cocos2d iphone - Chipmunk Physics Positioning -

i've looked around quite bit , haven't been able find concise answer question. cocos2d game have integrated chipmunk physics engine. on initialization, setup boundaries of 'playing field' establishing series of bodies , static shapes follows:

- (void)initplayingfield {     // physics parameters     cgsize fieldsize = _model.fieldsize;     cgfloat radius = 2.0;     cgfloat elasticity = 0.3;     cgfloat friction = 1.0;      // bottom     cgpoint lowerleft = ccp(0, 0);     cgpoint lowerright = ccp(fieldsize.width, 0);     [self addstaticbodytospace:lowerleft finish:lowerright radius:radius elasticity:elasticity friction:friction];      // left     cgpoint topleft = ccp(0, fieldsize.height);     [self addstaticbodytospace:lowerleft finish:topleft radius:radius elasticity:elasticity friction:friction];      // right     cgpoint topright = ccp(fieldsize.width, fieldsize.height);     [self addstaticbodytospace:lowerright finish:topright radius:radius elasticity:elasticity friction:friction];      // top     [self addstaticbodytospace:topleft finish:topright radius:radius elasticity:elasticity friction:friction]; } 

this setup works, except 1 thing-- positions of boundaries appear off significant amount of pixels right. after initializing player, , sending him towards boundary walls, not bounce off edges of screen (as expect setting bounding box @ 0,0), rather number of pixels (maybe 30-50) inwards edges of screen. gives?

i thought issue came rendering player sprite, after running through debugger, looks good. there special protocols follow when using chipmunk's positions render sprites? need multiply sort of pixels_per_meter? matter if in sprite sheet? sprite batch node? here how set sprite position in view:

player *player = [[rootmodel sharedmodel] player]; self.playersprite.position = player.position; self.playersprite.rotation = player.rotation; 

and here how set position in model:

- (void)update:(cgfloat)dt {     self.position = self.body->p;     self.rotation = self.body->a; } 

(yes, know it's redundant store position because chipmunk inherently.)

also, bonus question for finished reading post. how might move body @ constant velocity in whatever direction facing, allowing turn left or right @ given time, never slow down?

what's shape of physics body? perhaps forgot consider making shape same size of sprite. if consider position, sprite able move halfway outside screen @ border because position @ center of sprite.

to move body @ constant velocity, set velocity while disabling friction (set 0?) , allowing rebound off of collisions impact speed (in box2d restitution = 1, don't know chipmunk).