20 replies to this topic

[Kid Prozac]

Ive been trying to get my head around shock setups and was wondering if the following is true. Now this is just my interpratation of reading from books so if im wrong it'd be great if someone could tell me why.

assuming shocks with shim stacks for bump and rebound:

when it syas that the rebound is stiffend, it means that the force required to deflect the shim stack is greater than if there the rebound is backed off. ie a thinner shim stack will bend more easily than a thicker one.

so if the rebound is stiffened, more rebound is added?

also this slows down the time for the spring to return toi its original open position cos more force is required to open the shims.

if there is less rebound, because the force required to open the shims is less, the spring takes a longer time to reach a steady state, ie the decay curve has a very gentle gradient, so the car is more 'bouncy'

having done a fair bit on theoretical damping this semester, am i right to assume that the rebound adjustment, just alters the decay of the response?


Mark Hales:

'there was not enough rebound on the dampers to prevent the body rising and picking a wheel from the road'

am i right in thinking that cos there is not enough rebound the car is 'bouncing'

this is just from mostly reading inside racing technology book, but was just trying to get it in my own words for me to understand

[AAH]

stiffening refers to increasing (softening/decreasing would be the oppisite) the force vs. wheel velocity. Now depending on how the shock is adjusted, the force can be affected at different wheel velocities and not at others. In a typical 2 or 3-way adjustable shock, the rebound shim stack is fixed and rebound damping is changed via a needle in a orifice where the needle is screwed in or out to decrease or increase the area of the orifice. This type of adjustment effects the entire force vs. velocity curve.

The second type of adjustment normally seen on the bump side, and which you refer to, is the preload adjustment on the shim stack. Shims are just springs and open when the fluid pressure is great enough to deflect them and allow fluid to pass. When preloaded, the force at which they deflect becomes greater, therefore the damping increases. This preload adjustment effects the higher velocity ranges only (it's the reason for the 'knee' in the damping curve that many people refer to)

Instead of 'decay of the response' the term normally used is damping ratio where a damping ratio = 1 is critical damping; there are lots of theories on this subject.

I don't follow your 'bouncy' reference, but what I think Mark Hales is referring to is the that the body has rolled to a point where the wheel cannot extend further and therefore a wheel is picked up off the ground. You could prevent body roll by increasing the rebound on the inside wheel. I have oppions on using shocks to prevent body roll, but that's a long story so I'll pass for now.

Check out the penske or ohlin shock manuals available from a dealer for how a shock works. And Milliken and Milliken for theory. Racing technology gets you in the ball park, but doesn't have enough specifics.

Aaron

[RDV]

having done a fair bit on theoretical damping this semester, am i right to assume that the rebound adjustment, just alters the decay of the response?

It depends whether we are talking of road or racing cars. By classic damping practice racing cars are massively overdamped, low speed damping is mainly used to control the car attitude in transients, IE the rate of change in pitch or roll , and weigth transfer , this is more important as downforce levels due to underbody aerodynamics increase.

In steady state cornering dampers control the car over the bumps, to maintain contact patch pressures ( IE max -min Fz over rippled surface ) , but during braking , turn-in and power -out phases the car is initialy pitching (nose-down) then rolling (still nose-down ) then as power is applied rolls back and nose goes up at same time. Given the influence of aero effects ( even at relatively low speeds (>100kph-<150) the rate of change of CP is controled mainly on dampers , alowing car to be tuned as to response. At the risk of making this very long lets run thro a corner...

as we brake and car assumes positive rake , many factors affect car= CG goes up , but more mass is lifted at rear (wheels go into droop more than fronts) , car rakes nose down , shifting CP forwards....front wheel camber goes more negative, rear wheels more positive....front Fz on tyre increases ( increasing cornering force aviliable , reverse for rears..) and a host more, all this giving a positively oversteer tendency in a positive feedback loop ,exacerbated by the aero effect.... without going through middle phase of corner , as you put power on all this happens in reverse where,apart from degradation of cornering force (on rear tyres assuming rear wheel drive , 4wd and Fwd being another can of worms) due to amount of power available, will tend to make car understeer. So controling transients enables car to be more controlable, and be tailored to driver preference...

