28 replies to this topic

[mariner]

As a non ride/handling person reliant on published expert opinion and logic I would say - always , yes - because they have a constant resistance rate for even small movements.

So in a slalom test with its rapid right/left changes dampers would seem to be important. However looking at a very old Road and track magazine from November 1974 there is a test result which goes right against damper importance on sharp transients.

The magazine ran a test of various dampers including OEM, Koni, Bilstein etc. using both a damper dyno and three track tests, braking, slalom with 24 metre pole spacing and a 180 dgree 55 mtre raduis corner with 50mm*100mm wooden beams to provide ride excitation. the cars were a Volvo saloon and a 1974 (C3) Corvette, boith instrumented with invidual wheel movement potentiometers,lateral G and track timing.

As a comparision each car was run through the tests with no dampers at all.

Most of the results were what you might expect, the Bilsteins and Koni's gave a slight edge in slalom speed and braking but the OEM's were a good combination of control and low price.

However two results for the C3 Corvttte were truly odd. There was no difference in braking distance with no dampers fitted and also no difference in the slalom time versus having shocks. In fact the slalom times were identical for all types of shock as well.

R+T offered two explanations - the C3 had so much anti dive and squat that there was no dip on braking so the dampers could have no effect and the total roll rate of the springs was so high that it overwhelmed the damper impact on slalom transient response. their evidence to support the "high roll resistance form the springs" conclusion was that on the softly sprung Volvo saloon removing the dampers did increase times and different damperrs did affect the slalom time.

I know all this was long ago but basic principles don't change and the tests seemed very well measured so my question to the ride/handing experts is can the roll stiffness of springs be so high that it completley overwhelms the damper impact on rapid roll response?

The Corvette may have been stiffly sprung but nowhere near the spring rates of a racecar so why didn't the dampers affect the slalom speeds at all

[DaveW]

Dampers certainly will affect the way a vehicle responds to manoeuvring inputs - provided that they move. Logic would suggest, therefore, that the dampers didn't move when fitted to the Corvette. Two obvious possibilities are geometry & friction. Geometry could explain lack of movement for driver inputs, but the 180 degree corner with 50mm(!) inputs should have sent the car flying if it was attacked at any speed (the parameter missing from your post), particularly with no dampers fitted. I don't like it, but I have to favour the notion that the C3 suspension had significant levels of friction.

Incidentally, as a dumb foreigner, I have concluded that US suspension specs. for sedans throw away all in pursuit of ride (except California spec. vehicles, perhaps). 20 year old sportscar specs, on the other hand, appeared to be designed to ensure the owner knew that he was in a sportscar by making the dampers as close to "square" (frictional) as possible. I don't know the '74 Corvette, but perhaps Chevrolet had taken that idea to its logical conclusion.

One of my abiding memories of the USA is sitting in the back seat of an actively suspended Buick Park Lane negotiating a handling loop at the GM proving ground at Phoenix (with rumble strips embedded in the corners), watching a Corvette fall off the track whilst trying to follow. Might not have been the best riding car in the world (for reasons not entirely the fault of the suspension), but the active Buick had its moments.

[jmorris]

As any autocross (U.S.) participant knows, after tires it is shock/damper upgrades that make the biggest improvement. Autocross is all about transient response. Yes, some cars gain more than others with shock upgrades. Vehicles with poor camber control and/or very stiff springs tend to benefit less, for instance Corvettes and Minis.

[Greg Locock]

This sort of came up before, in exasperation one of the race engineers took the shocks off a car and sent it out, the laptime was more or less unaffected. Someone with a better memory than me should be able to dig it up.

[Fat Boy]

It was 1974, so we're talking about really, really bad dampers to start with. Second, we must also be talking about massive friction in the suspension. Didn't those cars have the axle as an upper rear locating link?

It's entirely possible that those cars were _so_ bad that you could take the dampers off of them and not notice. That fact has no relation to present dampers and/or racecars, however.

[mariner]

I don't think that dampers were THAT bad in 1974 the basics don't change that much. Also on the test with 180 degree curve with the wooden "rumble strips" there was a noticable difference both without dampers and between dampers. As the actual wheel movement was small I don't think the suspension friction argument is the only answer.

A possible explanation is that the dampers are of most value in controlling wheel excitation at the critical spring frequency (hence "damper"!). On this particualr test the Volvo had a wheelhop frequency of 11cps presumably a function of the spring rate, and this frequency was encountered on the rumble strip test. For the Corvette the estimated frequency was 9 cps front and 10.5 cps rear.

To answer a question posed above the speed difference in the 180 degree rumble strip test between no dampers and best damper was 6.3 mph or 28% so dampers did matter but not on a slalom test.

[gruntguru]

Did that model Corvette have leaf springs? Leaf springs can do quite a bit of damping depending on inter-leaf friction and lubrication.

