Droop Limiters?

[BritishV8]

I wondered if you gentlemen might educate me about droop limiters - their history, their effects on handling, pros and cons of various droop limiter devices/approaches, etc.

I'm currently studying a March 822 (i.e. 1982 model Formula Two car) and I understand that its owner had its KONI front shock absorbers "droop limited" internally when they were rebuilt. What does that entail? At first I imagined a nylon spacer inside the shock, but I guess a rubber spacer or a spring might work. What's the usual practice? Is this something that might have been installed from 1982, or is it a more modern technique? How common are droop limiters on newer Formula cars?

Obviously there are other ways to skin a cat. For example, a chain or cable could be attached to the mounting tabs I spotted on this older Brabham BT29:

Is a droop limiter something that you'd want one weekend (at one racetrack) but maybe not the next (at another)?

Thanks in advance!

[Steve Demeter]

Ask Keith Averill.

He gave me an explaination the other week that still has my head reeling.

He knows them well to say the least.

[Purple Frog]

Richard Pare wrote in 2001:

Droop limiting can work well on softly sprung older cars, but what they are really telling you is that your car is poorly set up and poorly balanced to begin with.

The article is nicely written, but horribly wrong. I don't have a printout of the article to refer to while writing this, so I may miss a point or two, but I'll attempt a simple explanation as to what is really happening.

First off, NOTHING you can do to the car will prevent weight transfer - it's a fact of physics that can't be undone. You CAN lessen weight transfer a bit by lowering the car, or lowering the car's CG, or by widening the track, but you can't stop it from happening.

Weight is transferred by 2 main means - Geometric Transfer, and Roll Transfer.

Geometric transfer happens instantaniously when the wheel is turned or the brakes applied. It is a transfer of weight directly thru the suspension arms from one side to the other, and is a function of roll center height and CG height. It is not the same as the lever arm effect of the CG around the roll axis.

Think of it as being like what happens when you push with our hand on the side of a box. The force that you are pushing with is instantaniously transferred to the contact points between the box and whatever it is sitting on.

Roll transfer is what happens as you continue to push on the box. Assuming that you are pushing at some height above the bottom of the box, your hand acts as a force against a lever, creating a torque moment arond the base of the box. That levering effect transfers the weight of the box to the side away from your hand.

The CG of the car is essentially the same thing as your hand in that example - it is a force located at a certain distance above the roll axis. How effective it is in producing roll is determined by how far above the roll axis it is located. That's the 'lever arm' part of the analogy. The higher the roll center, and/or the lower the CG, the less leverage there is (more geometric, or "built in" roll resistance) and the less the car will roll for any given cornering force. BUT WEIGHT IS STILL TRANSFERRED ACCORDING TO HOW STRONG THE CORNERING FORCE IS.

So what happens in a non-droop limited car when you turn the wheel?

1 - A certain amount of weight is immeadiately transferred thru the suspension arms. G-loading starts, and roll begins.

2 - Roll continues until the forces balance each other. How far the car rolls is determined by CG height, roll center height, track width, roll resistance, and cornering force. How much weight is transferred is a function of the same.

If the car is softly sprung, the car will roll a lot. As a consequence, the roll center position changes according to the suspension geometry. At the front, it will generally get lower,( becase of the "dive" from brking) and will usually move sideways a bit (sometimes quite a bit). As the roll center goes lower, the built in roll resistance is decreased, and the car rolls farther, and so on. At the same time, the rear roll center will rise as the suspension rises from the braking forces. As it rises, it's roll resistance increases, but more importantly, the amount of weight transfer to the front also increases.

If the transfer is too much, the front outside tire has too much weight transferred onto it, and at the same time the front suspension is also being asked to contribute more than it's fair share to the overall roll resistance couple. The result is a sloppy feeling car with a lot of push.

In the droop limited car, Geometric weight transfer also takes place immeadiately, but then things change a bit.

Since the inside wheel is already at full droop, the car can no longer roll about the roll center. If the car is to roll at all, the pivot point becomes the contact patch of the inside tire. This actually will raise the amount of Geometric roll resistance a fair amount, and the car won't roll as far. But more important is what happens to the effecive rate of the sway bar - it is cut in HALF! How?

Remember that the inside wheel cannot go into droop so the lever arm on that side of the bar is now in a fixed position. The only twist introduced into the sway bar is now solely thru the outer lever arm, and so the bar's contribution to roll resistance is cut in half. The overall roll resistance couple is biased more to the rear now, and the car doesn't develop as much push.

Another phenomenon that happens is that more weight stays on the inside wheel, so it can now contribute more traction than it could before. Remember that the contact patch is now the roll pivot point - for the car to roll at all, a certain amount of weight has to be on that patch. If the tire is completely unloaded, the roll pivot point then becomes the outer contact patch. When that happens, the car will stop rolling, and raise the inside tire off of the ground, at which point you can't corner any faster.

If you add preload to the springs, you accomplish 2 things :

1- the nose won't dive as far under braking. Rear-to-front weight transfer still happens,but will be decreased slightly - the max pitch attitude is decreased (the nose doesn't dive as much) so the front roll center stays higher, because the weight transferred first has to overcome the amount of the preload.

2- Roll stops sooner as the inside tire has less droop available.

