zero droop behaviour

[clausen]

Hi guys,

This might not be something that people here have thought about much, as FSAE cars must run at least 25mm of droop. However I am running a late model formula ford now, and I am having trouble finding anyone to discuss the finer points of their behaviour with that has such a grasp of vehicle dynamics theory as the clever people that frequent this forum.

I am trying to understand exactly what is happening as far as elastic roll stiffness goes when these cars run with zero droop on the front. I believe this is common practice on a number of different types of open wheel racecars. Setting them up with any substantial amount of droop causes them to understeer.

As far as I can work out, if the car is somewhere in a corner, and the inside front is at full droop, then as far as resisting roll moments goes it is behaving like an infinitely stiff spring (not wanting to extend with reduced load). This should mean that a zero droop setup would have effectively more elastic roll stiffness during a corner. But that doesnt make sense because a zero droop setup understeers less.

Does anyone care to put forward a theory?

[IsheeM]

Without thinking too much or too long, it sounds as though it might be working similar to an anti-roll bar. From my experience, increasing the ARB rate increases grip. Something to think about anyway.

[D J Yates]

As far as I can work out, if the car is somewhere in a corner, and the inside front is at full droop, then as far as resisting roll moments goes it is behaving like an infinitely stiff spring (not wanting to extend with reduced load)

Correct. Only problem is that it'll act like an infinitly stiff spring all the time, not just roll. That sounds bad to me, would probably be detremental to grip and handling in all situations.

Regarding understeer. When you lift the inside you'll be reducing the grip across the wheel pair and depending on wether at the front or rear, increasing or decreasing understeer respectivly.

Severly reducing the grip in this way sounds like a very crude way of balancing the car. Playing with anti-roll bars, spring dampers and tire pressures (we'll see a lot of this in F1 this year) would be much better. If your still stuggeling for front or rear grip why not try different tires alltogether.

[BryanH]

Paul, at at turn-in the roll centre migrates to to the inside tyre contact patch, and to the outside tyre c.p. on corner exit. (like a kart)
Maybe this can be modeled in susprog? Has been used on many types of cars since early eighties.

[Kevin Hayward]

Paul,

I haven't worked with Formula Fords before but I am aware of the setup you are talking about and have a few ideas ... but they are not backed up by much testing. Although we have run our FSAE cars droop limited at some tests to investigate the effects.

My first point would be that mechanical vehicle balance is not just a function of roll moment distribution. Grip at any wheel is a function of the accelleration of the wheel (load variation), camber angle, F/R tyre size ratio etc etc.

One of the obvious trade-offs are between the load variation and the camber angle. Stiffer anti-roll bars etc improves camber properties but will generally increase tyre load variation. An increase in grip but also a decrease. I would suggest that by droop limiting you effectively limit roll and improve camber properties. At the same time you have restricted roll without requiring stiffer springing. This does not hurt tyre load variation (at least not on the heavily loaded outside wheel). Seems like a bit of a free increase apart from the effect to roll moment distribution. The front is effectively a lot stiffer in roll with droop limiting as you mention.

We use old formula Ford tyres for driver training. I haven't done a lot of setup with them but I would take a stab that they are a little camber sensitive. If I was to design a Formula Ford I would probably want moderately long swing arm lengths in the front with not a lot of static camber. This requires small amounts of roll. However given that the vehicles are very mechanically grip limited I would want soft springing. Droop limiting the front allows this to happen. I would be less inclined to do this at the rear and would probably just run a roll bar. I would then try to balance the car primarily through camber control and tyre size.

I don't know if this is how the Formula Ford Guys do it ... to be truthful I haven't looked too hard at a FF for quite a while ... Not so much a fan of open wheelers.

But basically I think with the reasons above and the low centre of gravity it is probably a case of other factors having more importance than the load dependance of the tyres. We have found this happening more than once on our FSAE cars during tuning.

This is also not considering any toe effects, bump steer etc which may have a bearing on the problem as well. Truth be told I'm as interested in Paul to find out what reasons the Formula Ford guys use.

Cheers,

Kev

[RobCMT]

i'm familiar with this also in formula fords but don't have an answer sorry. Zero droop should mean more lateral load transfer therefore less overall grip available on the front tyres... Perhaps the geometry on these cars is such that the camber change in droop is enough to cancel any advantage that would be available if the inside tyre was more firmly on the ground??

