[!4KORNRZ!]

More interesting reading...I love the last reply.

http://forums.bimmerforums.com/forum.../t-286721.html

[Turninconcepts.com]

Originally Posted by import111

As has been said by a couple other people.....I am not as much into the whole mathematical calculations. However this thread has been a great read. There is 1 thing that was said in this thread that I strongly believe in. Someone mentioned that the smoother the surface, the less suspension travel you need (probably wasn't those exact words but close to it). There is 1 main reason why I have modified the suspension on every car I have owned......reduce body roll to keep the car as planted and stable as possible through twisties. I have learned that vehicle weight plays a HUGE roll in this. My last car (2001 2.5RS-T) weighed 2800 with me in it. It had JIC FLT-A2 coilovers with 8k/6k F/R spring rates, and 18x7.5 wheels with 215/40/18 rubber. That car felt more nimble and almost as stiff as my current car (2005 STi weighing around 3250lbs with me in it, with 255/40/17 tires on 17x8.5 wheels, Tein Flex coilovers 10k/8k F/R rates, Cobb 25mm hollow F/R swaybars rear set to middle setting). I hear a lot of people complain about how Flex's are over damped. For a very bumpy road this would be true, but for those smooth roads and smooth sections on a road course, I feel like they could be even stiffer. I guess I think there is no perfect suspension set up for every situation. There are gonna be times you want it softer, there are gonna be times you want it stiffer. I guess you gotta find that balance for the situations your car sees.

Wow, that was a lot of ranting, lol. Looking at the plot for the Flex's on full stiff, I am not shocked at all. I run mine 1 notch from full stiff always, and I know there is almost no suspension travel at all in both bump and rebound. For daily driving, small bumps are not a big deal and I didn't notice much of a difference going from stock to Espelir GT springs to Flex's. For big, or abrupt bumps, the Flex's hit them VERY hard and there is a huge difference from stock and Espelir GT's to the Flex's.

I don't have any "hard" data to add, but I thought I would share my thoughts and experiences with this type of thing.

Take a look at F1 cars in the late 60's and well into the 70's before aerodynamics came into play. You'll find that the suspension did darn near nothing in the way of travel or compliance. The suspension was basically the tires.

If you were given a road or track that was smooth as glass in all situations then you could run a suspension that's pretty much infinitely stiff and all you'd really have to worry about is the amount of grip you have available.

Sadly, a glass smooth road just isn't going to happen.

Bottom line given a performance standpoint is that the suspension is supposed to be reactive and supple enough to maximize the amount of grip you can get from the tires due to road conditions. That's it in a nutshell.

Now, where it gets complicated is how stiff can you run before you start to see performance degrade. At what point does the tire start to skip? At what point do you overload the lateral grip of the tire due to too fast of weight transfer? At what point do you lose potential grip due to slow weigh transfer? At what point does it upset the driver so much that it's hard to concentrate and control?

4Banger and Stretch - have you considered taking chassis flex into account on your numbers? I know that without a chassis dyno and real numbers we can't get exact, but given a factor of flex we should be able to at least account for it to some degree.

[stretch]

Originally Posted by Turninconcepts.com

4Banger and Stretch - have you considered taking chassis flex into account on your numbers? I know that without a chassis dyno and real numbers we can't get exact, but given a factor of flex we should be able to at least account for it to some degree.

Nope, not yet. I'd like to model that, but I wouldn't predict its effects to be significant in the context of this discussion. I think the major flex is torsion which would be from extreme load transfer to one or opposite corners of the chassis.

Originally Posted by import111

I hear a lot of people complain about how Flex's are over damped. For a very bumpy road this would be true, but for those smooth roads and smooth sections on a road course, I feel like they could be even stiffer.

That's just it, you don't need a suspension on a perfectly smooth road with no elevation changes. That's why Go Karts don't need them. You need a suspension for bumps and dips, and that's why you should optimize for that- it's there to prevent the reduction of grip over a bump. If you want more roll stiffness, that is the job of the springs and sway bars to provide. It's also the springs that need changing to optimize for a bumpier or smoother surface. The dampers need match the springs, but you pick the springs first.

I've told this story before, but I remember the first time I thought I could change handling bias and control body roll via my dampers. I cranked my Koni's all the way up to 3/4 stiff, but only in the rear. I firmed the fronts, too, just not as much. Everything felt great until, while autocrossing, I crested a very gentle hill mid-turn. The rear tires couldn't extend back towards the ground quickly enough and thus the rear tires lifted while the front stayed planted. Snap oversteer- the rear had no grip. It was bad even when slowing down a bit. And this was not on a bump, this was on a very gentle slope! This was the first time I realized just how badly overdamping reduces grip.

Try it yourself. Put your fronts on full stiff and rears on full soft or close to it. See if the car pushes on every bump. Switch the valving front and rear. Getting a twitchy rear over bumps? That's because increasing the damping is reducing grip. (You could also have less grip by underdamping, but that's not much of a possibility on the Teins.) The best part is that the faster (softer) settings are also the more comfortable settings.

