[stretch]

Originally Posted by 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...

Well, you're talking about a different thing then- steering inputs are a totally separate issue. Those things tend to happen below 3in/second from what I've seen and read. What I'm saying is that it might make sense to continue that higher low-speed damping rate beyond that point to improve both comfort and grip over bumps. Whether this is the case will depend on the magnitude of the bump. This might explain why Koni and others seem to do just that- the kink in the dyno chart you linked appears beyond 3in/sec (closer to 5in/second, but again these Koni dynos don't have nearly enough granularity to know where it is for sure). Basically I'm trying to find a justification for the way we see some dampers valved.

I imagine the low-speed bumps we're discussing are most often seen in real life as a speed-bump taken slowly or a gradual elevation change taken at speed. Since you do mostly race car stuff, it's only the latter case you need consider, but I'm personally interested in abrupt speed hump damping since I have to do three every time I leave my house.

Edit: I just did some quick estimates of what a low-speed bump would be (added to original post). Low-speed bumps in the real world aren't going to look like bumps at all since they'd be too gradual (they'd have to be very rounded), and I'm not sure if they are even worth worrying about. Anything that looks like a bump (like a manhole cover, pothole, etc) gets way into the high-speed damping really quick- even at just a few miles per hour. I still suspect uneven asphalt is worth analyzing since it is likely the only practical case of a 2.26hz bump, but I doubt it's important relative to other criteria.

I guess my tentative conclusion then is to go for 50-70% CD in the car control region (3in/sec and below) and as little bump damping as possible thereafter (providing you don't need high-speed bump to prevent the car from bottoming out). That 3-6in/sec region might have some comfort ramifications but I suppose I now see that merely as the transition period to the target high-speed damping. Rebound damping should have a strong fall-off too, but with a lower limit. The reason for this, I've read, is that you don't want a wheel accelerating back towards the ground too quickly or it'll essentially try to bounce back off of it when it hits. I'm not sure if this is better or worse than slowing (delaying) the tire's descent towards the ground.

From my modeling I really like the idea of being 60-70% damped for low-speed bump and rebound, 25-35% damped for high-speed bump, and 35-50% damped for high-speed rebound- with the transition point being at 3-4in/second. I'm actually having my TiC front dampers revalved to get within these ranges as an experiment in damper theory, so we'll see how it goes. (They already feel great, so if I can actually find room for improvement I'll be quite proud.)

[zzyzx]

Originally Posted by stretch

Basically I'm trying to find a justification for the way we see some dampers valved.

I think you'll find that the justification for the damper characteristics of a motorsports-oriented damper revolves around handling performance, not comfort. It would probably be beneficial to compare these dynos to a purely street/comfort damper to see where they differ.

Originally Posted by stretch

I imagine the low-speed bumps we're discussing are most often seen in real life as a speed-bump taken slowly or a gradual elevation change taken at speed.

If you go slow over a speed bump, you don't involve the dampers very much (=~ 100% transmissibility). If you go fast, it's no longer a low speed bump, but would have something less than 100% transmissibility. That leaves me scratching my head as to where these low speed bumps truly live in the real world...

For your given setup of dampers/springs/corner weights, there's an optimal speed to go over that speed bump. If you really want to get over those 3 speed bumps with the lowest transmissibility possible, then with all the information you have at your disposal you could model the speed bumps and calculate your optimal speed over them.

My hunch tells me that this speed is probably greater than the posted speed limit in that area.

[4banger]

Just incase anyone was curious about what a X Hz bump relates to:

[4banger]

Here is a graph for a Stock '05 STi. I labeled the cross over point, although it is really a moot point once you look at what size bump applies to the "low speed" region. The percents next to the damper speed labels are the %Critically Damped (beta in stretch's graph).

[zzyzx]

Thanks, 4Banger.

I saw your EDIT, Stretch.

So, I think the apt conclusion is to refer to low speed damping as "handling" and high speed damping as "comfort". That's not to say that high speed damping doesn't affect handling - it does, the degree to which is a function of the surface irregularities. On road courses that aren't smooth and certainly any off road situation, high speed is as critical as low speed for car control.

