I've been doing some suspension upgrades on my M3 and in the process taking lots of measurements. It has been a pretty eye-opening experience. All throughout my E46 ownership, I thought there was some small amount of lowering that will not result in a practical performance decrease. That's why I went with the Dinan spring/Koni shock combination, because it's viewed as a conservative upgrade.
As it turns out, lowering your car ALWAYS makes it worse. Unless you are working with a specially set up track-only car with modified chassis geometry, aerodynamics, etc, it will make your car less comfortable, worse handling, and less practical (all else being equal).
Why is that?
Travel
The E46 M3 only has a small amount of suspension travel. On a stock car in the front, you only have about 1/2-inch before the strut touches the bump stop. This is normal and there are a number of reasons for doing it. It's BMW's way of creating a progressive spring and a "flat" handling car, minimize camber loss, plus probably to meet specific government handling requirements. Stock ride height up front is 14.25" from the hub center to the fender edge on my '05. By going to a "conservative" 13.75", it's now resting on the bump stop before you even encounter a bump.
Here you can see a graphic of the strut body (yellow), stock bump stop (green) at stock ride height. Koni Sport shocks are the same length as stock, or at least within a few mm.
Here you can see what a Koni/Dinan setup looks like while just sitting there...
Many aftermarket shocks are shorter, so that lowering can occur without immediately running on the bump stop. For example, the KW V3 strut is a full inch shorter! Here you can see how it looks at the same stock ride height. They also use a different bump stop geometry.
Of course, there is a practical limit to how short you can make the strut before you actually need it to hit the bump stop, and prevent your wheel from crashing into the fender. This number is going to vary based on your particular wheel/tire specs and alignment, so it's hard to say what that limit is. On my car, that number is 11.5 inches from the hub center to fender lip. At that point, the wheel hits the inner fender. (stock alignment, 18x8.5 et38 with 245/40 PS4S).
So as you can see, absolute theoretical maximum travel is only 14.25" - 11.5" = 2.75". In reality, the max it can achieve while still allowing some buffer is about 12". So 2.25 inches of travel. It's not much! By lowering your car that "conservative" 1/2", you've given up almost 25% of the available travel!
I focused mostly on the front here, because the MacPherson strut is such a fickle design. The rear has some more travel available (but still only around 3 inches).
Ride comfort
It should be no surprise that by reducing travel, you're going to encounter the bump stop more frequently on bumpy roads. This transition and increase in spring rate will give the car a "crashy" feeling. I know for me personally, I felt as though I was hurting my car because it constantly was sending shockwaves through the chassis as the bump stop was just being pummeled. There were several roads here in California that I simply avoided because it was so unpleasant.
Practicality
I don't know about you guys, but even with a stock height M3 you have to think about parking curbs, driveways, speed bumps. As soon as you start lowering, you'll find yourself scraping on so many more things.
Handling
As I mentioned earlier, the MacPherson strut is a fickle design that is mostly chosen due to cost. It's a fantastic design when properly engineered, but suffers from a "roll center" change when you start lowering your car. This is well documented and there are plenty of remedies or compromises to reduce it. Basically - as you start lowering the car you start increasing the amount of body roll. The effects of a lower center-of-gravity are quickly negated by a body that rolls, camber that is lost, and it has effects on your steering feel.
Anyway, I thought I'd share. I've decided to raise my car back up to stock height and enjoy all the benefits it brings, even though it looks dorky.
As it turns out, lowering your car ALWAYS makes it worse. Unless you are working with a specially set up track-only car with modified chassis geometry, aerodynamics, etc, it will make your car less comfortable, worse handling, and less practical (all else being equal).
Why is that?
Travel
The E46 M3 only has a small amount of suspension travel. On a stock car in the front, you only have about 1/2-inch before the strut touches the bump stop. This is normal and there are a number of reasons for doing it. It's BMW's way of creating a progressive spring and a "flat" handling car, minimize camber loss, plus probably to meet specific government handling requirements. Stock ride height up front is 14.25" from the hub center to the fender edge on my '05. By going to a "conservative" 13.75", it's now resting on the bump stop before you even encounter a bump.
Here you can see a graphic of the strut body (yellow), stock bump stop (green) at stock ride height. Koni Sport shocks are the same length as stock, or at least within a few mm.
Here you can see what a Koni/Dinan setup looks like while just sitting there...
Many aftermarket shocks are shorter, so that lowering can occur without immediately running on the bump stop. For example, the KW V3 strut is a full inch shorter! Here you can see how it looks at the same stock ride height. They also use a different bump stop geometry.
Of course, there is a practical limit to how short you can make the strut before you actually need it to hit the bump stop, and prevent your wheel from crashing into the fender. This number is going to vary based on your particular wheel/tire specs and alignment, so it's hard to say what that limit is. On my car, that number is 11.5 inches from the hub center to fender lip. At that point, the wheel hits the inner fender. (stock alignment, 18x8.5 et38 with 245/40 PS4S).
So as you can see, absolute theoretical maximum travel is only 14.25" - 11.5" = 2.75". In reality, the max it can achieve while still allowing some buffer is about 12". So 2.25 inches of travel. It's not much! By lowering your car that "conservative" 1/2", you've given up almost 25% of the available travel!
I focused mostly on the front here, because the MacPherson strut is such a fickle design. The rear has some more travel available (but still only around 3 inches).
Ride comfort
It should be no surprise that by reducing travel, you're going to encounter the bump stop more frequently on bumpy roads. This transition and increase in spring rate will give the car a "crashy" feeling. I know for me personally, I felt as though I was hurting my car because it constantly was sending shockwaves through the chassis as the bump stop was just being pummeled. There were several roads here in California that I simply avoided because it was so unpleasant.
Practicality
I don't know about you guys, but even with a stock height M3 you have to think about parking curbs, driveways, speed bumps. As soon as you start lowering, you'll find yourself scraping on so many more things.
Handling
As I mentioned earlier, the MacPherson strut is a fickle design that is mostly chosen due to cost. It's a fantastic design when properly engineered, but suffers from a "roll center" change when you start lowering your car. This is well documented and there are plenty of remedies or compromises to reduce it. Basically - as you start lowering the car you start increasing the amount of body roll. The effects of a lower center-of-gravity are quickly negated by a body that rolls, camber that is lost, and it has effects on your steering feel.
Anyway, I thought I'd share. I've decided to raise my car back up to stock height and enjoy all the benefits it brings, even though it looks dorky.
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