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Damper tuning for Race Applications

 
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tErbo b00st



Joined: 24 Mar 2006
Posts: 10

PostPosted: Wed Jun 21, 2006 12:45 pm    Post subject: Damper tuning for Race Applications Reply with quote

I recieved a shock dyno for a certain shock. I dont want to say which one as I'm not sure I can disclose that information, but anyways. Here it is (rebound on top, bump on bottom):

http://www.driftsauce.com/forum/index.php?action=dlattach;topic=1059.0;id=379;image

I have always thought that a digressive curve for both rebound and bump was the proper way to valve a shock (atleast one that is only bump adjustable ie most street coilovers/dampers). I know that you want about 2.5 times the rebound as bump as well. This shock feels awesome on the car, but I dont have much experience with different coilovers, let alone full race dampers.

However, I was told by the designer of these shocks that for race applications you want linear rebound and digressive bump. Linear rebound because that mostly controls the energy that is stored in the spring when it is compressed and a spring is a linear device, hence you want a linear damper force to control it. That makes perfect sense to me. Moreover, a digressive rebound will give a better ride for a street car, and that is why most shock dynos that you see online have a regressive curve.

Is he generally correct in his statement, or was my original thinking correct? Mabye its 100% dependant on driver and track conditions. I also read somewhere that for autocross digressive is generally better, while for roadracing a linear plot is better. I have been trying to do a lot of research on the subject, but its so hard to find credible information on the internet (I'm going to buy a book REAL soon).

Please discuss basic damper tuning for race applications. I am fully aware of the different ways to tune a damper on the track, but what about the theory in desiging a damper.

BTW, he also showed me the CVP plot, which I dont have, and there was very little hystrisis.
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Wiisass
Head suspension nerd


Joined: 09 Nov 2005
Posts: 131
Location: Philly

PostPosted: Sat Jun 24, 2006 1:49 pm    Post subject: Reply with quote

I think I know what shock that is. At least I'm hoping it's the one I think. Is it the one Rob was working on getting tested for the past couple months? Why'd did whoever dynoed the shock switch the axis? The plot looks upside down. Who dynoed the shock? And what dyno are they using? I know it's Roehrig, but do you know what model.

Well if it's a front strut for an S13, with a 8kg/mm spring, then it looks pretty good. If not, then I couldn't tell you what I think about it because I wouldn't have enough info. But if I'm right, it looks like the Blue line for rebound is about critically damped and compression looks to be around 50% of critical throughout the range. I would've like to see a little more out of compression, but that's just me.

Now about the theory, I agree with the guy who designed those shocks, linear rebound and digressive compression. Digressive compression because that will allow you to run a much stiffer damper for low-speed stuff and then for high speed situations the damper will soften and allow the spring to absorb more of the force. Rebound on the other hand, well I think that you shouldn't see much high speed rebound unless something else is wrong and that would need to be fixed first, so rebound doesn't matter as much whether it's linear or digressive. But I think it should be at least linear through the low-speed region of the curve. A rebound curve that will digress too quickly could give a bad feel to the car. But as long as none of it's progressive, you should be alright.

I don't agree about the ratio of rebound to compression. That depends on a lot of things and doesn't always come out like you would expect. Especially with a digressive damper, you have more freedom to run a lot stiff low speed and still maintain a soft high speed, so the ratio of rebound to compression for the low-speed region could be 1:1, but for the high speed it could be 2:1, assuming a linear rebound curve and digressive compression. It's seemed to me that the reason for the higher ratio has been that in the past, digressive shock curves didn't happen as much, so if you're stuck with a linear curve, you're going to have to run a lot softer throughout the range because it's all compromise and sacrifice. And having a more underdamper low-speed region is a much better choice than having a higher damped high-speed region.

It seems like the shock designer knows what he's talking about. I still really think that high-speed rebound shouldn't happen. When I was using 4-way adjustable dampers, I never felt the need to adjust it and the driver's loved the way the car handled. When you were talking about the differences between autox and road racing where you just talking about the rebound side of the curve or the whole curve in general? About a better ride, rebound doesn't really effect the ride too much, the biggest part that will affect ride quality is high speed compression. This is why would want high speed compression to be soft. The higher the damping ratio the more force is transmitted through the damper into the chassis.

Which book are you planning on getting? I haven't been able to find a good book about race dampers yet. Shock Absorber Handbook by Dixon is said to be the best, but I wasn't too impressed with it. I have found several papers and other smaller articles that are very well written and all seem to contain small parts of what I would like to see in a book. I need to get around to putting them up the site somewhere, I'll hopefully have some time next week to do this. Most of them are very technical, but there are a couple less technical articles that are very good to read.

