RC Crawler Steering
 

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:
http://i171.photobucket.com/albums/u...g/Steering.jpg

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.

Re: RC Crawler Steering
 

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*:
http://i171.photobucket.com/albums/u...g/Caster-2.jpg


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:
http://i171.photobucket.com/albums/u...g/P1050930.jpg
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:
http://i171.photobucket.com/albums/u...g/P1050932.jpg

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:
http://www.freepatentsonline.com/6532062-0-large.jpg

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.

Re: RC Crawler Steering
 

Scrub Radius Comparisons

Here is a stock XR10 setup (using stock c-hubs, knuckles and wheels) with a set of Boss Claws mounted:
http://i171.photobucket.com/albums/u...g/P1050919.jpg
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:
http://i171.photobucket.com/albums/u...g/P1050921.jpg

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:
http://i171.photobucket.com/albums/u...g/P1050925.jpg

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):
http://i171.photobucket.com/albums/u...g/P1050938.jpg

And here is a line drawn to show scrub radius:
http://i171.photobucket.com/albums/u...g/P1050937.jpg

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.

Re: RC Crawler Steering
 

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:
http://i171.photobucket.com/albums/u...g/P1050945.jpg

Here is a picture showing the caster angle:
http://i171.photobucket.com/albums/u...g/P1050940.jpg

The combination of the caster angle and the King Pin Inclination of 8* allows the steering to look like this at full lock:
http://i171.photobucket.com/albums/u...g/P1050943.jpg

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:
http://i171.photobucket.com/albums/u...g/P1050942.jpg

**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:
http://i171.photobucket.com/albums/u...g/P1050944.jpg




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)
http://i171.photobucket.com/albums/u...g/P1050943.jpg
(rear)
http://i171.photobucket.com/albums/u...g/P1050944.jpg

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.

Re: RC Crawler Steering
 

Save 4

Re: RC Crawler Steering
 

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.

Re: RC Crawler Steering
 

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

Re: RC Crawler Steering
 

Subscribed ."thumbsup"

Re: RC Crawler Steering
 

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

Re: RC Crawler Steering
 

good work !!! "thumbsup"

Re: RC Crawler Steering
 

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

Re: RC Crawler Steering
 

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

Re: RC Crawler Steering
 

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).

Re: RC Crawler Steering
 

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

Re: RC Crawler Steering
 

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"

Re: RC Crawler Steering
 

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

I'll be back.

Re: RC Crawler Steering
 

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

Re: RC Crawler Steering
 

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.

Re: RC Crawler Steering
 

VERY well done sir "thumbsup"

Re: RC Crawler Steering
 

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.

Re: RC Crawler Steering
 

I'll see what I can do Jeremy--I do have a hard time getting whats in my head into words vs. showing a customer with examples "thumbsup"

Re: RC Crawler Steering
 

Great write up Jeremy"thumbsup" Lots of helpful info. I'm subscribed for sure...anxiously awaiting the BTA steering discussions to show up so I can glean some more info. Also interested in the Rear steer setup options as you mentioned.
Thanks for your time creating this thread!

Re: RC Crawler Steering
 

very very very informative. needs to be stickied

Re: RC Crawler Steering
 

This is a great idea, putting it in all in one place like this"thumbsup"
scrub radius, king pin inclination, & zero ackermann are my stumbles, this will be great reading tomorrow night when i came back to work:mrgreen:
Awesome write up Jeremy well done"thumbsup"

Re: RC Crawler Steering
 

Tagged this one....

Thanks for shedding light on this very confusing subject... "thumbsup"

Re: RC Crawler Steering
 

thanks Jeremy its finally starting to sink in, the descriptions and pics are a big help."thumbsup""thumbsup"

Re: RC Crawler Steering
 

First, I'll apologize for the large pictures, I use fairly big monitors at work and home and forget that not everyone does as well.

Yes, I would say that absolutely both options have their advantage......in fact, I would venture to say that every change made to steering will have both positive and negative effects. That is one of the reasons I started this thread....so that we can discuss these options and folks can understand what these changes will do and how it will benefit them.

