the how to go fast thread
06-04-2008, 04:39 PM
#31
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06-04-2008, 04:45 PM
#32
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TRAIL BRAKING
In the last installment, JoElla John described the art of Chauffeur Braking, that of using the most pressure on the brake pedal when the speed is the highest and gradually releasing the pressure as the speed decreases. With this skill well mastered you should be able to transfer from the brake pedal to the throttle without the car’s chassis or your passengers realizing the exact instant the transfer is made. Now you are ready to implement this skill into the satisfying process of weight transfer management.
Articles and ads written about cars may mention that a certain car has near perfect 50/50 weight distribution. Does this mean that the car is always balanced? No, only when it is stationary or at constant velocity. Some weight is dynamically shifted around the car’s center of gravity, toward the rear when accelerating and toward the front when decelerating.. This is not a bad effect, since a rear wheel drive car such as ours actually gets better tire grip for acceleration due to the rearward weight shift under acceleration. Likewise the front tires get better grip under braking due to the forward weight transfer. Used correctly, this forward weight bias is a handy condition for the optimum control of our cars.
When braking for a fast, sweeping turn the braking period should be executed so that the subtle transfer from the brake to the throttle is accomplished at or near the point of turning the steering wheel into the turn. The application of the throttle transfers the weight gradually to the rear of the car, causing a safe, stable, slightly understeering condition. The use of both the steering wheel and the throttle together guide the car smoothly through the turn, accelerating toward the turn exit.
However, when braking for a slow, tight turn the braking period should be shifted somewhat toward the turn-in point, so that the last bit of braking is done as you begin turning the wheel into the turn. This is known as TRAIL BRAKING. You do this all of the time during street driving, even to the point of applying the brakes in the middle of a turn, right? And it’s no big deal. That’s because at street speeds there is little weight transfer to unbalance the car. When you purposefully unbalance the car at more elevated speeds the weight transfer can be used to your advantage, because your Chauffeur Braking pedal technique keeps the front tires gripping well into the slow turn. Done incorrectly by jumping off the brakes while entering the turn or lifting off the brake pedal before entering the turn, the car will take on an undesirable understeering attitude and take a much wider line than you intended. Usually folks blame the car or the tires for the car understeering too much, but the car is only doing as it’s directed by the driver. Entering the turn properly, the front of the car is somewhat heavier than normal and the rear is lighter than normal, therefore the rear tires have subsequently less grip. The car is unbalanced, but for a reason. You are inducing oversteer to help point the car into the turn, and it’s imperative to use your eyes to look where you want the car to go.
WARNING: If the car stays in this unbalanced condition for just slightly too long, a spin will surely follow. That’s why the eyes must tell the foot the instant when the transfer from soft brake pressure to the throttle should take place. The application of the throttle transfers the weight from the front to the rear tires, regaining the car’s balance, and as a benefit the car is in an accelerating mode, perfect for a fast exit. If the car enters the turn understeering, then the application of the throttle is much later in time, and the exit speed is lower. The basic braking technique for all turns is the same, it’s only the timing that is different. The faster the turn, the sooner you release the brakes; the tighter the turn, the later you release them. Remember, as in all driving, the eyes are the key, and the earlier the throttle application in the turns, the quicker you will be.
Those of you who have participated in our recent Driver Schools at VIR can relate this discussion to Turn 4 (Left Hook) and Turn 11A (Oak Tree) and even to turn 1. Remember the fight involved trying to get the car to turn in enough to get to the apex? Well, it should never be a fight, because you and the car both lose. It is much better to work in harmony with the car. Use that Chauffeur Braking technique while Trail Braking, and enjoy the bliss of the rapid exit.
Articles and ads written about cars may mention that a certain car has near perfect 50/50 weight distribution. Does this mean that the car is always balanced? No, only when it is stationary or at constant velocity. Some weight is dynamically shifted around the car’s center of gravity, toward the rear when accelerating and toward the front when decelerating.. This is not a bad effect, since a rear wheel drive car such as ours actually gets better tire grip for acceleration due to the rearward weight shift under acceleration. Likewise the front tires get better grip under braking due to the forward weight transfer. Used correctly, this forward weight bias is a handy condition for the optimum control of our cars.
