# Why does the rear suspension compress when the e brake is applied, but not when the foot brakes are replied?

The title explains the question fairly well. The rear suspension also dips when power is applied (in the case of launch control). Does this phenomenon occur on independent suspensions, and why exactly does it happen?

• I've always noticed the rear suspension stretching (rear of vehicle goes up) when either the foot brake or the parking brake is applied. It's particularly noticeable on my motorcycle which has separate rear brake pedal (equivalent to e-brake), and very squishy suspension. This occurs because the vehicle is decelerating, thereby shifting the weight forward due to acceleration in the negative where the tires touch the ground. This causes the front spring to compress, and takes some weight off the rear springs, causing them to decompress. – the_storyteller Sep 14 '20 at 18:44
• This web page provides a beautiful illustration. Select "free body diagram" to show the force and velocity arrows, and click through the "accelerate" and "brake" steps to show the forces on the car. Use the second example, "2D rigid body model", to see the forces on each tire. dynref.engr.illinois.edu/ava.html – the_storyteller Sep 14 '20 at 18:47

It depends on the rear suspension of the vehicle as to whether it will "dip" or not under the circumstances you're talking about. The type of rear suspension I'd suspect you are describing is a cantilever design, where there is a forward pivot point at the body, then the two hub ends project towards the back of the car. This forms a large "U" shape if looked at from above. Springs support the car at the centerline of the wheel hubs and near the outside close to the hubs. When brakes are applied, the car can move slightly in either direction (forward or aft). When the car moves forward, the tires rotate and the axle moves with it (because the brakes have locked the wheel/tire stationary to the axle), rotating up towards the body, which compresses the support springs, thus making the back end dip. If you did the same in reverse, the axle would rotate away from the body, making the body push up.

Other types of suspensions (like independent rear suspension (IRS) or straight axle) don't normally do this because the axle itself cannot rotate (or in the case of the IRS, there's no axle there, but rather suspension parts much like what is in the front end of a front wheel drive car).

EDIT:

Here is the design I'm talking about in an image:

Part #6 is the axle I'm talking about.

• What I'm trying to understand is why a car would squat when the e brake is applied and, in the case of newer cars, why a car would squat in the rear when launch control is activated. The way I understand the squatting phenomenon is that it only happens (in an automatic car) when the transmission is in drive. It seems like the car wants to gradually accelerate, but can't since the wheels are stopped. Eventually once the wheels break free of the brakes, the squatting somewhat stops since the torque is being transferred to the wheels instead of the suspension. – Yri Sep 14 '20 at 21:06
• Well, along with what I've already described, you have what's called weight transfer. When you accelerate, the weight is shifted to the back of the vehicle. This happens with almost ANY vehicle. – Pᴀᴜʟsᴛᴇʀ2 Sep 14 '20 at 21:08
• Also, the reason why this doesn't happen when the brakes are applied, yet it does with the e-brake is for the reasons I've stated in my answer. When the brakes are applied, the front doesn't roll or allow the squat. It's the movement allowed which will cause it to squat/lift (depending on the direction of the movement). It doesn't actually take much movement for it to occur ... it just takes an inch or so. – Pᴀᴜʟsᴛᴇʀ2 Sep 14 '20 at 21:10

Simple geometry - when the wheel is locked (by brakes), it will apply the force to the trailing arm (in most suspensions) compressing the spring. Think of it as a lever.