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Suspension Dynamics Pt. 3: Final Notes

Posted 03-04-2010 at 02:59 AM by color0
Updated 03-08-2011 at 09:30 AM by color0
This week, we'll cover some final things to round out our theoretical view of suspension tuning. It's really just two items so this will be a quick read.

First: Ride Height. The general rule for on-road racers (i.e. Mini-Z's) is that lower is better. Let's look at our equation one final time:

Mx'' + Cx' + Kx = F(x)

Lower CG means less weight transfer, which means that the absolute value of F(x) is reduced. That lends itself nicely to reducing the amount that F(x) fluctuates, which means that the tires can give you more grip. Logically, it follows that whichever end of the car is lower will receive slightly more traction under driving conditions.

Second: RCP Characteristics. Not all on-road rules, however, apply to RCP. RCP surfaces are made of foam, are textured, and are produced in tile form, which means 1) the surface is actually somewhat soft, and 2) the surface is not smooth AT ALL. This in turn means that RCP actually seems a bit like off-road racing for Mini-Z's, which is why tuning cars for RCP tracks can be counter-intuitive and sometimes even frustrating.

One of the recent discoveries the Mini-Z community has made is that a rear pod with a slightly higher CG (PN Racing 94MM LCG V1/2/3, or Atomic 94MM LCG V5) is faster than the lowest rear pod possible (PN Racing 98MM LCG V3/4) when running in the same 98mm wheelbase. Why is this so? A higher CG should, by the above analysis, make the car slower.

My theory is that because RCP is a soft surface, when you transfer weight onto one wheel, the RCP foam actually compresses and forms an indent, which means that there's suddenly more surface area for the tire to grip onto -- hence, more grip. In fact, it actually seems like the more weight transfer the better for RCP, so long as you avoid 1) the inside rear wheel lifting off (losing power) and 2) traction rolling from too much weight transfer. The fastest Mini-Z's (corner speed wise) I've ever driven around an RCP track always seemed to be at the very edge of traction roll, so there is evidence to support my speculations.

But the implications of this observation point to the possibility that a car with a higher ride height might actually be faster than a lower one on RCP -- it might not be wise to slam our Mini-Z's down, and perhaps we should only lower them when we start to traction roll. That's something I'd like to test out sometime, and food for thought in the meantime.
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