When braking for a fast, sweeping turn the braking period should be executed so that the subtle transfer from the brake to the throttle is accomplished at or near the point of turning the steering wheel into the turn. The application of the throttle transfers the weight gradually to the rear of the car, causing a safe, stable, slightly understeering condition. The use of both the steering wheel and the throttle together guide the car smoothly through the turn, accelerating toward the turn exit.
However, when braking for a slow, tight turn the braking period should be shifted somewhat toward the turn-in point, so that the last bit of braking is done as you begin turning the wheel into the turn. This is known as TRAIL BRAKING. You do this all of the time during street driving, even to the point of applying the brakes in the middle of a turn, right? And it’s no big deal. That’s because at street speeds there is little weight transfer to unbalance the car. When you purposefully unbalance the car at more elevated speeds the weight transfer can be used to your advantage, because your Chauffeur Braking pedal technique keeps the front tires gripping well into the slow turn. Done incorrectly by jumping off the brakes while entering the turn or lifting off the brake pedal before entering the turn, the car will take on an undesirable understeering attitude and take a much wider line than you intended. Usually folks blame the car or the tires for the car understeering too much, but the car is only doing as it’s directed by the driver. Entering the turn properly, the front of the car is somewhat heavier than normal and the rear is lighter than normal, therefore the rear tires have subsequently less grip. The car is unbalanced, but for a reason. You are inducing oversteer to help point the car into the turn, and it’s imperative to use your eyes to look where you want the car to go.
WARNING: If the car stays in this unbalanced condition for just slightly too long, a spin will surely follow. That’s why the eyes must tell the foot the instant when the transfer from soft brake pressure to the throttle should take place. The application of the throttle transfers the weight from the front to the rear tires, regaining the car’s balance, and as a benefit the car is in an accelerating mode, perfect for a fast exit. If the car enters the turn understeering, then the application of the throttle is much later in time, and the exit speed is lower. The basic braking technique for all turns is the same, it’s only the timing that is different. The faster the turn, the sooner you release the brakes; the tighter the turn, the later you release them. Remember, as in all driving, the eyes are the key, and the earlier the throttle application in the turns, the quicker you will be.
Those of you who have participated in our recent Driver Schools at VIR can relate this discussion to Turn 4 (Left Hook) and Turn 11A (Oak Tree) and even to turn 1. Remember the fight involved trying to get the car to turn in enough to get to the apex? Well, it should never be a fight, because you and the car both lose. It is much better to work in harmony with the car. Use that Chauffeur Braking technique while Trail Braking, and enjoy the bliss of the rapid exit.
06-04-2008, 05:39 PM
#33
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Join Date: Jul 2003
Location: in your mommy's bung hole
Posts: 1,204
Car Info: 04 Red Impreza, 84 Levin GT-SR5
06-05-2008, 06:33 AM
#34
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i need more seat time in a RWD car.
racing at say, moran probably won't automatically make you a better driver, but you can translate a lot of it to driving a car. they are two very different experiences.... obviously.
WTF is bad juju driving?
racing at say, moran probably won't automatically make you a better driver, but you can translate a lot of it to driving a car. they are two very different experiences.... obviously.
WTF is bad juju driving?
06-05-2008, 06:48 AM
#35
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Location: <3 tai mei
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suspension travel
One of the most important criteria for both good handling and ride quality is having sufficient suspension travel. Suspension travel is what allows your suspension to do its job, and without enough, you'll find the ride jarring and your tires will feel equally overworked. This is a rare case of too little travel hurting both the car's comfort and its grip, so its importance cannot be underestimated regardless of the vehicle's intended purpose.
