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In my constant effort to bore you all senseless I thought I would write a few thousand words on a subject that has not been exhausted yet..
Springs. I thought I'd write about it as there have been discussions about rear suspensions. The basis to which is a spring. Front suspensions too are based on springs. First things first.
A Spring is a device that stores kinetic energy. Any substance that can be deformed by force then return that energy by reforming completely is a spring. Some household examples:
Rubber erasers.
The little springs inside ball point pens.
Rubber bands.
Rubber bands:
By pulling with XX amount of force you can deform the rubber band (stretch it) and when you release it it releases the exact* same amount of force when it returns to its original shape. The stretching is possible due to stressing or deforming the links of rubber on the atomic level..
(wow) That leads to;
*Not the exact amount as there are loses in friction and heat etc..
Modulus.
The modulus of a material in simple terms describes its springing ability.
What are considered "soft" metals have low a modulus. When deformed to a point they retain that deformation. They stay bent or in the case of a balloon they burst or once deflated do not return to their original shape. Now at birthday parties you can say "don't exceed the balloon's modulus!!" Materials with higher moduli return to the original shape. The amount of force required to deform a high moduli material, what we will now call a spring is referred to as it's rate.
Spring rate:
This is expressed in force/distance. For example a spring with a rate of 100Lb/ inch requires 100 pounds of force to change it's length 1 inch.
This is really the effective spring rate. I'll expand on that later.
Types of springs:
As I described it anything that can be deformed and return the force thus returning to shape is a spring. Lets look at a common expansion spring. A rubber band. Take a rubber band in your hands and you can explore a lot of the concepts around springs. First off loop it around the fingers of each hand. You can see that by increasing the distance between your fingers you can stretch the band. OK duh, but bare with me. Once the slack is taken up, that is once the band is tight, increasing the distance, stretching the band, requires a defined force. This force is pretty small but it is liner. Liner refers to an amount that increases the same amount over a distance. So if it takes say 1oz of force to stretch the band 1 inch then the second inch takes another ounce. So to stretch the band 3 inches that would require 3 ounces. If the force were non-liner (or as we tech guys say "on a curve"), the amount to stretch each successive inch would require greater force. So 1 ounce for the first inch, 2 ounces the second inch 3 ounces the third. So instead of requiring 3 ounces for 3 inches of stretch it would require 6 ounces. The other thing that you should notice is that the whole length of the spring stretches the same amount. That is that you cannot stretch the middle of a liner spring while keeping the center unchanged. This verifies it is liner.
Compression springs work the same way. The rubber eraser when bent will return to shape unless you exceed the modulus and then it breaks.
A coil spring (like inside the ball point) is a compression spring and an expansion spring. Since it has spaces between the coils. Coil springs work by twisting the bar stock they are made from. A 3 foot piece of round spring steel works similarly as a coil spring. It takes the same amount of force to twist a length of 100 pound spring stock no matter the shape. The coil spring just saves a lot of space. Older Porches and other rear engine cars employ these types of springs. They are referred to as torsion springs as this describes the motion required to deform them. Imaging what the rear of a motorcycle would look like with a torsion spring. I can't
So to compact the spring we coil it up.
Application.
Lets now look at applying the coil spring to a motorcycle. The job of the spring is to keep the tire on the ground. So when we hit a bump the tire doesn't jump, it moves up with the bump keeping contact. If it were just for your comfort we would just spring the seat like in the olden days. Here is a simple example. The old twin shock layout:
But for simplicity we will imagine only one spring and ignore the damper for now. Also we will pretend there is no front suspension.
Lets say this spring has a rate of 50lb/inch and a length of 10 inches (just to keep it round) also we will not include any weight of the motorcycle.
If the rider weighs 200 pounds and he sits on the bike. His 200 pounds deforms our liner rate spring 4 inches. There is still 6 inches of travel so the spring can absorbs another 300 pounds. But we have used up 4 valuable inches of travel. Now the tire is pretty close to the fender! So to retain that travel we need to increase the unloaded height of the bike 4 inches. Now our guy needs a box to get on the thing.. Imagine if he want to take a friend along! Enter...
