Quote:
Originally Posted by 135
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.
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These numbers are not consistent with what I have generally read, although I think I recall a Julian Edgar article that talked along these lines. These numbers are high, not typical.
If you want to explore the upper end of the envelope, it should be based on what actually works (i.e. based on testing) not just on math. My point in raising the subject of suspension ride frequencies is that it is a tool for seeing if you are in the right ballpark. Let's say you are bottoming out your front suspension. It is natural to think you need to go stiffer up front, but if your ride frequencies are "right", the problem is actually your ride height. The Ohlins kit as delivered is glaringly out of balance, with a rear ride frequency much lower than the front. I suspect this was a necessary concession to the soft rear subframe bushings.
Speaking of which, once you go really stiff, other compliances (tires, bushings, suspension arms, chassis) will defeat your attempt to get the ride frequencies you think you are getting. Race cars are caged, ball-jointed and reinforced to reduce these compliances.
I suggest finding someone who actually races a race-prepared 135i and find out what springs they are running. Whatever it is, you probably don't want to go that stiff for a dual duty car.
For a practical street driven car I would be skeptical of any car that does not permit at least 4" of total front wheel travel. The OE 135i doesn't have much more than that. It can't afford to lose much (in my opinion).