Unfortunately, the issue of suspension travel is very frequently overlooked by aftermarket suspension manufacturers- especially those making lowering springs. Many products do not or cannot properly address the issue of suspension travel, and owners may attribute the associated loss of handling and comfort to be a byproduct of stiffer springs. Often, this changes made by lowering springs are primarily made by the new ride height, not the spring rate change. Even many premium coilover sets have very limited suspension travel, which limits their usefulness except on smooth race tracks.
This series of articles will discuss how to measure your suspension travel, how it is used, and why having sufficient travel is so important.
Determining Suspension Travel in the Damper
To determine how much suspension travel your vehicle has, do the following:
1) Measure the distance from the center of your wheel (or, if you prefer, the upper lip of the wheel) to the center of the fender. We'll call this your ride height measurement.
2) Jack up your car until your wheels are off the ground. Measure again. We'll call this your droop height measurement. The two measurements should look somewhat like this:
3) Remove a shock and/or strut. We will need to see how much travel it has. If you can compress the shock by hand and measure its compression, that's great. If not (the gas pressure in dampers can be quite high), just measure the shaft of the damper, as shown. This is our damper travel measurement.
4) If you have a bump stop (sometimes called a jounce bumper) on the damper, measure to where that is, too. This is your linear damper travel. We'll elaborate on this later.
5) Determine your motion ratio. (See: Motion Ratios)
6) Reassemble.
Now, you have four measurements- your ride height, droop height, damper travel, and motion ratio. From these measurements, you can quite easily calculate how much suspension travel you have.
First, we need to figure out how wheel movement translates into damper movement. This is what the motion ratio tells us. The equation for this is:
wheel travel * motion ratio = damper travel
Or, going the other direction, we can figure out how much wheel travel the car has based on available damper travel:
damper travel / motion ratio = wheel travel
In the first damper pictured above, we see about 6.5 inches of suspension travel. The motion ratio for this car is 0.97, so the wheel travel permitted by the damper is 6.5 / 0.97 = 6.7 inches.
In the picture above illustrating wheel travel, a difference between ride height and full droop of 3 inches was recorded. Since we know there is 6.7 inches of total travel, and 3 inches of that is droop travel, we know that there is 3.7 inches travel remaining as bump travel. 3.7 inches of travel in the bump direction is quite a bit for a road car!
However, there are other things that can limit suspension travel. Among them are tire clearance, spring travel, and bump stops (jounce bumpers). Continue reading for more on those!
Bump Stops, Bump Travel, and Lowering Springs
The modern bump stop is more properly called a jounce bumper. Made from urethane and cut to varying sizes, today's bump stops are an integral part of most factory suspensions. Knowing their effects on the suspension is as important- if not more so- than that of your main springs. This page explores their effects when installing springs that lower a production car.
Below is a stock strut from a 2005 Impreza WRX STI:
The strut has about 5.75 inches of total travel, but it also has a 2.25 inch bump stop (also called a jounce bumper) installed. This bump stop is very stiff to prevent the car from damaging itself under extreme stress, but this also makes the ride uncomfortable and a bit unpredictable if the bump stops engage in regular driving.
With only 3.5 inches of travel before the damper engages the bump stop, it's important to see how much of that travel is in bump travel and how much is in droop travel. Upon measuring, the Subaru Impreza WRX STI was found to have nearly 3.5 inches droop travel on the front wheels, meaning the car is resting just above its bump stops at its ride height. Any bump the car hits- or even the weight of the driver- will engage the bump stops.
As it turns out, this is quite common- many popular cars actually ride on their bump stops at their factory ride height. Manufacturers use long bump stops to create a very progressive spring rate. This allows the manufacturer to use much softer main springs for better overall ride quality. Usually, the bump stop itself is very progressive- starting out soft but increasing in resistance until it won't compress anymore.
Because bump stops are designed to be very progressive, the behavior of a car drastically changes if a person replaces the factory springs with springs that lower the vehicle's ride height. This is because lowering springs don't just put the car onto the bump stops at ride height- they put the bump stops past their initial soft phase.
