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30 Dec 2005, 22:41 (Ref:1492217) | #1 | |
Racer
Join Date: Dec 2005
Posts: 246
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Dampers
Hi all!
I need your opinion about a damper problem. I am trying to use the damping ratios to know something about the dampers to use on a car. I'd like to have a idea of which kind of curve the dampers should have before to go on the track; what i want to obtain is a damper curve that needs only to be adjusted on the track after the first test days. My questions are: 1) the tipical curve of force/speed of a damper shows a decreasing value of the ratio "c=force/speed" as the speed grows up. In which zone i have to analyze the damping ratio (damping ratio = c/cr where cr is the critical damping)? 2) which values the damping ratio should have? thx in advance for the replies! |
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30 Dec 2005, 23:19 (Ref:1492242) | #2 | ||
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What sort of suspension travel are you talking about?
We'll assume this is a circuit car. |
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__________________
"...full of sound and fury, yet signifying nothing...." |
30 Dec 2005, 23:21 (Ref:1492243) | #3 | |
Racer
Join Date: Dec 2005
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yes...
it's a formula car... we talk about short suspension travels, more or less 15 - 20 mm at the maximum... |
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3 Jan 2006, 15:15 (Ref:1494187) | #4 | |
Racer
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up...
nobody can help? |
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3 Jan 2006, 15:28 (Ref:1494194) | #5 | ||
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For most of the circuit dampers I have seen, it's relatively linear in the first part of the velocity range (though you may be completely different). Given the body modes (pitch, roll, heave and warp) usually are low velocity motions I could initially study the lower velocity regions.
If you were lucky, 0-50 mm/s would be relatively linear. Anyone else? |
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3 Jan 2006, 18:18 (Ref:1494272) | #6 | |
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yes, the curve of the dampers i use is almost linear for the first part (the low velocity zone). Then it decreased its slope in the zone of the high speeds.
What i'd like to know is if anybody knows the way to adjust the dampers curve on the base of the critical damping and of the damping ratio.If you use this way you should be able to reproduce the right conditions also when motion ratios, masses, and spring rates are changed. Something like the adimensional numbers of the aerodynamics (Reynolds, etc) I think this is the only "scientifc" way to prepare the "right" damper before to go on the track. Then on the track you will adjust it using data logging..But in this way you should be able to chose the right window. Anyone knows how to use this methods and some values for damping ratio? |
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4 Jan 2006, 00:25 (Ref:1494481) | #7 | |||
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Quote:
IRL tracks are smooooooooth so you can throw a heap of damping at them, whereas street circuits require a lot more damping. As much as we (people who try and tune cars) would like to use dampers to control the body motion, on most realistic tracks, controlling the high frequency input is very important. I'd consider a damping ratio of 0.6-0.7 for a track car if you were looking mostly at low speed though. |
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4 Jan 2006, 10:20 (Ref:1494650) | #8 | |
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i have analyzed some dampers and i have seen that at low speed, in the first part of the curve, we have damping ratio of 1.5 with the softest solution and of about 3 with the harder one...it makes any sense?
I'd like to ask of it because analyzing also other dampers i have seen similar situations... obviously, in the zone of the high speeds the damping ratio goes down, but with the hardest solutions it is always higher than 1. Somebody who works on formula cars has any idea?? |
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4 Jan 2006, 10:48 (Ref:1494671) | #9 | ||
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Quote:
2) see what I have written in 1) above. Damping ratio can be in the range 1:1 through to 3:1 rebound:bump for race damping. Softer springs give longer travel and will require more rebound damping at higher piston speeds - ratios of 5:1 are found in rallying where large displacements and relatively soft springs give high spring energy and high piston velocities. |
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14 Jan 2006, 16:02 (Ref:1501192) | #10 | ||
Racer
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Quote:
Allan Staniforth book "Competion Car Suspension" tells you how its all done if you want free advise speak to the UCLAN crowd at NW FF rounds they do design stuff upto degree level they poeple change every year but the third year students know about that kind of stuff . There is a damper technician who contributes to this forum If I remember who it is I will contact you and tell you his name .Regards Martin |
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15 Jan 2006, 18:55 (Ref:1501807) | #11 | ||
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Quote:
However, I would also dispute that the book 'Competiotion Car Suspension' tells you 'how it is all done'. In fact - and no offence at all to the amazing Mr Staniforth - this book leaves many un-answered questions about damping. |
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16 Jan 2006, 08:26 (Ref:1502145) | #12 | ||
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Sorry mate, I took damping ratio as the ratio of damping to critical damping, not the ratio of bump to rebound.
