Choosing a suspension -- Setup discussion

[james 408]

Street vs Track:
Generally, softer for street, stiffer for track (unless bumpy).
Looser (oversteer) for track, stable (understeer) for street.

Spring rates:
more bumps = softer (keeps contact with ground). stiffer prevents body roll, used on smooth tracks.
Stiffer rear than front = oversteer

Dampers:
more bumps = softer (keeps contact with ground).
Compression : contraction :: rebound : extension.
High speed compression : big bumps at higher speed :: low speed compression : handling, small bumps, and parking lot
Digressive rates: high rates of compression/rebound in low speed compression can be used to create stiffness in situations like impacting handling dynamics.
Combined with vehicle weight, spring rate, compression rate, and rebound rate, dampers are responsible for allowing the springs to push the car back to neutral float. The rate at which this occurs is determined by the amount of damping. This impacts tire contact, handling, and ride quality.
Stiffer rear than front = oversteer; this is adjusted in the low speed compression range.

Roll bars:
roll bars provide spring rate to unequal forces (for instance 1 wheel hits a pothole, or body roll while cornering), by applying a twisting force through the bar and applying the opposite force on the other side. Inherently, this is used for several reasons instead of spring rate/damper, as it allows lower compression rates on the suspension for bumps while fighting body roll. The downside is it reduces the independence of the suspension.
Stiffer rear than front = oversteer; this is adjusted in the low speed compression range.

lowering the car
reduces center of gravity, requires stiffer rates to prevent bottoming out.
reduces drag/lift

which is right for you?
It's complicated.
There's lots of talk about the proper setup, matching struts to springs, and various price points. It's just not that simple. The expensive options are often adjustable and the research has hopefully already been done so you're not making valving changes and spring rate changes -- often even effective on a wide range of adjustment. But what works for one may not necessarily be the best for another (either person, or purpose). While I'm not an expert, I aimed merely to shed some light on the matter without writing a novel. This is not comprehensive, merely notation, be careful in applying.

Everything is a trade-off here, it's not black and white; each driver, and each application requires a different mix to please.

Who set up the damper? do they even own a shock dyno? How much track time vs street time? What comprimises did they make? How good are they? How good is the product?

why are some coilovers perceived better than others?
Well there's stroke, thickness, heat retention viscosity compensation, orifice/shim shape, materials, and build quality.
Further, it's the way they've set it up; that it allows pretty good range of adjustability, and that the range actually makes sense. Now if you look at various dampers and what they do to the adjustment, there's a giant range of what they do or do not do. If compression and rebound are adjustable, are they linked? do the ratios make sense? Often, they just don't. Then there's high speed compression vs low speed compression, are they linked? do these ratios make sense? What you'll find is that unless the suspension is fully adjustable, you better hope they tested it thoroughly, knowledgeably, and for your given application.

interesting resources:
Stasis/Ohlins Manual
AST dyno
ohlins dyno (slow speed/rebound adjuster)
Dyno chart video
A subjective experts opinion (don't agree with all of his perspectives, but it's a good overview; obviously it's a marketing dyno video from a business owner, so like the ohlins is shown on full rebound stiff and who knows what compression setting -- couldn't find a better vid on short notice.

omitted topics:
alignment
drag racing
differentials
________________________________________
Application to the s4 and shortcuts:
name brands or quality clones (track experience, proper expertise)

subjective ballparks, track oriented:
linear spring rates in the range of 600-1300, with the rears always 200 or 400lbs higher (linear spring rate not required, just a reference point)
Swaybar between 19mm and 25mm. i'd say 19mm for 800F 1200R spring rates, or 25mm for stocklike spring rates -- as ballparks. Personally I'd go 22mm rear with 800F 1200R spring rates, but that may be a bit loose depending on your taste.

[oppositelock]

Why higher spring rate in the rear? The rear is lighter. I guess to control weight transfer to the rear under acceleration?

[MikeyB3]

Just be honest with me. Are you going to fiddle and dial in for EVERY TRACK you go to, or are you going to find a particular set up you like how it feels and then leave it?