Below is the measured resistance from the bump stop pictured above:
You can see these particular bump stops offer a somewhat linear resistance of 300lbs in the first inch. There is no very soft first progression to be found here- the main springs are 224lbs per inch, so the bump stops are roughly 33% stiffer than the main springs! The total spring rate here is:
224lb/in + 300lb/in = 524lb/in
Beyond an inch, though, the jounce bumper quickly becomes four times stiffer still! This is the range of the bump stop designed to prevent the car from damaging itself under extreme stress. Most bump stops become extremely stiff by half of their full height and reach their terminal compression (they won't compress further with any amount of load) at 75% compression. The particular bump stop measured follows these trends.
So, what do lowering springs do? On this car, most lowering springs lower the car about an inch. This means the bump stop has also compressed nearly an inch, and it is now supporting 300lbs per wheel of the vehicle's weight. Furthermore, the suspension will not compress further except with extreme forces since the spring rate in the bump direction is around 1600lb/in just from the bump stop alone. Below is a graph illustrating how the spring rate stiffens significantly (the slope of the line gets steeper):
What does such a car feel like? It'll depend on the road, tires, and more. When the car hits a bump, the suspension will not be able to absorb the bump by compression. Instead, large forces will be transmitted to both the tires and the chassis, putting stress on the tire and accelerating the chassis upwards into its droop range. The chassis has lots of travel in that direction (and the spring rate gets much softer), so the car will once again feel soft after a harsh initial impact. The car will then settle down onto its bump stops again until the next bump. This extra chassis movement means a change in the car's center of gravity and inertia, which can create unpredictably and instability that the driver must cope with.
Furthermore, because the effective spring rate will have risen, the vehicle's dampers may now be too soft. This, along with the tendency of bumps to be transmitted to the chassis, can create a feeling of bounciness. Because the chassis is heavy, movement of the chassis contains a lot of potential energy that must be dissipated by the damper. The extra movement means your dampers are working longer, which causes them to generate more heat. This heat generally leads to wear.
So, there are three huge drawbacks to this design. All are present to some degree on a factory car that rides on its bump stops, but all get much worse as bump travel is reduced via lowering springs.
* Harsh initial impact on bumps, even small ones
* Ride height raises over bumps, creating unpredictability
* Potential for faster wear on the chassis, bushings, tires, and dampers
spring travel and coil bind
Coil bind happens when all the coils of a spring are stacked on top of one another and the spring cannot compress any further. When the spring cannot compress any further, any additional force gets transmitted directly on the chassis and tires, leading to very high amounts of stress on those items (and the driver). A spring that cannot compress any further is said to be at its solid length or bound length.
Regretfully, this aspect of design is quite frequently overlooked those who create aftermarket suspension packages- both in lowering springs and in full suspension packages.
Shown below is a strut from a 2005 Subaru WRX STI:
In this picture, the distance between the upper and lower spring perches has been shown. At its shortest point, the two spring perches are 7.5 inches apart. However, we know that (from a measurement in the previous article) that this particular strut has 5.75 inches of suspension travel. Thus, when the strut reaches full compression, the spring will be compressed to a full length of just 1.75 inches at its shortest point. This is quite a small space to compress to!
As it so happens, the factory springs installed on this strut compress to just 2.25 inches. Why the discrepancy?
Because of the bump stop, the strut should never reach full compression. We also saw in the previous article that a full-length bump stop has gotten very stiff at 5 inches into the damper travel, and its terminal compression isn't much further. At 5.25 inches into the damper travel the coils of the spring will all collapse together, and this (not surprisingly) is also the terminal amount of travel the bump stop will allow. Thus, while the spring is short enough to have coil bind problems without a bump stop, it is exactly short enough for use with this particular bump stop.
