Does offset affect suspension goemetry???
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From: Milwaukee
Does offset affect suspension goemetry???
Alright, I got new wheels/tires for my 08' f250(Diamo17k;M/T 305/65/17 ATZ) from 4wheelparts. Now, I was adamant in him verifying that these would fit properly. I have not put them on yet, Winter here. Now I read the article below which states different offsets will cause problems. True??? The offset on my wheels is -12, backspace is 5.43". I can't find the factory wheel offset, at present. Please give opinions on this. Mind you, I've changed rim/tire width too. Thanks, Lance
The whole suspension geometry is designed around the wheel offset. In somewhat simplistic terms, once the suspension designer decides what kind of wheel bearing/hub assembly they will use, the offset is calculated to spread the load that the wheel supports evenly across the wheel bearing. The load is not only the weight of the car, but also cornering loads. As was mentioned by others, too little offset will shift the load to the outside of the wheel bearing, and too much offset will shift the load to the inside of the wheel bearing, causing premature failure. But bearing loading/wear is only one of many variables that are affected by offset. Castor is the angle through the suspension kingpin bearings. These are the bearings that the wheel/tire turns on when you move the wheel left or right. The angle through the kingpins (Castor) is designed to end up on the ground in the middle of the tire's contact patch. If you move the offset very far this scrub radius will be adversly affected. This will make steering more difficult, and less responsive because you won't be using all of the tire's contact patch to steer with. The length of steering components like A arms also depend on wheel offset, so that camber changes as the wheel/tire moves up and down rotate around the center of the wheel/tire. All of these features of suspension design rely on where the centerline of the wheel/tire is. So moving that point very far will not only adversly affect wheel bearing life, but wear on other suspension components as well. I have read that you can get away with + or - 5mm from the factory designed offset before you start to cause damage and adversly affect the handling of your car.
The whole suspension geometry is designed around the wheel offset. In somewhat simplistic terms, once the suspension designer decides what kind of wheel bearing/hub assembly they will use, the offset is calculated to spread the load that the wheel supports evenly across the wheel bearing. The load is not only the weight of the car, but also cornering loads. As was mentioned by others, too little offset will shift the load to the outside of the wheel bearing, and too much offset will shift the load to the inside of the wheel bearing, causing premature failure. But bearing loading/wear is only one of many variables that are affected by offset. Castor is the angle through the suspension kingpin bearings. These are the bearings that the wheel/tire turns on when you move the wheel left or right. The angle through the kingpins (Castor) is designed to end up on the ground in the middle of the tire's contact patch. If you move the offset very far this scrub radius will be adversly affected. This will make steering more difficult, and less responsive because you won't be using all of the tire's contact patch to steer with. The length of steering components like A arms also depend on wheel offset, so that camber changes as the wheel/tire moves up and down rotate around the center of the wheel/tire. All of these features of suspension design rely on where the centerline of the wheel/tire is. So moving that point very far will not only adversly affect wheel bearing life, but wear on other suspension components as well. I have read that you can get away with + or - 5mm from the factory designed offset before you start to cause damage and adversly affect the handling of your car.
Last edited by ommegang; Jan 24, 2009 at 02:11 PM.
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The article is correct in saying that changing the offset from that of factory will change the loads seen by the bearing during operation. Now keep in mind that if you change wheels you can't keep the original backspacing. You need to keep the correct ratio as stock. For example if you take your stock 7" wide rim and the WMS is set at 40/60 (40% of the 7" is inside the WMS, and 60% is outside of it) you need to maintain the 40/60 on your new rims. One other thing to keep in mind is that when going to larger wheels and/or tires your rotating mass gets exponentially greater. This takes a toll on bearing and suspension component life as well.