Great Find on Roll Centers

Found this on Rctech.net on roll centers courtesy of the member "ray_munday": 

 

The important thing to know is what the roll centre actually means. When you are cornering, the side force from the tyre is directed to the chassis via the suspension links (the 'geometry'). Basically, the roll centre is the point in space at which the tyre force 'points to'. If the roll centre is high, when you turn, the side force from the outer wheel will point upwards and this actually pushes the chassis up, reducing roll angle. If the roll centre is below ground, the side force will push the chassis downwards, increasing roll angle. When the roll centre is at ground level, there is no up or down force applied to teh chassis from the geometry.

So in mid corner, a low roll centre acts like having a softer spring at that end of the car (ie more grip at that end) while a higher roll centre acts like a siffer spring at that end (ie less grip at that end). But thats only part of the story.

What makes roll centres so important is that they act immeduiately when you turn. When you first turn the wheel, the front tyres develop a slip angle and build up grip very quickly. This means that the roll centre force acts almost immediately. The chassis then starts to yaw, which creates a slip at the rear, so the rear tyre force builds up an instant later, meaning the front roll centre acts first then the rear roll centre. The chassis is starting to roll, so the dampers are starting to work, but dampers provide more force as they move faster, so they have a delayed action from the roll centres. Finally, the springs start to have an effect as their force is proportional to displacement, so it takes time for them to build up a significant force (you dont have instant displacement). As you exit the corner, these things happen somewhat in reverse. (this is obviously a simplification, but should give you the general idea).

So basically the roll centre allows you to tune the way that the vertical tyre force builds up through the turn. A high roll centre makes the force build up quickly, which can be good for low grip tracks where you need the tyre to bite; a low roll centre makes the force buildup more progressive, which is more useful for high grip tracks where you need the car to be a bit less responsive.

If we look at a high front roll centre, when we first turn the wheel, the outer front tyre will have a very fast build up of vertical force (it is trying to push the chassis up which in turn plants the tyre into the ground). This can make the front of the car feel very responsive, but once the car is in the middle of the corner, it will add some weight transfer at the front and take away a bit of front grip. A low front roll centre will tend to make the car less responsive to initial steering inputs (as the rate of force buildup is slower) but in mid corner provides more front grip.

Both the height of the ballstud and the length of the arm are used to adjust roll centre. If we raise the inner ballstud, we lower the roll centre (see diagrams below) and vice versa. The length of the upper arm affects how the roll centre changes as the suspension compresses / extends. (As the suspension moves, the roll centre moves up and down as well - it is an unavoidable fact of independent suspensions.) A longer upper arm keeps the roll centre lower as the suspension compresses, while a short upper arm keeps the roll centre higher as the suspension compresses. As Wild Cherry pointed out, the ballstud height is less sensitive than the length of the arm, but it varies a little from setup to setup. It also acts in a slightly different part of the corner.

To answer your question about the rear - adding a washer to the rear ballstud will lower the roll centre, which will give more rear traction. This is most noticable when applying power to the car as you are exiting the corner.

Sorry for the long post, but its not an easy subject to give a 5 line answer to!

Quote:

Originally Posted by Krio  Sorry, but that's not the correct way to find roll center. You're tracing the lines the wrong way and in your setup the roll center is crazy high, not low. Everything else you said is right though, just try a picture like this: http://www.mitchellsoftware.com/ForceB1.jpg

actually, richards diagram is correct - it just shows a geometry that is not usually shown for full size cars (ie the upper links pointing upwards at the inner end of the car). Having this layout gives a very low roll centre as shown, but can also make the camber become positive in compression. This is a big no-no for full size cars, but our tyres are much less sensitive to camber and much more sensitive to roll centre. unning 2 or more washers on the front B4 ballstud gives you a similar geometry.