No real book covers dampers that well. Milliken doesnt, neither does Gillespie. Apparently Dixon does in the shock absorber handbook but I must not get the point. The way I see it, it's the battle between using dampers to control the body during weight transfer and the need to reduce transmission of the road to the chassis. Unfortunately, its very difficult to get both right on a non-oval track. Also, how do you guys use data for damper adjustments? I find the roll and pitch speeds quite useful for determing the control of the body, and the PSD for the control of the unsprung mass over bumps (ie higher frequency). |
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16 Jan 2006, 09:09 (Ref:1502168) | #13 | |
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If I'm the 'mate' you are referring too, I did mention that the damping compared to cr would likely be in the range 0.5 cr to 0.7 cr - in ratio terms that would be expressed as between 2:1 and 1.4:1 (cr:actual damping force)
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16 Jan 2006, 09:14 (Ref:1502169) | #14 | ||
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Quote:
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16 Jan 2006, 09:23 (Ref:1502178) | #15 | |||
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Quote:
During a transient the dampers are a huge part of determining the load transfer. What purpose are you referring to? One of the main steps forward in the last decade has been the use of sophisticated dampers to allow relatively high low speed damping without excessive high speed damping that would reduce compliance over bumps. This is what digressive dampers are for. If you can use the dampers correctly to control body motions you can get away with lower spring rates which would give further improvements in wheel load control. Ben |
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16 Jan 2006, 10:18 (Ref:1502225) | #16 | ||||
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Quote:
I was referring to silente: Quote:
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16 Jan 2006, 12:32 (Ref:1502330) | #17 | ||
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Quote:
The main reason for the need for dampers which offer higher resistance at lower piston speeds and less at high piston speeds is the huge relative increase in spring rates which are used on cars with aerodynamic downforce over those with little or no downforce. This results in less displacement at much the same (loaded) wheel frequencies and so slower piston speeds. If the initial stiffness required for low piston speeds increased at the same rate as earlier dampers, as piston speeds increase - as the car travels over bumps or kerbs for example, then the dampers would become over hard and more energy would be put into the chassis than is desirable. Dampers with different high and low speed valving and/or adjustment therefore become more important the more downforce your car generates. If you have little or no downforce then dampers with a digressive rate should offer no advantage. As for your last point, I always prefer to think of the damper as controlling the energy in the spring: Bump damping determines how much and at what speed energy is 1) put into the spring, 2) how much is fed to the chassis and 3) how much is turned into heat. Rebound how much and at what speed the energy in the spring is released back to 1) the wheel/tyre and 2) the chassis and 3) how much is turned into heat. Maybe I'm wrong? Last edited by phoenix; 16 Jan 2006 at 12:33. Reason: spelling! |
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16 Jan 2006, 15:39 (Ref:1502436) | #18 | ||
Racer
Join Date: Dec 2005
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Quote:
In my opinion too was strange to find that values; but they was C/Cr... In the first part of the curve we have a damping ratio C/Cr of about 3 in compression at low speed with the hardest solution. about 1.5 with the softest one. I have to say it's a monodamper solution. In your opinion which is a good value for C/Cr in both compression and rebound for low and high speeds? Because at high speed i have found some dampers with the 0.7 you were talking about. But only at high speed. |
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16 Jan 2006, 17:03 (Ref:1502501) | #19 | ||
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Quote:
In real force terms (lb/ft or Nm) what are the magnitude of the forces you have calculated for cr, and the maximum and minimum damping forces your dampers provide and at what piston speed? |
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16 Jan 2006, 18:50 (Ref:1502577) | #20 | ||
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Claude Rouelle is looking at seting up the spring/damper unit in his latest newsletters. If you visit www.optimumG.com, you can find the newsletters in pdf format. The company is looking every issue (month) at 1 step in getting the right setup by calculus. Even if it is not finished, you may find it interesting to tis point.
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17 Jan 2006, 09:06 (Ref:1502929) | #21 | |||
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Quote:
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17 Jan 2006, 21:33 (Ref:1503390) | #22 | ||
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Parts 1-3 are in the archives/tech tips section of the optimumg web site.
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18 Jan 2006, 06:17 (Ref:1503552) | #23 | |||
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Quote:
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18 Jan 2006, 09:16 (Ref:1503609) | #24 | ||
Racer
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Quote:
I have calculated Cr as Ns/m (= N/m/s) not as Nm, because dampers work on speed, not on the change of position from a certain start position. After i had calculated Cr as showed by Mr Rouelle, i had also calculated my c as the ratio Force/speed referring to the first part of the curve (low speed) and to the last one (high speed). And i had found that C/Cr have that values i have said. I am not sure to have done this calculations in the right way, but i think it's the most correct way, accroding to Mr. Rouelle and to some books. Did i make mistakes? |
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18 Jan 2006, 09:50 (Ref:1503623) | #25 | ||
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I can't tell if you have made a mistake without knowing your numbers, as I cannot check your calculation - and that you are using the correct units. However, in the aformetioned articles by Matt Giaraffa, he does say in the October newsletter that C/Ccr ratios of 0.65 - 0.7 are a good starting point for a race setup, but that some teams end up using a ratio higher than 1 - i.e where C is larger than Ccr - so maybe with your mono setup the damping is correct. But he does not mention ratios where C is 3 times as great as Ccr. In my experience, with that much damping the car would jack down over a series of bumps and kerbs and the car would end up sitting on the bump stops after less than a lap. |
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