My understanding of Ackermann Angle comes from the 1:1 side. Having the tires following different arcs in a high traction setting (road) is very much necessary to minimize scrubbing. Anybody with a permanently locked rear end on their 1:1 will tell you about the tire chirping when on the street. Here is a picture that I found on Wikipedia that describes the need for correct Ackermann angle:
http://upload.wikimedia.org/wikipedi...nn_turning.svg

It is easy to see that the way to determine "correct" Ackermann angle is to adjust the angle of the knuckle so that both turn in arcs that have the same center....even if those arcs follow different paths (which they most certainly do).

I noticed that Wikipedia also had an image of a simple method on how to find the proper Ackermann angle for a vehicle:
http://upload.wikimedia.org/wikipedi...ple_design.svg

I am curious now how this method will apply to our vehicles. When I get home this evening, I plan to get out my berg and snap a few photos to see what the correct Ackermann angle should be.

Like you said, Zero Ackermann will force the outside tire to scrub and the truck will push against that tire causing it to slip, but the tires will turn at the same angle and the arc will be smaller which should result in a tighter turning radius. Correct Ackermann will allow the outside tire to follow a larger arc than the inside tire, but should minimize scrub and the rear of the vehicle wont tend push the front along in straight line.

I think that there are multiple differences between our crawlers and most 1:1 road vehicles. First, we are dealing with fully locked axles......no differentials....which means that the center of both front tires will spin at the same speed and that the rear is pushing the front equally from both sides of the vehicle. I also think that we are dealing with a setup that has less traction than a 1:1 road vehicle (this is just my opinion). How much the outside tire scrubs during turning will depend on the amount of traction between the tire and the rock.....the less traction, the less scrub.....more traction, more scrub.

Re: RC Crawler Steering
 

Haha....I feel your pain. I regularly answer very technical questions at my job and the hard part is describing it in a way that doesn't confuse the patient even more...:ror:

Not a problem. Like I said, my intentions with this thread were not only to help others, but gain more information for myself.

As for the BTA, I can certainly give more information.....what would you like to see?

Re: RC Crawler Steering
 

That brings up a good point.

When you run a setup that has a positive scrub radius (the kingpin line is on the inside of the tire), the tires will tend to toe out when turning. I read a good example of this. It was to take a piece of paper and put it on the table....this paper will simulate your tire. Now, take a pencil...this will simulate your steering linkage. Put the pencil (eraser facing down) in the dead center of the paper and push the tire forward. The "tire" should move only in a straight direction. Now, put the pencil toward the "inside" of the paper (what would be closer to the center of the vehicle) and push the paper.....you'll notice that it spins outward (toe out). Try the same but this time put the pencil toward the outside of the paper and push.....you'll notice that it spins inward (toe in). These are the effects of Positive Scrub Radius and Negative Scrub Radius, respectively.

Re: RC Crawler Steering
 

That's an excellent point I hadn't considered. The inside tire wants to roll slower. So even proper ackerman won't allow the tires to roll along scrub-free.

BTW, you mentioned the outside tire doing the scrubbing. In actuality I think it will be the tire with the least weight on it which scrubs. For example due to terrain unevenness. On an off-camber side hill when trying to turn uphill, the lower tire should have the bulk of the weight and the uphill (inside) tire will do the slipping.

On another note, here's a small point of contention. You previously tied ackerman to the draglink connection.

(you wrote "To achieve this, the point at which the drag link attaches to the knuckle must be in line...")

I think the draglink has no role in ackerman. I expect you meant tie-rod pivots. I couldn't see your Wiki sketches but I expect one shows the two steering arms with lines passing through the steering axis and tie-rod hole each side then converging at the center of the rear end. That's a typical descriptor for ackerman steering. The draglink can connect anywhere without affecting ackerman. Some people even attach it inboard along the tierod.

Re: RC Crawler Steering
 

As far as the scrub radius, I belive your doing the pencil and paper trick the wrong way same positions but twist the pencil. The paper (tire) pivots in the center when the pencil is in the center, but when it is to the edge the paper (tire) will travel in an arc. The problem with this is in different situations it can move the vehicle, because the wheel base is changing. Watched this at a comp a few weeks back. After the course was done, a guy was trying a crazy line but every time he tryed to turn the tires it moved the whole truck. The rear of the truck would slide over and fall off the rock.
Also if its traveling in an arc you are causing your servo to work harder. With these crawlers having locked diffs you are always going to have the inside and out side tire spinning the same speed causing one to loose traction for a second. With that being said a lot of these adjustments are going to be personal preferance.

