RC Crawler Steering

[JeremyH]

First, I’d like to thank Michelle at Krawler Konceptz for help with some of these graphics.

Second, if you’d like more in depth descriptions and lessons about vehicle dynamics, then check out Milliken’s “Race Car Vehicle Dynamics”:
Milliken Research Associates, Inc. -- "Race Car Vehicle Dynamics"

I haven’t been able to find a general thread about RC Crawler steering that had everything in one place, so I thought I’d go ahead and start one. I have read some VERY insightful posts from some extremely intelligent members of this board and I am hoping they will chime in here with more information.

I’ll start off by saying that I don’t claim to know all of this information and that I started this thread to get more insight into how all of these different steering adjustments add up to one package. I think this will help me and others figure out the best setup for their crawlers.

I have read some VERY insightful posts from some extremely intelligent members of this board and I am hoping they will chime in here with more information.

I hope this post is well thought out and doesn’t ramble too much….sometimes my fingers work a bit slower than my brain does. :lol:


Now, most of us all know the basics, but I will briefly go over them for those who might not.
Here is a picture of my super. I have labeled the basic parts of the crawler steering setup:

Now that we are past that, I would like to discuss each one in depth and how changes in it can affect the steering system of a crawler.


Servo Horn:
The longer the horn, the more throw you will get, but you’ll decrease force being applied to the wheels from the servo.
Basic physics: Torque= Force x distance (when the force is being applied at a 90* angle). So, as you increase the distance, the force must decrease.


Drag Link:
To best apply the torque provided by the servo to your wheels, your drag link should be at a 90* angle with the servo horn and parallel to the axle shafts (otherwise, some of the force is being applied in different directions and is simply being wasted). Adjust the length of the drag link to get proper throw from your servo horn.


Tie Rod:
The tie rod is probably the simplest device in this setup. It runs from knuckle to knuckle and should be parallel to the drag link. Make sure it doesn’t bind on the axle housing or differential and you are good to go. You can lengthen or shorten the tie rod to change the angle of toe of the steering (to be discussed later).


C-Hub:
The c-hub bolts to, or is part of, the axle housing. Its main function is to give a place for the knuckle to bolt to. It can be rocked backward (as you see in the picture above) to provide Positive Caster, or rocked forward to provide Negative Caster.


Kingpin:
The kingpin is a device, in the case of my super, a shoulder bolt, that attaches the knuckle to the c-hub. Sometimes there is a simple screw and a flanged bushing that perform this task.


Knuckle:
Now, we get to the most complex device of the steering system. The knuckle can have many options and performs many different functions. Slight changes in the knuckle position and manufacture can have a large effect on how your crawler steers. I have a feeling that most of this thread will be focused around these options and changes.

 

[JeremyH]

Toe:
Lengthen or shorten the tie rod to make the tires “bow-legged” (Toe Out) or “pigeon toed” (Toe In). Most street cars run with a slight toe in to get good tire wear and to help from keeping the tires from fighting one another as the vehicle drives down the road. This really should not matter too much on a crawler. I would guess that most crawlers run with zero toe, however, some might run with a bit of toe-in for tire clearance issues.


Caster:
Caster is the angle of the pivot line (created by the king pins) when viewed from the side of the tire with respect to a vertical line. Some crawlers allow the builder to rotate the c-hub to adjust the caster. Rock it backward (top kingpin toward the back of the truck) to gain positive caster or forward to gain negative caster. Most will either run a zero or positive caster angle. A positive caster angle will help center the servo and will help lay back the tires which is helpful when turning or digging uphill.
Here is a picture of the front of my super showing a positive caster angle of 22*:

Camber:
When looking at the tires from the front, the lean toward or away from the center of the axle. Most crawlers (and most 1:1s without adjustable ball joints) do not have the ability to actively adjust the camber.


Ackermann Steering Geometry:
In basic terms, this is the angle at which the front tires turn relative to the center of the turn radius. Since the outside tire turns through a different circle than the inside tire, Ackermann angle is used to keep the tires on a 1:1 from dragging along their path as the vehicle turns. On RC Crawlers, this is not a big factor and many builders will use knuckles that provide zero Ackermann angle. To achieve this, the point at which the tie rod attaches to the knuckle must be in line with the upper kingpin. See the picture below for an example of a zero Ackermann setup:

The yellow line indicates the Ackermann angle.

Here is a picture of the rear of my super showing that both tires turn at the same angle with a zero Ackermann setup:

A "correct" Ackermann steering setup will have a common center point of the circle that each tire follows during the entire steering cycle.


