RC Crawler Steering

[crash]

Originally Posted by JeremyH

We can discuss whatever you'd like.

Awsome---is bacon better really,lol---just kidding...


All of my opinions are based off the crawler stuff and I know allot of it does not pass over (settings) to high speed stuff....

[JeremyH]

Originally Posted by crash

All of my opinions are based off the crawler stuff and I know allot of it does not pass over (settings) to high speed stuff....

Yes, as I have been reading more about this topic, I realize that most information is related to high speed situations or daily driver situations.....it is not easy to separate what matters to our crawlers and what doesn't.

[CREEPERBOB]

Damn Jeremy, Impressed again my friend , keep the jewels coming.

[ffro]

Originally Posted by JeremyH

Yes, as I have been reading more about this topic, I realize that most information is related to high speed situations or daily driver situations.....it is not easy to separate what matters to our crawlers and what doesn't.

same here, just like thinking out loud. The reason I asked about dynamics is that the relation of the axle widths can help turning radius. Does anyone think about this? Also say in in a comp you use reverse 5% of the time, do you set caster where it works 95% better going forward? Or 50/50..... Or like a daily? Sorry guys at work in a large room by myself, get to thinking about something ............and it sticks!

[opek]

Originally Posted by JeremyH

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.

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.

Front and rear antisquat as well as torque twist play a huge roll in this, as I have experienced in my rig. With a low antisquat value and lots of throttle my AX10 would turn right sharper than it would turn left.

How about some discussion of reverse ackerman?

[ROCKEDUP RICKY]

Originally Posted by JeremyH

We can discuss whatever you'd like.

Your my Hero.
Well Duuuuuuuude too.

[JeremyH]

Originally Posted by JeremyH

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.

Ok, I snapped a picture of my berg just to see what "correct" Ackermann would be (according to the method used above). Here are the results:


The yellow line indicates the axis of rotation of the axle, the blue dot is the center of the axle (I didnt trust that the link mount was the center, I measured and confirmed) and the green lines are drawn from the kingpin to the center.

The only other thing I'd like to know is the actual angle of Ackermann using this method. I am going to try to import this image into a program that I have at work that will find the angle on the dicom images that we use there....hopefully it will work on this image as well.

[JeremyH]

Originally Posted by ffro

same here, just like thinking out loud. The reason I asked about dynamics is that the relation of the axle widths can help turning radius. Does anyone think about this? Also say in in a comp you use reverse 5% of the time, do you set caster where it works 95% better going forward? Or 50/50..... Or like a daily? Sorry guys at work in a large room by myself, get to thinking about something ............and it sticks!

Haha....well, I dont break it up into percentages....I try to shoot for using revers 0% of the time, so maybe I am just being optimistic.

Originally Posted by opek

How about some discussion of reverse ackerman?

I read a little about reverse Ackermann angles (for those reading, this means that the outer tire will turn sharper than the inner tire), but I don't see any benefits except in low turning angles (high speed applications). I would think that in our application (slow speed with high turning angles), this would produce a lot of push on the outer tire and it would easily lose traction. Does this sound correct to you?

Originally Posted by ROCKEDUP RICKY

Your my Hero.
Well Duuuuuuuude too.

[Hardline]

Originally Posted by JeremyH

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.

I guess I'm not seeing that. What with tires forced into wrong paths and the lockers forcing tires to wrong speeds, I think scrub is required to happen by those circumstances. Higher frictional grip won't reduce the amount of scrub, but it will build more stress in the wheels and suspension pieces during the scrub.

Hmmm, overdrive/underdrive gearing on our shafties probably works entirely by generating scrub and the resulting stress. Jeez, now we're doing it on purpose.

I wonder if we should be calling it all "scrub". You started this thread talking about getting all the words and labels right. I suspect scrub from a wheel's scrub radius is different from scrub from tires being forced to run at different angles, is different from scrub because an overdrive front axle is turning faster than the underdrive rear axle. They're all related, but not all the same thing.

[JeremyH]

Originally Posted by Hardline

I wonder if we should be calling it all "scrub". You started this thread talking about getting all the words and labels right. I suspect scrub from a wheel's scrub radius is different from scrub from tires being forced to run at different angles, is different from scrub because an overdrive front axle is turning faster than the underdrive rear axle. They're all related, but not all the same thing.

You are correct. However, every example I can find still refers to this slippages as scrub.....so I don't know what else to call it.

BTW, I found this cool video while doing some reading. Only the first few minutes are interesting, after that, the company gets into technical details about how long their tires lasted:
AxleTech Tire Scrub Video

[Hardline]

Reverse ackerman has to do with high-speed cornering such as a track car. There's such a thing as slip-angle -- the difference between the direction of the rim from the tire patch on the ground. The tire kinda squirms/twists up, trying to align itself to the tire's direction over the ground. The more weight on a tire patch, the more slip angle is developed, which in effect toes an outside (more weighted) tire out. Reverse ackerman compensates for that and tries to keep the tire patches from fighting each other. Carroll Smith, a great (and gone) race engineer writes about it in his books. Highly recommended reading for those interested. Start with Prepare to Win. Compared to Smith, Milliken is a difficult and boring read (ie math). But it's a textbook, so it figures.

