That’s because adjustable spring perches are much easier to design if they’re smaller diameter. But @Mustangman ‘s answer goes into more detail about the other reasons.
Did you miss this?
And this:
And this;
Hope that clears it up
If smaller dia springs then improve ride handling and/or ride quality, then why don’t manufacturers build them that way? No one needs adjustable ride height so they don’t have to include adjustable spring seats.
I read it the opposite way, larger diameter better.
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MacPherson struts have large diameter springs, offset to align with the force/load from the wheel/tire. See red arrow below.
With coil-spring-over-shock absorber suspensions, the force/travel is delivered from the lower control arm.
OEM coil over shocks don’t have adjustable spring perches, the adjustment is not a factor in the different suspension types.
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Found my missing 24mm socket. Which I need for knuckle.
factory springs are bulky because they are made for maximum ride comfort and to handle heavy loads over years of driving without sagging
aftermarket coilovers use narrow stiffer springs mostly to save space so you can fit wider wheels and get better suspension geometry
car manufacturers just want a smooth quiet daily driver while aftermarket brands focus on tight handling and maximum tire clearance
Last pic shows strut attached to lower control arm but mounting point is not inline with axle centerline. Does that affect suspension geometry? How close does inside edge of tire come from coil spring in full lock situation?
The last picture shows a coil-over shock, NOT a strut
There’s sufficient clearance
No worries
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Since the forum is slow and this spring design Q&A seems to have run it’s course, here is a question about suspension travel vs tire size that even a lot of off roaders get wrong or don’t understand… I know Mustangman will get it, I am just curious what others think…
I responded about this debate on another forum in a couple of post…
This is for coil spring front suspensions only;
Using a 4x4 2nd and 3rd gen Tacoma truck as an example, on a stock suspension with the largest tire possible on stock wheels and no wheel spacers, and OEM bump stops with no tire rub anywhere while off roading and or potholes and speed bumps…
Does a lift kit allow for bigger tires without rubbing/interference??
You can even add, does jacking up the front wheel under the lower control (closest to the ball joint) arm alone compress the suspension enough to test for tire rub/interference??
I know a big lift kit allows for less interference between a fragile ego and reality. 
Isn’t one of the points of lifting a vehicle to allow for greater clearance between the bumpers and the ground, to allow for increased approach/departure angles?
Think what happens when you are articulating the suspension, example going over very large rocks, or like dips while turning off a main road on to a service road, anything that compresses the suspension while also being able to turn the wheel without the tire rubbing… Maybe think about what happens while going over a washboard dirt road while driving fast, or jumping some railroad tracks…
I think if you just do a body lift then it will not affect your suspension.
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A body lift, like a lift kit is only part of the equation… A body lift still doesn’t clear the body mounts… But your thinking…
Hint, adjusting caster can also help with tire rub…
Also, the vehicle still has to be (interstate) street legal, even in Cali…
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Lifting independent suspension trucks is a pain. No really good way to do it. Longer springs add height but b.u.g.g.e.r. up the cv shaft angles making them more likely to vibrate and break. This is for both front and rear independent suspensions.
Lifting solid rear axles only gets dicey with short wheelbase trucks with lots of lift. Otherwise the driveshaft angles can be adjusted a bit or use cv joint shafts or double cardan joint shafts.
Bigger tires gives you ground clearance as long as they fit the wheelhouse. Full rebound (droop) is easily tested for interference. Full compression (bump) is a bit more difficult. Jacking under the ball joint of one front wheel can usually get you close but it is almost impossible to turn the wheel lock to lock to test clearances.
Longer shocks give more rebound travel but can break ball joints that are at the limit of their travel so those need to change. That affects cv shaft angles as well.
A body to frame lift avoids lots of these problems. Some details need to move - like the steering shaft and some linkages. Brake lines to the master need to get a little longer.
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Mustangman of course is correct on his explanation, and kinda touched base on what my question was all about…
But the question was does adding a lift kit allow for bigger tires without rubbing during full compression??
See picture below.. He is running 33" tires without rubbing, 34" tires would rub without making some changes to the vehicle…
With a 100% stock vehicle, Yes a lift allows for a bigger tire at normal ride height for said suspension, on a flat surface, but the problem is when you start to compress said suspension going over speed bumps and turning, dips while turning off the main road and whatnot, add off roading and with everything else being equal, when fully compressing the suspension it puts the tire back up deep in the fender (stuffing the tire as the off road guys call it), that space is what truly determines how big of a tire you can run on a given vehicle… The reason for a lift is to gain wheel travel, approach angle, departure angle, and breakover clearance, but when they are stuffed up into the wheel wells, they need to move just as well as any other point in your suspension travel. And that is where a lift kit is not the complete answer…
A lift kit does not prevent larger tires from rubbing when the suspension is fully compressed (stuffed). While a lift increases ride height, the maximum compression is still limited by the bump stops, meaning tires will likely rub in the same places as a stock vehicle. Proper clearance for a larger tire, requires cutting fenders and or the bumper, or even doing the CMC (cab mount chop) or cab mount relocate on the Tacoma for example, or extending bump stops to limit suspension compression. When the suspension is fully compressed, the tire sits in the same spot relative to the frame/fender as it did before the lift, causing rubbing depending on the tire size itself.
As far as jacking up the wheel under the lower control arm to compress the suspension to test for tire rub question??
What I mean is that raising one wheel up by jack or ramp is not fully compressing the suspension, it is static or sitting at a compressed ride height not full compression, unless the shock is bottomed out, or you have complete coil spring bind, or are sitting on the bump stop, or something is hitting or binding, you still have potential to compress the suspension even more, most likely if you have someone heavy open the door and stand like you are doing something to the roof while standing on the door sill and jump up and down or rock the truck you will still see some more compression due to the weight change, stuffing the tire/compressing the suspension even more…
The only way to truly compress the suspension for testing is by removing the coil spring from the shock on the front of your truck (in this example Tacoma), and even thin in the real world, you will still have flex in the joints and bushings and whatnot that will allow for even more suspension compression/movement… You really want to test out your suspension compression, take it out and jump it, get that baby airborne and see what happens when the weight of the truck stuffs the tires when landing… lol…
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100%
Where we 4 wheel, articulation, travel and clearance are big factors. No sense lifting when your differential or shock mounts are still going to rub rocks…
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It also doesn’t do you much good if you stuff the tire and it rubs or rips your fender off… lol
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Fortunately, I have been cautious enough to ensure that doesn’t happen. Lots of fitment checks to make sure there is adequate clearance under full travel and the driveline angles are acceptable. We do rock crawling (the operative word) although there is always the chance of an unseen obstacle at higher speeds. So it pays to be diligent in the initial set up.
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Dave is right about testing full compression… Remove the spring and drop the car to the bump stops, steer the wheels and see if it hits. Huge PIA for a DIYer, but absolutely effective.
Fortunately truck designers are usually off roaders, too, so they give you huge wheelhouses for those big tires. Sometimes they even offer those big tires from the factory.
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