[Ephry73]

Personally, I would love to keep the current stock ride height, but get better damping and less floating. I would love a more dialed in suspension with a firmer stance.


E

[james 408]

higher spring rate in rear increases oversteer. It's for car balance**. Same effect as stiffening a rear sway -- reducing weight transfer to that corner, thereby increasing grip to others.

**please note the inherent tradeoffs (like stiffer is worse for bumps, or that in terms of shock travel the heavier end will need to be properly sprung to prevent bottoming out.**

No i don't fiddle with the adjusters for every track, but sometimes i do for different tire setups, or just an overall tweak when I align the car. If i was competitively driving on different road surfaces I probably would though. Problem with no adjusters is that if you don't like it, you gotta revalve it to change it.

Also if I was daily driving and wanted it cushy and didn't track that often, i would likely have 2 setups. there are comprimises made in my setup, that while it's "tolerable" on the street and soaks high-speed bumps well for how stiff it is, it doesn't behave well in parking lot bumps (low speed compression is really stiff). All you do is crank down the rebound/compression adjusters, and for a true street car you'd dial down the spring rates as well.

[Ephry73]

Just be honest with me. Are you going to fiddle and dial in for EVERY TRACK you go to, or are you going to find a particular set up you like how it feels and then leave it?

Seems more sensible and practical. Then you can play with tire pressures


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[zatch_303]

We want increased oversteer on the b5 s4 since it naturally has a heap of understeer. So higher spring rate in the rear is good right?

[landfill]

^^^Another thing to increase oversteer on the b5 is to only upgrade the rear sway
bar and leave the front stock or try to find the smallest stock front bar. I learned this
one on the c4 chassis from downsizing from a sport 28mm front bar to the standard
26mm front sway bar and the apikol bar in the rear.

[james 408]

We want increased oversteer on the b5 s4 since it naturally has a heap of understeer. So higher spring rate in the rear is good right?

yes, but there are tradeoffs. higher spring rate will reduce grip over bumps. The reasons they say stiffer for heavier is to minimize bottoming out; therefore your front spring rate must be appropriate for the intended use. The ratio of rear to front, in order to reduce the understeer created by the heavier front end, skews towards puting more rear spring rate. Now balance can be affected by a great number of variables including: shifting weight/balast, rear sway bar, differentials and brake bias, tire size/pressure, etc. Each has its own pros and cons.

For a street car, i'd likely go more sway bar, rather than spring rate increase. The reason is that you can preserve some of the cushyness in the rear suspension. This is further compounded by the fact that most coilover kits do not provide adequate rear spring rate to resolve the understeer on its own.

Now i'm going to write a post here that I'll preface with this:
my intention here is to help share information i've gathered, and inform people so we can get a more educated discussion about different suspension options. The information on this stuff is so sparse that in my opinion the resulting 'impressions' of various suspension options isn't really helpful outside of very subjective things like 'rides nice' (with some exceptions).

Obviously where we should be is sharing shock dynos from various competitors and comparing notes. We're not there yet.

I'm going to add some of my subjective perspectives, and discussions about particular setups.

[james 408]

Suspension setup round 2:

Sway bar:
reduces independence because it links the suspensions together. Let's say you're cornering, the car rolls towards the outside tires, compressing the outside suspension. As it compresses, the force is resisted by the swaybar, and applied to the opposite side. This is why you get inside wheel lift on stiff sways.

now let's say 1 wheel hits a bump: same effect, but the benefit of applying the lifting force on the non-bump wheel doesn't aid anything like it does in resisting body roll

now let's say both wheels hit a bump: swaybar does nothing.

coilovers
now most coilover offering aim to hit the middleground: where they perform better on the track, can be lowered, but still preserve the daily drivability comfort. Now because of tradeoffs, it's not possible to do ALL of things better with one setting. Hence, adjustable suspension. Now if they've done their R&D appropriately, you'd know that softer springrates and settings are needed for street use, but that won't help you in resisting body roll on the track. But where the R&D really pays off, is that the suspension is adjustable along this range so that it suits the needs of both uses. Let's get into some specifics to help ground the conversation:

H&R Streets: progressive spring rates, mostly street focused but aimed to provide a range of ride heights and provide a performance increase as well. Now here's my review, and why i moved on:

Now for strictly street purposes this is a good setup, as it allows the occassional spirited corner, performs well, but is still supple enough for daily driving, and it works for different ride heights. That said, in strictly street scenarios, the suspension wasn't overly soft, particularly on the high speed bumps.

