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Downstop/Rebound Spring Tuning

Posted 09-23-2011 at 03:42 PM by color0
Updated 09-26-2011 at 05:15 PM by color0
Last week I had a conversation with fellow SoCal Z racer mugler about tuning the downtravel on the top shock of a pan car chassis -- as you might imagine our conversation was very related to my previous post here about the downstop adjustments on the PN and Kyosho shocks. I tried my best to explain why and how the downtravel adjustment works and my reply is repasted below.

Originally Posted by color0
When you brake for a corner, the weight transfers towards the front, this we know; but the amount that gets transferred forwards is dependent on both the front and the rear wheels, and we also have to make a distinction between weight and dynamic loading.

Say you have 0 downtravel -- full extension of the rear pod is the resting ride height. There's some weight on the rear wheels, and some weight on the front wheels. But the moment that weight begins to shift to the front, the downstop limiter will pick up the rear wheels off the ground, resulting in all of the car's dynamic load on the front wheels and 0 load on the rears. The weight hasn't shifted a whole lot though: action = reaction, so the full load on the front wheels means that the ground will push the car's weight back up through the front tires. Since the front wheels lie ahead of the car's CG, the net result is that the ground is pushing the car back to level, very hard. So a car with 0 downtravel will experience high load fluctuations on the tires (from full car weight to 0 car weight), but will corner very flat. The high load but low weight-shift on the front tires creates lots of front bite, and the zero load on the rear tires means you drift into the corner.

When you put a rebound spring on like the PN damper, you're still limiting the amount that the rear wheels can come up before they leave the ground, however, there's a middle ground in between where the rear tires are partially loaded, thanks to the rebound spring pulling the rear wheels up a little bit, but not all the way unlike a downstop would. So, when you brake, then a large majority of the load will go to the front wheels... but not all of it, since now your rears are still on the ground (just barely). Accordingly, more weight is allowed to shift to the front, but just barely. As a result, turn-in front bite is mildly reduced vs. a downstop arrangement, and rear grip is mildly increased since your rear tires are no longer leaving the ground as easily (they still might!) under braking.

When you leave the thing willy-nilly (no downstop at all), the rear suspension is free to extend as it pleases (spring action) or as gravity pleases. This means that the rear tires almost never leave the ground, and thus you get maximum rear traction. On the other hand, you will also allow more weight to shift to the front, which reduces turn-in performance since you're no longer taking load off of the rear wheels to give to the fronts, AND you're giving them more weight burden to cope with.

This is, of course, only smooth-track theory. On bumpy tracks, having a hard downstop and 0 downtravel is usually bad since the rear wheels will leave the ground on bumps as well as under braking. Hence why you see me use the PN top shock more often than the Kyosho. A rebound spring is IMO the optimal way to balance bump handling with turn-in response, since the black-and-white boundary of downstop tuning often leads to inconsistent handling: the rear wheels might leave the ground in one corner, but not for the next -- I noticed this a LOT this month at Inside Line, it's no longer predictable and precise like my old Kenon setup and I somewhat regret giving EMU my PN top shock.

I have one legitimate usage for a no-downstop rear shock setup, and that's the MRCG. I don't even understand the magic but the MRCG creates stupid amounts of steering up front, and can overwhelm the rear unless I artifically keep the rears on the road using infinite downtravel. I can get away with the increased weight transfer because the MRCG is super, super low.
Mugler got it, but I don't even begin to imagine that this was a comprehensive reply, so feel free to leave a comment if you'd like more detail on anything!
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