Question About Negative Camber: More Negative Camber == More Grip at Corner?

[ZDan]

Originally Posted by Draco-REX

I'm having trouble picturing how a MacPherson strut design can be gaining Negative camber after the LCA passes horizontal. Once above horizontal, the LCA is effectively shorter so it should be pulling the strut inwards, tilting it towards positive. And as the strut shortens, this should increase in rate.

This would be true only if the line of action of the strut were vertical. It's not, it is canted inward at ~12 degrees or so. The wheel will gain camber until the LCA is perpendicular to the line of action of the strut, about 12 degrees beyond LCA-horizontal.

[Draco-REX]

Originally Posted by Dimman

I drew this a while back while ZDan was helping to explain why a MacStrut isn't exactly a 1:1 motion ratio, but it also shows camber effects.




The discussion is in the 'Ask RCE and CSG Mike suspension question tag-team' thread.

Originally Posted by 7thgear

here ya go... a buddy of mine made this a long time ago.

Originally Posted by ZDan

This would be true only if the line of action of the strut were vertical. It's not, it is canted inward at ~12 degrees or so. The wheel will gain camber until the LCA is perpendicular to the line of action of the strut, about 12 degrees beyond LCA-horizontal.

Thanks guys, that makes a lot more sense to me. I was missing the perpendicular to the strut angle thing..


Hmm... Need to do some thinking.

[7thgear]

ALso, here is a MacPherson design used in a Lotus racing car.

I can guarantee you that under compression these thigns gain an ample amount of camber

i'd say pre-aero days this would have been a fantastic design.

-

[autobrz]

I only drive around on the street but even then I can appreciate a better setup suspension than stock. optimal static camber for maximum lateral grip level of the stock tires; more front roll stiffness from springs and less from sway bars will make steering response more linear and feel more supple. Having more bump travel in the rear will make taking bumpy corners so much more confidence inspiring. I've had quite a few instances on the street where the rear momentarily lost traction in the middle of a corner when hitting a bump, not to mention when the rear hits the bumpstops full-on, the chassis bounces off the bump stops and the whole car wiggles left to right.

there is so much I can improve on the stock setup that will make the car better without upsetting the balance and I only drive on the street... I see most bad setups start at very low ride height with not stiff enough spring rates or the use of too big sway bars...

[ayau]

Originally Posted by 7thgear

here ya go... a buddy of mine made this a long time ago.

Excellent gif. It took me a second to realize what I was looking at. I think the pause in the animation threw me off a little because it looks like I'm looking at the rebound effect rather than the compression effect.

Based on this animation, one can infer that macpherson struts gain negative camber under rebound when the LCA and strut are past the perpendicular point. Not really useful when you're cornering since you want the outside wheel to gain negative camber and not the inside wheel.

[ayau]

So all else equal, if you lower the front of the car, you'd want to increase spring rate to minimize the LCA and the strut from being perpendicular under compression.

Assuming @ZDan's suspension model is accurate, the front keeps gaining negative camber up until 4'' of compression. However, that doesn't take into account body roll, which changes the angle of the strut. That 4'' of bump travel is actually less when you take into account body roll, right?

Then you also have to take into account caster, which can change the amount of camber when cornering. More caster equals more negative camber under bump. I'm sure there are other variables as well.

Correct me if I'm wrong, but it appears you want the front to be as stiff as possible when tuning macpherson struts (ignoring all other factors). From a pure performance standpoint (ignoring cost, packaging, etc), double wishbone appears to be the superior design. I guess that's why Formula One still uses it.

[7thgear]

Originally Posted by ayau

So all else equal, if you lower the front of the car, you'd want to increase spring rate to minimize the LCA and the strut from being perpendicular under compression.

no, if you lower the car, the LCA angle will change period, upping the spring rate will do nothing for that because it is a function of mechanical geometry.

Originally Posted by ayau

double wishbone appears to be the superior design. I guess that's why Formula One still uses it.

F1 uses the design because it packages better

but otherwise, F1 suspension design laughs and farts and throws beer bottles in the general director of production vehicle suspension theory.

so you should NEVER draw any conclusions of F1 design and production cars. If you want to immitate a racing body.. watch what the rally guys do.

[ayau]

So by adding camber plates and adding negative camber, you're essentially changing the angle of the strut. This means you increased the negative camber bump travel to greater than 4", right? Let's assume zdan's model is accurate.

[7thgear]

Originally Posted by ayau

So by adding camber plates and adding negative camber, you're essentially changing the angle of the strut.

yes, but do it too much and wonky things start happening during steering

[Racecomp Engineering]

There's some debate about whether it's better to add camber at the topmounts (camber plates) or at the lower strut mounting point (camber bolts). Using plates pushed in gives you a higher roll center and can improve camber curve. However, it also increases the SAI which has a negative effect on camber when you turn the steering wheel.

- Andy

[PadSprouts]

Wow, this thread was very informative and exactly what I needed!

So since FR-S engineers added less negative camber to the front, this helps break out of a slide easier? (harder to oversteer?)