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05-29-2016, 10:32 AM | #1 |
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true rear coil-over: possible?
I'm wondering for those of you who are probably more in the know than me, do you think it is possible to put a coil-over spring on the shock in the rear? I realize this is a major change the from the OEM design, but there are good reasons to design a coil-over that has harmonized motion ratios.
Things I'm wondering: -top mount: how to keep the spring square? -lower mount: threaded sleeve over shock body? -rear shock towers: can they handle the stress? Please let me know why this is a horrible idea.
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05-29-2016, 01:06 PM | #2 |
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Top mount at body would have to be reengineered to support and withstand more force.
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05-30-2016, 01:30 AM | #3 |
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hmm yeah that's the major thing I was afraid of.
I guess maybe I'll look at rising rate custom rears instead.
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05-30-2016, 08:47 AM | #6 |
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You will not gain much from having a coilspring unit VS a spring that has adjustments and a good damper with a variable height.
Actually... only the spring preload is not available but you could engineer something around that.
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05-30-2016, 08:53 AM | #7 |
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I already have variable ride height springs in the rear (TC Kline perches). My point is that the motion ratio is so poor for the spring that it requires very high rates and this requires the shock to be valved for a completely different motion ratio.
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05-30-2016, 11:58 AM | #9 |
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no no, I'm trying to convey the fact that the shock has to be valved for a motion ratio in the OEM setup that is way different than the spring.
there is benefit to making them the same.
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05-31-2016, 06:05 AM | #10 |
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I can't really see any advantage in true coilover conversion. The stock setup just means using a higher spring rate than you otherwise would in a true coilover, and that's it.
There is one very minor point about the distortion of the spring perch as it is not directly aligned on axis, but it's not going to make any practical difference. |
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05-31-2016, 06:17 AM | #11 |
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The motion ratio I have found for the rear shock is 0.813. The rear spring is 0.563. These motion ratios are unrelated. (eg. the spring ratio is 69.25% of the shock). I fully understand that it is completely *possible* to valve shocks for a different motion ratio than the spring. What I'm saying is that its less than ideal. I would rather have a softer spring with more travel in the rear, with the same motion ratio as the shock.
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05-31-2016, 06:44 AM | #12 | |
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The springs attach in a different place, but you use springs with higher rate spec to compensate. The end result is that its Not going to make any difference to the shock valving. |
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05-31-2016, 07:16 AM | #13 |
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John_01 , let's try a thought experiment: Imagine a divorced suspension with a spring motion ratio of 0.1 and a shock motion ratio of 1.0. For every 1/10th" the spring moves, the shock will move 1". We all know that shocks work by displacement per unit time via the valving, so the shock will have to be valved comparatively very soft because it is experiencing so much motion relative to the spring. This means it will be hard to do things like high-speed and low-speed digressive dampening. Maybe not impossible, but certainly harder.
Disclaimer: I am far from an expert on shocks, but I know a little bit. Based on my calculations, I am looking at needing a 1700lb/in spring in back if I use the OEM position, which seems way extreme.
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05-31-2016, 07:08 PM | #14 | |
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1700lb/in seems high, although I think you wrote you will run some really sticky Hoosier tires on your car. Maybe you can consider adding a rear sway bar if you need that much spring rate. The stock spring location is actually good from a structural point of view because the spring perch is located directly below the rear chassis rail. I suppose the downside of the stock setup is that it is sensitive to movement of the rear subframe bushings. If you want to achieve really high rear wheel rates, you might want to consider solid rear subframe bushings. |
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05-31-2016, 11:06 PM | #15 |
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John_01 Yes, I am definitely considering a rear sway. The WhiteLine FSB showed up this afternoon, so that will be going on the car at some point in the future. I have some poly RSFB sitting in a box, so whenever I get the funds together for the LSD, I'll do those at the same time.
I realize 1700 seems, well, ridiculously high, but its what I calculate is about right for a NF of 2.5Hz. And NF of 2.5 is not crazy high. I know people who run NF's of higher than 3 on their track cars. The ideal front NF is, IMHO, 2.2Hz, which works out to be about 425lb/in which actually is very reasonable and is what a lot of the kits have targeted or come close to. The spring rate really doesn't have so much to do with the tires as it does with weight transfer; eg pitch and roll. Loads are moving through different points of the suspension. Camber is changing, toe may be changing, etc. Suspension does not create grip. Suspension merely allows grip to occur more often and for longer periods when done better. Yes a sticker tire allows us to pick a NF that is higher, but some of the street tires out today like the RE71's are so responsive from what I understand, that they demand as high of spring rates as any full-bore slick autocross tire. Of course the other thought here is to try to run progressive rears... I'm going to stick with the 350/700 setup this year almost certainly but I want my next spring change to hopefully be my last. Famous last words?
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06-01-2016, 08:13 AM | #16 |
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That is a *lot* of wheel rate. Maybe if you ran no bar at all or the stock bar (which at 15lbs of wheel rate is functionally the same as no bar, regardless what folks on here will say...)
Ive known zero successfully setup cars that were textbook built to natural frequencies. |
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06-01-2016, 08:17 AM | #17 | |
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2004 ASN/FIA Canadian D Modified Champion 2002 SCCA ProSolo Honda Street Challenge Series Champion 2002 SCCA ProSolo Street Modified Series Champion 2003, 2001, 2000 CENDIV Divisional Champion ProSolo ain't no joke. Yeah my math was without a rear bar. I simply ignored it to get to 1700lb/in. So I suppose I could back off those rates a little bit.
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06-01-2016, 08:53 AM | #18 |
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SM 15 years ago wasn't exactly a hard class.
It's pretty easy to win Pros with a half second advantage leaving the tree. Enjoy driving an STR S2000 with a higher rear NF than front. You'll die, so what kind of flowers do you want at your funeral? |
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06-01-2016, 08:56 AM | #19 |
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Hey bud, I didn't say it was the be-all, end-all! I'm trying out a theory here. Maybe it will work, maybe it won't. At least I'll be able to say I tried it.
I'm working on rigging up a ghetto-fabulous shock dyno right now.
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06-02-2016, 09:54 PM | #21 |
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Most high end dampers can come as true coilover rears. JRZ, MCS and Moton just to name a few.
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06-03-2016, 01:34 AM | #22 |
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I emailed JRZ-USA over the weekend, haven't heard anything. The MCS pictures on the Vorschlag site show a divorced rear setup, as do the Moton pictures on the HPA site. I'm confused...
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