Selecting The Correct Springs & Dampers For My Car

[BLiNK Motorsport]

I thought I would write a short summary of the above topic as it is a question that often arises yet no one ever seems to give a decent explanation.

Most Westfields are fitted with Dampers (commonly known as shock absorbers) that have some kind of adjustment that allows the owner to alter certain characteristics of the damper and therefore alter the handling of the car.

Tuning dampers is seen by many as a 'black art' and there is far less knowledge surrounding the selection of them than there is with, lets say; engines, however there is an engineering solution to our initial selection that at least can get us 'in the right ball-park'.

The below example actually doesn't relate to a Westfield but was a study carried out for SteveD and his new Lotus Elise Sprint car - he asked us for our opinion on what Springs and dampers he should select for his intended purpose.
(The graphs shown are typical and do not relate to his vehicle)


How do we select the right valving for the damper?

Firstly we have to select the right springs with which to 'suspend' our cars on. We do this by selecting a 'Bounce Frequency' - the lower the Bounce Frequency the more comfortable a car will feel and the more mechanical grip it will have but it will feel unresponsive to driver input.

Typical frequencies are:

0.5-1.5 Hz For passenger cars
1.5-2.0 Hz for Sports/Track cars
3.0-5.0+ Hz for Aero Cars

The bounce frequency is usually set different Front to Rear by a small margin; in a passenger car the front of the car hits the bump before the rear so the rear needs a higher frequency than the front to 'catch up'. Race Cars are often different, where comfort is no longer a consideration it can be advantageous to have a higher front bounce frequency as it can assist corner 'turn in' and improve corner exit grip.

Once we have a target 'Bounce Frequency' we can use formulas to calculate our spring rates using the sprung and unsprung mass supported by that wheel and the motion ratio of the spring (angle it's inclined at in simplistic terms).

This then allows us to select the correct springs for that vehicle dependant on it's use.

Now that the springs have been selected we can look at what damping is required to control that spring.

The first consideration we need to take into account is 'The Damping Ratio'.

The Damping Ratio

Critical damping - when a spring is un-damped, compressed and released it will continue to cycle until internal forces within the spring bring it to a stop. Damping of the spring will slow this cycle down. Critical Damping is when the spring is damped to a point when it returns to it's fully open length in the shortest time possible without any overshoot (doesn't extend beyond is free length); this is said to be a damping ratio = 1.

This initially seems desirable but the problem occurs when you 'hit' a series of small bumps such as the kerbing on a race track; the suspension 'jacks up' and you end up running on the bump stops as the spring can not return to it's open length quickly enough.

The lower the damping ratio the more comfortable the car will feel but the more unresponsive it will be to driver input.

The following are common damping ratios:

0.2-0.3 Comfortable road cars
0.4-0.6 Track cars
0.7-0.8 Race Cars

OK, now we have selected our target damping ratio we can now use it to equate our damper characteristics.

The Force Vs Velocity Graph

Some people may have seen these with the set of dampers they have been sold - most of them produced and sold with the dampers by the manufacturers are 'generic' and bear little resemblance to what you have in your boxes!

What is the Force Vs Velocity damping Graph?

This graph has nothing to do with the speed of your car! The velocity part relates to the shaft velocity of the damper itself. A damper works by flowing oil through orifices with 'shim stacks' controlling the flow of oil as well. The faster the shaft of the damper moves the more force it will generate.

A typical Calculated Force Vs Velocity Graph is Shown below:


(This Force velocity Graph was Generated from our own excel spreadsheet to produce ideal Force Vs velocity Curves)

Lets examine the axis of the graph:

Velocity; This axis is split into two areas; the low speed damping (Below 0.075m/s) and the high speed area. The low speed damping is what the driver feels when he turns the steering wheel or brakes/accelerates. As you turn into a corner the loaded wheel compresses and the unloaded wheel extends; the damper rod moves relatively slowly during these events.
The high speed area is when you hit a pot hole or a bump - the damper shaft moves very quickly under these circumstances.

Force; The graph is shown with the compression damping as positive and the rebound as negative. A damper works both in compression (as the rod is pushed into the damper body) and in rebound (as the rod is pulled from the damper body).


