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Originally Posted by Braineack
(Post 621310)
fwiw, I have koni sports with 400/250# springs and I leave them at full soft...
You don't race with them that way, or do you? |
autoxed with them, no track. i need more dampening in the front, bumps are very harsh, rears feel great.
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Great description of the effects of too much rebound damping:
Originally Posted by Thucydides
(Post 619707)
“dang; that’s a lot of rebound damping for stock springs”
The greatest opportunity for improvement is in ride quality. The chassis is in essentially constant motion over nearly all road surfaces, and over some particularly poor surfaces (Cataldo Road, Vacaville) this can be fatiguing and even punishing. The chassis follows moderate to large road undulations too closely which causes the chassis to pitch in response. The inside front tire can be made to skip over road surface over tight bumpy corners. The inside wheel feels like it “hangs” above the road surface before re-contacting and regaining grip. High-g cornering on moderately rough to rough corners can cause the steering wheel to saw back and forth as the inside tire gains and looses traction. https://www.miataturbo.net/showthrea...t=44328&page=3 Having lots of rebound damping got popular because it works well on tall plush cars/trucks with soft springs. It dynamically lowers them during transitions, improving grip and stability. Also for autox cars running grippy tires which are rules-limited to use stock springs. Doesn't work well on lower cars with stiffer springs and better suspension geometries, such as the miata. Unfortunately too many didn't realize that what works on tall sedans, doesn't work on sports cars. Works reasonably well on smooth racetracks, but awful on real-world bumpy backroads. Lots of people who've never tried a really good setup and have bad setups (e.g. stiff springs on OTS Konis), don't know what they're missing, and think, "I've got hot shit". |
Originally Posted by Thucydides
(Post 621386)
Hustler, are we talking NA or NB Bilsteins?
On a back road I'm not surprised at all. Shaikh pretty much said the same thing, though I think he might have been talking about autocrossing: "Surprisingly, the grip of the ’93LE with stock springs was higher than the ’91[450/350 springs]! The effect of spring rate reducing ultimate grip started to become apparent." And here's some more Shakh wisdom: "Three Key words: GRIP is KING! A fast suspension needs to have some compliance, otherwise grip is reduced. This means it also needs to be somewhat comfortable for the driver to have confidence. Thus, ultimate grip is tied to having a degree of compliance (ie comfort). Therefore, handling and comfort MUST co-exist!" I think I prefer stiffer springs over softer springs, regardless of which produces the lower lap time...however, I prefer the low lap time to "confidence." |
Originally Posted by JasonC SBB
(Post 621417)
Great description of the effects of too much rebound damping:
I had a feeling this was gonna happen. See my inputs in the GB thread, I was trying to be helpful, but I got smacked down by some holier-than-thou posts: https://www.miataturbo.net/showthrea...t=44328&page=3 Having lots of rebound damping got popular because it works well on tall plush cars/trucks with soft springs. It dynamically lowers them during transitions, improving grip and stability. Also for autox cars running grippy tires which are rules-limited to use stock springs. Doesn't work well on lower cars with stiffer springs and better suspension geometries, such as the miata. Unfortunately too many didn't realize that what works on tall sedans, doesn't work on sports cars. Works reasonably well on smooth racetracks, but awful on real-world bumpy backroads. Lots of people who've never tried a really good setup and have bad setups (e.g. stiff springs on OTS Konis), don't know what they're missing, and think, "I've got hot shit". OP can correct me if I am putting words in mouth. I don't want people to get the wrong impression about what is being discussed here. |
Originally Posted by djp0623
(Post 621449)
I hope i didn't give the impression that my car handles poorly. It's actually great. I think the OP was saying this is a really good product that can be made great.
I'm also very glad that folks like Jason are adding their input, and I'm hoping this will remain a smack down free thread. Experienced shock tuners are pretty rare, and I'm looking for all the help I can get. |
+1 to what Jason said. Shaikh would tell you the same, the high rebound DAMPING RATES on stock class autocrossers are intended to 'jack down' the car during transitional maneuvers. Ie, they don't allow the car to return to it's nominal ride height. Effectively, a proper stock class Miata will be utilizing the bump stops as their spring, and thus spring rate. Stop tuning becomes alpha-omega in these situations.
I, otoh, have the option to tune the suspension as I see fit, as do you guys with dd Miatas. We have to choose spring rates appropriate to our task, and work to keep the car off the stops except in extreme circumstances. For me, I am finding that the "appropriate" spring rate is the one which is stiff enough to control the car's body quickly enough to not allow it to wallow in very quick transitional maneuvers. I am finding that the car can carry enough speed, and has enough grip, to handle slaloms very quickly, but the body is still responding to the car's last move while in the next element. It doesn't take a set quickly enough to use the grip available. Hustler, I hope you KNOW that your dampers can handle the stiffer springs. 700 is a fairly substantial rate, and needs a strong shock to control it's movement. I feel that having the wrong damping is worse than having the wrong spring rate. I like my FCMs, don't regret the purch. What I do regret is that I wasn't sure what I would be doing with the car when I ordered them. They were spec'd as a sometimes street driven, sometimes autocrossed, sometimes tracked setup. Now I want a 100% autocross setup, which I feel would need to be stiffer that the others because of the transitional nature of the activity. |
What does "jack down" mean in layman terms?
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Shock is so stiff in rebound that when you hit a bump or the body rolls during transition, the rebound damping doesn't allow the suspension to return to it's normal static position. Since an autocross car is almost always in transition, with really strong rebound damping the car effectively becomes lower than it would be statically while on course.
Combine weak stock spring rates, high grip from r-comps, tall bump stops, and limited suspension travel in general, the "stock" autocrosser is going to use it's stops as springs. |
In non-autoxing situation you want all the rebound to "control" heavier springs from effectively making your car a bouncy honda or acting as a catapult when driving over speed bumps.