[RobCMT]

oops looks like Kev beat me too it! (and in much more detail)

[Denny Trimble]

I'll throw some conjecture in the ring as well.

Until the load on the outer spring is greater than its preload, it won't deflect. So, until the roll moment reaches that point, the front has an infinite roll stiffness (neglecting tire deflections here).

Depending on shock settings, this will yield a car with very stiff front roll resistance on turn-in, then a more reasonable amount of front roll resistance as the car settles into the corner. This increase in front roll stiffness during the turn-in phase has been claimed to help by Carroll Smith (and probably others).

That's my guess, at least.

[Denny Trimble]

Originally posted by Halfast:
Paul, at at turn-in the roll centre migrates to to the inside tyre contact patch, and to the outside tyre c.p. on corner exit. (like a kart)
...

How do you find the roll center on a kart? And, in a related area, how do you find the roll center in a "swingarm" rear suspension like Lehigh's (think ATV without independent suspension)? Both of these suspensions rely on structural stiffness to control roll moment resistance, and I haven't read anything about them in thevehicle dynamics books I have around.

[Kevin Hayward]

Denny,

That is a good point. Although for droop limiting all you have to have is your preload equal to your static load. In that case there would be outside deflection straight away. Although you could have more roll resistance.

However the transition between braking and turning is an interesting one. If your preload is equal to your static load then as you brake both front springs are partially compressed. This means that the inside wheel will be able to rebound slightly ... meaning that initial front roll resistance is decreased from the droop limited case.

Off course what you say is true if extra preload is used. So it appears by this that the amount of preload can be used to alter the turn-in characteristics of the vehicle one way or another. Could be a useful tuning technique.

There are also a couple of interesting ideas that occur to do with the Hallum Tyre model. Once we start talking transients and tyre temps etc. we get a whole new depth to the problem. I would love to see some test results from a Formula Ford hooked up with tyre temp sensors and wheel position sensors.

Anyone?

...

Oh well worth asking

Kev

[clausen]

Wow, good stuff,

Dj Yates, we use a control tyre in Australian FF.

Good point Denny. This makes it even more confusing. We run only just enough preload to zero droop at static ride height with the driver in it. So any load on the track will compress the spring. We do always (as far as im aware) turn in while braking, so at the moment of turnin, we would get some droop happening.

Kevin, I believe the tyres are pretty camber sensitive. Late model cars run very long instant centres, low RC's, and 1 - 1.5 deg static negative with 6-7 deg caster. I'm only new to these cars and havent done much testing myself, but im pretty sure that if running some droop and understeering, adding more front bar will worsen the u/s while reducing droop with reduce it.

I'm dying to get damper pots on it (soon) and tyre temp logging (when we get a new logger).

[clausen]

Kevin, could you please drop me a mail at
paul.clausen-at-lexicon.net ?

cheers

[BryanH]

Paul, zero droop is not particular to F.Ford, I doubt that you could find one single open wheeler built since the late 80's that doesn't run preloaded front and less than 25mm rear droop. Considerable preload is more the norm.
I have setup several "tin-tops" with 1-2mm front and 15-20mm rear droop for exactly the reasons desc by Kev,(poor design that can't be altered) & they have always been quicker,neater to drive with major improvement in turn-in.
A little rear droop is needed to stop wheelspin over bumps.
But seeing as single seaters are clean sheet designs the reasons why it is the fast setup are more likely as Denny described. I have no idea what the usable windows of adjustments are on a single seater or the science behind it.
One obvious reason is control of ride height for stable aero but there has to be more in it than that. A detailed post from an ex Reynard/Lola/Dallara or F1 suspension engineer would be appreciated!
Before you go experimenting be aware that the shocks need internal bump stops designed for preloaded installation, I found out the hard way on an FC RX-7. On the first few cars I used stainless cable & turnbuckles from sailboat shop.

[Denny Trimble]

So, are you using internal spacers in the shock (with rubber "bump stops" as well?) to limit droop? And / or tension cables?

Just trying to get a clearer picture of what you learned the hard way http://fsae.com/groupee_common/emoticons/icon_smile.gif

[clausen]

Cheers Bryan. What sort of tin-tops are you talking about? imp. prod? If you take one of these sorts of cars that is well balanced while running droop, and then limit the droop, how exactly does it's behaviour change? balance?