How do we choose a 'curve' for our Force Vs Velocity Graph?

If its a Track Day car driven to the Circuit we would be aiming for a damping ratio of, lets say 0.6 - this ratio can be entered into an equation to obtain an initial 'slope' for our damper graph.

We then need to modify our graph for intended use; The purpose of the damper is to control the spring.Compression damping is said to control the un-sprung mass whilst rebound damping controls the sprung mass; hence we normally run more rebound damping than compression damping.

The last part is tuning the high speed area; we want the low speed to 'feel' good to the driver - hence the damping ratio of 0.6 but we might want the high speed to feel comfortable and 'trade off' some performance; therefore we might choose a damping ratio of 0.3 for the high speed damping. (This may differ on a race car where we could maybe run as much rebound damping as possible before 'jackdown' starts to occur - damping does not allow spring to return to normal length quickly enough before next bump is hit)

This is why you might get two different gradients of lines on the damper curve in both rebound and compression.

The shape the curve makes are also given different names:

(1) Progressive (as Shaft speed increases so does damping rate)
(2) Linear (As shaft speed increases damping rate remains constant)
(3) Digressive (as shaft speed increases damping rate decreases)

HTH.

 

Further reading:

 

http://forum.wscc.co.uk/forum/index.php/topic/111860-damper-dyno-plots-explained/

 

(Mathematics & Description from Optimum G)

[7sRWild]

Oh...that hurt my head....this is why you do what you do so very well..and I have a Rum and coke while you do it..

[Rory's Dad]

Oh...that hurt my head

Mine too...

Great read tho'

Rory's Dad

[AdamR]

Great stuff, thank you!

I hope this doesnt come out wrongly/sound ungrateful, but will there be a 'part 2' about spring rates? Techy stuff like this is always a great read, especially when written with down to earth explanations like you always do

[s2rrr]

Fraser,

Awesome stuff, as Adam awaiting part 2 the spring rate issue. Coincidentally we discussed it at the last meeting.

Bob

[BLiNK Motorsport]

Adam, Bob:

Firstly we have to select the right springs with which to 'suspend' our cars on. We do this by selecting a 'Bounce Frequency' - the lower the Bounce Frequency the more comfortable a car will feel and the more mechanical grip it will have but it will feel unresponsive to driver input.

Typical frequencies are:

0.5-1.5 Hz For passenger cars

1.5-2.0 Hz for Sports/Track cars

3.0-5.0+ Hz for Aero Cars

The bounce frequency is usually set different Front to Rear by a small margin; in a passenger car the front of the car hits the bump before the rear so the rear needs a higher frequency than the front to 'catch up'. Race Cars are often different, where comfort is no longer a consideration it can be advantageous to have a higher front bounce frequency as it can assist corner 'turn in' and improve corner exit grip.

Once we have a target 'Bounce Frequency' we can use formulas to calculate our spring rates using the sprung and unsprung mass supported by that wheel and the motion ratio of the spring (angle it's inclined at in simplistic terms).

This then allows us to select the correct springs for that vehicle dependant on it's use.

Now that the springs have been selected we can look at what damping is required to control that spring.

[s2rrr]

Thanks Fraser,

I think my brain needs a simpler explanation, if I were to ask the question on here about what strength springs people use I would get just about every combination of weights possible. Maybe that's to satisfy personal driving styles etc and car/driver weights also where the car is to be driven, I note that from Westfields own data sheet/manual they recommend various strengths of coil springs front and rear and a basic average is sold to the punter.People used to tune there cars by stiffening up the suspension etc etc and the implication is that stiffer is better ( in some ways it is) but not necessarily . Our discussion if you recall was to do with my mates car bottoming out the gearbox hoop in he dip after Lodge at Oulton. I think we had 170lb springs on the rear. Now he has gone for 225s I believe. He tells me that on the road they make no real noticeable difference but thats an extra 110lb per inch ( 2 x 55 ) at the rear of additional suspended ability ( laymans term ).He hasn't tried it on the trak as yet.