Just a general note about damper design, it's hard to say what amount of damping you want. I mean it does depend a lot on the car and a lot on variables that can be measured, but that's not the whole story. It seems that you can usually get away with running somewhere between 60-80% of critical based on natural frequencies for rebound and compression in the low-speed region and then as long as you have a digressive compression curve, you'll be fine with high speed or you'll have to accept the slightly higher force transmissibility that you will have because of the increase in damper, but if it's for a race car, then it's going to have to be accepted, as long as it's not upsetting the chassis.

Simulations and computer work will also help a lot in the design phase. I've been working a lot on designing some shocks for a supra and I've spent way too much time staring at excel spreadsheets and matlab code, but it will hopefully pay off in the end. Unfortunately the dampers for that car are not adjustable and my dyno is 500 miles away from the car, so I need to get the curve right the first time.

You have to keep in mind, that past successes and failures also have a lot to do with design. And testing is another important part. I haven't had the opportunity yet, but a lot of people seem to like Claude Rouelle's damper speed histogram method for tuning dampers. But a very good test driver who knows what he's feeling and can be very objective is also a very important tool to have. Of course, all of this would happen after the initial design. And of course, damper settings can be different for each course and each driver, but if you're in the right range and the driver's have similar styles, you should be fine. And most people won't be able to tell the difference with small changes in the dampers.

Well, that's all for now, let me know what other questions you have. And I'll try and catch you on AIM somtime this week, I have some other questions for you.
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tErbo b00st



Joined: 24 Mar 2006
Posts: 10

PostPosted: Mon Jun 26, 2006 11:04 am    Post subject: Reply with quote

If your talkin about Rob Riter, you are correct. Its not upside down, the way the axis is oriented is up to the tester/machine. I have wondered myself why it isnt a standard orientation, but have fun it is strictly up the the tester and machine.

It is the front strut for an 8kg/mm spring, where the "recommended" setting is 8/15. Although I am running a 9kg/mm...not a big difference.

What is the critically damping that you are tlaking about? I've never heard that. If you dont want to explain it just tell me to go read a book...I will Smile.

First off, thanks for your take on the dyno plot. I was doubting the designers theory, but it seems I shouldnt be.

When I said a digressive is better for autox I was talking about rebound. From what I've read a digressive compression is always the best.

I dont really have much else to say on your explanation because I either agree with some of it 100%, or dont know enough to comment on it, hah.

I'm going to buy this book http://www.amazon.com/gp/product/1560915269/ref=pd_cp_b_title/103-3373722-3496620?%5Fencoding=UTF8&v=glance&n=283155 It seems like a very informative book. Its not just on dampers though, its on suspension tuning in general. The reviews look great.

I'm not on AIM too often, but deff try and hit me up if you do catch me on there.
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Wiisass
Head suspension nerd


Joined: 09 Nov 2005
Posts: 131
Location: Philly

PostPosted: Tue Jun 27, 2006 2:30 am    Post subject: Reply with quote

If Rob Riter is the guy at Touge Factory. The curve is upside down to me, I use a roehrig dyno and the same software but with compression on the top and rebound on the bottom, so I'm used to looking at it the other way. But it doesn't matter either way, it's still the same curve.

The 9k spring should work pretty well. Are you still running the recommended setting? If not what are yours set at and what spring are you running in the rear, 7k?

Criticallly damped is a vibrations term. It's when your damping ratio is 1. Basically, the lower the damping ratio, the longer a body takes to settle and the greater the overshoot of the steady state value. So when something is critically damped, there is 0 overshoot and the settling time is the lowest. With a damper, you can decide on the damping ratio for both compression and rebound seperately, this is where the guessing and the tuning comes in. When you adjust a shock, you're adjusting the damping ratio.

Race Car vehicle dynamics is a great book. It should be required reading for anyone who wants to discuss suspension. I actually had the chance to talk with Doug Milliken, the son, back in May. We actually talked about drifting for a little and the problems with greatly lowering a macpherson strut car. It was kind of funny talking about drifting with him, but he seemed interested. He had only heard of it, wasn't too familiar with setup, how competition worked. But I definitely recommend his book. Another good set of books is the Carroll Smith set. The tune to win, prepare to win, etc.
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MistaTwo



Joined: 09 Mar 2007
Posts: 18
Location: Pittsburgh, PA / London, England

PostPosted: Mon Mar 12, 2007 4:35 am    Post subject: Reply with quote

another book that has a decent section on Dampers in it is called "Competition Car Suspension" by Allan Staniforth. i'm halfway through it, and it is a very good read
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