Quote:
Originally Posted by bbnks2
Quote:
Originally Posted by Driven5
That's the problem with an off-the-shelf 1-piece design like the Ohlins. Oh, sure it has more total travel. However, it's inherently restricted to a manufacturer pre-selected range of ride height and spring rates. If you take it outside of that range, it becomes inherently compromised. If you want to properly run outside of that, it becomes a custom affair.
Meanwhile, yes the total travel is compromised on the 2-piece design, found on YCW/RedShift/etc. However, it can be combined for a wider range of ride heights and spring rates. The other drawback here is that if you don't know what you're doing, it's just as easy to get wrong as it is right.
I don't know what RedShift run standard, or what options they offer, but YCW have a 110mm travel shaft standard and can run 130mm travel shaft with the optional extended reservoir that extends down into lower mount cavity. However, as best I can tell, this is actually more valuable for a street setup than a track setup. Based on the Swift metric spring data, the 110mm setup can already meet or exceed the 'usable stroke' of the vast majority of spring we'd be discussing here. Even without going into coil bind, exceeding the engineered usable stroke of the springs can unnecessarily damage them. So going for 130mmm on those springs would merely allow you to do more potential damage to your springs more often. Thus to get the parts working together properly, there shouldn't be any need for longer stroke in a track application. If you're bottoming out with the 110mm, it simply means the springs and/or bars (depending when the bottoming is occurring) are not stiff enough.
That being said, it has crossed my mind (but I never had reason to ask) that for additional full-travel lowering on a track biased setup, they may also be able to combine the 110mm stroke shaft with a shorter body by using the extended reservoir.
|
Yes the ycw are about 110mm and the Ohlins appear to get down on the internal bump stop as early as 75mm with about 100mm total travel per fe1rx's data. Many other lighters cars have 6-7"+ of front wheel travel whereas we end up with about 4.5."
Based on that I've realized that you don't really have much of a choice but to run really stiff front springs with a short stroke strut. Many of the "mid-range" front spring rates will result in bump stop engagement. Especially so on a roadcourse where the cars velocity is higher.
I think 700lb/in is a good place to start up front. I have no hard data on this but I believe that in a 3300lb BMW you will see close to 2000lb of outside corner load in a turn. At 700lb/in your 4.3" of strut stoke will have used about 1" of static compression (1" of rebound travel) given a ~650lb sprung corner weight. That leaves an additional 3" or so of bump travel less your bump stop. Add in a sway bar and effective wheel rate in roll is closer to 900+ meaning you can support that 2000lb cornering load even with a small bump stop in place to prevent bottoming out the struts.
700lb is the cutoff where I would want to run a helper spring to re-gain rebound travel if I were to run any higher of a spring rate. 75/25 bump/droop distribution is already pushing it. I think 60/40 is where most people want to be. The Dedicated Ohlin's appear to be 12k (672lb)/18k (~1000lb) and also run a front helper. It will be interesting to see where the bump/droop travel lands and work the math backwards to see just how much it gets into the bump stops up front in a turn. Maybe they are a longe stroke strut... I am leaning toward the helper should be removed on this setup though and just deal with having limited rebound travel.
Take these numbers with a grain of salt but...
If 800/1100 pushes too much then I'd probably look to increase the rear spring rate to achieve closer to a 60% front roll couple distribution. To land close to 60% you would need something like a swift Z65-178-260 (1500lb). Not sure if anyone has experimented with going that high with the rear rate but that's how the basic math plays out. Also not sure on the spring length either. Maybe it doesn't actually take that much spring due to other complexities in calculating roll couple but that's what my understanding of the basic math shows...
Maybe my perception will change when I have the ohlins 12k/18k in hand which are heavily front biased on paper. If my car magically handles insanely well ( i know some very fast drivers are already using them) then I'll have to try to figure out where the magic comes from (maybe the damping?). |
Completely agree, math says to go with the 1500lb springs but seems insane to me lol