suspension modifcation talk

[nthfinity]

basic knowlege of phisics of handling say that if you want to turn at higher velocities, then lower the c/g, stiffen the chassis, increase the spring rate (stiffness) and get stronger dampers...... all this makes sence, but an article about suspension, specifically, from an engineer at porsche said it takes 18 months to get theh proper suspension set up for a car. proper possibly meaing its predictable in turns, doesnt rattle the frame, and has the "right" amount of over, or understeer.
if this is true, most aftermarket suspension mods could heavily interfere with the car's driving performance... i dono
anybody with any experience, both good/bad?

[graywolf624]

basic knowlege of phisics of handling say that if you want to turn at higher velocities, then lower the c/g, stiffen the chassis, increase the spring rate (stiffness) and get stronger dampers...... all this makes sence, but an article about suspension, specifically, from an engineer at porsche said it takes 18 months to get theh proper suspension set up for a car. proper possibly meaing its predictable in turns, doesnt rattle the frame, and has the "right" amount of over, or understeer.

Lowering c/g for instance isn't necessarily a good thing. The distance between roll center and c/g determines the amount of roll of the vehicle for instance. the larger the distance the more the moment arm. Then you have the issues with lowering screwing up both dynamic and static camber because of lowering.
Then you have the issue with properly matching springs to shocks as well as getting the appropriate rates for the surface you are running on.
Then you have the issue with tuning for over and understeer based on the stiffness of roll bars and the placement of braces.
You must be very careful when modifing supensions. Even more so as a company for the potential of lawsuits and the ability to mass produce.
It is never as simple as just bolt and go.

[hotgemini]

To add to what graywolf said, when the porsche engineer says 'suspension' don't think springs and shock absorbers, think ackerman angle, geometric and force-based roll centres, natural frequency, steering axis inclination , camber curves, bump steer, anti/pro-dive/lift/squat, roll-steer, NVH and so on.

[graywolf624]

think ackerman angle, geometric and force-based roll centres, natural frequency, steering axis inclination , camber curves, bump steer, anti/pro-dive/lift/squat, roll-steer, NVH and so on.

Your going to make the poor kids head explode.

[yin_cheong]

And stiffer springs don't necessary mean better. In normal everyday roads, springs that are too stiff do not allow all 4 wheels to be in contact with the road all the time. Lowering of suspension is generally good, but you need enough clearance to go over the common bumps on the road. For a street racer, the default setup of most cars is pretty good, maybe just lowering it a little to improve cornering stability. Unless your country has track quality roads everywhere....

[nthfinity]

well, unfortunately, we often do have track quality roads.... but they turn into the worst pot-hole infested roads due to our winters.
there is a good thing tho, some of our less traveled roads dont go bad quickly, and remain perfect for high speed twists and turns.
when i owned an 88 saab 9000t, id spend several hours a night taking various roads around the Ann Arobor area (U of M)
i had concidered modifying the suspension, but ultimately decided against it due to high miles/age.

[nthfinity]

oh, and also, no my head wont explode, i am an aerospace engineering major... heavy with phisics. i have decent understanding, and cirtainly wish to learn more

Your going to make the poor kids head explode

so use as much technical jargin as you wish; im here for keeps

[Epic2112]

One thing that seems to get overlooked very often is the fact that no matter what you do, the only thing that matters is where the rubber meets the road (hehehe). What I mean is that lowering the center of gravity in and of itself isn't important. What is important is the amount of your tire that is in contact with the road. As such, lowering the c/g will lessen the amount of body roll, and that in turn will generally allow more tire to stay in contact with the road.

Sorry if that was left unsaid because it is obvious, but I find many people that I talk to do suspension work without having any understanding of what it actually accomplishes.

[hotgemini]

Okay, here are my thoughts. So you start with a car, one designed for general commuter use, (think VW golf not Porsche 911 GT3RS) and you want to modify it for occaisional track use but mostly just spirited road driving on varied road surfaces. You've already done the sticky tyres on bigger wheels thing however the additional grip only serves to highlight the cars excessive bodyroll and the dominant understeer bias.

What are your options and what are the pitfalls?

Shock absorbers: Manufacturers cut BIG corners here, the only two pitfalls that people often fall into is overcapitalisation and over-damping. With care both are easy to avoid. The shock absorbers job is to control the motion of the suspension, without it the springs motion would be undamped and would oscillate uncontrolled.

