I'm certainly no expert of upgrading cars, but it seems like you can't go wrong when you start from the bottom-up: Tires, wheels, brakes, etc. before you start adding power.
Photography Credit: Ken Neher
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A pearl of wisdom from Wilwood’s Michael Hamrick: “When I’m asking about what someone wants out of a brake system, I need to know how the chassis, suspension and tires are all working together.”
The role tires play is particularly important: No matter how good your brakes are, it’s ultimately those four contact patches and their interface with the pavement that stop the car.
Let’s first take a look at some of the physics. Assume we have a 3000-pound car going 100 mph that needs to slow to 50 mph for an upcoming corner. The kinetic energy of that moving body is about 1.35 megajoules, or 0.375kWh. That’s enough energy to power the average American home, a/c blasting and stereo blaring Slayer from ceiling-high speakers, for about 20 minutes.
To slow that car to 50 mph, we need to reduce its kinetic energy to around 0.33 megajoules, or 0.09kWh.
“But wait,” you say, “that’s way less than half the energy even though you’re cutting the speed in half.” Good eye, and that’s because kinetic energy doesn’t scale linearly with speed; it scales with the square of the speed.
So to slow that car for the corner, we have to shed 0.285kWh of kinetic energy–about 15 minutes of the power needed to keep our house cool and tunes cranked–and turn it into something else. Remember that energy can’t be created or destroyed; it can only be turned into some other type of energy.
We could turn it into sound or light, but those are fairly efficient forms of energy, so we’d have to produce a lot of them in a short time to reduce speed enough to make this corner. We certainly don’t want to blind or deafen everyone within the entire ZIP code.
So we need to convert that kinetic energy into something more manageable for this situation. How about heat? Brakes do that by using friction.
Tires really can affect braking performance. We collected these two data traces during a recent tire test, and both show the same car braking in the same corner–but on different tires. The additional grip of the tire tested in the red trace allowed for a steeper deceleration curve, meaning the car slowed at a greater rate.
Heat is quick and easy to produce, but it does mean we have to then deal with the excess thermal energy in and around the brakes before the materials become thermally overloaded. That’s a discussion for a different day, but you get the basic principles we’re working within.
Now, a very similar relationship is taking place at your four relatively tiny contact patches. Friction between the tire and the road produces heat that transfers the kinetic energy out of the car, but many more complex operations are happening, too.
A tire’s relationship with the road plays out on a nearly microscopic level. Elastic deformation within the tread surface pulls and stretches the rubber to make more complete contact with the pavement’s countless tiny surface features. Every bit of deformation creates interference between the two that allows the tire to grip the road.
Very generally speaking, that’s why a softer tire has more grip than a harder tire: The softer rubber deforms more at that miniscule level, allowing it to better squirm into the cracks, bumps, ripples, grains and pockmarks in the road surface.
And ultimately, it’s the tires, not the brakes, that stop the car. They’re the final link in the braking chain. So when it comes to tuning a brake system, don’t shortchange this part of the operation. Everything from age, compound, pressure, camber settings and rate of weight transfer can affect a given tire’s relationship with the road and therefore affect braking performance.
Photography Credit: J.G. Pasterjak
To help preserve solid braking integrity, here are a few tire-focused questions to ask yourself:
I'm certainly no expert of upgrading cars, but it seems like you can't go wrong when you start from the bottom-up: Tires, wheels, brakes, etc. before you start adding power.
The brakes can only use the traction available at the contact patch - but they can also fail to use it. That's usually a bigger problem than a lack of grip.
This is the most important part of the article, but it also contains a significant error that's pretty common:
- Does the amount of dynamic weight transfer experienced under braking pair well with the front-to-rear brake proportioning pressure? When the rear wheels unload under braking, they have less contact patch and, therefore, less grip. Sending a lot of brake pressure to unloaded tires just takes away potential from tires with more load. Proportion tuning is an important part of testing. And remember, when you change spring rates, you might change the amount of pitch the car sees under braking, which could necessitate a proportion adjustment.
Pitch is a result of weight transfer, not a cause. Changing spring rates won't change your weight transfer. Changing ride height will, but not spring rate.
