Droop Limited Suspension

[johnc]

I've never thought intentionally droop limiting the suspension was a good idea and a lot of that thinking came from having to run on the bumpy tracks we have here in California. My thinking tended to follow Mark Ortiz's thinking but he has found a positive benefit from droop limiting along with others on this board (Cary).

 

MORE THOUGHTS ON ZERO-DROOP SETUPS, AND RELATED OVAL TRACK SPRING SPLIT ISSUES

 

In the February 2007 newsletter, I responded to a question regarding zero-droop setups. These are setups where the suspension is not allowed to have any droop travel from static position. I said that these were bad for everything except controlling ground clearance in order to have consistent aerodynamic ground effects. As a result of further reflection, and conversation with a number of people, I have concluded that there is another benefit.

 

That benefit is that when the car is in a rolled condition, the inside wheel pair has greater pitch resistance than the outside wheel pair. This has effects similar to left-stiff spring splits on an oval-track car. The car de-wedges when accelerating rearward while also accelerating laterally, and it gains wedge when accelerating forward while also accelerating laterally. That helps the car get itself rotating in yaw on entry, provided the driver is decelerating or braking while entering, and helps it put power down on exit.

 

This would apply to a car with zero-droop setup at both front and rear. If only the front is zero-droop, effects would be confined to corner exit.

 

This benefit does come at a price in the wheels' ability to follow bumps, but if the surface is smooth enough, it may be worth it.

 

A similar effect, but more subdued, can be had by making the wheel rate increase in droop, either using rocker geometry or using snubbers. Note that this does not mean that the spring load or force increases in droop; it means that the force decreases at a greater rate with respect to droop displacement, as droop displacement increases.

 

A third way of creating a rising rate in droop is used on dirt Late Models, and could also be applied to road racing cars, although I have yet to hear of it being tried for road racing. This third way is to have two springs stacked on top of each other on a coilover, separated by a slider. The slider is

 

arranged to top out against an adjustable collar on the coilover threads, at some point as the suspension extends. This adjustable collar has a smooth sleeve above the shoulder that the slider seats against. This provides a smooth surface for the slider to ride on, and protects the threads. These devices are usually used on the left front in oval racing.

 

The rate increase occurs in a single step. It would be possible to create two smaller steps by using two sliders and three springs, but I have never seen this.

 

Elsewhere in oval racing, one still often encounters the erroneous belief that the way to tighten the car (add understeer/reduce oversteer) on exit is to use a right-stiff front spring split. This belief stems from misunderstanding of the relationship between spring rate and spring force, in a situation that causes an extension displacement of the spring.

 

To understand this sometimes confusing concept, imagine a car on the drag strip, with the suspension on the left front wheel locked solid (spring rate effectively nearly infinitely large). When the car launches, the locked suspension will be unable to extend, and it will be quite easy to lift that wheel off the ground, while the right wheel, whose suspension can extend, will stay on the ground. All load remaining on the front wheels will be on the right front, and the car will have more than 50% of its weight on the right front and left rear.

 

The more we increase the spring rate, the more closely the suspension approximates a locked condition. The greater the spring rate, the more the load increases with compression, and the more the load decreases with extension. Consequently, a stiffer inside front spring increases load on the outside front and inside rear tires, and tightens the car (adds understeer) on corner exit.

[260DET]

Don't know about zero droop for production type cars but near zero droop seems to be worth a try. The (my) theory is that with a production car you will always get some roll, some would say the suspension should be soft enough to allow some roll, unlike an open wheel race car. So with a production type car with zero droop even on a dead smooth circuit one front wheel would be off the ground a lot.

 

On the heavy front springs comment, the 280ZX project car will have 400 pound front, 250 rear, to start with anyway. With 4-500whp and a helical LSD

I want to wedge weight onto the inside rear wheel in a corner. NB those spring rates are around the same as rates at the wheel.

 

Does this all make sense or am I talking ♥♥♥♥

[JMortensen]

Don't know about zero droop for production type cars but near zero droop seems to be worth a try. The (my) theory is that with a production car you will always get some roll, some would say the suspension should be soft enough to allow some roll, unlike an open wheel race car. So with a production type car with zero droop even on a dead smooth circuit one front wheel would be off the ground a lot.

I agree with Richard here that "near zero" droop might not be such a bad thing, and I think that is what Cary has always talked about. If you look at any "real" racecar, they just don't have 5 or 6 inches of droop like a lowered Z does. Why you need all that droop is beyond me, especially with really stiff springs, and I think that Cary is right that all it really does is let the car roll further in turns than it would otherwise. He can speak for himself, but that's the way I've always read his posts. I suspect that having a bunch of aero downforce like the open wheeler might make zero droop a more attractive prospect because you'd have some functional droop at speed that way.

[tube80z]

Don't know about zero droop for production type cars but near zero droop seems to be worth a try. The (my) theory is that with a production car you will always get some roll, some would say the suspension should be soft enough to allow some roll, unlike an open wheel race car. So with a production type car with zero droop even on a dead smooth circuit one front wheel would be off the ground a lot.

