Strut tower cut-off for camber adjustment

[kep]

If I'm wrong please point out in exactly what way I'm incorrect. I'm just here to gain and share knowledge, not win arguments.

You've just summarized what I displayed in my chart: A preloaded spring will not move until a force greater than the preload is applied to it. My entire argument is that on a car driven on a road course, your springs should be compressed passed 0" of travel at all times. The sole exception that I can think of is when a GTI/R lifts a rear wheel in a tight autocross corner. Mountain bike wheels are frequently off of the ground, so the same principal does not apply.

 

[victorofhavoc]

If I'm wrong please point out in exactly what way I'm incorrect. I'm just here to gain and share knowledge, not win arguments.

You've just summarized what I displayed in my chart: A preloaded spring will not move until a force greater than the preload is applied to it. My entire argument is that on a car driven on a road course, your springs should be compressed passed 0" of travel at all times. The sole exception that I can think of is when a GTI/R lifts a rear wheel in a tight autocross corner. Mountain bike wheels are frequently off of the ground, so the same principal does not apply.

The whole point of the discussion was why you want separate preload and height adjustment, right? If you agree preload changes the initial force needed to move the spring then I'm missing the point of how you're determining there's no benefit to having separate preload and height?

Is the benefit massive? No, but it does exist.
You will certainly reach full extension of the rear suspension in track settings btw. I've got a photo of me lifting the inside rear wheel on pss10 coils, and people have told me they've seen my car do it around various corners at my local track, even on stock suspension. Usually 60mph range corners. I've also dropped off curbs fast enough that when I had the mss springs that would lose preload you could hear them clunk as they left the upper seat and then came back into contact.

 

[kep]

Correct, preload changes the amount of force needed to move beyond 0" of shock travel. This will only occur on a wheel that has a force less than the preload force on it. The most probable instances in which that could happen are high roll corners or going off a steep decline like a curb.

In your curb example with MSS springs, I see no issue from a performance perspective. If they clunk when unweighted, that means they were installed with little/no preload and the resolution would be to add preload. Even if we did this and adjusted the shock body to keep a constant ride height, the performance would be the same. We would have exactly the same amount of bump travel and the droop travel would technically be reduced. However, on a spring with zero preload that last bit of droop travel would be unusable, so the effective droop travel would not change. As long as the loose springs do not bounce out of place or cause damage, I don't see how they would cause a performance issue. In my opinion the best solution for this problem would to install helper springs which would eliminate loose springs by regaining that last bit of droop travel.

With lifting a wheel in a corner the same principal would apply. I can imagine a spring with extreme preload could be slightly unpredictable since the shock would refuse to move unless a force greater than the preload was applied. The thing is, both of these cases only apply at the limit of droop travel. My belief is that a fixed length shock should be capable of a fitting a longer stroke than an equivalent adjustable body shock, therefore preventing the unweighted wheel scenario in the first place.

 

[Raguvian]

Does preload really matter if the corner weight of the car is higher than the preload? If you've got 1" of preload on a 400lb/in spring but your car is applying 800 lb over that spring, I'm not understanding how the preload would affect ride quality or handling.

 

[kep]

Does preload really matter if the corner weight of the car is higher than the preload? If you've got 1" of preload on a 400lb/in spring but your car is applying 800 lb over that spring, I'm not understanding how the preload would affect ride quality or handling.

No, the only time preload comes into play is when a shock is at full extension. The keyword is preload - once you've overcome the amount of force required to compress the spring, preload is no longer in effect. For example, imagine a strut an assembled strut off of a car. The preload is the force between the lower spring perch and the strut bearing, which is held together but the strut top nut. Now if I am able to push down on the assembly and compress the spring, I can safely remove the top nut, since all of the force is being supplied by me, not the nut which was responsible for the preload force.

