[MGizzle]

Originally Posted by DuncanG

confused and speeling dont help

5in/sec on a road car is nothing. On a cross-country hack I've seen (estimated by video monitoring) 40"/sec compressions and thats with 'overdamped' Ohlins. But of course thats bump damping not roll damping.

Imagine going through a chicane and transferring from 2" droop/bump to 2" bump/droop, so 4" travel. How long do you want to take for that to take a set? Thats mainly a function of the sprung frequency (unless very overdamped) so lets assume its sprung for 2Hz (in heave) and the bars roughly double roll-stiffness so we're talking 4Hz in roll. From trough to peak (or full-left to full-right) is 1/2 cycle so that gives us 0.5 secs for the transition - seems reasonable. Now 4" in 0.5 sec is 8"/sec.

I'm sure Stretch will check whether a 2Hz spring setup is consistent with 2" bump/droop at full tilt but I think its in the ball-park.

Don't get me wrong, I know there are damper velocities above 5in/sec, and I have seen quiet higher velocities too, but that is when I hit something while going fast such as a runing over the gator strips or hitting really bad pot holes.

All I was trying to say is that it doesn't make sense to dampen at 0.7crd at those velocities and it doesn't make sense to see those kind of velocities in roll or heave either, if you are road racing.

Sorry about the spelling, I type faster than I can speak..

[4banger]

Sort of like this data Hoosier Tire ?
The only other data I have been able to find is Cooper Avon Tyres Racing Website. Unfortunately I haven't been able to find any data on "normal" tires.

Originally Posted by MGizzle

What happened is that a lot of FSAE teams payed Milliken's company some money to do some tire testing for the 10'' and 13'' hoosier, good year tires etc. (slicks). What the test was is pretty much what the OEM's due to test tires.

[lackskill]

Originally Posted by 4banger

Sort of like this data Hoosier Tire?
The only other data I have been able to find is Cooper Avon Tyres Racing Website. Unfortunately I haven't been able to find any data on "normal" tires.

fixed the hoosier link.

[4banger]

Thanks. I fixed mine too.

[MGizzle]

That data is static and basic. I am talking about data while the tire is being loaded and rolling at high speeds while at the same time slip anagles etc. are placed into it. It gives you plenty of very fancy and complicated graphs etc. It is hard to compensate for all of them so you mostly focus on one aspect that you design to and just proceed with that.

As far as finding data on a regular passanger tire? That is THE trade secret of every tire company. It is like comparing BMW design philosophy to GM's design philosophy. You can't really say which one is "mathematically" right and which one is not. It is what ever "they each" think is more important in a car design, and at the end you get "vehicles" from both that drive & feel quiet differently.

[4banger]

Sounds interesting, MGizzle. Hopefully your friends will be able to share thier data. Not that I'd have any idea what to do with it, but I think it would be an interesting read.

[Turninconcepts.com]

Originally Posted by MGizzle

That data is static and basic. I am talking about data while the tire is being loaded and rolling at high speeds while at the same time slip anagles etc. are placed into it. It gives you plenty of very fancy and complicated graphs etc. It is hard to compensate for all of them so you mostly focus on one aspect that you design to and just proceed with that.

As far as finding data on a regular passanger tire? That is THE trade secret of every tire company. It is like comparing BMW design philosophy to GM's design philosophy. You can't really say which one is "mathematically" right and which one is not. It is what ever "they each" think is more important in a car design, and at the end you get "vehicles" from both that drive & feel quiet differently.

Milliken has a huge section on this. I'll reread it this weekend and give my thoughts, but I had a hard time understanding it when it got complicated. I can't claim to be able to explain it all that well even after reading it again.

[BigSky]

does the Hunter Road Force give any hint to spring rate- I know it has lots of different programs within in it and it does put a load on the tire?????

[stretch]

I was talking with Myles@RCE about dampers earlier today and it made me want to post something I've been looking into lately. Thanks goes to 4banger for starting me on this research.