Overdamping reduces grip by preventing your tire from extending back towards the ground quickly enough. In bends, it will cause your inside wheels to get light. And over bumps, the entire car will get skittish. For that last reason, often fast cars can feel slow (compared to other cars running the same spring rates) simply because they're more composed.

In my experience from what I've run and measured, any damper that isn't comfortable (within reason given the spring rates) is probably overdamped. Skittishness in turns is another huge clue of improper damping. So is the feeling of reduced body roll- it can mean the damper isn't letting the spring load up.

[4banger]

Originally Posted by Turninconcepts.com

4Banger and Stretch - have you considered taking chassis flex into account on your numbers? I know that without a chassis dyno and real numbers we can't get exact, but given a factor of flex we should be able to at least account for it to some degree.

I'm not even sure on where to start on modeling chassis flex. Right now I would settle for being able to model tire rate properly. In one of the links that BigSky posted up, one "expert" suggest that not accounting for tires can have up to 40% difference in natural frequency.

[Turninconcepts.com]

Originally Posted by 4banger

I'm not even sure on where to start on modeling chassis flex. Right now I would settle for being able to model tire rate properly. In one of the links that BigSky posted up, one "expert" suggest that not accounting for tires can have up to 40% difference in natural frequency.

WHA?! I'm going to have to through those links that Bigsky posted sooner than I thought.

That's ok on the modeling for now because I don't know anyone near here who has a shaker rig. Scooby921 had access to one at his old job. I'll shoot him a note and see if he knows of anyone up near him.

Heck, it'll be fun just to throw an STi on one and see what happens anyway.

Oh, and a side thought - I thought there was a touch of that info in the X-brace thread somewhere.

[stretch]

4banger, I noticed the damped frequency in the latest graphs, a nice touch. The natural frequency doesn't change as much as I thought it would unless you're way overdamped.

[4banger]

Originally Posted by stretch

4banger, I noticed the damped frequency in the latest graphs, a nice touch. The natural frequency doesn't change as much as I thought it would unless you're way overdamped.

If you are over damped, it doesn't exist. I don't know why people don't talk in things in terms of damped frequecy more often. It seems like that would be the more accurate description of your car.

[4banger]

I have a paper with an equation for natural frequency that includes tire spring rate as well, and for the stock setup the difference is only about 4 percent. Maybe the EVO guy was over estimated a little.

The X brace thread did have some info, but I don't really know what to do with it.

Originally Posted by Turninconcepts.com

WHA?! I'm going to have to through those links that Bigsky posted sooner than I thought.

That's ok on the modeling for now because I don't know anyone near here who has a shaker rig. Scooby921 had access to one at his old job. I'll shoot him a note and see if he knows of anyone up near him.

Heck, it'll be fun just to throw an STi on one and see what happens anyway.

Oh, and a side thought - I thought there was a touch of that info in the X-brace thread somewhere.

[Turninconcepts.com]

Originally Posted by 4banger

The X brace thread did have some info, but I don't really know what to do with it.

I'm not either, but I know a couple of guys who would...

[4banger]

Originally Posted by Turninconcepts.com

I'm not either, but I know a couple of guys who would...

I can email you the Car Labs graph if you need it.

[Susp Tuner]

Hey Guys,

Let me introduce myself, my name is Danny and I am actually the author of that ChassisSim paper. I have found this thread to be very informative and its cut and thrust I have found very refreshing.

To ally anyone's fears (particularly the moderators) I'm not hear to sell anything or to push any agenda. However there have been a few things said about this paper that have been completely taken out of context and given the fact that this is a serious technical discussion I feel honour bound to address these points.

*First things first if you read the paper from back to end, you will see in the damper guide I specify that low speed damping ratios should be set from 0.5 to 0.7, and the high speed damping ratios from 0.3 to 0.4. The smoother the circuit the higher you can go. Isn't it funny how the higher numbers correlate with the calculations and experience of some of the members of the forum.

*That Porsche was tuned for Australian circuits, and Australian circuits are extremely bumpy in comparison to their European and American counterparts. I have worked as a Professional Race engineer in a number of different categories, and different continents and it continues to amaze me how bumpy most Australian circuits are. I can tell you right know when the A1GP teams race at Mexico, Durban (South Africa) and Eastern Creek (Australia) the bumpy nature of these circuits post all manor of setup problems. However my point is because this car was having to drive over the racing equivalents of plough fields the damping had to be set to near road car values just to get the suspension to comply.

*Also remember the Porsche and the Subaru are fundamentally different vehicles. The Porsche has a cg height of approximately 0.4m and a total mass of 1255 kg with a weight distribution of 36.6% on the front axle. A Subaru in contrast (particular from MY01 and beyond) has a road weight of 1500 kg (at least), a cg height of approximately 0.5m (probably more) and a weight distribution of approximately 55% on the front axle. Consequently I am not surprised the damping values found in this forum are higher because particularly if you are running semi slick rubber and the circuits are smoother the chassis needs more support. Given these quite significant to go blindly applying numbers from a Porsche to a Subaru and to claim someone is clueless is completely out of context.

