Dampers and Bumpy Circuits

[tristancliffe]

I have a monoshock car, with Koni two-way adjustable dampers (bump and rebound, 9 'clicks' range on each, no high or low speed separate adjustment.

We've got a fairly good range of setups for most tracks and conditions now (although they don't vary from each other much or at all in most cases!) from testing and racing. But there is one track in particular that I've never been totally comfortable with - Oulton Park. I've never tested there, and most of the race weekends have been single headers (one 15min practice, one 15min race), often with damp sessions - not the best time to play with dampers. I am hoping to test this time, but thought some ideas beforehand would be useful. Forewarned is forearmed, or something.

Oulton is bumpy. So, I would say I'd need LESS bump damping to effectively soften the suspension over the bumps (dynamic rate). All well and good?

But I'm guessing that the rebound shouldn't be left alone, right? Too much rebound relative to bump and the car will tend to jack down over time, is that correct? Or does the rebound setting have more to do with the spring rate rather than the bump setting - i.e. as I'm not changing the springs, the rebound setting ought to be pretty similar to what it is everywhere else?

My guess is that I out to go 1 or 2 'clicks' softer on bump, and 0 or 1 'click' softer on rebound to get into the right ballpark.

For the purpose of this thread, I'm considering the whole car, rather than each end, and I appreciate that each end of the car might have different requirements.

Or, could it be the case that MORE damper is needed? I find that unlikely, but happy be told I'm wrong.

[phoenix]

As you have concluded, softening the bump settings is an obvious choice, as by doing so the chassis will have less opportunity to disrupt the contact patch when a wheel hits a bump. The rebound setting is there to control the expansion of the spring after compression, such that the chassis on the rebound doesn't upset the tyre contact patch. With softer bump settings the springs will compress more, so they will be 'charged' with more energy.

The elastic energy in the spring is given by the formula: Ve = 1/2 kx^2, where k is the spring rate and x is the spring displacement. So you can calculate that the energy in the spring increases with the magnitude of displacement. Therefore the rebound rate should be increased also - the maths says at a higher rate than the bump setting is softened, IF one click in bump = one click in rebound (which depends on your damper valving).

[tristancliffe]

Ah ok. More rebound with less bump, all other thing being equal, in simplistic terms.

At least I have a starting point, and can head in more or less the right direction from the get go - far quicker than starting from scratch with the dampers set fully soft!!

Thank you.

[elitemotorsport]

Koni dampers tend to have loads of low speed damping, often called nose because of how it looks on a damper dyno graph.

Nose is what you as a driver likes on turn in, a nice tight responsive feel, but it's not what you want on bumpy circuits.

A spring/car doesn't know whether it's bump or rebound, it just sees it as damping, so you need to keep the total damping for the spring the same. You as a driver will feel the difference so you do need to soften one and stiffen the other and testing/educated guesses will lead you to that setting. Without seeing the graphs i'm not sure if you'll achieve what you are after with the Koni's because of this inherent high low speed damping they have.

D.

[tristancliffe]

Why can't the spring/car tell which is bump and which is rebound? I thought that bump was when the spring/damper compressed, and rebound was extension. With the different valving, in conjunction with the internal construction, the damper 'knows' whether it's damping bump or rebound and uses the damping rates appropriately. I can't see how it doesn't know - what would the point of bump and rebound control be if it didn't matter.

Anyway, attached are the damper curves for our Koni as originally published. Nothing there that looks like a nose to me, but it's clear that the rebound force is quite a lot more than the bump force for a given piston speed. It's almost 2:1 by the looks of it.

[elitemotorsport]

You sort of misunderstood my point. A spring needs a damper to take out energy it doesn't matter how that damping is split between bump and rebound its just the total amount of damping overall that absorbs the springs energy. I was just using it as a point to illustrate that basically what you take out of bump put into rebound and vice versa.

If you put a car on a 4 post test rig the car will respond the same no matter what the split is between bump and rebound so long as the total amount of damping is the same. You as a driver however will feel the difference.

Your graph doesn't go through zero, but if you extend each curve to zero you'll see that at low shaft speeds the gradient (nose) of the graph is steeper than at the higher shaft speeds. Low shaft speeds are your transient movements like turn in and bump under braking or squat under acceleration. High speed is where you run over kerbs etc. Gentle undulations like the approach to Deers Leep? at Oulton respond well to stiffening up rebound and taking the low speed bump down. The car then tends to not want to leap of the crest of the undulations.

Most race cars have a ratio of about 2:1 rebound to bump. Road cars tend to be more like 10:1.

I used to design race car dampers for F1 cars, WRC and BTCC. Our dampers often replaced Konis or where used alongside them for different applications. Some drivers in BTCC used to like the Konis for qualifying because they built tyre temperature quickly on one lap shoot out style qualifying. Ours tended to look after the tyres for the longer distance races.

[tristancliffe]

I see. I misunderstood your point. In terms of handling (ignoring bumps), I like the car, and I'm probably not good enough, committed enough, consistent enough orpossessing enough feel to notice most changes either.