Preload:
Preload is virtual weight. Here is how it works;
Our guy gets a friend who weighs the same to sit on the bike. The suspension deforms. The guy now bolts a bracket to hold the spring in place. Then the friend gets off and the guy raises the spring mount 4 inches. The height is back to before and the spring is 4 inches shorter now. When the 200 pound load is added again the spring doesn't budge. Why?
Because it is still trying to get back to it's original shape with the 200 pound of force the friend loaded it with, it has been preloaded.
So lets now look at a modern shock:
That large nut arrangement at the right is the preload adjustment.
Now that we are back in the real world I'll reduce the numbers a bit. This spring probably has about a 1 Kilo /mm rate. So for every mm of compression we need 1 kilo (2.2 pounds per 0.039inches for you Imperialists.) Pretty stiff! As you can see this shock probably has 100 mm of preload available. That translates to 100 Kilos of preload.
I'm not going to get too far into actual suspension set up but I will say that some sag is looked for in the rear suspension. Something around 20 mm, that means when you get on the bike you want it to sag about 20 mm. On the above spring that means 20 Kilos removed from the preload needed. This in turn gives us even more preload available.
Getting complicated:
As I stated we are using liner springs. Problem is the load the spring is subjected to is not liner
In cornering the centrifugal forces are absorbed by the spring. This stinks because it takes up needed travel. Also a bad thing would be to use up all the travel cornering then need some more. Bad news! Further we need two types of springs. We need a soft spring to keep the tire on the ground while going straight. Then we need a stiff spring to keep the cornering from taking up all the travel. We could use two springs or we could use a non-liner rate spring. These were tried and the hard part is to change the rate you need to change the springs. Finding the right spring is hard and adjusting it for the rider harder. Enter the linkage.
Link suspensions:
There have been many types of link suspensions used all with cool sounding names. We older guys remember them. The most successfully is the one currently used in the Kat. So how do we make a single rate spring act like a rising rate spring and make it adjustable (if needed)?
Everyone knows Archimedes right? No? Bummer.. Cool guy. Anyway we all know what a lever is:
Because the arm on the left side of the fulcrum is longer than the right the amount of force needed to lift the weight is less than the actual weight. This ratio is based on the ratio of one arm to the other. So if you switch sides the force needed is more than the weight. Here is the link type suspension:
This is different than the Kat suspension but the same rules apply and it is a good view.
As you can see the shock is connected to the link and the link to the swing arm. The shape of the link creates a changing leverage point.
So for the first say 2 inches of travel the leverage is fairly even the farther it moves the leverage turns negative. It is on a curve and the shape and size of the link can determine the exact curve. For example on a cruiser the link can provide a soft spring moving up to stiff and for a sport bike it is stiff moving up to hard. This can get us exactly what we want. In fact we can have very soft to hard over a short part of the travel or very soft to hard over the whole travel. The more the swing arm moves the more force it takes to move it. Race bikes can have several mounting points to change the leverage allowing lightning fast changes in spring rate.
Complications still
As I stated earlier a coil spring is a coiled torsion spring. Compressing the spring is not folding it. Imagine a piece of zig-zagged shaped steel, when you compress this you are folding it. Compress a coil spring and you are twisting it along it's whole length to get the coils closer. This is why I mentioned the effective spring rate. The spring rate expressed as Kg/mm is based on the length of the coil spring. The actual spring rate of the material is torsional and would be Kg/ degree/mm. So imagine a 100mm length of spring stock. Lock one end in a vise and fit a handle to the other. In order to twist it 10 degrees it requires 100Kg. If you double the length it would only require 50Kg. That is why it is force per degree per length. Remember I said you are stressing the atomic link of the material in deforming it? Well you can only force those links so much. But you can double the amount of links. Back to coils. You have two coils springs;
One 10 inches long with 10 coils.
