Suspension Truth # 3:What Makes A Car "Fun To Drive"? Factors In Suspension Dynamics

In the last article, we explored what influences a suspension to ‘feel sporty’ vs. actually deliver better performance. A great car rewards us with a sublime driving experience while many (most?) let us down in a various ways. As part of that article, I dove into fairly technical terms without much introduction, so I’m taking a step back to do an overview and define a few terms. Then, I’ll get into the real meat of Suspension Truth and why we’re on The Truth About Cars – more seat-of-the-pants impressions tied to juicy technical details and real-world test data like shock dyno graphs, 3-axis accelerometer results – even raw shock velocity measurements from our Aim EVO system! From here we’ll be able to give metrics for different vehicles and see what we like and what we’d like to improve.

A number of you asked in the last article’s comments “is it possible to improve upon something so-so?” with the complementary question of “why does model X feel really good?” The same design factors are at work in both cases, differing only in how they’re managed. With a holistic approach, I’d like to convey my view of how those factors work. First, as independent parts and second, how they relate to the whole vehicle’s to create a ride or handling experience and your subjective experience of that.

Like any system with many variables, there are many interactions at play, but at least in my empirically minded way, a few are more dominant than others and also depend upon the road environment (like smoother vs. rougher). For simplicity’s sake, I’d like to use this breakdown of the behaviors we care about: a) grip and stability going around turns, b) comfort driving in a straight line and c) responsiveness to direction changes.

So what are the most important factors in how a car feels and responds? Some factors influence both straight line and cornering, which isn’t always intuitive. Dampers really do matter in cornering, because we all know (sing it with me now) ‘there’s no such thing as s smooth road’. The video is referenced in the previous article. Here’s my list of vital factors in a suspension design plus the impact on ride and/or handling:

• Total body roll or roll stiffness index, generally determined by overall roll stiffness normalized to the vehicle weight. The stiffness comes from springs and sway bars (though bump stops often contribute as well) divided by the vehicle’s weight which is what I call the “roll stiffness index”. You can directly measure the roll angle – a head-on photograph is wonderful for this. The roll stiffness index requires more calculations. Lots of body roll means less tire contact patch and less grip.
• The amount and build-up of shock damping force, especially at low-speed – the region where the shock changes direction, from extension to compression and back again – is what controls all driver-induced inputs (steering, brake, gas) and also small chassis movements. There can be a subtle quality of ease or dis-ease depending upon this hidden effect, which we talked about in the last article. It’s certainly possible to get good body control without undue harshness if the damper forces vary smoothly instead of having sudden changes in slope.
• A suspension geometry (MacPherson strut, double-wishbone, etc.) that provides a consistent tire contact patch, bushings and suspension members that behave predictably so the driver can be confident extracting the available grip without nervous handling.
• Bump stop interactions during cornering – this is the least understood, most often ignored and most important of all the factors, both in my opinion and experience. At the extremes of handling or ride is where the extremes of bump stop behavior comes into play. Most people think they’re not hitting them because modern bump stops feel very supple most of the time. Because of this, a behavior like terminal understeer (‘the car won’t turn and I’ve cranked the wheel all the way over!’) will almost always be due to the bump stop tuning. Because this part of the suspension is both hidden (under dust boots) and non-intuitive (how does it respond to a load?) it’s usually overly simplified, ignored, literally removed or cut in half, making matters even more. Plus, how the front and rear bump stops interact relative to each other greatly influences the steady-state cornering response. Simply attending to bump stop interactions and keeping in mind the suspension design can give one insights that turn a vehicle from pushy to playful!
• Chassis settling time in response to vertical disturbances – this comes via choice of ride frequencies with attention to ‘Flat Ride’, how the shock builds force in the velocity range associated with the time period of the chassis movement, bump stop length/stiffness to help manage big impacts, suspension bushing density and whether any micro-cellular urethane (MCU) has been incorporated into the design of the shock mounts or other areas of the chassis to help reduce NVH (noise-vibration-harshness) by absorbing high-frequency vibrations.
• Center of gravity height, which we have only a little control over via lowering or perhaps lightening. Generally, the more passenger- and cargo-oriented the vehicle, the heavier and taller it’ll be, the more weight transfer will occurs and the less grip available, not to mention horror-inducing body roll from usually low roll stiffness! But a tall, heavy vehicle can still behave reasonably well if the other factors are managed, or adjusted. As the expression goes, you can’t make a pig into a race car, but you can make a very fast pig.
• Chassis stiffness – having gone to a nearly full cage and hardtop on my ’95 convertible Miata convertible (‘Senna’), I was amazed at how much less of the road I felt. The suspension and tires communicated nuances so much better! On a convertible or other flexi-frame vehicle, shoring up the chassis rigidity helps in numerous ways. The higher resonant frequency means less ‘noise’ couples from the road through the chassis and into you.
• Seats – this was mentioned in the comments and is a great point I’m going to ‘borrow’! A bench seat won’t give us much confidence compared to a bucket, or a proper race seat. Being confident and not having to brace yourself all the time is really a good recipe for feeling more connected to the car. Conversely, a race seat with no cushioning is simply going to feel abusive.

I’ve thrown a lot of primary and sub-factors out, giving much to discuss in comments and additional blog posts. In that spirit, perhaps you can mention (even repeating yourself from the previous article is fine) a specific vehicle you have in mind and how you feel about it in regards to these primary factors:

1. Total body roll or overall chassis stiffness and center of gravity
2. Responsiveness to steering
3. Ride quality and chassis response on small, medium and big bumps
4. Chassis stiffness
5. Seats, confidence in cornering or support for long-distance driving

I will reply to your comments with my relevant experience or observations, forming a thread as I see already happens. Your feedback will help me learn more about the spectrum of vehicle’s you’re using, have owned, and are interested in buying or learning about.

Before the next article, I’ll record a video with my vision of how steady-state cornering operates combining many of the factors above. First, taking springs and sways together to create a certain dynamic balance – I use the term ‘front roll couple’ or ‘FRC’ which I calculate as the percent of roll stiffness across the front axle vs. the total vehicle roll stiffness. Then I’ll add the effect of bump stops, then throw in camber front vs. rear, and finally the impact of shock damping on the whole mess – for both smooth and rough roads! Building up step by step, I think this video (or two or three) will be fun and insightful! I’ve certainly wanted to create them for some time. Plus it’s necessary as I’ve been attempting to wave my hands around as I type, hence why this article took longer to write (and re-write and re-write…).