*As for that 4sec difference that was tested back to back at Eastern Creek. The car was run with the dampers originally fitted to the car, and then it was tested with the dampers specified from that paper. That particular anecdote was included to illustrate how using this tuning approach can get you into the ball park, particularly if the original specification simply does not work. You would be surprised in the world of data engineering how sometimes you are forced into using data from dissimilar circuits to arrive at a setup, particularly if a session gets washed out. Just remember Eastern Creek is 13 turns so if your gaining 0.3s a lap per corner it doesn't take long to add up.

Just remember people that paper was written as a guide as opposed to a plug this in and you'll go 15sec a lap quicker. The business of race engineering a car is not an easy one. You have many conflicting requirements, you need to control the body attitude, allow the suspension to be compliant over the bumps and kerbs, and as well as controlling tyre temperatures. There are no hard and fast answers. If there were we'd all get bored and do something else. However with the application of a few scientific principles, appropriate use of vehicle simulation and carefully reviewing the data from the circuit a base setup can be arrived at that will help us be competitive long before we hit the circuit.

Best Regards

Danny

[stretch]

I saw someone named "Susp Tuner" posted here and though, "Sweeeeeeeeet."

Welcome, Danny.

Not disagreeing with any of that, but I would like to discuss one bit further. Why would a heavier car with a higher CG require a higher %cd? I would think it'd need firmer springs and thus firmer dampers, however the proportions would stay similar and thus %cd would remain somewhat similar. Perhaps a little more low-speed would be necessary to combat the heavier sway bars inevitably used on the higher CG car, but why change the damping factor for high-speed valving?

[Turninconcepts.com]

Originally Posted by Susp Tuner

Hey Guys,

Let me introduce myself, my name is Danny and I am actually the author of that ChassisSim paper. I have found this thread to be very informative and its cut and thrust I have found very refreshing.


*First things first if you read the paper from back to end, you will see in the damper guide I specify that low speed damping ratios should be set from 0.5 to 0.7, and the high speed damping ratios from 0.3 to 0.4. The smoother the circuit the higher you can go. Isn't it funny how the higher numbers correlate with the calculations and experience of some of the members of the forum.

Best Regards

Danny

Hi Danny,

Glad to see you joining in here, and welcome.

I find it interesting that you say this. The Koni guy who tested the AST setup was surprised at how soft they looked, but from your feedback and experience it seems that you would most likely go even softer.

As such I'm betting that you also spec a fair bit of travel in the dampers and springs. What are you guys typically shooting for in the way of travel?

[Susp Tuner]

Hey Stretch,

Mate, I should have been a bit more specific in that example. You would have to up the low speed damping to give the chassis more support in pitch and roll, but the high speed stuff you wouldn't touch.

What is intresting though is when you analyse a V8 Supercar. Here's a car that weighs 1450 - 1500 kg, Runs a live rear axle and is effectively a NASCAR for street and road courses. However due to the nature of the tyres a supercar is very soft in both spring and damping. The reason is the Dunlop tyres have a very soft sidewall and need to be treated with great care.

However you do need some tyres that need a bit more of a workout. This could be due to a number of reasons, hardness of the compound, sidewall stiffness. Basically when the tyre needs to be worked that's when you see spring rates rise, and the low speed damping ratios get quite large (in the order of 0.7 to 0.8 sometimes 1.2! that's if the circuit can handle it) A classic example is that when V8 Supercars where running Bridgestone tyres their spring rates where double what they are now.

Danny

[Susp Tuner]

Hey Turninconcept

First of all thank you guys for your warm welcome. I appreciate it.

How soft you go is a function of three things. How bumpy the circuit is, how the suspension geometry reacts and what the tyre wants.

I've always been of the view you run as soft as you can. This helps the car ride the bumps and keeps the contact patch load as uniform as possible. If I'm dealing with a circuit that is pretty smooth i'll be thinking low speed damping ratios of 0.7 and the high speed and rebound in the order of 0.4. However if I know I'll be running on a goat track I'll tend toward the number on my paper because I know that road holding starts to take precedence, and If I need to work the tyres hard I'll do it with the bars.

That being said the Subaru WRX is an intresting case. Being a road car it has a high centre of gravity and it runs McPherson Struts front and rear. While McPherson struts are OK for a road car, once you start putting race grip into it, the McPherson strut runs out of steam very quickly. This is due to excessive roll centre variation and excessive camber gain. Match that to a tyre that has a hard compound and it needs to be worked hard, then the car needs some order of stiffness to keep this under control. You combine that with a tyre that needs to be worked then you need to go hard in the springing and damping.

How much will be a function of the suspension geometry movement and how the tyres need to be loaded. Typically If you see +/- 20mm that is OK but honestly I won't be nailed down on this figure. The best way to determine it, and I'm not giving anyone a sales plug is to use dynamic race car simulation to determine what the cambers and roll centres are doing and then you can go from there.

As I said before, this is a hard business with no hard and fast rules. I'm not saying that to dodge a bullet, but I am saying that because there are many compromises that must be entailed.

Best Regards

Danny