Ideally a spring's solid length should be short enough to never occur in practice except in the event of a bump stop failure- that is, if the bump stop were to split and become ineffective. In such a case, it may be desirable to have coil bind so that the damper itself does not get damaged, but either way this a scenario to hope never happens.
Pictured here is an aftermarket spring designed for this strut:
The spring has 1.5 extra coil winds compared to the factory spring, and each wind is thicker, too. Because of this, the spring's solid length is over 0.75 inches longer, and it easily reached its solid length in use. A shortened bump stop, as some tried, only made this problem more frequent and severe. There is no good way to correct this situation other than to replace the springs.
Not all springs are created equal. To show this, here are two springs of nearly equal spring rate and length. (Both are 9" long, but one is 300lb/in while the other is 280lb/in. That's close enough that we'll consider them both 300lb/in.) Notice how one uses thicker coils and many more of them:
Because of this, the blue spring has a significantly shorter solid length. The difference is not slight: the silver spring has about 4.2 inches of travel, while the blue spring has 5.7 inches. This is a 1.5 inch difference, or a whopping 35% more travel! Furthermore, for a car that puts 900lbs on each of these springs, we know that 3 inches of travel will be used up just by supporting the weight of the car. (This is the car's ride height.) At this point, the silver spring has 1.2 inches left for bump travel, but the blue spring has 2.7 inches remaining. This means the blue spring allows 225% more bump travel!
For factory-replacement lowering springs, you may have to take your own measurements, but do not assume the manufacturer has done that testing already. Often lowering spring manufacturers outsource the manufacture of the springs to a company that does not know the bound length requirements for the vehicle. For coilover springs, all good manufacturers list the available travel (or solid length) of their springs on their web site, so be sure to choose springs that are capable of delivering what your car needs.
source: build a faster car
One of the most important criteria for both good handling and ride quality is having sufficient suspension travel. Suspension travel is what allows your suspension to do its job, and without enough, you'll find the ride jarring and your tires will feel equally overworked. This is a rare case of too little travel hurting both the car's comfort and its grip, so its importance cannot be underestimated regardless of the vehicle's intended purpose.
Unfortunately, the issue of suspension travel is very frequently overlooked by aftermarket suspension manufacturers- especially those making lowering springs. Many products do not or cannot properly address the issue of suspension travel, and owners may attribute the associated loss of handling and comfort to be a byproduct of stiffer springs. Often, this changes made by lowering springs are primarily made by the new ride height, not the spring rate change. Even many premium coilover sets have very limited suspension travel, which limits their usefulness except on smooth race tracks.
This series of articles will discuss how to measure your suspension travel, how it is used, and why having sufficient travel is so important.
Determining Suspension Travel in the Damper
To determine how much suspension travel your vehicle has, do the following:
1) Measure the distance from the center of your wheel (or, if you prefer, the upper lip of the wheel) to the center of the fender. We'll call this your ride height measurement.
2) Jack up your car until your wheels are off the ground. Measure again. We'll call this your droop height measurement. The two measurements should look somewhat like this:
3) Remove a shock and/or strut. We will need to see how much travel it has. If you can compress the shock by hand and measure its compression, that's great. If not (the gas pressure in dampers can be quite high), just measure the shaft of the damper, as shown. This is our damper travel measurement.
4) If you have a bump stop (sometimes called a jounce bumper) on the damper, measure to where that is, too. This is your linear damper travel. We'll elaborate on this later.
5) Determine your motion ratio. (See: Motion Ratios)
6) Reassemble.
Now, you have four measurements- your ride height, droop height, damper travel, and motion ratio. From these measurements, you can quite easily calculate how much suspension travel you have.
First, we need to figure out how wheel movement translates into damper movement. This is what the motion ratio tells us. The equation for this is:
wheel travel * motion ratio = damper travel
Or, going the other direction, we can figure out how much wheel travel the car has based on available damper travel:
damper travel / motion ratio = wheel travel
In the first damper pictured above, we see about 6.5 inches of suspension travel. The motion ratio for this car is 0.97, so the wheel travel permitted by the damper is 6.5 / 0.97 = 6.7 inches.