Let me know when you find an ex Lola/Reynard/Dallara engineer http://fsae.com/groupee_common/emoticons/icon_smile.gif Actually that gives me an idea - Tauranac (spelling?) might be a good person to get an opinion on this from.

And are you saying that people use more preload in the front of open wheelers than is just required to maintain zero droop in static conditions?

Another point of interest is that at least some (fast)V8 supercar setups have significant droop in the front. But then again they're strange things.

[DJHache]

This is a good thread!

Denny, I don't understand what you mean when you say "Until the load on the outer spring is greater than its preload, it won't deflect". If you have a preloaded spring and you add 20lbs of load to it won't it deflect?

As far as the no-droop case it seems that if the inside wheel is effectively off the ground then you lose all the antiroll stiffness from the inside spring and your total front anti-roll stiffness would decrease. This would alter your front-rear antiroll stiffness distribution and the rear would handle more of the weight transfer resulting in less grip at the rear. The net result is a grippier front (because the load varies less) and a looser rear (because the load varies more).

It's funny how we can all reason something out to make it agree with our thoughts.

[Hoosier Daddy]

Originally posted by DJHache:

As far as the no-droop case it seems that if the inside wheel is effectively off the ground then you lose all the antiroll stiffness from the inside spring and your total front anti-roll stiffness would decrease.

This type of thing can happen with a big front ARB... Lifting the inside.


Originally posted by DJHache:
It's funny how we can all reason something out to make it agree with our thoughts.

lol

[Denny Trimble]

Originally posted by DJHache:
Denny, I don't understand what you mean when you say "Until the load on the outer spring is greater than its preload, it won't deflect". If you have a preloaded spring and you add 20lbs of load to it won't it deflect?

If the spring perch barely touches the spring when the shocks are off the car, that's "zero preload". Any force you apply will deflect the spring, neglecting friction in the shock.

If you crank the perch 10 turns tighter with the shock off the car, the spring is preloaded against the extension limit of the shock. It will take (10 * pitch of perch threads * spring rate) pounds to start deflecting the spring at all; the applied force only compresses the spring after it is equal to the magnitude of the preload. The force/displacement curve still has the same slope, but the Y-intercept is much higher because of the preload.

Grab a shock and a scale and try it.

[DJHache]

I see better now, Denny. What you're saying is that if your shock has 200lbs of preload, the extension stop will generate 200lbs of force in the opposite direction. If you then add compressive force on the shock it will simply transfer from the extension stop to the load and then it will start compressing the spring once 200lbs has been reached.

Hoosier Daddy, it's true that a big ARB can generate wheel lift but once the wheel has lifted the ARB has no effect and the roll stiffness on that end decreases.

[Z]

Getting back to the issue of "droop-limiting gives less understeer". Here are two factors missing from the above explanations.

Factor 1.
First the basics. Assume linear rate springs. Assume only just droop-limited, so no preload. Assume typical FF wishbones, so long and almost horizontal virtual swing-arm. Don't think about "roll-centre" (a dodgy concept for independent suspensions). Instead think about line from wheelprint to swing-arm IC - call it the "n-line" (normal to wheelprint up-down movement), or "control-arm-force-line" (as per Mark Ortiz). For the moment forget about ARB's, dampers, and rear suspension.

The droop-limited pair of springs will have twice the roll-stiffness of a normal setup (ie. for given roll moment a normal setup's 2 springs compress/extend "X" and roll angle is 2X/Track radians, while on the droop-limited setup only one spring compresses for roll angle = X/T.) So for given cornering G (and forgetting rear susp.) the droop-limited is stiffer and only rolls half as much. Stiffer should mean more understeer, but half roll angle means less adverse camber change so less understeer. So far this is as noted in above posts.

BUT!!! When the normal setup rolls there is no change in chassis centreline ride height. When the droop-limited setup rolls the centreline ride height DROPS by X/2. This drop in front ride height lowers the slope of the n-lines (= lower RC). Lateral load transfer (LLT) is a combination of spring forces (= elastic LLT), control arm forces (= virtual swing arm, or n-line, or kinematic LLT), and damper forces (= viscous LLT). When the n-lines slope down from the chassis to the wheelprints there is kinematic LLT off the inside wheel and onto the outside wheel. BUT! When n-lines slope up from car centre to the wheelprints (= below ground RC) then kinematic LLT is ONTO the inside wheel and OFF the outside wheel.

