17

This is something that I've always wondered. I'm no mechanic (web developer, actually) with very basic knowledge of vehicle mechanics so I can only guess at something like brake pipe pressure being involved, but that's about as far as I could guess. I can't find any article on the subject either, though I admit it's quite a niche question.

Case in point; I'm leaving the motorway via a downhill off-ramp and come to a stop in the queue using the foot brake (still downhill) - I shift into neutral, apply the handbrake and release the foot brake. As I release the foot brake the rear of the car sinks, as though the suspension was being elevated by the foot brake, though I've no idea how this is possible.

I apologise in advanced if this is rudimentary to those versed in vehicle mechanics and may be obvious. I've come to the assumption that it's normal behaviour and not an issue, but I can't remember whether or not this occurred with my previous two cars.

If it's relevant, here's my vehicle information:

Citroen Xsara Picasso Exclusive, 2006 (06), 1.6 HDI (Diesel) - 92HP, Manual Transmission

36

With the handbrake on, the rear wheel is not able to rotate. When the foot brake is released the car will try to move forward. This will cause a rotational force on the rear tire. Since the rear tire cannot turn, the rotational force will be transferred to the axle mounting point 'A' which will cause the road spring to compress, hence lowering the car body. If you did the same thing going up hill, you will find that the body raises instead.

enter image description here

  • 17
    The design of the rear is the main reason for this. Being on a swing arm causes the squat. Vehicles without this design, namely those with a solid 4-link or those with independent rear suspension (IRS) will not incur the "squat" you'd get here. – Pᴀᴜʟsᴛᴇʀ2 Jan 16 at 11:38
  • 6
    @Pᴀᴜʟsᴛᴇʀ2's comment is critical for this answer's explanation. I drive a pick-up and was confused when trying to think through this explanation in my head. (I was already confused by the question, but assumed it was just something I hadn't noticed before.) – Zach Mierzejewski Jan 16 at 14:44
  • Indeed. It's the trailing-arm suspension that causes this effect. IIRC my old Peugeot 106 did the same thing. – spender Jan 17 at 11:21
  • 4
    For us physics / engineer folks, Ya gotta live in the MOMENT... – zipzit Jan 17 at 19:18
19

When you stop the car using the footbrake, all four wheels are held stationary by the brake. When you apply the handbrake, this locks the rear wheels only (in most cars) - as you then release the footbrake, this releases the front wheels, allowing them to turn ever so slightly. Gravity is still trying to pull the whole car downwards, and so it settles down on the rear suspension - it's something that's hard to describe!

  • 1
    Ah, that makes perfect sense and, while I agree that is difficult to describe, I understand you. I was thinking the suspension was expanded and was settling back to rest, however you've shown that it was at rest and the cars front moving forward slightly is actually compressing the rear suspension - if I've got that right! Thanks again. – Kallum Tanton Jan 16 at 10:14
1

It's simpler than that. Your handbrake is actuated by a cable or sometimes a linkage. That linkage is applying a fair amount of force to set the brake, because the handbrake lever has a lot of leverage. This force is between the carbodt and the rear "axle".

It is designed to transmit this force straight, so there should be minimal body movement if the suspension is at normal height. Aiming downgrade, a little more weight is transferred forward, and the suspension on the back is a little lighter than normal. So it sits higher. That causes the shift when the handbrake cable is pulled taut.

-1

Well, it's only matter of inertia forces. If you start breaking - no matter if you use hand- or footbrake - the front suspension settles down as the inertia forces push the car (including you inside) to the front (yes, at the same time the rear suspension elevates a bit). If you release the brake the inertia forces disappear and the car returns to the normal position - you can feel this movement as sinking of the back.

  • The reaction of the car to the application of the foot brake (where all four brakes engage) is different from the reaction to the application of the hand brake (rear brakes only). – mike65535 Jan 17 at 16:55

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.