Practice - 02.20.2010

Yesterday I practiced all day at Nexus Racing and did a lot of testing. The track seems to have lost some traction which could be due to the lack of racers being on it. Last Saturday's race was cancelled due to the freak snow storm Georgia got so the track hasn't seen as much action as usual. Most of the time, I have to be careful of traction roll coming out of the sweeper but the car was planted all day.

I first played around with front roll centers as the car was pushing at corner entrance. I found that making the camber links steeper gave the car more turn in but lost some on power steering at mid and corner exit. When I dropped the front camber link to the outside inner hole, the car gained more front bite and was easier to drive. It really helped make the car feel balanced throughout the corners. A local racer suggested I also raise the rear camber link on the hub by 1mm to make the car roll more. This instantly made the car faster by 0.2 sec while keeping it balanced.

I reduced the rear toe from 3 deg to 1 deg. While it gave more steering throughout the corner, the rear end had no grip. I switched to 2.5 rear toe and this brought the rear end in more while giving more steering. I need to do more testing and see if this improved lap times and/or consistency.

Last, I made the wheelbase shorter which upset the balance of the car but made it faster. I'll probably stick to the shorter wheelbase and practice with it more to build consistency.

I couldn't believe how fast some of the Tekins are running now. Before the v2.03 update, the KO (which I run) and the Tekin were basically the same. Now, I'm getting blown by half way down the straight by the more well tuned Tekin setups. KO - c'mon guys give us more power!

Overall, it was a really fun day. It's such a stress relief to leave town for the day and focus on nothing but the car and hanging out with everyone. Everytime I return from the track, I feel so refreshed but then again it's hard to get back on focus because I'm still excited from the day!

Year of the Tiger - Introduction to my New Blog

This is my first blog of sorts and I decided it would be a good place to store my R/C racing adventures as well as some other part of my daily life. I was inspired to create this blog after reading Alexander Hagberg's blog http://hagberg.blogs.se/, a talented young European driver for Team Xray.

I have been racing on and off since 2003. My first 'real' R/C was an Associated TC3 which was a great platform to learn from but began to show its age when brushed motors and NiMH batteries were at their peak. I now drive an Xray T2R regularly at my local track, Nexus Racinghttp://track.nexusracing.com. This carpet season has been my most focused season, in all of the time I have spent racing, as I am working hard and practicing as much as I can to improve my skills and knowledge.

To begin, the carpet season has been quite a learning curve as it is my first experience with rubber tires on carpet. In the past I had dabbled in some foam carpet with the Tc3 but never taking it seriously. The T2R is quite a good platform for the traction-limited conditions as the fibreglass chassis flexes and thus creates more traction than its carbon fibre counterpart. Most run the Sorex 28's http://www.nexusracing.com/product_info.php?manufacturers_id=&products_id=2940 . They seem to provide more traction compared to the Jaco Blue's used at most U.S. major races although I have yet to test them. I have only used two sets of the Sorexes this season and they seem to hold up fairly well. I have noticed that it is critical to be sure they remain properly glued(especially the fronts) to avoid traction rolling and numb steering.

The base setup I have gathered from the local Xray drivers has never been too far from the mark but the car is still somewhat slow compared to the others. Talking a pro driver, it's important for the car to rotate in order to build corner speed. I will try to "free" up the car as much as possible in practice this weekend and report the result.

Another aspect of setup I learned during last practice was how an abundance of traction can slow a car down. Makes sense as what we are really imposing is friction and too much will slow the machine down. On the other than, took little the the car wastes time sliding around. With that said, I raised the front outer camber link from 1mm to 2mm and found the car to have less steering, less twichiness, and more of a "direct" feel from the steering. Interestingly, my lap times went down easily by 0.1 sec per lap which was amazing. I will experiment with this as well to try to figure out the relationship between steering and laptimes/corner speed.

Here's a picture of my regular track located just outside of Atlanta, Ga. I've been racing with many of the regulars there for a long time and they are some of the best people I know. I'm the guy closest to the camera on the driver's stand. I got to race the Xray XII car in that pic - such a great 12th scale!