But I go worse, relatively, at bumpy tracks compared to smoother tracks (though it could be other factors - lack of talent, corner profiles, tyre age that actually are making the difference), so wanted to improve performance over bumps.

I shall take a bit from bump, add some rebound, and start from there. My teammate has Ohlins dampers, I have Konis (that's what they came with), and I've seen others used as well. At our level, what you get used to is probably more important that what is technically better, especially given most won't, or can't afford to, ever change.

Thanks.

[Chris Wilson]

Elitemotorsport: Talking of Oulton, how would you respond to a powerful RWD car (RWD Skyline with plenty of grunt and double adjustable Eibach dampers all round) where it picks up both back wheels on the bump coming out of Druids? It has a good plate type LSD in it, which gives excellent traction, but at just this one point of the circuit traction breaks and the car can be very snappy, especially in the wet. I feel it's definitely a damper setting issue.

Tristan: I had a 96 Dallara F3 with monoshock front and could never really fathom out how to set the front damper up. The Belleville washer "Anti Roll Bar" affair seemed to respond to changes, but I was never really aware of much change in handling from damper rebound and bump changes. It put me off monoshock front ends on single seaters, to be honest, although your results show you seem to have no problem getting the times out of the machine

Interesting thread. I once toyed with buying a basic shock dyno in a bid to understanding better how valving worked, and to take some of the guesswork out of "is this damper ok" type questions.

[tristancliffe]

Originally Posted by Chris Wilson

Elitemotorsport: Talking of Oulton, how would you respond to a powerful RWD car (RWD Skyline with plenty of grunt and double adjustable Eibach dampers all round) where it picks up both back wheels on the bump coming out of Druids? It has a good plate type LSD in it, which gives excellent traction, but at just this one point of the circuit traction breaks and the car can be very snappy, especially in the wet. I feel it's definitely a damper setting issue.

Tristan: I had a 96 Dallara F3 with monoshock front and could never really fathom out how to set the front damper up. The Belleville washer "Anti Roll Bar" affair seemed to respond to changes, but I was never really aware of much change in handling from damper rebound and bump changes. It put me off monoshock front ends on single seaters, to be honest, although your results show you seem to have no problem getting the times out of the machine

Interesting thread. I once toyed with buying a basic shock dyno in a bid to understanding better how valving worked, and to take some of the guesswork out of "is this damper ok" type questions.

The front isn't damped in roll, so the front damper is only working with ride - bumps, braking, acceleration. The rear pair of dampers control the roll damping (in addition to friction in the belleville stack, but that's not as nice as valved hydraulic dampers!), so you have to control cornering balance on the rear dampers. There isn't a huge difference between, say, softest and hardest settings, but there is a difference. How much it influences laptimes at our level remains to be seen though! One school of thought suggests that if the car isn't touching the ground (other than the tyres), the tyres are from the last couple of years, and nothing is broken or smoking, then it's all from the driver. I don't believe, but I've heard experienced people claim it!

I guess with the Oulton bump problem, any change to improve performance over the bump is liable to make the car worse everywhere else, so it might be quicker to 'live with it'. If you want to try and get rid of it, do as they said above - softening bump and stiffening rebound (possibly just at the rear if that's where your hopping problem is) and getting it better over the bump. If you're quicker over the lap, then hurrah. If not, then revert to the old settings.

But I'm no damping expert (which is why I made this thread!), so feel free to ignore me until someone clever speaks (types).

[elitemotorsport]

Chris. I'd take the same approach, soften the bump settings and stiffen the rebound, try and get it so the bump upsets the car the least and the rebound tries to keep the wheels from lifting. You might want to try just softening bump first, just to see if softening it stops it reacting so badly.

When i've noticed the biggest difference in softening bump and stiffening rebound is over the faster undulating bumps.

You might have a situation where the reaction of the front is adding to the reaction of the rear. Not quite sure the best way to describe it, but, if imagine a speed bump, at some speed you will hit the rear wheels whilst the car is still moving about as a reaction to the front hitting. If the rear wheels hit at the same time the car is still going up from a bump wave caused by the front then it'll add to the amount of rear movement you get. If this is happening you might find that a softening of front bump and or rebound should change the frequency or the amplitude of the front movement such that it hits the rear out of phase / lower amplitude and helps reduce the effect of the rear hitting the bump.

Hope that's clear. Sorry it's not a definitive answer, but, hope it gives you something to try.

Dampers do make a big difference to the handling of the car, so long as you are working on an area that they have influence on. You have to try and keep in mind that dampers produce their "stiffness" as a result of how fast the damper rod moves not how far it moves. Springs and roll bars are the reverse. So when you mentally analyse what the car is doing try and think whether the wheel is bumping fast, slow or over a significant distance.

For example initial turn in is very low spring movement because the car hasn't had time to generate G and transfer weight and move the spring much. But its a fast movement so what travel you do have is enough for the damper to have an effect on the tyres load, so you would expect a damper adjustment to make an impact.

Mid corner is different because the car has taken a set position so the dampers won't be moving much but the spring will have been compressed so you would expect a change in springs/arb to have an effect, but dampers probably won't.

