I see many suspension struts have relatively large diameter coil springs. Like 6”-7” diameter OD. And many aftermarket coil over springs are much smaller or more compact. Say in the 4”-5” dia. Obviously the spring seat would match the spring dia but the wheel offset or backspace may not change. So, why are original springs so “bulky”?
I imagine @Mustangman can give a better explanation, but the small diameter ones have adjustable spring perches, meant for custom/performance installations.
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I was thinking the same thing…lucky for us to have a suspension engineer hanging around!
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I agree, the top picture is of an OEM or OEM (stock) type replacement…
The 2nd pic is of aftermarket adjustable coil overs, they have a ride high adjustability that the OEM or stock replacements don’t typically have…
We have a brake, steering and suspension engineer on here???
Only kidding sir, the upmost respect to and for Mustangman…
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I too await the more educated response from Mustangman. My limited knowledge is that the larger coil diameters are primarily used for ride comfort. A larger diameter coil is more compliant, has less lateral distortion in compression and is generally more linear in compression force throughout its range.
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@TwinTurbo has the right idea about ride comfort and @texases is spot on about height adjustability of the smaller spring on a “coil-over” shock.
The size of the spring is to get the spring stiffness, or spring rate, low enough to provide a good ride while producing enough load to hold the car up.
The diameter does one other important function; it offsets the sideload caused by the position of the strut relative to the tire. The load of the car held by the wheel is quite a distance to the centerline of the strut. That adds a bending load to the strut causing friction. Friction hurts the ride. The spring and spring seats are designed to add bending force in the opposite direction to reduce that friction. To do that the spring is offset toward the tire which is why the spring seat overhangs the top of the tire and why the diameter is so large. It is also why the lower coils of the spring have a specific position. The barrel shape also allows a shorter overall spring height as the coils tend to collapse inside each other (and not rattle against) so they can collapse further than those cylindrical coils.
Notably, in the 80s, both the Mustang and the Camaro used strut front suspensions with the spring on the lower control arm and not around the strut. Both great examples of cars NOT noted for their smooth ride!
The “common wisdom” in the performance car community is to swap in coil-overs, as they are called, because they are “high performance” and cooler. Yes you can lower the car a lot, but they ride like a buckboard because the friction is much higher and the springs are much stiffer.
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I believe that was “Modified McPherson strut” . . . ?
Benz had it, as well . . .
I remember replacing several failed coil springs and it was kind of hairy
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I also think that was correct.
The MacPherson strut was invented by a Scottish engineer working for GM named Earle MacPherson (Yes, I have often mistakenly spelled it McPherson, too). It was designed for a low cost Chevy project called the “Cadet” intended for all four corners with an independent rear suspension!
When GM killed the Cadet program, MacPherson quit GM and went to work for Ford. Ford of Britain first used it on the 1951 Ford Consul.
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Spelling isn’t that easy for some of us 
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May we assume this theoretical Chevy “Cadet” with independent rear suspension would have cost more engineering dollars than GM was willing to spend at the time?
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I think GM was awash in cash after WW2 and could easily have afforded the IRS…
GM did introduce independent rear suspensions on the 1960 small cars, the Corvair, and Pontiac Tempest, and later the 1963 Corvette. Ed Cole wrenched out some cash from the beancounters.
But Americans wanted BIG cars…. The site below has a good article on the Cadet.
Toyota in my opinion has done a great job with the Tacoma’s using a pretty small diameter coil spring with their struts, my truck rides much smoother than the 17 with much larger diameter coil spring Corolla…
I know the shocks (strut) & tires mostly makes that happen, but man with the torn up road (construction with steel plates) all over her road, the Corolla kills her back (twin 11" rods) vs my truck, she loves the truck…
And the truck still handles the curves very well, even with it’s stupid high center of gravity…
Technically your Taco has coil over shocks in the front although many do call them struts. The truck has a double A arm front suspension so it is not a MacPherson strut suspension. This is a pretty common front suspension design these days. Having the spring on the shock makes assembly easier.
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For the curious, here’s a 2020 Taco front suspension, reminds me of a rear shock/spring on a motorcycle:
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Notice the lower spring seat… the notch for the spring end and the helical rise staying in contact with the spring. This keeps the forces centered on the shock and it eliminates spring “clatter” that can happen if the coil rises off the seat too early. Bouncing can then cause noises.
The “coil over” shocks that are sold nearly always have flat seats requiring the springs to be squared and ground.
Yep, very true, reminds me of one of the higher ups (possible engineer?) from Tenneco during a training seminar, (lots of different reps and whatnots from them there), said, the easiest way to tell if it is a strut or a shock is to remove it and see if you can still drive the vehicle at speed down the road… lol
Meaning that a MacPherson strut is part of the suspension holding the wheel on the vehicle (takes the place of an A-arm/ball joint attaching point, sitting on the bump stops does not keep it from being driven), whereas a shock can be removed and the vehicle still driven…
But basically the internals are the same, the difference is how they are mounted to the vehicle…
Ahhh the Tacoma TRD PRO trim level with OEM Fox shocks…
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The shock/strut description is mostly correct… and a line I have used many times. The placement of the spring and bump stops can make that an issue.
A typical strut is beefier than a shock. The chrome rod is usually 7/8 to an inch in diameter to take the bending loads. Internal piston tends to be larger as well. A shock would have a 1/2 to 5/8 inch rod.
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And that is why I am very thankful for your knowledge and contributions on here… Never thought that deep into the diameter of the chrome rod between the 2, even though I have handled more than my fair share of shocks/struts, but thinking about it, I guess I knew that and or noticed it, just put it back in the back, rarely used, part of my brain… lol
The responses seemed to focus on adjustable spring perch/height design. Not so much overall coil diameter.