47 replies to this topic

[NRoshier]

I've been researching dampers and how they work - or are supposed to work instead of how they actually work for a magazine article...anyway after months of getting more confused than the Mad Hatter on LSD I've basically given up and am flying white flags! These things are just too complicated for my limited attention skills and time and most who work with them won't discuss their thoughts/experiences.
However I will need some dampers for my own project car quite soon and have sourced some Dynamic dampers...but get zero response from Dynamic in the USA and UK. In the mean time I have found is a local man who has built race dampers, mostly Bilstein, for the past 20 years and he is very helpful - but vague on specifics. I have also found that the good ol USA has quite a diverse range of Bilstein dampers in comparison to my own country and the Bilstein ASN dampers seem good value for money. However I note that Bilstein only list two different pistons for their dampers (digressive and linear) vs the multitude available for Penske/Ohlins etc.
So what are the learned opinions for of those on this forum for using linear vs digressive/linear vs digressive etc on a car?
I know this is a big question, but I would value input from those experienced in such matters.
My own car will be used 40% on the road (mostly going to events) and 60% on the track, 1000kg, 200KW.

Edited by NRoshier, 14 September 2009 - 11:50.

[cheapracer]

Welcome back Neil from the dark depths that you and Fat Boy have been too

The very latest and free copy of Race Car Engineering has an article that makes your timing quite ironic.

I suggest you read the article first.

Again and ironically I was considering starting a thread about shockers, I don't agree with them and how they currently perform but at this time have no alternative.

After you read the article i'll tell you something interesting about Vintage Motorcycles.

[cheapracer]

Magazine here - http://forums.autosp...w...113014&st=0

[NRoshier]

Ta, however the depths are of my own making and not related to Fatboy. WRT RCE I'll head over to the newsagent for a read...I stopped buying it years ago.
As far as how they perform, well some seem to perform well and others not. I got my hands on a set of 4 way adjustable Sachs dampers - however the local agent is not easy to deal with, so they have moved on to a friends sports sedan and he is happy. Interestingly the 'new' double adjustable shaft/piston in the Bilstein seems to be the same patters as the old SACHS dampers...I assume they have bought the design?
A similar lack of local support with Dynamics shocks has caused me to look at the Bilsteins - here the amount of quality local support may well be the decider rather than what may have technical advantages as I feel a well set up set of dampers is crucial.

[cheapracer]

You understand the magazine above is for free as in no money?

[Greg Locock]

Has any of our more instrumented members got data for shock force (via a strain gauge) vs time vs shock or wheel travel they can publish?

The plot of force vs velocity I get for real events like pave bears only the slightest resemblance to the nice little curve we get on a dyno. I'd add that our MTS only goes up to 1500 mm/s, whereas on real events I see as much as 3500 mm/s

[NRoshier]

Thanks Greg. I am interested in your comment as you are now the third person who has hinted/told me that their shock dyno is of lesser use then vehicle data...which makes sense, but makes me question the value of shock dyno data other than as a verification of damper settings/condition etc. However this does make initial damper selection a bit hit and miss or you have to rely on a person with significant experience.
Cheapy yes I know it is free, however I also get to look at how my own magazine sits on the shelf in comparison to others and frankly I hate looking at the damned computer screen all the time!

[gbaker]

Put simply, damper force is a function of velocity, not displacement, as is the case with springs.

Math will set you free.

[Fat Boy]

Thanks Greg. I am interested in your comment as you are now the third person who has hinted/told me that their shock dyno is of lesser use then vehicle data...which makes sense, but makes me question the value of shock dyno data other than as a verification of damper settings/condition etc. However this does make initial damper selection a bit hit and miss or you have to rely on a person with significant experience.
Cheapy yes I know it is free, however I also get to look at how my own magazine sits on the shelf in comparison to others and frankly I hate looking at the damned computer screen all the time!

I don't know if I'd say that a crank pin dyno is of lesser use than vehicle data, but it's important to know they're different beasts. Once you've picked a damper, a standard dyno is good for making incremental build changes and for verifying the build (as you said). On the initial build, it is hit-and-miss, though. You can have 2 dampers whose curves overlay perfectly on a crank pin dyno that act completely different on the car. This is when you need to get out the hydraulic or EM to really see what's happening. Another great option, as Greg alluded to is to strain-gauge the damper shaft.

gbaker...simply put, shaft velocity is but one variable in the damper force equation. Maybe lay off on the snarky comments until after you've actually been to the racetrack and/or built a damper. Math might set you free, but reality will knock you on your ass.

