Suspension - Sportbike Forum: Sportbike Motorcycle Forums
Reply
 
LinkBack Thread Tools Display Modes
post #1 of 28 (permalink) Old 07-24-2008, 07:52 AM Thread Starter
 
Join Date: May 2008
Posts: 867
Angry Suspension

Damn engineers have to make everything so complicated.

How does a bikes suspension work? I know how important it is to the overall performance and handling but what's it made of...? Damping, forks, something about fluid..? There's even pictures and I still don't get it!
RydeAddict is offline  
Sponsored Links
Advertisement
 
post #2 of 28 (permalink) Old 07-24-2008, 08:34 AM
 
Join Date: Mar 2004
Posts: 8,798
Alright, lets see how much of it I can explain without having to draw pictures. A common misconseption is that suspension controls handling, which is not entirely true. Suspension components control how the wheels move in relation to the frame. Lets keep this in regard to bikes, because it gets much more complicated with cars.

Wheels have to move in respect to the frame of the bike for three completely different reasons.
1. Comfort. bumpiness sucks. Since we are talking about sportbikes, we'll disregard this entirely.
2. Surface irregularities. There are bumps and things on the surface, the road can reach up in a bump, or drop away in a pothole.
3. Weigh transfer. The bikes weight shifts front or rear depending on wether the bike accelerates or brakes.

First thing first (And I'm may well be going over things you know full well) is the property of inertia and momentum. Basically, the heavier something is, the harder it is to get it going in whatever direction, and the harder it is to make it stop. This is pretty self evident. So if a bike had no suspension, and it hit a bump, the wheel has no choice but to travel up, taking the rest of the bike with it. Once the entire weight of the bike is traveling up, it would take it a great deal of time to come back down, a time during which your wheels will not have contact with the ground and you will have no control.
If, you let the wheel move independently of the bike, it weights much less, and thus takes much less time to come back down. This is one of the reason why it is so important to have very light wheels. It is much more important to save weight below the suspension components (wheels, tires, brake calipers, swing arm, fork castings) than it is above them (the rest of the bike)
This being the real world, there are limitations to moving the wheels independently of the bike, and the main one is travel distance. The wheels can only travel so far before the top of the wheel hits the body of the bike, of the bottom of the bike hits the ground. This distance is called the stroke. You can increase the stroke by moving the bike further away from the ground, like you see in dirtbikes. But doing this moves the center of gravity higher, making the bike harder to turn, and more likely to stoppie/wheelie. So like all things, this is a compromize. Sportbike have around 5"-6" of travel on their suspension.

Continued on next post



Vash is offline  
post #3 of 28 (permalink) Old 07-24-2008, 08:40 AM
 
Join Date: Mar 2004
Posts: 8,798
In order to deal with surface irregularities, wheels have to move quickly up and down, maintaining contact with the road as much as possible. The travel distance isnt as big of a deal, since bumps usually dont get too big.
However, when dealing with weight transfer, what is important, isnt how far the wheels travel, but that they continue traveling the whole time weight is being transfered. When the suspension reached the end of its travel while braking the bike stoppies, on the other end it wheelies. Either scenario leaves only one wheel on the ground and the bike becomes very difficult to control.

In street riding, surface irregularities is the bigger concern, since there is more of them, and less weight transfer. On the track, the pavement is better, and weight transfer is the bigger consern. But tracks arent made out of glass, and so there are still bumps.

Continued on next post



Vash is offline  
 
post #4 of 28 (permalink) Old 07-24-2008, 08:46 AM
 
Join Date: Mar 2004
Posts: 8,798
There are two ways in which suspension components control wheel movement. mechanically via springs, and hydraulically via oil.

A spring responds to force by compressing. It will compress a certain distance for a given ammount of force, and then it will extend back (overshooting abit and oscillating back and forth).
In order to control this oscilation, the fork or shock has a sylinder and a piston in it filled with oil. When the fork/shock compresses the oil if forced out of the cylinder thru a small hole. The oil can flow out of this hole at a certain rate, no matter how much force you put behind it.
So, the spring controls how far the wheel travels, and the size of the hole thru which the oil flows controls how fast the wheel will travel on the way there.



Vash is offline  
post #5 of 28 (permalink) Old 07-24-2008, 09:02 AM Thread Starter
 
Join Date: May 2008
Posts: 867
Quote:
Originally Posted by Vash View Post
A spring responds to force by compressing. It will compress a certain distance for a given ammount of force, and then it will extend back (overshooting abit and oscillating back and forth).
In order to control this oscilation, the fork or shock has a sylinder and a piston in it filled with oil. When the fork/shock compresses the oil if forced out of the cylinder thru a small hole. The oil can flow out of this hole at a certain rate, no matter how much force you put behind it.
So, the spring controls how far the wheel travels, and the size of the hole thru which the oil flows controls how fast the wheel will travel on the way there.
Okay I was with ya thru all of that... But the part I quoted just hit me like a freight train. That's what I didn't get, like what happens inside it. And the fork is the shock..? Is that right?
RydeAddict is offline  
post #6 of 28 (permalink) Old 07-24-2008, 09:06 AM
 
Join Date: Mar 2004
Posts: 8,798
This is the first place the suspensions two functions interfere with each other. To deal with bumps, the suspension needs to move quickly. To deal with braking the suspension needs to move slowly, or else it will run out of travel.

