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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Optimal tire pressure for Koni FSDs?



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      08-29-2011, 07:34 PM   #1
Drivaar
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Cool Optimal tire pressure for Koni FSDs?

What psi have people found to work best for optimal comfort?

I had two thoughts on this:

1. Lower tire pressure slightly so that the tire absorbs some shock also. But then that will end up reducing the frequency of the vibration to the shock... in some cases, possibly so much that the shock will damp less than optimal.

2. Keep tire pressure fairly high so that the vibration frequency is well transmitted to the shock, which will then do the damping as designed.

Thoughts?

Thanks!
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      08-29-2011, 10:38 PM   #2
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Quote:
Originally Posted by Drivaar View Post
What psi have people found to work best for optimal comfort?

I had two thoughts on this:

1. Lower tire pressure slightly so that the tire absorbs some shock also. But then that will end up reducing the frequency of the vibration to the shock... in some cases, possibly so much that the shock will damp less than optimal.

2. Keep tire pressure fairly high so that the vibration frequency is well transmitted to the shock, which will then do the damping as designed.

Thoughts?

Thanks!
I'm running the recommended pressures on the door jamb with non-RFTs. I ran 4 PSI higher before I went to the Konis.

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      08-30-2011, 06:07 AM   #3
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Very interesting slant on the FSD, what pressures work best?

From my perspective I always try and get a balance between tire wear and driving feel. With the E91 330d wagon that means front tires (17") are up a bit (2.4bar) from 2.2bar normal setting, and rears need setting for the loads carried, starting from a base of 2.6bar.

What is more enlightening is the tires themselves. Having run Bridgestone Potenza RE050 (RFTs), Goodyear Eagle F1 GS-D3 and currently a Falken FK 452, there is definitely a difference in the way the FSD and tire work in combination.

If we start from the fact a tire has a spring rate, variable at that, according to pressure and load carried, we can expect differences. I thought the car worked really well on the Goodyear, no real vices and pretty comfortable at the above pressures. But when I fitted the Falken the car seemed to come together in a different way, as if I'd "hit the spot" for suspension harmonics, clearly the frequencies of all parts harmonised, to give a more supple ride and greatly subdued the NVH in the car.

The only way to simply describe it, it feels better controlled, more precise, but 'rubbery' at the same time. I feel the suspension is working properly, makes a much more mature feeling to the drive and feedback.

I'm often driving the same road, where I have some undulating surfaces on some sweeping bends and feel the car is only just damped enough, then hit some cross ridges where I think, any firmer and the car would be crashing, so do feel it sits pretty much in the middle, for the right balance.

I've tried both higher and lower pressures, and it starts swinging the balance, either too soft, or too hard, hence sitting with the pressures I run.

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      08-30-2011, 04:04 PM   #4
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Each tire performs differently at a specified PSI. Soft feeling tires often benefit from higher pressures, while stiff tires do not need an increase. Every tire has a different spring rates and damping.

Know that you should adjust pressures to keep an even wear pattern on the tread. If you track drive, pressures should be efficient to maintain an acceptable temperature over the tread surface.

Ultimately, each tire/vehicle combination is different, and it needs to be experimented with. There's no rule of thumb to help you match any tire with your suspension configuration.

Good luck!
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      08-30-2011, 09:50 PM   #5
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anerbe: heh, that's the point of this thread... to see if there is someone who can help us think through whether you want the tires softer to absorb a little bit of vibration or whether that would end up reducing the frequency enough that the FSDs aren't able to do their thing optimally.

Seems like what we really need is the spec on the Koni FSDs. On the web site it says that the shocks are really differentiating between vibrations of ~10 Hz (where the shock behaves soft) and 1 Hz (where the shock behaves stiff).
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      08-31-2011, 07:32 AM   #6
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As I see it, a stiffer tyre should improve the way we benefit from the Koni FSD concept, as frequency typically increases with higher spring rates for a given load. As the frequency increases, the feature opens to soften the rebound. If we assume the tire spring rate (working in series with the suspension spring rate) increases the overall stiffness of the suspension, we should see greater benefit. But…. if we have a softer tire, there isn’t as much vertical force generated, the soft tire damps more of the impact energy, hence a softer start point and less work necessary to get the same effect from the FSD feature. Same result? Could well be, or at least similar.

I do believe the experimental route is the only one that makes for measuring our actual ride quality.

I totally understand what you are saying, trying to identify what tire damping frequency works best. And translating that to an increase or decrease in setting pressure.

Personally I don’t think it is that simple, as there are so many variables in tires. Are we looking at one size, or needing to accommodate different rim diameters, various sections and aspect ratios, and of course tire construction, like our friend the run-flat.

