The affects of Wheel offset and suspension performance

Shock Therapy

Suspension Tuning
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, XP 1000 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.

5-1-30-tire.jpg


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.

4-3-30-tire.jpg


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.

4-3-63-tire.jpg


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.

5-2-28-tire.jpg


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.

5-2-30-tire.jpg


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.

5-2-32-tire.jpg


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 got to Shocktherapyst.com
Thank you to Cutting Edge for the boxed arm drawings.
 
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Well written. Besides the effects listed above, widening the car with wheels also has the effect of softening the suspension. Basically the suspension becomes a longer lever arm giving it more leverage on the shock.
 
Well written. Besides the effects listed above, widening the car with wheels also has the effect of softening the suspension. Basically the suspension becomes a longer lever arm giving it more leverage on the shock.

I see what you are saying and it makes sense to me if we were talking a 4+ LT arm but I was under the impression that excessive scrub ratio essentially cause the force to be applied to the suspension from the wrong angle per se and causes the suspension to be harsh. Although wider wheels do have more leverage, it seems the leverage would be more applied to wheel bearings and ball joints than it would the suspension. Or maybe I am misunderstanding which is very possible. lol Either way, I'd love to learn some more on the topic.
 
Very good info. WHAT are most guys running, As far as offsets? I see Alot of 5+2 and very few 4+3.

On an XP, the best wheel offset up front is a 5/1 or 5/2. Probably 75% of the aftermarket wheels on XP's that we see here are 4/3's. It is starting to lessen as time and more wheel choices are available though.
 
I see what you are saying and it makes sense to me if we were talking a 4+ LT arm but I was under the impression that excessive scrub ratio essentially cause the force to be applied to the suspension from the wrong angle per se and causes the suspension to be harsh. Although wider wheels do have more leverage, it seems the leverage would be more applied to wheel bearings and ball joints than it would the suspension. Or maybe I am misunderstanding which is very possible. lol Either way, I'd love to learn some more on the topic.


You are correct with respect to leverage and the forces added to the system due to width. But, when it comes to scrub radius, the added force and leverage is applied to the steering components mostly and carries into the rest of the parts. As you increase scrub radius via wheel width or offset or both (long travel does not increase scrub radius) you are essentially giving the rock or tree or whoop more leverage to TURN the wheel in spite of you. This added leverage, or force, shoots into the steering components first. Since the tire was (for a millisecond) forced to turn slightly instead of accept the bump and go upward, all of those forces that went in to the steering manifest themselves into the arm mounts, bushings and ball joints and you feel this in the car as a jolt or sharp hit. Also, you feel this in the steering wheel when the car wants to follow the grooves in the road even though you are holding the wheel straight, or as the hits get larger the wheel wants to rip out of your hand instead of remain straight (power steering can cover this feeling up). Wider wheels may add more stress to the bearings and ball joints but added scrub due to wheel OFFSET will affect steering feel, longevity, suspension component life and overall driver feel and feedback. You know how a car with a ton of bump steer in the geometry will wander especially when you compress the suspension and it will wear out steering components? This is similar but worse on the rest of the parts too. Hope that helps.
 
You are correct with respect to leverage and the forces added to the system due to width. But, when it comes to scrub radius, the added force and leverage is applied to the steering components mostly and carries into the rest of the parts. As you increase scrub radius via wheel width or offset or both (long travel does not increase scrub radius) you are essentially giving the rock or tree or whoop more leverage to TURN the wheel in spite of you. This added leverage, or force, shoots into the steering components first. Since the tire was (for a millisecond) forced to turn slightly instead of accept the bump and go upward, all of those forces that went in to the steering manifest themselves into the arm mounts, bushings and ball joints and you feel this in the car as a jolt or sharp hit. Also, you feel this in the steering wheel when the car wants to follow the grooves in the road even though you are holding the wheel straight, or as the hits get larger the wheel wants to rip out of your hand instead of remain straight (power steering can cover this feeling up). Wider wheels may add more stress to the bearings and ball joints but added scrub due to wheel OFFSET will affect steering feel, longevity, suspension component life and overall driver feel and feedback. You know how a car with a ton of bump steer in the geometry will wander especially when you compress the suspension and it will wear out steering components? This is similar but worse on the rest of the parts too. Hope that helps.
Absolutely...that makes sense to me. Thanks for the clarification.

One of these days I am going to come see you and have you rework my car. Considering I am only about 30 minutes from you will you guys help me get it dialed in after the install? Rumor has you have a test track on your property?
 
Absolutely...that makes sense to me. Thanks for the clarification.

One of these days I am going to come see you and have you rework my car. Considering I am only about 30 minutes from you will you guys help me get it dialed in after the install? Rumor has you have a test track on your property?


You are correct on both accounts. We are on the outside of town for one reason, so we can test 10 times a day without having to load up and make a trip. When your ready, we will dial you right in. Have a good one.
 
