Handling Write-Up, Suspension Tuning, Aerodynamics and other blurbs Options

[JesterDC]

Write-up on Body-Roll, Suspension Tuning and other randomness
I think it's all covered-ish

At special request from jdg371 in my first thread, located here -> http://www.6gc.net/forums/index.php?showtopic=38530 , I bring you (whether you’re in the SCCA or AutoX) a semi-brief-ish, semi-in-depth-ish(no equations or too much physics jargon) explanation of (drum roll please datdatdatdatdat....) suspensions, suspension tuning, body roll and other things that do stuff that affect do-hickeys and such.

What sorta Suspension Set-up is in my 6th gen Celica?

[indent]Front Suspension

Macpherson Struts are what you will find under your car. They consist of a lower sub-frame, a wheel hub that the axle and steering linkage hooks up to, a strut(a spring with a shock absorber in the center, a couple of links and somewhere to mount. This type of suspension only works (without any need for good luck charms or holy water )on cars with unibody construction, that's because all force not absorbed by the suspension is transmitted right to the mount in our wheel well causing our chassis to flex and sometimes permanently warp. Typically not seen as a performance suspension because of its limited adjustability, though it still works its way into 911's, Mustangs, and all BMW's. Really the only advantages this suspension has are a typically roomier front engine compartment due to lack of upper A-arms, and it's cheap to make but expensive to fix(I a smell corporate conspiracy).

[indent]Rear Suspension

An independent multi-link suspension system is set-up in the back and I was all set to explain it until I saw the in- in front of de-. Can't really wrap my head around with out some diagrams to help, but from my very limited knowledge I know that independent multi-links are very adjustable. It’s possible to adjust either camber, toe-in, or caster of the wheel attached without affecting the other two planes of adjustment.


So now we grasp basic knowledge of our suspension, but if some one could expand on the rear suspension that would be sweet.

[indent]Tuning the Suspension and Other stuff you need to know that affects Handling.

Suspension tuning is one of the easiest things you can do to make your car go FASTER. Ok so tuning a cars suspension won’t make your car accelerate any faster or give it a higher top speed, but it will let you go faster through the turns and in turn faster on the straights (because you’re exiting the turn faster at a higher speed).

Modern road cars typically have gas-filled shock absorbers and a low-rate (soft) spring. This combination gives a smooth ride with springs providing the opposing force against movement and the shocks dampening (absorbing) the movement of the suspension.

To properly tune a suspension you’ll need either a coil-over type suspension (preferably one that can adjust stiffness, of at least the shocks, without affecting ride height, like these http://www.6gc.net/index.php?action=parts&itu=439) or a whole lot of different rate springs and the time to swap them after every run.

Suspension tuning is FRICKIN’ EASY. If you learn nothing else from this write-up learn this:

If turning and the inside REAR wheel is coming off the ground or trying to, you need to adjust the FRONT suspension to increase dampening force (shocks) or increase rate of stiffness (springs) or both, in the FRONT.

If the inside FRONT wheel is coming off the ground or trying to, you need to adjust the REAR suspension to increase dampening force (shocks) or increase rate of stiffness (springs) or both, in the REAR.

The reason that your car is suffering from body roll is because of the strut(spring and shock), created by manufacturers for factory cars, is designed to make your drive “comfortable”, not “fun”. So when you’re turning (at speed) with your “comfortable” suspension set-up your suspension is trying to be “comfortable”, but I seriously doubt you’ll be very comfy riding on 2-3 contact patches when you should be on 4. The forces that are really screwing you over in a turn are the centrifugal forces acting on your car. These forces are what roll your car, normally pivoting on the outer front tire because you shifted your cars weight forward when you hit the breaks before entering a turn. Think about trying to do a stoppie, you know like those crotch rockets, with your Celica. Now that that’s pictured in your head think about trying to do a stoppie with your Celica and trying to turn at the same time, yeah… still not happening.