In the picture above illustrating wheel travel, a difference between ride height and full droop of 3 inches was recorded. Since we know there is 6.7 inches of total travel, and 3 inches of that is droop travel, we know that there is 3.7 inches travel remaining as bump travel. 3.7 inches of travel in the bump direction is quite a bit for a road car!
However, there are other things that can limit suspension travel. Among them are tire clearance, spring travel, and bump stops (jounce bumpers). Continue reading for more on those!
Bump Stops, Bump Travel, and Lowering Springs
The modern bump stop is more properly called a jounce bumper. Made from urethane and cut to varying sizes, today's bump stops are an integral part of most factory suspensions. Knowing their effects on the suspension is as important- if not more so- than that of your main springs. This page explores their effects when installing springs that lower a production car.
Below is a stock strut from a 2005 Impreza WRX STI:
The strut has about 5.75 inches of total travel, but it also has a 2.25 inch bump stop (also called a jounce bumper) installed. This bump stop is very stiff to prevent the car from damaging itself under extreme stress, but this also makes the ride uncomfortable and a bit unpredictable if the bump stops engage in regular driving.
With only 3.5 inches of travel before the damper engages the bump stop, it's important to see how much of that travel is in bump travel and how much is in droop travel. Upon measuring, the Subaru Impreza WRX STI was found to have nearly 3.5 inches droop travel on the front wheels, meaning the car is resting just above its bump stops at its ride height. Any bump the car hits- or even the weight of the driver- will engage the bump stops.
As it turns out, this is quite common- many popular cars actually ride on their bump stops at their factory ride height. Manufacturers use long bump stops to create a very progressive spring rate. This allows the manufacturer to use much softer main springs for better overall ride quality. Usually, the bump stop itself is very progressive- starting out soft but increasing in resistance until it won't compress anymore.
Because bump stops are designed to be very progressive, the behavior of a car drastically changes if a person replaces the factory springs with springs that lower the vehicle's ride height. This is because lowering springs don't just put the car onto the bump stops at ride height- they put the bump stops past their initial soft phase.
Below is the measured resistance from the bump stop pictured above:
You can see these particular bump stops offer a somewhat linear resistance of 300lbs in the first inch. There is no very soft first progression to be found here- the main springs are 224lbs per inch, so the bump stops are roughly 33% stiffer than the main springs! The total spring rate here is:
224lb/in + 300lb/in = 524lb/in
Beyond an inch, though, the jounce bumper quickly becomes four times stiffer still! This is the range of the bump stop designed to prevent the car from damaging itself under extreme stress. Most bump stops become extremely stiff by half of their full height and reach their terminal compression (they won't compress further with any amount of load) at 75% compression. The particular bump stop measured follows these trends.
So, what do lowering springs do? On this car, most lowering springs lower the car about an inch. This means the bump stop has also compressed nearly an inch, and it is now supporting 300lbs per wheel of the vehicle's weight. Furthermore, the suspension will not compress further except with extreme forces since the spring rate in the bump direction is around 1600lb/in just from the bump stop alone. Below is a graph illustrating how the spring rate stiffens significantly (the slope of the line gets steeper):
What does such a car feel like? It'll depend on the road, tires, and more. When the car hits a bump, the suspension will not be able to absorb the bump by compression. Instead, large forces will be transmitted to both the tires and the chassis, putting stress on the tire and accelerating the chassis upwards into its droop range. The chassis has lots of travel in that direction (and the spring rate gets much softer), so the car will once again feel soft after a harsh initial impact. The car will then settle down onto its bump stops again until the next bump. This extra chassis movement means a change in the car's center of gravity and inertia, which can create unpredictably and instability that the driver must cope with.
Furthermore, because the effective spring rate will have risen, the vehicle's dampers may now be too soft. This, along with the tendency of bumps to be transmitted to the chassis, can create a feeling of bounciness. Because the chassis is heavy, movement of the chassis contains a lot of potential energy that must be dissipated by the damper. The extra movement means your dampers are working longer, which causes them to generate more heat. This heat generally leads to wear.
