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Lowering past 1” effects on roll center?

 
Old 02-27-2019, 10:32 AM
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Default Lowering past 1” effects on roll center?

What’s up guys. So right now I’m on Öhlins DFV at their recommended ride height (1” drop). Correct me if I’m wrong, but that recommendation has to do with suspension geometry/roll center?

Looking to drop another 10mm, will this dramatically affect roll center?
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Old 02-28-2019, 03:51 AM
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Don't see that anyone has responded to this, so I will chime in with my limited knowledge. Yes, the s2000 roll center becomes increasingly lower with the more suspension lowering, which appears to affect body roll. 10mm is not an extreme amount, so I don't know that it would be noticeable. Rob Robinette explains roll center on his site. Other than seeing where you are at visually, I don't know how you figure this out.

https://robrobinette.com/S2000RollCenters.htm
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Old 02-28-2019, 06:07 AM
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Roll center is part of the problem but riding the rear bump stops is probably the more limiting factor. My race car liked having the rear as low as possible until the bump stops became a problem. That happened around 1.2 inches below stock but varies with spring stiffness.
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Old 02-28-2019, 07:49 AM
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Originally Posted by robrob View Post
Roll center is part of the problem but riding the rear bump stops is probably the more limiting factor. My race car liked having the rear as low as possible until the bump stops became a problem. That happened around 1.2 inches below stock but varies with spring stiffness.
As said, the limits on the S2000 are a function of its design. In common practice, these constraints become immutable because of time and cost or because of racing rules (e.g. NASA TT that limits/prevent relocating suspension mounting points or changing the spindles.)

The short rear shock is one of those limits. It was so short Honda went to an external piggyback reservoir for their OEM shock. Very unusual for a mass production car at those price points.

Typically the only geometry modifications are lengthening and dropping the spindles with a "roll center adjuster" and lowering/raising the height of the spindle with a different diameter tire. I believe there are also offset bushings available for the upper control arm, but they seem primarily used to add camber rather than change geometry. So the compensation is often in the form of stiffer springs and shocks. It seems S2000s run with fairly low additional downforce; as a result, spring rates for track day cars are very similar to the rates for autocross cars.

Wisefab has a suspension kit with fabricated spindles and suspension arms (sold through Evasive) that lowers the car while maintaining geometry. It also allows for very easy alignment adjustments. However, at $6k it is also very rare and runs afoul of the rules for many racing/time trial classes.

Many purpose-built race cars allow extensive adjustment of the geometry by changing spindles, raising and lowering mounting points, and changing the length of suspension arms. NASCAR used to feature Panhard bar adjustments, raising and lowering the rear roll center, during mid-race pit stops. They also adjusted spring rates by inserting or removing spring rubbers. Now the drivers have a knob to adjust the Panhard bar with a remote electric motor. Rates are often adjusted with tire pressure. Other than the spring rubbers, the S2000 doesn't have any of those options.

Note that making calculations off virtual points adds to the simplicity of calculation, that those simplicities hide the calc in a few free body diagrams of the suspension that can show you the actual forces involved. Video game and F1 simulations can produce very accurate vehicle dynamics, it isn't dark science. So much so there is actually a group doing an iRacing series in Spec Racer Ford at tracks in advance of the actual SCCA Hoosier Series races there.
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Old 02-28-2019, 11:41 AM
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Awesome, thanks a lot for all of your input.
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