Scrub Radius:
Scrub radius is the distance on the ground from the center of tread of the tire to the point where a line drawn through the kingpins would touch the ground. Here is a diagram (not one that I made) that describes scrub radius:

Zero scrub radius occurs when a line drawn through the king pins intersects the tire centerline at the road surface. This setting requires the least amount of effort to turn the wheel. During turning, however, this setting can cause the tire to grab and fight itself as different forces are applied to equal widths of the tire contact patch (because parts of the tire are having to travel faster than other parts).

Positive scrub radius occurs when these two lines intersect at a point below the surface of the road. This setting allows the tires to push and pull the truck, essentially making the wheelbase longer. For larger values of scrub radius, this setting allows the tire to roll and turn easily during steering at lower speeds.

Negative scrub radius occurs when these two lines intersect at a point above the surface of the road. This setting will help stabilize a vehicle at higher speeds, as weight is not heavily transferred from one side to the other on uneven terrain.

Both Positive and Negative scrub radius have different effects on the vehicle when in motion (especially on a 1:1 vehicle and these effects will differ between front wheel and rear wheel drive vehicles). I believe that most of these effects wont make a big difference at the low speeds our crawlers travel. What we are most interested in is the effects that scrub radius has on the vehicle when turning.

Also, all of these settings will change according to tire roll and other dynamic factors.

Axial’s Brandon did a great job of describing Scrub Radius and it’s place in both 1:1 and RC crawling in his blog:
Brandon's Formula Offroad Build: Part 3. Insight into the AR60 Axle Design.

Scrub radius can be changed by changing the offset of the wheel or by changing the angle of the king pins (or a combination of both).


King Pin Inclination (also called Steering Axis Inclination):
King Pin Inclination is the angle that the king pins make with respect to a vertical line. The purpose of angling the king pins is to help reduce the scrub radius while being able to use a wheel without a deep offset. This is used on 1:1 vehicles because it is difficult to fit most typical Macpherson Strut type suspension components (similar to the coilover setup that we use on our crawlers) and braking components inside the wheel while using a zero degree KPI setup. Using a KPI setup will help reduce scrub radius while allowing the builder to use a wheel with more of a neutral offset. A KPI setup will change the height of one side of the suspension of a crawler while cycling the steering.

 

[JeremyH]

Scrub Radius Comparisons

Here is a stock XR10 setup (using stock c-hubs, knuckles and wheels) with a set of Boss Claws mounted:

You can see that the knuckles and angled king pins tuck nicely into the wheel and should provide a somewhat neutral scrub radius with a pivot point that is close to the centerline of the tire.

Here is the scrub radius on this truck:

You can see that it still has a positive scrub radius. This setup is looking good for scrub, however, the stock setup will contact the front lower control arms which will limit steering throw.

As a contrast, here is the rear of my super, which runs vertical king pins:

You can see here that the intersection of SAI (steering axis inclination) and the tire centerline are parallel so they will never intersect. The scrub distance from this would be measured from somewhere on the ground inside of the tire to the tire centerline.

Here is a shot of the king pins in my 2.2 berg (which runs VP 8* c-hub and hardware, bigron weights and is spaced out just enough to clear the countersunk hardware on my inner bead ring):

And here is a line drawn to show scrub radius:

As you can see, the scrub radius is NOT zero, but some positive value (as is the stock XR10 setup). I believe that the stock XR10 setup is closer to a zero scrub radius, but not by much.



I think the lesson learned here is that finding the perfect balance of wheel offset, knuckle or wheel weight, tire clearance, foam density (as it affects tire roll during turning), king pin inclination and caster is a delicate balance and not an easy task to get perfect. The setup I run in my 2.2 berg seems to be working well for me.

 

[JeremyH]

It was asked for me to post pictures of a Wraith steering setup. I will start with the steering on the front axle.

The front steering setup on my Wraith uses the Axial Wraith Universals, VP C-Hubs and Wraith Knuckles with Zero Ackermann knuckle arms. I have the caster set at the most positive caster setting (meaning that the hubs are rotated back and the screw is furthest forward in the stock slot). Here is a picture of the setup:

Here is a picture showing the caster angle:

The combination of the caster angle and the King Pin Inclination of 8* allows the steering to look like this at full lock:

You will notice that the inside tire is laid back quite a bit more than the outside tire. This is the 8* KPI at work! "thumbsup"


Now, we get to a whole new ballgame.........REAR STEER. :shock:

The rear steering on my Wraith includes Axial Wraith Universals, Axial C-hubs and Axial Wraith/XR knuckles.

