Toy Car Racing.. Or Is It?

Toy Car Racing.. Or Is It?: May 2010

Sunday, May 16, 2010

Great Find on Roll Centers

Found this on 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!

Originally Posted by Krio View Post
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:
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.


Tuesday, May 11, 2010

Relaxing Fun

I had a great time racing offroad for the first time at Turtle Trax Raceway. I competed in the SCT class with a friend's stock Slash among Associated and Kyosho SCTs.Although they had either 17.5 or 13.5 motors and proper racing tires, I was surprised how close the stock Slash came to the more sophisticated trucks. Driving an off road car felt like I was going R/C for the first time all over again and it was a blast! Bumping into each other, sailing over jumps, and shooting up rooster tails was so much fun and a nice change from onroad. Although I had a great time, it doesn't replace onroad and my enthusiasm for onroad could not be higher. While at Turtle Trax, the 2wd buggies really caught my eye and I could see myself getting one in the future if I happen to live close to a good electric off road track. Luckily, the owner of Turtle Trax is considering building an indoor facility closer to where I live :). We will see what happens. The atomosphere was very relaxed and it was nice not to worry about 10ths of seconds or having the most speed. It was nice to throw the truck on the track, run around, and most importantly have fun.

This Sunday will be the nexst race at Hobbytown. During the last couple of weeks, bought a Tekin RS and sold the KO. I still need to get the latest software for the Tekin but I'm thrilled to finally have a decent amount of speed instead of being destroyed on the straight. On top of that, I haven't done much to the car. I have been thinking for awhile about the constant lack of traction on my car, both front and rear. I compared setups with the guys who seem to have a decent handling car and we are nearly the same. In some cases I should have even more steering compared to them as I am running 2.8 front springs and 6 deg caster. I'm now thinking that a fresh pair of Jaco Blues would help. MY tires aren't abnormally worn or tearing yet but at this point, I'm running out of ideas. At the very least, I will have a fresh set when/if the current tires are toast. I really hope tires will help my car. I'm about a second per lap and two laps total off pace. This is with no speed and an ill-handling car so I'm hoping for a big improvement Sunday.If the tires help, I can focus on other details to make the car even better rather than chasing non-existent traction all day.

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Friday, May 7, 2010

The Next Step

After a long week of finals, I'm finally done with this semester. It's going to be an interesting summer, on the track and off. I will be starting my internship in about a month and have to deal with the changes that associate with it. Hopefully it will lead to fruitful endevors past this summer.

This weekend in racing will be interesting because I am going to attend my first off road race EVER! The onroad racing here only happens twice per month which obviously isn't enough for me as I'm running off road just to keep myself occupied. I will be  racing in the SCT class that has been so popular over the last couple of years in the States. I honestly never thought I would be racing offroad but I'm excited to try something different.

Unfortunately, onroad has taken a social life of its own and not in a good way. Only two races in and I'm finding myself irritated by the amount of bickering and lack of enthusiasm over the whole deal. Part of the reason to do R/C i to relax and it makes it difficult when someone always has a problem or complaint about how something is being managed. I'm curious to see if this is true in off road as well or if onroad just seems to attract the whiners.


In other news, ROAR recently announced that they will begin regulating ESCs for Sportsman classes. In short, no timing advance, turbo, etc - it basically brings the ESCs back to where they were about a year ago. This is something that the R/C community has been asking for from ROAR. Hell, we have been asking for something, anything from ROAR on the subject. I think it's good that ROAR is finally speaking up after a year of the industry pumping out these super ESCs but I'm not so sure this is the best way to deal with it. I've never attended a ROAR race but in my mind, the "sportsman" guys were usually pretty decent at racing. Before the economic downturn, you would see people from all over the country coming to attend the ROAR nationals of XYZ. If you have this kind of desire to race at a national event, is getting the right speedo really that big of an issue? It's not like only certain people can buy the speedos or anything; plus, some manufacturers (read:Tekin) offer their software updates free of charge. I think that the real issue comes down to the new speed we have found - people are always going to want to go faster and faster but what we really need to do is redefine what "stock" and "mod" is. The speedo wars aren't just at the national level but at the club level these days, especially in onroad and running the newer speedos are so common that it's pointless to scale the power back, espeically at a national level.

With that said, I ended up buying a Tekin RS for my T2'008 :)