mechanical grip and controling "bounce" as such is only important at very low speeds , and mainly to keep rubber in contact with ground, ride comfort not being the issue in racing cars.

this is just from mostly reading inside racing technology book, but was just trying to get it in my own words for me to understand

if you manage to understand dampers you are a better man than me , gunga din, I give classes in vehicle dynamics and data analysis , and have given seminars at Koni on racing dampers and still dont understand them... damping along with tyres are the black art side of racing, and need an in depth investigation to relate what theory gives us and what we find in practice. seven poster rigs are very usefull in separating many elements of damping and grip, but until we can have a seven poster with lateral G many aspects are still unclear...

[Kid Prozac]

thanksalot, thats answered a lot of questions

if you manage to understand dampers you are a better man than me , gunga din, I give classes in vehicle dynamics and data analysis , and have given seminars at Koni on racing dampers and still dont understand them... damping along with tyres are the black art side of racing, and need an in depth investigation to relate what theory gives us and what we find in practice. seven poster rigs are very usefull in separating many elements of damping and grip, but until we can have a seven poster with lateral G many aspects are still unclear...

yeah sorry shouldn't have really come out like that, i remmeber a quote in Allan Staniforths 'Competition Car Suspension'

'There are only four things left: more power, less weight, more downforce. Dampers will do nearly everytning else.' This from a top designer.

I'm just trying to get a reasonable understanding as I'm very much interested in Suspension, hoping to work in that field when i graduate. I'm also redoing the suspension on a formula SAE car, so was trying to get a decent knowledge for testing purposes. And as the final year team seems to know about as much as me, I'm doing my own research.

You could prevent body roll by increasing the rebound on the inside wheel. I have oppions on using shocks to prevent body roll, but that's a long story so I'll pass for now.

If its not too much asking, would you be able to tell me why, or is it a very complex issue, i seem to have trouble getting my head around how rebound actually helps keeps wheels on the ground, as you say, rebound reduces body roll, i have come across this fact in many books but none seems to give a reason.

Instead of 'decay of the response' the term normally used is damping ratio where a damping ratio = 1 is critical damping; there are lots of theories on this subject.

I'm studying this at the minute, both in Free, Forced vibration cases as part of my DYnamics course, and also in Control Systems for controlling servo systems etc

One final thing, is there any damper companies in the UK that anyone knows who take engineering students for industrial placement years ;)

[AAH]

KP, increased rebound doesn't 'hold' the wheel on the ground - the weight of the car and gravity does that for us! The subject is not really intuitive and the easiest way that I can think to explain it without confusing you and me is to think of the longitudinal example (pitch) instead of the lateral example (roll): Brakes are applied car pitches forward about the center of gravity - front shocks compress (bump), rear shocks extend (rebound). Weight transfer is dependent on CG hieght, wheelbase, and long. acceleration (brake force) - not shock damping. What the shocks are contributing to is how fast the weight is transfered (how fast the front shocks compress and the rears extend).

Assume the braking event is really long and the rear shocks extend to their limit, unless you are on a motorcycle and you are standing on the pegs the rear wheel(s) are not going to lift of the ground. Now, INCREASE your rear rebound force and reduce the braking time keeping the braking force the same so that now your rear shocks don't extend all the way by the time you finish braking. The rear wheels are still in contact with the ground and because they haven't extended all the way, the rear body is closer to the ground.

I think similar thinking could be applied to roll conditions. Like I said, it's not really intuitive, but I hope this helps.

Aaron

[Kid Prozac]

thanksalot, that helpeed a great deal. dunno why but i always try to think of things in the case of roll, if maybe i apply things to pitch it might be easier!

[Rat_Fink]

I think you are concentrating on the almost steady state aspect of dampers. The damping force created is a function of velocity and as such the dynamic application of dampers is the main issue. The response generated to a dynamic input at the wheel and the way the damper slows the motion after the event rather than during it.

[Kid Prozac]

I'm just trying to get a basic understanding of how dampers actually operate. i find it easier if im told how something works, its all very well reading in a text book that if you turn stiffen x, the car will understeer, that means nothing to me, i have to understand why it does that.

does anyone know any good books on dampers? or more specifically race car handling? not bothered at all if its full of techy stuff, im an engineering student so im here to learn!! i already have all the Allan Staniforth books, IRT and Race car chassis design by Forbes Aird.

