This has me thinking about why why my conservatively lowered 205 car is misbehaves so much under braking, acceleration, and especially down uneven or steeply crowned lanes and uneven roads. Granted it turns in well, and mid corner sticks like poo to a blanket, but even then it feels nervous. I'm no expert but what they are saying does make sense, and when you combine all the factors together it really does make me feel a bit silly. If you can't be arsed to read the whole post, in a nutshell, lowering a standard car without making changes to counter act the knock on effect upon it's geometry has essentially made it unstable, together with neutral toe and a number of other factors. Our mcphereson strut suspension and trailing arms seems very sensitive to lowering...and i think i've done too much, and without sounding controversial... if what i've done is too much... then the generally accepted common lowering ammount may mean the majority of lowered 205's are also not working to their optimum
Up until now my understanding was the the following:
Lower the center of gravity by lowering the front and rear of the car
Reduce the body roll by fitting stiffer anti roll bars
Reduce the body roll by fitting stiffer springs
Up the damping considerably to critically damp the springs for their length and stiffness
use negative camber front and rear to keep the tyre flat when the car and consequently wheel rolls over in the corner
Use neutral toe at the front to give a more direct steering feel, and reduce the dead zone.
Run reasonably low tyre pressures (32f/30r)
get the lightest wheels possible to reduce unsprung mass.
Of course lowering was within reason as I didn't want to run out of suspension travel. I'd also heard it was not ideal to run with level lower arms (ie not pointing down like standard cars) although mine are level. and that neutral toe can make it twitchy, which mine is, but it was the movement over bumps and crashy ride that is most worrying me.
Going through their article i've picked out some quotes to demonstrate why lowering could have caused my dodgy handling.
Understeer... and tyre pressure
Originally i thought lower pressures meant more grip.. however this does not seem to be the case. It would seem that the slip angle (direction tyre points vs direction it's actually going when cornering) can be reduced by increasing pressure.
Nose heavy front wheel drive car... like a 205 mi-16, perhaps increasing front pressures might have helped reduce understeer instead of changing for softer springs and stiffer rear torsion bars. also saw this just now, nice coincidence.
How does a tuner manipulate tire loading and slip angle? By tweaking the spring rates, anti-roll bar rates, tire sizing and pressure, and to a lesser degree, the shock damping. The first option a tuner has is to increase the tire pressure. The harder a tire is inflated, within reason, the smaller slip angle it develops. In the case of a nose-heavy front-wheel-drive car, if you add several psi to the front tires and take some pressure out of the rear, the front tires will run a smaller slip angle while the rear tires' slip angle will increase. This alone can do quite a bit to reduce understeer.
Understeer and spring rates... explained with slip angle rather than "because it's stiffer"
So more weight will be transferred (and possibly more quickly) to the stiffest bit of suspension, hence understeer if you run a stiff front end and soft standard rear end.
Changing the spring and anti-roll bar rates has a large impact on slip angle. Running a stiffer spring or anti-roll bar on one end will cause more weight to be transferred onto the outside tire as the car tries to roll in a corner. The softer end will compress and the more stiffly sprung end will resist compression, putting more weight into the tire and causing it to run at a bigger slip angle.
Various damping settings have been used on my car, but i've ended up running about 12-20/40 at the front and 14-30/40 at the rear to make it turn in properly, much more than is actually needed to critically damp the springs, it would seem this is not ideal.
So if set way hard.. the shock will be delaying the real weight transfer happening, but have lightning fast turn in, which is what i originally found when checking the shocks out when first fitted. If you were to run them at 5.. they are just about critically damped but the turn in was squidgy, run them hard and it was on rails... but not necessarily better when it got to the middle of the corner.
Shocks can improve response and help balance the car right after the initiation of a turn; soft shocks get the car to a steady point of weight transfer faster. When stiff, they can delay weight transfer. Thus, shocks affect how the car feels at turn-in and also how it feels past mid-turn. A car with the shocks set fairly hard will turn in sharply. If the shocks are set too hard, the balance might change later in the turn in an unpredictable way as the heavy damping slows the body roll and weight transfer.
However... just to be a bit contradictory. Lowering the center of gravity is good because it reduces weight transfer by lowering the center of gravity, stiffening the suspension to cope as previously mentioned increases the speed of any weight transfer so the car responds more quickly to inputs.. now...
now... mine does bottom out alot... so either i'm running too much damping and it's packing down, or the springs are too short... or both. I've got another two inches of adjustment on the coilovers.. and if i ran a two inch longer 10" slightly harder spring and raised the ride height half an inch plus slacked off the damping a fair smidge, i would not have either issue.. It's on gaz coilovers and 200lbs 8 inch springs.
Of course, stiffer springs have more rebound energy. To prevent your car from bouncing like a pogo stick, you need shocks with more damping. Shocks don't affect how much a car rolls, but they do affect how the suspension responds to bumps and steering input. More rebound damping keeps the car from bouncing and floating over bumps and undulations. More damping also makes the car more responsive to steering input. Too much rebound damping can prevent the suspension from returning once compressed, causing it to pack down and gradually bottom out.
