Jack's Not Here, Man!

A lot of ink has been spilled in bike magazines about brake jack. The idea is that the use of the rear brake applies a force to the rear suspension, which causes the suspension system to extend toward the top-out position. This reduces suspension performance and control and upsets the bike's handling and fore-aft balance. But here's the thing: In most cases, brake jack simply isn't what's happening.

The only jack-suffering bikes in the disc-brake era that made it to any sort of wide-scale production were some of the Mert Lawwill bikes that lacked a floating brake linkage (like Schwinn's Four Banger and Tomac's 204 Magnum). For every other bike out there, something else is happening.

THE MYTH OF JACK
Single-pivot bikes, in particular, have been maligned for brake jack. But most single-pivot bikes don't jack. Instead, like almost every other full-suspension bike, they do the opposite: They squat. When you hit the brakes (for the purposes of this article, we're talking only about disc brakes), the braking force is applied to the tire's contact patch at the ground (where the majority of forces applied on the rear suspension are generated), opposite the direction of motion. This creates a force that acts on the pivot (or, on a linkage bike, on the instant center of the linkage). Depending on the placement of the pivot or linkage instant center, this will be either a compressive force or an extensive one. And most suspension bikes these days compress, or squat, to varying degrees. There's no such thing as completely "neutral" or "independent" braking. And, much like a bike's anti-squat characteristics when pedaling, the influence of braking can change depending on where the bike is in its travel. It's true that different bikes have different braking characteristics, but it's all different tunings of the same force.

While we're at it, let's do away with this myth: By simply changing the position of the rear caliper relative to the disc rotor (when the rear wheel and caliper are attached to the same frame member), a builder can change the braking characteristics of the bike. Not true. The braking force that influences the suspension is generated where the tire meets the ground, not at the caliper. Where the caliper grips the rotor along its diameter makes no difference in braking or suspension behavior.

PUSHING FORWARD ON A SQUISHY FORK
If most of the bikes we ride are compelled to squat when the rear brake is applied, why does the rear end extend when we use the brakes? Weight transfer. Essentially, this is the mirror image of accelerative forces, though more abrupt and severe. During braking, a rider's weight is up high on the bike and is trying to continue its forward motion; at the ground, there is an opposite force. This tosses that weight forward, changing its distribution and causing the fork to dive. The rider's forward motion also takes some weight off the rear suspension, so it extends—but not because the rear brake is creating an extension force.

There's another factor that accentuates fork dive, too. Some percentage of the braking force at the front axle is turned into fork dive, though how much varies with a bike's head angle: The slacker the angle, the more "in line" the fork is with the braking force and the more dive that occurs.

And then there's fork setup. A super-plush, long-travel fork with a soft spring and light damping will have a lot of fork dive, which can feel like brake jack, especially if the terrain is steep. Stiffer fork springs or added damping will reduce dive and, to a degree, lessen rear-end extension. But these adjustments take away from the fork's plushness and sensitivity, so it's up to the rider to find his preferred balance.

The lesson: Even if the braking reaction is compressive, rider weight transfer and fork dive are greater influences, so the net effect may still be rear-suspension extension. The combination of fork dive and rear-end extension make for a hell's stew of handling: Not only is there a lot of weight on the front wheel, there's less fork travel available, and the bike's head angle is effectively steeper—exactly what you don't want when trying to execute a controlled stop or descend a steep or tricky line.

Tony Foale, renowned expert in the area of motorcycle suspension and chassis engineering and author of Motorcycle Handling and Chassis Design, offers this opinion on his website ( tonyfoale.com): "The problem of rise on the rear is less severe than that of dive on the front. The springing is stiffer on the rear and so for a given weight transfer the rise will be less than the dive, particularly with the additional dive effect due to the raked fork legs anyway."

Forward weight transfer also takes weight off the rear wheel; in effect, this makes for a stiffer shock spring and less-sensitive suspension. Also, as weight is removed from the rear wheel, rear-tire traction is reduced. Even if we constructed a theoretical bike with a rear brake that had absolutely zero influence on the rear suspension, there would still be suspension extension, stiffening and reduced rear-wheel traction because of weight transfer.

IS SOME SQUAT GOOD SQUAT?
It's almost universally agreed that, if brake jack actually existed, it would be bad. But there's still some debate about brake-induced squat—in particular, how much is the right amount? Some can be a good thing, advocates say, because it can limit forward weight transfer. However, squatting the suspension influences bump sensitivity.

"If you're on a bike that squats during braking, that means the linkage is influencing the spring rate to keep the bike level," says Specialized's senior suspension engineer, Jason Chamberlain.

Dylan Howes, Trek's director of frame suspension technology, looks at it differently. "It's working to hold the suspension in a more compressed state," Howes says, "preventing the suspension from rebounding, which would make the suspension feel stiffer."

On the surface, it appears to be a balancing game. It's easy to envision engineers tweaking their suspensions for the ideal ratio of weight-transfer-limiting brake squat to suspension sensitivity for maximum performance and rider benefit. However, suspension systems are very interconnected: Changing the braking characteristics will have an effect on the suspension's pedaling and bump performance. And, nobody is designing suspensions around braking only. Pedaling efficiency and suspension performance are too important to be sacrificed.

According to Joe Graney, Santa Cruz's director of engineering, "Telescopic forks are the main culprit of braking-induced changes in suspension, and there is nothing rear suspension will do to change that. Though he's modeled and analyzed his bikes' braking characteristics, he doesn't do anything specifically to tune braking performance. In his opinion, it is what it is: in the same ballpark as his competitors' bikes and, to his way of thinking, there's nothing he would do to change it that wouldn't negatively affect the bike's other characteristics.

THE SQUAT YOU WANT
As pointed out earlier, single-pivot bikes have been incorrectly maligned for brake jack—but there is a nugget of truth buried deep in the criticism. Single-pivot bikes often have more brake squat than bikes with a linkage instant center, which means they're the least sensitive suspension when braking (though, in theory, the single pivot has better control of weight transfer). The differences aren't as huge as many manufacturers would like you to believe, but there is a difference—enough for some people to feel and others to obsess over.

But, just like drive-force-induced anti-squat ("You Don't Know Squat"), brake squat is just one of many factors that a rider can consider and weigh against his needs and preferences. But it's not important enough, or even relevant enough, to obsess over, because all the commonly available suspension systems have inherent braking characteristics that we're stuck with for the foreseeable future. Basically, we're fighting a losing battle against rear suspension extension: No matter what we do, as long as we love our longer-travel, telescopic suspension forks, we're going to have forward weight transfer, fork dive and rear-end extension, and the associated effects on rear-suspension sensitivity and rear-wheel traction. Thankfully, all the systems are fairly close in braking performance, and they all seem to work well. If you're fixated on changing a bike's braking performance, floating brakes (see "Brake Control" ) can help, but changing brake-squat characteristics also changes weight-transfer characteristics. When you get, you give; when you give, you get. You just can't jack with that.