At the track: The progressive rates and damping permitted the car to 'porpose' over rolling bumps and significant body roll. Now as the suspension compressed, it grew in stiffness, and even though the car was all rolled over, when it hit a bump, it didn't soak it very well at all.

Stasis/ohlins:
The beauty of this setup is in its versatility. Custom spring rates, with knowledgeable sales staff guiding you to make the right choice. Once you've selected your place along the bumpy/street vs smooth track spectrum with your spring rates, the dampers are setup so that you can adjust them along that very wide spectrum. This is why so many people have positive reviews of them... they're extremely versatile and Stasis did an excellent job in advising rates. Now because of this versatility, I'll just address my particular setup/review:

800/1000; the spring rates are stiff, too stiff for DD. But because of the softer high speed compression (and lack of progressive spring rates), it soaks up high-speed bumps well. And for full smooth track setup, race teams were running around 1200/1600 on some tracks. My complaint: as i have it set now, the rebound rates are a bit higher than i'd like, but since the low speed compression is linked to that adjuster, i can't really do anything about it as I like the stiffer low speed compression. Overall I'm very happy with the suspension; I don't think they made a mistake in setting the rebound that stiff for that low speed compression setting, here's why:

If I was on a flat/smooth track, that additional rear rebound might be just what the doctor ordered. I think they setup their ratios extremely well. But I run on very bumpy track often, with jumps. I'd like my suspension to droop faster to maintain contact with the ground -- there are times where it feels like it's floating. Now maybe i'd run into problems with the suspension unweighting and getting spring lift, i dunno -- but i suspect if I had the fully adjustable independent adjustment of low/high compression/rebound, I could find some marginal benefits.


So here's the deal:
I can always revalve mine, as you can on many other setups (might want to look into this, that's what that experts video above is of, a guy that custom valves). So it's quite possible just springs and struts, and just spending a bit of time with a guy willing to revalve your crap 2-3 times for your ideal setup will fit the bill. Or maybe you just want the ride height adjustment and a reasonable comprimise and not care to learn or adjust it. Lots of ways to slice this. But what I found was a complete lack of comprehensive data on the subject matter, so I thought I'd graze the topic and let you know how far down the rabithole it goes.

[james 408]

Other interesting links:

Chassis: https://www.youtube.com/watch?v=xYYGkWVy32I
Suspension geometry: https://www.youtube.com/watch?v=x0oi...0ygfR7&index=5
dive squat: https://www.youtube.com/watch?v=UhjQXJ6Yegs

[2Snails]

Thanks for the info. This is good stuff to form the basis of a reasonably intelligent decision - something I'm rarely accused of making.

[m47thew]

Great info man.

Can we talk about wheels and tires too?

Lately I've been researching on different wheel/tire setups and I'm having a hard time finding good information (Although I did find this helpful in regards to understeer http://www.tirerack.com/tires/tirete....jsp?techid=58)

In terms of choosing wheels for the track I'm finding mixed reviews. But so far I have gathered the following:

Smaller rims should have lower moment of inertia (rolling resistance) and therefore lead to faster accelerations. With more rubber on the tire, you can theoretically have more surface area contact too. However, with smaller rims you will have tires with a larger volume of air which are subject to flexing in the corners and thus leading to a rolling of the vehicle (acts like a spring).

Wider rims should have more grip too because of the larger contact area (better cornering/braking). Furthermore additional exposed surface area means better cooling. But again more weight can take away from acceleration.