[Greg Locock]

I don't know if I'd say that a crank pin dyno is of lesser use than vehicle data, but it's important to know they're different beasts. Once you've picked a damper, a standard dyno is good for making incremental build changes and for verifying the build (as you said). On the initial build, it is hit-and-miss, though. You can have 2 dampers whose curves overlay perfectly on a crank pin dyno that act completely different on the car. This is when you need to get out the hydraulic or EM to really see what's happening. Another great option, as Greg alluded to is to strain-gauge the damper shaft.

gbaker...simply put, shaft velocity is but one variable in the damper force equation. Maybe lay off on the snarky comments until after you've actually been to the racetrack and/or built a damper. Math might set you free, but reality will knock you on your ass.

I wish I could post my data but it isn't mine. It was described as a hairball plot, force vs velocity in real events. I drew three straight lines through it and called it done. Incidentally a reasonably good MBS model of a shock absorber has something like 50 discrete elements, many of them non linear. I think I saw it in an SAE paper.

One thing I just realised, when the wheel is in 'freefall' into a four inch pothole its velocity is about 2000 mm/s, you racers must see that as you come off kerbs?

[gruntguru]

One thing I just realised, when the wheel is in 'freefall' into a four inch pothole its velocity is about 2000 mm/s, you racers must see that as you come off kerbs?

Slightly OT. My 2007 Focus axle tramps (left side only) if it breaks traction on a wet road. 60,000 km. Bounce the mudguard shockie test seems OK (I know - thats next to useless). Is this common? Is it most likely the shock?

(Obviously from the universe where Ford sell the Focus but I thought I'd best mention it JIC)

Edited by gruntguru, 16 September 2009 - 10:11.

[Greg Locock]

Slightly OT. My 2007 Focus axle tramps (left side only) if it breaks traction on a wet road. 60,000 km. Bounce the mudguard shockie test seems OK (I know - thats next to useless). Is this common? Is it most likely the shock?

(Obviously from the universe where Ford sell the Focus but I thought I'd best mention it JIC)

Check your engine mounts. Powerhop on FWD cars is usualy an interaction between the rigid body modes of the engine, and a complex forced response of the tire, basically a stick slip positive feedback loop.

Any significant change in the driveline's compliance could affect it. Typical frequency is 6 hz or so, so it isn't simple wheelhop. I never had much luck curing it with shocks, but if they were stuffed then that might be it.

[gruntguru]

Any significant change in the driveline's compliance could affect it. Typical frequency is 6 hz or so, so it isn't simple wheelhop. I never had much luck curing it with shocks, but if they were stuffed then that might be it.

Yeah, 6Hz sounds about right, scary loud thumps as the suspension hits the stops.

[Fat Boy]

One thing I just realised, when the wheel is in 'freefall' into a four inch pothole its velocity is about 2000 mm/s, you racers must see that as you come off kerbs?

I'd say generally no. The highest damper velocities I remember off the top of my head is around 1/2 that. Generally speaking, 1/4 of that is more common. Keep in mind, we're dealing with much smaller displacements and in general don't see big square-edged things. If a curb is square-edged, then you don't use it. If it has reasonable entry and departure angles, it can be quite high and still usable. We're much more apt to see that than a real city street curb.

[Lukin]

2000-2500 mm/s is usually a pre-cursor for a broken suspension part with us!

[Greg Locock]

2000-2500 mm/s is usually a pre-cursor for a broken suspension part with us!

same here, because at the far end of the pothole is a 4 inch kerb the other way. That's the dreaded Square Edge Pot Hole test, designed to rip suspension arms out of subframes.

[gruntguru]

2000-2500 mm/s is usually a pre-cursor for a broken suspension part with us!

It has a lot to do with travel. If your racecar suspension has only 1/4 the travel of Greg's roadcar, similar accelerations will only produce 1/4 the peak velocity in the race car suspension. (assuming rigid tyres and the suspension doesn't reach travel limits)

Edited by gruntguru, 17 September 2009 - 10:51.

[gbaker]

...gbaker...simply put, shaft velocity is but one variable in the damper force equation. Maybe lay off on the snarky comments until after you've actually been to the racetrack and/or built a damper. Math might set you free, but reality will knock you on your ass.

I can't wait to get to a race track, or design a damper.

[Tony Matthews]

I can't wait to get to a race track, or design a damper.

What's first, then? Keep us posted...

[Greg Locock]

I think he was being sarcastic, but would dispute that F=c.v is more than a first order stab at what is going on.

For instance, consider the subtleties in these potato plots. forums.vwvortex.com/zerothread?id=1648157

You can see stick slip in the seal, and see how the kneepoint moves through the range in different ways.

