The Affects of Wheel Offset and Suspension Performance
By: Shock Therapy (www.ShockTherapyst.com)
Tuning off road suspension on a daily basis allows us to drive many vehicles with many set ups and different environments. UTV’s, sand cars, super lite, Pro 4 and Trophy Truck are just a few. So it may be safe to say that we haven’t quite seen it all but surely we are pretty close. With the increase in UTV traffic at the shop there is one thing that we see a lot that is a serious issue people should know about. Wheel offset and tire height.
For the purpose of this subject we are going to talk about a Polaris XP1000 but these things translate to all cars and trucks. They just may be different numbers.
One of the first accessories that people purchase for their new RZR or any UTV for that matter are wheels and tires. Better traction, taller tire, better ride, cooler wheels and wider wheels are just some of the reasons which are all good. But, if you choose the wrong wheel offset for the tire height you will get some serious negative issues that most people are not prepared for. Put simply, if the wheel is too wide or the wheel offset places the tire wider by too much, it will negatively affect the front suspension geometry and cause problems. Some of these issues are:
- Excessive feed back into the steering wheel
- Steering tracking with any line in the road or trail, wandering
- Stiff steering feel, slow to respond
- Overall stiff, rough ride over all obstacles
- Premature wear on all front end and steering parts
How does this happen? Too much “scrub radius” in the front end geometry caused by wheel offset. What is “scrub radius”? Stand in front of your front tire and imagine a line running through the upper and lower ball joints of the spindle that continues to the ground. This imaginary line is called “king pin inclination”. This is also the line that your wheel and tire pivot on when you turn the wheel. Now imagine a line running through the centerline of the tire from top to bottom where the tire contacts the road. The distance between where these two lines where they contact the road is called “Scrub Radius”. We have made some CAD drawings of this to illustrate what we are talking about. These drawings show the 4130 boxed arms and spindle system on the Jagged X, SCORE and BITD car. Thanks to Cutting Edge for the boxed arm drawings. This is a completely STOCK geometry system depicted. In the first drawing we have a stock 5/1 wheel offset with a 28” tall tire. The scrub radius is shown at the bottom at .427”. This means there is less than half an inch between the king pin inclination line and the tire center line where the tire contacts the road. Shown here where the red and green lines end at the tire contact patch.
The smaller the scrub radius the less feedback to the wheel you get. The larger the scrub radius the more feedback and shock load you get. Some road feel in the steering wheel is a good thing. This can let your hands tell your mind what the car is doing. Too much scrub radius causes all of the problems I listed in the beginning.
How much scrub radius is just right and how much is too much? This is pretty tough to answer because every form of motorsport requires different amounts for each intended use. For instance, road racing, go carts or even the new Vette off the dealer lot use a lot of scrub. As much as 1.5” to 2.0”. This is fine if you don’t hit any rocks. Smooth road courses and lots of power steering and less than 3” of wheel travel can cover up the negatives and allow you to turn a bit better and feel more through the wheel in this situation. But, off road is a completely different animal. The last thing you want to do is feel a 6” rock or 10” ledge or 2 foot tall whoop every time you hit it and worry about losing a thumb to the steering wheel in the process. Additionally, taller, heavier wheels and tires used in off road for strength and for better ride quality place way more stress on the front end components and that stress needs to be managed. Too much scrub radius magnifies this stress exponentially. As the tire height and weight goes up, the scrub must go down. Industry norms are as follows:
- Sand cars 1/2 to 1” of scrub radius
- Class 1 off road, 1/2 to 3/4” of scrub radius
- Trophy Truck 1/4 to1/2” of scrub radius
Only three things can change scrub radius. One, spindle design which also means new upper and lower arm design. Since most people are not going to design a whole new front end for their RZR, this option really isn’t an option.
Two, wheel offset. The wider the wheel is the more scrub radius you can get. More importantly, the more wheel offset you get (less to the inside and more to the outside) the more scrub radius you get. Stock RZR 1000 front wheels are 5” inside and 1” outside. This is measured from the hub mounting flange or where the wheel bolts to the car. A 4/3 wheel has 4” inside and 3” of wheel outside of the hub flange.