So, there are three huge drawbacks to this design. All are present to some degree on a factory car that rides on its bump stops, but all get much worse as bump travel is reduced via lowering springs.
* Harsh initial impact on bumps, even small ones
* Ride height raises over bumps, creating unpredictability
* Potential for faster wear on the chassis, bushings, tires, and dampers
spring travel and coil bind
Coil bind happens when all the coils of a spring are stacked on top of one another and the spring cannot compress any further. When the spring cannot compress any further, any additional force gets transmitted directly on the chassis and tires, leading to very high amounts of stress on those items (and the driver). A spring that cannot compress any further is said to be at its solid length or bound length.
Regretfully, this aspect of design is quite frequently overlooked those who create aftermarket suspension packages- both in lowering springs and in full suspension packages.
Shown below is a strut from a 2005 Subaru WRX STI:
In this picture, the distance between the upper and lower spring perches has been shown. At its shortest point, the two spring perches are 7.5 inches apart. However, we know that (from a measurement in the previous article) that this particular strut has 5.75 inches of suspension travel. Thus, when the strut reaches full compression, the spring will be compressed to a full length of just 1.75 inches at its shortest point. This is quite a small space to compress to!
As it so happens, the factory springs installed on this strut compress to just 2.25 inches. Why the discrepancy?
Because of the bump stop, the strut should never reach full compression. We also saw in the previous article that a full-length bump stop has gotten very stiff at 5 inches into the damper travel, and its terminal compression isn't much further. At 5.25 inches into the damper travel the coils of the spring will all collapse together, and this (not surprisingly) is also the terminal amount of travel the bump stop will allow. Thus, while the spring is short enough to have coil bind problems without a bump stop, it is exactly short enough for use with this particular bump stop.
Ideally a spring's solid length should be short enough to never occur in practice except in the event of a bump stop failure- that is, if the bump stop were to split and become ineffective. In such a case, it may be desirable to have coil bind so that the damper itself does not get damaged, but either way this a scenario to hope never happens.
Pictured here is an aftermarket spring designed for this strut:
The spring has 1.5 extra coil winds compared to the factory spring, and each wind is thicker, too. Because of this, the spring's solid length is over 0.75 inches longer, and it easily reached its solid length in use. A shortened bump stop, as some tried, only made this problem more frequent and severe. There is no good way to correct this situation other than to replace the springs.
Not all springs are created equal. To show this, here are two springs of nearly equal spring rate and length. (Both are 9" long, but one is 300lb/in while the other is 280lb/in. That's close enough that we'll consider them both 300lb/in.) Notice how one uses thicker coils and many more of them:
Because of this, the blue spring has a significantly shorter solid length. The difference is not slight: the silver spring has about 4.2 inches of travel, while the blue spring has 5.7 inches. This is a 1.5 inch difference, or a whopping 35% more travel! Furthermore, for a car that puts 900lbs on each of these springs, we know that 3 inches of travel will be used up just by supporting the weight of the car. (This is the car's ride height.) At this point, the silver spring has 1.2 inches left for bump travel, but the blue spring has 2.7 inches remaining. This means the blue spring allows 225% more bump travel!
For factory-replacement lowering springs, you may have to take your own measurements, but do not assume the manufacturer has done that testing already. Often lowering spring manufacturers outsource the manufacture of the springs to a company that does not know the bound length requirements for the vehicle. For coilover springs, all good manufacturers list the available travel (or solid length) of their springs on their web site, so be sure to choose springs that are capable of delivering what your car needs.
source: build a faster car
08-30-2008, 02:04 PM
#39
Registered User
iTrader: (13)
Join Date: Mar 2008
Location: Pleasanton /La Jolla
Posts: 3,184
Car Info: 2005 Stg.2 Wrx
To double-clutch during a downshift, perform the following steps:
* Let off the throttle, press in the clutch, and shift the “stick” to neutral.
