Preloading, how to tune it?

[Muzz]

I understand that it maybe best to run 0 preload, so that there is MAXIMUM suspension travel. But what about in the old days where the only way to lower a car on coilovers was to preload the spring?

Lowering requires you to lower the spring perches, preloading involves raising the perches to the point where the spring starts compressing, as the shock cant extend further.

The thing that I think is important to understand, and hasn’t yet been discussed is this.
With these common coilovers that you raise and lower ride height by the spring perches, the ones that don’t offer separate independent height adjustment, the preload is basically almost always at zero, here’s why.

When you install these coilovers in your car, it is very likely you will be lowering your car an inch or more. I believe and have heard that when the coilovers adjustable spring perch is set between its recommended ride heights, that the spring is basically loose in the perches. The only time the spring is preloaded is when you raise the car above what the manufacturer recommends.

If you set these coilovers to have zero preload, just enough to hold the spring firmly, and then install them on the car, the car will usually be raised UP an inch or so above stock ride height. Almost always the perches are lowered from there, to lower the car to the desired ride height. Meaning that there is no preload on the spring.

Once again, the only benefit of using Coilovers that allow separate height and preload adjustment is the ability to alter the amount of bump and droop travel separately from the height.

So basically if you are using coilovers that adjust height using the spring perches, and your not reaching your bump or droop maximum travel limits, there is absolutely no benefit at all to running the same coilovers which instead offer both height and preload adjustment.

I don’t believe any of these types of coilovers would ever be reaching there droop limit on the street (maby on the rear wheels of a fwd track car, but as ive mentioned, its not a bad thing) there is not enough weight transfer to completely remove all weight from one corner and transfer it to the opposite corner on the same end, that would take more than 1.5g’s of cornering force.

The only concern is running out of bump travel (bottoming out), however, if the coilover is designed well, with correct spring rates, and the ride height is sensible, there should be no worry of this at all, even under really hard breaking.

From calculations, say the chosen spring rate will mean that the spring might compress a very maximum of 2” from extreme potholes etc. And droop a maximum of a inch, the coilover would then be designed with say 5” of total available shock travel, so that the car should never reach the maximum bump or droop travel (unless the user decides to drop the car on its guts). The coilover is then designed so that at the optimum ride height say a 2” drop, the shock sits right in the middle of its stroke.
What im trying to say, is that if you install any decently designed coilover, that adjusts height by the spring perches, and you keep your car at a decent ride height, you should never be reaching those bump and droop travel limits.

I understand that it maybe best to run 0 preload, so that there is MAXIMUM suspension travel.

Its not so much that that you get maximum travel, the travel always stays the same. What is altered, is where the shock lies in its stroke, when the car is at its static ride height.

Do you know what makes all this really hard???

The fact that our cars aren't perfectly balanced. *bangs head on wall*

How do they go about adjusting corner weights? Do they adjust the preload or the height at the base of the coilover?

If the coilover allows, adjust the height, at the base of the coilover.
In saying that, it’s perfectly fine to do it using the spring perches if that’s your only option. It won’t negatively effect your handling in the slightest, the only way it would, is if the shocks were now reaching there bump or droop travel limits after you adjusted the corner weights via the spring perches.

Preload will give more bump travel but less droop?
And vice versa?

Correct

But wouldn't preloading, shorten the overall length of the coilover?

No, the shock can only extend so far, to preload the spring involves you to tighten the spring perch upwards, compressing the spring. Since the shock cannot extend further, the overall length of the coilover will remain the same when adding preload from zero. This is done with the coilover out of the car.

Ok, I understand that regardless of how much preload is used, the spring rate will be the same. Both Linear and Progressive springs?

No, progressive springs make things complicated. All of what I’ve been talking about so far is regarding linear springs. Adding preload to progressive springs will effect where the car sits along it progressive spring rate curve, at static ride height. Meaning, that adding preload to progressive springs will progressively make the ride stiffer.

Say an un-preloaded progressive spring starts at a rate of 5kg/mm which increases as the spring compresses, the spring may start at 8kg/m along the springs same progressive rate curve, if the spring is preloaded.

Can't we use preload to tune how the car initially shifts its' weight?

Not really, see below.

Using my example from before, the springs won't compress till 250kg+ is put on each corner. So if we knew how much force was thrown onto the fronts when 50% brakes are applied @ 100km/hr. Then couldn't we tune it to minimise nose diving/lift/body roll? By doing this, we can balance the cars' weight out or add bais front/rear, therefore giving us maximum tyre grip?

Good question, unfortunately the answer is no, not to any worthwhile extent.

The formula for calculating lateral (cornering) weight transfer is this:

Lateral load transfer (lbs)=
(Lateral acceleration (g’s) x weight (lbs) x CG height (inches)) / track width (inches)

When using the formula, say your calculating the weight transfer between the rear pair of wheels, you use the weight over both wheels.

Longitudinal load transfer (lbs)=
(Longitudinal acceleration (g’s) x weight (lbs) x CG height (inches)) / wheelbase length (inches)

For weight in this formula, you use the total weight of the car.

As you can see, the suspension wont change this, it is not a part of the equation. However in saying that, the suspension controls body roll etc, and this effects how far the CG moves therefore very slightly effecting weight distribution.

It is better to control body roll with stiffer springs or swaybars, than to do it by limiting how far the suspension can compress or extend.

As for diving and lift, controlling that is what anti dive and anti lift geometry is for, it effectively controls the stiffness of the suspension under brakes/acceleration without resorting to mechanically limiting suspension travel, that isn’t done as per normal, smoothly by the spring.

I greatly recomend these books to anyone keen improve their knowledge on suspension:
Tune to win, the art and science of race car development and tuning. By carrol Smith
Race car engineering and mechanics, by Paul Van Valkenburg

The top one is more in depth and is a supurb read, the next one covers lots more areas, so less of the technical suspension stuff, it has a good section on fwd racing though.