 

[Raguvian]

No, the only time preload comes into play is when a shock is at full extension. The keyword is preload - once you've overcome the amount of force required to compress the spring, preload is no longer in effect. For example, imagine a strut an assembled strut off of a car. The preload is the force between the lower spring perch and the strut bearing, which is held together but the strut top nut. Now if I am able to push down on the assembly and compress the spring, I can safely remove the top nut, since all of the force is being supplied by me, not the nut which was responsible for the preload force.

Right, I understand what preload is, but what I've always had trouble understanding is how it affects handling with a linear spring. I get that the shock will take more force to start moving to overcome the preload, but if the corner weight of the car is higher than the amount of preload I'm not sure I get how handling will be affected. When going around a corner, the inside wheel(s) may have less weight than the amount of preload on the springs, but the cornering load isn't on those wheels either.

 

[kep]

As long as the dynamic weight on a wheel is greater than the preload force, there will be no performance difference between different preload values. If the weight is less than the preload, for example the inside wheel on a corner, then that wheel will be at full droop travel, and differences can be observed.

Forces	State	Handling effect of preload
Wheel weight > preload	Spring is under compression, wheel is on the ground	none
Wheel weight < preload < 0	Shock is at maximum extension, but wheel is still on the ground	The shock is effectively rigid
Wheel weight = 0	Wheel is off the ground	none

 

[victorofhavoc]

Right, I understand what preload is, but what I've always had trouble understanding is how it affects handling with a linear spring. I get that the shock will take more force to start moving to overcome the preload, but if the corner weight of the car is higher than the amount of preload I'm not sure I get how handling will be affected. When going around a corner, the inside wheel(s) may have less weight than the amount of preload on the springs, but the cornering load isn't on those wheels either.

Correct, it's the transition point when the car swings from loading one side to loading up the other. That's when you feel the handling difference because the car goes from full droop to loaded in a very short period, and varying preload per side can affect how the car settles during that transition.

 

[victorofhavoc]

Correct, preload changes the amount of force needed to move beyond 0" of shock travel. This will only occur on a wheel that has a force less than the preload force on it. The most probable instances in which that could happen are high roll corners or going off a steep decline like a curb.

In your curb example with MSS springs, I see no issue from a performance perspective. If they clunk when unweighted, that means they were installed with little/no preload and the resolution would be to add preload. Even if we did this and adjusted the shock body to keep a constant ride height, the performance would be the same. We would have exactly the same amount of bump travel and the droop travel would technically be reduced. However, on a spring with zero preload that last bit of droop travel would be unusable, so the effective droop travel would not change. As long as the loose springs do not bounce out of place or cause damage, I don't see how they would cause a performance issue. In my opinion the best solution for this problem would to install helper springs which would eliminate loose springs by regaining that last bit of droop travel.

With lifting a wheel in a corner the same principal would apply. I can imagine a spring with extreme preload could be slightly unpredictable since the shock would refuse to move unless a force greater than the preload was applied. The thing is, both of these cases only apply at the limit of droop travel. My belief is that a fixed length shock should be capable of a fitting a longer stroke than an equivalent adjustable body shock, therefore preventing the unweighted wheel scenario in the first place.

Agree on the first two parts, but the issue with mss springs was that droop slop was only eliminated by going to slightly above stock ride height and there was zero room for a proper helper spring... But that was just their crap product.

I think you're forgetting s transitions. When you have varying preload and varying shock compression travel you're going to have the car settle "funny". This does make for some unpredictable behavior at the limit when transitioning directions. You'll also have some compression variance under hard braking that will result in one side hitting bump stop before the other.

Again, I'm not saying this makes for a totally undrivable car or that you should stay away from these cheaper designs (I run the pss10 myself, currently), just that the added benefit of keeping shock range and height adjustment separate is a benefit worth paying for.

 

[Oversteermybagel]

Victor. Adjustable body coil overs allow us to adjust the ratio of bump to drop at the loss of some total travel-- spring perch adjustable systems do not. Which begs the question. Is the ratio of bump to droop important and what ratio are we trying to achieve? If it is not important then adjustable shock bodies have no benefits but if it is important then adjustable spring perch systems only have one 'optimal' height. I suspect this ratio is important but I don't know how to to best set it (not important to me because my car is sitting on perch adjustable MT21 as of currently, but generally speaking).