SHOCKS Shocks control the energy in the spring and the timing of the release of this energy. When the spring is compressed the energy of the initial motion is stored and released back in the opposite direction. The shock absorbs, or dampens this release of energy. Without them the car would oscillate uncontrollably. Just take a look at what is driven on the public highways. A well-balanced spring-shock system keeps the chassis steady and in constant contact with the road surface. Shocks only work when weight is being transferred. They have no effect in steady state running or cornering when there is no suspension travel. This is rare. When the suspension is in transition and the shock is extended (rebound) or compressed (bump), they effect the car's handling. Shocks are attached to the car's suspension so that the damper shaft moves when the suspension moves, and to the car's chassis. When the driver responds to the car's movement, the driver is responding to what the shock is doing. Compression controls the motion of the unsprung weight. Rebound controls the motion of the sprung weight. If the car has too much shock the ride will be harsh and will not absorb road irregularities. Too much bump and the car slides, rather than sticks; too much rebound and the wheels will not return quickly to the road surface and the car may "jack down" over long corners. Soft shocks allow the suspension to move fast enough to keep the tire in contact with the road. But too little shock and the car floats, oscillates, dives, rolls and feels generally unresponsive. Jacking down is a situation that occurs when the shock, after hitting a bump and compressing the spring, does not allow the spring to return to its neutral position before the next bump. Repeated bumps will lower the car, there will be a dramatic increase in roll stiffness and the car will understeer or oversteer depending which end is effected. Adjust for bump (compression) first. Bump controls the upward movement of the suspension when hitting a bump. It should not be used to control downward movements, roll or bottoming. Bump is set when "side hop" or "walking" in a bumpy corner is minimal and the ride is not overly harsh. The car should feel positive on turn in. If bump is too soft the nose will dive under braking. Rebound adjustments has the greater effect on the drivers feel of the car. Rebound controls roll and lean when entering or exiting a corner and limits how fast the motion occurs. Rear shock rebound effects corner entry, front shock rebound effects corner exit. Set rebound for smooth entry without excessive leaning. The driver can set the rebound for their preference of oversteer or understeer in corner entry without effecting corner exit. Too much rebound will cause initial loss of lateral acceleration (understeer or oversteer depending which end) and will lead to jacking down. Shocks will control the timing of this motion. Shock rates are a measurement of resistance, determined by the movement of the piston inside the shock body. Stiff shocks slow down the motion and speed up the rate of transfer. Soft shocks allow the suspension to respond quickly and slow the rate of transfer. The total amount (weight) transferred is determined by the spring. |
Originally Posted by webby459
(Post 621723)
+1 to what Jason said. Shaikh would tell you the same, the high rebound DAMPING RATES on stock class autocrossers are intended to 'jack down' the car during transitional maneuvers. Ie, they don't allow the car to return to it's nominal ride height.
Too much rebound will cause initial loss of lateral acceleration (understeer or oversteer depending which end) and will lead to jacking down. Shocks will control the timing of this motion. |
I'm going to post my shock curves later for you guys to analyze. I'm on FM springs.
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I was whoring around at Cobb/AST a couple weeks ago for the "open house" and the AST guy discussed how little information you get from a shock dyno. He went on to say that he's put shocks together on an Evo that were rock hard, revalved them and produced an almost identical dyno, and they worked properly.
Its all over my head though. |
Originally Posted by hustler
(Post 621868)
Its all over my head though.
welcome to my life. |
Originally Posted by Braineack
(Post 621869)
welcome to my life.
Originally Posted by hustler
(Post 621868)
I was whoring around at Cobb/AST a couple weeks ago for the "open house" and the AST guy discussed how little information you get from a shock dyno. He went on to say that he's put shocks together on an Evo that were rock hard, revalved them and produced an almost identical dyno, and they worked properly.
Next, I really don't get how that could be, though. Seriously, can someone surmise how this could be? The only thing that I can think of is it being different in terms of when it was actually mounted in the car, like the suspension was bound up, or something. |
Originally Posted by webby459
(Post 621874)
Next, I really don't get how that could be, though. Seriously, can someone surmise how this could be? The only thing that I can think of is it being different in terms of when it was actually mounted in the car, like the suspension was bound up, or something.
It was pretty cool to hold all the shock parts in your hand and make sense of the mysterious metal tube we spend so much cash on. It also quickly became apparent that I'll never buy another twin-tube shock. I'm looking forward to 949 Club Sport Xidas...especially with AST 30-minutes away. |
While I'm not ready to discount the value of shock dyno graphs, one thing Bernie pointed out was that if you look at a shock test taken to the 5 in/sec. velocity, verses the same shock taken to the 10 in/sec. velocity, they'll produce slightly different curves. The graph I've posted called "Jim Lowe" shows this pretty clearly.
I guess the lesson here is that like a lot of tools the shock dyno isn't perfect, and it's certainly not a substitute for testing on the car. It's still a cool tool, and I'll bet an experienced tuner gets some pretty useful information out of it, but maybe you just can't take it too literally. |
one thing they are useful for is making sure they match side to side...
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I suspect part of the problem with comparing shock dyno graphs is the same problem you have comparing chassis dyno graphs... no two read identically. So if Shaikh builds a shock that works, dynos it on HIS dyno and then makes 100 of them that dyno the same they'll probably all work reasonably similarly.
If AST or Bernie or Moton or whomever takes Shaikh's graph and match it on THEIR shock dyno, odds are the shock will work differently. |
Doesn't he used to valve for Shaikh (FCM)?
I've always heard great things from FCM. I'd assume his valving should be similar or the same to FCMs, or am I wrong? |
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