So for typical FF wishbones a lower front ride height means LESS kinematic LLT and hence less understeer (from this factor). Lower ride height will also, typically, mean more negative camber on outside wheel, again less understeer. Of course, the car is only interested in the "bottom line". It takes account of all (hundreds+..) factors, their sizes and signs (+ or -), then adds them all to decide whether it is going to under/oversteer. So really you also have to know what the dampers, rear susp., etc. are doing...

Droop-limiting is like fitting falling rate springs (the force/deflection curve bends down to the right). Rising rate springs (curve bends up to right) are great for absorbing bumps, and are often used as a justification for rockers (use linear coils and make rising rate with rocker geometry). Falling rate springs pull the car down during roll, but rising rate lifts the car! A lifted car (higher CG) is a BAD thing, hence most setups that work well are either linear or perhaps falling rate (at least in roll, "third spring" acts only in bounce so can have rising rate). (This is another justification for rockers out the window.)

Factor 2.
A lower front ride during cornering will pull the nose closer to the ground. Formula Fords aren't supposed to be "aero" but a lower nose will probably create some suction under it. Since this is cantilevered out the front of the car it may (?) increase front download AND reduce rear download, hence less understeer.

Z

[Kevin Hayward]

Z,

Unless I am mistaken the lower kinematic LLT is countered by a higher elastic LLT which has more understeer (from that factor). Pretty much a balance but a different way of counter-acting those forces.

However your mention of the force lines is an interesting point. The slope of the n-lines during droop limiting will have a negative jacking effect. This in turn lowers the COG height further (added to the loss of height due to one spring compressing, one not). I am not aware of how pitch sensitive the aerodynamics of the Formula Fords are but it is certainly a possibility.

Excuse my ignorance but what sort of floor area does a Formula Ford have? and is it flat all the way to the rear? These factors could show whether there is any advantage in raking the vehicle body.

Kev

[clausen]

Good Stuff Z, I hadn't considered the effects of this setup's effect on ride height.

Kevin, the floor is dead flat, but it is quite big. It runs from the nose to the engine bellhousing.

We find that running 5mm of rake makes a significant improvment to turn-in. After this thread was finished I was going to start talking about why this is. I figured that it might be to do with RC height (n-line angle) and that I could make a similar change without lifting the cog. It could quite possibly be aerodynamic.

When we get our damper pots on we'll see if we can measure any downforce changes with pitch.

Does anyone have any equations for ballpark figures for flat bottom ground effect? I guess I'll borrow Kats and have a look.

[BryanH]

Many thanks "Z" You have filled in a few of the blanks.
Denny, "Kart roll centre" is Pat d'Rats theory so he will have to cough up the diagrams.
Lehigh used same brand rear axle components we use, the bearing mounts a self aligning and contribute very little to roll stiffness. So it was pure trailing arm geometry. btw lack of a brake torque reaction arm means that it would have unloaded the R/R wheel under heavy braking...
The RX-7 was fitted with custom built Koni struts, but I omitted to tell the Koni Enginner that I intended to run them pre-loaded.
The piston valving got beaten to death!
Koni have internal bump stop components that were built into "mk11" versions and I also reduced the pre-load. All good now.
The cable limiters were for experimentation - easy to fit and adjust at the track. Shortening the shock shaft is the proper way to do it.
Remembering more about the changes in the cars behaviour, it fits in with "z",s exp. very closely. Has anyone else noticed that BMW road cars run very little front droop? and 996 Porsche.
Interesting experiment; put about 8 team members in your big old chevy then weld droop limit chains to all four corners
Maybe a job for Myth Busters!
Bryan H.

[BryanH]

Paul, I know it's a hot day but shouldn't you be working? I'm meant to be fabing a PP manifold.
RMIT's RO4 is ride height/rake sensitive and you would think it has nothing to do with G.E. on their car. F-ford downforce could probably be measured on kitchen scales.

[PatClarke]

Bryan,
Kart Roll Centres ??? The only time I ever heard that term used was in a RaceCar Engineering mag article by 'Rocket' Rod Stevenson...Not me.
I cant see how you can have a roll centre without having some suspension articulation, unless you consider the roll axis to be between the loaded contact patches.

