the 2012 steering angle, roll center, ackermann and more thread

[swim]

Thought it'd be interesting to talk suspension geometry, since it's getting to be that time where everyone sets their cars up. I know a LOT more people are going to be running cut knuckles and doing other geometry mods this year. Could be nice to get an idea of what we'll be seeing this season, and sharing thoughts/experience could help everybody with their own setups.


I'll go first

Currently: zero toe, uneven and unknown caster, ~1* camber. haven't measured angle, but it's shallow. Decent contact at lock, though.



soon as I pull my motor, I'm planning on cut knuckles, extended LCA's, different steering rack, and different inner and outer tie rods. I know it will get it competitive steering angle, and it may end up a little lower, but I have NO IDEA what will happen to the ackermann, kingpin axis, or roll center. I'll post up results whenever that happens.

[swim]

if you're not familiar with ackermann, it's geometry designed to reduce scrub while parking...



basically, the more ackermann you have, the more toe-out at full lock you'll have. it helps with parking, but in the middle of holding a drift it means that the following wheel is at a less-than-optimal angle, which scrubs speed. the closer to zero ackermann your steering is, the faster you will go at a given drift angle, but ackermann also contributes to stability at speed, and helps the steering return to center. you're not supposed to drive cars with zero-ackermann knuckles on the street, due to safety considerations at highway speeds.

you know how the steering arm on your knuckle curves in a little bit? if you curve it in more, you increase ackermann, and if you make it straighter, you reduce it. if you've chopped your knuckles down, you've probably altered this geometry, unintentionally or not.

[swim]

camber at lock is another big thing... steering axis, caster, and static camber all play into it. at 55* of lock, shitty geometry can multiply into a ton of positive camber.

flat contact:




excessive positive camber at lock:

[kfidelity]

You don't have kingpins

[swim]

You don't have kingpins

sure I do. just, instead of being a physical part of the suspension, it's an imaginary line drawn from the ball joint pivot to the strut bearing.

[Idriftjunk]

Great topic. I'll have to get some pics up tomorrow of my set up and how I got there.

[chicoson]

good shit! sticky this moe foe!

[BENAE86]

Warning nerd content, for real though I'm excited to keep seeing what's posted here. Ill post up when my shit changes

[swim]

ROLL CENTER



The roll center is the point at which roll movement gets transferred into the body... If you think of the car as a point mass at it's center of gravity, the roll center is the point at which the roll force acts on that body. So,with a longer distance (roll couple) (lever arm) between the center of gravity and roll center, it creates more torque, and causes the car to roll more.

This is why people cry about lowering your car too much. People blow this shit out of proportion all the time. Still, it is an important effect, and you will certainly have more grip if you address it. Remember, the front and rear suspension have their own roll centers independent of each other, and you should correct them together, to keep the roll axis (imaginary line between the front and rear centers, the axis the car rolls about) tipped forward. If you slam your car, then mod the knuckles for roll center correction, but do nothing in the rear, your roll axis will be tipped back, the car will transfer weight to the rear with roll, which increases understeer.

How do you tell what your roll center is? Easiest way to get an idea is looking at your lower control arms. If you're a man about lowering your car, your FLCA's probably point up at static ride height. This means your roll center is somewhere underground. Get your FLCA's closer to horizonal, or even pointing down like they would if the car was still stock ride height, and your roll center will be better.

How do you fix it? It's one of the most involved things to adjust. Either, you cut the knuckle and weld in more material between the spindle and balljoint mounting point (I've seen people weld solid steel blocks in to make the knuckle 1" or more taller than stock), or get a balljoint with a longer shank. Stock-style extended balljoints are typically expensive, and beware, some of them don't do shit. The roll center is corrected by moving the spindle UP relative to the balljoint PIVOT POINT, so you need a balljoint with an extended shank. Megan and some other bullshit companies make balljoints with an extended lower section, which moves the FLCA DOWN relative to the balljoint pivot point, which doesn't do anything. Best thing to do is get the Parts Shop MAX FLCA and tension rod combo with a pillowball instead of a normal ball joint. They sell different length shanks to go in their pillowball to adjust roll center on this setup. Also, people have modded ebay pillowball FLCA's for roll center correction with good results, but even that's a substantial DIY project, with all the shit you have to fix on the ebay parts.