From my reading, we are looking at unsprung mass shake frequencies, and tire characteristics definitely affect those frequencies. But as tires of the same size and specification can have up to about a 20% difference in spring rate, at a given pressure, we obviously don’t have any fixed baseline, even for a given wheel size.

We are into complex factors, such as natural damping frequencies, dynamic stiffness and amplitude decay rates, all variable with pressure, load and speed.

From my studying this subject, the fact that the tire spring rate is of an order of magnitude higher than the spring rate of the suspension system, we shouldn’t be impinging on the natural frequency of the FSD valve feature, with a little variation in tire pressure.

IMO, as an engineer, (but not a tyre/suspension engineer), I see any potential variation in ride quality with the Koni FSD fitted to a given suspension system, being down to tire size, specification, and pressure setting, rather that it ‘changing’ the FSD dynamics.

From my understanding, the greater changes to the suspension frequencies would be the actual spring rate, of OEM/aftermarket springs. That is where I’d be giving most attention to how the FSD feature is affected.

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      08-31-2011, 09:43 AM   #7
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If you fill the tires too much, the FSDs can feel too bouncy over bumps.
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      08-31-2011, 09:45 AM   #8
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Quote:
Originally Posted by Drivaar View Post
anerbe: heh, that's the point of this thread... to see if there is someone who can help us think through whether you want the tires softer to absorb a little bit of vibration or whether that would end up reducing the frequency enough that the FSDs aren't able to do their thing optimally.

Seems like what we really need is the spec on the Koni FSDs. On the web site it says that the shocks are really differentiating between vibrations of ~10 Hz (where the shock behaves soft) and 1 Hz (where the shock behaves stiff).
Drivaar - I understand your point. However, asking for a smoking gun answer to whether running any tire at a higher or lower PSI will work on your suspension will not work. There's too many variables and unknown technical details to determine an optimum setup for all tires, all road conditions, etc...

Like I said earlier, and what HighlandPeter explained, you need experimentation / trial and error with your current configuration. +/-5 PSI does make a considerable difference in dynamics, but know that what you do gain in some performance areas, you trade off in others.

Lets just say that I'm a tire engineer who's currently at the Green Hell doing testing to approve a tire spec for a certain OEM...
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      08-31-2011, 11:25 AM   #9
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Quote:
Originally Posted by GaryS View Post
If you fill the tires too much, the FSDs can feel too bouncy over bumps.
It may be a factor, but as the same thing happens with many other suspension setups, it could be a simple result (cause and effect) of changing the pressure of the tire, rather than changing the FSD operation.

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      08-31-2011, 01:01 PM   #10
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Quote:
Originally Posted by HighlandPete View Post
It may be a factor, but as the same thing happens with many other suspension setups, it could be a simple result (cause and effect) of changing the pressure of the tire, rather than changing the FSD operation.

HighlandPete
I doubt that too much air pressure changes the FSD operation at all. But it can make the shocks work too hard. It's worth noting that Koni sells the same FSD (same part number) for a 128i coupe with 108 lb/in front springs and a 335i convertible with 145 lb/in front springs.
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      08-31-2011, 04:47 PM   #11
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Great points from everyone!

GaryS: that is very interesting info. Based on that, my gut feeling is to reduce the tire pressures a little bit to soak up a few more bumps and then leave the rest to the suspension.

I agree that it's hard to say for sure what will work best, aside from trial and error as most people have stated. That said, I got a loaner with 17-inch low-performance RFTs and non-sport pack, and DAMN, the FSDs with 18-inch wheels and high-performance tires are MUCH more comfy. So yes, this question is a little silly in the sense that it's about fine-tuning from one good situation to an even better one.
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      09-01-2011, 05:48 AM   #12
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Best option is to do a test and tune day at a local track. Besides the subjective evaluation from the feel and inputs of the tires, your track times will provide you data on what the best pressure to run is.

Of course what conditions run on the track vs. the road will vary. Since you aren't setting times on the public road, drive the same course and test different pressures. The ride that is the most compliant and stable under impact/shock will probably be best for public roads. The last thing you want is a tire that bump steers at every uneven surface.
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      09-01-2011, 07:14 AM   #13
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anerbe: Agreed if that is the priority. In my case, I'm trying to tune for optimum street comfort. I was wondering if you had any thoughts on that with respect to tire psi and the shock working together.

Basically the question is whether a 10% drop in tire psi will help the tire do a better job of absorbing impacts than keeping the tire psi higher and letting the shock do it.

Thinking further, the answer may be obvious... changing the shocks did FAR FAR more for my ride quality than messing with tire pressure with stock shocks.

So clearly, the shocks are the ones that are best at absorbing bumps. Therefore... maybe it makes sense to have the tire pressures at stock so they transmit more of the actual road undulations to the shocks, letting the shocks do their thing.

Thoughts?
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