 

My practical experience would say your theory may need a little correction. I can run true zero droop or limited droop and I don't pick up tires. The key is running spring rates that actually support the forces the car deals with, which are generally stiffer than most people think are needed (close to 1:1 corner weight/wheel rate). I ran suspension logging a couple of times to see what was going on and I could see that once the limiters topped out (very easy to see as the suspension displacement is now a straight line) the car would then roll by compressing the outer suspension. The net effect of this was a reduction in ride height. Without limiters the ride height would actually increase.

 

I usually have .5 to .75 inches of front droop, although I've used zero at times but only when the course is extremely smooth. In the rear I usually have around 1 to 1.25 inches of droop. I've been able to even out rear tire temps and tune at mid-corner push with adjustments to rear droop travel.

 

Here's some info from Richard Pare if ICP that I found useful. These guys gave me the push needed to try this out.

 

To reiterate the meanings of all of these terms:

 

Zero Droop : Suspension is set up so that the travel in rebound is topped out with the car just sitting there ( and possibly, but not necessarily, still topped out with some amount of aero load). Rebound stops can be internal or external to the shock. If external, preloading the spring may or may not be necessary. If limited internally, preloading the spring to at least the force it sees at rest is required. In either case, the point at which the suspension moves off of the limiter will depend on how hard the spring is pushing against the limiting devise - ie - the "preload" against the limiter.

 

Droop Limited: Suspension is set up so that the shock tops out in rebound before all of the weight is taken off of the spring. Can be done with either internal or external limit devices. If done internally to the shock, it will require that the spring be Preloaded. If done externally, preloading the spring may not be required. The car will actually compress the springs slightly when set on the ground and have a "limited" amount of droop available.

Preload: With the shock fully extended, the spring is still compressed. The measurement is in pounds, but usually expressed in "turns of the perch", which varies with the shock thread pitch.

 

Zero drooping will give an instantaneous response at initial turn in. What happens after weight starts transferring depends on how much preload is used. For a simple math exercise, we'll assume that the MR is 1:1, and therefore the force ratio is 1:1.

If the corner weight is 300#, and the spring is preloaded to 300#, any increase in vertical load on the suspension will result in shock and spring compression. If a car has downforce, this will mean that for most situations on the track, the suspension is already compressed off of the limiter somewhat at turn in, and the car will behave as if it had regular rebound capabilities UNTIL enough weight is transferred off of the inside tire to top out the shock (or hit the limiter). At that point, the actual roll center will start to move towards the inside tire. How much it will move will depend on how much more weight is removed, and what the tire spring rate is.

 

If that same spring was preloaded to 600#, the outside suspension will not compress until at least 300# of weight is transferred ( ie - 300# of combined source load - weight transfer plus aero load - is added to that corner). The car will handle like a go kart up to that point.

Droop limiting is a means of swiftly changing the actual force-based roll center towards the inside tire at a pre-determined amount of lateral loading. The roll center shift occurs when the inside shock tops out in rebound, sometime before full lateral g loading occurs.

The reason for the use of any of these is to combat a push that cannot otherwise be cured with the springs that are being used. The need for it usually goes hand-in-hand with either the car being sprung too softly, and/or above ground geometric roll centers.

 

Above-ground geometric roll centers produce a high degree of Jacking Effect - simply put, a self-stiffening of the suspension from the lateral loads that are fed thru the a-arms. This self-stiffening can result is an overloading of the outside front tire, and therefore a bad push.

 

By shifting the roll center towards the inside tire at some point in lateral loading (usually before the push becomes noticable to the driver), with any further increase in lateral loading ( and the accompanying weight transfer) the car is forced to roll about a point near the center of the inside tire contact patch (it will be AT the center only when all weight is taken off of that tire). For the outside suspension to compress as it accepts that weight, the chassis actually is lowered, which in turn drops the height of the geometric roll center, and in turn decreases the rate at which the jacking effect increases - ie - the self-stiffening effect doesn't get as bad as it would have.

 

 

We have used droop limiting on the rear of some cars - most notably the old Indy Lights cars - to help with a slight mid-corner push at some tracks. On those cars, the preload was set to about 100 pounds, and the topping of the inside shock could be seen on the data at 5 or more points in the Keyhole at Mid Ohio as just a small "flat" in the shock positioning graph.

[260DET]

Well Cary you've been there, practical experience > theory. Have you had any problems with front brake lockups, particularly when trail braking?

[tube80z]

Normally the suspension will be compressed under breaking (or trail braking) so this shouldn't be an issue. I guess you could have a problem if the track dropped away from you but you'd have the same issue with a front anti-roll bar holding the wheel up. So to answer your question I didn't have anymore lockup than usual when pushing it very hard.

 

Generally the cars I see picking up wheels all the time are the guys running a lot less spring. That would be one concern I'd have about your proposed setup. The soft rear may allow it to compress enough to lift your front tire off the ground. I had a car that was setup that way a long time ago and that's what it did.

 

Cary

[260DET]

Yeh, the trend where I am is towards softer springs to give better grip, particularly at the rear with a lot of power. Maybe heavier springs at the rear than 250's and no ARB?