[Tony Matthews]

I think he was being sarcastic,

I gave him the benefit of the doubt, Greg, my response could be read as ironic or supportive!

[cheapracer]

I can't wait to get to a race track.

You need help with the map? The roads are often not quite as they appear on paper.

Edited by cheapracer, 18 September 2009 - 06:26.

[DaveW]

I think he was being sarcastic, but would dispute that F=c.v is more than a first order stab at what is going on.

...and the "Industry Standard" Peak Force vs Peak Velocity plots produced by most Dyno software is even more remote from reality.

[cheapracer]

All these posts just support what I said at the beginning "I don't agree with them and how they currently perform but at this time have no alternative".

But I'm thinking about the problem.

Edited by cheapracer, 18 September 2009 - 15:53.

[DaveW]

All these posts just support what I said at the beginning

So they do cheapracer. Think on..

Bear in mind that all dampers include a series "spring" (oil compliance) that will differ between compression & rebound, most include a parallel spring (unequal CSA) & many (especially road vehicle dampers) are frequency-dependent. They are also temperature-dependent, and vary with charge pressure. Most will cavitate at sufficiently high velocities, causing characteristics to change dramatically. Shock waves can be generated when control valve open (damper chuckle). Some types display a characteristic time delay when responding to a discrete input (not to be confused with conventional frequency response), and control valve friction can introduce (additional) non-linearities, and even instabilities. Fluid galleries contribute to the characteristics (be worried when remote reservoirs are connected by long flexi-pipes). I believe that many are impedance-sensitive (their observable characteristics depend upon the impedance they are driving into). The last, if true, is a problem for conventional damper dynamometers, which attempt to present an infinite impedance. The peak load-peak velocity plot generated by all dyno software (I think) can be particularly misleading & should be assigned to the rubbish bin. Most dyno software can generate more representative plots, but these are held to be "more difficult" to interpret. "Welcome to the real world" is a tempting reply.

[DaveW]

I'd say generally no. The highest damper velocities I remember off the top of my head is around 1/2 that. Generally speaking, 1/4 of that is more common. Keep in mind, we're dealing with much smaller displacements and in general don't see big square-edged things. If a curb is square-edged, then you don't use it. If it has reasonable entry and departure angles, it can be quite high and still usable. We're much more apt to see that than a real city street curb.

I agree in general, Fat Boy. However, I recall one Champ car race (in San Jose?) that caused a few broken top eyes & (allegedly) 3.5 m/s at the rear dampers, although that number may well have been augmented by the potentiometers....

[Lukin]

Is that the track with the big 90 step input surface change in the track?

Most of our issues are with kerbs as well. The kerbs at Hamilton in the first year were real bad, there were a lot of broken dampers. For this year they took the 4th apex kerb out so the drivers were 20 km/h quicker and hitting the first three kerbs harder.

In most other cases it's if the driver uses too much kerb and drops over the back of it, then you end up with a 30-40mm step input coming back on. Oran Park and Hidden Valley are bad for this.

[Greg Locock]

One of my analytical models to solve my pothole problem for the shock hit a max force of 6000N, 1500 lb. I don't think they'll even try and build that one! Now we've started complaining about the bush at the top of the shock absorber. The one at the bottom is a thing of beauty and a joy forever, or at least I have persuaded Neil to use them instead of spherical ball joints in his car, but the top one is quite soft for NVH reasons -if you make them too hard the passengers can hear the shock fluid squooshing around, which I quite like, managers don't.

Edited by Greg Locock, 19 September 2009 - 01:36.

[McGuire]

I agree in general, Fat Boy. However, I recall one Champ car race (in San Jose?) that caused a few broken top eyes & (allegedly) 3.5 m/s at the rear dampers, although that number may well have been augmented by the potentiometers....

Don't remember the velocities but they were big. Streetcar tracks. No joke. Damnedest thing I ever saw. At the start, 18 cars in single file launching into the air, as though they were flying in formation.

[gruntguru]

Check your engine mounts. Powerhop on FWD cars is usualy an interaction between the rigid body modes of the engine, and a complex forced response of the tire, basically a stick slip positive feedback loop.

Would you expect a problem with engine mounts on a low-K Focus - never been thrashed?

The plot of force vs velocity I get for real events like pave bears only the slightest resemblance to the nice little curve we get on a dyno. I'd add that our MTS only goes up to 1500 mm/s, whereas on real events I see as much as 3500 mm/s

Do you test bare shocks on the MTS or is it always suspension assemblies?
What sort of input(s) would you suggest for testing bare shocks (FSAE) on an MTS?

