Last week on Corvette Forum, we published the first portion of this in-depth conversation with Shaikh Ahmad of Fat Cat Motorsport about the complex nature of shocks. If you haven't read part 1 of the conversation, be sure to check it out.
Flat Ride Recap
If a car’s shocks were removed and it went over a bump at a given speed, both the front and rear would bounce up and down at a given speed until they return to rest. The frequency of this bouncing can be measured in Hertz (Hz) and this is the first aspect of advanced suspension tuning that Shaikh utilizes. As the front encounters undulations in the road before the rear, the two oscillate out of sync with each other which can cause fundamental handling problems regardless of how good your other components are. Choosing spring rates that lock these frequencies together is the first part of the equation the guys at FCM use to dial in any machine from road to race.
Don’t Just Tune With Shocks
Getting caught up in the latest triple adjustable, 30-way suspension is often a trap. “You don’t want to only use shock adjustments to tune oversteer and understeer in a vehicle,” Shaikh said when asked how important of a role high-end multi-way adjustable shocks play in a suspension setups. “You’re going to end up overusing something. I don’t think you should ever just look to one component. Think of it as a whole.” This is great advice that has been given to me by names like Steve Nichols who designed the McLaren MP4/4. Shaikh is definitely onto something with the way he approaches modifying cars. I’ve driven one of his early setups that featured modified Bilstein dampers and can attest to the fact that his stuff really works. Performance over bumps was unbelievable on back roads that rattle your fillings in even the most exotic of sports cars.
“You need to see what your contact patches are telling you. They’ll tell you almost everything you need to know,” the shock tuning guru advised me. “And, along with video of your driving, you can cure and diagnose almost any handling problem you will run into. A lot of cars from the factory have terminal understeer— Porsche, Audi, and even Mazda are designed this way because of the bump stops. You’ll see higher front tire temperatures, generally, the outside is much hotter than the inside.” If you track your car and don’t own a pyrometer it’s time to buy one. To be proactive, you want to measure temperatures at each track day event you run.
Bump Stops—The Over Looked Modification
Bump stops prevent your shocks from bottoming out and being damaged. Think of them as a rubber door stop that can be tuned just like your springs can. Different stiffnesses allow for different handling characteristics when the shock comes into contact with them. Understanding what they contribute is vital to knowing how to tune. It’s a part you almost never hear about in magazines or in automotive reviews. Shaikh was quick to highlight a great example of how they can contribute to lap times. “A stiffer rear bump stop can help the rear of a car want to rotate which allows you to avoid ramping up your rebound adjustment to the point of reducing grip,” he informed. Every component has to be looked at when you upgrade your vehicle especially if you chose to use a mix of stock components and aftermarket components as many of us do. Understanding minor details like this can make a huge difference in racing classes where tenths of a second are the difference between 1st and 5th in qualifying.
Front Roll Couple Percentage
Front roll couple percentage describes the amount of total vehicle weight that the front tires hold up during cornering. The higher the percentage, the more work the front end is doing in a corner. And, the more you increase this number, the more likely you are to experience understeer. “Springs, sway bars and bump stops go into creating these splits,” Shaikh said before adding “In the case of a front engine rear wheel drive car, you want a bit more weight transfer up front so you can get on the throttle earlier. You have to bias the front roll couple towards the front suspension so you can use the throttle to transfer weight onto the rear when you’re ready to do that.” For reference purposes, NASCAR racers will run around 75% FRC to help combat the massive power their cars produce. Increases in horsepower generally favor higher front percentages which anchor the car around the front end creating a positive and stable driving feel as the driver gets more and more aggressive coming off of corners.
Shock fluid has large implications on performance. Not only do you need to make sure the body of your damper can displace enough fluid, but there’s the fine art to how much to pressurize it. “No one tells you about this one,” Shaikh said. “Every shock has a spring inside of it. That’s what nitrogen is—another spring inside the shock. The suspension has to overcome this gas force every time it needs to oscillate and move. It’s one of the most overlooked features in suspension tuning. Having too much pressure creates unnecessary forces on the suspension.” Shaikh uses advanced internal piston designs to mitigate this, allowing the shock to move freely and improve driving feel while still delivering outstanding feedback to the driver cornering on the limit. It’s all down to a science at FCM.