BTW, check your PM
Pat

[clausen]

Bryan, I was on my lunch break! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Hey have you had much to do with NSU's? I've got an RO80 as a sunday car these days. It's a pretty amazing thing to drive.

Also droop limiting with cables as a quick and rough experiment is a cool idea. I'll try it on my RX7 if I get around to it http://fsae.com/groupee_common/emoticons/icon_smile.gif

[clausen]

It just occured to me that all this zero droop stuff is why mono shock doesnt suck on F3 cars and things. I always though mono shock was crap because I thought you were'nt able to use damping to adjust behaviour, because the bellcrank thing would most straight sideways in the roll springs. But with a zero droop setup the damper is always going to be compressed in roll because the inside wheel can't rebound while the outside is compressed.

That might explain why monoshock doesnt suck, but not why it might actually be better.

[Dan B]

This is a small thread (on rebound springs) with some info that might parallel this thread.

http://www.eng-tips.com/viewthread.cfm?qid=107676&page=4

[clausen]

Cheers Dan. That is an interesting thread. There's still more to talk about on this one though. The next peculiarities to do with running zero droop are the effects of rebound damping.

It's fairly obvious that rebound damping will do nothing or next to nothing on turn-in (depending on whether or not you're braking). So the question is how much (or how little) rebound damping you can run. I've come to the conclusion that it's probably a good idea (as far as dealing with bumps goes) to use a damping coefficient of around 0.7 because that gives the quickest settling time. But on a zero droop car, you don't have to worry about overshoot. It will just hit full droop, and unless the rebound is super quick, it will stop there. So in this situation, how quick is too quick when it comes to rebound? What sort of damping ratio?

[clausen]

aww come on, we were having such a good discussion for a while!

[Richard Pare]

Just heard about this thread & thought I'd chime in to get things corrected.

"Zero Droop" and "Droop Limiting" are 2 pages of the basicly same thing. Both/either are done by many to combat push.

The simplest explanation is that both will change the degree of Jacking Effect at some amount of lateral loading. Jacking Effect is basicly a dynamic self-stiffening of the suspension in reaction to lateral loads fed thru the suspension arms. Think of it as being the same as anti-dive or anti-squat, except laterally rather than longitudinally.

Generally, either will be done to combat a push in a car that is not well set up - generally too soft.

Drivers will start to feel push generation at some specific level of lateral load. The idea is to decrease the self-stiffening effect starting at that load level or just before it is reached. (The car is already pushing, but the driver can't feel it yet - it is too subtle).

Droop limiting - either via preloading the spring when installing on the shock, or thru the use of a mechanical stop devise - allows the car to accept lateral loading and roll a specific amout before the inside spring tops out. At that point there is a certain amount of vertical load still left on the inside tire. For any further load transfer ( and the necessary increase in lateral loading) the actual roll center moves to the center (almost) of the inside tire contact patch. For any further roll (from increased lateral loading) the car now pivots about that point, and lowers itself. That lowering in turn decreases the Jacking Effect. Roll stiffness from the springs and bars is not changed at all, so the net effect is a decrease in roll stiffness (or at a minimum, no further increase)

Zero Drooping just starts things immediately, rather than after the lateral load builds to a certain level. It can have the benefit of making the car VERY reactive on initial turn-in, but will also decrease the ultimate grip potential (that ultimate grip loss is present in droop limiting also).

Most cars push from being set up too soft, allowing the car to either roll out of the correct camber for those tires, and/or transfer too much weight to the outside front tire. Both of those situation can be present, or just one - the trick is to figure out which you've got.

Too soft can be either springs, bars, or both. Push can come from the front bar being too soft, the rear springs too soft, or ALL of the springs too soft.

A note on spring rates - spring rate by itself has nothing to do with grip generation. Obviously, wayyyy too stiff a spring will hurt grip from how it hammers the tire contact patch, but that isn't the range we are speaking about.

[clausen]

Ta for the post Richard,

Why do you say that zero droop reduces the ultimate grip potential? Do you mean of the front wheel pair or the whole car?

And are you sure that roll stiffness from springs doesnt increase once the inside tops out? I think i need to get a pencil and paper out http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Richard Pare]

Roll stiffness from the springs and bars stays the same. Remember that for every further (x)degree of roll, only the outside ones move, but they now move twice as far.