There was some talk earlier in this thread about how to define what is high and low speed damping. It's 1.414 * your natural spring frequency.

Below this point, higher damping produces lower transmissibility to the chassis. That is, you'll have more comfort AND grip. Above that point, lower damping produces lower transmissibility. Softer springs reduces transmissibility everywhere.

A transmissibility of one means your chassis moves exactly as much as your wheels do. You see this at very low speeds because the chassis just follows the slope of the road. This is akin to going over a speed bump as slowly as possible. Speed up just a little, and that speed bump will feel bigger than it really is. Everyone has felt this before. But speed up enough, and- assuming you have enough suspension travel- the bump will smooth out again.

This is because there are two distinct bump velocity ranges for a spring system: the amplification range and the isolation range. The amplification range peaks when you hit a bump or series of bumps that cause your wheel to move at exactly the same frequency as your natural frequency. The transition to isolating bumps happens at the square root of 2 (1.414) times your natural frequency. Thus, where we want this division of damping forces depends on the bump you are hitting. You can see this in the graph below as the point in which transmissibility declines. Bumps beyond this frequency will feel more muted to the extent the damper allows.


(image source)

So when someone recommends 50% CD for low-speed and 25% CD for high-speed, they're aiming for a curve that'll match the light blue line in the amplification range and match the red line in the isolation range. You're trying to minimize the amplification of a low-speed bump while maximizing the isolation of a high-speed bump. As always, there's more to it than that (since there are many sources of spring in your car), but applying these rules should get you pretty close to maximizing comfort and performance simultaneously.

However, to know where that switch between amplification and isolation lies, you must have data on the size of bump you are hitting. This magic switchover point happens at a different point (in terms of damper velocity) depending on the amplitude of the bump we hit. If I'm reading this graph correctly, the damper rates do not matter whatsoever when hitting a bump of exactly sqrt 2 times your natural frequency, which for a stock STI would be a 1.414 * 1.6hz = 2.26hz bump. Below that, we want more damping, and above that we want less.

To put in perspective what a 2.26hz bump is, that means the wheel will complete its up stroke in 1/(2*2.26) of a second. (Why multiply by 2? Because a full cycle happens in 1/2.26=0.44 of a second, which consists of an up and down stroke.) So, that's 0.22 seconds for the bump stroke to complete. Thus-

A 2.26hz bump that is 1 inch tall will result in a damper speed of 1 / 0.22 = ~4.5 inches per second.
A 2.26hz bump that is 2 inch tall will result in a damper speed of 2 / 0.22 = ~9 inches per second.

The stock dampers have a broad damping coefficient peak around 5in/second, so I think I can make a conclusion that they are optimized for 1-1.5 inch low-speed bumps at 2.26hz. However, notice that 0.25 inch bumps at 10hz would still put the damper in that peak even though we'd want a lower damping coefficient there. I suppose this is where Koni's FSD shock technology would come in handy, if I understand it correctly- those dampers change their rate based on bump input frequency, which is exactly what we'd want.

But what does it mean to hit a 2.26hz bump? Translating that to vehicle speed (in miles per hour) is a little complicated, but I estimate that for an abrupt bump (like a manhole cover) the vehicle velocity would be roughly 6in per 0.22 seconds or roughly 1.5 miles per hour. Pretty slow, eh? I suppose this means that when hitting an abrupt bump, the vehicle is always going to be in its high-speed valving range. However, a rounded bump- which most speed bumps are- would change that quite a bit. Basically, bump with square edges will produce much higher velocities than a bump with rounded edges even if it's the same height. What if you hit a gradual change of slope in the asphalt? Any time the road is not perfectly level, you could have a damper compression of a 0.22 second duration. I believe this is the most realistic example of low-speed damper transmissibility translating to increased grip and comfort even though it's a rather vague definition.

