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# Misunderstood aspect

### Tech Articles

Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals

In the Spring Rates module around 1:45, Andre said "A subtle aspect here that's often misunderstood is that assuming we have a linear spring rate, if we add another 250 pounds to the mass already on the spring, it will be compressed by another inch or two inches in total."

What is the misunderstood aspect?

It would help if you gave a link to the module in question, but my guess is it's referring to pre-loading the spring or moving the seat potition and hence ride height? Spring rate is fixed, regardless of the way the spring is installed.

Hi Hairul,

What Andre is getting at here is that for a linear spring, the rate does not change regardless of how much force is applied to it. That applies to whether that extra force comes from having more mass applied on top of it, or if preload is added to it.

Cheers,

Tim

I'm sorry for a dumb question, but what is preload?

There's no dumb questions here Hairul!

Spring preload is a term we use to describe how compressed the spring is with the coilover is in its fully extended position. Let's say you jack your car up so it's sitting on axle stands and the suspension is hanging down and fully extended. If the spring perch (what we use for ride height adjustment) is set such that the spring is almost loose, then there is no preload on the spring. If we wind the spring perch up and start compressing the spring (with the suspension still fully extended) then this adds preload to the spring, I've added a picture below to help explain it.

I actually do an explanation of this in our live Wheel Alignment Fundamentals Webinar. If you come along to the one tomorrow you can watch along and ask some more questions if you like. Here's a link to sign up for tomorrow's one.

I like this Tim guy, he explains things far simpler than I ever could. I tend to like the motorcycle example for preload. The bikes unsprung weight, with just me is ~500 lbs, if I bring my roomate along for the ride, it's ~700 lbs. So in this case I'd preload the suspension more to account for his additional weight, and keep the suspension as similar as possible for the increased load. With cars there's alot less dramatic weight change for extra people, and less sensitive anti-squat geometries to keep in line so the effect is often lost, or misused.

Thought I'd commented a few days ago - couldn't have taken?

As Tim said it's the initial compression of the spring when fitted, but not actually loaded. It can be expressed as a distance such as 1" or 25mm, or as a force such as 25lbs force or 100N, or as a load such as 10kg or 25lbs.

The confusion seems to be that the pre-load is achieved by moving one, or both, spring platforms. To use the motorcycle example above, the bottom spring platform is raised at the bottom end and this changes the 'start point' of the spring assembly, which lifts the top of the spring the same amount under load and hence the rear of the motorcyle. So it follows that when a pillion sits on the 'cycle, while the suspension will move the same amount the 'cycle will be higher because it started higher. Back in the day, some people swore by increasing the pre-load to impove handling, whereas it would most likely be due to a slight change in steering rake.

Something else to bear in mind is that the amount of pre-load at full extension is going to affect the amount of force required to start compressing the spring assembly. For example, if you had one at 100N and one at 200N preload, the first wouldn't move until 100N of force was applied, and the second 200N of force. This is important as it also limits the droop, or extension from the normal spring position, available and it acts as a 'check strap' - this affects both roll and bump absorption properties AND can put a lot of stress on the damper assembly as it tops out.

Man...... I’m so confused as well. Am I confusing pre-load with ride height adjustment? Setting preload via the spring perches at full extension of the Coilovers is a completely different thing from raising or lowering the ride height vie spring perches right? I mean now matter how much you increase the preload with the spring perches, that shouldn’t affect the static ride height right?

It can be VERY confusing at first, as there are a lot of different things to consider - but you'll get the hang of it.

Yes, with conventional 'coil-over' designs changing the spring platform postion will change the ride height - it's the standard way of doing it as it's changing the position the spring is applying the force on the vehicle from. because this can mean a lot of preload, and reduced droop (spring extension from the normally loaded position) another method is using different length springs with the same spring rate - the rate isn't related to the actual length, just how much force is required to compress (or extend) the spring a nominal amount.

I said 'conventional' above, because there is a design of coilover that uses two threaded adjustments, one more for spring pre-load and one more for setting the ride height and corner weights.

Brian,

I understand it can be a little confusing to start with, if you have a car or motorbike you can play around with, the best way is to experiment for yourself and it will become pretty clear.

Changing the preload on the spring will change the ride height, this is the correct way to do it in fact. The stiffness (rate) of the spring (assuming a linear spring) does not change with preload. However, if you increase the preload of the spring, this will increase the ride height.

Again, I encourage you to see this effect for yourself if you have a way to test it!

Hope that helps,

Tim