Stock class shock valving
12-13-2005, 08:44 PM
#11
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Mike,
What you say is true, but I'm not sure where you were going with it. It is worth noting the implication of your post:
The springs and dampers share the force carried to the wheel. The faster the wheel is moving, the stiffer the damper makes the effective spring rate.
A very good point. I was able to keep from rubbing fenders on my stock class Subaru by running really stiff compression damping rates. The tire would still rub, but only at the exit of long sweepers. The damper would carry so much force that it would keep the spring from compressing very quickly... slowing the compression down to the point that a rub wouldn't happen until about 4 seconds into a corner.
The same principle applies in rebound where you can actually jack the car down onto the bumpstops by using soft compression and high rebound damping rates. What happens here is that the spring is not stiff enough to extend the damper before it is re-compressed by transient load. So you effectively run at a lower ride height and a stiffer spring rate becase the car must overcome the internal force in the damper before it can compress the spring further.
Andy
What you say is true, but I'm not sure where you were going with it. It is worth noting the implication of your post:
The springs and dampers share the force carried to the wheel. The faster the wheel is moving, the stiffer the damper makes the effective spring rate.
A very good point. I was able to keep from rubbing fenders on my stock class Subaru by running really stiff compression damping rates. The tire would still rub, but only at the exit of long sweepers. The damper would carry so much force that it would keep the spring from compressing very quickly... slowing the compression down to the point that a rub wouldn't happen until about 4 seconds into a corner.
The same principle applies in rebound where you can actually jack the car down onto the bumpstops by using soft compression and high rebound damping rates. What happens here is that the spring is not stiff enough to extend the damper before it is re-compressed by transient load. So you effectively run at a lower ride height and a stiffer spring rate becase the car must overcome the internal force in the damper before it can compress the spring further.
Andy
12-15-2005, 07:45 AM
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Originally Posted by shaggy,Dec 12 2005, 11:10 PM
For more money, I could get the same type of adjustment out of the compression side of my Penskes... its called the 8760 series canister.
When you say that you are not able to tune the compression using the 8100 cainster, are you saying that the range of adjustment is not enough or that the change in valving from detent to detent is too great?
12-15-2005, 09:17 AM
#13
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The range of compression adjustment on the 8100 reservoir is somewhat limited, and the range gets narrower as you add compression damping via the main valve stack (on the piston). For that reason I switched to the 8660 for the '05 season, and was happy with the decision.
Last spring I debated going with the 8760, dialing in the shocks, and then epoxying one of compression adjusters to make the shock only 2-way adjustable. I decided this was a gray area and lot of money to gamble on a protest, so I asked the SEB for a rule clarification. Meanwhile the clock was ticking, so I played it safe and went with the 8660.
Come July-ish the clarification was finally issued (after MUCH internal debate, I'm told) and you can "fix" an adjuster to make a triple into a double or a quad adjustable into a double. It is also in the 2006 rule book, which is now online.
I have no regrets, however, as my shock guy told me that most people just crank on the high speed compression adjuster, as it also affects low speed, and that in his opinion the 8760 low speed adjuster is a very fine tuning tool.
Steve
Last spring I debated going with the 8760, dialing in the shocks, and then epoxying one of compression adjusters to make the shock only 2-way adjustable. I decided this was a gray area and lot of money to gamble on a protest, so I asked the SEB for a rule clarification. Meanwhile the clock was ticking, so I played it safe and went with the 8660.
Come July-ish the clarification was finally issued (after MUCH internal debate, I'm told) and you can "fix" an adjuster to make a triple into a double or a quad adjustable into a double. It is also in the 2006 rule book, which is now online.
I have no regrets, however, as my shock guy told me that most people just crank on the high speed compression adjuster, as it also affects low speed, and that in his opinion the 8760 low speed adjuster is a very fine tuning tool.
Steve
12-15-2005, 01:08 PM
#15
Originally Posted by shaggy,Dec 13 2005, 08:29 PM
As a general rule, dampers do not change the rate of weight transfer front to rear or left to right.
It's a mass/spring/damper system, right? And the angular acceleration of the body is proportional to the difference between the input torque (due to braking or cornering force) and the resisting torque that's transmitted through the shocks and springs from the ground. And the weight transfer comes directly from that resisting torque.
Imagine a step function being applied to this sytem (as in, you step on the brakes, har har har). With low damping the initial resistance comes only from the spring and the angular acceleration of the body is high due to the initial resistance. Lower resistive force means less weight transfer. At some point the rate of dive and the amount of dive will reach a point where the spring and damper torques exceed the braking torque and start to decelerate the dive. In the extreme case of no damping, that won't happen until the dive angle has reached the "static" dive angle, and you will get overshoot -- along with weight transfer that's greater than the static amount of weight transfer.
With greater damping, there will be more resistance earlier on, resulting in more gradual dive, and a more gradual buildup of the resistive torque as the car reaches the static amount of dive.
Maybe if I'm bored tonight I will crank out the force vs time curves for the step response of a damped & sprung mass that is hit with a step input.
12-15-2005, 04:23 PM
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OK, I see where you're coming from. I've been oversimplifying the problem (I'm a civil engineer, its what we do!) and forgetting about boundary conditions.
Can we agree that for practical purposes this has no significant effect on the balance of the car? It does have some effect on absolute braking / acceleration efficiency but it is still the cross axle effects that determine balance.
Andy
Can we agree that for practical purposes this has no significant effect on the balance of the car? It does have some effect on absolute braking / acceleration efficiency but it is still the cross axle effects that determine balance.