Edited by gruntguru, 04 August 2009 - 08:27.

[Greg Locock]

Just run a test and shocks make no difference to yaw velocity/swa in step steer or frequency response.

They do affect the roll response but that is irrelevant for racers.

[RDV]

Low speed damping is ALL about transients and platform control, and yes, for racing they respond bloody well nowadays...

[cheapracer]

Did that model Corvette have leaf springs? Leaf springs can do quite a bit of damping depending on inter-leaf friction and lubrication.

All Corvettes have leaf rear but only a single but nice pickup as many don't realise that the interaction of leafs give good damping.


In my rallying a couple blew out during events over the years - feels like you have a flat tyre but of course rallying has one or too more bumps than a slalom course

[Greg Locock]

I should explain that my (computer based) tests were linear range and open loop at constant speed, on smooth roads, at only 0.1-0.2 g. The agreement between my calculations and the tests on the car is very good.

[Fat Boy]

I don't think that dampers were THAT bad in 1974 the basics don't change that much. Also on the test with 180 degree curve with the wooden "rumble strips" there was a noticable difference both without dampers and between dampers. As the actual wheel movement was small I don't think the suspension friction argument is the only answer.

A possible explanation is that the dampers are of most value in controlling wheel excitation at the critical spring frequency (hence "damper"!). On this particualr test the Volvo had a wheelhop frequency of 11cps presumably a function of the spring rate, and this frequency was encountered on the rumble strip test. For the Corvette the estimated frequency was 9 cps front and 10.5 cps rear.

To answer a question posed above the speed difference in the 180 degree rumble strip test between no dampers and best damper was 6.3 mph or 28% so dampers did matter but not on a slalom test.

If you had already decided on the answer, why did you ask the question?

[mariner]

I asked the question because while I can guess at reasons I really do not know and the data ( that the dampers didn't matter on the slalom ) seemed so opposite to most of what I have read that I wondered if the experts here could explain it.

The slalom length was 450 feet and the there were 7 poles at 75 foot spacing including one at entry and one at the exit. The slalom elapsed time was 7 seconds ( 43.8 mph).
If somebody is better at maths then me and has some actual experience on the radius required to weave between the 75 foot poles then it would give a rough lateral acceleration value to compare to Greg's 0.2g

[gruntguru]

I asked the question because while I can guess at reasons I really do not know and the data ( that the dampers didn't matter on the slalom ) seemed so opposite to most of what I have read that I wondered if the experts here could explain it.

The slalom length was 450 feet and the there were 7 poles at 75 foot spacing including one at entry and one at the exit. The slalom elapsed time was 7 seconds ( 43.8 mph).
If somebody is better at maths then me and has some actual experience on the radius required to weave between the 75 foot poles then it would give a rough lateral acceleration value to compare to Greg's 0.2g

I get an average lateral acceleration of 0.56g which sounds too high considering the peaks would have to be significantly higher.

[mariner]

Thank you for the calculation.

They reported 0.73g for the C3 Corvette on a 100ft radius skid circle test as part of the evaluation so I suppose 0.56 could be right for 75ft radius pole swerviing.

[Fat Boy]

Forget speed and G's. You've got a 3 right to left driver inputs in 7 seconds. That's a 0.43 Hz input, which is no doubt lower than the roll crossover freq. of the car. Damping, and of lot of it, would have to make it better (assuming you're on smooth pavement).

In 1974, hell...1984, racing dampers weren't good. In 1974 road dampers were ****, as were all the friction and compliances around them. If they took them off the car and didn't notice a change, then they obviously weren't doing much.

Greg, I think roll response would be quite important for racers, but it's entirely possible we're using the same term differently.

[Greg Locock]

Greg, I think roll response would be quite important for racers,

Oh, I'm sure it is, but smooth road manouevres around poles at constant speed is not the best place to demosntrate the difference.

Let's see, 1/2g say 2 degrees of chassis roll, say 25mm wheel motion in 0.5 seconds, that's 50 mm/s wheel motion. Well, according to my shock absorber database at 50 mm/s the maximum road car damping in rebound at that is 40 kgf, 10 is more typical. So, is my model going ot notice a change of maybe 20 kgf in 300 in vertical load? Given that the other wheel on the same axle may be unloading a similar rate? No.

Actually I thought it was you that ripped the shock absorbers off the car before sending it back out!

[Fat Boy]

Actually I thought it was you that ripped the shock absorbers off the car before sending it back out!

Honestly, that sounds like something I'd do, but I don't remember ever doing anything quite like that. At one point I did set up a car on a shaker rig with no valving or oil in the dampers, but that was more to see tire issues. You can get some great wheel hop bouncing like that.