Three, tire height. The taller the tire gets on a given front end and wheel offset the less scrub radius you get because the two lines (tire center line and king pin inclination) have a longer distance to intersect as the tire gets taller. The CAD drawing below shows a stock wheel at 5/1 with a 30” tire on it. The scrub radius has gone down from .427” with a stock tire to .343” with a 30” tire. This set up would be very smooth feeling. It would have a very low stress level on the front end parts and would never add to a rough ride. Road feel is less but by such a small amount it would be hard to notice.
This brings up the question “what is the right amount of scrub radius for a UTV”? The answer is purely subjective and open to debate but if you want our opinion we would say that no less than 1/4” and no more than 1”. The reason we say this is because the wheel and tire package on most UTV’s are comparably lite. Between 28lbs and 65lbs and not that tall. The loads developed by these are way under a Class 10, Class 1 or Trophy Truck. You can get away with a little more scrub (1”) than these other cars can. We can also say this because we have driven UTV’s with scrub numbers in this range and we would say anything in this range is fine. This means that a 5/1 or 5/2 wheel offset is great for tires from 28″ to 31″ tall on an XP. Just remember that 1” of scrub radius has more road feel in the wheel and more force applied to the front end. 1/4” of scrub has less of both but may not have enough road feel for some people. Factory is just under 1/2” at .427.
Now let’s take a look at how bad the scrub can be with some of the popular wheel and tire combinations out there. Below is a CAD drawing of a 4/3 offset wheel with a 30” tire. It has a 1.843” scrub radius! Wow that is a lot. This is the wheel and tire combination that we see the most problems with.
Customers complain of rough ride, wandering steering, arm pump and worn out front ends. A lot of people bring their cars to us hoping that our suspension kits will fix their issues not knowing that it isn’t the suspension’s fault. But, sadly the answer is no, out kits won’t fix their geometry problems. The suspension will work better and feel smoother but the underlying issue is still there and will continue to create problems.
Some of you may already have a 4/3 offset or more and wonder if going taller with the tire will help you. The answer is yes, but, not nearly enough. If you jump up to a 32” tall tire you will only lose .083 of an inch of scrub. The scrub radius only dropped down to 1.760”. Still way too much. As a matter of fact, you would have to run about a 50” tall tire to get it under an inch of scrub and all the way up to a 63” tall tire to get it back to stock scrub radius. The drawing below shows a 63” tall tire and .423” of scrub radius. Ridiculous I know.
Here are some combinations that work just fine and drive great. Below is a 28” tall tire (stock) on a 5/2 wheel. .927” of scrub. This is pushing it a bit close to the 1” max but this ok.
This is a 30” tire on a 5/2 wheel. This has .843” of scrub. Better than the stock tire and well within the “happy” range.
This one shows a 32” tire on a 5/2 wheel. It has .760” scrub. Very good combination. Clearly you would be just fine with any of these.
You may wonder why, the wheel company’s offer a 4/3 or even bigger offset wheel if there are so many issues with it. After all, they should know all about scrub radius right? Well, the reason we have heard from some in the industry is that the 4/3 offset was perfect for the RZR 900 front spindle. When the 1000 came out with a wider axle length in order to get more travel, the extra width went into the spindle design and into the wheel offset of 5/1. Since the wheel companies already had the molds and tooling for the 4/3 offset they continued to sell them while they developed a 5/1 or 5/2 offset. Good or bad, that was it.
It is also good to note that if you are duning only, you can run more offset and scrub radius than in an off road scenario. The reason is the dunes are pretty smooth with the exception of around camp so the excess feed back from rocks or ruts doesn’t exist in the dunes. The extra scrub radius will help with turning a bit as well. Also, wider can be more stable in the dunes but when you hit the chop and whoops you will feel that extra scrub radius in the wheel for sure. A lot of personal preference in the dunes.
In conclusion, the reason we have listed all this information is to help inform and educate. Because if you know why something acts the way it does you can make better decisions and more informed purchases. Any wheel and tire combo will work and drive. Just understand the difference in drive-ability and longevity that come with those choices no matter which way you go.
We hope this helps,
– Shock Therapy
For more articles and videos on these and other subjects go to Shocktherapyst.com
Thank you to Cutting Edge for the boxed arm drawings.