Ultimate grip potential is controlled by a couple of factors - the load on the contact patch, and the rate of change of any change in that load. Until there is no load at all on the inside tire, it is still contributing lateral grip. However, once the shock tops out, the rate of change in response to all of the tiny imperfections in the track surface goes up dramaticly (only the tire carcass is flexing now in response), breaking the bonds sooner than if the rate of change was slower. Think of the stretched tire rubber as behaving like taffy - if you stretch it slowly, it will stretch forever. If you stretch it too fast, it will just snap.

[Z]

Roll stiffness from springs doubles, when one of them becomes droop limited. Roll stiffness from (normal) ARB stays the same. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

[Richard Pare]

Nope - spring resistance per degree of roll stays the same. Remember that, since the car is now pivoting about the inside tire contact patch instead of near the center of the car, it takes twice as much movement of the outside suspension. in other words, the spring moves twice as much, but so does the suspension, so it is a wash.

[Z]

This would be much easier with some sketches, but here goes.

Consider only two wheels at one end of the car, springs directly above wheelprints (ie. motion ratio = 1:1), linear rate springs (up until droop-limiting), etc., etc.
Track = T
Spring rate = K
Roll-Moment on half-car due to a given cornering G (and assume this to be a pure couple) = M

Normal suspension:
Roll-Moment M causes vertical forces F and -F at the two wheels such that M = F.T
One side of car rises D, other side drops D (ie. car rolls about centreline), where F = K.D
Roll-Angle = 2D/T (radians).
So Roll-Stiffness = Roll-Moment/Roll-Angle = F.T/(2D/T) = K.D.T/(2D/T) = K.T.T/2


Droop-limited suspension:
Roll-Moment M causes vertical forces F and -F at two wheels such that M = F.T (as before).
Only one side of car moves! Since F and K as before this side drops by D.
So droop-limited Roll-Angle = D/T (radians) (ie. half of normal).
So Roll-Stiffness = Roll-Moment/Roll-Angle = K.T.T (ie. double that of normal).


Or looking at it from "same roll-angle" point of view;
Normal suspension has two springs deflected by D, giving moment M = 2 x F.(T/2) = F.T
Droop-limited has one spring deflected by 2D (so giving force = 2F), giving moment M = 2F.T (ie. twice as stiff).

Z

[Richard Pare]

Small problem with your math logic there - the inside spring does not resist roll, only the outside spring, therefore does not figure into the equations.

[Z]

True, the inside spring doesn't resist roll, but whatever is limiting droop does! It is because the inside wheel doesn't droop downwards that the body only rolls half as much for a given applied roll-moment.

Anyway, the easiest way to find out for sure is to apply a given roll-moment to the two setups (half-car), and then measure roll-angles. I'll leave it at that. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

[ben]

Z, I'm happy that I understand and agree with your explanation of the doubling in roll stiffness due to zero droop. However I'm less clear with your explanation the roll stiffness from an ARB stays the same.

If we have the case with droop then the outer wheel's ARB arm moves up by ˜D' and the inner wheel's down by ˜D' we have a total angular twist of the bar of ˜2D' multiplied by the lever arm length acting on the length of the bar.

In the zero droop case the inner ARB mount doesn't move and the outer one moves by ˜D' hence we now have half as much twist in the bar as the droop case. This would suggest the roll stiffness of the bar is less in this situation.

Could you enlighten me as to what I've missed here?

Ben

[Z]

Ben,

The important point to remember is;

Roll-Stiffness = Roll-Moment/Roll-angle (ie."stiffness = "force" divided by "deflection")

You say - "In the zero droop case the inner ARB mount doesn't move and the outer one moves by ˜D' hence we now have HALF AS MUCH TWIST in the bar as the droop case." (My emphasis)

So the bar exerts HALF AS MUCH FORCE at each wheel. So 0.5xForce/0.5xDeflection = 1xF/D = same "Roll-Stiffness" of the ARB.

In the zero-droop case the ARB is exerting smaller forces at the wheels, but this is because, for a given cornering load, the total forces at the wheels are fixed and the zero-droop wheel springs are carrying a greater proportion of these forces (because their roll-stiffness has increased from the normal case).

