The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - cupped lower a-arm?

*lakeroadster* · 01-09-2011, 04:19 PM

Hottrucks post gets us considering the worst case scenario of having a deflated or blown air bag.

Basically we are discussing "Scrub Line". The following is from a recent Hot Rod magazine article discussing if the government can "outlaw Hotrodding"

Pennsylvania has developed a standard applicable to street rods, specially constructed, or reconstructed vehicles that uses what's called a scrub line. (The state enforces a bumper height law for all other passenger vehicles.) A scrub line is an imaginary surface created if lines were drawn from the bottom of the wheel rim on one side of the car to the bottom of the tire on the other side. Lines drawn from both sides using a taut string create an X under the vehicle's suspension. A suspension or chassis component, except exhaust systems and sheetmetal, may not hang below the top portion of this X.

Think of it like this. What happens if you have a front tire blow out? If your lower a-arm hangs lower than the wheel rim, and you have a tire blow out, then the a-arm is shoveling up pavement, it turns into an anchor, if you will.

I saw a real world application of this about 15 years ago. I drove up on a guy in a "patina'd" 47 Ford whose car was literally facing the wrong direction up on a sidewalk. His front right tire went flat, and the car had stepped lower a-arms. When the tire went flat the lower a-arm dug into the asphalt and it spun him up onto the sidewalk, in his own words "in an instant". You could see where the a-arm had dug out the asphalt.

There is a lot of thought that goes into the design of stock suspensions by Engineers that go through these scenarios. Scrub lines are one of those designed in features.

I have never understood why anyone would intentionally build a suspension that when totally aired out -and/or- when a tire goes flat, that the suspension would rub the road. This leaves you with little, if any, control over the steering of the vehicle. I guess I am more of a function drives form kind of guy, with safety taking job one priority when making modifications. Life is to short to trust an air bag or a tire with you or your loved ones life.

Something to think about?

01-09-2011, 04:19 PM	#43
lakeroadster Account Suspended Join Date: Sep 2009 Location: High Plains of Colorado Posts: 2,485	Re: cupped lower a-arm? Hottrucks post gets us considering the worst case scenario of having a deflated or blown air bag. Basically we are discussing "Scrub Line". The following is from a recent Hot Rod magazine article discussing if the government can "outlaw Hotrodding" Pennsylvania has developed a standard applicable to street rods, specially constructed, or reconstructed vehicles that uses what's called a scrub line. (The state enforces a bumper height law for all other passenger vehicles.) A scrub line is an imaginary surface created if lines were drawn from the bottom of the wheel rim on one side of the car to the bottom of the tire on the other side. Lines drawn from both sides using a taut string create an X under the vehicle's suspension. A suspension or chassis component, except exhaust systems and sheetmetal, may not hang below the top portion of this X. Think of it like this. What happens if you have a front tire blow out? If your lower a-arm hangs lower than the wheel rim, and you have a tire blow out, then the a-arm is shoveling up pavement, it turns into an anchor, if you will. I saw a real world application of this about 15 years ago. I drove up on a guy in a "patina'd" 47 Ford whose car was literally facing the wrong direction up on a sidewalk. His front right tire went flat, and the car had stepped lower a-arms. When the tire went flat the lower a-arm dug into the asphalt and it spun him up onto the sidewalk, in his own words "in an instant". You could see where the a-arm had dug out the asphalt. There is a lot of thought that goes into the design of stock suspensions by Engineers that go through these scenarios. Scrub lines are one of those designed in features. I have never understood why anyone would intentionally build a suspension that when totally aired out -and/or- when a tire goes flat, that the suspension would rub the road. This leaves you with little, if any, control over the steering of the vehicle. I guess I am more of a function drives form kind of guy, with safety taking job one priority when making modifications. Life is to short to trust an air bag or a tire with you or your loved ones life. Something to think about? Attached Images Last edited by lakeroadster; 01-09-2011 at 06:36 PM.