This is also why DG (an autoxer...) focuses on 2 in/sec. In autox most (not all...) parking lots are smooth, so high speed damping becomes somewhat irrelevant. The low speed adjustment is where you're really going to feel it in slaloms, corner entry, etc. It's when you drive home from the event that you'll have to deal with real-world road conditions and your high speed damping comes into play. In practical terms, if you have to drastically change your shock settings for the trip home to avoid having your teeth chatter, then they're probably not digressive.

[BigSky]

this may not be totally germane to the discussion, but my actual experience w/ the Koni SA 8610's- the high speed damping (most likely bump- which is non-adj w/ the 8610) appears to be at "fault" for my only complaint about these dampers. Small seams/cracks/bridge abutments and the like are the only time I would describe my ride as not comfortable. It seems to be picky as some seams/cracks have no effect, others do. I've talked w/ a couple of folsk that have had the 8610's revalved to dial out some high speed compression and they've been pleased.

Also I was told that as you INCREASE spring rates, LESS high speed compression is needed- is this true?

[4banger]

Steve,
I would agree with you on calling the "low speed" region something more appropriate like the handling or car control region.

I would encourage everyone to run their own numbers and not rely on what you have seen posted on the internet.

This level of analysis is great for getting the big picture and drawing some conclusions, but if anyone is interested in taking it a step further, refer to Fundamentals of Vehicle Dynamics by Gillespie page 156, figure 5.19 for how the graph of transmissibility changes once you account for the tire spring. Steve - I thought you would appreciate the reference.

[4banger]

Originally Posted by BigSky

Also I was told that as you INCREASE spring rates, LESS high speed compression is needed- is this true?

I wouldn't make a blanket statement like that because you need to treat things as a whole system. I've come across some SAE papers that suggest for our vehicles 30-40% critically damped at high damper speeds. Either increasing spring rate or decreasing damping will get you going in that direction. I would start with the damping first.

[3RotorRX7]

Does anyone have the equation for critical damping?

Thanks,
Erik

[4banger]

Originally Posted by 3RotorRX7

Does anyone have the equation for critical damping?

Thanks,
Erik

I'll trade you the equation for the front and rear motion ratios and a stock damper plot of an RX-7

[3RotorRX7]

Originally Posted by 4banger

I'll trade you the equation for the front and rear motion ratios and a stock damper plot of an RX-7

The motion ratio of the front is 0.60 and the rear is 0.68.

Erik

[3RotorRX7]

Here is a link to a site with stock dyno curves:
Suspension Upgrades

Originally Posted by 4banger

I'll trade you the equation for the front and rear motion ratios and a stock damper plot of an RX-7

[4banger]

OMG, you are awesome. I was really just kidding about the trade, but I am analyzing various cars noted for their excellent stock handling and finding the data has been a royal pain. Judging by your post count, I'd say a welcome to the site is in order. Are you considering an STi, or are you just curious about suspension theory?

Stretch has a really good spreadsheet with pretty much every equation you could want to know. I'll pm you a link to it in a sec.

[3RotorRX7]

Originally Posted by 4banger

OMG, you are awesome. I was really just kidding about the trade, but I am analyzing various cars noted for their excellent stock handling and finding the data has been a royal pain. Judging by your post count, I'd say a welcome to the site is in order. Are you considering an STi, or are you just curious about suspension theory?

Stretch has a really good spreadsheet with pretty much every equation you could want to know. I'll pm you a link to it in a sec.

Thanks for the welcome. I am working on setting up a friends STi for STU autocross, and will be using the information on his car. But, I am wanting the equation to see if my settings on our 2800 series Koni's we use on our RX7 is correct, also. I saw Stretch's link, but it didn't work any more.

Thanks,
Erik

[stretch]

Ruh-roh. It should work! I wonder if there's a typo'd link somewhere, since the file location hasn't changed: http://www.fromsteve.net/carstuff/su...ensionCalc.zip

Edit: crap, you're right. I'll fix it shortly.
Edit #2: I just moved my site to a new server; it might take a few more hours for your machine to see the new location, but it'll work soon.