[JeremyH]

Originally Posted by CREEPERBOB

Damn Jeremy, Impressed again my friend , keep the jewels coming.

Thanks Bob.

Originally Posted by opek

Front and rear antisquat as well as torque twist play a huge roll in this, as I have experienced in my rig.

Torque twist?!?! Who the hell deals with that these days.....

Originally Posted by Hardline

Reverse ackerman has to do with high-speed cornering such as a track car.

Yeah, that's what I've found as well....which is the reason I didn't include it in the original post.

[JeremyH]

Well, I wasnt able to upload the image to my dicom viewer here at work, but I was able to find an on-screen protractor that seems to have worked well.

Here is what should be the "correct" Ackermann Angle of my 2.2 berg (with 12.5" WB at maximum when cycling the suspension):


You can see that it should be 15* according to the simple method quoted in the wikipedia article on Ackermann. I am sure that there is a slight amount of user error here, but that appears to be correct. When I get home this evening, I'll measure the set of VP Ackermann knuckles and see what angle they used.

[Hardline]

On the subject of toe and ackerman: Suppose we turn the steering all the way to one stop. Say the tire into the spring. Now the angle of the other tire depends on initial toe and ackerman. Even if it toes out some, like ackerman wants, it will still scrub due to the locker/wheel speed so no help there. If it's going to scrub anyway, why not toe that wheel in, maybe a lot, and get a tighter turning radius? What I'm doing here is making a case for static toe in or reverse ackerman. Just a thought, I'm not necessarily a believer.

[JeremyH]

We had a club member that ran a setup like that once....and it seemed to push the outer tire a lot during turning and cause a LOT of tire fold. He needed very stiff foams to minimize that...

[JeremyH]

Well, waddya know, looks like VP nailed the "correct" Ackermann angle for their Berg knuckle:


This is for the outer hole, the inner hole looks to be a bit less of an angle.

So, for those of you reading, it looks like if you want "correct" Ackermann steering, then your best bet is to run these knuckles.

[Manning]

Wow, all this steering talk and it took until post 51 to mention slip angles.

And yes, the Carroll Smith series of books are excellent. Very down to earth and "fun" reads for the technogeek. I'm an engineer and the Milliken book bored me to tears.

[opek]

Originally Posted by Manning

Wow, all this steering talk and it took until post 51 to mention slip angles.

And yes, the Carroll Smith series of books are excellent. Very down to earth and "fun" reads for the technogeek. I'm an engineer and the Milliken book bored me to tears.

Yep, that is one reason I never stress having a bunch of toe out. That and I've haven't yet felt like my excessive toe has kept me out of a valley or from making it through a gate.

Reverse ackerman to me is a packaging thing. I can run my shocks and links wide while still having my outside tire turn to lock. I can set the inside tire to rub just enough to not upset the truck.

I've thought about trying a lower roll center up front to get the rig to really put some weight on the outside tire... Maybe it's a worthwhile compromise in a sporty.

Driving uphills or or off of hills you would never notice you had reverse ackerman and not 0 ackerman. Hasn't it been well discussed on this forum that with a locked differential correct ackerman is not correct?

[JeremyH]

Originally Posted by Manning

Wow, all this steering talk and it took until post 51 to mention slip angles.

Sorry that I didnt bring it up, however, when writing a lot of the first few posts, I did not see an easy way to describe the differences in slip from all different setups. Slip with a crawler tire is not as easy to describe as a street tire. The tread patters vary WIDELY across different tires and a different foam setup will cause differences in slip. On a 1:1 street tire, the tread patterns are all mostly the same (with similar lug depth and spacing) and the material that provides support (air) is, of course, pretty much the same density and pressure (until you get into much heavier vehicles). Then we start talking about different types of terrain and it gets even more complex.

If you'd like to give an explanation, I'm sure everyone would be up for a bit of a read.

[opek]

When the rim tire combo is rolling in one direction the sidewall for the tire is lined up with the rim is lined up with the contact patch, or if it just stopped after rolling forward with no turning. As the rim starts to turn (change direction) the contact patch is held in place on the ground. The sidewall flexes to make up the difference. The bigger and softer your sidewall is the more you have to change the direction of the wheel before the sidewall pulls the contact patch and makes it move.

The way the sidewall pulls and pushes on the contact patch changes how the rubber on the contact patch meets the road, and almost all tires make more traction with some slip angle. I believe this is called shouldering of the tread, it's how the tread bunches up onto itself. The softer the sidewall is the more slip angle you need to improve traction. Street tires generate maximum traction anywhere from 7-15* and race tires usually do it at much lower values like 3-6*. I'd be money the extra traction has to do with more than just sidewall stiffness. AFAIK most tires will generate an extra 4-15% traction at slip angles. Usually bigger sloppier tires will get a bigger boost in traction.