amemiya

ok lets set the records straight her because there are alot of good idea's and in away everyone is right,


the 3mm apex seals are alot heavier..(think about marry-go-round when you go to the outside of it it trys to through you off) this causes better seal at high rpm because of sheer inersha pushing the apex seal up agianst the rotor housing. The contact patch is not such a big ishue as the force being used. They are infact stronger and will give you I little bit longer life for the begening tuner. they have there ups and downs it all comes down to how hard you drive you car and how much tunning mistakes you might make. You will have to make a sacerfice of engine longitivaty to get the stronger seal. The exact where increase I don't know but using synthetic may help I don't know of anyone who has documented this.

If you can aford it go ceramic but if you detonate hard an seal of any size or matieral will fail.....

rx7_ragtop

Actually 3mm seals don't seal as well. They will "float" at lower RPM than 2mm seals also, because they are heavier. The only reason for going to 3mm seals is either the rotors are worn beyond spec and HAVE to be milled, or VERY high (400+) hp applications where tuning might be a concern (and hence detonation.)

Brad

RX-7Impreza

i am just being a nitpick and this in no way matters but it isnt gravity it is inertia

Justin

amemiya

who needs seal at low rpm?????

amemiya

you know what I be talkin about

krackerx7

iTrader: (1)

im not trying to get in your little argument but with thicker seals you can run more power.

HUGH

Just thought I'd get my 2 bits in. When they installed my Hurley appex seals they used a dual spring setup, both were beafier than the stock units. This must be because of the heavier seal I guess.

scathcart

Argue physics all you want. Call me an idiot all you want.
Force of friction = coefficient of friction x normal force.
Try to explain it to me, I'm pretty sure I'll understand. The reason I'll stick to my explanation is because I have equations to back it up.
Your examply of velcro is absolutely rediculus. Put a piece of velcro on the wall. Now stick another piece of velcro to it. There is no force of gravity on the force perpendicular to the materials, so there is no friction. The two stick together because the fibres interlock, not due to friction.
You want a better example than velcro? Try this. Get a mass, say a block of wood 2" wide on one edge and 40 " wide in the other side. Now drag it across a table at a constant velocity on the 2" surface and the 40" surface using a pull scale. You'll get the same reading.
Sean Cathcart
Sorry dude, but you're wrong. Talk to anyone who knows about this stuff.

scathcart

Inertia is the wrong term as well. The force generated is caused by the spring and moreso by the centrifugal force created by rotatinal motion.
Sean Cathcart

scathcart

I agree. Centrifigal or centirifcal force doesn't exist. Centripetal force and centrifugal force are one and the same; interchangable terms.
Sean Cathcart

scathcart

Inertia is created by mass x velocity
p=mv
since the apex seal has no velocity perpendicular to the rotor housing, it has no inertia. It has inertia rotationally, but this does not effect apex seal force.

GTUsGUY

they aren't the same, one works in one way and the other in the opposite sense. Like a car making a curve, the force that makes the car turn is the centripetal force made by the front wheels(in the direction that are pointing) and the centrifugal force is the one that pulls the car to the outside of the turn.

the force that applies here more is centrifugal force. Because the movement of the rotor will make them try to go out, in a straight line in relations with the rotational shaft of the rotor. This macking them press them self against the housing.(this in high RPM) And if you visualize this, it will tell you that in some way they really have some type of perpendicular force against the housing.

That's what I believe they told me. I have no formula, because I don't remember them

scathcart

I agree. Its the same force that will push water to the bottom of a bucket if you swing it upside down, in a circle.
Centripetal and centrifugal force use the same equation. They are equal forces. They act on one another. They are interchangable terms b/c of Newton's law: for every action force there is an equal and opposite reaction force.
You seem to know what you're talking about. Glad there is another physics guy among us.
Sean Cathcart

RX-7Impreza

i dont even know why i try doing physics on this forum. no one ever gets it.

centrifugal force is a myth it does not exist. the idea behind centrifugal force is that there is a force pulling directly out from the center of a spinning object. this force does not exist. there is a force pulling in towards the center of it. in this case the centripetal force comes from the side housings holding the rotor housings in and thus retaining the seals. the force pushing the seals to the outside is in the form of inertia of the seal continually trying to accelerate at a 90 degree angle to the radius of the circle.

oh and about the part about the hooks holding it together and not friction. what do you think keeps the hooks from slipping off of each other, friction maybe.

with something as light as a wooden block you wouldnt be able to notice the difference. (yeah i remeber when i was in high school and we did that experiment) i dont know if you have ever worked construction (in fact i doubt it) but take a 1/2 inch piece of plywood 8x4. put it on a concrete basement (relatively even surface) now anyone who has pushed one across a floor knows that it is much easier to push it when it is on one end than when it lays flat.

it is rediculous to say that increased surface on the ground wont increase friction.

how can you even say that a mass that is moving doesnt have inertia?

you dont understand physics. will someone pleeeeeeeeease back me up on this. i dont want to get into another long argument with someone who doesnt understand physics