Toe... now mine is set neutral...
hmm, this all sounds very familiar, perhaps the "neutral toe" is actually with everything else and the bump steer that might be occuring turning into lots of toe out...hence chicken oriental handling.
Below are guidelines for setting toe and how it can affect feel and handling.
Reduced understeer at turn-in Improved steering response Counteracts natural tendency for front- and all-wheel-drive cars to toe-in under throttle load
Instability during braking Straight-line instability, especially over single-wheel bumps or split-traction surfaces Unrecoverable understeer
Talking about lowering again...specifically how the center of gravity relates to the car's roll center. Roll center is the center about which all the suspension rolls when looking front on, and crucially usually higher than the center of gravity. I didn't really understand exactly what a roll center is.. but their explanation makes sense.
The distance between the roll center and the center of gravity is called the roll couple. The roll couple is the lever arm that centrifugal force working on the CG uses to make a car lean over in a turn around the roll center.
so when we lower the center of gravity a bit by compressing the suspension we lower the roll center a s*it load, which makes the car respond more slowly to steering inputs and more susceptible to roll, therefore needs overly stiff suspension... which does not actually suspend the car, as much as make it bounce about... yet more argument to run somewhere nearer standard ride height...
The longer the roll couple, the more weight is transferred to the outside wheels during cornering and the more the car will want to roll in a turn. A longer roll couple makes cars slower to respond to steering input. The resulting weight transfer from a long roll couple also uses the inside tires less effectively during cornering, thereby reducing the available grip.
now... a bit on our type of front suspension and how it reacts to being set in a compressed state (lowered)
Moving the suspension through a wide range of travel can also result in another problem. Most factory vehicles have compromised suspension geometry and several problems can occur when a car heels way over in a turn. First, the suspension can gain positive camber. This is worse in cars with MacPherson strut suspensions. With strut-type suspensions, the car rolls, but the tires don't. This forces the tire to roll onto its outside edge and reduces its contact patch-clearly not the best way to use the tire.
Negative camber is good, and 309 bones help give this... but if the lower wishbone at rest points upwards to the wheel as the suspension compresses as in pic 2... the blue compressed state shows that you get positive camber... not good
So all that money on 309 bones is out the window as soon as the suspension compresses. Think about it, when the arm is flat it is at it's longest, it will only get shorter as it goes up.pulling the bottom of the wheel inwards, so ideally you want most -ve camber and flat arms when the suspension is in compression.
one would assume that the peugeot designers made sure that around normal amounts of suspension movement, bumpsteer was kept to a minimum.. however as the suspension pivot points for the track rods are not in the same arc as the suspension linkage, bump steer will occur.
The other evil effect of roll is bump steer. Bump steer is caused when the steering linkage and the rest of the suspension travel in different arcs throughout the range of motion. As a result, the tires can give steering input even if the steering wheel isn't moved when the car heels over. This translates to the driver as a twitchy and unstable chassis. Combine dive and squat and all of these problems add up to a serious lack of control.
This image shows the ideal McPherson strut set up..
If memory serves our inner track rod linkage is higher and further inboard, and the outer is also higher..
Thinking back, someone already mentioned raising the rack up to help lowered cars achieve better bump steer characteristics.. The pulsars used to mount the track rods upside-down, to the same effect..
In summary lowering the car which has.
Moved the suspension into a compressed state geometry out of ideal position - Inducing more bump steer
Made things worse with neutral toe - Magnifying any bump steer issues.
Moved the wishbone into a level (or possibly slight upward facing ) resting position - meaning any compression will cause a reduction in -ve camber and possibly positive camber!
Lenghened the roll couple by lowering the roll center lots in comparison to reduction in center of gravity - dulling turn in, making the car react more slowly to input and roll with more force.
Stiffened the damping to try and improve turn in - causing slow weight transfer and uneven mid corner handling.
Stiffened the spring rates to reduce roll - increasing the weight transfer to the front wheels and causing increased slip angle and understeer, decreased ride comfort
Stiffened the rear spring rates to counter act the front slip angle - his actually helped neutralise the understeer somewhat..
Stiffened the rear anti roll bar stiffness ALOT - this helped front end grip on corner exit, and turn in... i'm not sure there are any negative effects, except the rear does not travel over one wheel bumps as well.
increased rear -ve camber - this also helps when the car rolls to keep the rear in contact.
Decreased rear toe in - if there is any inherent toe out through roll and suspension compression, it will oversteer more... eek.
and this is without taking into account a few other major factors...
All in all it feels after reading this that the generally trodden path of lower stiffer is not necessarily going to make your car handle better.
Now... there are two options..if my understanding is somewhere near correct.
1 - try to go back to a more standard front geometry, increase spring rates slightly, and lenghten the ammount of travel, increase front pressures, put a bit of toe in to the mix. Live with the higher center of gravity, in favour of a shorter roll couple, less damping, more travel, better ride.
2 - keep it low and try to redress some of the bump steer issues by moving either the rack, or the track rod ends position, lower the bottom arm outer joint to raise the roll center nearer the new center of gravity, increase spring rates considerably at the front to stop it bottoming out, reduce front damping to just enough to cover the damping of the springs.
Edited by kyepan, 24 May 2010 - 11:16 PM.