Monday, December 29, 2014

Information Series Part 2. Roll Center Correction Ball Joints

Since I've been on the subject of roll center correction I figured why not keep the ball rolling and move on to another roll center correction part.  For part 2 of the series I will be discussing roll center correction ball joints.  A friend of mine brought this up with me when he read part 1.  After talking about it with him I feel like this is something that should be brought up before moving on to another subject as it's a simple modification to do and it's related to the previous subject.

Before reading part 2 I strongly urge you to read part 1 first.

Lets begin!

All a roll center correction ball joint is, is a ball joint with a longer "shank" on it.  Now the point of the longer shank is to move the pivot point (the actual ball portion) of the ball joint down further.  Moving this down further moves it farther away from the spindle and in turn helps bring the lower control arm to a more desired angle.  This helps correct roll center when lowering a vehicle.

Above I have drawn an example showing what is meant by a longer shank and what exactly a roll center correction ball joint looks like.  Now lets take a look at how this can be used.

Here's a comparison to show how a roll center correction ball joint works.  For the comparison I decided to use a roll center correction knuckle.  As you can see the ball joint effectively works the same as a roll center correction knuckle only its adding material to the ball joint instead.  Both setups would have the same roll center when set at the same height.

This all sounds great, replacing a ball joint is rather cheap and is a very easy job to do.  However, there are of course some limitations and some downsides to this option, you can of course also add too much.

Lets talk about this a little more in depth starting with some limitations.  The biggest issue is clearance.  With knuckles like the S-Chassis has the ball joint doesn't go straight down off the knuckle, it comes off at an angle.  If I remember correctly, on the S-Chassis it's about a 22 degree angle.  This means when you go longer with the ball joint it'll also move the pivot point of it farther out.  If gone far enough it'll run into the brake rotor, so you can only go so far with it.

Here is an example showing the clearance on my personal car.  I had issues with my setup hitting the brakes and was forced into spacing my brake rotors and calipers outwards in order for them to clear the ball joint.

Another clearance factor might be wheel clearance.  The lower you go with it the closer it'll get to your wheel and it will eventually run into it.  With that in mind, on cars with the outer tie rods set at the same height as the lower ball joint you'll have to factor the clearance for that as well.  This is another issue I've personally come across as well as a friend of mine.  The only option in those situations is run poor bump steer settings or get bigger wheels.

Next lets talk about their strength.  Ball joint shanks aren't exactly big and beefy, so when you go longer with them you start to run into a factor of how much load they can handle.  I'm not exactly well qualified to tell you how long you can go before this becomes an issue but I can tell you if you're only doing an inch or two or if you're buying an aftermarket ball joint from a company, you wont have anything to worry about here, so don't let this become something that you'll be bothered about.

Also with double wishbone style front suspension you'll have the same issue as if you were using a roll center correction knuckle, doing this will change the upper and lower control arm's angles in relation to each other which will in turn effect the cars camber change rate and other characteristics.  In fact when doing this you could make the angles of the arms bad enough where it would have an extremely negative effect on your roll center.  So if you're doing this with double wishbone, don't do a lot, because the way most cars are setup from the factory are pretty close to ideal.  After all, manufacturers don't just slap stuff together and call it good, they do a lot of engineering and testing before making something and I can assure you they know more about suspension than you do.

Hopefully I've covered enough on this subject for now.  There's surprisingly a lot to talk about when it comes to ball joints and I'm sure we could keep talking more and more about them, but for the time being I think this is a good stopping point for part 2 and roll center correction parts.

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