Quote:
Originally Posted by fe1rx
Quote:
Originally Posted by 135
I know you've previously recommended against a higher spring rate but I thought those comments may have been based on the limitations of an Ohlins coilovers setup and, considering the absence of coil bind in a range of a scenarios with my Moton coilovers, I would be interested to hear your thoughts on the above options, especially regarding the 6" vs 5"+Helper decision?
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Based on the undamped natural frequency of the car, I would not suggest a front spring stiffer than 70 N/mm. At that, your undamped natural ride frequency at the front is about 2 Hz (when tires are considered in the calculation), which is in the high range of normal for a sedan based race car. Suspecting that your car, like mine, does dual duty, that is really the upper limit of reasonable. If the car doesn't feel stiff enough at that, I suggest that something else is actually the problem. If it is roll stiffness, that should be addressed with bars not with springs. |
From my reading, there are various opinions on what the undamped natural frequency of the car should be and there appears to be general consensus that a comfortable ride is around 1.0 Hz and a race car with downforce is around 3.5 Hz+. So what's in between? It depends on how you want to define the categories but you could have:
1.0-2.0 Hz - Sports cars (street)
2.0-2.5 Hz - Dual duty street/track cars
2.5-3.5 Hz - Race cars without downforce
The problem is that it is all subjective. What one person deems harsh may be acceptable to another - within reason.
While not exact measurements for my car, I have reused the same tyre rate (2200 lbf/in), front bushing rate (7 lbf/in) and rear bushing rate (23 lbf/in) that you quoted. In addition, I have decided to use a uniform 120 lbs for the front/rear unsprung mass, more so because my wheels and brakes are not OE and, while I am not currently in a position to measure the individual components, the lighter wheels and larger (and heavier?) BBK will most likely cancel each other out. Based on this, my current natural frequencies are:
Front 1.98 Hz ± 0.03 Hz
Rear 1.80 Hz ± 0.04 Hz
The quoted figures are left/right averages with the deviation indicating the spread from average for the left and right.
My preference is for the rear to be 10% less (or within a range or 0.2-0.3 Hz less) than the front, so the current figures are very close to being within those limits.
As per your suspicions, my car is dual duty, although I have made compromises on comfort to target a more track-oriented ride.
The Z60-152-100 (6") or Z60-127-100 (5"+Helper) Swift springs options that I was considering would put the front natural frequency at 2.29 Hz ± 0.05 Hz, which is not too extreme for the front but, without also increasing the rear spring rate, the difference between front and rear is ~0.5 Hz, which is more than target. The rear springs would need to be increased to 180-200 N/mm to get back within the desired 0.2-0.3 Hz range.
Following are the average front and rear natural frequencies (deviation, front ± 0.04 Hz, rear ± 0.05 Hz) for my car for the specified spring rates:
Front
N/mm ... Hz
80 ..... 2.09
90 ..... 2.19
100 .... 2.29
Rear
N/mm ... Hz
150 .... 1.85
160 .... 1.90
170 .... 1.95
180 .... 1.99
190 .... 2.03
200 .... 2.07
Based on my target front/rear differential, the following front/rear spring rate pairings could be used:
F 70, R 140 - current configuration
F 80, R 140-160
F 90, R 160-180
F 100, R 180-200
I was advised that my Moton dampers are valved to support spring rates up to 1100 lbf/in (~192 N/mm), although, I'm not sure if that was before or after the motion ratio was applied. To provide a safety buffer, this limit could be reduced by a token 100 lbf/in (~9%), resulting in 1000 lbf/in (~175 N/mm). If the damper valving was based on spring rate before the motion ratio was applied, then this would rule out the rear 180-200 N/mm spring rates and, therefore, the front 100 N/mm spring rate, which I was considering.
My decision would then revolve around whether I should maintain the existing spring rates or step them up. Based on spring rates and frequencies alone, I would most likely select
Front: 90 N/mm, 2.19 Hz
Rear: 180N/mm, 1.99 Hz
While the rear 180 N/mm (~1028 lbf/in) spring rate does extend into my self-imposed safety buffer, it is only by a small margin. The alternative would be to use a 170 N/mm (~971 lbf/in) 1.95 Hz rear spring rate.
Following all this, I would then need to go back and consider stroke to ensure coil-bind was still not going to be an issue.
Thank you for your responses, which made me pay more attention to the ride frequencies. I'd be happy to hear your thoughts.