In reply to Colin Wood :
Colin, if you dial in an awesome suspension and grip with no power, you can end up with a pretty uninspiring car. I went to the track one time with two ND Miatas running the same wheel/tire/suspension setup. One had 150 hp and one had 525. I was basically at WOT all the time in the low power car, it had a very limited number of tricks. It was not as much fun as the car that was working the suspension harder. The most fun I ever had with an NC Miata also involved a supercharger and indifferent tires. Good handling, just not a lot of traction so I could play with it.
I also once had a 1999.5 2.0 Golf that had good suspension and tires and a stock engine - the car didn't really have handling. It couldn't go fast enough to have handling. It just had grip.
So yeah, if you're mostly looking for speed above all else, you want to make sure you've got the chassis sorted. But stapling a car to the ground isn't always the most fun way to build a car.
Adding more grip and braking power can definitely impact brake proportioning. I've got an adjustable prop valve waiting to go into the Jeep, as with added grip and better pads, it'll lock rear first at times and get a bit sketchy. Just too much weight transfer compared to what the original brake proportioning was designed for.
In reply to rslifkin :
Grip yes. Any braking system should have the ability to lock any wheel, so more braking power isn't really going to affect proportioning unless you are only changing it at one end.
Things that affect proportioning due to weight transfer:
- ride height (higher = more transfer)
- grip (tires, weather, surface: more grip = more transfer)
- wheelbase (unlikely to change, but longer wheelbase = less transfer)
- changes in static weight distribution
Things that will change proportioning:
- changing pads at one end
- changing rotor diameter or piston sizes
- changes to the proportioning valve (duh)
- master cylinder changes if the diameter is different front/rear
What did I forget?
In reply to Keith Tanner :
Thank you for that insight. I like the mental image of "stapling the car to the ground."
In reply to Keith Tanner :
Agreed. But going from low grip all seasons and anemic stock replacement brake pads that could barely generate enough power to lock said all seasons (while nearly lifting myself out of the seat) to grippier tires and much better pads added a whole lot more braking force (and therefore weight transfer).
In reply to Keith Tanner :
You're correct that changing spring rates won't change the amount of weight transfer, but it will change the pitch. When weight is transferred to the front of the car, the springs will compress due to the increased load on them. How much they compress is determined by the spring rate. Stiffer springs will compress less, softer springs will compress more.
Why worry about pitch, instead of just weight transfer? There are a couple of reasons. As the front suspension compresses, the camber and toe curves come into play. More compression generally means more toe and camber change. Camber curves are typically negative in compression, so any static negative camber angle will only become more negative, adversely affecting the contact patch and reducing grip. On production cars, toe usually moves towards toe-out in compression, which "wastes" some of the tire grip by producing lateral forces.
I have seen formula cars with really low ride heights where the front wing endplates contact the ground under hard braking. Every pound of contact force between the wings and the ground takes away from the load on the front tires, while not adding much braking. Reducing the pitch (and ensuing endplate / ground contact) will improve braking under this condition.
I'm well aware of camber curves and bump steer, but you'd have to have a lot of movement before it became significant enough to require proportioning adjustments. I suspect it's one of those true in theory but minor in practice situations. It's far more likely that if you change spring rates on a passenger car, you're also changing the static ride height and thus the CG height, which is going to have more of an effect.
There are always edge cases. With enough suspension movement, you can end up hard on bumpstops in braking and thus your spring rate goes through the roof and you lose suspension compliance and grip. I've worked on a race car that had weird braking balance problems that weren't explained until we discovered the front tires were making contact with the inside of the fenders under maximum braking. You could construe both of those as "changing spring rate requires different brake proportioning", but really it's "the car had a problem that was solved by changing spring rates". Same with a formula car that's grounding out. You're not having to change brake bias because of a change in spring rates, you've got a fundamental setup problem that needs to be fixed before you set the bias.
In reply to Keith Tanner :
Agreed. Toe change under braking would have to be extreme to meaningfully affect grip. The Jeep has more toe change with suspension movement than a lot of vehicles, but it still doesn't toe out enough to be an issue. You can feel it in the steering if you come into a turn on the brakes too hard, but in a straight line it's not very noticeable. It runs very close to 0 static toe, so not going from toe in to toe out might help mask it.
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