Welcome to Marshall Pruett's new Racing Tech Mailbag on RACER.com. If you have questions about the technical side of the sport for Pruett, who spent most of his life working as a mechanic and engineer in open-wheel and sports car racing, send them to PruettsTechMailbag@Racer.com. We can't guarantee your letter will be published, but Marshall will always reply.
Q: First off, I want to say thank you for doing this mailbag. Being fascinated with car technology is what led me to study engineering, and it can be difficult to find straight answers on racing technology.
I wanted to ask about the third spring/damper system used in (at least) F1 and IndyCar, as I still don't fully understand it. I understand a spring/mass/damper system, and I understand how roll bars work, but I still don't get third springs.
Aaron Foster, West Lafayette, IN
MP: Thanks for the kind words, Aaron, but don't thank me too quickly. I might disappoint you before this first one is done…
The conventional damper system with two shocks up front and two in the back worked just fine until downforce started having a major influence on performance. With air pushing cars lower and lower as they go faster and make more downforce, teams used the one option they had to combat the problem by stiffening the suspension with heavy coilover spring rates.
The big, stiff springs kept the downforce cars from bottoming out and also helped to maintain the correct ride heights to make optimal downforce– especially in the fast corners. But in the slow corners, those heavy springs provide very little suspension movement, which hurts mechanical grip. If you've ridden in a car that's so stiff it rattles your teeth over every bump, this is essentially what teams had to do to keep from burying the floor of their car into the racing surface at high speeds.
Think of Long Beach and the hairpin leading onto the straight. With big springs on an Indy car or a prototype, that car is going to struggle for grip turning in at the hairpin and again when leaving the corner as the driver tries to put the power down. It won't turn and it will light up the rear tires when you touch the throttle.
I'm exaggerating, but in the hairpin, you want the thing to roll around like an old Cadillac, yet the big springs make it handle more like a tank. Then, while accelerating down Shoreline Drive, a massive amount of downforce is piled onto the car, and in that situation, the big springs keep the ride height – and the underside of the car – low and under control.
Engineers want the best of both worlds for their drivers – supple handling in the slow corners without giving up solid and stable handling in the fast stuff – that will produce the fastest lap time. The first evolution came with limiting shock travel, but it had its drawbacks.
Hard plastic shims cut to various thicknesses (known as packers), along with softer bump rubbers, were attached to the exposed portion of the shock's shaft to limit ride height once it was compressed a certain amount. It allowed softer springs to be used, but once the shocks were compressed enough to hit the shims and/or bump rubbers and could compress no farther, the suspension went solid. Crashing over curbs, hard landings and other big movements could mean all of the cornering forces were being managed by the tire sidewalls instead of the shocks and springs. That might work for karts, but not for heavy cars with gobs of downforce.
The solution was the third spring system, which attaches through links on the bell cranks that connect the suspension pushrods to the shocks, and acts as a friendlier ride height limiter.
Instead of using the main coilover springs to control ride height or a mix of springs and packers, the third spring system took over that responsibility, allowing teams to use softer coilovers, which improves mechanical grip.
As you can see in the photos, teams have their own take on the best method – for some it's a small spring with packers and bump rubbers, while others go for a series of packers and bump rubbers. Even hydraulic ride height control is used in some series.
It's all done to keep the front and back of the car from going below the engineer's desired height, and with the t-swivel in place, the inter-linked suspension is allowed to work independently.
Engineers will call for adjustments to the shim stack on the third spring as they make ride height adjustments, leading mechanics to pick from bins to find the exact thickness needed.
Some teams rely on third springs more than others, and at some tracks, they may only be used up front. Running without them is also an option. And stiffer springs are still used at certain tracks…
I don't mean to be vague, but the third spring system isn't a magic cure-all. It works great at tracks where low speed mechanical grip is needed and high speed cornering is also crucial. Like the rest of the suspension and aerodynamic tuning options, it's an important tool, but not the only one for teams to consider. (Continued with 11 more questions and answers.)
If you have a racing tech question, send it to: PruettsTechMailbag@Racer.com