I see what you are saying and it makes sense to me if we were talking a 4+ LT arm but I was under the impression that excessive scrub ratio essentially cause the force to be applied to the suspension from the wrong angle per se and causes the suspension to be harsh. Although wider wheels do have more leverage, it seems the leverage would be more applied to wheel bearings and ball joints than it would the suspension. Or maybe I am misunderstanding which is very possible. lol Either way, I'd love to learn some more on the topic.

There is two different things going on here and I was simply stating another consequence of wider wheels. Yes, the increase scrub puts additional strains on bearings, ball joints and other components as well as induces additional forces into the steering as others have stated.

But widening the track does soften the suspension on an A-ARM suspension. I should have clarified that. And yes, the rear of an XP behaves like an a-arm even though it looks more like a trailing arm. What we are looking at here is the ratio between the distances from the a arm pivot to the shock and from the shock to the center of the tire. As the length of the shock to tire centerline grows in proportion to the mount to shock distance, the teeter-totter starts to favor the the force a bump introduces over the force the shock spring can produce. Along that same note, wider tires with the same scrub radius will also induce these same forces to both the shocks and the bearings, ball joints and steering because bumps are not always along the center of the tire!
 
I don't know who you are so if I am talking to a suspension guru bear with me. I'm not trying to be argumentative. The wheel applies the force to the hub and spindle which is in the same place regardless of what offset wheel you are running. I don't see how a high offset wheel can add more leverage on the suspension when it's force is pushing on suspension in the exact same spot (hub and spindle) as the lower offset wheel? Justin or Ernie, can one of you jump back in here?
 
I don't know who you are so if I am talking to a suspension guru bear with me. I'm not trying to be argumentative. The wheel applies the force to the hub and spindle which is in the same place regardless of what offset wheel you are running. I don't see how a high offset wheel can add more leverage on the suspension when it's force is pushing on suspension in the exact same spot (hub and spindle) as the lower offset wheel? Justin or Ernie, can one of you jump back in here?

Yes its applying pressure to the same spot but with a longer lever. It has more leverage on everything including the shock. If you look at the green line on the pics above the farther away it is from the shock mount the more leverage.
 
A larger offset wheel is like a cheater bar on the end of the suspension. Yes its connected to the same place but that does not matter. The tire and wheel are part of the whole suspension system.
 
A larger offset wheel is like a cheater bar on the end of the suspension. Yes its connected to the same place but that does not matter. The tire and wheel are part of the whole suspension system.

Yes its applying pressure to the same spot but with a longer lever. It has more leverage on everything including the shock. If you look at the green line on the pics above the farther away it is from the shock mount the more leverage.

I don't know who you are so if I am talking to a suspension guru bear with me. I'm not trying to be argumentative. The wheel applies the force to the hub and spindle which is in the same place regardless of what offset wheel you are running. I don't see how a high offset wheel can add more leverage on the suspension when it's force is pushing on suspension in the exact same spot (hub and spindle) as the lower offset wheel? Justin or Ernie, can one of you jump back in here?

You guys are all correct. Any time you add length to an A arm system (wheel, arm, spacers) and keep the shock to frame pivot distance the same you will add leverage to the system and make it feel softer. Think in extreme terms when thinking about the wheel width. What if it was a 40" wide wheel and it made the RZR twice as wide but you didn't change the suspension at all. Now hit a bump or rock with the outside of the tire. The shock will clearly have an exponential harder job to control that.
With respect to the article and scrub radius. Imagine you are standing straight and as stiff as you can. I came up and grabbed your shoulders and try to twist you in a circle, spinning you on your feet. It would be a decent amount of work for me to spin you and slide your feet across the ground. Now, hold a 10 foot long bar across your shoulders and try not to spin. I now grab the bar on the end (10 foot from you) and try to spin you in a circle on your feet, only by pushing on the end of the bar. I think I could probably spin you just applying light pressure to the bar with one hand only. This is what excessive scrub radius does to the spindle and steering. YOU are the spindle facing forward, trying not to turn when you hit something (me). When you add the leverage of more wheel offset (added scrub radius) it is like me trying to turn you when you have that 10 ft bar. Easier for me (the bump or rock) way harder for you to stop it (spindle, steering, tie rods, arm mounts) Way more stress.
I hope that helps a bit and didn't just make it more confusing.
 
Thanks for the explanations guys. I've got a buddy with 4+3 wheels on his 1000. I'm gonna have to drive it back to back with mine over the same rough section of road.
 
At the off road expo this weekend I talked to the Raceline guys and they said that Method doesn't measure back spacing correctly and that the Raceline 4+3 wheels aren't that much wider than stock and pretty close to the Method 5+2. Anybody know if there is any truth in that?
 
So if I read this correctly I did good by going with a 5+2 wheel on my jagged x? Or is a 4+3 better for the 900's? That keeps me close to stock right if not better with 30" tires.


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