Applying Handbreak - Press Release button or Not

BeenThroughItAll

Car_54 wrote: »

I am reminded of Godwin's law: "As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches 1".

There must be a corollary involving motoring forums, and Morris Minors, MGs or Austin 7s.

Are you suggesting that one shouldn't be allowed a comparison involving MGs etc?

Stop trying to gag us all! You'll never bring us to heel under your Nazi jackboot.

Zandoni

Samsonite1 wrote: »

You think you can, that's why it's better to use the ratchet. There is a difference and you just don't know it. That's ok though.

There's no difference at aall but I'm happy to let you go on believing.

Samsonite1

Zandoni wrote: »

There's no difference at aall but I'm happy to let you go on believing.

There is no difference between two different things? Why of course!

GabbaGabbaHey

EssexExile wrote: »

I drive an automatic, what's a parking brake?

It's the thing that you're meant to use at traffic lights instead of holding your car on the footbrake and dazzling the poor driver behind you with your high intensity rear brakelights (especially at night and/or in the rain)!

Zandoni

Samsonite1 wrote: »

There is no difference between two different things? Why of course!

They are not different things at all, the button is just there to engage and disengage the pawl from the ratchet.

Samsonite1

Joe_Horner wrote: »

Two levers and a cable in between. You pull one lever, it supplies mechanical advantage according to its dimensions so that the load in the cable is greater than you could apply yourself. The other end of the able pulls another lever, which also provides mechanical advantage so that the force aplied to the shoes is, perhaps, 50 to 100 times the force you apply with your hand but acting through a few mm of travel instead of the 20 or so cm of travel your hand goes through

Thanks for the physics & engineering lesson. So by virtue of mechanical advantage, if you applied a relatively small amount of force like 10KG, you are actually applying 1 tonne of force to the mechanism?

Just asking a question...

reeac

Returning to the fray after my contribution re MGAs, Although personally I would always press the button to avoid nasty rasping noises and associated wear, foot operated parking brakes (which I dislike) don't give you the option of avoidance and presumably give a satisfactory service life.

Samsonite1

It does seem that there are other more modern ways, although the old ratchet system is fairly reliable and simple to fix in many cases.

Joe_Horner

Samsonite1 wrote: »

Thanks for the physics & engineering lesson. So by virtue of mechanical advantage, if you applied a relatively small amount of force like 10KG, you are actually applying 1 tonne of force to the mechanism?

Just asking a question...

You'd be applying that sort of force by the time it gets to the brake shoes (ignoring friction along the way). The simple calculation for mechanical advantage is:

force applied at other end = the force you apply x the distance you move / the distance the other end moves.

With a handbrake that's hard to measure because the actual movement once the shoes touch the drums (which is what matters) is very small, but you can do it in stages by each lever in the system.

A typical handbrake lever will give about 3 or 4 to one advantage (the bit you pull is 3 to 4 x as long as the bit that pulls the cable) and the lever at the other end, that operates the brakes, will give about the same - the part attached to the cable is usually about 4 - 5x as long as the part that moves the shoes. Ignoring friction you multiply the two to get the total advantage.

So my top-of-head guess earlier was a bit on the high side - total advantage will be around 20:1 through the system, giving 200kg for a 10kg pull. But the cable itself will only "see" about 1/4 of that, around 50kg tension for every 10kg of pull, because the rest is multiplied by the lever at the other end.

Samsonite1

Thank you, the revised figures seem good. My friend is building a car and reckoned total advantage on his handbrake assembly was 22:1, so seems realistic!

So then a 70kg tug on the handbrake will apply around 350kg of force on the cable which seems like it could damage it? I understand force and levers, but I do not really understand why some parts of the handbrake would not suffer from hard yanks (material stress limits) - could you explain that part?

Apologies for being too inquisitive and thanks for the answers.