Car Pitch Quiz

[johnc]

Let's say the front of your car lifts under acceleration (pitches up) and you want to reduce that behavior because its unloading your splitter and increasing understeer as you exit the corner. Would increasing the front spring rate reduce the pitch, and if so, why?

[Gavin]

Is your car really lifting at speeds where your splitter would be making downforce?

Impressive.

 

Or are you just talking about the weight shifting?

[johnc]

Its a hypothetical quiz.

[bjhines]

Increasing front spring rate will reduce pitch... This is not immediately intuitive...

 

There is more "windup" or length of compression in a weaker spring...

In effect.. the wheel will still have some spring tension on it even at relatively long extension...

 

Where the stiff spring will move only a small amount before it begins to rattle loosly on it's perches...

 

When you jack up a race sprung car this becomes instantly obvious... you only get a few pumps of the handle before the weight is completely off the wheels...

 

Under acceleration the front end lift gets very little "help" lifting because there is so little windup in the stiff springs..If it moves a little more than an inch then the spring is no longer helping at all... where the stock springs have several inches of windup.. the front end gets a "lift" from the springs all the way until the struts fully extend... even further because you actually have to compress the springs to remove them at FULL EXTENSION...

[katman]

I wouldn't want to upset the overall balance of the car with a spring change, but I might be willing to risk a transitional behavior with a shock adjustment. I say increase either rear bump or front rebound. Man, I wish I had enough power for this hypothetical to be a problem

[JMortensen]

If you have 50 in/lb springs in front for a total front spring rate of 100 in/lbs, and a weight of 500 lbs on the front axle, the front end would sag 5". Now if you had one hundred percent weight transfer to the rear, then the front end would come up 5" on acceleration. If you changed the spring rate so that the front was 500 in/lbs combined, then the sag in front would total 1". With the same 100% weight transfer the front end would climb 1". So yes increasing the spring rate would keep the nose down.

[johnc]

A shock will reduce the rate of pitch change, but ultimately the weight is going to the back and the front is going to climb. If you're lucky you will be on the straight by then so its not an issue.

 

BJ is right.

 

Think of it this way...

 

For sake of discussion, let's say that because of wheelbase and CG height the car will transfer 500 lbs. of load to the rear under .8G of acceleration regardless of what springs are on the front and back. That 500 lbs. comes off the front wheels.

 

EXAMPLE 1 (Front 250 lb. in. rate springs with a 1:1 motion ratio in the suspension)

Under load the front of the car will extend (pitch up) 1" under acceleration.

 

EXAMPLE 2 (Front 350 lb. in. rate springs with a 1:1 motion ratio in the suspension)

Under load the front of the car will extend (pitch up) .71" under acceleration.

 

The front will pitch up the numbers listed above even if you install steel rods in the rear suspension replacing the rear springs. That 500 lbs. of load is moving to the rear and coming off the front of the car.

 

From Mark Ortiz's latest chassis engineering newsletter:

 

 

ZERO-DROOP SETUPS

 

The next question I have is about suspension droop travel limit fwd and aft. I have seen that modern single seaters have no droop at all. Now I am not sure about a historic racecar like my March F2 (year 1971).

 

The shock stroke (fwd and aft) during hard driving is about 20mm. I know aft shocks should have more travel than fwd shocks but how much?

 

Corner exit on acceleration the car is pitching fwd up and with no droop can help to get more weight on the car fwd because the unsprung weight will help to hold the nose down. Until now I made the shock adjustment with more rebound but I think that's the wrong way to fix the problem.

 

The only reason it makes any sense to not let the suspension move freely in droop is to control ground-clearance-sensitive aerodynamic elements. From every other perspective, making the suspension top out prematurely is a bad thing.

 

For best mechanical grip, we want the suspension to extend freely until the springs reach zero load, and then stop. If the suspension extends further, so that the springs hang loose, that doesn't hurt grip but it can cause the springs to beat up the shocks, or the spring retainers or adjusting collars, or other pieces, if it happens very often.