How in your opinion will that alter the track behaviour of the car bearing in mind that he weighs about 10 stone soaking wet but normally has a passenger on the track. Is it something where an approximate spring rate for front and rear will suffice or does it need to be calculated for every car.driver combination. I guess due to driver weights it must, but we did come to the conclusion that corner weight setting ( which could be relative) is a waste of money if you are swapping drivers or taking passengers etc.He is using the higher rate springs for trackdays and reverted to the "standards" for the road. Just trying to understand whether its trial and error or any golden rules applying. Do you have various springs in stock for your geo setting up or is it done on prior knowledge/experience.

Need to get my head around the subject to at least get the car as good/safe as it can be without busting the bank. Probably going to get a geo check but will discuss at the next WSCC meet I can get to.

Cheers

Bob

[Kevin Jones]

Fraser, great article as usual, always wondered what all this valving was about.

Maybe worth warning that a lot of spring advise should be taken with a large pinch of salt because there are many types of front geometry, ARBs allow you to take liberties and at least speed series runners have different objectives, such as maximising traction.

[BLiNK Motorsport]

Fraser, great article as usual, always wondered what all this valving was about.

Maybe worth warning that a lot of spring advise should be taken with a large pinch of salt because there are many types of front geometry, ARBs allow you to take liberties and at least speed series runners have different objectives, such as maximising traction.

That's correct Kevin, anybody who states 'xxx lbs/inch' are the best hasn't taken the other factors into consideration.
When selecting a ride frequency it does take all the factors into consideration - the sprung mass, the unsprung mass and the motion ratio of the spring so it can be applied to any suspension design; not just different Westfield designs but any vehicle.

Here is a summary when selecting Springs:

(1) Decide what Ride Frequency you want the vehicle to have (Higher frequencies have less mechanical grip but more response to driver input)
(2) Never use a wheel spring to control Body roll
(3) Select an ARB stiffness to control Body Roll (measured in Degrees/g)
(4) ARB stiffness will effect Ride Frequencies (Everything is a compromise but suspension parts are linked!)
(5) You can not effect total load transfer accross the vehicle but you can change the percentage the front and rear axles transfer.
(6) The stiffer the axle in relation to the other axle the more load & the faster it will transfer.
(7) The more load the axle transfers in proportion to the other axle the less overall grip it will have. (An axle has it's most grip when the tyres are evenly loaded)

Bob,

with refference to the above you can see what will happen to your friends car; he has stiffened the rear axle (by quite a large percentage) in relation to the front axle so the car will tend to be more likely to oversteer and less likely to understeer than it was before. (more weight is being transferred by the rear axle and it therefore has less grip).
As a caveat - ride heights are very important in relation to the damper having enough rebound and compression stroke; if the car is set to low there will not be enough compression and will hit the bump stops - this makes the axle solid and weight will transfer very quickly during a corner causing a probable spin.

The ride frequencies, as stated get you in the right 'ball park' closing in on the perfect settings is down to testing.

When testing you need to datalog; often a driver will think the higher frequencies feel faster because the car feels more 'alive' when, infact it is slower as the grip has been reduced.

Incorrectly selected dampers can also have very adverse effects on handling; they can cause the car to pitch and roll as they are unable to properly control the spring.

HTH

(Just as an aside; F1 & other full Aero Cars run at ride frequencies just over 5Hz. This is required because the aero force generated compresses the springs so much that they need to be such a high 'poundage' to maintain ride heights. The human body can just stand this - the ligaments holding your internal organs in place start to suffer around 4.5Hz and you organs start to bang into your skeleton causing bruising!)

[s2rrr]

Thanks Fraser

Will discuss over a pint at the next meet. Seems more to it than meets the old eye.

Bob

[Captain Colonial]

Well done Fraser! This would make a cracking article if it was submitted for inclusion in Westfield World...(hint, hint)

[CedricTheBrave]

some interesting calculation info here LINK

[CedricTheBrave]

Actually page one of that series give more info LINK

endorses what the OP is on about

[CedricTheBrave]

I worked out to achieve a wheel frequency of 2hz with an unsprung mass = 157kg I would need a 220lb spring

any advances on 220

[BLiNK Motorsport]

I worked out to achieve a wheel frequency of 2hz with an unsprung mass = 157kg I would need a 220lb spring

any advances on 220

What motion ratio's are you using for your front suspension? (I would be surprised if your unsprung mass = 157kg's?)