Another 4 inches long with 4 coils.
Both made with the same material. This makes the effective spring rate of the shorter one higher. Compressing the long one 1 inch is only compressing it 1/10 it's length. Compressing the short one 1 inch is 1/4 its length. So you can change the effective rate of the spring by removing coils. However you also reduce the maximum stroke.
So where is all this going? Beside in one ear and out the other?
Here:
1988 until 2006 the 750 Katana has a claimed dry weight of 200-211 Kg.
GSX-R 1 176-181Kg
GSX-R 2 187-208Kg
GSX-R 3 179Kg
GSX-R 4 166- 163Kg
Lets take the second worst case scenario. (you "fat" 98+ are on yer own!) A 2006 GSX-R shock on my 1995 Katana. I won't ever argue about the weight, thats what I nice guy I am. That leaves me with a weight delta of 37Kg. Remembering that some of the weight is taken by the forks.. say for argument 40% But tell you what I'm gonna do. I'll take it all! 37Kg. OK.. I will fit this shock to my Katana.
Then in my careful set up I will dial in another 37Kg of preload. If the GSX-R spring has an effective rate of 100Kg/mm. I'm guessing more. I've messed with a lot of springs and the GSX-R spring is stiff. So that equals 37mm (probably less) extra preload. Pfft. A pittance..
If I'm still worried? I'll lop off a coil and fit a spacer. Increasing the spring rate (with a small loss in stroke) and there you go. 21st century rear shock technology on my old Kat..
I hope you learned a little about springs and how they work in motorcycle suspensions. I also hope that from now on no one will say:
"But that (insert suspension component here) is rated for the GSX-R.. Not the heavier Katana.. "
I'll leave the damper stuff to some other hardy soul (I can tell you're all fighting for the chance!) and move on to something more fun for next time. Rear brakes!
Other installments in the "more than you wanted to know" series:
A lot of information about Hydraulics
A lot of information about Vacuum
A lot of information about Geometry
Springs. I thought I'd write about it as there have been discussions about rear suspensions. The basis to which is a spring. Front suspensions too are based on springs. First things first.
A Spring is a device that stores kinetic energy. Any substance that can be deformed by force then return that energy by reforming completely is a spring. Some household examples:
Rubber erasers.
The little springs inside ball point pens.
Rubber bands.
Rubber bands:
By pulling with XX amount of force you can deform the rubber band (stretch it) and when you release it it releases the exact* same amount of force when it returns to its original shape. The stretching is possible due to stressing or deforming the links of rubber on the atomic level..
(wow) That leads to;
*Not the exact amount as there are loses in friction and heat etc..
Modulus.
The modulus of a material in simple terms describes its springing ability.
What are considered "soft" metals have low a modulus. When deformed to a point they retain that deformation. They stay bent or in the case of a balloon they burst or once deflated do not return to their original shape. Now at birthday parties you can say "don't exceed the balloon's modulus!!" Materials with higher moduli return to the original shape. The amount of force required to deform a high moduli material, what we will now call a spring is referred to as it's rate.
Spring rate:
This is expressed in force/distance. For example a spring with a rate of 100Lb/ inch requires 100 pounds of force to change it's length 1 inch.
This is really the effective spring rate. I'll expand on that later.
Types of springs:
As I described it anything that can be deformed and return the force thus returning to shape is a spring. Lets look at a common expansion spring. A rubber band. Take a rubber band in your hands and you can explore a lot of the concepts around springs. First off loop it around the fingers of each hand. You can see that by increasing the distance between your fingers you can stretch the band. OK duh, but bare with me. Once the slack is taken up, that is once the band is tight, increasing the distance, stretching the band, requires a defined force. This force is pretty small but it is liner. Liner refers to an amount that increases the same amount over a distance. So if it takes say 1oz of force to stretch the band 1 inch then the second inch takes another ounce. So to stretch the band 3 inches that would require 3 ounces. If the force were non-liner (or as we tech guys say "on a curve"), the amount to stretch each successive inch would require greater force. So 1 ounce for the first inch, 2 ounces the second inch 3 ounces the third. So instead of requiring 3 ounces for 3 inches of stretch it would require 6 ounces. The other thing that you should notice is that the whole length of the spring stretches the same amount. That is that you cannot stretch the middle of a liner spring while keeping the center unchanged. This verifies it is liner.
