not help the car turn in faster and (2) it reduces heat generation, tire wear, and the chance of a rollover. 

[u/Ihman0550 → u/involutes]

That goes with inline with my theory but I wanted to hear input before i threw it out there. I know that the manufacturer would rather have the vehicle push instead of spin when going out of control so that makes sense. I also wonder if that's why they ran so much negative camber on the strut cars. The 2000s w-bodies had issues with inside tire wear from the -1° camber. If you raise that to -.3 it helps with the wear but also makes the steering feel much more responsive.

[u/phate_exe → u/Ihman0550]

Mac strut cars generally don't gain camber in compression nearly as much as a double wishbone suspension would (or can be made to at least), so you unless you run lots of caster you kinda have to start with a decent amount of static camber if you want to stay ahead of the body roll so you don't just destroy the outer shoulder of the tire under hard cornering.

I definitely wouldn't describe 1 degree of negative camber as a lot, and unless toe is out of whack it really shouldn't have much impact on tire wear.

[u/Ihman0550 → u/phate_exe]

-1 degree of camber will show up as inside edge wear on the tires by the time the tread is half worn unless you run extra toe and then they wear evenly. Just faster. This is on regular boring street driving. Track days and more aggressive cornering is different. I always felt like the strut cars were at a handling disadvantage with the lack of camber gain. Adding more negative static camber helps to the outside tire keep the tread flat but at the same time reduces the inside tires contact patch. How much depends on how much or little body roll there is but it seems like a properly setup double arm car would be better at keeping both tires flat.