Dampers

[silente]

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!

[Chucky]

What sort of suspension travel are you talking about?

We'll assume this is a circuit car.

[silente]

yes...

it's a formula car...

we talk about short suspension travels, more or less 15 - 20 mm at the maximum...

[silente]

up...


nobody can help?

[Lukin]

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?

[silente]

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?

[Lukin]

Originally Posted by silente

I think this is the only "scientifc" way to prepare the "right" damper before to go on the track.

Given dampers are as much for ride control and reducing contact patch load variation over bumps, can you really work out ideal damping before you get to a track without a 7 post rig?

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.

[silente]

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??

[phoenix]

Originally Posted by silente

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!

1) initial values for rebound should be calculated at static ride height. Knowing the weight of the car, the spring rate and the deflection you can calculate the energy stored in the spring which needs to be damped. Then you can calculate the piston speed should the weight of the car instantaneously be removed from the spring and damper. Once you have calculated the critical damping set the rebound at 0.5 of critical damping. Then test. Smoother tracks and not using kerbs will allow more damping - maybe up to 0.7 cr. Damping may have to be < 0.5 cr on less smooth tracks or where kerbs are used. Bump damping will be in the range of 1:1 of rebound damping at low piston speeds but as low as 1:2.5 at higher piston speeds, but, as you have noted, your dampers already show this characteristic. For setting purposes, and assuming your dampers are valved correctly for your cars weight and springing, start on the same click on bump as you have calculated for rebound, then adjust in testing.

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.

[maddogf3]

Originally Posted by silente

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!

I would have suggest that you first work out the wheel frequency at the front and the rear then find out what valves blocks are in your dampers .
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

[phoenix]

Originally Posted by maddogf3

I would have suggest that you first work out the wheel frequency at the front and the rear then find out what valves blocks are in your dampers.

I had assumed that wheel frequency would be known, as without this information it is impossible to calculate piston speed.

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.

[Lukin]

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).

[phoenix]

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)

[phoenix]

Originally Posted by Lukin

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.

Try using something other than dampers to control weight transfer, then use the dampers for the purpose they were designed.

[ubrben]

Originally Posted by phoenix

Try using something other than dampers to control weight transfer, then use the dampers for the purpose they were designed.

*Confused*

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

[Lukin]

Originally Posted by phoenix

Try using something other than dampers to control weight transfer, then use the dampers for the purpose they were designed.

I probably said that bit wrong. I know damper's don't control the amount of weight transfer, I was referring the the speed of load transfer during braking, turn in, mid corner and exit and how it can be used to tune the car in transient conditions (especially if your playing with roll centre heights and changing the amount of instant load transfer through the suspension arms etc).

I was referring to silente:

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 assume he means a ratio of 1.5-3 for compression:rebound, not C/Cr, or else he must be doing something wrong, or using cement instead of oil.

[phoenix]

Originally Posted by ubrben

*Confused*

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

I must have misunderstood what was meant by weight transfer. Yes, of course dampers are critical in instantaneous, transient weight transfer, but they can't be used to sort out load transfer imbalances per se.

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?

[silente]

Originally Posted by Lukin

I was referring to silente:



I assume he means a ratio of 1.5-3 for compression:rebound, not C/Cr, or else he must be doing something wrong, or using cement instead of oil.

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.

[phoenix]

Originally Posted by silente

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.

So, let me get this right. At low piston speed your dampers set to maximum offer 3 times the resistance of the required critial damping force? And on the lowest setting they offer 1.5 times the critical damping force? I am afraid that you have made an error in your calculations of cr.

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?

[Andrei]

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.

[tzei]

Originally Posted by Andrei

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.

Allright. Now where do i find parts 1 to 3? Anyone?

[MIdsm]

Parts 1-3 are in the archives/tech tips section of the optimumg web site.

[tzei]

Originally Posted by MIdsm

Parts 1-3 are in the archives/tech tips section of the optimumg web site.

Thanks a bunch... and i feel stupid

[silente]

Originally Posted by phoenix

So, let me get this right. At low piston speed your dampers set to maximum offer 3 times the resistance of the required critial damping force? And on the lowest setting they offer 1.5 times the critical damping force? I am afraid that you have made an error in your calculations of cr.

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?

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?

[phoenix]

Originally Posted by silente

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?

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.