rob.ok

Anyone watch the video I posted. I've read up on this issue quite a bit, but I still learned a thing or two from that video. It's done really well for what it is.

-Rob

theblackpearl

but it's 25 minutes long lol, ok ok, i'll watch it now

edit: nice vid but very long winded, they should really get some cliffnotes or something, i think everything could have been said in a page or two of reading...

ZDan

It's not due to reduction of contact area. I'm sure there's some release agent in the mold for the pads, that would cause initial loss of grip, much like with brand brand new tires. Also have to consider if the new pad material is the same or compatible with the old. If not, there will be a change in performance until a layer of the new pad material is deposited on the rotor. For sure any loss of braking with new pads is going to be down to temporarily reduced coefficient and not reduced contact area.

ZDan

I'd like to talk to your physics teacher(s)...

Ff = Fn * c.f.

Frictional force = normal force times coefficient of friction.

Yeah, that is a simple model, but in the case of new pads on a worn rotor, at normal temps, it's going to hold. There may initially be a lower c.f. with new pads/old rotor, but once pad/rotor c.f. is established (usually only a couple of ~30mph stops for me with street pads), you'll get the same braking force per pedal effort well before you've got full pad/rotor contact.

theblackpearl

Hmm, yeah I'm an idiot and was thinking about a statics equation, this is defnitel not a static situation lol

darylcha

I watched it
Super educational vid. Kind of learned something new that I thought I always already knew about! Bedding in pads+rotors correctly is so important and yet so hard to do right?

ahrmike

ZD, This is the same theory as why do wide tires have better grip on asphalt, when technically only the compound should matter?

Its because mu, the coefficient of friction is dependent at a microscopic level on contact area.

The reason why for most purposes mu doesnt change is because of the way surfaces deform. Nothing is ever totally touching so if you put weight on it, The tips elastically deform. THIS deformation is the actual area of contact - which is why mu*normal force doesnt change.

Wide area of seeming "contact" but only a small area is actually contacting.

If we're talking about a really soft material (rubber) OR a strong clamping force (brakes) the contact area does start mattering...if the normal force is strong enough to plastically deform the entire surface area (or most of it) then increasing the surface area does actually increase friction.

Not sure if I'm being perfectly clear, but the rubber tire analogy makes sense, right?

ZDan

Actually, it's not the same, because wider tires don't necessarily give greater contact patch area. Contact patch area is going to approximately be the load on the tire divided by the inflation pressure (note the absence of tire size in this approximation). Sidewall and carcass stiffness will play a role as well. But contact patch area is not a strong function of tire width and in fact you can have wider/lower-profile tires that have *less* contact patch area from increased sidewall stiffness.

[QUOTE]The reason why for most purposes mu doesnt change is because of the way surfaces deform.

EngineerX

I thought it's "Bed-in"

http://www.stoptech.com/tech_info/wp_bedintheory.shtml

xb1

Unnecessary nit-picking. You has it.