The above graph may make transmissibility look low at all damping coefficients as the bump frequency increases, but remember that bump velocities are much higher too. Having a transmissibility of 0.1 instead of 0.2 may seem like a small change, but it's a doubling in comfort and very important at highway speeds. At 60mph, your wheel will pass over a bump in just a hundredth of a second, so a 1 inch bump can accelerate your wheels at 50in/second or more depending on how much your tire absorbs. You wouldn't want to feel that through the seat- you wouldn't even want to feel 0.2 of that. This is why very low damper forces are desirable at high damper velocities. Furthermore, I've read that peak bump velocities are frequently double that of peak rebound, so digression (a low damping coefficient at high damper speeds) is more important there.

Another thing to consider which this graph does not take into account transmissibility due to your springs reacting on sprung and unspring weight. I believe the transmissibility here is simply the ratio of your sprung to unspring weight. Any acceleration upwards on your sprung weight will require an equal but opposite reaction from your unsprung weight to push it back down towards the ground. If we have 200lbs of acceleration moving upwards and 1400lbs to resist it (as in the rear of the car), the compressed springs will end up using 6/7ths of their stored energy pushing the wheels back down and 1/7th pushing the sprung weight up. You can improve this ratio by reducing your unsprung weight; something racers and OEM's have always known but it deserves mention again here.

Ugh, after reading my own post, I'm afraid I didn't really answer anything! Hopefully this'll just breathe new life into this thread since it is very relevant in understanding what critical damping values to look for.

[toogood]

me likes the words being exchanged....thats why for less head-ache...i leave the factory suspension........................for now

[zzyzx]

Stretch, could you credit that graph?

When I get a chance tonight, I'm gonna grab Milliken and look into a few points you make.

[Matt-]

You can find that graph in any Mechanical Vibrations textbook. It's a displacement transmissibility graph.

http://www.me.mtu.edu/courses/meem3700/lecture_10.pdf

Edit: Not to nitpick, but the transition occurs at sqrt 2, not pi.

[DuncanG]

Originally Posted by stretch

...There was some talk earlier in this thread about how to define what is high and low speed damping. It's 1.414 * your natural spring frequency.....
The stock STI spring rates produce a natural frequency of 1.6hz front, 1.8hz rear (roughly). Thus, a damper for those struts should switch from their low to high-speed valving at roughly 60mm/second front, 65mm/second rear.

Sorry to be be dim but, how do you get from 1.414*1.6Hz to ~60mm/sec?

[stretch]

Originally Posted by zzyzx

Stretch, could you credit that graph?

When I get a chance tonight, I'm gonna grab Milliken and look into a few points you make.

As Matt said, that graph is lots of places. I just did a Google image search for one (since my original sources were embedded in PDFs), which happens to be borrowed from something unrelated to suspensions. I edited the post to note the source, but Matt's link has much, much better graphs- great find.

But Matt- where did I say the transition happens at pi? I don't see the error, but maybe I'm blind. (Edit: duh, I am blind. Fixed.)

I'd love to hear some commentary on the subject if you have any!

Originally Posted by DuncanG

Sorry to be be dim but, how do you get from 1.414*1.6Hz to ~60mm/sec?

Holy crap, you're totally right. I had my units wrong and the numbers you see included a metric conversion that... well, nevermind what I was thinking, it was wrong. I completely changed my original post. Good thing you caught that, because I was drawing conclusions that I shouldn't have. (I'm just learning this stuff myself.)

[zzyzx]

Stretch,

I think the only thing I'd suggest is that you refer to "low speed bumps" as "steering inputs", "body control", "transitional control (e.g. slaloms...)" or the like. This will make it much more tangible to those trying to understand how this translates into improving the handling performance of their suspension. In the real world, I'm not sure just how many low speed "bumps" exist...

Furthermore, I've read that peak bump velocities are frequently double that of peak rebound, so digression (a low damping coefficient at high damper speeds) is more important there.

KONI seems to think so: Graph of 8611