Scrub radius...little clarification?

[Red coupe]

ok so sitting in suspention class yesterday talking about scrub radius, teacher talks about the intersecting point of kingpin inclination and a true vertical, but doesnt mention the referance point for this vertical line so I ask him and he anwers by re explaining that its the intersection of these two lines and the dangers of just changing wheel size/offset or lowering to much but doent realy answer, finaly he gives us its through the center of the tire, at the bottom were it meets the road, so....

just double checking to make sure I got it scrub radius is the distance between the spindle, and the intersection point of the king pin inclination angle and a verticle line drawn though the print of the tire?

[Exar-Kun]

its the distance between the centerline of the tire/wheel (determined by offset, width, spindles design..) and the intersection of the line drawn by the kingping inclination (determined by other factors such as suspension design, type, steerling linkage desnign, etc) and the ground.

theoretically, if you had zero scrub radious the force on the steering adis would be nil, enabling you to not need power steering and reducing the force input back to the hub, etc to a minimum.

there are a few ways to reduce the scrub radious-increase kingpin inclination (this assumes a positive scruib radious)...this isnt a good option because you want to keep it between 9 and 14 degrees IIRC for steering geometry issues.-Use a more positive offset wheel (bleh....theres dissadvantages to this to, and clearance issues)-reduce how far the spindle sitst the hub assembly outboard (yeah..good luck with that)

See..now you know why you want to keep your offsets and wheel widths (in 1337 JDM drifter speak) "wussy" or (in educated peoples opinions) 'propper' or close to factory spec as possible while still maintaing the advantages of wider/lower spect ratio tires (better turn in, greater cornering force at a given slip angle...stickier compounds).

I'll try and dig up an illustration sometime if you're still confused later, after I finish moving and reading my GMAT prep stuff...-Chet

[skylndrftr]

scrub radius is the distance between the physical center of the contact patch, and the point that would be created if you projected a line from the top suspension mount through the bottom mount until it intersects the ground plane it is a function mainly of caster and kingpin inclination

[Red coupe]

yeah, its mainly the use of the term "center line off the tire" that throws me off because in any situation were your camber <> 0 it can mean differnt things, as a line drawn directly though the tire wouldnt be verticle, and a vertical line through the center of the top of the tire would give a smaller scrub radius then a verticle drawn though the bottom of the tire when camber is negitive. To make matter even worse one of the slides that was incorrect and shown showed the vertical line drawn compleatly wrong (it wasnt though any center point of anything...)

Exar-Kun you would love my teacher he relates every thing back to why you shouldnt slam or raise your car with out procaution and why you need to watch tire size/offset....

[Exar-Kun]

Proof yet again, the more you know...the more you'll realise the guys who BUILT the car in the first place probably did a better job than you'd think :p

I don't mind the larger wheels, low ride heights..I mind what it does when you don't take into acount the 30 things that happen when you DO it-and don't correct for them...(99.9% of the population).

-Chet

[deezlins]

what should you do to correct all those things?

say i lowered the car like 1.5" and put on 17x8's all the way around or 17x8 front 17x9 rear, what would occur and how would you fix it?

sorry to get a little off topic, just tryin to learn
Modified by deezlins at 3:44 AM 2/21/2005

[Red coupe]

well as far as the lowering as ive been told 1.5 is about the max acceptable drop before you start messing things up...what is happening is the drop is that it is changing the angle the arms are at while the car is moving down the road, were the lower control arm would normaly be parallel with the ground now its at an angle. when the arm moves it moves in an arc, so as it moves up the end moves up, and inward as it circles its mounting point. if the arm is straight out moving up or down a little wont cause very much movement inward, but as the arm is angled the same amout of upward movement will move it in alot more(Kinda like piston dwell and how when your turning an engin over by hand it get hard then easy then hard as it passes TDC...

the offset thing is kinda like this(I didnt take this from a 240 website so "stock wheel" probably wouldnt apply and I wouldnt assume a 0 scrub radius)

see were the red line intersects the the black line is were the center line of your tire(and tire forces) intersect the axis your wheel turns around, if the two intersect right at the road surface the the force on the left and right sides of the tire is the same, if you move the wheel outward they intersect bellow the road and I belive the tire pulls right? that what Ive taken from it at least...

[deezlins]

cool, thanks for helping me understand this better. That picture helps understanding scrub radius.

so how do you know when the wheel width and offset is too much without actually measuring the angles and stuff, and how would you correct that if they were?