Compression springs work the same way. The rubber eraser when bent will return to shape unless you exceed the modulus and then it breaks.
A coil spring (like inside the ball point) is a compression spring and an expansion spring. Since it has spaces between the coils. Coil springs work by twisting the bar stock they are made from. A 3 foot piece of round spring steel works similarly as a coil spring. It takes the same amount of force to twist a length of 100 pound spring stock no matter the shape. The coil spring just saves a lot of space. Older Porches and other rear engine cars employ these types of springs. They are referred to as torsion springs as this describes the motion required to deform them. Imaging what the rear of a motorcycle would look like with a torsion spring. I can't
So to compact the spring we coil it up.Application.
Lets now look at applying the coil spring to a motorcycle. The job of the spring is to keep the tire on the ground. So when we hit a bump the tire doesn't jump, it moves up with the bump keeping contact. If it were just for your comfort we would just spring the seat like in the olden days. Here is a simple example. The old twin shock layout:
But for simplicity we will imagine only one spring and ignore the damper for now. Also we will pretend there is no front suspension.
Lets say this spring has a rate of 50lb/inch and a length of 10 inches (just to keep it round) also we will not include any weight of the motorcycle.
If the rider weighs 200 pounds and he sits on the bike. His 200 pounds deforms our liner rate spring 4 inches. There is still 6 inches of travel so the spring can absorbs another 300 pounds. But we have used up 4 valuable inches of travel. Now the tire is pretty close to the fender! So to retain that travel we need to increase the unloaded height of the bike 4 inches. Now our guy needs a box to get on the thing.. Imagine if he want to take a friend along! Enter...
Preload:
Preload is virtual weight. Here is how it works;
Our guy gets a friend who weighs the same to sit on the bike. The suspension deforms. The guy now bolts a bracket to hold the spring in place. Then the friend gets off and the guy raises the spring mount 4 inches. The height is back to before and the spring is 4 inches shorter now. When the 200 pound load is added again the spring doesn't budge. Why?
Because it is still trying to get back to it's original shape with the 200 pound of force the friend loaded it with, it has been preloaded.
So lets now look at a modern shock:
That large nut arrangement at the right is the preload adjustment.
Now that we are back in the real world I'll reduce the numbers a bit. This spring probably has about a 1 Kilo /mm rate. So for every mm of compression we need 1 kilo (2.2 pounds per 0.039inches for you Imperialists.) Pretty stiff! As you can see this shock probably has 100 mm of preload available. That translates to 100 Kilos of preload.
I'm not going to get too far into actual suspension set up but I will say that some sag is looked for in the rear suspension. Something around 20 mm, that means when you get on the bike you want it to sag about 20 mm. On the above spring that means 20 Kilos removed from the preload needed. This in turn gives us even more preload available.
Getting complicated:
As I stated we are using liner springs. Problem is the load the spring is subjected to is not liner
In cornering the centrifugal forces are absorbed by the spring. This stinks because it takes up needed travel. Also a bad thing would be to use up all the travel cornering then need some more. Bad news! Further we need two types of springs. We need a soft spring to keep the tire on the ground while going straight. Then we need a stiff spring to keep the cornering from taking up all the travel. We could use two springs or we could use a non-liner rate spring. These were tried and the hard part is to change the rate you need to change the springs. Finding the right spring is hard and adjusting it for the rider harder. Enter the linkage.Link suspensions:
There have been many types of link suspensions used all with cool sounding names. We older guys remember them. The most successfully is the one currently used in the Kat. So how do we make a single rate spring act like a rising rate spring and make it adjustable (if needed)?