Justin

scathcart

A moving mass doesn't have inertia in relation to the rotor face. The velocity is perpendicular. Try reading my post. I said it has rotational inertia.
The coefficient of friction on the edge of a board is different than on the flat surface of a board. Try my experiment with a huge mass on top. Just try it. I'm not using HS physics here.
You say you are using physics but you have no eq's to back it up. Surface area has no effect on friction. If you knew physics, you would know that. Sorry dude.
Your friction example is still flawed, and you pointed it out. Velcro is a series of hooks. Pulling velcro apart tears the polymer hooks out of the fibre material. Hang a hook in a loop of rope in the ceiling. Friction holding it on? Nope.
For every action force, there is an equal and opposite reaction force. The centrifugal force is the reaction force to the centripetal force. If it didn't exist, when you held onto a mass and swung your arm in a cricle, your arm would fly off.
Sean Cathcart

scathcart

I find it funny that you're saying I don't know physics. I am a physics major in univeristy. I am the same age as you are, you can't have any more chance to learn than I have. I think I have more knowledge in this area since I am applying physics, and you're only using everyday situations as an argument. Prove me wrong when you come up with actual calculations to prove your claims.
Sean Cathcart

pweizman

scathcart is correct...as much as it seems wrong...surface area has no effect on Friction. All that matters is the weight, and the coefficant of friction between the two materials. It seems backward...but college and empirical data has taught me otherwise.

However, Impreza is correct that the 3mm seals are stronger....mainly because of the increase in thickness. This equals more strength. The 3mm seals will resist failure from detonation better than a 2mm seal. This is entirely due to the difference in thickness....and probably a bit to do with the actual modulus of the material.

If you want a good write up, go check out Pineapple racing's page.

http://www.pineappleracing.com/TechLinks.html

I trust Rob's opinion totally when it comes to rotary engines.

Later,
Patrick

Cheers!

Dude hate to burst your bubble but mass*velocity is not inertia. Mass * velocity gives you linear momentum.

Angular momentum is defined by the cross product of the radial arm *cross* mass*velocity vector

Futhermore, interia is defined as the integral(z^2)dA where dA is the change in area, which = bdZ assuming the equalateral triangle has a side lengths = b and height = h.

the moment of interia through the centre of the triabgle is then I = (bh^3)/36

btw i'm assuming a triangle with no thickness... ( i know its not like this in reality but makes calc easier)

So i think you might have some stuff mised up guys

btw you should think of the rotors as like blades on a propeller air plane...

F=ma! force = mass * aacceleration

accel = angular velocity * radius

and the greater the angular the velocity the faster the rotor/blades spin. So when u spin at high RPM you will generate tremendous amounts of force at the tip of the rotor/blade.

That is why the rotor/blades of airplanes need to be made out of high yeild strength metallic alloys such as aluminum (7075).

aluminum is a good choice because the density is low, therefore mass is lower, therefore less force is exerted on teh rotor!



where

amemiya

jesus christ everytime we start talking about something you guys take the RX7forum and turn it into the god damn physicsforum. I am glad you guys are trying to keep us up to date on terms but everyone knows for the most part what we are talking about.. jeez have you read the nos discousion it was the same thing.

SOLORX7

I'll add my 2 cents ,I have had more problems with 2 mm seal than 3 mm seals,the idea of 3 mm seals spreading the load over a larger area is spot on,think of it as an elefant stepping on your foot,with a sandal on it spreads the load over a larger area,with a pair of high heels on the weight is spread over a much smaller point and it would punch right in. crude example, but the same idea.

amemiya

man that gave me a mental picture thanks:rofl

RX-7Impreza

ok, starting to doubt myself.

if surface area doesnt matter, then why do we get larger tires. i know that it isnt seans idea of catching more of the rough spots.

you are still wrong about centrifugal force. what holds your arm on is centripital force. centrifugal force is supposed to be a force pulling directly away from you. the only force is one moving perpindicular to your arm. inertia.

if you dont understand that i dont know how you can be a physics major. i have known that centrifugal force doesnt exist since the 6th grade.

i posted the thing about inertia not existing because you werent clear in your explanation. you just blatantly said it has none.

Justin

pweizman

The big difference between tires and most other things sliding over each other is this.... When we say that area doesn't matter, it is strictly in planar motion to which this applies...or better yet, in situations where the two surfaces are non deformable...

Tires are deformable. They kind of "wrap" around the road. Pavement has many bumps and ridges...and the tire can deform and wrap around those bumps and ridges. This creates more traction, and better grip because the tire can react forces against these ridges and bumps.

I am not sure what the argument is with Centripetal forces... When swinging a string with a rock attached to the end...the force acting directly toward you is the centripetal force.

Later,
Patrick

David88vert

Centripetal Force
http://hyperphysics.phy-astr.gsu.edu/hbase/cf.html

Rotational Motion and Centrifugal Force
http://observe.ivv.nasa.gov/nasa/spa...rifugal2a.html

Inertia
http://www.calphysics.org/inertia.html

RX-7Impreza

yeah i was just wondering why rock crawlers deflate their tires in order to get more grip on the rocks. they deflate them so that the contact patch is larger. anyone????

alright i will say this again. we all once thought that there was a force called centrifugal. i read your nasa thing and this is what i have to say.

Centrifugal force- a force pulling out away from the center of a spinning object parallel to the string holding a spinning rock.

now if you spin that rock around and cut the string. centrifugal force says that it will travel in a trajectory directly away from the center. you can try this at home. it wont work.

when you cut the string the object follows a trajectory which is tangent to the circle it is spinning in. or at a 90 degree angle to the string. in order for you to sling a rock at someone directly in front of you you have to release it when it is perpendicular to the line between you and him.

yes centrifugal force has a definition, but that does not mean it exists.

I hope to god you understand.

Justin