[Idriftjunk]

So I swapped my car to rack and pinion, redesigned the steering knuckles, built in Autodesk then cnc'd. I got my ackerman by drawing a line from the center of the rear axle, through the balljoint. On this axis is where your steering tie rod should be. The distance to the ball joint will give you more or less steering angle/ faster slower ratio. Unfortunatly I'm stuck using what my car has for other geometry. Not the final setup though, as my car will never be complete, eventually going to build tubular control arms and get the width out a little more. I'm going in for a proper alignment next week and can give actual specs, I ran all last year aligned with a tape measure. Thickness also acts as a spacer to give me flat lca's when dropped. http://www.youtube.com/watch?v=iTq4T...1&feature=plcp

[swim]

it's weird to see a front-steer car with a tension rod... does that beefy custom tie rod outer ever smack the tension rod or swaybar on the following wheel?

[kfidelity]

it's weird to see a front-steer car with a tension rod...

S13s have them, found that out after I broke one.

[OMG Ninja]

S13s have a rearward steering rack.

[Idriftjunk]

it's weird to see a front-steer car with a tension rod... does that beefy custom tie rod outer ever smack the tension rod or swaybar on the following wheel?

Everything clears, the only thing that hits is my rim on the tension rod. I cant wait to change the whole set up. Fd rx7s and a lot of bmws are front steer systems also.

[swim]

Everything clears, the only thing that hits is my rim on the tension rod. I cant wait to change the whole set up. Fd rx7s and a lot of bmws are front steer systems also.

Right but FD's have a wishbone LCA instead of a tension rod I thought... not that it matters, if mitsu managed to package all that up there with no rubbing at lock or under bump then more power to them.

[Idriftjunk]

Yes your right. My car was a rear steer system, thats why i had to make new knuckles. Because of how far back i put the engine, i had very few options. Im going to try to do fd brakes suspension etc, and see if i can make it work. Same bolt pattern, more brake options, lighter components.

[OMG Ninja]

^ and MORE JDMNESS!!!1!!!!111!!

[onijake]

I'm really interested in suspension settings more so this year than any other year. When running against similiar powered cars last year at MDU events and the ECB, proper suspension settings became more apparent.

It seems the going trend is to push the roll center to the rear of the car under load. Coupled with toe-in and zero camber under compression, grip in the rear is exagerrated. Cars with a low torque monster power-house, such as a v-8, and suspension settings such as this, will pull out of a dig faster. In competition settings, this is huge because it increases the gap from the trailing car. Thus, the trailing car has less impact on the tandem.

When roll center is pushed to the back of the car, the front of the car becomes unstable. At least, from my experience in gold car. One of the ways I'm countering that is by following threads such as this. I'm now running a low caster, 3.5* (compared to 7.5* stock). 2* of toe out, and 1.5* of negative camber in the front. High front damper settings and roll bar stiffness adds to keeping the car balanced during transitions. If drifting the proper way, the increased grip on the rear means you can throw it into the initiation faster with more sustainable angle.

Right?

[240 GTR]

Do you mean 11* of caster? As in you're pulling the wheel farther forward than the stock setting?

Yes, you are right, but to a point. You need to find a good balance between having enough grip that you can do sick entries, while keeping the car loose enough that your angle isn't looking inconsistent through the rest of the course. I had that problem at the beginning of last season- I set the car up with a good amount of rear toe-in and had big, really sticky tires. The car was a rocket ship mid drift when you got on the power. Once you had to start modulating the throttle, the grip would become overwhelming and cause the car to lose angle quickly. It was a back and forth juggling act of clutch kick-modulate-clutch kick-modulate. I had a judge (and well established FD driver) tell me that I should loosen the car up a bit since my angle was noticeably inconsistent. Sure enough, he was right.

If you're worried about following a v8 car in tandem, start using left foot braking instead of pulling the ebrake if you get too close. It takes some getting used to, but it will definitely help you keep up with a higher powered car.

Some rules of thumb:
Increase rear toe-in=increased side (+alittle forward) bite
Decrease rear tire pressure=increased forward bite

[onijake]

no, I mean 3.5* of caster, as in 3.5* of caster.

edit: to achieve this, I beat the hell out of my fender well.