Now we know what’s happening and how to fix it, but what’s so bad about it?

Have you ever heard the phrase “Man, this car really grips the road!” while your tires play a big role in that, they’d be nothing without the supporting cast. Performance suspensions are built to help distribute weight evenly left-to-right, and sometimes front-to-back (not the case in 6GC’s). Traction is a force tied in with friction, and in the case of traction friction is very good. A Contact Patch is the area of tire that actually touches the road; it’s a very small space. When your car’s contact patch isn’t a contact patch any more you don’t have any traction. Body Roll pulls weight off of tires, those tires aren’t contacting or contacting as much as they should with the road.

So say your inside rear is coming up on hard turns, that means that your pivoting on the outside front wheel. The entire car is pivoting on your outside front, so the inside front and outside rear are also being pulled up (maybe not off the ground but still getting pulled up). So with weight coming off all wheels except the outside front, all that transferred weight will be placed on one wheel’s contact patch, the outside front. Now the car is going to experience massive oversteer with the back end whipping out around the outside front because it’s basically the only wheel getting traction right now. Or if your center of gravity is high enough (ex. SUV’s) you’ll probably flip off the road, and onto those people trying to make me donate money to non-existent charities at stop-lights.

Strut Bars

Strut bars are bars (duh) that connect the strut towers together to negate chassis flexing forces being transferred through the wheel well strut mounts. Reducing chassis flex helps to reduce understeer when turning at high speeds.

Anti-Sway Bars

Basically an ASB is a torsion spring connected to the lower arms of the suspension. A good ASB matched to your car will not affect vertical suspension movement, so you’ll still have a smooth ride. But ASB’s do affect roll-related movement, as one arm moves up and another arm moves down the distance between them gets bigger and the torsion spring helps keep that distance from getting to big so you stay more level. An ASB that is not matched to your vehicle can cause vertical motion of the suspension to be transferred side-to-side because of the torsion spring being to strong, this results in a rocking motion much like a hammock.

Aerodynamics

Downforce is our friend, Lift is not our friend. Downforce increases traction, Lift does not. Lift is VERY BAD. Most cars on the road actually freaking produce lift, WTF ! That is BAD! So now it’s up to you to fix it, ‘cause I don’t wanna be in a flying car on a slalom. Not much to be done but put an actual (functional) spoiler on the back, and get a low suspension combined with a body kit that has a bottom line parallel to the ground. The lower you are to the ground the smaller the gap thus less air under car, and if there’s less air there the lift of the car is reduced. The front of 6GC’s engine bay is pretty roomy, if you put a GT-4 front bumper with a custom fabricated hood and a little more custom fabricating connecting the top grill on the bumper to an opening in the hood and you could have a front spoiler similar to a Lotus or a Ferrari!

Drag is also bad. Our car has a coefficient drag of .38. No fake hood scoops or fake brake cooling scoops, all they do is act like a parachute. Smooth surfaces are good, everything we can see on the Celica is smooooooth, but what about those surfaces we can’t see? The under-carriage is rough, bumpy, jagged, and has parts sticking-out all over the place, the Lotus Elise (powered by the same engine as 7GC’s) has carbon fiber sheets covering the under-carriage to reduce drag and turbulent air produced there. A diffuser mounted on the bottom back bumper, normally as a body kit or add-on skirt, will help direct air up into the low-pressure area behind your car, further reducing unwanted drag.

One thing you need to know is that more downforce=more drag, but it is a very worth while trade. F1 cars have a coefficient drag of .7 to 1.1 that varies based on their needs at the track. If it’s a track full of slaloms then they need more downforce, if it’s mostly straights they need less drag.


Blurbs list

Cars with lower centers of gravity roll less. Other than lowering suspensions not much can be done to lower your C of G.

Tires with less sidewall flex and bend less, so their contact patch stays steadier. But they also make more road noise and absorb less force themselves, for example holes, rocks, and speed bumps (at speed) are more likely to damage your rims or pop your tires or both, trust me my sister has lost 3 rims and popped her tires 5 times on her Jetta equipped with the factory performance suspension.

