RC Crawler Steering

[crash]

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"

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"

 

[Mr.Joshua]

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!

 

[manalow88]

very very very informative. needs to be stickied

 

[prusik333]

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"

 

[Rckcrwlr]

Tagged this one....

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

 

[crawlhog]

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).

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

 

[JeremyH]

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?

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:

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:

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.

 

[JeremyH]

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"

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:

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!

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?

 

[JeremyH]

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.

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.

 

[Hardline]

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

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.

 

[ffro]

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.

 

[JeremyH]

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.

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.

 

[Erin]

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:

 

[Hardline]

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.

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.

 

[crash]

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"

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.

 

[crash]

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.

 

[ffro]

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.

 

[JeremyH]

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.

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"

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.

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...

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.

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.

 

[ffro]

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

 

[JeremyH]

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