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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.... |
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Re: RC Crawler Steering Damn Jeremy, Impressed again my friend "thumbsup", keep the jewels coming. |
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How about some discussion of reverse ackerman? |
Re: RC Crawler Steering Quote:
Well Duuuuuuuude too."thumbsup" |
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http://i171.photobucket.com/albums/u...g/P1050946.jpg 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. |
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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. |
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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 |
Re: RC Crawler Steering 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. |
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Re: RC Crawler Steering 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): http://i171.photobucket.com/albums/u...gleforBerg.jpg 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. |
Re: RC Crawler Steering 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. |
Re: RC Crawler Steering 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... |
Re: RC Crawler Steering Well, waddya know, looks like VP nailed the "correct" Ackermann angle for their Berg knuckle: http://i171.photobucket.com/albums/u...eAckermann.jpg 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. |
Re: RC Crawler Steering Wow, all this steering talk and it took until post 51 to mention slip angles. :mrgreen: 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. |
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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? |
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If you'd like to give an explanation, I'm sure everyone would be up for a bit of a read. "thumbsup" |
Re: RC Crawler Steering 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. |
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