[Jezztor]

RDV: do you have any of those seminars etc ready to send ? If so, would you mind sending to my email addy? Would like a good read on dampers & suspension etc.

By the way, has anyone got a pic of a F1 bump rubber?

Jezz

[Kid Prozac]

RDV: do you have any of those seminars etc ready to send ? If so, would you mind sending to my email addy? Would like a good read on dampers & suspension etc.

and if its not too much trouble could i possibly request a copy, my email is kidprozac@hotmail.com

thanksalot

[RDV]

....do you have any of those seminars etc ready to send ? If so, would you mind sending to my email addy? Would like a good read on dampers & suspension etc.

sorry chaps, was a couple of years ago, might have some of the notes at home , but away from home at moment and the papers are probably under a couple of tons of bumph..

but the olins manuals and penske also cover the ground sensibly , also there is a lot of stuff on the net , only a bit difficult to sort out as most of the advice is formula ( racing class) specific. a lot of the basics are interchangeable but whole thing is a minefield.

For the calc side Gillespie has an extensive section Fundamentals of Vehycle Dynamics (SAE ISBN 1-56091-199-9, and Dixon has a book entirely dedicated to dampers, also lots of sae papers, will try to find list, and of course Milliken Race Car Vehicle Dynamics ( ISBN 1-56091-526-9)
Any good data I have on F1 fall under hefty non-disclosure clauses in contracts . Will try to find anything appropriate not too sensitive and email accordingly .

and jezztor if you are into bumprubbers I can only say " Abandon all hope you who enter here..." , they are fun but as bad as tyres or dampers, everything is non-linear.......... have some interesting excel sheets , how can I post them ?

[Jezztor]

Well, the reason I ask is, during the Brazillian qualifying, on Digital + I heard Montoya speaking to his mechanics as he was entering the pitlane after he smacked off his turning vane. They said "we're going to replace the turning vane, and possibly make a rear suspension change, if we do, it will be left bump". I didn't know F1's still used bump rubbers...?

If someone can display the excel sheets, it would be great - very interested.

Ulrico

[AAH]

I think 'left bump' is probably left compression damping. I was under the impression that most of teams were using non-adjustable shocks, so changing the damping would require a shock change in the rear. I suppose this may be possible depending on the orientation of the shocks (fronts would be much harder).

I don't doubt that bump rubbers are used (they are used in CART). They are mostly used to control the attitude of the underbody and just look like really thick rubber washers. The ones I've seen are straight cut and don't have a taper to alter the spring rate - the spring rates are so high and are less of a concern on bump rubbers because the tire spring rate is much softer in comparison.

Aaron

[RDV]

...if we do, it will be left bump". I didn't know F1's still used bump rubbers...?

the change was on the rear low-speed compression, it helps reduce squat power-on , and increases rr grip on corner exit. and yes they use bumprubbers , mainly as way of having progressive springing rate on a linear spring or torsion bar. changing bump-rubbers is a time consuming change ,usualy done between sessions, damper clicks are much faster, altering the gap between brub and damper slightly longer , this changes the point where rate goes from linear to progressive, driver would say change gap. as for non adjustable dampers , you use them when setup is fixed , if still setting car up or on known bumpy track (Interlagos , Estoril,Barcelona..) during free practice you arrive with adjustables..

[Jezztor]

Well my original request stands - anyone got pics of F1 bump rubbers?

Interesting info RDV, good work. Adjustables are slightly heavier than non-ajustables so that does make sense.

Jezz

[RDV]

F1 bump rubber shots .......yes how do I post them?

[desmo]

RDV, ifyou can e-mail them to desmo@fineartscrimshaw.com, I'd be happy to post them here.

[Jezztor]

Thanks guys

[desmo]

Bump rubber installation on the F12000 Three of 'em! Courtesy RDV

[Jezztor]

Thanks boys!

[desmo]

If anyone is interested in how an elastomer bump rubber affects the overall rate curve, here is a link to a rate curve in Excel, again courtesy RDV.