Understanding shocks and springs

[cowboyup3371]

Okay, so I am looking for new rear shocks and front springs for the bike. Before I get into my question, please know that I am calling that Dave Quinn place to ask about getting rear shocks like someone mentioned here. But I also want to compare prices since I am still on a tight budget over the next several months.

With that in mind, I already know what shocks I need for the bike. I can get the ones from Progressive for about $300 or I also found some on Z1 that I can order for less. However, Z1's site says I need to choose the springs but I don't know how to read the information on it:

Progressive Suspension Rear Springs - 75/120lb - Black
PS03-1394B

OEM: 03-1394B
One pair of black rear springs - 75/120 spring rate - 9.75" length, 1.85" inside diameter. Select Standard Springs if the rider is less than 200lbs and there is limited passenger use.

What exactly does the 75/120lb mean in this? I also saw some for 145/190. Is one of those numbers for someone's weight? How do I know what to choose (AGAIN I will call that other individual)?
Thanks.

[tkent02]

Okay, so I am looking for new rear shocks and front springs for the bike. Before I get into my question, please know that I am calling that Dave Quinn place to ask about getting rear shocks like someone mentioned here. But I also want to compare prices since I am still on a tight budget over the next several months.

With that in mind, I already know what shocks I need for the bike. I can get the ones from Progressive for about $300 or I also found some on Z1 that I can order for less. However, Z1's site says I need to choose the springs but I don't know how to read the information on it:

What exactly does the 75/120lb mean in this? I also saw some for 145/190. Is one of those numbers for someone's weight? How do I know what to choose (AGAIN I will call that other individual)?
Thanks.

Spring rate, the amount of force required to squeeze the spring a certain distance, in this case pounds/inch. Sometimes it's KG/mm or some other measure. When there are two numbers it's a dual rate or progressive rate spring.

[cowboyup3371]

So if the spring moves one inch after 75 pounds sits on it and the max it can support is 120 pounds, I would bottom this particular spring out? Should I get one with a larger number?

[Steve]

No, there is about 4 inches of movement in the shock. The first inch will require 75 pounds to move one inch. It will require more than 75 to move a second inch, more than that for the third inch and 120 pounds for the last inch. Following that logic, 150 pounds will move it less than two inches, because the rate is higher for that second inch.

Also keep in mind that those numbers are per spring, and there are two on the bike. Also, the forces on the spring are not just what you put on the seat. They have to hold up the bike, too, and there are leverage factors due to angles and the fact that the shocks are not mounted at the axle.

When all is said and done, you should be looking for a proper amount of what is called "sag". Usually accepted as 20-25% of the available travel. This means that when you are sitting on the bike, ready to ride (this includes you, your gear, your backpack full of books, whatever), you compress that suspension no more than one inch. If you regularly carry a passenger, you will want to be able to adjust the suspension so that it does not "sag" more than an inch with the additional weight, if possible. Same thing with the front. Not as much adjustability, but you can choose your spring rate, then use different preload spacer lengths to set the ride height. Most forks have about 6 inches of travel, so a proper "sag" would be between 1 and 1.5 inches.

.

[tkent02]

There's no max, it moves at 70 lbs/inch until the small coils are squeezed together, then it moves at 120 lbs/inch until the shock travel is used up. If you have three inches of shock travel, it would take a few hundred lbs of force to bottom out, per shock. Maybe 600 or so with both shocks on the bike. This is the combined weight of the bike and you and the impact of hitting the bump. For you and your bike, you may want something bigger than 70/120, but you definitely don't want the 145/190 unless you weigh 380 lbs, or are loading the bike for an around the world tour. Every bump or hole in the road would be painful with those.
You really need to try the Hagons I'm getting for the 650G, see if they work for you or maybe see if you want a slightly different spring rate. Your bike is a little lighter, slightly less shock absorber angle, so a given spring would feel slightly stiffer on your bike but you will be able to get an idea. For instance, those Ohlins felt a little too stiff on my old 1000, but felt much too hard for the 750, almost painful. They will be great on the heavier 1000 with fairing and hard bags and packed for the road. We can put these 650 shocks on my 550 and you can bounce up and down on it. I'll even loan them to you to try on your bike if you want. I will let you know when they get here, he was going to ship them out yesterday so they will be here pretty soon I hope.