To Cartridge Or Not To Cartridge . . . That Is The Question

[MCRIDER007]

Yes, springs will be the same. You may need a different length spacer, but that is trivial. 1.0 kg/mm is a good target for your spring weight for solo riding. If you plan a lot of 2-up you might go 1.05 or 1.10 max.

Why would you put heavier springs on the front if you plan to ride 2-up? All of the passengers weight is behind the shock which increases the sag at the back end but makes the front end lighter (ever notice how easy it is to lift the front wheel when you have a passenger). If you are a very heavy rider than a third to a half of your weight is going to be transferred to the front end and that would justify heavier springs.

That is a good question. Here's my thinking on it, tell me where I'm wrong...

While statically the passenger weight will be primarily supported by the rear shock springs, it will not dynamically be suspended by the rear only. The front forks do not just respond to vertical (gravitational) input from the undulating road surface, they also respond to the forward vector of inertial force riding forwards into the bumps. Since the total weight of the riders, luggage, bike and gear will be increased, with the same fork spring rate the forks would travel more over a particular bump amplitude.

I believe that the suspension springs' rates should not be selected to achieve a certain ride height alone, but rather to achieve a particular amount of suspension travel (over the expected terrain). Counter to the thinking of some folks who believe the stiffer the better so the suspension doesn't move as much and change the bike's attitude, I think that the optimum spring weight would be one that allows the suspension to move the maximum, entire available travel when used over the worst case terrain without bottoming (or topping) out.

If a particular weight fork springs allowed full stroke on the worst case bumps, adding more weight anywhere on the bike would cause the same springs to bottom out. Yes, you could dial up increased compression damping to over come that tendency, but that would not be optimum since it will also harshen the ride over smaller bumps.

It may be that 1.0kg/mm is already stiffer than my definition of optimum, in which case there may be no need to increase the spring rate any higher.

Since I really don't know the answer I can't tell you where (or if) you are wrong, but I have never felt the need for heavier springs on the front when riding double except maybe in emergency braking conditions when the passenger would be pushing against the rider transferring too much weight to the front end making easier for the forks to bottom (which is the primary reason that lightweight track bikes often use relatively heavy fork springs).

When I first visited GP Suspension I asked about the fork spring weights and was told that they use a universal weight (at that time it was .95 KG, they later went to 1.0 KG) unless the rider weight exceeded 350 lbs, they were not concerned about passenger weight and it you think about it, the weight distribution on the suspension is going to be quite different for a 350 lb rider than a 200 lb rider with a 150 lb passenger.

Most riders do not realize the effect that speed has on suspension due to the lift that is created by air pushing against the faring and under the motorcycle but the bike gets lighter with speed, the suspension unloads, and on most bikes the front is impacted more than the rear. As the front gets lighter relative to the rear, do you need heavier springs to maintain contact with the pavement or does that just change the steering geometry in a negative way? I don't know the answer, when I have a passenger I adjust the shock the best I can, the forks never seem to need adjustment. Food for thought.

 

[yamafitter]

FredW is sort of on the correct course in his analysis on spring rates. I do disagree on his discussion about selecting spring rate however. Ride height is one of the most important considerations when selecting spring rates since it puts the bike in the correct range for the damping to be effective. If the roads you travel are really rough you should be backing off the rebound and compression damping to control it from being too harsh. It is only if the roads are so rough that damping adjustment alone can't control the travel should you consider stiffer springs.

When riding two up the extra mass of the pillon will be transferred forward during braking causing more fork "diving" and therefore require more compression damping. You should also increase both fork and rear spring preload when possible when carrying extra loads or passenger. Suspension tuning is part science and part witchcraft. The witchcraft part comes from the fact that you are relying on what the rider is feeling in the way of feedback as to what the bike is doing which is very subjective.

 

[Fred W]

Yeah, I did not mean to say that ride height was unimportant. It obviously is.

Thanks for the replies. More stuff to think about.

 

[VanHarlan]

Making Flippy Floppy . . . and being of the mind that one should do things right the first time . . . I'm going to do the complete AK-20 install all at once for a variety of reasons . . . which means I need to get back to work to pay for it.