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      09-10-2021, 05:25 PM   #1
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Drives: 2007 Z4MC
Join Date: Feb 2019
Location: New York, NY

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2007 BMW Z4MC  [0.00]
Suspension enlightenment and engineering - philosophical post

Had some thoughts as I've grown on my car modding journey and thought I'd share.

I started out, as most do, reading stories of people modding their suspensions and getting many perceived benefits: i.e. upgrading to coilovers improving the ride quality and performance, RTABs, alignment settings, etc etc. I installed coilovers, changed my alignment, and I loved the responsiveness, the reduced body roll, the perceived performance improvements. But as I got a lot of experience both street driving, track driving, and autocrossing with this setup, I wanted more. I read and researched more about suspension from many sources (youtube videos, fatcat motorsports content, technical articles on alignment, scrub radius, steering axis, dynamic alignment changes like camber gain, weight transfer, roll center, damping), and as a result, my view on car modding has evolved a lot.

I used to think, given all the experience and stories on this forum, I'll mod my car and it'll become better. Now, I'm realizing, there's a ton of nuance when it comes to tuning a car's suspension, and it's all designed holistically. BMW engineers specify all the components to get certain suspension parameters close to targets in order to get desired, safety, grip, understeer/oversteer, characteristics. Everything is engineered with everything else in mind. The springs and sway bars determine roll stiffness, the front/rear roll stiffness have a target ratio, amount of roll and dynamic camber curve is designed together, the wheels are chosen for fitment as well as scrub radius with the whole setup.

Additionally engineers have a ton of institutional knowledge from years of releasing many models, and they have specific philosophies of their own. I.e. BMW sports cars feel different to drive than do Porsches or Toyotas. This is a result of differences in philosophies of what makes a car good to drive.

When we mod our cars, we're creating our own driving characteristics in an unknown direction. Example: you add negative camber by moving your strut mounting position, it also alters the steering axis inclination and scrub radius. What's the overall effect going to be? We'll get more negative camber, but what do we give up?

I haven't even mentioned the added complexity of designing dampers. They have a huge impact on the dynamics, because they add (or reduce) the stiffness on the order of 100s of pounds, and they do so *dynamically* i.e. in proportion to shock travel speed. BMW designs their dampers to make a BMW feel like a BMW, whereas Toyota designs their dampers differently for a different feeling with different priorities in mind. When you change to an aftermarket damper, who knows how it'll feel and what priorities they optimized, and what they gave up. I.e. adjustable shock bodies give up suspension travel in order to get a wide range of ride height adjustability. Some coilovers have a huge amount of rebound damping to give you responsive turn-in and better road feel at the cost of grip on rough road surfaces. And finally how the alignment changes dynamically is changed as you change roll stiffness.

This post might seem anti-mod, but it's not. The purpose of this post is to balance out the glowing mod posts on this forum with more context. I'm actually pro-mod for the uninitiated because it puts you on a journey of learning more about cars, and you learn a lot more than you do by reading the forums. Also I don't want to discount the information on the forums - they are a huge trove of information, although imperfect, but still useful!

It's been a very insightful journey for me. But at the end of it, I'm leaning more towards leaving cars as stock as possible, and only making minor adjustments where I am confident they're needed. The right way to mod essentially requires you to become an automotive engineer and consider the whole system holistically. To test a hypothesis, capture data, and iterate. Which is totally possible and many race teams do it successfully, but it's not something that the every day forum member has the resources for.

Last edited by filtercoffee; 09-10-2021 at 05:31 PM..
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