The ARB and the wheel-springs are two independent systems that act in parallel to resist roll. The ARB exerts an equal and opposite force at each wheel depending on the difference of wheel heights relative to the chassis. It doesn't know, or care, what the wheel springs are doing, or how hard the wheel springs are working to resist roll.

So, putting it another way, if you fit very stiff, or very soft wheel springs, will the ARB's roll-stiffness change? No, it is an independent system that exerts a roll-moment that depends only on the roll-angle that it sees. Its "percentage contribution" to total roll-stiffness changes, but its absolute roll-stiffness stays the same.

Hope the above makes sense.


Z

[ben]

Oops, I think I made the same mistake as Richard that I'd just corrected. Of course the force is still divided between the two wheels and not just applied to the one that moves.

Note to self: repeat Newton's laws (III in this case) before hitting the reply button :-)

Ben

[Richard Pare]

You are still forgetting one thing : once the inside suspension tops out, the outside wheel has to move TWICE AS FAR for every additional degree of roll, compared to when both wheels are moving, so the ARB still twists the same amount per degree of roll.

As such, the ARB contributes the SAME stiffness per degree of roll as before, even though one end is now "fixed".

This is a common known fact amoung seasoned engineers.

[ben]

It can't move twice as far because it doesn't see twice the load for a given roll moment. You can't just add all the load transfer to the outside wheel just because the inside doesn't move. This is the point I'd missed that Z has explained. Richard - would you care to put some numbers to your claim regarding the actual load changes and wheel deflection for a given roll moment?

Ben

[Richard Pare]

You arr confusing yourself! The units we are concerned with are PER DEGREE OF ROLL, not per unit of weight transfer!

[ben]

Surely we need both as Z explained, you need to calculate the roll angle per unit roll moment to get the stiffness. Your approach seems to ignore the additional load required to generate the roll angle if one side can't droop. As I said a worked example with some numbers would be good.

Ben

[clausen]

I still haven't sat down and done any calcs on this topic, but for what it's worth Claude Rouelle says that running zero droop to improve performance is an australian myth and he has data that shows that it's not worth doing....

[PPAM20]

Although Mr Pare makes some good points reguarding how the front reacts to zero droop, I think the main advantage in using zero droop is to control the REAR weight transfer. Instead of the weight of the front wheel assembly being supported by the ground, imagine the wieght distibution between the inside and outside rear wheels if the inside front wheel is cantalevered off the ground (or almost off the ground). The result is a more even distribution of weight between the 2 rear tires and thus better acceleration potential.

But a few notes.
The car has to be designed to use zero droop - usually a lightly loaded, wide track front.

Zero droop tends to turn in quickly (loads the outside front quickly) but the tire is not going to like spending alot of time in the middle of the corner. Downforce can help in this area but the best idea is to "turn in and get out" - do your best to minimize the middle of a corner.

These two points bring up a third - zero droop is not made for every car in every class on every course. Quick turns, lots of HP, and downforce to get you through the sweepers seems to be the ideal situation.


First time post, hope this helps. You guys and gals have and awesome site - keep up the good work.

BTW Lotus Sevens RULE!!

[ben]

Maybe a slightly different way of putting it, but isn't the load more equal on the rear tyres because the roll stiffness is at the front is higher therefore the LLTD is more front biased?

If you need better transient response at the front, might it be better to run more %anti-roll and some toe out rather than zero droop?

Ben

[PPAM20]

Very true - but I think in practice zero droop will allow you to run less total roll stiffness while still giving you the "bite" needed for initial turn in. This is especially important if you are running alot of front roll stiffness (90% plus of total roll stiffness). Again, it's part of a total vehicle package for specific tracks. What works with F1/Champ Cars/SCCA formula cars/etc on smooth tracks with mostly 80-170 mph corners may not be appropriate for FSAE.

I hesitate giving an opinion on FSAE cars because I am not as qualified as most of you on these cars. However my guess is if you are running alot of front roll stiffness and not spending alot of time in the "middle" of a corner or otherwise feel you are not overloading the outside wheel continuously, I would consider zero droop. If you are setting up your car with a more traditional percent of front roll stiffness and overall roll stiffness, I would hesitate on the zero droop issue.

BTW, I think (not sure) most autocross/solo cars are set up in the more traditional way.