Quote:
Originally Posted by fe1rx
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). |
I had built in a lot of assumptions into my calculations, some of which I know may not be as close as what is preferred but, for the purposes of my example, they would have to suffice. As an example, all my suspension components have been replaced with M3 and aftermarket and I use medium compound R-comp / semi-slick tyres on the track that can handle (or should be matched with) a higher spring rate.
I realise theory is just that and it's no substitute for real-world experiences but it's a good starting point to know in what direction I need to head. Being outside of the US, the cost involved in testing even a small range of options is exorbitant, given our poor exchange rate and high shipping costs, so theory and research play an important part in attempting to get it right the first time.
There are two or three 135i race cars in the national professional categories but I don't think they'd provide me with the details of their suspension setup. I'd think those secrets would be tightly held. Other than at a professional level, there aren't many 135s, going to the same extreme, being raced here. From my research, I have come across several other US-based 135i race cars that have published their spring rates and they were as follows:
Front / Rear
650 / 700
560 / 800+
550 / 850
700 / 850
Using the second and third spring rates, since they have the most commonality, equate to a front/rear spring rate of 100/150 N/mm, which would have a 0.45 Hz difference, which is more than my target range but it worked for these cars. To get back to a maximum 0.3 Hz target ride frequency difference, the spring rates would need to be increased to a 100/180 N/mm pairing or decreased to 90/160 N/mm.
All in theory, of course.