I have read a bit about what to do with the caster on the rear axle and I am undecided which direction (positive or negative caster) would be most helpful. Because of this, I chose to set the c-hubs at a neutral setting (screw in the middle of the slot). Here is the resulting caster setting:

**Note** the lowers on the rear of my Wraith are just a hair too long which is rotating the axle down a tad and giving it a bit of caster (which is undesirable, IMO).

Here is a picture of how the tires sit a full steering lock:

Using the following two images, we can see the difference between the positive caster setting (on the front axle) and the neutral caster setting (on the rear axle):
(front)

(rear)

You can easily see the two differences between the two setups. In the front, the inside tire is much more laid over and the outside tire turns less sharp. On the rear, the inside tire remains vertical and the outside tire turns in an arc equal to the inside tire.



Now, one of the things I would like to accomplish with this thread is to start a discussion about the effects of caster on a rear axle on a crawler with rear steer.

 

[JeremyH]

Save 4

 

[johnnyh66]

Good timing, I was just reading up on Zero Ackermann... I believe I want to go VP Zero Ackermann on my scaller and standard Axial on my FOFF.

 

[JeremyH]

Good. As this thread progresses, I plan to put up additional images of different settings as well as make comparisons of the different settings.

 

[hellbounder]

Subscribed ."thumbsup"

 

[crawlhog]

First off nice write up, helps alot with the wording and the pics. If possible could you post a pic of how to have the C Hubs of your Wraith setup. Ive tried it a few different ways but am not sure whats the best. Right now I have it like the directions say but it just dosent seem right.
Thanks

 

[dude26]

good work !!! "thumbsup"

 

[JeremyH]

The post regarding scrub radius comparison was upgraded with info about my 2.2 berg. "thumbsup"

 

[Cheatin]

Yeah man, great info all in one place, needs a sticky.

 

[JeremyH]

First off nice write up, helps alot with the wording and the pics. If possible could you post a pic of how to have the C Hubs of your Wraith setup. Ive tried it a few different ways but am not sure whats the best. Right now I have it like the directions say but it just dosent seem right.
Thanks

I just updated the 4th post of this thread with pictures and information regarding both steering axles on my wraith (that have different steering setups).

 

[crash]

Nice job Jeremy. Its my pet peeve when folks don't know the proper wordage for steering parts "thumbsup"

 

[JeremyH]

Yeah man, great info all in one place, needs a sticky.

Nice job Jeremy. Its my pet peeve when folks don't know the proper wordage for steering parts "thumbsup"

Thanks guys.

Crash, yeah, it doesnt bug me, but one of the goals of this post was to give the basics. One of the other goals is to get more of the technical details out there and promote discussions about what works better on the different trucks on this board. I do know that you are not the only one it bugs.....last time I talked with Almighty Malach, he was telling me how bad it pisses him off to hear people say "chub" instead of "c-hub"...:ror:

BTW, you are one of the folks I was talking about in the very first post.....as someone who builds 1:1 rigs (for a living I believe), I think you can help provide some insight on some of these topics..."thumbsup"

 

[Szczerba]

Good thread. Looks like a good saturday morning read with some coffee.

I'll be back.

 

[Meatwad]

Subscribed to share at some point, its one of those things you know will come up later and wanna reference. Thanks for this "thumbsup"

 

[Hardline]

This is going to be useful. But do you think you could resize some of those pics and graphics? Gotta tell you on a laptop I have a hard time just seeing that sketch. And the text length matches the pics, so I have to scroll back and forth trying to read it.

Not such a big deal, but since this is destined to become a reference, why not get it in good shape from the get/go?

On the subject of ackerman, my understanding is:

Zero ackerman (uncorrected geometry) has the advantage of maximizing the steering angle on both sides. But forces one front or the other to scrub in a tight turn. A forced scrub can't be good for getting maximum adhesion to the rock.

Correct ackerman allows each front to follow it's proper path, thus eliminating a source of forced scrub and improving grip. But when one tire is all the way against the springs, or links or whatever stops it, the other tire isn't turned as far as it might be, so some turning angle is lost.

So both techniques have pros and cons. Is that how you see it?

EDIT: Now that I've posted to this thread, the pictures and dwg have resized and everything reads right. I wonder what that's all about? Can a thread reader do something to cancel out oversize pics? Maybe my browser is fubar.

 

[ittybitty]

VERY well done sir "thumbsup"

 

[crash]

I'll see what I can add to this.

Scrub Radius ---The more you increase the scrub radius(increase the distance from the center of the kingpin to the center of the tire) the more power you will need to turn (increased leverage force). One of the down sides to increasing this is when turning hard on traction less surfaces the likely hood of having the whole rig slide a bit is increased. Instead of the tire being pivoted(less scrub) its being pushed(more scrub) which can easy push the whole rig.