Stiffer Springs: Reduces the body roll and given appropriate front:rear rates, will reduce the understeer. Also reduces the comfort level... "So what this is just my fun car"... unfortunately stiffer springs also reduce the ability of the suspension to do its a big part of its job, which is to allow the tyre to follow the road surface irrespective of changes in the distance between the road surface and the car.

Lower Ride Height: Lowers the centre of gravity (C of G) and usually comes in the form of shorter stiffer springs. Will affect the static camber, dynamic camber (the position of the static ride height along the camber curve) and roll centre. Its usually easy to fix the static camber problem but the other two usually get ignored. IMHO lowering a car should generally be regarded with suspicion, mistrust and considered primarily an aesthetic issue. Note: I'm not saying that there is no advantage in lowering a car, nothing of the sort and I'll spit on the first person who tries to take me out of context. Rather when a suspension saleperson is trying to sell you a set of springs, ask him about camber curves and see if he can provide any data (say camber gain/loss at 50mm bump/rebound).

Swaybars: Increases the roll-stiffness at one end of the car, thereby helping reduce transfer of load at the other end. To put that another way, a bigger rear swaybar helps the front of the car turn more efficiently. Manufacturers have generally fitted quite small swaybars, bodyroll serves as a good warning to the schmuck driver that he shouldn't be trying so hard. Excessive increases in roll stiffness will make the progression across the limit of adhesion more sudden but sensible increases allow many of the benefits of stiffer springs without most of the drawbacks. A real winner for a hard driven road car in my humble opinion.

Beyond this we are delving beyond the realm of lightly modified road cars, at which point I get to roll out many of those technical terms I mentioned in my earlier post.

[coombsie66]

I agree, in most road car applications the lowering of the car is mainly for asthetical purposes. I would not consider lowering a car any more than 35mm, not only due to camber issues but also (especially in a FWD car) the added stress on the driveshafts, CV joints, gearbox seals, etc.
I cannot believe that the majourity of 'tuning' companies (i mean, spax, G-max etc) spend a suitable amount of time R&Ding a lot of their suspension products for individual cars. I think that the majority of the time you are purchasing something that will be detrimental to the cars preformance in most situations! (There are exceptions to this).

[graywolf624]

Ok if you think you can handle it. This will get complex really fast.

I have to address this real quick
"lowering the c/g will lessen the amount of body roll, and that in turn will generally allow more tire to stay in contact with the road. "
Did you even read what I said about c/g. Lowering cg by lowering a car can potentially increase the distance from roll center. It isn't necessarily a good thing. Increasing the distance of c/g from roll center can increase roll. The section between the roll center and c/g is a moment arm around which the car rolls. The longer this moment arm the more the resulting roll(obviously from physics). Furthermore it can royally screw your dynamic camber. This can come about because by lowering the car you increase sai(steering access inclination).

I recently edited a friends article on this subject. Unfortunately I can't post it here until it is released. That being said I can provide some links (these aren't my own, just sites I found by searching google for the appropriate terms. I did read them though and there doesnt seem to be any bad information.):

general. The basics:
http://eaglewoman.racesimcentral.com...amics-susp.htm

Ackerman angle:
http://www.mech.uq.edu.au/courses/me...teering/s1.htm

Lowering (I'll try to find a better definition of how to find roll center.):
http://e30m3performance.com/myths/We..._transfer2.htm

[graywolf624]

Found you a piece on roll center. It is almost correct. The only thing it misses is the point where the suspensions on either side of the car are asymetrical. At this point the intersectionpoint of the lines from ic to tire centerline on both sides of the car are the roll center as opposed to the intersection with the car centerline.

http://members.aol.com/sccacuda/cars/5SmthTa.html

[nthfinity]

thanks for the good links/articles, greywolf, hotgemini, this is just the kind of literature ive been longing to learn 8)

[deth]

damn, thanx i've always wanted to look into this, but could never find time or a reliable source!

i might be opening another can of worms, but how does passive rear steering work? i've hear on some cars when the rear bushings wear to teh point where this is play this can lead to rear steer. can this be designed for, and how beneficial is it?

[hotgemini]

Passive rear steering is just where you deliberately design the suspension to deflect under load in a predictiable and hopefully beneficial manner, from my understanding and experience with peugeots it mostly seems to be toeing out the outside rear tyre. Mind you, it is quite hard to measure toe during high speed cornering with the traditional tools