[DaveW]

Greg, Like this, perhaps?

http://www.davew.webs.com

Apologies, I wasn't as adept as you. I failed to get the site to play the video. Hence I suggest you save the video file locally & play it yourself. BTW the red car is active (in case you don't spot it) & the two slaloms were executed at 60 mph & 65 mph, respectively.

Edited by DaveW, 07 August 2009 - 09:49.

[DaveW]

Perhaps the video file I posted was mischievous (apologies to Greg) & a little unfair because the active vehicle didn't roll much (& therefore had little roll response to deal with). It was also much better damped, ran an active lateral balance algorithm, & included hub control algorithms intended to reduce CPL variation**.

However, the video is instructive, I think. The slalom trajectory of the passive (white) vehicle was disturbed by roll motion (specifically, the inertia forces generated by roll overshoot) and apparently by hub motion. The latter (particularly) would probably be caused by surface irregularities and, in this case, by variations in available grip (the surface was damp in places). The implication is that more damping would improve modal stability and hence slalom performance, but would probably have had an adverse effect upon vehicle NVH characteristics. It is quite common to find road vehicle suspension set-ups compromised in this way, particularly so for "high performance" (boy racer) versions, where modal damping ratios are often lower than "standard" versions of the same vehicle (springs increased, but dampers untouched or reduced).

Not mentioned in this thread so far (I think), is the effect damping "style" can have on transient response. It is common for "road" race vehicles to adopt an aggressive low velocity shape (nose) for the steered axle dampers to provide "feel" (whatever that is) and to shorten steering time constant. In theory, this would be expected to reduce ultimate cornering performance, but appears consistently to give a driver the confidence to make better use of what remains. An alternative (& counter-intuitive) strategy for "oval" race vehicles is to use aggressive rear dampers with a more linear front "style". This appears to allow corners to be negotiated with higher exit speeds (less steering-induced drag), possibly because the set-up style induces transient oversteer.

** Equivalent to Greg's "preview" but without requiring a microwave sensor (which would probably be unable to distinguish between a brick & a paper bag, I suspect).

Edited by DaveW, 10 August 2009 - 08:24.

[RDV]

^ What the man said....high "nose" or "knee" for rear dampers also very useful for power down on 500hp+ cars...

[Greg Locock]

anybody else got a problem downloading dave's video?

[wrighty]

anybody else got a problem downloading dave's video?

worked for me try right-clicking on the video link on the page and 'save target as'

[Tony Matthews]

worked for me try right-clicking on the video link on the page and 'save target as'

I just left-clicked, then 'play', no probs. But then I'm 'intelligent' enough to see the number 74...

Edited by Tony Matthews, 10 August 2009 - 10:12.

[Greg Locock]

OK, got it now, in Dilbertesque style the guy who runs his cpu hardest at work is on the oldest PC, and can't read wmv files. Probably should have done it at home!

Sorry the Active Excel proves nothing in the context of this thread. It'd be funnier if you had film of the times it ended up in the ditch.

Edited by Greg Locock, 11 August 2009 - 10:39.

[DaveW]

OK, got it now, in Dilbertesque style the guy who runs his cpu hardest at work is on the oldest PC, and can't read wmv files. Probably should have done it at home!

Sorry the Active Excel proves nothing in the context of this thread. It'd be funnier if you had film of the times it ended up in the ditch.

Sorry you feel that way, Greg.

I have nothing to add to my post #20, I'm afraid, other than the fact that the only successful contemporary ditch hunter I can recall was not driving an active vehicle.

[cheapracer]

It'd be funnier if you had film of the times it ended up in the ditch.

Was there particular/consistant reason it went south or was it just a mixed bag?

[DaveW]

Was there particular/consistant reason it went south or was it just a mixed bag?

Greg L will say (has said) NVH. Actually, there was a general belief with the OEM fraternity that active suspension could cure all design/build issues in road vehicles. This was not (& never could be) the case. However, Lotus modified a range of existing road vehicles on an "experimental" basis. Some did have NVH issues, but a fair number were perfectly acceptable by anybody's standards & were extremely good fun (& safe) to drive. The Lotus system (much to everybody's surprise) really did offer the handling improvement suggested by the video I posted.

The technology was taken over by Bill Moog and developed (in the production sense) by a small band of very capable engineers he recruited in Stuart (Florida). So far as I know, they solved most of the outstanding issues with actuation & measurement. This was sold (ultimately) to TRW, who, I believe, funded the activity for a while before putting the whole thing into cold storage. I don't know what happened subsequently (if anything).

[Greg Locock]

Greg L will say (has said) NVH.

Once you switched to a polyV drive belt for the pump instead of that toothed horror, and insulated the pipes from the body suddenly the whole world was a cheerier place. Sorry I didn't mean there was nothing to be learned from the video, I just thought that a video of a slaloming passive car looks like a video of a slaloming passive car.