I switch my wheels in winter and notice quite a difference in ride quality from my 17x7.5 to my 18x8. While the 17s are comfortable to drive in. The 18s are much more responsive and I really feel all the features present on the road.

Now I'm trying to justify getting a 3rd set (track only) but cant decide on what's best...

[james 408]

sorry for the delay was awol.

Wheels/tires:

Yeah you're in the right ballparks on the wheels. Wider = grippier in the dry, more likely to hydroplane. heavier (rotational inertia) is impacted by size and weight. Tires have vastly different weights as well, fyi. So yes a taller wheel, even if weight is the same, is more inert because of the weight distribution - distance to the center, but it might be partially offset by reduced tire weight/sidewall depending on your choice.

old school wisdom was some light 17s on 13" discs was the way to go, although i suspect that in longer sessions 18s on 14" discs would likely be better. (that was my theory anyway)

so yeah, weight is key, size is key, fitment is key, and cost is key. This is one of those areas where you just take a good wheel like an rpf1 or superleggera or whatever floats your boat, make sure it clears your brakes, you want it slightly wider than the tire range suggests, but still within the range on tire rack for your chosen tire.

Also keep in mind that ET of 1mm = 2x section width mm (in other words if you make your tire's section width 10mm wider, you would need a 5mm change in offset in order to maintain the distance to the inside upright. This would mean you lose all 10mm of space on the fender side, so you'll need to keep fender rolling in the back of your mind. Keep in mind rolling diameter of the tire will also impact clearance on your inside uprights. This will also widen your 'track' and therefore impact suspension measurements like scrub radius. Generally I've found wider is better, but there are tradeoffs, and without re-engineering your suspension, there is a point at which going too wide will start to hurt you. Where that point is i dunno, but probably not til around 285 or more by my random guesstimate. It's also harder on wheel bearings to have more leverage on the hub (as well as increased grip).

and weight isn't just about acceleration, it affects braking, and as you are kinda describing in your post, it also affects the suspension quite dramatically. I don't think the rotational distribution matters on the suspension front just raw weight... but basically it works like this:
If the wheel is heavier, it has more inertia as it is compressed by the road towards the bump stops (and same for rebound as well). This means that your effective damping force is increased, the bump affects the car more, and more heat is generated in the damper. So lighter wheels will quite literally react significantly better over bumps, other things equal.

Shortcut suggestion: 8.5" section width on 225 competition tires = about et35. Grab some PSCups, some 19lb oz's in 18x8.5, roll your fenders a smidge, and go melt some rubber. 255's have been fit, but in competition tires the amount of fender rolling and offset really necessitates a widebody or some really agressive rolling and ET20.

subjective/objective impacts of wider tires:
Slightly more numb steering in terms of road feel
increased tramlining/darting
reduced hydroplane resistance
increased temperature/wear resistance
increased grip
heavier steering
costs more
generally heavier so read above about impacts to accel/brake/suspension

[james 408]

This might help you understand the war on understeer. Pleasure and speed vs predictable. While control is best achieved somewhere in the middle, and depends on the driver's skill and preference.

Stiff vs soft, on the other hand, is more determined by the track/application. And driver preference plays a roll here too, as stiffer might load up faster and provide benefits in certain high speed transitions, but doesn't provide as much feedback or grip on some tracks. Plus there's comfort to consider off-track.

[james 408]

To illustrate the importance of a rear diff, see 2:10. Or anywhere else in the video really to a less obvious extent.



with 1 wheel in the air, virtually zero power goes to the ground, unless you have that rear LSD. And that is correct, the torsen does not fix that because of the zero load condition.

[Ephry73]

Great video.


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[james 408]

Just gonna throw this here: Tire choice and rim width study by tire rack, reinforcing what's said above, but there's some nuance if you're interested. remember your fenders under compression at lowered/track ride height are extremely limiting, despite what you might read/see elsewhere.

https://www.youtube.com/watch?v=1MDq23scaWw

[FLACO X3]

Very nice information