[DaveW]

Slightly OT. My 2007 Focus axle tramps (left side only) if it breaks traction on a wet road. 60,000 km. Bounce the mudguard shockie test seems OK (I know - thats next to useless). Is this common? Is it most likely the shock?

Always interesting to speculate on the cause of a problem with (almost) zero information. The one piece you did supply was "breaks traction". When this happens the tyre recovers from its laterally deformed state smartly, injecting an incremental vertical load into the suspension. With no (or insufficient) damping the wheel will retract & then slam down hard again, causing the cycle to repeat. It is analogous to something that can happen to, particularly, tin top race cars (with high roll inertias) in the middle of a flat turn. The solution, in their case, is to move stiffness from bar to spring & increase damper settings. My guess is that a replacement damper on the offending corner should work for you. A crude test, if you are interested, might be to unbalance both front wheels deliberately (add the same offset weight to both sides) & drive the vehicle in a straight line. The undamped corner (assuming I am correct) should be upset more than the other.

[Ross Stonefeld]

Don't remember the velocities but they were big. Streetcar tracks. No joke. Damnedest thing I ever saw. At the start, 18 cars in single file launching into the air, as though they were flying in formation.

What I remember, when it wasn't drowned out by my laughter, was the horrible noise it made when they went over them with full tanks. As you said when they did it in a pack it was ridiculous.

And what did they do immediately afterwards? Go through the best first turn on a street course EVER. I was transfixed by the first race at San Jose, but I couldn't believe what I was watching.

[Greg Locock]

Would you expect a problem with engine mounts on a low-K Focus - never been thrashed?


Do you test bare shocks on the MTS or is it always suspension assemblies?
What sort of input(s) would you suggest for testing bare shocks (FSAE) on an MTS?

Drive your proto round a track and record the wheel travels with stringpots. That will give you more data than any rule of thumb.

FSAE is mostly smooth circuit work, i doubt you need to go above 500 mm/s (but that is a guess).

We test anything that will fit on the MTS, that's why they are so overbuilt. Haven't broken it yet. The good thing about testing coilovers is that you can check the the side load calulations out.

[gruntguru]

Always interesting to speculate on the cause of a problem with (almost) zero information. The one piece you did supply was "breaks traction". When this happens the tyre recovers from its laterally deformed state smartly, injecting an incremental vertical load into the suspension. With no (or insufficient) damping the wheel will retract & then slam down hard again, causing the cycle to repeat. It is analogous to something that can happen to, particularly, tin top race cars (with high roll inertias) in the middle of a flat turn. The solution, in their case, is to move stiffness from bar to spring & increase damper settings. My guess is that a replacement damper on the offending corner should work for you. A crude test, if you are interested, might be to unbalance both front wheels deliberately (add the same offset weight to both sides) & drive the vehicle in a straight line. The undamped corner (assuming I am correct) should be upset more than the other.

The car is a stock daily drive (driven mostly by my wife). "Breaks traction in the wet" means very little grip - street tyres on public road. I posed the question because Greg knows this breed inside out and I suspect the problem might be common.

[McGuire]

Engine mount, probably rear. If you crawl underneath and look you may even see the rubber smooshed around. "They" make a polyurethane replacement if you want to take the thug route and give up NVH.

[Lukin]

My first car had a similar thing which was engine mount related. If I put it in reverse and drop back quick made a monster noise!

[Greg Locock]

I posed the question because Greg knows this breed inside out and I suspect the problem might be common.

I've never worked on Ford's C platform, so any advice is generic. I have worked on several FWD high power applications and the cure for powerhop is usually damping on engine rigid body modes. Powerhop in the rain is classic, for whatever reason you get a nice 'resonance' between the tire and the driveline.

[DaveW]

I've never worked on Ford's C platform, so any advice is generic. I have worked on several FWD high power applications and the cure for powerhop is usually damping on engine rigid body modes. Powerhop in the rain is classic, for whatever reason you get a nice 'resonance' between the tire and the driveline.

Interesting... I recall a "Powerhop" mode on my diesel Mondeo. Happened in any road conditions (so far as I recall). Left foot parked, 1700 (ish) rpm in any gear, & the correct throttle setting would send the power train ballistic. More throttle simply increased the intensity. The only cure was to change gear (closing the throttle was the stabilizing action). A friend's similarly powered Focus had identical characteristics. He passed his problem on, but I learned to live with it. The problem disappeared after an engine re-chip (for a totally unrelated problem). I actually have a fondness for the Mondeo, despite is quirks & internal styling. My good friend JM (of Elan fame, etc., Greg) hates it with a passion, however (as he does most vehicles with undamped engine mounts). Having said that, he is currently trying to find a Focus he can live wth (3rd in 3 weeks at the last count).