If the springs are preloaded enough, the amount of Geometric weight transfer is increased, which results in the faster reactions at the front

If the springs are preloaded too much (such as that the nose doesn't dive at all under braking), then the Geometric weight transfer phenomemon becomes the sole means of weight transfer , and quicky overloads the tire contact patches, resulting a "skatey" and "darty" car.

The "sweet spot" is where the car is still able to dive a bit under braking, leaving a bit of droop available for the inside tire. The car will still roll a bit, according to the amount of cornering force produced, but the inside tire will run out of droop travel at exactly the right g-loading where push would usually start ( remember the halving of the sway bar rate & the resultant roll couple balance)

Whew! Sorry I couldn't make that any shorter - it's a really complex set of relationships that is going on when you droop limit. I over-simplified some of it for the sake of brevity, so the description isn't 100% accurate.

While you CAN get good results by droop-limiting, it really is not the right way to balance out the car - it's a crutch. You are only masking the symptom, not curing the problem.

It also can be a real bitch to find that "sweet spot", especially if you aren't really, really consistant in your setups.

But, if you think that is hard, try droop limiting on the rear to cure a loose condition in a particular corner only!

[BritishV8]

Great! Thank you.

I admit this still doesn't match my intuition very well, but perhaps if I sleep on it a little more... In the meantime, I've written four rough draft photo captions for an article. Have I synthesized the information above correctly? PLEASE let me know what you'd change to make these captions more accurate.

(caption for the first photo)
Phil Harris of Truechoice installed internal spacers to shorten the stroke of these KONI shocks.

(caption for the second photo)
"Droop limiting" is a common technique for altering the the dynamic behavior of the front suspension on relatively modern Formula racecars. When the chassis rolls far enough to engage the droop limiter, several interesting things happen: (a) the roll center shifts to the inside tire's contact patch, (b) roll resistance increases sharply, (c) further weight transfer is somewhat inhibited.

(caption for the third photo)
So what effect does droop limiting have from a driver's perspective? Droop limited front shocks made Justin's March 822 seem more "neutral" (less understeer) in fast corners and also gave him a little more margin for confident trail braking.

(caption for the fourth photo)
Some racers dismiss droop limiting as a band-aid remedy for suspensions with too-soft wheel rates. I don't know about that... but I do know that the wheel rates on Justin's March 822 are ~800 #/inch (front) and ~950 #/inch (rear). Justin advises that the car feels VERY stiff until you get it up to full speed, but that the suspension works over a larger range of motion than he expected.

[marty7]

Drooop limiting or Zero drooping the front is a very valid handling mod. I don't like discussing it much as it is a very good tweak. almost all modern single seaters are now zero droop at the front.

I had long discussions with Bill Stone and Malcolm Oastler on this when rebuilding my formula fords. It was discribed to me as something for nothing. Most tracks are getting smoother. I have limited the droop om my Lotus 69. The 69 was designed with a certain amount of anti dive. Droop does not affect this as the front is in bump when under braking. Under acceleration the car will squat and try to lift the front, Zero droop stops the front from lifting, there is a traction advantage. On cornering you get the outside front to work more. The Lotus is still nice to race and forgiving. Limiting droop has not given it any nasty vices.

To limit droop you need to look at the suspension design. With my cars that have outboard dampers I have achieved it by shortening the damper shafts. You have to be meticulous on set up to get lengths right. that is both front wheels come off the ground at the same time when you lift it.

If you have an inboard pushrod type car you can preload the springs, then adjust the pushrod till the car is droop limited. Rocker suspension requires shortening of the damper shaft. I have seen simple methods of wire and clevis, even chain from top to bottom damper mount.

martin

[kea]

BritishV8,
You have a lot of good information here, but if you wish to talk, you can call me at, 248-585-9139.

[Purple Frog]

(caption for the second photo)
(a) the roll center shifts to the inside tire's contact patch,

Is it the inside tire's contact patch? or outside?

[BritishV8]

Inside, if Richard Pare's explanation (above) is correct. Note where he wrote:

Since the inside wheel is already at full droop, the car can no longer roll about the roll center. If the car is to roll at all, the pivot point becomes the contact patch of the inside tire. (emphasis added)

Sitting here at my dining room table, pretending I'm a Formula Two car's front suspension, I'm not really sure I believe that my roll center has moved to my right hand (i.e. "inside wheel"). But I had a long talk with Keith on Friday morning, and he did help me understand what's going on much better generally. I'm re-writing captions now because I have a lot more information to cram into them.

Marty's comment about accelerating forward & the car's tendency to squat has certainly helped my understanding. Anyone else have something to add?

[R. Pare]

Think about it for a second - when the inside suspension cannot droop any further, if the car is to roll about any point other than the contact patch, the tire will have to either decrease the squash from whatever there is left for load on the tire, or, once the tire is fully unloaded, it would have to lift of of the ground. In our cases, the roll movement is all in the outboard suspension (except for what little tire squash can take place), so the car actually lowers its ride height, which in turn decreases the progression of the suspesion self-stiffening fron the jacking effect, in turn decr4easing the push that the car would have developed if it were not droop-limited.

It still is a piss-poor way to get rid of push, but in many cases, because of the geometry, low motion ratios, not so hot shocks, etc, you may have no other choice, so then it is indeed a valid way to tune the car to get what you want for an end result.

Don't be afraid to use it if you have no other, better, choices.

[sccadsr31]