Re: RC Crawler Steering
 

Ahh....yes, good catch. You are exactly correct. I meant "tie rod". I have gone back and fixed that. Yes, the position and length of the drag link does not change the Ackermann settings. In fact, on many 1:1s (for example, the TJ line of jeeps) a y-steering setup is used and the drag link is not connected to the knuckle at all, but instead connected to the tie rod. There are quite a few crawlers that run a similar setup. "thumbsup"

As for the scrubbing, that is a good point. It should be both tires that will scrub (especially if they have a non-zero scrub radius).

Just thinking about it, I cant tell which tire would slip more....the inside tire that has less downward force being applied, or the outside tire that is being pushed more from the rear.

This might be something that needs to be determined empirically rather than theoretically.

Re: RC Crawler Steering
 

It would seem to me the lower tire on a sidehill trying to turn up would be the one scrubbing the most, atleast it appears that way from the push generated by the rear. It always seems to be this tire thats losing the most traction and allowing the push to occur. Idk, maybe I'm seeing it wrong as well:roll:

Re: RC Crawler Steering
 

My guess is "it depends". On the slippage caused by steering angle differences, the heavy tire wins. On front slippage caused by rear push, both tires will want to scrub. We'll generally get various combinations of those.

All I know is I feel better about using zero ack now. Tire scrub seems unavoidable on these rigs.

Re: RC Crawler Steering
 

Actually not true.

On the Y setup the draglink goes from the steering box to the passenger knuckle. The tie rod connects from the draglink near the passenger knuckle to the driver knuckle. The down side to this setup is the possible varying toe due to suspension travel.

Re: RC Crawler Steering
 

Caster-- I have always had some caster in my rigs (7 degrees positive is what I always shoot for in 1:1 stuff I build) but not until the last few months I have really experimented with allot of caster. Some good benefits of more caster is the ability to actually turn sharper.

When you increase caster you are basically doing what a street bike does and turning on the edge of the tire more (you can simply turn a street bike by leaning a tire). Another nice aspect is being able to carve/pull yourself up steep inclines kinda similar to how scrub acts without the loss of power effort in steering due to the angle in which the tire pivots.

One down side is more wear on the outer edges of your tires and similar to scrub you might have some adverse affects where the whole rig will be moved. Another is like being able to carve up steep stuff it can also cause you to loose traction.

Re: RC Crawler Steering
 

Increased caster will help in foward motion, we all know that in crawling. But it will hurt in reverse, I know we all hate to reverse. The terminology is corect, but looking at racing sights for the benefits do no actualy apply to us. Those settups are for racing, mostly cars with one diff driving, maybe one diff with a locker. We are 4wheels locked, at least one diff locked at a time (moa). The game changes again with under/over drives. As for the y-link on the steering of a 1:1 that is mostly for clearence issues with the frame, and bump steer. We dont have that problem. Our steering box (servo) is on the axle.

Re: RC Crawler Steering
 

Sorry, I missed your post earlier.

I did not specify in my example earlier, but I was referring to a situation where the tire (paper) was going straight, not turning (which I believe you are referring to).

The points you mention are correct. A non-zero scrub radius (both positive and negative) will force the servo to work harder. And yes, we are dealing with an entire setup as a whole here so some effects might be caused by a combination of steering principles.

I also like that you mentioned when the steering would move the entire truck. IMO, that is one of the benefits to running noninclined kingpins on the rear of a front and rear steer crawler. You CAN move the truck back and forth, making it longer or shorter, to help reach out and grab a ledge.

Oh, and I agree.....it is personal preference. "thumbsup"

Ahh yes, you are correct. I swapped in the Currie steering setup into my TJ long ago and it has been a while since I've messed with it...:oops:

Ya know....I noticed this exact property on my front tires recently. I have been running the same boss claws on my 2.2 berg for over a year now and it had been a while since I changed the location or direction of the tires. I started to notice increased wear on them.....not on the outer edge, but on the inner edge. I knew that the caster is what caused it, but I was intrigued to see it on the inside more than the outside. I am sure there are plenty of factors that will determine which side wears faster, but I did find it interesting.

Re: RC Crawler Steering
 

Is this discussion on just the steering and setup of the steering, or also the whole dynamics of turning the crawler?

Re: RC Crawler Steering
 

We can discuss whatever you'd like. "thumbsup"