Andy
12-15-2005, 04:27 PM
#17
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Originally Posted by shaggy,Dec 15 2005, 05:23 PM
I've been oversimplifying the problem
12-15-2005, 08:00 PM
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Originally Posted by Orthonormal,Dec 15 2005, 02:08 PM
Lower resistive force means less weight transfer.
In your example of stepping on the brakes, with low resistive force (I'm assuming your talking about the damper), the damper slows the rate at which the weight is transferred. At high rates, (low resistive forces) the weight transfer rate is high and the weight transfer will shoot past the point where the torque generated by the spring/tire combination will equal the weight transfer. Whereas, if the rate of weight transfer is slower (high resistive force), the weight transfer will reach the balance point, and may slightly overshoot, but not near the amount of overshoot if the resistive force is low.
12-15-2005, 10:07 PM
#19
Don't confuse the body motion with the weight transfer. The body rolls or dives because there's a torque due to braking or cornering that isn't instantly balanced by the springs and shocks.
The sooner that the spring/shock force builds up to resist the motion, the slower that motion will be.
Stiff shocks/springs = builds force quickly = slower body motion
The weight transfer comes directly from the additional forces applied by the springs and shocks during cornering. So when the damping and spring forces build quickly, the weight transfer occurs more quickly.
Andy, I think that the fore/aft rate of transfer does matter, because the quicker you shift weight forward when you lift or tap to tuck in the nose, the more effective it will be. I'm seeing that there are some situations where it's not relevant -- like turn-in at the end of a braking zone, where the fore/aft transfer has already occurred, and the effect of front compression is limited solely to how it affects the lateral weight transfer distribution.
The sooner that the spring/shock force builds up to resist the motion, the slower that motion will be.
Stiff shocks/springs = builds force quickly = slower body motion
The weight transfer comes directly from the additional forces applied by the springs and shocks during cornering. So when the damping and spring forces build quickly, the weight transfer occurs more quickly.
Andy, I think that the fore/aft rate of transfer does matter, because the quicker you shift weight forward when you lift or tap to tuck in the nose, the more effective it will be. I'm seeing that there are some situations where it's not relevant -- like turn-in at the end of a braking zone, where the fore/aft transfer has already occurred, and the effect of front compression is limited solely to how it affects the lateral weight transfer distribution.
12-16-2005, 03:36 AM
#20
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I think there may be some confusion between RATE of weight transfer and AMOUNT of weight transfer in this discussion. Dampers only affect the rate, not the amount. Ortho is right, in that high damping rates (stiff shocks) increase the rate of weight transfer.
Initially, this concept is usually very counterintuitive, don't feel bad if you don't get it right away (I know I didn't, and I'm a mechanical engineer). People want to look at what the body of the car is doing, and correlate that to weight transfer. It is the internal forces that matter.
Work through this logic exercise with me. We have a spring that is one mile long. It has no mass and we won't worry about coil bind. It has a rate of 100 lb/mile. One mile up in the air there is a 100 lb weight at the end of the spring. You are standing at the bottom, holding up the spring when someone drops the weight onto the spring. The weight begins to accelerate downward, but initially, you hardly feel any weight in your hands, because the weight has to travel a significant distance to build force in the spring. In fact, the weight has to travel 1 mile before the spring is compressed enough that you are holding up the entire 100 lbs.
Now, we put a massless damper in parallel to the spring. This is a very stiff damper, and 100 lbs only moves it 0.5 in/sec. You are at the bottom, again, holding up the spring and damper when someone drops the weight. Nearly instantly, you will feel the entire 100 lbs in your hands. The damper is "supporting" the 100 lb weight, and lowering it slowly to you. Over the next minutes (hours?) the weight will continue to travel downward until it travels the 1 mile, and the spring force balances out the gravitational force on the 100 lb weight.
In the end, the weight travels the same 1 mile distance, whether the damper was there or not. In this example, the weight is the sprung mass (the body of the car), and your hands are the road surface. The first example is analogous to the car on soft shocks. The second example is an extreme case of putting on very stiff shocks. The stiff shocks increase the rate of weight transfer, but not the total amount.
HTH
Steve
Initially, this concept is usually very counterintuitive, don't feel bad if you don't get it right away (I know I didn't, and I'm a mechanical engineer). People want to look at what the body of the car is doing, and correlate that to weight transfer. It is the internal forces that matter.
Work through this logic exercise with me. We have a spring that is one mile long. It has no mass and we won't worry about coil bind. It has a rate of 100 lb/mile. One mile up in the air there is a 100 lb weight at the end of the spring. You are standing at the bottom, holding up the spring when someone drops the weight onto the spring. The weight begins to accelerate downward, but initially, you hardly feel any weight in your hands, because the weight has to travel a significant distance to build force in the spring. In fact, the weight has to travel 1 mile before the spring is compressed enough that you are holding up the entire 100 lbs.
Now, we put a massless damper in parallel to the spring. This is a very stiff damper, and 100 lbs only moves it 0.5 in/sec. You are at the bottom, again, holding up the spring and damper when someone drops the weight. Nearly instantly, you will feel the entire 100 lbs in your hands. The damper is "supporting" the 100 lb weight, and lowering it slowly to you. Over the next minutes (hours?) the weight will continue to travel downward until it travels the 1 mile, and the spring force balances out the gravitational force on the 100 lb weight.
In the end, the weight travels the same 1 mile distance, whether the damper was there or not. In this example, the weight is the sprung mass (the body of the car), and your hands are the road surface. The first example is analogous to the car on soft shocks. The second example is an extreme case of putting on very stiff shocks. The stiff shocks increase the rate of weight transfer, but not the total amount.
HTH
Steve