Was playing around with one of the on-line spring rate calculators. For fun, I was imputing corner weights for a stock Miata. The results indicated an ideal spring rate in the 400-450lbs/inch range. The calculator takes into consideration corner weight, motion ratio of the a-arm and shock angle.
When I looked to see what the stock Miata spring rates were for the NA, I was surprised to see that they were like 170 lbs/in? That is a huge difference.
What does Mazda know that the calculators dont?
Comfort for a wide range of drivers and NVH.
NOHOME wrote: What does Mazda know that the calculators dont?
Bumpstops add rate.
IIRC SpecMiata rates are 700F-350R, although the rear is considered undersprung by most experts. Wait for Keith to pipe up, his Miata/MGBGT should use rates close to what you would want. The 302 might need some front help.
As an experiment I threw a set of 400 / 275 springs over the stock dampers on my NB. The car cornered flat but NVH increased dramatically. It would require a more expensive set of dampers to cope with those spring rates and expensive development time to get such a shock/spring package tuned to OEM standards. Mazda's ROI on such an exercise would be 0; hence the aftermarket.
DeadSkunk wrote: IIRC SpecMiata rates are 700F-350R, although the rear is considered undersprung by most experts. Wait for Keith to pipe up, his Miata/MGBGT should use rates close to what you would want. The 302 might need some front help.
Warren:
You are cheating cause you know where I am going with this! I was just surprised by the discrepancy between stock and "Calculated" rates.
I expect that the Volvo wont be much different than Keith's MGB in weight and could make a good starting point. Now that you mention it, I bet I could find the rates on his "Slow Cars Fast" site.
When the time comes. I am inclines to buy the FM coil-over set-up anyways, so I can make it their (Keith's) problem at that point!
I was taught that higher spring rates -- for a constant motion ratio and sprung/un-sprung weight -- increases frequency (the cycles per minute the suspension will compress and rebound through, expressed in Hz). No, the relationship is not linear to the increasing spring rate (except on a hypothetical car).
So if I'm changing my springs and dampers (but nothing else), I pick front and rear frequencies I'm aiming for and work the algebra backwards to get the desired spring rates, taking into account the limitations of the suspension travel I'm working with. Then I select dampers that can deal with that rate.
The ideal frequency for a consumer road car is typically half that (ball park) of an enthusiast track car. The lower spring rates for a lower frequency suspension allow a lot of body roll and fore/aft pitching, which are fine if I'm holding the car in a particular attitude for steady periods but not so good if I am constantly changing attitudes like I do on the track. My wheels have trouble maintaining contact with the pavement, and my CG keeps blipping around, resulting in unpredictable things.
I'm guessing your website was targeting an "ideal" frequency for sporting use; thus the higher numbers.
Some times you can actually raise the suspension frequency on a consumer passenger car (as in, street-going sports car) a fair amount without getting too bad of a ride, assuming you have available suspension travel (or tricks with progressive springs). But there's always a limit.
But this is pretty much book learning in my case. (Puhn's book is great if you take the time to study it.) I won't be starting with applications until next year, when the Fubarri gets through the first stage (driveline replacement) and moves on to suspension evaluation.
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