Everyone knows Archimedes right? No? Bummer.. Cool guy. Anyway we all know what a lever is:
Because the arm on the left side of the fulcrum is longer than the right the amount of force needed to lift the weight is less than the actual weight. This ratio is based on the ratio of one arm to the other. So if you switch sides the force needed is more than the weight. Here is the link type suspension:
This is different than the Kat suspension but the same rules apply and it is a good view.
As you can see the shock is connected to the link and the link to the swing arm. The shape of the link creates a changing leverage point.
So for the first say 2 inches of travel the leverage is fairly even the farther it moves the leverage turns negative. It is on a curve and the shape and size of the link can determine the exact curve. For example on a cruiser the link can provide a soft spring moving up to stiff and for a sport bike it is stiff moving up to hard. This can get us exactly what we want. In fact we can have very soft to hard over a short part of the travel or very soft to hard over the whole travel. The more the swing arm moves the more force it takes to move it. Race bikes can have several mounting points to change the leverage allowing lightning fast changes in spring rate.
Complications still
As I stated earlier a coil spring is a coiled torsion spring. Compressing the spring is not folding it. Imagine a piece of zig-zagged shaped steel, when you compress this you are folding it. Compress a coil spring and you are twisting it along it's whole length to get the coils closer. This is why I mentioned the effective spring rate. The spring rate expressed as Kg/mm is based on the length of the coil spring. The actual spring rate of the material is torsional and would be Kg/ degree/mm. So imagine a 100mm length of spring stock. Lock one end in a vise and fit a handle to the other. In order to twist it 10 degrees it requires 100Kg. If you double the length it would only require 50Kg. That is why it is force per degree per length. Remember I said you are stressing the atomic link of the material in deforming it? Well you can only force those links so much. But you can double the amount of links. Back to coils. You have two coils springs;
One 10 inches long with 10 coils.
Another 4 inches long with 4 coils.
Both made with the same material. This makes the effective spring rate of the shorter one higher. Compressing the long one 1 inch is only compressing it 1/10 it's length. Compressing the short one 1 inch is 1/4 its length. So you can change the effective rate of the spring by removing coils. However you also reduce the maximum stroke.
So where is all this going? Beside in one ear and out the other?
Here:
1988 until 2006 the 750 Katana has a claimed dry weight of 200-211 Kg.
GSX-R 1 176-181Kg
GSX-R 2 187-208Kg
GSX-R 3 179Kg
GSX-R 4 166- 163Kg
Lets take the second worst case scenario. (you "fat" 98+ are on yer own!) A 2006 GSX-R shock on my 1995 Katana. I won't ever argue about the weight, thats what I nice guy I am. That leaves me with a weight delta of 37Kg. Remembering that some of the weight is taken by the forks.. say for argument 40% But tell you what I'm gonna do. I'll take it all! 37Kg. OK.. I will fit this shock to my Katana.
Then in my careful set up I will dial in another 37Kg of preload. If the GSX-R spring has an effective rate of 100Kg/mm. I'm guessing more. I've messed with a lot of springs and the GSX-R spring is stiff. So that equals 37mm (probably less) extra preload. Pfft. A pittance..
If I'm still worried? I'll lop off a coil and fit a spacer. Increasing the spring rate (with a small loss in stroke) and there you go. 21st century rear shock technology on my old Kat..
I hope you learned a little about springs and how they work in motorcycle suspensions. I also hope that from now on no one will say:
"But that (insert suspension component here) is rated for the GSX-R.. Not the heavier Katana.. "
I'll leave the damper stuff to some other hardy soul (I can tell you're all fighting for the chance!) and move on to something more fun for next time. Rear brakes!
Other installments in the "more than you wanted to know" series:
A lot of information about Hydraulics
A lot of information about Vacuum
A lot of information about Geometry
. 
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