Obviously anywhere you have bumps then the dampers will make a difference.

These are massively generalised statements, but, they do point you into a direction that should provide positive changes.

D

[Belatti]

Almost all tracks are bumpy arround here, and what has worked for me is to use softer springs. I have to tell you first that I engineer underpowered formula cars (125HP) modest downforce that weight 540Kg including driver, ballast and fuel.

[Belatti]

Another cheap quick test you can do, if you have got or can borrow an N2 bottle, is to play a little bit with nitrogen pressure from dampers. More pressure will mean an additional progressive gas spring in bump and more extension preload. That can go in hand with spring stiffness reduction, the progression helping a little bit to prevent bump bottoming and helping extension another bit without changing clicks if you soften damper bump.

Otherwise, you can leave springs as they are and try lowering pressure, only if you are sure dampers wont cavitate. That will be similar to use a little less spring stifness, after all you may do other things to have some safety progression for very severe bumps.

I have experience with penske dampers and they can work well between 150 and 250 psi. I use the feature for finetune wet weather set up.

[elitemotorsport]

The Koni's will cavitate at under 12 ish bar. The Penske dampers can run a lot lower gas pressure as they have a remote reservoir and the bump valving in the head of the reservoir puts in some of the resistance that the gas pressure is there for.

If you have single tube dampers they tend to have very low gas volumes because of the packaging so the gas spring effect is much bigger than on remote reservoir dampers. It also adds to the nose characteristic that i spoke about before. It's probably another reason why Konis aren't a lot of people preference on the bumpy stuff, but, are liked on smooth circuits and one shot quali laps.

[NJH]

Same subject area I have a car with double adjustable remote reservoir shocks (GAZ). I am really really happy with the cars dynamics now as in roll is nice and smooth and weight transfer is just right for me such that I can slow it up a touch by holding the weight of my foot on the brake a touch after turn in or not do that and get a more aggresive turn where you can feel the rear tyres squirm. Balance between understeer and oversteer in different corner speeds really nice as well. However I got a nasty shock (literally) when pressing on at Silverstone the other week when running over "the bump" just after the apex on the right hander after the new pits. That bump knocks back through the steering wheel pretty heavily and it causes the whole car to skip left a touch. Now I drive my car on the road sometimes as well and that made me think the high speed bump was low as the car feels a touch soft/bouncy over motorway bumps yet on the track surely the above experience is indicating the opposite?

[elitemotorsport]

Your problem is the GAZ dampers.

The style of valving that they have is such that you basically have orifice damping, so the damping force increases exponentially with shaft speed. If you look at the Koni graphs shown earlier in the thread you'll see that as shaft speed increases the graphs begin to flatten off, so you get proportionately less damping at very high shaft speeds. With an orifice damper you get the reverse.

So basically when you run that kerb you are effectively running on a solid bar and all the force is being transmitted into the tyres and chassis.

Proper racing dampers have high rates of low speed damping and low rates of high speed, so you get stiffness for feel in transient movements and softness to run over kerbs etc. They achieve this by opening up more holes for the fluid to pass through as the shaft speed increases. Check www.nitron.co.uk for some diagrams showing how the internals work.

Gaz, Spax, Avo, and Protech all share the same internal components and they basically have one hole with a spring loaded valve controlling how much fluid comes past. Once that hole starts to choke with the amount of fluid trying to rush through it then the damping rises exponentially. See the theory of dash pot dampers for more details and the maths.

[NJH]

Thanks thats interesting stuff. I thought they would have had a shim stack like on the diagrams from here?
http://www.blackartdesigns.com/id4.html

... I have never seen the insides of one so wouldn't know.

[elitemotorsport]

The mono tube dampers work as per that decription from Black art, although i hope their high speed adjuster isn't like their sketch, i hope they simiplified it for the sake of the document.

What you have is the old Leda product that Gaz took over which woorks in a very different fashion.

D.

[Chris Wilson]

elitemotorsport: Are you still active in damper rebuilding? I have a set of Eibach double adjustables off an endurance race Skyline that need the piston rods lengthening (I suspect they just have spacer tubes within to shorten their open length), and an O ring leak at where the nitrogen chamber arm seals to the main body. I also need the valving testing and a damper plot producing. Thanks, e-mail me if you prefer at chris@chriswilson.tv

[elitemotorsport]

Hi Chris,

I could do it but it would involve me making use of Nitrons dyno etc. You may be better off asking Guy at Nitron if he would be prepared to do it for you. He's a really helpful, nice guy so i wouldn't think it would be a problem.

D.

[andy robinson]

Chris I too have a powerfull but heavy car & consider Oulton to be the most difficult ( but one of the best circuits in the uk) to drive/set up for.
Having raced on the Island circuit, the braking for this hairpin is really difficult to get right purly due to all the bumps, the car is really nevervous down cascades & the whole watertower through druids is a 'big accident' waiting to happen area.
Yet my car was fine around cadwell & a dream on the new twisty but flat snetterton 300