 

If the suspension tops out before the springs unload, that abruptly unloads the tires, but it also keeps the ground clearance from growing, at least as long as we don't pull the wheels off the ground. That's bad for mechanical grip, but it's good for aerodynamics if we've got a floor, a valance, a splitter, or a wing that has to be near the ground to work well.

 

From what I can tell by pictures on-line, the March 712 has a fairly broad chisel-shaped nose, with two small nose wings on the sides of the chisel. The radiator is in the nose, fed by an intake below the leading edge of the chisel, and exhausting through an outlet on the top of the chisel. The little wings are up fairly high compared to later cars. It doesn't look like the car would be highly sensitive to ground clearance at the front. The car pre-dates tunnels and diffusers; the floor isn't designed to make downforce. It has a rear wing, but this would not be highly sensitive to ride height either. So I don't think the car would realize the same advantages as a more modern car from zero-droop suspension.

 

There is no hard and fast rule for the relationship between front and rear shock travel, as shown by travel indicators on the shock shafts. In tail-heavy, rear-engined cars, it is common for the front suspension to have a higher natural frequency and smaller static deflection than the rear. That is, the front suspension is stiffer in ride than the rear, relative to its sprung mass. This normally results in more suspension travel at the rear than at the front. Also, it is normal to have more aerodynamic downforce at the rear than at the front. This will result in the shock travel indicators showing more travel at the rear than at the front, if the motion ratios are similar. Looking at photos, it appears that the motion ratios at the front and rear of the March 712 are fairly similar. So if the travels are similar, it may be that the rear springs are a bit stiff relative to the front – but not necessarily.

 

Keeping the front end from lifting under power will only add a little bit of load to the front tires – and concomitantly reduce rear tire loading a little. The amount of rearward load transfer, for a given forward acceleration, depends entirely on the height of the center of gravity and the wheelbase. How much the nose lifts wouldn't matter at all, except that more nose lift does result in a slightly higher c.g. If the front of the car seems to lift excessively under power, stiffer front springs will reduce that. You may want to combine the stiffer springs with more rear anti-roll bar or less front anti-roll bar.

 

Some readers may find it surprising that the front end lifts less with stiffer springs, but that is indeed the case. A stiffer spring doesn't mean more upward force. It means less travel for a given load change – hence less extension travel for a given load decrease.

Ordinarily, we don't want to keep load on the front tires under power; we want load to transfer to the rear so we can put power to the ground. This is true unless the car understeers excessively on corner exit. This is not uncommon in rear-engined cars, but a more common complaint in racing generally is that the car spins the wheels and/or oversteers on exit. A car that is too tight (understeers too much) power-on is a problem that racers in many classes would kill to have. If the back tires stick too well, just feed them more power, goes the reasoning.

 

This doesn't always work, however. There may be no more power available; the driver may have the throttle wide open already. This is often encountered in Formula Fords. Or the understeer may simply increase until power breaks the rear tires loose completely, whereupon the rear end suddenly snaps out.

 

The March appears to have very short control arms, especially the uppers. That means that either the camber recovery goes away markedly as the suspension extends, or the roll center rises as the suspension extends, or both. Having that happen at the front would worsen a power push, and stiffer front springs would reduce the lift. Restricting droop travel would keep the nose down too, but the effect would be abrupt, and the roll resistance would abruptly increase, worsening the push. More spring and less bar keeps the front end down more, without that problem.

[tube80z]

The following is from Richard Pare and I think addresses droop limiting and zero droop suspension better than what Ortiz wrote. And as many know I've played with this a lot on my Z and found it to work for me, YMMV

 

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.

 

Cary

[nat0_240_chevZ]

yes, droop limiting and 'anti-droop' device of sorts is the best way to combat the pitching'

 

limited rebound (movement) in the shock 'sitting closer to the fully unloaded pos'n of the shcoks travel' I have seen done with chains and other sorts.

 

You can still improve the lift more by droop limiting moreso than with a stiffer spring rate, (could still run 250lb/" with buger all lift), Also reducing the open stroke length of the shock, its huge, and its also running a couple of inces pre-load on the perches, hence when u reduce the weight by accelerting (loading the rears ie tipping) it wants to lift

[260DET]

Interesting stuff, thanks to all the contributors.