Re: Brake design question - Ray? Anyone?

Danny G. writes:

B-but...if theparking brake were incorporated into the rear service brakes in the way that Ray describes, how would it be independent?

Geoff

I don't think so Ray...far be it for me to argue with an expert like you but I think you're wrong here. I've had a lot of very technical theory studies in the Air Force to do with 'Weight and Balance' of aircraft and 'moment arm' etc. I think that the only factors governing this 'transfer' thing is the vertical 'moment arm' of both the 'weight above the axle' (how much weight and how high above the front axle) and the horizontal moment arm (how much and how far behind the front axle).

The only thing that the suspension contributes is it's height, it's firmness has almost no effect.

There are basically 2 types of parking brakes for vehicles with rear disc brakes. Some older Toyotas used a cable mechanism that made the rear calipers squeeze the rotor. Most late model Toyotas with rear disc brakes have a drum brake inside the crown of the rotor that is cable actuated and does not rely on the caliper to brake. In other words, the braking surface of the drum normally gets little or no wear because the parking brake is usually applied when the vehicle is stationary.

Gord, I think we're saying the same thing a little differently. The "moment arm" (I call it the "moment of movement") is what is affected by suspension stiffness and travel. During braking, soft springs with a lot of travel will allow the rear suspension to rise and the front suspension to dive more than stiffer springs with limited travel. Raising of the rear of the vehicle affects the moment arm and balance, putting more weight on the front axle and reducing the weight on the rear axle more than the effects of inertia alone. In other words, it is difficult to overcome the weight transfer that occurs from inertial, but you can overcome the weight transfer from the moment arm by keeping the body and payload closer to the rear axle instead of letting it become elevated.

Imagine crazy Danny (not me some other Danny. LOL) Goes out to his car and cuts off all four disk brake calipers with a cutting torch. Parking brake would still work.

In real life I would imagine none of the parking brake types really are worth talking about unless your the type of driver that yanks them at 60 to spin a uturn in the middle of the road.

True but it's a miniscule amount in the big picture...

I see it as a very small amount...think of it like this, if the springs were soft 'enough' then the car body could stand upright over the front axle and 'all' the weight would be transferred to the front axle...BUT...how much travel are we talking about?...maybe a foot?...what's that in angular movement?...maybe a few degrees?...wouldn't matter a hill of beans would it?...and we need to think of the 'difference' in movement between soft suspension and firm suspension...doesn't seem to me to be very significant...

I see your point - a foot of travel on a 15 or 20 foot long vehicle would results in about 3 or 4 degrees of angular movement at the front of the vehicle, I think my previous explanation was poor and now that I think about it, I'm not even sure why the effect of jacking up the rear of the vehicle has such a great effect on the front brakes. There is a formula I'm missing here. I do know that a little bit of weight transfer can make a big difference in handling and braking - that is why the little bit of weight jacking on Indy cars can make a difference in how the car handles.

Ah I think I see were your coming from. I've been looking this from the how it effects handling side of the fence.

Sounds right to me Ray...

BTW, let me say that it's always a pleasure to have a technical discussion with you sir...'nuff sed... :)

effects handling side of the fence.

Ok Danny, seeing as how I know just about zip about handling (despite having worn out two RX-7's) I'll not comment further!...

Same back at ya! ;-)