I relate this story because it seems to fit your description fairly well, but not gg's, I thought.

[gruntguru]

I've never worked on Ford's C platform, so any advice is generic. I have worked on several FWD high power applications and the cure for powerhop is usually damping on engine rigid body modes. Powerhop in the rain is classic, for whatever reason you get a nice 'resonance' between the tire and the driveline.

I wonder if it might be sensitive to tyre pressure? Wish I had time and inclination to fiddle . . .

[Greg Locock]

My good friend JM (of Elan fame, etc., Greg) hates it with a passion, however (as he does most vehicles with undamped engine mounts). Having said that, he is currently trying to find a Focus he can live wth (3rd in 3 weeks at the last count).

I relate this story because it seems to fit your description fairly well, but not gg's, I thought.

There seemed to be a period of about 2 years where all I did was fit those little dampers to engine mounts. The ones off the Omega were quite good, but a bit too stiff, the best ones were from Isuzu. I'm a bit surprised that Focus and Mondeo get away without a hydromount, I know Fiesta has one and it needs it. Or is it that they have one and it is not tuned for that mode?

[gruntguru]

Interesting... I recall a "Powerhop" mode on my diesel Mondeo. Happened in any road conditions (so far as I recall). Left foot parked, 1700 (ish) rpm in any gear, & the correct throttle setting would send the power train ballistic.

Was this a "wheelhop" mode with the wheel bouncing between the road suface and the bump stops or a fore-aft lurching (Aussies refer to as Kangaroo hop)?

[DaveW]

There seemed to be a period of about 2 years where all I did was fit those little dampers to engine mounts. The ones off the Omega were quite good, but a bit too stiff, the best ones were from Isuzu. I'm a bit surprised that Focus and Mondeo get away without a hydromount, I know Fiesta has one and it needs it. Or is it that they have one and it is not tuned for that mode?

Forgive me, I spoke loosely. The primary engine mode on the Diesel Mondeo is well-controlled (6.4Hz/20%), but there appears to be a floater, excited better in pitch, at 9.3Hz/3%.

[Greg Locock]

Forgive me, I spoke loosely. The primary engine mode on the Diesel Mondeo is well-controlled (6.4Hz/20%), but there appears to be a floater, excited better in pitch, at 9.3Hz/3%.

Sounds reasonable, a pitch mode will be easily excited by torsionals in the halfshafts.

[DaveW]

Was this a "wheelhop" mode with the wheel bouncing between the road suface and the bump stops or a fore-aft lurching (Aussies refer to as Kangaroo hop)?

"Kangaroo hop" would be a good description - similar to a "learner" trying to master a clutch, except that the clutch was not involved. The only rational explanation I could think of was that the engine pitch mode was interfering with fuelling.....

[gordmac]

More evidence to support the idea that front wheel drive is an unnatural invention of the devil!

Edited by gordmac, 21 September 2009 - 10:25.

[McGuire]

I wonder if it might be sensitive to tyre pressure?

Sure, also tread wear and tire spec. Truth is wet-pavement hop is a common trait in fwd. Also on dry pavement given enough wheelspin. If a car doesn't exhibit it new, it may well drive into it as the miles stack up. For example, engine mount wear will be accelerated by hard-ish driving or hot garage shift. Wheelhop is often encountered on the North American Focus; aftermarket home remedies include poly engine mounts and "traction-bar" devices. I would not underestimate tires in a variety of fwd behaviors. I drive Cadillacs and when the tires get to around 70 percent tread remaining, suddenly torque steer becomes a lot more apparent.

[Bill S]

More evidence to support the idea that front wheel drive is an unnatural invention of the devil!

.... really?
I remember running a FWD Corolla in the Malaysan MME 12 Hour race a few years ago and it absolutely bucketed down for a few hours. When it was very wet, I can remember that at times we were up to fifteen seconds a lap faster than the Lotus Elise's.
I agree that a high-power FWD is a nasty thing but for up to very roughly 200hp they can be a pretty reasonable thing if set-up well.

[Fat Boy]

.... really?
I remember running a FWD Corolla in the Malaysan MME 12 Hour race a few years ago and it absolutely bucketed down for a few hours. When it was very wet, I can remember that at times we were up to fifteen seconds a lap faster than the Lotus Elise's.

15 seconds a lap ain't the car. If you're dealing with that kind of time, look behind the wheel.