* Let out the clutch.
* Bump the throttle to make the engine “blip”. Sometimes in my MG I’ll press in the accelerator all the way to the floor for a fraction of a second.
* Press in the clutch.
* As the engine speed decreases to match the transmission speed, throw the stick into the next lower gear. Since you actively “matched the revs, it should fall right in!
* Let out the clutch. The downshift should have been as smooth as butter!
why do you need the 2nd step of letting out the clutch? cant you just keep it in and bleep the throttle?
* Let off the throttle, press in the clutch, and shift the “stick” to neutral.
* Let out the clutch.
* Bump the throttle to make the engine “blip”. Sometimes in my MG I’ll press in the accelerator all the way to the floor for a fraction of a second.
* Press in the clutch.
* As the engine speed decreases to match the transmission speed, throw the stick into the next lower gear. Since you actively “matched the revs, it should fall right in!
* Let out the clutch. The downshift should have been as smooth as butter!
why do you need the 2nd step of letting out the clutch? cant you just keep it in and bleep the throttle?
01-07-2009, 11:48 PM
#40
Registered User
iTrader: (19)
Join Date: Apr 2006
Location: Bay Area
Posts: 1,112
Car Info: Sti hatch
do a track event and get an instructor, they can teach you a lot due to many events worth of practice, that is if you get a competent one whom enjoys the leisure of teaching, not just in it for their benefit of a free track day..
01-08-2009, 12:11 AM
#41
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iTrader: (17)
Join Date: May 2005
Location: Sunnyvale, CA
Posts: 22,776
Car Info: '13 BRZ Limited / '02 WRX
To double-clutch during a downshift, perform the following steps:
* Let off the throttle, press in the clutch, and shift the “stick” to neutral.
* Let out the clutch.
* Bump the throttle to make the engine “blip”. Sometimes in my MG I’ll press in the accelerator all the way to the floor for a fraction of a second.
* Press in the clutch.
* As the engine speed decreases to match the transmission speed, throw the stick into the next lower gear. Since you actively “matched the revs, it should fall right in!
* Let out the clutch. The downshift should have been as smooth as butter!
why do you need the 2nd step of letting out the clutch? cant you just keep it in and bleep the throttle?
* Let off the throttle, press in the clutch, and shift the “stick” to neutral.
* Let out the clutch.
* Bump the throttle to make the engine “blip”. Sometimes in my MG I’ll press in the accelerator all the way to the floor for a fraction of a second.
* Press in the clutch.
* As the engine speed decreases to match the transmission speed, throw the stick into the next lower gear. Since you actively “matched the revs, it should fall right in!
* Let out the clutch. The downshift should have been as smooth as butter!
why do you need the 2nd step of letting out the clutch? cant you just keep it in and bleep the throttle?
This really does become second nature, rather quickly too. I was up in Napa at the end of the summer, exploring some backroads. I had been double-clutch downshifting for a few weeks by then, and all of a sudden I realized I had been doing it the entire time I was out there! It really does make shifting so much smoother. I'm still working on doing double-clutch upshifts smoothly though (I haven't put as much effort into it).
10-31-2012, 04:04 AM
#45
Registered User
Join Date: Oct 2012
Location: USA, Chicago illinois
Posts: 5
Car Info: Mazda rx 8
it makes for an interesting research. doesn't damage to try and understand how drivers can make their automobiles go really fast around a schedule.
i think i should put up a little lawful essential observe, i dont' announce this will make you any faster than you really are, nor do i announce that me posting this will make me faster than you. so research (or don't read) at your own interest
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i think i should put up a little lawful essential observe, i dont' announce this will make you any faster than you really are, nor do i announce that me posting this will make me faster than you. so research (or don't read) at your own interest
campervan hire gold coast
Last edited by Johnlawrencew; 11-03-2012 at 03:04 AM.
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