Soft tires have better traction but wear out faster; hard tires last longer but have less traction.

Roll cages are good for when your suspension craps out (or if you’re a bad driver who pretends your suspension crapped out) .


Any questions, comments, corrections, and/or discussions would be cool. I’d also be happy to help with any otherwise noobish questions.

[rdyzz]

i have ran my car without a swaybar with up rated springs, it takes bumpy roads way better since the wheels are independent. with the swaybar it seems like transitions are alot better during turn-in and steering wheel input.

as for rear swaybars, i dont like it. really stiff rear swaybars seem to always make one wheel lift. i run without one and compensate for this with spring rate, it makes the rotation alot more predictable for me. i can still rotate with lift throttle.

on the 7th gen celica the swaybar endlink seems to be mounted to the strut because of space limitations. if you notice the tie rods are really low, this doesnt allow the sway bar links to be mounted under and onto the control arm without it hitting the tie rod. there isnt a reason other than this, there is no gains imo since a swaybar only really needs to be mounted to the chassis and two ends mounted to the suspension parts you want to connect it to, to have it work.

most modern cars seem to have more caster into its engineering than our older cars have, but i still dont feel caster alone makes a production level car handle better. if the car is not engineered with alot of caster adjusting caster can affect a cars bumpsteer, or the amount the wheels toe in and out during compression and rebound. this can make you turn the steering wheel more and more as the suspension compress during a turn, or work in the opposite fashion where you may have to unwind the steering wheel from the direction you are turning. this is mainly due to the way the steering arm is casted into the knuckle. so anytime it is tilted up or down further than factory specs it runs in a different sync than the lower control arm causing bump steer. lowering can affect this to, thats why over lowered cars have alot of bump steer. i've had to correct my bump steer by grinding 14mm off the knuckle, toe in now moves less then .006" from static height to full compress, but not from being over lowered mainly from using a RCA kit which changed the geometry a good bit.

as for alignment i have found that macphersons front work well with alot of negative camber, even though my car has a RCA to correct the geometry my car performed best with -2.5-2.7 degree's of camber. i have ran -1.2 before as a conservative setting and it showed alot in how it performed and on the tires. i usually run .10 to .15 toe out up front and 0 to .05 toe in on the rear. i like this better since my rear suspension is sprung alot stiffer than my front suspension.

what i have also found out to is running less SAI has helped with my cars handling, i know its supposed to make caster and other geometry changes count more, i experimented with this and running more SAI, since more would slightly benefit camber curve but would reduce wheel rate due to the angle of the strut. i liked the feel of the car best with less SAI, i used a eibach camber bolt and dialed in as much camber as i could, -4.3 degree, and pulled my upper top mount to the positive position. i ended up with -2.7

i also tested running with a wider front track than rear, 50mm total. i used 2 25mm front H&R spacers. i knew this would bring my scrub radius into the 2" positive range. since with my current 8" wheel setting with a +20 offset, my scrub radius measured in the positive 1.2" side. i was ready for the torque steer of running more positive scrub radius, but it didnt feel to bad actually. not as much as what the moto iq site would make you believe.

all in all at the last pdx here it was really fun and actually transformed my handling quite a bit. i run on direzza star specs and i kept up with some pretty good drivers driving sti's and s2k's with better tires, the only disadvantage i had was out of corner exit since i didnt have an lsd i could not throttle out as early. and any time i tried it was one wheel wonder.

sorry to rant so much, i just thought id share some of my experience and maybe some knowledge, but its still to each his own, thats the thing that makes suspension tunning so interesting, because there is no one way that works best (but you can make it worse), you have to find out what works for you and your driving style. i can also saying out of the whole aspect of tuning a car i think suspension tuning is somthing im into the most.

This post has been edited by rdyzz: Jan 16, 2013 - 6:00 PM