I have read numbers anywhere from 2:1 bump-droop to 1:1 bump-droop as a target. However, maybe we are actually targeting some sort of number (>3inch bump & >3 inch droop for example). How do we go about targeting how much shock travel do we need? Do we target as much bump travel as we can get, pick springs to be appropriate for the cornering loads we expect to see, and then set droop travel such that un-laden wheels never fully leave the ground? Or do we target some specific value for bump travel and droop travel based on the geometry of the circuit we run?

 

[xXDavidCXx]

"preload changes the amount of force needed to start moving the shock"

I experienced this phenomenon recently when I raised my car via adding preload, that end of the car became stiffer and less comfortable as each bump now had to generate the pre-load force before compressing the spring.

 

[victorofhavoc]

Victor. Adjustable body coil overs allow us to adjust the ratio of bump to drop at the loss of some total travel-- spring perch adjustable systems do not. Which begs the question. Is the ratio of bump to droop important and what ratio are we trying to achieve? If it is not important then adjustable shock bodies have no benefits but if it is important then adjustable spring perch systems only have one 'optimal' height. I suspect this ratio is important but I don't know how to to best set it (not important to me because my car is sitting on perch adjustable MT21 as of currently, but generally speaking).

I have read numbers anywhere from 2:1 bump-droop to 1:1 bump-droop as a target. However, maybe we are actually targeting some sort of number (>3inch bump & >3 inch droop for example). How do we go about targeting how much shock travel do we need? Do we target as much bump travel as we can get, pick springs to be appropriate for the cornering loads we expect to see, and then set droop travel such that un-laden wheels never fully leave the ground? Or do we target some specific value for bump travel and droop travel based on the geometry of the circuit we run?

I love it! Now you're really getting into the weeds of designing suspensions. The simple answer is that it all depends. It depends on where the suspension sits at static, how much roll you generate, the amount of bar you're using, your tire, brake, total wheel movement on each track, etc.

The type of driving, competition limitations, etc will all add complexity. There's a great speed secrets article that goes over mcstrut front end cars specifically and one of the larger points is limiting camber gain in the front. Usually this is done through increasing stiffness of springs or adding more bar. More bar allows you to keep more motion in the shock while also limiting the independent nature of the suspension. More spring allows more independent movement of the wheels but also decreases total traction under braking. Without digging too deep into other areas, the fact of limiting suspension roll angle has to be a balance between spring and bar in most setups. Yes, some can go extreme and go barless but there are other things in play there... In most of our cases you'd be targeting a 2 to 3 degree roll as optimal and then work out the range of shock motion needed.

Long and technical, but great read: https://nasaspeed.news/tech/suspens...ck-performance-for-strut-suspension-cars/?amp

 

[xXDavidCXx]

More practically, I created a thread where I discuss roll and camber loss and another thread comparing springs and bars. Both back up what is being said about the limitations of our McPherson suspension.

links in my sig

 

[victorofhavoc]

More practically, I created a thread where I discuss roll and camber loss and another thread comparing springs and bars. Both back up what is being said about the limitations of our McPherson suspension.

links in my sig

You did a great job compiling info, and thank you for doing so!

 

[kep]

Correct, it's the transition point when the car swings from loading one side to loading up the other. That's when you feel the handling difference because the car goes from full droop to loaded in a very short period, and varying preload per side can affect how the car settles during that transition.

Unless your weight transfer involves completely unloading the wheels on one side of your car, this has nothing to do with preload. The force of a linear spring is simply F = -kx, where k is the spring rate and x is the displacement. The spring force does not depend on preload, or velocity, or how what part of the stroke you are in (until you coil bind).

How long it takes for a car to settle after an impact depends on wheel rate (which depends on spring rate) and damping force. If you believe that any of this is affected by the preload of a spring, please show me mathematically how that works.