First lets deal with setting the SAG, which is the first step of suspension tuning. Basically, when the bike is in its ready form (with you and gear and fuel) you want the suspension to be partway compressed. If it is all the way compressed, then the wheel would not be able to move up when you hit a bump. If it is all the way extended, the wheel will not be able to reach out to the bottom of a pothole. The more a wheel can "reach" the less it can "retract" (and it will need to retract both to deal with bumps and to deal with braking forces).
On the street, pot holes are a bigger concern than braking forces. So the accepted number for sag (how much you compression is compressed when handling the full weight) is 1.25" to 1.5". The ammount it can retract is (Total travel)-Sag. For track duties, pot holes are a much smaller concern, and braking performance is a much bigger concern, so the common track Sag is 1" to 1.25". If you want to sounds really smart, you can express that in millimeters.
Sag is adjusted by changing the preload knobs (collars on the rear shock). This drives a screw into the top of the spring. The name of the adjuster is incorrect, because it doesnt preload anything. The spring doesnt compress anymore, because it still has the exact same weight sitting on it. Instead the screw/collar raises the bike up. So to set the proper sag, you first measure the fully extended suspension (lifting the bike so there is no weight on the wheel) and then measure the how much the suspension compresses when you sit on your bike with all your gear on.
Keep in mind, the preload adjust does not change how much the suspension compresses for a given weight (this is called spring rate and you can only change it by changing the spring) it just chages where in its stroke the suspension is located when you are on the bike and there are no bumps. Measuring it is fairly streight forward on the front, but you have to get abit more creative on the rear.

Some bikes use progressive rate springs. These are springs that are wound tighter at one end than the other. They have a certain spring rate for part of their distance, and then they have a higher spring rate for the rest of their travel. The idea is that the bike can quickly respond to a small bump (with the weaker spring rate) but when braking the spring gets to its stiffer rate to keep you from bottoming out longer. Its like two springs in one! Sounds great on paper, but in reality it sucks. With the spring rate changing, tuning this suspension is enormously more difficult.
Some bikes (motoGP for sure, maybe other race bikes, but no production bikes) use digressive rate springs. These springs get easier to compress the more they are compressed. The idea is that they can take the enormous force applied to them while braking, but they can deal with small bumps while being that loaded. They cant deal with bumps when not being harshly loaded, and they are a serious tuning nightmare, but we are talking about motoGP teams here, they can deal with hard math.



Vash is offline  
post #7 of 28 (permalink) Old 07-24-2008, 09:13 AM
 
Join Date: Mar 2004
Posts: 8,798
Quote:
Originally Posted by RydeAddict View Post
Okay I was with ya thru all of that... But the part I quoted just hit me like a freight train. That's what I didn't get, like what happens inside it. And the fork is the shock..? Is that right?
Forks are the front suspension, the shock it the rear. They are very simular in how they are built, Except that up front there are two forks so the springs are weaker, and in the back there is only one shock. Also the forks hold the wheel directly, while the shock acts on the wheel thru a level, being the swing arm. There is ussually some complicated linkages in the back, but all it does is changes a ratio. In other words the front suspension compresses one inch for every inch the wheel moves. On the rear, the suspension compresses some fraction of what the rear wheel moves. But that fraction is always the same, it never changes, so dont worry about it.

As for oil. Ok, take a saringe and fill it with water. Now push the water out by pressing on the plunger. Do this several times, using a different ammount of force. You will see that there is a cieling to how fast you can push the water out. You can push it slower, but all the force in the world wont push it faster.
There is something very much like it inside your fork. Instead of water it uses oil.



Vash is offline  
post #8 of 28 (permalink) Old 07-24-2008, 09:15 AM
 
Join Date: Mar 2004
Posts: 8,798
Now imagine taking that saringe, and attaching a spring to it, one end to the body the other end to the plunger. Put the whole thing in a dish, so that when the spring pushes the plunger back after you compressed it, it can suck in whatever water it pushed out, and never get filled with air.

That is basically what your suspension looks like.



Vash is offline  
post #9 of 28 (permalink) Old 07-24-2008, 09:19 AM
 
Join Date: Mar 2004
Posts: 8,798
Notice that if the hole in the plunger is small enough, this will also keep the spring from oscillating back and forth. It will compress, retract and stop. If you make the hole real big, the spring can still oscillate. If you make the hole real small it will take a damn long time for the plunger to go down and come back up.

With me?



Vash is offline  
post #10 of 28 (permalink) Old 07-24-2008, 09:38 AM Thread Starter
 
Join Date: May 2008
Posts: 867
I can't believe I was able to keep up but I think I understand it now. I mean, I had to read that like six times but woohoo! Awesome analogy too, even better than a picture. So it's really not that complicated Parks just needed your help writing that chapter. Thanks a lot Vash!

I know I'm slacking but I haven't set the sag on my bike yet (lame, I know). I can't do it myself but I suppose the dealer could. I don't notice the bike move that much when I get on it though, I guess that means it may be stiff... I like to sit way up by the tank when I ride (keeps me from leaning on my arms) and I normally get up and lean over the front of the bike some when I encounter bumps on the road. So it doesn't beat me up too bad. Come to think of it, I always wondered if it was okay to not slow down for railroad tracks... Might be my dirt skills taking over but can't you just hit them at speed and hang on? Things seem to bump you around more going slow sometimes.
RydeAddict is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Sportbike Forum: Sportbike Motorcycle Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome