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Chassis Stiffness in Detail

Posted 05-17-2011 at 01:46 AM by color0
A recent development on the forums regarding the Mini-Z MR03's chassis stiffness inspired me to write about it a little this week, and give my own perspective on it: what does chassis stiffness do, why is it important, and how do you know how much you need? Let's discuss.

Generally, when we talk about Mini-Z setups, we talk about front end, and rear end; tune the front end for steering and response, tune the rear end for traction and forgiving behavior, etc. etc. But the art of car setup, after all, is that the front and rear have to work together to produce optimum cornering! That's where chassis stiffness comes in -- it connects the front and rear ends, allowing them to "communicate" to a certain degree. Naturally, since the chassis is just like a very stiff spring, the stiffer the chassis in between the front and rear suspensions, the faster and more direct said "communication" will be. Additionally, the chassis provides a "baseline" stiffness that can be subtracted by softening the suspension, but cannot be added by stiffening the suspension past the stiffness of the chassis itself (think about it: if the spring is stiffer than the chassis, the entire chassis will flex rather than the suspension if you load one end of the car!). So in a sense, the chassis does have to be tuned in accordance with the rest of the suspension: balance is everything.

So why is this communication important? As I said, the front and rear suspensions have to work together to produce maximum grip and a good handling feel. Say we're on a low-traction track: say the front end is super-stiff and built for quick response, and the rear end is super-soft and built for ultimate traction. Our initial mode of thinking would dictate that in a corner, the rear will tend to roll more than the front, which means the car will simply lift up its inside front tire every corner, which means we're obviously not going to be cornering at maximum potential. But here comes the magic: if we use a soft chassis between the front and rear ends, we can reduce the communication and dampen the respective properties of the front and rear end, allowing the front end to have fast response (it's still stiff!) and the rear end to have good traction (it's still soft!), but the rear end rolling action will not be translated as much to the front end, preventing the front tires from lifting off the ground as much and increasing or cornering potential. In this sense, chassis stiffness is somewhat of a "crutch": as the example shows, you can use it to get around what otherwise might be a non-ideal setup.

Now the previous example showcased how chassis stiffness affects the strength of interaction between front and rear, but not the speed. So say now that we're on a tight, quick, high-traction track: the front and rear suspensions are both relatively stiff, but the car's just not rotating as fast as you'd like it to during corner entry, and if you stiffen the suspensions any more you'll start to skip over the track surface. Again, chassis stiffness to the rescue: if you stiffen up the chassis, the weight transfer forces from the front end will be translated to the rear end faster, resulting in faster response from the rear suspension, which upsets the rear tires a little bit more and gives you that extra bit of rotation you desire, ALL without needing to stiffen the actual suspensions! Now chassis stiffness isn't a crutch so much as a method of fine-tuning the split-second dynamics of the car without touching other things that have already been optimized.

Modern 1/18 and larger pan and touring cars have used this concept for several years now, and it's not really surprising why: a softer chassis seemingly gives the user a larger setup window, since you can implement front and rear setups that theoretically wouldn't work well together, but you "persuade" them to cooperate using a soft chassis that isolates their individual behaviors. This makes finding a decent setup easier for the 90+% of the RC hobbyist population that isn't absolutely godly at setting up a racing car. On the other hand, a stiffer chassis is another tuning option to help cars adapt to ever-faster and ever-grippier tracks, improving their handling response to keep them ahead of the corner. All this is achieved with a simple swap of the chassis, and so it's an easy way for manufacturers to introduce new tuning items.

So how does this translate to Mini-Z? Recall, if you will, that the Mini-Z MR-02 has a beefy, stiff grey chassis from the factory:

This chassis is super-stiff, as MR-02 aficionados will agree, and provides the best response and stability on high-grip tracks. But on low-grip tracks you'll start seeing a lot more of the smoked-grey SP chassis, because the softer clear plastic material provides more mechanical grip and forgiving handling when it matters the most.

What about the MR-03? As soon as we got our hands on it, we almost universally agreed that the black chassis was a limp noodle compared to the MR-02 box structure.

Here was a chassis that you could visibly flex with your bare hands, and on the track it showed: in low-traction conditions, the MR-03 was a wonderful drive! On carpet tracks, my personal experience was that I could keep up with tuned MA-010 AWD's with a fairly stock MR-03, very impressive. But in high-grip conditions it just didn't work so well, and the MR-02 remained king. As a response to this complaint, Kyosho specially developed a carbon-laminate "underfloor" stiffener, to give the black chassis the stiffness it needed while taking away a little ride height; there was also the MR-03 SP grey chassis, which took the material from the MR-02 to cast the MR-03 box chassis, resulting in a much stiffer structure.

And yes, as the racing community for Mini-Z's has matured, you'll see that these specialized parts DO show up more and more often when the track conditions demand it. I personally just bought an SP chassis for my MR-03 to deal with the high HIGH traction at RC Kenon, but in the meantime, there's another trick being used by racers to boost the MR-03 black chassis' stiffness.

As it turns out, the slots in the MR-03 chassis are just large enough that you can epoxy an entire 2mm-dia. carbon rod inside! Doing this increases the stiffness of the black chassis to approximately SP-chassis levels; I imagine it's still not as stiff as it would be with the Kyosho carbon underfloor, however, which I don't intend to test given that my ride heights are routinely less than 1.5mm. On the track the results are immediate: the car feels more solid, a little bit more "urgent" is the word I'd use to describe the feeling. Turn-in is tighter, weight transfer is faster, so long as the chassis isn't too stiff and hurting overall grip, these are good things to have. The same effect is observed with the SP chassis, which I used at RCX and will continue to use for now.

Anyways, this was my short look into chassis stiffness tuning that I want to share with everyone. Next time I'll continue through my setup from RCX, laying out most everything I know about Mini-Z into this series of articles.
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